**Part 1**

**Metallic Nanoparticles** 

**1** 

*Poland* 

**Metallic Nanoparticles Coupled with** 

 *Institute of Physics, Nicolaus Copernicus University, Torun* 

Plasmon excitations in metallic nanoparticles provide an efficient way to manipulate electromagnetic fields at the nanoscale (Maier, 2004). While the interactions between plasmons and simple nanostructures such as organic dyes or semiconductor nanocrystals is relatively well described and understood, application of metallic nanoparticles to multipigment structures has started just recently (Carmeli, 2010; Govorov, 2008; Kim, 2011; Mackowski, 2008; Nieder, 2010). Light-harvesting complexes, or more generally, photosynthetic complexes, are quite appealing in this regard as they not only provide an interesting biomolecular system for studying plasmon effect on both the optical properties of pigments and the energy transfer between them, but also they could offer attractive potential application route in photovoltaics (Atwater & Polman, 2010; Mackowski, 2010).

Extending concepts and methods that have been developed for describing the coupling of single organic chromophores with plasmon excitations in metallic nanoparticles (Anger, 2006; Chettiar, 2010; Govorov, 2006) to multi-chromophoric biological systems has not been completely straightforward from both theoretical and experimental points of view. On the one hand, organic molecules or semiconductor quantum dots are much more robust nanostructures than pigment-protein complexes, therefore, the sample preparation in the latter case should be more gentle, so that the protein itself maintains its structure. Preserving protein structure implies that the function of the complex as a whole also remains intact. This assures conservation of the energy transfer pathways between various chromophores comprising the complex as well as identical optical properties, including absorption and fluorescence, to that of the isolated (decoupled from a metallic nanoparticle) biomolecule. On the other hand, from the theory standpoint, biomolecules, and in particular lightharvesting complexes, render themselves a real challenging system to model due to multitude of interactions between chromophores such as chlorophylls and carotenoids (Blankenship 2002), which results in many energy transfer pathways and formation of strongly coupled excitonic systems, as well as conformational changes of the protein itself. Nevertheless, driven by the continuous development of optical spectroscopy and microscopy techniques (Polivka & Sundstom 2004) as well as more efficient modeling tools, significant progress has been achieved in understanding interactions and functions of lightharvesting complexes. It has also been helped by high-resolution crystal structures of the

**1. Introduction** 

**Photosynthetic Complexes** 

*Optics of Hybrid Nanostructures Group* 

Sebastian Mackowski

## **Metallic Nanoparticles Coupled with Photosynthetic Complexes**

## Sebastian Mackowski

*Optics of Hybrid Nanostructures Group Institute of Physics, Nicolaus Copernicus University, Torun Poland* 

## **1. Introduction**

Plasmon excitations in metallic nanoparticles provide an efficient way to manipulate electromagnetic fields at the nanoscale (Maier, 2004). While the interactions between plasmons and simple nanostructures such as organic dyes or semiconductor nanocrystals is relatively well described and understood, application of metallic nanoparticles to multipigment structures has started just recently (Carmeli, 2010; Govorov, 2008; Kim, 2011; Mackowski, 2008; Nieder, 2010). Light-harvesting complexes, or more generally, photosynthetic complexes, are quite appealing in this regard as they not only provide an interesting biomolecular system for studying plasmon effect on both the optical properties of pigments and the energy transfer between them, but also they could offer attractive potential application route in photovoltaics (Atwater & Polman, 2010; Mackowski, 2010).

Extending concepts and methods that have been developed for describing the coupling of single organic chromophores with plasmon excitations in metallic nanoparticles (Anger, 2006; Chettiar, 2010; Govorov, 2006) to multi-chromophoric biological systems has not been completely straightforward from both theoretical and experimental points of view. On the one hand, organic molecules or semiconductor quantum dots are much more robust nanostructures than pigment-protein complexes, therefore, the sample preparation in the latter case should be more gentle, so that the protein itself maintains its structure. Preserving protein structure implies that the function of the complex as a whole also remains intact. This assures conservation of the energy transfer pathways between various chromophores comprising the complex as well as identical optical properties, including absorption and fluorescence, to that of the isolated (decoupled from a metallic nanoparticle) biomolecule. On the other hand, from the theory standpoint, biomolecules, and in particular lightharvesting complexes, render themselves a real challenging system to model due to multitude of interactions between chromophores such as chlorophylls and carotenoids (Blankenship 2002), which results in many energy transfer pathways and formation of strongly coupled excitonic systems, as well as conformational changes of the protein itself. Nevertheless, driven by the continuous development of optical spectroscopy and microscopy techniques (Polivka & Sundstom 2004) as well as more efficient modeling tools, significant progress has been achieved in understanding interactions and functions of lightharvesting complexes. It has also been helped by high-resolution crystal structures of the

Metallic Nanoparticles Coupled with Photosynthetic Complexes 5

fluorophore due to plasmon excitation in the metallic nanoparticle. On the other hand, the quantum efficiency, when approaching the range of distances shorter than 20 nm, starts to drop significantly, due to the non-radiative energy transfer from the fluorophore to the nanoparticle. The net result of these two processes in displayed in Fig. 1, where a clear nonmonotonic dependence of the intensity of fluorescence emitted by the fluorophore upon the distance to the metallic nanoparticle can be seen. Importantly, the strongest plasmon induced enhancement of the fluorescence occurs for distances between 10 and 30 nm; for smaller distances non-radiative fluorescence quenching dominates, while for longer

distances the fluorophore barely feels the presence of the metallic nanoparticle.

Fig. 2. Comparison between absorption spectra of spherical end elongated gold

the radiative rate should also increase as a result of plasmon excitation.

in this contribution.

nanoparticles with the fluorescence and absorption of the LH2 complex. In the first case plasmon excitations should influence mainly the absorption in the visible range, in the second case the effect should be visible for absorption and emission in the infrared.

Another critical parameter that influences the interaction between metallic nanoparticle and fluorophore is the relation of their spectral properties. This is shown schematically in Fig. 2. In the first case scenario the absorption of metallic nanoparticles overlaps significantly with absorption of a biomolecule (in this case this is the LH2 complex from purple bacteria) in the spectral range of about 530-550 nm, while featuring virtually no overlap with the fluorescence. One may expect that for such a combination the major influence of metallic nanoparticles is due to absorption enhancement. In contrast, for a hybrid nanostructures built of components characterized with spectral properties as those displayed in Fig. 2b, there should be absorption enhancement both around 560 nm as well as in the infrared spectral region, around 800 nm. In addition, since there is a spectral overlap between plasmon band and fluorescence emission,

Optical spectroscopy provides variety of techniques that allows for distinguishing between various processes that determine the net effect of plasmonic excitations in metallic nanoparticles on a fluorophore. Indeed, in an ideal situation, where only absorption rate is affected by the plasmon excitation, there should be no change in the fluorescence decay time, while an additional band should appear in the fluorescence excitation spectrum. In contrast, when only radiative rate increases as a result of plasmon coupling, the fluorescence excitation spectrum for a hybrid nanostructure should be identical to the reference structure, with much shorter fluorescence decay time. Several experimental configurations exhibiting these various aspects of plasmon coupling with pigment – protein complexes are discussed

complexes (Hofmann 1996; McDermott, 1995), enabling thus direct association of the pigments as well as their interactions both with themselves and the protein with the actual structure and spatial arrangement of the pigments in these systems.

The purpose of this chapter is to review recent research carried out on hybrid nanostructures composed of metallic nanostructures and light-harvesting complexes. In general, the research is focused on improving the light absorption of the light-harvesting complexes through properly designed plasmonic nanostructures. However, before we start discussing particular hybrid nanostructures fabricated in the context of plasmonically enhanced absorption of light-harvesting complexes, we describe two basic concepts of metal-enhanced fluorescence: distance dependence of the fluorescence intensity and the influence of spectral properties of metallic nanoparticles and placed nearby molecules (Anger, 2006). This brief introductory discussion is essential for understanding the rationale behind designing hybrid nanostructures that involve biological fluorescing complexes.

Fig. 1. Dependence of the quantum yield and non-radiative fluorescence quenching upon the distance. Fluorescence intensity of a chromophore displayed as a function of the distance between the chromophore and spherical metallic nanoparticle with 40 nm diameter.

The optical properties of a fluorophore placed in the vicinity of a metallic nanoparticle are strongly affected by plasmon excitations induced in the latter by a laser light. Without a metallic nanoparticle, a fluorophore is characterized with three rates: absorption rate, radiative rate, and non-radiative rate. Since the oscillation of electrons in the metallic nanoparticle results in creation of local electromagnetic field, in principle all three rates can be changed (Lakowicz, 2006). In addition, another process related to non-radiative energy transfer from the fluorophore to the metallic nanoparticle could also take place in such a hybrid nanostructure. The influence of plasmon excitations upon the quantum yield of a fluorophore and non-radiative energy transfer between the fluorophore and metallic nanoparticle has been recently studied theoretically. In particular, the dependence on the separation distance between the two nanostructures has been analyzed in detail. It turns out that the distance between the fluorophore and metallic nanoparticle is of critical importance in regard to the process that plays dominant role in such a system. In Fig. 1 we show the dependence of the excitation rate and quantum efficiency of a fluorophore upon the distance to the metallic nanoparticle. In this example we consider a metallic nanoparticle with diameter of 40 nm**.** The excitation efficiency increases exponentially with reducing the distance, which is a clear manifestation of stronger electromagnetic field felt by the

complexes (Hofmann 1996; McDermott, 1995), enabling thus direct association of the pigments as well as their interactions both with themselves and the protein with the actual

The purpose of this chapter is to review recent research carried out on hybrid nanostructures composed of metallic nanostructures and light-harvesting complexes. In general, the research is focused on improving the light absorption of the light-harvesting complexes through properly designed plasmonic nanostructures. However, before we start discussing particular hybrid nanostructures fabricated in the context of plasmonically enhanced absorption of light-harvesting complexes, we describe two basic concepts of metal-enhanced fluorescence: distance dependence of the fluorescence intensity and the influence of spectral properties of metallic nanoparticles and placed nearby molecules (Anger, 2006). This brief introductory discussion is essential for understanding the rationale behind designing hybrid nanostructures that involve biological fluorescing complexes.

Fig. 1. Dependence of the quantum yield and non-radiative fluorescence quenching upon the distance. Fluorescence intensity of a chromophore displayed as a function of the distance

The optical properties of a fluorophore placed in the vicinity of a metallic nanoparticle are strongly affected by plasmon excitations induced in the latter by a laser light. Without a metallic nanoparticle, a fluorophore is characterized with three rates: absorption rate, radiative rate, and non-radiative rate. Since the oscillation of electrons in the metallic nanoparticle results in creation of local electromagnetic field, in principle all three rates can be changed (Lakowicz, 2006). In addition, another process related to non-radiative energy transfer from the fluorophore to the metallic nanoparticle could also take place in such a hybrid nanostructure. The influence of plasmon excitations upon the quantum yield of a fluorophore and non-radiative energy transfer between the fluorophore and metallic nanoparticle has been recently studied theoretically. In particular, the dependence on the separation distance between the two nanostructures has been analyzed in detail. It turns out that the distance between the fluorophore and metallic nanoparticle is of critical importance in regard to the process that plays dominant role in such a system. In Fig. 1 we show the dependence of the excitation rate and quantum efficiency of a fluorophore upon the distance to the metallic nanoparticle. In this example we consider a metallic nanoparticle with diameter of 40 nm**.** The excitation efficiency increases exponentially with reducing the distance, which is a clear manifestation of stronger electromagnetic field felt by the

between the chromophore and spherical metallic nanoparticle with 40 nm diameter.

structure and spatial arrangement of the pigments in these systems.

fluorophore due to plasmon excitation in the metallic nanoparticle. On the other hand, the quantum efficiency, when approaching the range of distances shorter than 20 nm, starts to drop significantly, due to the non-radiative energy transfer from the fluorophore to the nanoparticle. The net result of these two processes in displayed in Fig. 1, where a clear nonmonotonic dependence of the intensity of fluorescence emitted by the fluorophore upon the distance to the metallic nanoparticle can be seen. Importantly, the strongest plasmon induced enhancement of the fluorescence occurs for distances between 10 and 30 nm; for smaller distances non-radiative fluorescence quenching dominates, while for longer distances the fluorophore barely feels the presence of the metallic nanoparticle.

Fig. 2. Comparison between absorption spectra of spherical end elongated gold nanoparticles with the fluorescence and absorption of the LH2 complex. In the first case plasmon excitations should influence mainly the absorption in the visible range, in the second case the effect should be visible for absorption and emission in the infrared.

Another critical parameter that influences the interaction between metallic nanoparticle and fluorophore is the relation of their spectral properties. This is shown schematically in Fig. 2. In the first case scenario the absorption of metallic nanoparticles overlaps significantly with absorption of a biomolecule (in this case this is the LH2 complex from purple bacteria) in the spectral range of about 530-550 nm, while featuring virtually no overlap with the fluorescence. One may expect that for such a combination the major influence of metallic nanoparticles is due to absorption enhancement. In contrast, for a hybrid nanostructures built of components characterized with spectral properties as those displayed in Fig. 2b, there should be absorption enhancement both around 560 nm as well as in the infrared spectral region, around 800 nm. In addition, since there is a spectral overlap between plasmon band and fluorescence emission, the radiative rate should also increase as a result of plasmon excitation.

Optical spectroscopy provides variety of techniques that allows for distinguishing between various processes that determine the net effect of plasmonic excitations in metallic nanoparticles on a fluorophore. Indeed, in an ideal situation, where only absorption rate is affected by the plasmon excitation, there should be no change in the fluorescence decay time, while an additional band should appear in the fluorescence excitation spectrum. In contrast, when only radiative rate increases as a result of plasmon coupling, the fluorescence excitation spectrum for a hybrid nanostructure should be identical to the reference structure, with much shorter fluorescence decay time. Several experimental configurations exhibiting these various aspects of plasmon coupling with pigment – protein complexes are discussed in this contribution.

Metallic Nanoparticles Coupled with Photosynthetic Complexes 7

energy transfer dynamics as well as inter-pigment interactions in a well-defined geometry

Fig. 3. Pigment structure of the PCP complex reconstituted with Chl *a* together with absorption (black line) and fluorescence (red line) measured in water solution at room

spectral range, but it extends it into the blue-green spectral region.

The absorption spectrum of the Chl-PCP displayed in Fig. 3 has an intense, broad band between 400 to 550 nm that is mainly due to Per absorption, and two Chl – related bands at 440 nm (Soret) and 660 nm (QY). One example of chlorophylls used for reconstituting PCP complexes is [3-acetyl]-chlorophyll a (acChl). Chemically, it differs from Chl only by the C-3 substituent, but the absorption and fluorescence spectra of acChl-PCP, the PCP complex reconstituted with acChl, are red shifted as compared to the PCP complex containing Chl. At the same time the Per absorption in the blue-green spectral range is affected very slightly. For the PCP complexes reconstituted with acChla the absorption of the Qy band of the chlorophyll molecules is shifted by approximately 20 nm to the red. The fluorescence emission of the PCP complex originates from weakly coupled Chl molecules and it appears at 670 nm for Chl-PCP and 690 nm for acChl-PCP. Upon absorption of light, peridinins in PCP transfer their electronic excitation to Chl a. The efficiency of this excitation energy transfer is higher than 90% [20]. Subsequently, Chl a passes the energy on to membranebound light-harvesting complexes and the Photosystem II. Clearly, the absorption spectrum of PCP enables the photosynthetic apparatus to harness the sunlight not only in the red

Optical spectroscopy studies of both native and reconstituted PCP complexes have been carried out on the ensemble (Akimoto, 1996; Kleima, 2000; Krueger, 2001) and singlemolecule levels (Mackowski, 2007; Wormke, 2007a; Wormke, 2008). Using transient absorption in femtosecond timescale main energy transfer pathways have been described, it has also been demonstrated that the two Chl a molecules interact relatively weakly with characteristic transfer time between them to be of the order to 12 ps (Kleima, 2000). These observations were also corroborated with fluorescence studies of individual PCP complexes: it has been shown that it is possible to distinguish emission originating from each of the two Chl a molecules and using the property of sequential photobleaching of the Chl the energy splitting between the two molecules in the monomer were determined (Wormke, 2007a). Recent work on PCP complexes reconstituted with both Chl a and Chl b provided coherent description of the energy transfer pathways and dynamics in this unique antenna

given by the protein (Mackowski, 2007, Polivka, 2005).

temperature.

(Mackowski, 2007).

## **2. Materials and methods**

In this section we introduce the structure and the optical properties of light-harvesting complexes used in our studies as well as present basic characteristics of metallic nanostructures, including their morphology and plasmon characteristics. Next we present experimental techniques employed for investigating the interactions between plasmon excitations and chromophores embedded in the proteins. These include standard absorption and fluorescence/fluorescence excitation spectroscopy, both in solution and in a layered geometry, as well as confocal fluorescence microscopy coupled with time-resolved capability and spectrally-resolved detection. Combination of all these experimental techniques allows for comprehensive description of plasmon-induced effects on the complex biomolecular systems.

## **2.1 Light-harvesting complexes**

Pigment-protein complexes that take part in photosynthesis can be generally divided into two groups: complexes containing reaction centers, which carry out charge separation, and complexes responsible solely for harvesting the sunlight and transferring it to the reaction centers. Large proteins of Photosystem I and Photosystem II fall into the first category, while light harvesting complex 2 (LH2) from purple bacteria and peridinin – chlorophyll – protein (PCP) complex from algae belong to the second group. As the structure and function of all these biomolecules has been described in detail previously, we focus here only on the aspects that are relevant for understanding the influence of plasmon excitations on the optical properties of light-harvesting systems, PCP and LH2.

## **2.1.1 Peridinin-chlorophyll-protein**

Light-harvesting complexes were developed in the course of evolution in order to enhance and broaden the absorption of photosystems for the efficient use of sunlight in photosynthesis. Their major function of these pigment-protein complexes is to harvest the sunlight and transfer the energy to the Photosystems. Peridinin-chlorophyll-protein (PCP) found in Dinoflagellates *Amphidinium carterae* is one of many such complexes. It is a watersoluble protein employed as an antenna external to the membrane. The structure of the PCP complex, shown in Fig. 3, has been determined with 1.3 Å resolution using X-ray crystallography (Hofmann, 1996). The native form of PCP consists of two chlorophyll a (Chl) and eight peridinin (Per) molecules embedded in a protein matrix. All the pigments are arranged in two almost similar clusters and embedded in the hydrophilic protein capsule. The conjugated portion of each Per is close to the chlorophyll tetrapyrrole ring at a van der Waals distance (3.3 to 3.8 Å), the distance between Mg atoms of the two Chl a in one monomer is 17.4 Å and intercluster edge-to-edge distances between Per are in the range of 4-11 Å. The ratio of Per to Chl a of 4:1 indicates that PCP utilizes the carotenoids as its main light-harvesting pigments. It has been shown that the PCP complex can be reconstituted with other Chl derivatives which exhibit different optical properties (Brotosudarmo, 2006). Importantly, the folding of the protein used in the reconstitution procedure takes place over almost identical pathway as in the native system, which results in very similar structures of the reconstituted systems. Since each of these chlorophyll molecules features specific absorption and emission characteristics, it became possible to construct and study the

In this section we introduce the structure and the optical properties of light-harvesting complexes used in our studies as well as present basic characteristics of metallic nanostructures, including their morphology and plasmon characteristics. Next we present experimental techniques employed for investigating the interactions between plasmon excitations and chromophores embedded in the proteins. These include standard absorption and fluorescence/fluorescence excitation spectroscopy, both in solution and in a layered geometry, as well as confocal fluorescence microscopy coupled with time-resolved capability and spectrally-resolved detection. Combination of all these experimental techniques allows for comprehensive description of plasmon-induced effects on the complex

Pigment-protein complexes that take part in photosynthesis can be generally divided into two groups: complexes containing reaction centers, which carry out charge separation, and complexes responsible solely for harvesting the sunlight and transferring it to the reaction centers. Large proteins of Photosystem I and Photosystem II fall into the first category, while light harvesting complex 2 (LH2) from purple bacteria and peridinin – chlorophyll – protein (PCP) complex from algae belong to the second group. As the structure and function of all these biomolecules has been described in detail previously, we focus here only on the aspects that are relevant for understanding the influence of plasmon excitations on the

Light-harvesting complexes were developed in the course of evolution in order to enhance and broaden the absorption of photosystems for the efficient use of sunlight in photosynthesis. Their major function of these pigment-protein complexes is to harvest the sunlight and transfer the energy to the Photosystems. Peridinin-chlorophyll-protein (PCP) found in Dinoflagellates *Amphidinium carterae* is one of many such complexes. It is a watersoluble protein employed as an antenna external to the membrane. The structure of the PCP complex, shown in Fig. 3, has been determined with 1.3 Å resolution using X-ray crystallography (Hofmann, 1996). The native form of PCP consists of two chlorophyll a (Chl) and eight peridinin (Per) molecules embedded in a protein matrix. All the pigments are arranged in two almost similar clusters and embedded in the hydrophilic protein capsule. The conjugated portion of each Per is close to the chlorophyll tetrapyrrole ring at a van der Waals distance (3.3 to 3.8 Å), the distance between Mg atoms of the two Chl a in one monomer is 17.4 Å and intercluster edge-to-edge distances between Per are in the range of 4-11 Å. The ratio of Per to Chl a of 4:1 indicates that PCP utilizes the carotenoids as its main light-harvesting pigments. It has been shown that the PCP complex can be reconstituted with other Chl derivatives which exhibit different optical properties (Brotosudarmo, 2006). Importantly, the folding of the protein used in the reconstitution procedure takes place over almost identical pathway as in the native system, which results in very similar structures of the reconstituted systems. Since each of these chlorophyll molecules features specific absorption and emission characteristics, it became possible to construct and study the

**2. Materials and methods** 

biomolecular systems.

**2.1 Light-harvesting complexes** 

**2.1.1 Peridinin-chlorophyll-protein** 

optical properties of light-harvesting systems, PCP and LH2.

energy transfer dynamics as well as inter-pigment interactions in a well-defined geometry given by the protein (Mackowski, 2007, Polivka, 2005).

Fig. 3. Pigment structure of the PCP complex reconstituted with Chl *a* together with absorption (black line) and fluorescence (red line) measured in water solution at room temperature.

The absorption spectrum of the Chl-PCP displayed in Fig. 3 has an intense, broad band between 400 to 550 nm that is mainly due to Per absorption, and two Chl – related bands at 440 nm (Soret) and 660 nm (QY). One example of chlorophylls used for reconstituting PCP complexes is [3-acetyl]-chlorophyll a (acChl). Chemically, it differs from Chl only by the C-3 substituent, but the absorption and fluorescence spectra of acChl-PCP, the PCP complex reconstituted with acChl, are red shifted as compared to the PCP complex containing Chl. At the same time the Per absorption in the blue-green spectral range is affected very slightly. For the PCP complexes reconstituted with acChla the absorption of the Qy band of the chlorophyll molecules is shifted by approximately 20 nm to the red. The fluorescence emission of the PCP complex originates from weakly coupled Chl molecules and it appears at 670 nm for Chl-PCP and 690 nm for acChl-PCP. Upon absorption of light, peridinins in PCP transfer their electronic excitation to Chl a. The efficiency of this excitation energy transfer is higher than 90% [20]. Subsequently, Chl a passes the energy on to membranebound light-harvesting complexes and the Photosystem II. Clearly, the absorption spectrum of PCP enables the photosynthetic apparatus to harness the sunlight not only in the red spectral range, but it extends it into the blue-green spectral region.

Optical spectroscopy studies of both native and reconstituted PCP complexes have been carried out on the ensemble (Akimoto, 1996; Kleima, 2000; Krueger, 2001) and singlemolecule levels (Mackowski, 2007; Wormke, 2007a; Wormke, 2008). Using transient absorption in femtosecond timescale main energy transfer pathways have been described, it has also been demonstrated that the two Chl a molecules interact relatively weakly with characteristic transfer time between them to be of the order to 12 ps (Kleima, 2000). These observations were also corroborated with fluorescence studies of individual PCP complexes: it has been shown that it is possible to distinguish emission originating from each of the two Chl a molecules and using the property of sequential photobleaching of the Chl the energy splitting between the two molecules in the monomer were determined (Wormke, 2007a). Recent work on PCP complexes reconstituted with both Chl a and Chl b provided coherent description of the energy transfer pathways and dynamics in this unique antenna (Mackowski, 2007).

Metallic Nanoparticles Coupled with Photosynthetic Complexes 9

Metallic structures with nanometric sizes have been the subject of intense research in recent years due to mainly unique optical properties of these systems that can be used for manipulating light at the nanoscale, designing biosensors and artificial chiral nanostructures, enhancing the optical properties of semiconductor nanocrystals and organic fluorescent dyes, enabling detection of emitters characterized with low fluorescence quantum yields, such as carbon nanotubes or DNA. In addition to efforts aimed at exploiting plasmon effect in metallic nanoparticles, significant research have been carried out to achieve almost perfect control of the morphology of metallic nanoparticles, and thus the plasmon properties thereof. Among many techniques of fabrication of metallic nanostructures are evaporation of metallic film on a corrugated substrate (Chettiar, 2010; Mackowski 2008), nanosphere lithography (Hulteen, 1995), electron beam lithography, electrochemical deposition, direct formation of silver island film (Ray, 2006), and chemical synthesis (Link, 1999). Each of these methods requires particular technical capabilities, frequently the experimental setups for nanostructure fabrication is expensive making them hardly accessible. Chemical synthesis of nanoparticles however, is quite simple, at least on the basic level, and, when mastered, provides a way to obtain highly monodisperse nanoparticles with tailored morphology and surface functionalization. This results in welldefined optical properties such as energies of plasmon resonances, and conjugation capabilities to other nanostructures or surfaces. In this contribution we describe the interactions present in hybrid nanostructures composed of light-harvesting complexes PCP and LH2 and metallic nanostructures fabricated using chemical deposition of silver island film on a glass substrate, electron beam assisted deposition of silver island film on a glass substrate, as well as chemically synthesized gold spherical nanoparticles and nanorods. We show that by careful design of a hybrid nanostructure we can control the impact of plasmon excitations in metallic nanoparticles upon the absorption and emission of the chlorophyll-

One of the simplest to fabricate metallic nanostructures is a corrugated metallic film. The method to obtain such a film with islands characterized with sizes of tens of nanometers has been previously applied to study the impact of plasmon interactions upon the fluorescence of various organic dyes, semiconductor quantum dots, and a few proteins, including the green fluorescent protein (Lakowicz, 2006; Ray, 2006). Silver island films used in our experiment were prepared by reducing an aqueous silver nitrate solution. All chemicals were purchased from Sigma-Aldrich and used as received. First, freshly prepared aqueous NaOH (1.25 M) was added to a silver nitrate solution. The precipitate was re-dissolved by adding NH4OH, and the solution was cooled to ~5°C under stirring. After adding Dglucose, clean microscope cover slips were dipped in the solution, which was then heated up to 30°C. The resulting Ag-covered glass coverslips were examined using absorption spectroscopy and atomic force microscopy (AFM). In order to change the morphology and thus the properties of the silver island film, we fabricated several samples with varied dipping time in the reaction solution: the coverslips were kept in solution for 1 and 3 minutes. In Fig. 5 we show AFM image of the SIF obtained by dipping the coverslip for 1 minute in the reaction solution. The islands are characterized with average sizes of about 40-

**2.2 Metallic nanostructures** 

containing light-harvesting complexes.

**2.2.1 Silver island film** 

#### **2.1.2 Light-harvesting complex 2**

Another example of a pigment-protein complex employed to harness sunlight energy and transfer it efficiently to reaction centers is a light-harvesting complex 2 (LH2) from purple bacteria Rhodopseudomonas palustris. This protein is placed in the thylakoid membrane, where many of LH2 complexes surround relatively widely spaced LH1 complexes, to which they transfer excitation energy. The BChl a molecules in LH1 have a single strong nearinfrared absorption band of 875 nm, while LH2 has two strong BChl a absorption bands at 800 nm and 850 nm (Fig. 2). In this way the energy gradient is formed, which facilitates efficient energy transfer from LH2 to LH1 complex, and then further to the reaction center. Both the structure and the optical properties of the LH2 have been a subject of intense studies in recent years (van Oijen, 1999; Hofmann, 2003; Bopp, 1997). It has been shown using atomic force microscopy technique, that LH2 complexes arrange around a LH1 lightharvesting complex, in the middle of which is a reaction center (Scheuring, 2004). The spatial arrangement of Bacteriochlorophylls (BChls) and carotenoids in this complex is displayed in Fig. 4. The X-ray crystallography studies of the LH2 complex have shown that out of the 27 BChl molecules 18 form a strongly coupled ring with average distances between the molecules less than 1 nm (McDermott, 1995). This excitonically coupled ring is responsible for the absorption band at 850 nm. The remaining 9 molecules form a ring of weakly coupled BChls as they are spaced by more than 2 nm. All pigments, BChls and carotenoids, are embedded in a hydrophobic protein (not shown). Single molecule investigations (van Oijen, 1999) proved that the B 850 ring are in fact not fully symmetric, and the exciton levels feature significant splitting.

Fig. 4. Pigment structure of the LH2 complex together with absorption (black line) and fluorescence (red line) measured in buffer solution at room temperature.

The absorption spectrum of the LH2 complex is shown in Fig. 4. It consists of two prominent bands at 800 nm and 850 nm which correspond to absorption of the two rings of BChl molecules. The carotenoids are in close contact with both BChl rings, and are mainly responsible for a broad absorption between 390 nm and 550 nm. Importantly, the fluorescence of LH2, which originates exclusively from the strongly coupled ring (named B850) has therefore an excitonic character. The presence of strong absorption bands and fluorescence emission in the infrared spectral range requires – in order to influence the optical properties of the LH2 complex - application of metallic nanoparticles that feature plasmon resonances in the near infrared.

#### **2.2 Metallic nanostructures**

8 Smart Nanoparticles Technology

Another example of a pigment-protein complex employed to harness sunlight energy and transfer it efficiently to reaction centers is a light-harvesting complex 2 (LH2) from purple bacteria Rhodopseudomonas palustris. This protein is placed in the thylakoid membrane, where many of LH2 complexes surround relatively widely spaced LH1 complexes, to which they transfer excitation energy. The BChl a molecules in LH1 have a single strong nearinfrared absorption band of 875 nm, while LH2 has two strong BChl a absorption bands at 800 nm and 850 nm (Fig. 2). In this way the energy gradient is formed, which facilitates efficient energy transfer from LH2 to LH1 complex, and then further to the reaction center. Both the structure and the optical properties of the LH2 have been a subject of intense studies in recent years (van Oijen, 1999; Hofmann, 2003; Bopp, 1997). It has been shown using atomic force microscopy technique, that LH2 complexes arrange around a LH1 lightharvesting complex, in the middle of which is a reaction center (Scheuring, 2004). The spatial arrangement of Bacteriochlorophylls (BChls) and carotenoids in this complex is displayed in Fig. 4. The X-ray crystallography studies of the LH2 complex have shown that out of the 27 BChl molecules 18 form a strongly coupled ring with average distances between the molecules less than 1 nm (McDermott, 1995). This excitonically coupled ring is responsible for the absorption band at 850 nm. The remaining 9 molecules form a ring of weakly coupled BChls as they are spaced by more than 2 nm. All pigments, BChls and carotenoids, are embedded in a hydrophobic protein (not shown). Single molecule investigations (van Oijen, 1999) proved that the B 850 ring are in fact not fully symmetric,

Fig. 4. Pigment structure of the LH2 complex together with absorption (black line) and

The absorption spectrum of the LH2 complex is shown in Fig. 4. It consists of two prominent bands at 800 nm and 850 nm which correspond to absorption of the two rings of BChl molecules. The carotenoids are in close contact with both BChl rings, and are mainly responsible for a broad absorption between 390 nm and 550 nm. Importantly, the fluorescence of LH2, which originates exclusively from the strongly coupled ring (named B850) has therefore an excitonic character. The presence of strong absorption bands and fluorescence emission in the infrared spectral range requires – in order to influence the optical properties of the LH2 complex - application of metallic nanoparticles that feature

fluorescence (red line) measured in buffer solution at room temperature.

**2.1.2 Light-harvesting complex 2** 

and the exciton levels feature significant splitting.

plasmon resonances in the near infrared.

Metallic structures with nanometric sizes have been the subject of intense research in recent years due to mainly unique optical properties of these systems that can be used for manipulating light at the nanoscale, designing biosensors and artificial chiral nanostructures, enhancing the optical properties of semiconductor nanocrystals and organic fluorescent dyes, enabling detection of emitters characterized with low fluorescence quantum yields, such as carbon nanotubes or DNA. In addition to efforts aimed at exploiting plasmon effect in metallic nanoparticles, significant research have been carried out to achieve almost perfect control of the morphology of metallic nanoparticles, and thus the plasmon properties thereof. Among many techniques of fabrication of metallic nanostructures are evaporation of metallic film on a corrugated substrate (Chettiar, 2010; Mackowski 2008), nanosphere lithography (Hulteen, 1995), electron beam lithography, electrochemical deposition, direct formation of silver island film (Ray, 2006), and chemical synthesis (Link, 1999). Each of these methods requires particular technical capabilities, frequently the experimental setups for nanostructure fabrication is expensive making them hardly accessible. Chemical synthesis of nanoparticles however, is quite simple, at least on the basic level, and, when mastered, provides a way to obtain highly monodisperse nanoparticles with tailored morphology and surface functionalization. This results in welldefined optical properties such as energies of plasmon resonances, and conjugation capabilities to other nanostructures or surfaces. In this contribution we describe the interactions present in hybrid nanostructures composed of light-harvesting complexes PCP and LH2 and metallic nanostructures fabricated using chemical deposition of silver island film on a glass substrate, electron beam assisted deposition of silver island film on a glass substrate, as well as chemically synthesized gold spherical nanoparticles and nanorods. We show that by careful design of a hybrid nanostructure we can control the impact of plasmon excitations in metallic nanoparticles upon the absorption and emission of the chlorophyllcontaining light-harvesting complexes.

#### **2.2.1 Silver island film**

One of the simplest to fabricate metallic nanostructures is a corrugated metallic film. The method to obtain such a film with islands characterized with sizes of tens of nanometers has been previously applied to study the impact of plasmon interactions upon the fluorescence of various organic dyes, semiconductor quantum dots, and a few proteins, including the green fluorescent protein (Lakowicz, 2006; Ray, 2006). Silver island films used in our experiment were prepared by reducing an aqueous silver nitrate solution. All chemicals were purchased from Sigma-Aldrich and used as received. First, freshly prepared aqueous NaOH (1.25 M) was added to a silver nitrate solution. The precipitate was re-dissolved by adding NH4OH, and the solution was cooled to ~5°C under stirring. After adding Dglucose, clean microscope cover slips were dipped in the solution, which was then heated up to 30°C. The resulting Ag-covered glass coverslips were examined using absorption spectroscopy and atomic force microscopy (AFM). In order to change the morphology and thus the properties of the silver island film, we fabricated several samples with varied dipping time in the reaction solution: the coverslips were kept in solution for 1 and 3 minutes. In Fig. 5 we show AFM image of the SIF obtained by dipping the coverslip for 1 minute in the reaction solution. The islands are characterized with average sizes of about 40-

Metallic Nanoparticles Coupled with Photosynthetic Complexes 11

be synthesized, even with little resources: sizes of the nanoparticles range from a few to a few hundreds of nanometers. They can also be of essentially any shape: spherical, elongated, triangular, cube or star-like. Furthermore, since the nanoparticles are synthesized in the colloidal form, it is possible to functionalize their surface with functional group suitable for specific attachment to surfaces or conjugation with other nanostructures. Lastly, there have been many examples for self assembly of metallic nanoparticles in complex structures with

Fig. 6. Scanning electron microscopy images of gold spheres and nanorods. The structural data is accompanied with absorption spectra measured for these two samples at room

In this contribution we report on synthesis of spherical gold nanoparticles and gold nanorods with the purpose to match absorption bands of light-harvesting complexes. While spherical gold nanoparticles, which feature plasmon resonances around 500-500 nm correspond to carotenoid absorption both in the PCP and LH2 complexes, the nanorods have their resonance also in the infrared, as shown in Fig. 6. By controlling the reaction we tune the energy of the plasmon resonance exactly to 800 nm, thus matching perfectly the

Gold nanoparticles were synthesized using a reduction reaction and dispersed in toluene. The average diameter of the nanocrystals was 5 nm, which results in a plasmon resonance maximum at 530 nm. The synthesis of Au nanorods was based on seed-mediated growth in water solution. All chemicals (HAuCl4×3H2O (99.9%), NaBH4 (99%), L-Ascorbic Acid (99+%), hexadecyltrimethylammoniumbromide (CTAB) (99%), and AgNO3 (99+ %)) were purchased from Aldrich and used without further purification. Deionized water (Fluka) was used in all experiments. In order to prepare Au seeds CTAB solution (4.7 ml, 0.1M) was mixed with 25 μl of 0.05 M HAuCl4. To the stirred solution, 0.3 ml of 0.01 M NaBH4 was added, which resulted in the formation of brownish yellow solution. Seeds solution was kept at room temperature until further used. For the synthesis we use Au seeds prepared beforehand. The "seed-mediated" method was developed previously; it is carried out in aqueous solution at atmospheric pressure and near room temperature. Appropriate quantities and molarities of CTAB (150 ml, 0.1 M), HAuCl4 (1.5 ml, 0.05 M), L-Ascorbic acid (1.2 ml, 0.1 M), 0.01 M AgNO3 (1.6 ml, 1.8 ml, 2 ml) and seed (360 μl) water solutions were added one by one in a flask, followed by a gentle mixing. Addition

new properties and functions (Link, 1995).

B800 absorption of the LH2 complex.

temperature.

50 nm laterally and the surface density is very high. In addition we also include in Fig. 5 the absorption spectrum measured for the SIF structure which features plasmon resonance with a maximum at 450 nm and the linewidth of about 150 nm, and thus matches the absorption of Per in the PCP complexes.

Fig. 5. Atomic force microscopy image of a silver island film fabricated by chemical synthesis together with absorption spectrum of the sample obtained for 1 minute-long dipping time of the glass substrate in the reaction solution. The lateral size of the AFM image is 5 microns.

## **2.2.2 Semicontinuous metallic layer**

The semicontinuous silver film has been fabricated using electron beam-assisted evaporation of silver on glass substrate (Chettiar, 2010). While this nanostructure may look similar to the SIF discussed above, the substrates obtained with the e-bam technique are typically more homogeneous, the sizes and shapes of the silver islands is controlled to higher degree. Proper adjustment of the parameters during the evaporation process leads to corrugated metallic films with designed optical properties. For instance, it has been shown that by changing evaporation time it is possible to obtain morphologies ranging from roughly isolated islands to the strongly coalescing ones. Such differences in morphology resulted in strong shift of plasmon energies towards the red and near infrared spectral ranges, opening thus completely new possibilities for applying these structures for controlling the optical properties of infrared – emitting systems. Yet another important advantage of semicontinuous metallic films fabricated using e-beam assisted evaporation is the capability of uniform coating of such films with dielectric layers with thicknesses ranging from a few nanometers up to tens of nanometers. As plasmon induced effects depend crucially upon the separation between metallic nanoparticles and optically active molecules, such structures render themselves a highly suitable system for investigating processes that occur in plasmonic hybrid nanostructures. In particular, the results included in this contribution have been obtained for a semicontinuous silver film covered with a 25 nm-thick SiO2 layer evaporated in the same process without exposing the structure to ambient conditions.

#### **2.2.3 Colloidal metallic nanoparticles**

Among metallic nanostructures, ones of the most studied are colloidal metallic nanoparticles. It is triggered mainly by enormous variety of metallic nanoparticles that can

50 nm laterally and the surface density is very high. In addition we also include in Fig. 5 the absorption spectrum measured for the SIF structure which features plasmon resonance with a maximum at 450 nm and the linewidth of about 150 nm, and thus matches the absorption

Fig. 5. Atomic force microscopy image of a silver island film fabricated by chemical synthesis together with absorption spectrum of the sample obtained for 1 minute-long dipping time of the glass substrate in the reaction solution. The lateral size of the AFM

The semicontinuous silver film has been fabricated using electron beam-assisted evaporation of silver on glass substrate (Chettiar, 2010). While this nanostructure may look similar to the SIF discussed above, the substrates obtained with the e-bam technique are typically more homogeneous, the sizes and shapes of the silver islands is controlled to higher degree. Proper adjustment of the parameters during the evaporation process leads to corrugated metallic films with designed optical properties. For instance, it has been shown that by changing evaporation time it is possible to obtain morphologies ranging from roughly isolated islands to the strongly coalescing ones. Such differences in morphology resulted in strong shift of plasmon energies towards the red and near infrared spectral ranges, opening thus completely new possibilities for applying these structures for controlling the optical properties of infrared – emitting systems. Yet another important advantage of semicontinuous metallic films fabricated using e-beam assisted evaporation is the capability of uniform coating of such films with dielectric layers with thicknesses ranging from a few nanometers up to tens of nanometers. As plasmon induced effects depend crucially upon the separation between metallic nanoparticles and optically active molecules, such structures render themselves a highly suitable system for investigating processes that occur in plasmonic hybrid nanostructures. In particular, the results included in this contribution have been obtained for a semicontinuous silver film covered with a 25 nm-thick SiO2 layer evaporated in the same process without exposing the structure to

Among metallic nanostructures, ones of the most studied are colloidal metallic nanoparticles. It is triggered mainly by enormous variety of metallic nanoparticles that can

of Per in the PCP complexes.

image is 5 microns.

ambient conditions.

**2.2.3 Colloidal metallic nanoparticles** 

**2.2.2 Semicontinuous metallic layer** 

be synthesized, even with little resources: sizes of the nanoparticles range from a few to a few hundreds of nanometers. They can also be of essentially any shape: spherical, elongated, triangular, cube or star-like. Furthermore, since the nanoparticles are synthesized in the colloidal form, it is possible to functionalize their surface with functional group suitable for specific attachment to surfaces or conjugation with other nanostructures. Lastly, there have been many examples for self assembly of metallic nanoparticles in complex structures with new properties and functions (Link, 1995).

Fig. 6. Scanning electron microscopy images of gold spheres and nanorods. The structural data is accompanied with absorption spectra measured for these two samples at room temperature.

In this contribution we report on synthesis of spherical gold nanoparticles and gold nanorods with the purpose to match absorption bands of light-harvesting complexes. While spherical gold nanoparticles, which feature plasmon resonances around 500-500 nm correspond to carotenoid absorption both in the PCP and LH2 complexes, the nanorods have their resonance also in the infrared, as shown in Fig. 6. By controlling the reaction we tune the energy of the plasmon resonance exactly to 800 nm, thus matching perfectly the B800 absorption of the LH2 complex.

Gold nanoparticles were synthesized using a reduction reaction and dispersed in toluene. The average diameter of the nanocrystals was 5 nm, which results in a plasmon resonance maximum at 530 nm. The synthesis of Au nanorods was based on seed-mediated growth in water solution. All chemicals (HAuCl4×3H2O (99.9%), NaBH4 (99%), L-Ascorbic Acid (99+%), hexadecyltrimethylammoniumbromide (CTAB) (99%), and AgNO3 (99+ %)) were purchased from Aldrich and used without further purification. Deionized water (Fluka) was used in all experiments. In order to prepare Au seeds CTAB solution (4.7 ml, 0.1M) was mixed with 25 μl of 0.05 M HAuCl4. To the stirred solution, 0.3 ml of 0.01 M NaBH4 was added, which resulted in the formation of brownish yellow solution. Seeds solution was kept at room temperature until further used. For the synthesis we use Au seeds prepared beforehand. The "seed-mediated" method was developed previously; it is carried out in aqueous solution at atmospheric pressure and near room temperature. Appropriate quantities and molarities of CTAB (150 ml, 0.1 M), HAuCl4 (1.5 ml, 0.05 M), L-Ascorbic acid (1.2 ml, 0.1 M), 0.01 M AgNO3 (1.6 ml, 1.8 ml, 2 ml) and seed (360 μl) water solutions were added one by one in a flask, followed by a gentle mixing. Addition

Metallic Nanoparticles Coupled with Photosynthetic Complexes 13

In order to image fluorescence of light-harvesting complexes coupled to metallic nanoparticles we constructed a confocal fluorescence microscope based on Olympus infinity-corrected microscope objective LMPlan 50x, characterized with a numerical aperture of 0.5 and working distance of 6 mm (Krajnik, 2011). The resulting laser spot size is about 1 μm for the excitation laser of 485 nm. The sample is placed on a XYZ piezoelectric stage (Physik Instrumente) with 1 nm nominal resolution of a single step, which enables us to raster-scan the sample surface in order to collect fluorescence maps. They are formed by combining fluorescence intensity measurements with the motion of the XY translation stage. For excitation of fluorescence, we use one of four diode-pumped solid-state lasers with wavelengths of 405, 485, 532 and 640 nm. Typical optical power of the laser sources is about 5 mW, but in the case of actual measurements it needs to be strongly reduced in order to prevent photobleaching of the molecules. We used the excitation powers in the range of 0.004 to 0.04 mW. Gaussian beams of the lasers are achieved by using a spatial filter. The fluorescence is detected in a back-scattering geometry and focused on a confocal pinhole (150 μm) in order to reduce stray light coming out of the focal plane. The emission of PCP complexes is extracted with HQ 650LP (Chroma) dichroic mirror and HQ 670/10 (Chroma) bandpass filter. In order to extract fluorescence of LH2 complexes we used a longpass filter

Our experimental configuration, described in detail in (Krajnik, 2011), allows for measuring fluorescence intensity, spectra and lifetimes. The spectrum, dispersed using the Amici prism is measured with a CCD camera (Andor iDus DV 420A-BV). The spectral resolution of the system is about 2 nm. Fluorescence intensity maps are collected with an avalanche photodiode (PerkinElmer SPCM-AQRH-14) with dark count rate of about 80 cps. Fluorescence lifetimes are measured using time-correlated single photon counting module (Becker & Hickl) equipped with fast avalanche photodiode (idQuantique id100-50) triggered

In this section we describe experimental results obtained for five architectures of hybrid nanostructures comprising metallic nanoparticles and light-harvesting systems. As for metallic nanostructures we used silver island film, semicontinuous silver film, spherical gold nanoparticles and elongated gold nanoparticles (nanorods). We coupled them with chlorophyll and carotenoid molecules embedded in the PCP complex from *Amphidinium carterae* and in the LH2 complex from *Rhodopseudomonas palustris*. The results of optical spectroscopy and microscopy show that the optical properties of light-harvesting systems are affected by the plasmon excitations in metallic nanoparticles both in th visible and infrared spectral ranges. Depending on the actual design of a nanostructure, either

Generally, the effect of plasmon excitations in metallic nanoparticles on the optical properties of nearby emitters is monitored by measuring the fluorescence intensity. When the geometry of a hybrid nanostructure leads to plasmon-induced enhancement, the fluorescence intensity of such a hybrid structure is increased. When, on the other hand, nonradiative energy transfer from the emitter to the metallic nanoparticles plays the dominant

HQ850LP (Chroma) and a bandpass filter D880/40m (Chroma).

by a laser pulse. Time resolution of the TCSPC setup is about 30 ps.

absorption or fluorescence radiative rate enhancement is obtained.

**2.4.1 Fluorescence imaging** 

**3. Experimental results** 

of ascorbic acid, as a mild reduction agent, triggered a mixture color change from dark yellow to colorless. After addition of the seed solution, the mixtures was put into water bath and kept at constant temperature of 28 ºC for 2 hours. Obtained products were separated from unreacted substrate and spherical particles by centrifugation at 9.000 rpm for 60 minutes. The supernatant was removed using a pipette and the precipitate was redissolved in pure water.

## **2.3 Sample preparation**

In the research described in this work, we have used several sample architectures, from a very simple ones, where light-harvesting complexes were deposited directly on the surface of the metallic layer being either in the form of silver island film or colloidal gold nanoparticles spin-coated on a glass coverslide, to more advanced, where metallic nanoparticles were separated from the light-harvesting complexes by a thin dielectric layer. For that purpose we used SiO2 deposited using e-beam assisted evaporation, the thickness of the SiO2 layer was changed from 5 nm to 40 nm. The light-harvesting complexes were dispersed in a PVA and then spin-coated on the substrate.

## **2.4 Experimental techniques**

The optical properties of hybrid nanostructures comprising light-harvesting complexes and metallic nanostructures have been studied using absorption and fluorescence spectroscopy in the visible and infrared regions. spectral region. Absorption spectra were obtained using a Cary 50 spectrophotometer. Fluorescence and fluorescence excitation spectra of both structures were measured using the FluoroLog 3 spectrofluorimeter equipped with specially designed mount suitable for holding planar samples. A Xenon lamp source with a double grating monochromator was used for excitation and the signal was detected with a thermoelectrically cooled photomultiplier tube characterized by a dark current of less than 100 cps.

Fluorescence spectra of samples comprising light-harvesting complexes and Au nanoparticles were measured in a standard optical setup with a back-scattering geometry. The laser excitation beam (=485 nm, 640 nm, or 405 nm) was focused, using a lens with a focal length of 30 mm, on the sample surface and the excitation power was controlled using notch filters. Typical excitation powers used were in the range of 200 W. The emission was guided through a 150 m pinhole and focused on a slit of a 0.5 monochromator (Shamrock 500, Andor) coupled with a charge coupled device detector (iDus 420BV, Andor). Fluorescence decays were studied using time-correlated single photon counting. For excitation, a diode-pumped solid state laser emitting at 405 nm, 640 nm, or 485 nm and generating 30 ps pulses at 80 MHz repetition rate was used. Detection was carried out with an ultrafast avalanche photodiode detector (idQuantique). The experiment was controlled using a time-correlated single photon counting card (SPC 150 Becker & Hickl). Emission spectra as well as fluorescence decays were collected for ten different spots across the sample in order to check for the reproducibility and homogeneity of the sample. Fluorescence of light-harvesting complexes was extracted using appropriate long-pass and band-pass optical filters.

## **2.4.1 Fluorescence imaging**

12 Smart Nanoparticles Technology

of ascorbic acid, as a mild reduction agent, triggered a mixture color change from dark yellow to colorless. After addition of the seed solution, the mixtures was put into water bath and kept at constant temperature of 28 ºC for 2 hours. Obtained products were separated from unreacted substrate and spherical particles by centrifugation at 9.000 rpm for 60 minutes. The supernatant was removed using a pipette and the precipitate was

In the research described in this work, we have used several sample architectures, from a very simple ones, where light-harvesting complexes were deposited directly on the surface of the metallic layer being either in the form of silver island film or colloidal gold nanoparticles spin-coated on a glass coverslide, to more advanced, where metallic nanoparticles were separated from the light-harvesting complexes by a thin dielectric layer. For that purpose we used SiO2 deposited using e-beam assisted evaporation, the thickness of the SiO2 layer was changed from 5 nm to 40 nm. The light-harvesting complexes were

The optical properties of hybrid nanostructures comprising light-harvesting complexes and metallic nanostructures have been studied using absorption and fluorescence spectroscopy in the visible and infrared regions. spectral region. Absorption spectra were obtained using a Cary 50 spectrophotometer. Fluorescence and fluorescence excitation spectra of both structures were measured using the FluoroLog 3 spectrofluorimeter equipped with specially designed mount suitable for holding planar samples. A Xenon lamp source with a double grating monochromator was used for excitation and the signal was detected with a thermoelectrically cooled photomultiplier tube characterized by a dark current of less than

Fluorescence spectra of samples comprising light-harvesting complexes and Au nanoparticles were measured in a standard optical setup with a back-scattering geometry. The laser excitation beam (=485 nm, 640 nm, or 405 nm) was focused, using a lens with a focal length of 30 mm, on the sample surface and the excitation power was controlled using notch filters. Typical excitation powers used were in the range of 200 W. The emission was guided through a 150 m pinhole and focused on a slit of a 0.5 monochromator (Shamrock 500, Andor) coupled with a charge coupled device detector (iDus 420BV, Andor). Fluorescence decays were studied using time-correlated single photon counting. For excitation, a diode-pumped solid state laser emitting at 405 nm, 640 nm, or 485 nm and generating 30 ps pulses at 80 MHz repetition rate was used. Detection was carried out with an ultrafast avalanche photodiode detector (idQuantique). The experiment was controlled using a time-correlated single photon counting card (SPC 150 Becker & Hickl). Emission spectra as well as fluorescence decays were collected for ten different spots across the sample in order to check for the reproducibility and homogeneity of the sample. Fluorescence of light-harvesting complexes was extracted using appropriate long-pass and

dispersed in a PVA and then spin-coated on the substrate.

redissolved in pure water.

**2.3 Sample preparation** 

**2.4 Experimental techniques** 

100 cps.

band-pass optical filters.

In order to image fluorescence of light-harvesting complexes coupled to metallic nanoparticles we constructed a confocal fluorescence microscope based on Olympus infinity-corrected microscope objective LMPlan 50x, characterized with a numerical aperture of 0.5 and working distance of 6 mm (Krajnik, 2011). The resulting laser spot size is about 1 μm for the excitation laser of 485 nm. The sample is placed on a XYZ piezoelectric stage (Physik Instrumente) with 1 nm nominal resolution of a single step, which enables us to raster-scan the sample surface in order to collect fluorescence maps. They are formed by combining fluorescence intensity measurements with the motion of the XY translation stage. For excitation of fluorescence, we use one of four diode-pumped solid-state lasers with wavelengths of 405, 485, 532 and 640 nm. Typical optical power of the laser sources is about 5 mW, but in the case of actual measurements it needs to be strongly reduced in order to prevent photobleaching of the molecules. We used the excitation powers in the range of 0.004 to 0.04 mW. Gaussian beams of the lasers are achieved by using a spatial filter. The fluorescence is detected in a back-scattering geometry and focused on a confocal pinhole (150 μm) in order to reduce stray light coming out of the focal plane. The emission of PCP complexes is extracted with HQ 650LP (Chroma) dichroic mirror and HQ 670/10 (Chroma) bandpass filter. In order to extract fluorescence of LH2 complexes we used a longpass filter HQ850LP (Chroma) and a bandpass filter D880/40m (Chroma).

Our experimental configuration, described in detail in (Krajnik, 2011), allows for measuring fluorescence intensity, spectra and lifetimes. The spectrum, dispersed using the Amici prism is measured with a CCD camera (Andor iDus DV 420A-BV). The spectral resolution of the system is about 2 nm. Fluorescence intensity maps are collected with an avalanche photodiode (PerkinElmer SPCM-AQRH-14) with dark count rate of about 80 cps. Fluorescence lifetimes are measured using time-correlated single photon counting module (Becker & Hickl) equipped with fast avalanche photodiode (idQuantique id100-50) triggered by a laser pulse. Time resolution of the TCSPC setup is about 30 ps.

## **3. Experimental results**

In this section we describe experimental results obtained for five architectures of hybrid nanostructures comprising metallic nanoparticles and light-harvesting systems. As for metallic nanostructures we used silver island film, semicontinuous silver film, spherical gold nanoparticles and elongated gold nanoparticles (nanorods). We coupled them with chlorophyll and carotenoid molecules embedded in the PCP complex from *Amphidinium carterae* and in the LH2 complex from *Rhodopseudomonas palustris*. The results of optical spectroscopy and microscopy show that the optical properties of light-harvesting systems are affected by the plasmon excitations in metallic nanoparticles both in th visible and infrared spectral ranges. Depending on the actual design of a nanostructure, either absorption or fluorescence radiative rate enhancement is obtained.

Generally, the effect of plasmon excitations in metallic nanoparticles on the optical properties of nearby emitters is monitored by measuring the fluorescence intensity. When the geometry of a hybrid nanostructure leads to plasmon-induced enhancement, the fluorescence intensity of such a hybrid structure is increased. When, on the other hand, nonradiative energy transfer from the emitter to the metallic nanoparticles plays the dominant

Metallic Nanoparticles Coupled with Photosynthetic Complexes 15

indicates that the PCP complexes that interact with metallic nanoparticles preserve their overall functionality. We can also see that the change in the fluorescence intensity is accompanied with sharp reduction of the fluorescence lifetime. In fact, the emission of PCP complexes on the glass substrate features a monoexponential decay while upon coupling to the SIF substrate the fluorescence decay curve is more complex. First a rapid decay takes place, which is probably due to efficient quenching of the PCP complexes that are very close to the metallic layer. After a first nanosecond the decay time of fluorescence gets longer,

thus becoming similar to the decay observed for the reference structure.

Fig. 7. Images of PCP fluorescence measured for the complexes deposited on the SIF substrate with 1 minute-long dipping time and 3 minute-long dipping time in the reaction solution. The maps were obtained at room temperature for the laser excitation wavelength

of 485 nm, the laser power was 40 W. The size of the images is 100x100 microns.

Fig. 8. Comparison between fluorescence spectra and fluorescence decay curves measured for PCP complexes on the glass and SIF substrates. For all measurements the excitation

Overall, the results obtained for PCP complexes embedded in PVA matrix on the SIF layer demonstrate that high inhomogeneity of the structure leads to quite complicated behavior. Indeed the enhancement of absorption rate is entangled with enhancement of fluorescence rate, and in addition, signatures of non-radiative energy transfer from the chlorophylls to metallic structure are present. In the case of the hybrid nanostructure studied here, there is no control over the morphology of the SIF itself as well as over the separation between light-

wavelength was 485 nm.

role, the emission is efficiently quenched. However, fluorescence spectrum alone gives only limited information about the actual processes responsible for the enhancement of the emission intensity. In order to elucidate the mechanisms in detail, it is important to combine standard fluorescence spectroscopy with fluorescence excitation spectroscopy and timeresolved fluorescence spectroscopy; these two experimental techniques provide a way to separate the plasmon-induced increase of the radiative rate from an induced increase of the absorption.

## **3.1 Peridinin-chlorophyll-protein on silver island film**

Initial experiments on hybrid nanostructures composed of light-harvesting complexes and metallic nanoparticles have been carried out on PCP complexes deposited directly onto the silver island film layer (Mackowski, 2008). In order to change the spectral properties of the metallic film, we fabricated SIF substrates with 1 and 3 second long dipping time in the reaction solution. Next, PCP complexes diluted in PVA were spin coated in ensemble concentration on the SIF layer. Since the thickness of the PVA layer is approximately 100 nm, the structure formed in this way spans over all relevant ranges of plasmon-pigment interaction. For PCP complexes located very close to the SIF the non-radiative energy transfer to the metallic nanostructure should play a dominant role and thus fluorescence quenching is expected. In contrast, when the distance between light-harvesting complexes and the SIF is larger than 40-50 nm, there is virtually no interaction between the two components of the hybrid nanostructure. Yet, the optical properties of all in-between molecules should be affected by the plasmon excitations in metallic nanostructure.

In Fig. 7 we display fluorescence images obtained with our confocal fluorescence microscope for the PCP complexes spin-coated onto two SIF substrates characterized with different time of deposition. Bright areas correspond to the higher fluorescence intensity. In the case of the SIF substrate that was kept in the reaction solution for 1 minute only the image is relatively homogeneous, the variation of fluorescence intensity is moderate. On the other hand, for the second structure, which was kept in solution two minutes longer, the areas of high and low fluorescence intensity are clearly separated from each other. We attribute the areas characterized with high fluorescence intensity to regions where the SIF layer was formed during the reaction, while the low fluorescence intensity suggests that the metallic layer detached from the glass substrate during the reaction.

The structure where both SIF and glass surfaces are present at once provide an easy and straightforward means to compare the fluorescence properties of PCP complexes coupled to plasmon excitation to the uncoupled ones. In Fig. 8 we show fluorescence spectra as well as fluorescence decay curves measured with the laser focused on either one of the two areas. As expected, for the PCP complexes placed on the SIF substrate the intensity of the emission is substantially higher that for the reference structure. The enhancement factor estimated from these two spectra is about fourfold. It correspond well to the average enhancement factor obtained for this structure. Importantly, as demonstrated in previous report (Mackowski, 2008), the maximum emission of the PCP complexes as well as the shape of the fluorescence spectrum remain unchanged upon coupling the light-harvesting complexes to the metallic nanoparticles. Also, since we use a 485 nm laser wavelength for the excitation, the observation of the intact fluorescence emission for both substrates indicates that the efficiency of the energy transfer from carotenoids to Chl molecules is comparable. This

role, the emission is efficiently quenched. However, fluorescence spectrum alone gives only limited information about the actual processes responsible for the enhancement of the emission intensity. In order to elucidate the mechanisms in detail, it is important to combine standard fluorescence spectroscopy with fluorescence excitation spectroscopy and timeresolved fluorescence spectroscopy; these two experimental techniques provide a way to separate the plasmon-induced increase of the radiative rate from an induced increase of the

Initial experiments on hybrid nanostructures composed of light-harvesting complexes and metallic nanoparticles have been carried out on PCP complexes deposited directly onto the silver island film layer (Mackowski, 2008). In order to change the spectral properties of the metallic film, we fabricated SIF substrates with 1 and 3 second long dipping time in the reaction solution. Next, PCP complexes diluted in PVA were spin coated in ensemble concentration on the SIF layer. Since the thickness of the PVA layer is approximately 100 nm, the structure formed in this way spans over all relevant ranges of plasmon-pigment interaction. For PCP complexes located very close to the SIF the non-radiative energy transfer to the metallic nanostructure should play a dominant role and thus fluorescence quenching is expected. In contrast, when the distance between light-harvesting complexes and the SIF is larger than 40-50 nm, there is virtually no interaction between the two components of the hybrid nanostructure. Yet, the optical properties of all in-between

molecules should be affected by the plasmon excitations in metallic nanostructure.

metallic layer detached from the glass substrate during the reaction.

In Fig. 7 we display fluorescence images obtained with our confocal fluorescence microscope for the PCP complexes spin-coated onto two SIF substrates characterized with different time of deposition. Bright areas correspond to the higher fluorescence intensity. In the case of the SIF substrate that was kept in the reaction solution for 1 minute only the image is relatively homogeneous, the variation of fluorescence intensity is moderate. On the other hand, for the second structure, which was kept in solution two minutes longer, the areas of high and low fluorescence intensity are clearly separated from each other. We attribute the areas characterized with high fluorescence intensity to regions where the SIF layer was formed during the reaction, while the low fluorescence intensity suggests that the

The structure where both SIF and glass surfaces are present at once provide an easy and straightforward means to compare the fluorescence properties of PCP complexes coupled to plasmon excitation to the uncoupled ones. In Fig. 8 we show fluorescence spectra as well as fluorescence decay curves measured with the laser focused on either one of the two areas. As expected, for the PCP complexes placed on the SIF substrate the intensity of the emission is substantially higher that for the reference structure. The enhancement factor estimated from these two spectra is about fourfold. It correspond well to the average enhancement factor obtained for this structure. Importantly, as demonstrated in previous report (Mackowski, 2008), the maximum emission of the PCP complexes as well as the shape of the fluorescence spectrum remain unchanged upon coupling the light-harvesting complexes to the metallic nanoparticles. Also, since we use a 485 nm laser wavelength for the excitation, the observation of the intact fluorescence emission for both substrates indicates that the efficiency of the energy transfer from carotenoids to Chl molecules is comparable. This

**3.1 Peridinin-chlorophyll-protein on silver island film** 

absorption.

indicates that the PCP complexes that interact with metallic nanoparticles preserve their overall functionality. We can also see that the change in the fluorescence intensity is accompanied with sharp reduction of the fluorescence lifetime. In fact, the emission of PCP complexes on the glass substrate features a monoexponential decay while upon coupling to the SIF substrate the fluorescence decay curve is more complex. First a rapid decay takes place, which is probably due to efficient quenching of the PCP complexes that are very close to the metallic layer. After a first nanosecond the decay time of fluorescence gets longer, thus becoming similar to the decay observed for the reference structure.

Fig. 7. Images of PCP fluorescence measured for the complexes deposited on the SIF substrate with 1 minute-long dipping time and 3 minute-long dipping time in the reaction solution. The maps were obtained at room temperature for the laser excitation wavelength of 485 nm, the laser power was 40 W. The size of the images is 100x100 microns.

Fig. 8. Comparison between fluorescence spectra and fluorescence decay curves measured for PCP complexes on the glass and SIF substrates. For all measurements the excitation wavelength was 485 nm.

Overall, the results obtained for PCP complexes embedded in PVA matrix on the SIF layer demonstrate that high inhomogeneity of the structure leads to quite complicated behavior. Indeed the enhancement of absorption rate is entangled with enhancement of fluorescence rate, and in addition, signatures of non-radiative energy transfer from the chlorophylls to metallic structure are present. In the case of the hybrid nanostructure studied here, there is no control over the morphology of the SIF itself as well as over the separation between light-

Metallic Nanoparticles Coupled with Photosynthetic Complexes 17

Fig. 9. Comparison between fluorescence excitation spectra measured for acChl-PCP on glass substrate and semicontinuous silver film. The detection energy was 690 nm. An enhancement dependence on the wavelength obtained by subtracting both fluorescence

It is important to note that the fluorescence excitation spectra measured for the reference structure and for the PCP complexes deposited on the silver film are not in any way adjusted or normalized. Yet, they are very comparable for wavelengths longer than 475 nm, in particular in the absorption range of low energy Per molecules. This suggests that the number of PCP complexes probed in both experiments is almost identical, which makes the estimation of the enhancement factor remarkably straightforward. Also the fluorescence spectrum measured for the hybrid nanostructure is is identical, for all excitation wavelengths, to that of the reference structure, which supports our previous observation that the preparation of the hybrid nanostructure has no measurable effect on the protein or

The fluorescence excitation data point towards increase of the absorption rate of the lightharvesting complex as being the dominant mechanism responsible for the enhancement of the fluorescence intensity. This suggestion is also helped with analysis of the spectral properties of both the PCP complexes and the semicontinuous silver film: they overlap mainly in the blue-green spectral range. In order to verify this we carry out time-resolved fluorescence experiment with the excitation wavelength of 405 nm, which corresponds to the maximum of the enhancement curve displayed in Fig. 9. The result if this experiment in shown in Fig. 10. The decay time of the control sample on glass is equal to 3.2 ns, while for the hybrid nanostructure a shortening of the lifetime to 2.3 ns when plasmons in the silver island film are excited. This less than 30 percent reduction of the lifetime, while measurable, is relatively small compared to previous results on fluorescent dyes (Dulkeith, 2002) and light-harvesting complexes (Mackowski, 2008), where order-of-magnitude changes have been measured. The small change of the fluorescence lifetimes in the case of the acChl-PCP complexes coupled to the semicontinuous silver film supports our conclusion that the enhancement measured in the fluorescence excitation is predominantly due to the enhancement of the excitation rate in the light-harvesting complexes. We also note that the fluorescence decay curve measured for the hybrid nanostructure features a monoexponential behavior, in contrast to the results obtained for PCP complexes deposited on the SIF. Such a uniform characteristics suggests improved homogeneity of the distance

excitation curves is also shown.

pigment properties.

harvesting complexes and metallic surface. Therefore, other approaches need to be devised, aimed at better control of sizes or shapes of metallic nanoparticles and the distance between the proteins and metallic structures.

## **3.2 Peridinin-chlorophyll-protein on semicontinuous silver film**

In previous sections we pointed out the important role of the separation between lightharvesting complexes and the metallic nanoparticles. In the case of sample geometry involving inhomogeneous silver island film and PCP complexes spin-coated directly on top of it in a relatively thick PVA layer, we have observed that the increase of the fluorescence emission is a combined product of absorption and fluorescence rate enhancement. In addition, the signatures of non-radiative energy transfer from the PCP complexes to the SIF layer have been observed in the time-resolved spectra.

In order to minimize the influence of the processes that lead to fluorescence quenching and at the same time to achieve uniform distance from the metallic layer to light-harvesting complexes, we have fabricated a hybrid nanostructure based on semicontinuous silver film (Czechowski, 2011). Such a corrugated metallic surface can be made using e-beam assisted evaporation under high-vacuum conditions. Scanning electron microscopy studies of similarly prepared samples have indicated improved uniformity of the islands, that resulted in narrowing of the plasmon resonance measured in the absorption experiment (Chettiar 2010). Furthermore, on top of the silver film we deposited a 25-nm-thick silica layer. The layer serves two purposes: on the one hand it protects the silver surface against oxidation, on the other hand it provides a uniform spacer between metallic nanoparticles and lightharvesting complexes. The final change compared to the structure where PCP complexes were spin-coated in a PVA matrix on top of the SIF, concerned direct deposition of the PCP water solution on the SiO2 surface of the spacer. In this way we can assume that all the complexes are at approximately identical distances from the silver islands. Here we used PCP complexes reconstituted with acChl a as they offer the largest energy separation between their fluorescence and plasmon resonance of silver islands. In addition, the concentration of PCP complexes is much higher than for samples prepared with spincoating, which makes it possible to study the plasmon induced effects using standard fluorescence excitation spectroscopy.

The fluorescence excitation spectrum measured for the detection wavelength of 690 nm for acChl-PCP on glass substrate is shown in Fig. 9. It is compared with with the result obtained for acChl-PCP complexes placed on the semicontinuous silver film. The excitation spectrum for the reference structure is similar to previously published (Brotosudarmo, 2008) it features strong absorption due to Per in the spectral range from 400 nm to 550 nm, and corresponds roughly to the absorption spectrum. This suggests that the sample preparation leaves no effect on either the protein or the pigments. In contrast, the maximum of fluorescence excitation spectrum is blue-shifted by ~40 nm for the PCP complexes deposited on the silver island film and separated from the metallic nanostructures by a 25-nm thick SiO2 layer. The difference between the two cases is seen after subtracting both curves and evaluating the enhancement of the emission. We find that the enhancement curve is a welldefined band with a maximum at 407 nm and linewidth of about 35 nm. We attribute this enhancement to plasmon excitations in the metallic layer that impact the absorption of the PCP complexes.

harvesting complexes and metallic surface. Therefore, other approaches need to be devised, aimed at better control of sizes or shapes of metallic nanoparticles and the distance between

In previous sections we pointed out the important role of the separation between lightharvesting complexes and the metallic nanoparticles. In the case of sample geometry involving inhomogeneous silver island film and PCP complexes spin-coated directly on top of it in a relatively thick PVA layer, we have observed that the increase of the fluorescence emission is a combined product of absorption and fluorescence rate enhancement. In addition, the signatures of non-radiative energy transfer from the PCP complexes to the SIF

In order to minimize the influence of the processes that lead to fluorescence quenching and at the same time to achieve uniform distance from the metallic layer to light-harvesting complexes, we have fabricated a hybrid nanostructure based on semicontinuous silver film (Czechowski, 2011). Such a corrugated metallic surface can be made using e-beam assisted evaporation under high-vacuum conditions. Scanning electron microscopy studies of similarly prepared samples have indicated improved uniformity of the islands, that resulted in narrowing of the plasmon resonance measured in the absorption experiment (Chettiar 2010). Furthermore, on top of the silver film we deposited a 25-nm-thick silica layer. The layer serves two purposes: on the one hand it protects the silver surface against oxidation, on the other hand it provides a uniform spacer between metallic nanoparticles and lightharvesting complexes. The final change compared to the structure where PCP complexes were spin-coated in a PVA matrix on top of the SIF, concerned direct deposition of the PCP water solution on the SiO2 surface of the spacer. In this way we can assume that all the complexes are at approximately identical distances from the silver islands. Here we used PCP complexes reconstituted with acChl a as they offer the largest energy separation between their fluorescence and plasmon resonance of silver islands. In addition, the concentration of PCP complexes is much higher than for samples prepared with spincoating, which makes it possible to study the plasmon induced effects using standard

The fluorescence excitation spectrum measured for the detection wavelength of 690 nm for acChl-PCP on glass substrate is shown in Fig. 9. It is compared with with the result obtained for acChl-PCP complexes placed on the semicontinuous silver film. The excitation spectrum for the reference structure is similar to previously published (Brotosudarmo, 2008) it features strong absorption due to Per in the spectral range from 400 nm to 550 nm, and corresponds roughly to the absorption spectrum. This suggests that the sample preparation leaves no effect on either the protein or the pigments. In contrast, the maximum of fluorescence excitation spectrum is blue-shifted by ~40 nm for the PCP complexes deposited on the silver island film and separated from the metallic nanostructures by a 25-nm thick SiO2 layer. The difference between the two cases is seen after subtracting both curves and evaluating the enhancement of the emission. We find that the enhancement curve is a welldefined band with a maximum at 407 nm and linewidth of about 35 nm. We attribute this enhancement to plasmon excitations in the metallic layer that impact the absorption of the

**3.2 Peridinin-chlorophyll-protein on semicontinuous silver film** 

layer have been observed in the time-resolved spectra.

the proteins and metallic structures.

fluorescence excitation spectroscopy.

PCP complexes.

Fig. 9. Comparison between fluorescence excitation spectra measured for acChl-PCP on glass substrate and semicontinuous silver film. The detection energy was 690 nm. An enhancement dependence on the wavelength obtained by subtracting both fluorescence excitation curves is also shown.

It is important to note that the fluorescence excitation spectra measured for the reference structure and for the PCP complexes deposited on the silver film are not in any way adjusted or normalized. Yet, they are very comparable for wavelengths longer than 475 nm, in particular in the absorption range of low energy Per molecules. This suggests that the number of PCP complexes probed in both experiments is almost identical, which makes the estimation of the enhancement factor remarkably straightforward. Also the fluorescence spectrum measured for the hybrid nanostructure is is identical, for all excitation wavelengths, to that of the reference structure, which supports our previous observation that the preparation of the hybrid nanostructure has no measurable effect on the protein or pigment properties.

The fluorescence excitation data point towards increase of the absorption rate of the lightharvesting complex as being the dominant mechanism responsible for the enhancement of the fluorescence intensity. This suggestion is also helped with analysis of the spectral properties of both the PCP complexes and the semicontinuous silver film: they overlap mainly in the blue-green spectral range. In order to verify this we carry out time-resolved fluorescence experiment with the excitation wavelength of 405 nm, which corresponds to the maximum of the enhancement curve displayed in Fig. 9. The result if this experiment in shown in Fig. 10. The decay time of the control sample on glass is equal to 3.2 ns, while for the hybrid nanostructure a shortening of the lifetime to 2.3 ns when plasmons in the silver island film are excited. This less than 30 percent reduction of the lifetime, while measurable, is relatively small compared to previous results on fluorescent dyes (Dulkeith, 2002) and light-harvesting complexes (Mackowski, 2008), where order-of-magnitude changes have been measured. The small change of the fluorescence lifetimes in the case of the acChl-PCP complexes coupled to the semicontinuous silver film supports our conclusion that the enhancement measured in the fluorescence excitation is predominantly due to the enhancement of the excitation rate in the light-harvesting complexes. We also note that the fluorescence decay curve measured for the hybrid nanostructure features a monoexponential behavior, in contrast to the results obtained for PCP complexes deposited on the SIF. Such a uniform characteristics suggests improved homogeneity of the distance

Metallic Nanoparticles Coupled with Photosynthetic Complexes 19

effect should be much stronger. In order to evaluate that we carried out fluorescence imaging experiment on PCP complexes deposited on the three Au nanoparticle samples with varied thickness of the SiO2 spacer. In the first step a fluorescence map was acquired of 100x100 micron sample area. The fluorescence maps were in all cases very uniform, variations of fluorescence intensity were below 15 percent. Next, approximately 50 fluorescence spectra we collected, each off a different spot on the sample surface. Finally, the same procedure was applied for measuring fluorescence decay curves. In this way statistically significant information about fluorescence intensity as well as fluorescence

Fig. 11. Typical fluorescence spectra and fluorescence decay curves of Chl-PCP complexes deposited on Au nanoparticle substrates with different thickness of the SiO2 spacer: 40 nm

In Fig. 11 we compare representative fluorescence spectra of Chl-PCP deposited on plasmonic substrates with Au spherical nanoparticles. The continuous-wave results are accompanied with time – resolved data. The intensity of fluorescence emission for 12 nm spacer is dramatically (fivefold) enhanced as compared to the reference structure with 40 nm thick SiO2 spacer. For the smallest spacer (4 nm) the fluorescence decreases rapidly due to non-radiative energy transfer from chlorophylls embedded in the PCP complexes to metallic nanoparticles. Importantly, in analogy to all previously described experiments, the fluorescence spectrum of the light-harvesting complexes remains unchanged, indicating that

Fluorescence decay curves that accompany the spectra provide means for understanding the mechanism of fluorescence enhancement. The decay time measured for the reference structure (40 nm) is equal to 3.3 ns, a typical value for PCP complexes reconstituted with Chl *a* (Mackowski, 2007). As the SiO*2* spacer gets thinner, the fluorescence lifetime gets shorter, and for 12 nm thick spacer is equal to 2.5 ns. Further reduction of the fluorescence lifetime is seen for the thinner, 4 nm, spacer. In this case the decay time is approximately 50 percent of the reference value. However, the mechanism of lifetime reduction is in both cases (4 nm and 12 nm) completely different. In the first case the shortening of the fluorescence decay time indicates enhancement of radiative rate of PCP complexes. This effects contributes to the observed increase of the emission intensity seen in the fluorescence spectra. On the other hand, for the 4 nm thick SiO2 spacer, the lifetime reduction is due to excitation quenching. Thee results obtained for PCP complexes coupled to Au nanoparticles demonstrate clear

(blue), 12 nm (red), and 4 nm (black). The laser excitation wavelength was 485 nm.

the biomolecules are intact upon interacting with metallic nanoparticles.

decay time is obtained.

between the light-harvesting complexes and the metallic layer, as indeed expected for our preparation procedure. We have also carried out time-resolved experiments with other excitation energies, in particular with 640 nm. This wavelength corresponds to direct excitation of Chl molecules and excites no plasmons. In this case the fluorescence decay shows no dependence upon either glass or metallic substrate.

Fig. 10. Comparison of fluorescence decay curves measured for acChl-PCP on glass substrate and semicontinuous silver film. The excitation wavelength was 405 nm.

Finally we comment on another aspect of fluorescence decay time reduction observed for the 405 nm laser excitation. Since this reduction is attributed to the increase of the radiative rate of emission, it implies that there are plasmons excited in the semicontinuous silver film with wavelengths around 690 nm, where acChl-PCP emits. As 405 nm laser excites no such plasmons directly, this observation could be indicative of plasmon propagation in terms of energy relaxation. This hypothesis requires further experimental evidence but when proven correct, it could open another pathway in te field of plasmon engineering.

The results of fluorescence spectroscopy on acChl-PCP complexes deposited on semicontinuous silver film spaced by 25 nm SiO2 layer confirm that by careful design of plasmonic hybrid nanostructure it is possible to selectively enhance the absorption of the light-harvesting complexes. The next step is to devise and fabricate a hybrid nanostructure, which would allow for systematic studies of plasmon induced effects as a function of the separation layer thickness.

#### **3.3 Peridinin-chlorophyll-protein on spherical gold nanoparticles**

The influence of the distance upon the interaction between PCP complexes and metallic nanoparticles requires fabrication of structures with precisely controlled thickness of the SiO2 spacer. Such structures were fabricated in an analogous way as described previously, with the thickness of SiO2 layer equal to 4, 12, and 40 nm. In this case however the metallic nanostructure used was a monolayer of uniform gold nanoparticles characterized with plasmon resonance at 530 nm.

While at the distances of 40 nm the influence of plasmon excitations on the optical properties of light-harvesting complexes is expected to be minimal, for thinner spacers the

between the light-harvesting complexes and the metallic layer, as indeed expected for our preparation procedure. We have also carried out time-resolved experiments with other excitation energies, in particular with 640 nm. This wavelength corresponds to direct excitation of Chl molecules and excites no plasmons. In this case the fluorescence decay

Fig. 10. Comparison of fluorescence decay curves measured for acChl-PCP on glass substrate and semicontinuous silver film. The excitation wavelength was 405 nm.

correct, it could open another pathway in te field of plasmon engineering.

**3.3 Peridinin-chlorophyll-protein on spherical gold nanoparticles** 

separation layer thickness.

plasmon resonance at 530 nm.

Finally we comment on another aspect of fluorescence decay time reduction observed for the 405 nm laser excitation. Since this reduction is attributed to the increase of the radiative rate of emission, it implies that there are plasmons excited in the semicontinuous silver film with wavelengths around 690 nm, where acChl-PCP emits. As 405 nm laser excites no such plasmons directly, this observation could be indicative of plasmon propagation in terms of energy relaxation. This hypothesis requires further experimental evidence but when proven

The results of fluorescence spectroscopy on acChl-PCP complexes deposited on semicontinuous silver film spaced by 25 nm SiO2 layer confirm that by careful design of plasmonic hybrid nanostructure it is possible to selectively enhance the absorption of the light-harvesting complexes. The next step is to devise and fabricate a hybrid nanostructure, which would allow for systematic studies of plasmon induced effects as a function of the

The influence of the distance upon the interaction between PCP complexes and metallic nanoparticles requires fabrication of structures with precisely controlled thickness of the SiO2 spacer. Such structures were fabricated in an analogous way as described previously, with the thickness of SiO2 layer equal to 4, 12, and 40 nm. In this case however the metallic nanostructure used was a monolayer of uniform gold nanoparticles characterized with

While at the distances of 40 nm the influence of plasmon excitations on the optical properties of light-harvesting complexes is expected to be minimal, for thinner spacers the

shows no dependence upon either glass or metallic substrate.

effect should be much stronger. In order to evaluate that we carried out fluorescence imaging experiment on PCP complexes deposited on the three Au nanoparticle samples with varied thickness of the SiO2 spacer. In the first step a fluorescence map was acquired of 100x100 micron sample area. The fluorescence maps were in all cases very uniform, variations of fluorescence intensity were below 15 percent. Next, approximately 50 fluorescence spectra we collected, each off a different spot on the sample surface. Finally, the same procedure was applied for measuring fluorescence decay curves. In this way statistically significant information about fluorescence intensity as well as fluorescence decay time is obtained.

Fig. 11. Typical fluorescence spectra and fluorescence decay curves of Chl-PCP complexes deposited on Au nanoparticle substrates with different thickness of the SiO2 spacer: 40 nm (blue), 12 nm (red), and 4 nm (black). The laser excitation wavelength was 485 nm.

In Fig. 11 we compare representative fluorescence spectra of Chl-PCP deposited on plasmonic substrates with Au spherical nanoparticles. The continuous-wave results are accompanied with time – resolved data. The intensity of fluorescence emission for 12 nm spacer is dramatically (fivefold) enhanced as compared to the reference structure with 40 nm thick SiO2 spacer. For the smallest spacer (4 nm) the fluorescence decreases rapidly due to non-radiative energy transfer from chlorophylls embedded in the PCP complexes to metallic nanoparticles. Importantly, in analogy to all previously described experiments, the fluorescence spectrum of the light-harvesting complexes remains unchanged, indicating that the biomolecules are intact upon interacting with metallic nanoparticles.

Fluorescence decay curves that accompany the spectra provide means for understanding the mechanism of fluorescence enhancement. The decay time measured for the reference structure (40 nm) is equal to 3.3 ns, a typical value for PCP complexes reconstituted with Chl *a* (Mackowski, 2007). As the SiO*2* spacer gets thinner, the fluorescence lifetime gets shorter, and for 12 nm thick spacer is equal to 2.5 ns. Further reduction of the fluorescence lifetime is seen for the thinner, 4 nm, spacer. In this case the decay time is approximately 50 percent of the reference value. However, the mechanism of lifetime reduction is in both cases (4 nm and 12 nm) completely different. In the first case the shortening of the fluorescence decay time indicates enhancement of radiative rate of PCP complexes. This effects contributes to the observed increase of the emission intensity seen in the fluorescence spectra. On the other hand, for the 4 nm thick SiO2 spacer, the lifetime reduction is due to excitation quenching. Thee results obtained for PCP complexes coupled to Au nanoparticles demonstrate clear

Metallic Nanoparticles Coupled with Photosynthetic Complexes 21

thinnest SiO2 layer of 4 nm the fluorescence intensities are all very similar. In fact, the measured distribution is even less pronounced than in the case of the reference sample. Such behavior may well be due to the dominant role of the fluorescence quenching caused by metallic nanoparticles, which takes over below a certain thickness of the spacer between the metallic nanoparticles and light-harvesting complexes. In such a case any fluctuations of either LH2 concentration or SiO2 spacer thickness may be of lesser

Fig. 13. Fluorescence decay curves measured for LH2 complexes on Au nanoparticles separated by SiO2 spacer. Excitation wavelengths of 405 nm and 485 nm were used.

In order to determine the possible origin of the observed fluorescence enhancement, time-resolved fluorescence was measured on identically prepared samples. The fluorescence decay curves obtained for the structure with 4 and 12 nm thick SiO2 layer is compared in Fig. 13 with the one measured for LH2 complexes deposited directly in glass substrate (Bujak, 2011). Apparently, upon coupling to the plasmons localized in the Au nanoparticles the fluorescence decays show virtually no change. Therefore, we assume that the fluorescence enhancement is predominantly due to an increase in the absorption in the carotenoid region of the LH2. The observation of exclusive increase of the absorption efficiency in the LH2 complexes coupled to Au nanoparticles was rendered by two factors. On the one hand, the difference in energy between plasmon resonance and the fluorescence emission is almost 400 nm, thus the overlap between low-energy tail of the plasmon resonance with the emission spectrum of the LH2 is minimal. This is much larger energy difference than for PCP complexes deposited on the semicontinuous silver film or Au nanoparticles. On the other hand, spherical gold nanoparticles are very uniform in size. This inhibits any possibility of energy relaxation in plasmonic structure, as it was observed for PCP complexes on the highly

The results described so far point clearly towards strong dependence of the plasmon induced effects upon the excitation energy. In most cases achieving strong coupling requires direct excitation of plasmons in metallic nanoparticles. In order to illustrate this, the fluorescence lifetimes were measured for LH2 complexes on Au nanoparticles with the excitation energy of 405 nm. In contrast to the 485 nm excitation, this energy excites

significance.

inhomogeneous SIF substrate.

dependence of the fluorescence enhancement upon the distance between chlorophyllcontaining proteins and metallic nanoparticles. While most of the effect is due to increase of absorption, there is also significant contribution associated with increase of the radiative rate. This approach can be then used for optimizing the geometry of plasmonic hybrid nanostructure for the most efficient performance.

#### **3.4 Light-harvesting complex 2 on spherical gold nanoparticles**

Light-harvesting complex LH2 from the purple bacteria is characterized by relatively weak absorption in the visible spectral range with its main absorption bands appearing in the near infrared, at 800 and 850 nm. By coupling LH2 to spherical gold nanoparticles we attempt to enhance the absorption between 400 and 550 nm. The geometry of the hybrid nanostructure was identical to discussed previously: monolayers of Au nanoparticles were covered with SiO2 dielectric layers with thickness of 4, 12, and 40 nm. During the experiment the fluorescence spectra excited into carotenoid absorption (Wormke, 2007b) were measured at ten different locations across the sample. In this way it was possible to account for any inhomogeneities due to the preparation of the hybrid nanostructures. The fluorescence spectra measured with SiO2 spacers between 4 and 40 nm and are shown in Fig. 12.

Fig. 12. Fluorescence spectra measured for LH2 complexes deposited on Au spherical nanoparticles on SiO2 spacers with thicknesses as indicated. The spectra were obtained for ten different locations on each sample.

There are several interesting observations worth pointing out. First of all, for the reference sample with 40-nm-thick SiO2 spacer the scattering of the measured intensities can be attributed to local fluctuations in the LH2 concentration due to spin-coating approach. In contrast, for the sample with the 12-nm-thick SiO2 spacer the spread of fluorescence intensities is significantly greater and the observed variation cannot be due to fluctuations of the LH2 concentration. Since plasmon interactions are expected to be significant for such a separation between the metallic nanoparticles and light-harvesting complexes, we attribute the distribution of fluorescence intensity to variation in plasmon coupling between the LH2 complexes and Au nanoparticles. Such variations can be caused for instance by interface roughness of the SiO2 layer, even small variations of the spacer thickness would result in measurable changes of the fluorescence intensity. Finally, for the

dependence of the fluorescence enhancement upon the distance between chlorophyllcontaining proteins and metallic nanoparticles. While most of the effect is due to increase of absorption, there is also significant contribution associated with increase of the radiative rate. This approach can be then used for optimizing the geometry of plasmonic hybrid

Light-harvesting complex LH2 from the purple bacteria is characterized by relatively weak absorption in the visible spectral range with its main absorption bands appearing in the near infrared, at 800 and 850 nm. By coupling LH2 to spherical gold nanoparticles we attempt to enhance the absorption between 400 and 550 nm. The geometry of the hybrid nanostructure was identical to discussed previously: monolayers of Au nanoparticles were covered with SiO2 dielectric layers with thickness of 4, 12, and 40 nm. During the experiment the fluorescence spectra excited into carotenoid absorption (Wormke, 2007b) were measured at ten different locations across the sample. In this way it was possible to account for any inhomogeneities due to the preparation of the hybrid nanostructures. The fluorescence spectra measured with SiO2 spacers between 4 and 40 nm and are shown in

Fig. 12. Fluorescence spectra measured for LH2 complexes deposited on Au spherical nanoparticles on SiO2 spacers with thicknesses as indicated. The spectra were obtained for

There are several interesting observations worth pointing out. First of all, for the reference sample with 40-nm-thick SiO2 spacer the scattering of the measured intensities can be attributed to local fluctuations in the LH2 concentration due to spin-coating approach. In contrast, for the sample with the 12-nm-thick SiO2 spacer the spread of fluorescence intensities is significantly greater and the observed variation cannot be due to fluctuations of the LH2 concentration. Since plasmon interactions are expected to be significant for such a separation between the metallic nanoparticles and light-harvesting complexes, we attribute the distribution of fluorescence intensity to variation in plasmon coupling between the LH2 complexes and Au nanoparticles. Such variations can be caused for instance by interface roughness of the SiO2 layer, even small variations of the spacer thickness would result in measurable changes of the fluorescence intensity. Finally, for the

nanostructure for the most efficient performance.

ten different locations on each sample.

Fig. 12.

**3.4 Light-harvesting complex 2 on spherical gold nanoparticles** 

thinnest SiO2 layer of 4 nm the fluorescence intensities are all very similar. In fact, the measured distribution is even less pronounced than in the case of the reference sample. Such behavior may well be due to the dominant role of the fluorescence quenching caused by metallic nanoparticles, which takes over below a certain thickness of the spacer between the metallic nanoparticles and light-harvesting complexes. In such a case any fluctuations of either LH2 concentration or SiO2 spacer thickness may be of lesser significance.

Fig. 13. Fluorescence decay curves measured for LH2 complexes on Au nanoparticles separated by SiO2 spacer. Excitation wavelengths of 405 nm and 485 nm were used.

In order to determine the possible origin of the observed fluorescence enhancement, time-resolved fluorescence was measured on identically prepared samples. The fluorescence decay curves obtained for the structure with 4 and 12 nm thick SiO2 layer is compared in Fig. 13 with the one measured for LH2 complexes deposited directly in glass substrate (Bujak, 2011). Apparently, upon coupling to the plasmons localized in the Au nanoparticles the fluorescence decays show virtually no change. Therefore, we assume that the fluorescence enhancement is predominantly due to an increase in the absorption in the carotenoid region of the LH2. The observation of exclusive increase of the absorption efficiency in the LH2 complexes coupled to Au nanoparticles was rendered by two factors. On the one hand, the difference in energy between plasmon resonance and the fluorescence emission is almost 400 nm, thus the overlap between low-energy tail of the plasmon resonance with the emission spectrum of the LH2 is minimal. This is much larger energy difference than for PCP complexes deposited on the semicontinuous silver film or Au nanoparticles. On the other hand, spherical gold nanoparticles are very uniform in size. This inhibits any possibility of energy relaxation in plasmonic structure, as it was observed for PCP complexes on the highly inhomogeneous SIF substrate.

The results described so far point clearly towards strong dependence of the plasmon induced effects upon the excitation energy. In most cases achieving strong coupling requires direct excitation of plasmons in metallic nanoparticles. In order to illustrate this, the fluorescence lifetimes were measured for LH2 complexes on Au nanoparticles with the excitation energy of 405 nm. In contrast to the 485 nm excitation, this energy excites

Metallic Nanoparticles Coupled with Photosynthetic Complexes 23

In Fig. 15 we show the result of fluorescence imaging experiment carried out on LH2 complexes deposited directly on gold nanorods with plasmon resonances at 550 nm and 800 nm. The maxima of the resonances match ideally with absorption bands of the LH2 complex, attributed to carotenoids and bacteriochlorophylls, respectively. In the experiment we probe the fluorescence enhancement for these two excitation wavelengths, importantly for these two excitations the same sample area was monitored. It can be seen in particular for the maps shown in the upper row of Fig. 15, areas with low fluorescence intensity are clearly correlated. In the case of LH2 complexes on glass substrate fluorescence maps acquired for both excitation wavelengths are very uniform, as shown below the maps with intensity histograms. In both cases the histograms oare of Gaussian shape with maxima at 7500 and 21000 cps for 556 and 808 nm excitation, respectively. The picture changes

Fig. 15. Fluorescence images of LH2 complexes deposited on glass substrate (upper row) and Au nanorods (lower row). For exciting carotenoid absorption a 556 nm laser was used, whereas for exciting B800 BChl ring – a 808 nm laser was used. The maps for a given structure were obtained from the same sample area. The size of the images is 50 x 50

microns.

plasmons very inefficiently while still populating excited states of carotenoids. The results included in Fig. 13 show that the fluorescence lifetime shows no dependence upon the thickness of the SiO2 spacer. However, the actual enhancement factor measured for 405 nm laser is substantially reduced compared to 485 nm laser, which very efficiently excites plasmons in metallic nanoparticles. The comparison is displayed in Fig. 14. For both excitation wavelengths the dependence of the enhancement factor on the distance between light-harvesting complexes and metallic nanoparticles is qualitatively the same. Yet, under the condition of efficient excitation of plasmons the maximum enhancement observed for the spacer with 12 nm thickness is 2.5 times greater.

Fig. 14. Comparison of distance dependence of the fluorescence intensity enhancement for PH2 complexes deposited on Au spherical nanoparticles with different spacer thickness. The data was obtained for 485 nm and 405 nm laser excitations.

In conclusion, results of fluorescence spectroscopy carried out on hybrid nanostructures composed of light-harvesting complex LH2 and gold nanoparticles demonstrate the strong impact of plasmon excitations upon the optical properties of the biomolecule. For a spacer with a thickness of 12 nm substantial increase of the fluorescence intensity is observed, which is due to an enhancement of absorption of the carotenoids in this light-harvesting complex. Furthermore, we observe strong dependence of the fluorescence enhancement on the laser wavelength: for efficient excitation of plasmons in metallic nanoparticles (=485 nm) the enhancement is approximately 2.5 times stronger than for the out-of-plasmonresonance excitation wavelength (=405 nm).

#### **3.5 Light-harvesting complex 2 on gold nanorods**

The final example of a hybrid nanostructure composed of light-harvesting complexes and metallic nanoparticles is a system where we combine Au nanorods with LH2 complexes from purple bacteria. From the previous discussion we know that by using Au nanorods we gain a tunability of plasmon resonances that reach near infrared spectral region (Bryant, 2008). In this way then we can affect the spectral properties of 800 and B850 absorption bands of the LH2 complex as well as its fluorescence emission.

plasmons very inefficiently while still populating excited states of carotenoids. The results included in Fig. 13 show that the fluorescence lifetime shows no dependence upon the thickness of the SiO2 spacer. However, the actual enhancement factor measured for 405 nm laser is substantially reduced compared to 485 nm laser, which very efficiently excites plasmons in metallic nanoparticles. The comparison is displayed in Fig. 14. For both excitation wavelengths the dependence of the enhancement factor on the distance between light-harvesting complexes and metallic nanoparticles is qualitatively the same. Yet, under the condition of efficient excitation of plasmons the maximum enhancement observed for

Fig. 14. Comparison of distance dependence of the fluorescence intensity enhancement for PH2 complexes deposited on Au spherical nanoparticles with different spacer thickness. The

In conclusion, results of fluorescence spectroscopy carried out on hybrid nanostructures composed of light-harvesting complex LH2 and gold nanoparticles demonstrate the strong impact of plasmon excitations upon the optical properties of the biomolecule. For a spacer with a thickness of 12 nm substantial increase of the fluorescence intensity is observed, which is due to an enhancement of absorption of the carotenoids in this light-harvesting complex. Furthermore, we observe strong dependence of the fluorescence enhancement on the laser wavelength: for efficient excitation of plasmons in metallic nanoparticles (=485 nm) the enhancement is approximately 2.5 times stronger than for the out-of-plasmon-

The final example of a hybrid nanostructure composed of light-harvesting complexes and metallic nanoparticles is a system where we combine Au nanorods with LH2 complexes from purple bacteria. From the previous discussion we know that by using Au nanorods we gain a tunability of plasmon resonances that reach near infrared spectral region (Bryant, 2008). In this way then we can affect the spectral properties of 800 and B850 absorption

the spacer with 12 nm thickness is 2.5 times greater.

data was obtained for 485 nm and 405 nm laser excitations.

resonance excitation wavelength (=405 nm).

**3.5 Light-harvesting complex 2 on gold nanorods** 

bands of the LH2 complex as well as its fluorescence emission.

In Fig. 15 we show the result of fluorescence imaging experiment carried out on LH2 complexes deposited directly on gold nanorods with plasmon resonances at 550 nm and 800 nm. The maxima of the resonances match ideally with absorption bands of the LH2 complex, attributed to carotenoids and bacteriochlorophylls, respectively. In the experiment we probe the fluorescence enhancement for these two excitation wavelengths, importantly for these two excitations the same sample area was monitored. It can be seen in particular for the maps shown in the upper row of Fig. 15, areas with low fluorescence intensity are clearly correlated. In the case of LH2 complexes on glass substrate fluorescence maps acquired for both excitation wavelengths are very uniform, as shown below the maps with intensity histograms. In both cases the histograms oare of Gaussian shape with maxima at 7500 and 21000 cps for 556 and 808 nm excitation, respectively. The picture changes

Fig. 15. Fluorescence images of LH2 complexes deposited on glass substrate (upper row) and Au nanorods (lower row). For exciting carotenoid absorption a 556 nm laser was used, whereas for exciting B800 BChl ring – a 808 nm laser was used. The maps for a given structure were obtained from the same sample area. The size of the images is 50 x 50 microns.

Metallic Nanoparticles Coupled with Photosynthetic Complexes 25

particular Dr. Dawid Piatkowski, Dr. Radek Litvin, Lukasz Bujak, Nikodem Czechowski, Bartosz Krajnik, Maria Olejnik, Kamil Ciszak, and Mikolaj Schmidt for their excellent work

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Atwater, H & Polman, A.(2010), Plasmonics for improved photovoltaic devices, *Nature* 

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and vital contribution.

**6. References** 

qualitatively for the LH2 complexes deposited on gold nanorods. The most pronouncing effect is much larger inhomogeneity of the fluorescence maps. There are regions of a few micron size that feature much stronger emission intensity. We can attribute them to either favorable separation between LH2 and Au nanorods or orientation/geometry of gold nanorods that would lead to formation of hot-spots of strongly localized electromagnetic field.

In addition to highly homogeneous fluorescence images, there are also significant differences of the distribution of fluorescence intensity, in spite of using the same excitation powers for a given laser wavelength. Indeed, the maximum of fluorescence intensity measured for 556 nm appears roughly at the same value as for the reference sample, but the histogram features substantial high-intensity tail of intensities, which is due to plasmoninduced enhancement in the hybrid nanostructure. Conversely, for the excitaiton of 808 nm the we also observe a broad tail towards higher intensities, but in this case the average intensity is also twice the average intensity measured for the reference sample. These preliminary results demonstrate that by using gold nanorods we are able to modulate the optical properties of multi-chromophoric systems such as light-harvesting complexes, which absorb in the infrared spectral range. Further work is required to coherently describe the complexity of plasmon interactions in this system.

## **4. Summary and conclusions**

We have described various geometries of hybrid nanostructures composed of lightharvesting complexes from algae or purple bacteria and metallic nanostructures in the form of silver island films or monolayers of metallic nanoparticles synthesized chemically. The samples were studied with numerous optical spectroscopy and microscopy techniques including fluorescence excitation, time-resolved fluorescence, and fluorescence imaging with high spatial resolution. In all fabricated structures we observe strong effects attributable to plasmon induced effects on the optical properties of the light-harvesting complexes. Depending on the actual geometry we are able to increase fluorescence or absorption rate, in most cases however both effects are entangled. The results demonstrate that plasmon excitations in metallic nanostructures can be efficiently applied for controlling the light-harvesting capability of photosynthetic complexes, possibly paving the road towards novel photovoltaic architectures based – at least in some degree - on natural photosynthesis.

## **5. Acknowledgment**

Research in Poland has been supported by the WELCOME project "Hybrid Nanostructures as a Stepping Stone towards Efficient Artificial Photosynthesis" funded by the Foundation for Polish Science and EUROCORES project "BOLDCATS" funded by the European Science Foundation. I am indebted to my friends and colleauges, with whom I have a great pleasure to collaborate on this project: I thank Wolfgang Heiss (Linz University), Eckhard Hofmann (University of Bochum), Richard J. Cogdell (University of Glasgow), Nicholas A. Kotov (University of Michigan), Hugo Scheer (LMU Munich) and the members of their research groups involved in parts of this research. Last but not least, I also acknowledge members of my research group at the Institute of Physics, Nicolaus Copernicus Unviersity in Torun, in particular Dr. Dawid Piatkowski, Dr. Radek Litvin, Lukasz Bujak, Nikodem Czechowski, Bartosz Krajnik, Maria Olejnik, Kamil Ciszak, and Mikolaj Schmidt for their excellent work and vital contribution.

## **6. References**

24 Smart Nanoparticles Technology

qualitatively for the LH2 complexes deposited on gold nanorods. The most pronouncing effect is much larger inhomogeneity of the fluorescence maps. There are regions of a few micron size that feature much stronger emission intensity. We can attribute them to either favorable separation between LH2 and Au nanorods or orientation/geometry of gold nanorods that would lead to formation of hot-spots of strongly localized electromagnetic

In addition to highly homogeneous fluorescence images, there are also significant differences of the distribution of fluorescence intensity, in spite of using the same excitation powers for a given laser wavelength. Indeed, the maximum of fluorescence intensity measured for 556 nm appears roughly at the same value as for the reference sample, but the histogram features substantial high-intensity tail of intensities, which is due to plasmoninduced enhancement in the hybrid nanostructure. Conversely, for the excitaiton of 808 nm the we also observe a broad tail towards higher intensities, but in this case the average intensity is also twice the average intensity measured for the reference sample. These preliminary results demonstrate that by using gold nanorods we are able to modulate the optical properties of multi-chromophoric systems such as light-harvesting complexes, which absorb in the infrared spectral range. Further work is required to coherently describe the

We have described various geometries of hybrid nanostructures composed of lightharvesting complexes from algae or purple bacteria and metallic nanostructures in the form of silver island films or monolayers of metallic nanoparticles synthesized chemically. The samples were studied with numerous optical spectroscopy and microscopy techniques including fluorescence excitation, time-resolved fluorescence, and fluorescence imaging with high spatial resolution. In all fabricated structures we observe strong effects attributable to plasmon induced effects on the optical properties of the light-harvesting complexes. Depending on the actual geometry we are able to increase fluorescence or absorption rate, in most cases however both effects are entangled. The results demonstrate that plasmon excitations in metallic nanostructures can be efficiently applied for controlling the light-harvesting capability of photosynthetic complexes, possibly paving the road towards novel photovoltaic architectures based – at least in some degree - on natural

Research in Poland has been supported by the WELCOME project "Hybrid Nanostructures as a Stepping Stone towards Efficient Artificial Photosynthesis" funded by the Foundation for Polish Science and EUROCORES project "BOLDCATS" funded by the European Science Foundation. I am indebted to my friends and colleauges, with whom I have a great pleasure to collaborate on this project: I thank Wolfgang Heiss (Linz University), Eckhard Hofmann (University of Bochum), Richard J. Cogdell (University of Glasgow), Nicholas A. Kotov (University of Michigan), Hugo Scheer (LMU Munich) and the members of their research groups involved in parts of this research. Last but not least, I also acknowledge members of my research group at the Institute of Physics, Nicolaus Copernicus Unviersity in Torun, in

complexity of plasmon interactions in this system.

**4. Summary and conclusions** 

photosynthesis.

**5. Acknowledgment** 

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Metallic Nanoparticles Coupled with Photosynthetic Complexes 27

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**2** 

 *India* 

N. Venkatathri

**Hollow Nano Silica: Synthesis,** 

**Characterization and Applications** 

*Department of Chemistry, National Institute of Technology, Andhra Pradesh,* 

Since the discovery of mesoporous silica molecular sieves by Beck et al. (Beck et al., 1992; Kresge et al., 1992), mesoporous materials have opened many new possibilities for application in the fields of catalysis (Tanev et al., 1994), separation, and nanoscience (Wu & Bein, 1994; Agger et al., 1998; Li et al., 2003; Yu et al., 2005]. In recent years, fabrication of silica materials with designed structure (e.g. thin films, monoliths, hexagonal prisms, toroids, discoids, spirals, dodecahedron and hollow sphere shapes) is an important research in modern materials chemistry. Among them the fabrication of monodispersed hollow spheres with control size and shape is fastest developing area (Schacht et al., 1996; Bruinsma et al., 1997; Fowler et al., 2001). It is generally accepted that hollow sphere with mesopores will exhibit more advantages in mass diffusion and transportation as compared with conventional hollow spheres with solid shell. They can serve as a small container for application in catalysis and control release studies (Mathlowitz et al., 1997; Huang & Remsen, 1999). The methods currently used to fabricate a wide range of stable hollow spheres include nozzle reactor processes, emulsion/phase separation, sol-gel processing, and sacrificial core techniques. The fabrication of hollow spheres has been greatly impacted by the layer-by-layer (LbL) self-assembly technique (Decher, 1997). This method allows the construction of composite multilayer assemblies based on the electrostatic attraction between nanoparticles and oppositely charged polyions. By varying the synthetic methodology and reactants, it is highly probable to achieve the materials with interesting

The presence of pores of uniform size lined with silanol groups confers these mesoporous materials as a potential candidate for hosting a variety of guest chemical species, such as organic molecules, semiconductor clusters, and polymers (Moller & Bein, 1998). For example, MCM-41 was reported as a drug delivery system (Vallet-Regi et al., 2001). Ibuprofen has been shown to readily adsorb from an *n*-hexane solution into the porous matrix of MCM-41, and to slowly release into a solution simulating physiological fluid. Furthermore, it has been found that in this host/guest system there is a strong interaction between the silanol groups and the carboxylic acid of the ibuprofen molecule. Having proven the feasibility of this system for drug retention and delivery, further effort should be made in gaining control of the amount of drug delivered, and its release rate. It can be thought that this delivery rate could be modulated by modifying the interaction between the

**1. Introduction** 

morphology and properties.

through energy transfer: Application to light-harvesting complexes, *Applied Physics Letters*, Vol.90, No.19, (May 2007), pp.193901, ISSN 0003-6951

Wörmke, S.; Mackowski, S.; Schaller, A.; Brotosudarmo, T.; Johanning, S.; Scheer, H. & Bräuchle C. (2008), Single Molecule Fluorescence of Native and Refolded Peridinin-Chlorophyll-Protein Complexes, *Journal of Fluorescence*, Vol.18, No. 3-4, (2008), pp. 611-617, ISSN 1053-0509

## **Hollow Nano Silica: Synthesis, Characterization and Applications**

N. Venkatathri

*Department of Chemistry, National Institute of Technology, Andhra Pradesh, India* 

## **1. Introduction**

28 Smart Nanoparticles Technology

Wörmke, S.; Mackowski, S.; Schaller, A.; Brotosudarmo, T.; Johanning, S.; Scheer, H. &

*Letters*, Vol.90, No.19, (May 2007), pp.193901, ISSN 0003-6951

611-617, ISSN 1053-0509

through energy transfer: Application to light-harvesting complexes, *Applied Physics* 

Bräuchle C. (2008), Single Molecule Fluorescence of Native and Refolded Peridinin-Chlorophyll-Protein Complexes, *Journal of Fluorescence*, Vol.18, No. 3-4, (2008), pp.

> Since the discovery of mesoporous silica molecular sieves by Beck et al. (Beck et al., 1992; Kresge et al., 1992), mesoporous materials have opened many new possibilities for application in the fields of catalysis (Tanev et al., 1994), separation, and nanoscience (Wu & Bein, 1994; Agger et al., 1998; Li et al., 2003; Yu et al., 2005]. In recent years, fabrication of silica materials with designed structure (e.g. thin films, monoliths, hexagonal prisms, toroids, discoids, spirals, dodecahedron and hollow sphere shapes) is an important research in modern materials chemistry. Among them the fabrication of monodispersed hollow spheres with control size and shape is fastest developing area (Schacht et al., 1996; Bruinsma et al., 1997; Fowler et al., 2001). It is generally accepted that hollow sphere with mesopores will exhibit more advantages in mass diffusion and transportation as compared with conventional hollow spheres with solid shell. They can serve as a small container for application in catalysis and control release studies (Mathlowitz et al., 1997; Huang & Remsen, 1999). The methods currently used to fabricate a wide range of stable hollow spheres include nozzle reactor processes, emulsion/phase separation, sol-gel processing, and sacrificial core techniques. The fabrication of hollow spheres has been greatly impacted by the layer-by-layer (LbL) self-assembly technique (Decher, 1997). This method allows the construction of composite multilayer assemblies based on the electrostatic attraction between nanoparticles and oppositely charged polyions. By varying the synthetic methodology and reactants, it is highly probable to achieve the materials with interesting morphology and properties.

> The presence of pores of uniform size lined with silanol groups confers these mesoporous materials as a potential candidate for hosting a variety of guest chemical species, such as organic molecules, semiconductor clusters, and polymers (Moller & Bein, 1998). For example, MCM-41 was reported as a drug delivery system (Vallet-Regi et al., 2001). Ibuprofen has been shown to readily adsorb from an *n*-hexane solution into the porous matrix of MCM-41, and to slowly release into a solution simulating physiological fluid. Furthermore, it has been found that in this host/guest system there is a strong interaction between the silanol groups and the carboxylic acid of the ibuprofen molecule. Having proven the feasibility of this system for drug retention and delivery, further effort should be made in gaining control of the amount of drug delivered, and its release rate. It can be thought that this delivery rate could be modulated by modifying the interaction between the

Hollow Nano Silica: Synthesis, Characterization and Applications 31

Fourier transform Infrared (FT-IR) spectra in the framework region were recorded in the diffuse reflectance mode (Nicolet 60SXB) using 1:300 ratio of sample with KBr, pellet. Ultraviolet – visible (UV-Vis.) spectroscopic analysis were carried out using Shimadzu,

Ibuprofen (IBU) drug (Ranbaxy Chem. LTD., 99%) was dissolved in hexane solution at a concentration of 30 mg/ml. 1.0 g nanocuboids or MCM-41 was added into 50 ml IBU hexane solution at room temperature. Sealing the vials to prevent the evaporation of hexane, then the mixture was stirred for 24 h. The nanocuboids or MCM-41 adsorbed with IBU was separated from this solution by centrifugation and dried under vacuum at 60 0C. Filtrates (1.0 ml) was extracted from the vial and diluted to 10 ml, and then was analyzed by UV/vis

The X-ray diffraction pattern calcined MCM-41 and hollow cuboids are given in Fig. 1a,b. The pattern shows their identity. The pattern from as-synthesized sample did not change much on calcinations. Both the as-synthesized and calcined patterns of hollow cuboid shows three Bragg diffraction peaks, which can be assigned to the (1 0 0), (1 1 0) and (2 0 0) reflections of a hexagonal symmetry structure (*P6mm*) typical for MCM-41. *d* spacing and unit cell parameter (a0) calculated from the XRD data are 12.6 nm and 14.54 nm respectively

 a b Fig. 1. X-ray diffraction pattern of calcined mesoporous silicas a) Nanohollow cuboids and

UV-2450 spectrometer.

spectroscopy at a wavelength of 235-320 nm.

**3. Results and discussion** 

(Grun et al., 1999).

b) Nanocrystalline MCM-41

confined molecule and the mesoporous matrix with different morphology. Here, one of the advantages of nanocuboids compared to conventional mesoporous materials is reflected in their much higher storage capacity. Ibuprofen with the molecules size of 1.0 x 0.6 nm was used to examine the storage capacity.

Nanomaterials are the talk of today's Materials researchers. Mesoporous hollow silica spheres were recently invented. It is important due to the drug storage property. Synthesis of mesoporous silica nano hollow cuboids is the very recent advancement (Venkatathri et al., 2008) in this category. In the present invention, the physicochemical property of mesoporous silica's, Nanocrystalline MCM-41 and Nanohollow cuboids were compared. It is found that Nanohollow cuboids store much more drug molecules say Ibuprofen.

## **2. Experimental**

Silica Nanohollow cuboids are synthesized as follows. 3.57 ml of triethanolamine (TEtA, 98%, Aldrich, USA) was added to a solution containing 74 ml of ethanol (99%, Aldrich, USA) and 10 ml of deionized water. 6 ml of tetraethoxyorthosilicates (TEOS, 98%, Aldrich, USA) was added to the above prepared mixture at 298 K with vigorous stirring. The reaction mixture was stirred for another 1 h. A solution containing 5 ml of TEOS and 2 ml of octadecyltrimethoxy silane (C18TMS, 90 %, Aldrich, USA) was added to the above solution (11.4 SiO2: 6 TEtA: 1 C18TMS: 149 H2O: 297.5 EtOH) and further reacted for 24 h. The resulting octadecyl group incorporated silica nanocomposite was retrieved by centrifugation. The sample was washed several times with distilled water, dried and calcined at 823 K for 8 h in air to obtain hollow cuboids silica material.

Nanocrystalline Silica MCM-41 is synthesized as follows. Cetyltrimethylammonium bromide was dissolved in 120 g of deionized water to yield a 0.055 mol l-1 solution, and 9.5 g of aqueous ammonia (25 wt%, 0.14 mol) was added to the solution. While stirring, 10 g of tetraethoxy silane (0.05 mol) was added slowly to the surfactant solution over a period of 15 min resulting in a gel with the following molar composition: 1 TEOS: 0.152 cetyltrimethylammonium bromide; 2.8 NH3: 141.2 H2O. The mixture was stirred for one hour then the white precipitate was filtered and washed with 100 ml of deionized water. After drying at 363 K for 12 h, the sample was heated to 823 K (rate 1 K min-1) in air and kept at this temperature for 5 h to remove the template.

X-ray diffractograms (XRD) were recorded on Rigaku Multiplex diffractometer using Cu K radiation and a proportional counter as detector. A divergence slit of 1/328 on the primary optics and an anti-scatter slit of 1/168 on the secondary optics were employed to measure data in the low angle region. The particle size and shape were analyzed by a Scanning electron microscope (SEM), Topcon, SM-300. Transmission electron micrographs (TEM) of the samples were scanned on a on a JEOL JSM-2000 EX electron microscope operated at 200 kV. The samples for TEM were dispersed in isopropyl alcohol, deposited on a Cu-grid and dried. Thermogravimetry (TG) analysis of the crystalline phase was performed on an automatic derivatograph (Setaram TG 92). The specific surface area (BET) of the samples was determined using a Micromeritics ASAP 2010 volumetric adsorption analyzer**.** Before N2 adsorption samples was evacuated in vacuum at 573 K. The data points of p/p0 in the range of about 0.05–0.3 were used in the calculations. The Fourier transform Infrared (FT-IR) spectra in the framework region were recorded in the diffuse reflectance mode (Nicolet 60SXB) using 1:300 ratio of sample with KBr, pellet. Ultraviolet – visible (UV-Vis.) spectroscopic analysis were carried out using Shimadzu, UV-2450 spectrometer.

Ibuprofen (IBU) drug (Ranbaxy Chem. LTD., 99%) was dissolved in hexane solution at a concentration of 30 mg/ml. 1.0 g nanocuboids or MCM-41 was added into 50 ml IBU hexane solution at room temperature. Sealing the vials to prevent the evaporation of hexane, then the mixture was stirred for 24 h. The nanocuboids or MCM-41 adsorbed with IBU was separated from this solution by centrifugation and dried under vacuum at 60 0C. Filtrates (1.0 ml) was extracted from the vial and diluted to 10 ml, and then was analyzed by UV/vis spectroscopy at a wavelength of 235-320 nm.

## **3. Results and discussion**

30 Smart Nanoparticles Technology

confined molecule and the mesoporous matrix with different morphology. Here, one of the advantages of nanocuboids compared to conventional mesoporous materials is reflected in their much higher storage capacity. Ibuprofen with the molecules size of 1.0 x 0.6 nm was

Nanomaterials are the talk of today's Materials researchers. Mesoporous hollow silica spheres were recently invented. It is important due to the drug storage property. Synthesis of mesoporous silica nano hollow cuboids is the very recent advancement (Venkatathri et al., 2008) in this category. In the present invention, the physicochemical property of mesoporous silica's, Nanocrystalline MCM-41 and Nanohollow cuboids were compared. It is found that Nanohollow cuboids store much more drug molecules say

Silica Nanohollow cuboids are synthesized as follows. 3.57 ml of triethanolamine (TEtA, 98%, Aldrich, USA) was added to a solution containing 74 ml of ethanol (99%, Aldrich, USA) and 10 ml of deionized water. 6 ml of tetraethoxyorthosilicates (TEOS, 98%, Aldrich, USA) was added to the above prepared mixture at 298 K with vigorous stirring. The reaction mixture was stirred for another 1 h. A solution containing 5 ml of TEOS and 2 ml of octadecyltrimethoxy silane (C18TMS, 90 %, Aldrich, USA) was added to the above solution (11.4 SiO2: 6 TEtA: 1 C18TMS: 149 H2O: 297.5 EtOH) and further reacted for 24 h. The resulting octadecyl group incorporated silica nanocomposite was retrieved by centrifugation. The sample was washed several times with distilled water, dried and

Nanocrystalline Silica MCM-41 is synthesized as follows. Cetyltrimethylammonium bromide was dissolved in 120 g of deionized water to yield a 0.055 mol l-1 solution, and 9.5 g of aqueous ammonia (25 wt%, 0.14 mol) was added to the solution. While stirring, 10 g of tetraethoxy silane (0.05 mol) was added slowly to the surfactant solution over a period of 15 min resulting in a gel with the following molar composition: 1 TEOS: 0.152 cetyltrimethylammonium bromide; 2.8 NH3: 141.2 H2O. The mixture was stirred for one hour then the white precipitate was filtered and washed with 100 ml of deionized water. After drying at 363 K for 12 h, the sample was heated to 823 K (rate 1 K min-1) in air and

X-ray diffractograms (XRD) were recorded on Rigaku Multiplex diffractometer using Cu K radiation and a proportional counter as detector. A divergence slit of 1/328 on the primary optics and an anti-scatter slit of 1/168 on the secondary optics were employed to measure data in the low angle region. The particle size and shape were analyzed by a Scanning electron microscope (SEM), Topcon, SM-300. Transmission electron micrographs (TEM) of the samples were scanned on a on a JEOL JSM-2000 EX electron microscope operated at 200 kV. The samples for TEM were dispersed in isopropyl alcohol, deposited on a Cu-grid and dried. Thermogravimetry (TG) analysis of the crystalline phase was performed on an automatic derivatograph (Setaram TG 92). The specific surface area (BET) of the samples was determined using a Micromeritics ASAP 2010 volumetric adsorption analyzer**.** Before N2 adsorption samples was evacuated in vacuum at 573 K. The data points of p/p0 in the range of about 0.05–0.3 were used in the calculations. The

calcined at 823 K for 8 h in air to obtain hollow cuboids silica material.

kept at this temperature for 5 h to remove the template.

used to examine the storage capacity.

Ibuprofen.

**2. Experimental** 

The X-ray diffraction pattern calcined MCM-41 and hollow cuboids are given in Fig. 1a,b. The pattern shows their identity. The pattern from as-synthesized sample did not change much on calcinations. Both the as-synthesized and calcined patterns of hollow cuboid shows three Bragg diffraction peaks, which can be assigned to the (1 0 0), (1 1 0) and (2 0 0) reflections of a hexagonal symmetry structure (*P6mm*) typical for MCM-41. *d* spacing and unit cell parameter (a0) calculated from the XRD data are 12.6 nm and 14.54 nm respectively (Grun et al., 1999).

Fig. 1. X-ray diffraction pattern of calcined mesoporous silicas a) Nanohollow cuboids and b) Nanocrystalline MCM-41

Hollow Nano Silica: Synthesis, Characterization and Applications 33

a

**200 400 600 800**

**Temperature (0C)**

b

**40**

Fig. 4. Thermogravimetric profile of as-synthesized mesoporous silicas,

a) Nanocrystalline MCM-41 and b) Nanohollow cuboids.

**50**

**60**

**70**

**% Weight loss**

**80**

**90**

**100**

Fig.2a,b shows the scanning electron micrograph of MCM-41 and hollow cuboids. MCM-41 particle size is 200 – 500 nm with spherical shape. Hollow cuboids are aggregate of cuboids with 500 nm particle size.

Fig. 2. Scanning electron micrograph of calcined mesoporous silicas a) NanocrystallineMCM-41 and b) Nanohollow cuboids

Transmission electron micrograph of MCM-41 and hollow cuboids are given in Fig. 3a,b. MCM-41 shows hexagonal array of channels characteristic of Mesoporous structure. By Fast Fourier Transform (FFT) of the TEM images, we estimate a unit cell dimension of 3.3 nm. TEM of cuboids shows core and shell structure. It can be seen from the images that the average inner diameter of the cuboids are nearly 100 nm, with outer shell thickness 50 nm. The particle sizes are uniform similar to SEM results. This distinguished pore channel arrangement with most of them running through the shell, are favorable for the access of guest molecules.

a b

a b

Fig. 3. Transmission electron micrograph of calcined mesoporous silicas a) NanocrystallineMCM-41 and b) Nanohollow cuboids

Fig.2a,b shows the scanning electron micrograph of MCM-41 and hollow cuboids. MCM-41 particle size is 200 – 500 nm with spherical shape. Hollow cuboids are aggregate of cuboids

a b

Transmission electron micrograph of MCM-41 and hollow cuboids are given in Fig. 3a,b. MCM-41 shows hexagonal array of channels characteristic of Mesoporous structure. By Fast Fourier Transform (FFT) of the TEM images, we estimate a unit cell dimension of 3.3 nm. TEM of cuboids shows core and shell structure. It can be seen from the images that the average inner diameter of the cuboids are nearly 100 nm, with outer shell thickness 50 nm. The particle sizes are uniform similar to SEM results. This distinguished pore channel arrangement with

a b

Fig. 3. Transmission electron micrograph of calcined mesoporous silicas

a) NanocrystallineMCM-41 and b) Nanohollow cuboids

most of them running through the shell, are favorable for the access of guest molecules.

Fig. 2. Scanning electron micrograph of calcined mesoporous silicas

a) NanocrystallineMCM-41 and b) Nanohollow cuboids

with 500 nm particle size.

Fig. 4. Thermogravimetric profile of as-synthesized mesoporous silicas, a) Nanocrystalline MCM-41 and b) Nanohollow cuboids.

Hollow Nano Silica: Synthesis, Characterization and Applications 35

a

b

Fig. 5. Nitrogen adsorption/desorption isotherms of calcined mesoporous silicas, a)

Nanocrystalline MCM-41 and b) Nanohollow cuboids.

The Thermogravimetry of MCM-41 and hollow cuboids were given in Fig. 3a,b. MCM-41 shows the 30 % loss at 25 - 625 oC due to the loss of template. The initial endothermic loss is due to loss of physisorbed water. Later the exothermic loss is due to oxidative decomposition of template. According to the curve the cuboids began to lose its weight at the beginning of heating, likely because of desorption of the physisorbed water and ethanol. It eliminates almost 25 % of its weight in the temperature range 25-200oC and losses almost 30 % weight in the temperature range of 200-500 oC. The later weight loss is due to the oxidative decomposition of the template.

Typical nitrogen sorption isotherms for MCM-41 and hollow cuboids are shown in Fig. 5a,b. In case of MCM-41, the nitrogen isotherms indicate a linear increase of the amount of adsorbed nitrogen at low pressures (P/Po = 0.35). The resulting isotherm can be classified as a type IV isotherm with a type H2 hysteresis, according to the IUPAC nomenclature (Fujiwara et al., 2004; Brunauer et al., 1940; de Boer, 1958; IUPAC, 1957). The steep increase in nitrogen uptake at relative pressures in the range between P/Po = 0.40 and 0.60 is reflected in a narrow pore size distribution. Thus, the variation of the catalyst in the solution during the growth process enables one to adjust and to control pore structural parameters such as the specific surface area (900 m2/g), the specific pore volume (1.29 cm3/g), and the average pore diameter (239 Ao) and medium pore width (302 Ao). The nitrogen adsorption/desorption isotherms of nanocuboid is of type IV nature (Fig. 5b) and exhibited a H1 hysteresis loop, which is typical of mesoporous solids (Wu et al., 2002). Furthermore, the adsorption branch of the isotherm showed a sharp inflection at a relative pressure value of about 0.68. This is characteristic of capillary condensation within uniform pores. The position of the inflection point indicates mesoporous structure, and the sharpness of these steps indicates the uniformity of the mesoporous size distribution. Correspondingly, the pore size distribution of the calcined sample shows a narrow pore distribution with a mean value of 1.90 nm. The sample with a specific surface area of 792 m2/g and pore volume of 0.51 cm3/g was obtained using the Brunauer–Emmett–Teller (BET) and Barrett– Joyner– Halenda (BJH) methods, respectively.

The Fourier transform Infrared spectra of as-synthesized MCM-41 and hollow cuboids are shown in Fig. 6a,b. Peaks around 1700 and 3430 cm-1 corresponding to the carboxyl and hydroxyl groups (Li et al., 2002) respectively. The adsorption peak belonging to the Si-O stretching vibration of Si-OH bond appears at 960 cm-1(Shan et al., 2004). The weak peaks at 2855 and 2920 cm-1 belong to the stretching vibrations of C-H bonds, which show a few organic groups are adsorbed on the spheres. The peaks for carboxyl, hydroxyl and C-H vibrations are weak in MCM-41, shows the lesser organics, resulting of organic template. The strong peaks near 1100, 802 and 467 cm-1 agree to the Si-O-Si bond which implies the condensation of silicon source (Agger et al., 1998).

Fig. 7 shows the UV ray absorbance spectra of 30 mg/ml ibuprofen hexane solutions (Zhu et al., 2005) before (a) and after (b) the interaction with nanocuboid and (c) MCM-41. The drug put in contact with nanocuboid and MCM-41 does not show any sign of degradation, since the positions of the absorbance maxima remain unchanged after the interaction and no new bands appear. The Ultraviolet ray absorbance intensity of filtrate decreases after Ibuprofen solution interaction with nanocuboids and MCM-41. This shows the remaining Ibuprofen is adsorbed over the molecular sieves. It was calculated that 561.8 mg and 270.5 mg ibuprofen

The Thermogravimetry of MCM-41 and hollow cuboids were given in Fig. 3a,b. MCM-41 shows the 30 % loss at 25 - 625 oC due to the loss of template. The initial endothermic loss is due to loss of physisorbed water. Later the exothermic loss is due to oxidative decomposition of template. According to the curve the cuboids began to lose its weight at the beginning of heating, likely because of desorption of the physisorbed water and ethanol. It eliminates almost 25 % of its weight in the temperature range 25-200oC and losses almost 30 % weight in the temperature range of 200-500 oC. The later weight loss is due to the

Typical nitrogen sorption isotherms for MCM-41 and hollow cuboids are shown in Fig. 5a,b. In case of MCM-41, the nitrogen isotherms indicate a linear increase of the amount of adsorbed nitrogen at low pressures (P/Po = 0.35). The resulting isotherm can be classified as a type IV isotherm with a type H2 hysteresis, according to the IUPAC nomenclature (Fujiwara et al., 2004; Brunauer et al., 1940; de Boer, 1958; IUPAC, 1957). The steep increase in nitrogen uptake at relative pressures in the range between P/Po = 0.40 and 0.60 is reflected in a narrow pore size distribution. Thus, the variation of the catalyst in the solution during the growth process enables one to adjust and to control pore structural parameters such as the specific surface area (900 m2/g), the specific pore volume (1.29 cm3/g), and the average pore diameter (239 Ao) and medium pore width (302 Ao). The nitrogen adsorption/desorption isotherms of nanocuboid is of type IV nature (Fig. 5b) and exhibited a H1 hysteresis loop, which is typical of mesoporous solids (Wu et al., 2002). Furthermore, the adsorption branch of the isotherm showed a sharp inflection at a relative pressure value of about 0.68. This is characteristic of capillary condensation within uniform pores. The position of the inflection point indicates mesoporous structure, and the sharpness of these steps indicates the uniformity of the mesoporous size distribution. Correspondingly, the pore size distribution of the calcined sample shows a narrow pore distribution with a mean value of 1.90 nm. The sample with a specific surface area of 792 m2/g and pore volume of 0.51 cm3/g was obtained using the Brunauer–Emmett–Teller (BET) and Barrett– Joyner–

The Fourier transform Infrared spectra of as-synthesized MCM-41 and hollow cuboids are shown in Fig. 6a,b. Peaks around 1700 and 3430 cm-1 corresponding to the carboxyl and hydroxyl groups (Li et al., 2002) respectively. The adsorption peak belonging to the Si-O stretching vibration of Si-OH bond appears at 960 cm-1(Shan et al., 2004). The weak peaks at 2855 and 2920 cm-1 belong to the stretching vibrations of C-H bonds, which show a few organic groups are adsorbed on the spheres. The peaks for carboxyl, hydroxyl and C-H vibrations are weak in MCM-41, shows the lesser organics, resulting of organic template. The strong peaks near 1100, 802 and 467 cm-1 agree to the Si-O-Si bond which implies the

Fig. 7 shows the UV ray absorbance spectra of 30 mg/ml ibuprofen hexane solutions (Zhu et al., 2005) before (a) and after (b) the interaction with nanocuboid and (c) MCM-41. The drug put in contact with nanocuboid and MCM-41 does not show any sign of degradation, since the positions of the absorbance maxima remain unchanged after the interaction and no new bands appear. The Ultraviolet ray absorbance intensity of filtrate decreases after Ibuprofen solution interaction with nanocuboids and MCM-41. This shows the remaining Ibuprofen is adsorbed over the molecular sieves. It was calculated that 561.8 mg and 270.5 mg ibuprofen

oxidative decomposition of the template.

Halenda (BJH) methods, respectively.

condensation of silicon source (Agger et al., 1998).

Fig. 5. Nitrogen adsorption/desorption isotherms of calcined mesoporous silicas, a) Nanocrystalline MCM-41 and b) Nanohollow cuboids.

Hollow Nano Silica: Synthesis, Characterization and Applications 37

**240 260 280 300 320**

a

b c

**Wavelength (nm)**

Fig. 7. The Ultraviolet – visible absorbance spectra of 30 mg/ml ibuprofen hexane solutions before (a) and after (b) the interaction with calcined mesoporous silica Nanohollow cuboids

**0.0**

and (c) mesoporous Silica Nanocrystalline MCM-41.

**0.2**

**0.4**

**0.6**

**0.8**

**Absorbance**

**1.0**

**1.2**

**1.4**

**1.6**

Fig. 6. Fourier transform Infrared spectroscopic analysis of as-synthesized mesoporous silicas, a) Nanohollow cuboids and b) a) Nanocrystalline MCM-41.

400 900 1400 1900 2400 2900 3400 3900 4400 4900

Wavenumber, cm-1 (1/cm)

a

b

Fig. 6. Fourier transform Infrared spectroscopic analysis of as-synthesized mesoporous

silicas, a) Nanohollow cuboids and b) a) Nanocrystalline MCM-41.

0

5

10

Transmittance (%)

(%)Absorbance

15

20

25

30

Fig. 7. The Ultraviolet – visible absorbance spectra of 30 mg/ml ibuprofen hexane solutions before (a) and after (b) the interaction with calcined mesoporous silica Nanohollow cuboids and (c) mesoporous Silica Nanocrystalline MCM-41.

Hollow Nano Silica: Synthesis, Characterization and Applications 39

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molecules can be stored in per gram nanocuboid and MCM-41, respectively from Ultraviolet ray absorbance according to Beer–Lambert Law (Jeffery et al., 1997). The surface area and pore volume of MCM-41 and nanocuboid are very close to each other, but much more ibuprofen molecules can be stored into nanocuboid than into MCM-41. This illustrates that the hollow cores could hold more than half drug molecules of total storage amount.

Tetraethylorthosilicate (TEOS) was hydrolyzed in the presence of basic triethanolamine. However the hydrolysis rate of TEOS using triethanolamine is very slow as compared to hydrolysis with NH3. For example, using the molar ratio described above, TEOS can be hydrolyzed in 2h using NH3 whereas triethanolamine took 24 h to hydrolyze the TEOS. In the present synthetic recipe, triethanolamine not only act as a catalyst for the hydrolysis but also it acts as a reactant. The hydrolyzed silica monomers react with triethanolamine to give respective oxide. Such silicate-triethanolamine adduct are held together with hydrogen bonding. The triethanolamine sandwiched silica layer condensed and form nanocuboids. MCM-41 is reported to crystallize by self assembly of surfactant/template (Grun et al., 1999) in similar to nanocuboids.

## **4. Conclusion**

A novel procedure was invented to synthesize mesoporous Silica Nano hollow cuboids with uniform size and morphology. It is characterized by various physicochemical techniques. The results are compared with Nanocrystalline silica MCM-41. Transmission electron micrographs shows, 150 nm hollow diameter and 50 nm shell thickness in hollow cuboids. Further, the mesoporous silica Nanohollow cuboids were found to store much more guest molecules than conventional mesoporous silica Nanocrystalline MCM-41.

## **5. Acknowledgement**

The author thanks Director, National Institute of Technology, Warangal, India for constant encouragement throughout the course of work.

## **6. References**


molecules can be stored in per gram nanocuboid and MCM-41, respectively from Ultraviolet ray absorbance according to Beer–Lambert Law (Jeffery et al., 1997). The surface area and pore volume of MCM-41 and nanocuboid are very close to each other, but much more ibuprofen molecules can be stored into nanocuboid than into MCM-41. This illustrates that

Tetraethylorthosilicate (TEOS) was hydrolyzed in the presence of basic triethanolamine. However the hydrolysis rate of TEOS using triethanolamine is very slow as compared to hydrolysis with NH3. For example, using the molar ratio described above, TEOS can be hydrolyzed in 2h using NH3 whereas triethanolamine took 24 h to hydrolyze the TEOS. In the present synthetic recipe, triethanolamine not only act as a catalyst for the hydrolysis but also it acts as a reactant. The hydrolyzed silica monomers react with triethanolamine to give respective oxide. Such silicate-triethanolamine adduct are held together with hydrogen bonding. The triethanolamine sandwiched silica layer condensed and form nanocuboids. MCM-41 is reported to crystallize by self assembly of surfactant/template (Grun et al., 1999)

A novel procedure was invented to synthesize mesoporous Silica Nano hollow cuboids with uniform size and morphology. It is characterized by various physicochemical techniques. The results are compared with Nanocrystalline silica MCM-41. Transmission electron micrographs shows, 150 nm hollow diameter and 50 nm shell thickness in hollow cuboids. Further, the mesoporous silica Nanohollow cuboids were found to store much more guest

The author thanks Director, National Institute of Technology, Warangal, India for constant

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molecules than conventional mesoporous silica Nanocrystalline MCM-41.

the hollow cores could hold more than half drug molecules of total storage amount.

in similar to nanocuboids.

**5. Acknowledgement** 

**6. References** 

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1958.

**4. Conclusion** 


**3** 

**Synthesis of Titanate and Titanium** 

**Dioxide Nanotube Thin Films and** 

**Their Applications to Biomaterials** 

Recently, titanium compounds with one-dimensional nanostructures, such as nanotubes and nanofibers, have recently attracted much attention. Among these 1-D compounds, nanotubes composed of titanium dioxide and titanate are now being studied actively. Titanium dioxide nanotubes can be synthesized using porous anodic alumina membranes (Imai et al., 1999; Yamanaka et al., 2004), organic molecules (Jung et al., 2002), or polycarbonate membranes (Shin et al., 2004) as templates, or methods involving anodization of titanium metals (Macak et al., 2005). Since the interesting reports by Kasuga et al. (Kasuga et al., 1998; Kasuga et al., 1999) and Chen et al. (Chen et al., 2002), titanate and titanium dioxide nanotubes synthesized using the hydrothermal method have found a wide range of potential uses in photocatalysis (Tokudome et al., 2004; Jiang et al., 2008), dye sensitizing solar batteries (Uchida et al., 2002), hydrogen storage (Bavykin et al., 2005), electrochromism (Tokudome et al., 2005), bonelike apatite formation (Kubota et al., 2004), proton conductors (Thorne et al., 2005), electron field emission characteristic (Miyauchi et al., 2006),

In order to maximize the characteristics of the nanotube and to use them efficiently, preventing their excessive aggregation and arrangement at larger than micrometer or centimeter size are considered important. Especially, it is important to fabricate thin films composed of nanotubes. Kasuga et al. (Kasuga et al., 2003) reported the fabrication of titanate nanotube thin films by coating a titanate nanotube dispersion liquid to a substrate, and then calcinating the substrate. Tokudome et al. (Tokudome et al., 2004) and Ma et al. (Ma et al., 2004) also reported the fabrication of titanate nanotube thin films using a layerby-layer method. However, neither study had transformed titanate nanotube thin films into titanium dioxide thin films. Kim et al. (Kim et al., 2007) used electrophoretic deposition (EPD) to fabricate 2-μm-thick titanate nanotube thin films, and they transformed the titanate nanotube thin films into titanium dioxide nanotube thin films by calcination. However, these methods involve complicated processes, including (1) synthesis of nanotubes, (2) preparation of a liquid in which the synthesized nanotubes are dispersed, (3) coating of the nanotubes onto a substrate using the prepared liquid, and (4) fixation of the coated nanotubes onto the substrate surface by calcination. Since it is generally difficult to prepare

photoinduced hydrophilicity (Tokudome et al., 2004), etc.

**1. Introduction** 

Mitsunori Yada and Yuko Inoue

*Saga University* 

*Japan* 


## **Synthesis of Titanate and Titanium Dioxide Nanotube Thin Films and Their Applications to Biomaterials**

Mitsunori Yada and Yuko Inoue *Saga University Japan* 

## **1. Introduction**

40 Smart Nanoparticles Technology

Wu P., Tatsumi T., Komatsu T., & Yashima T., 2002, Postsynthesis, Characterization and

Yu K., Guo Y., Ding X., Zhao J., & Wang Z., 2005, Synthesis of silica nanocubes by sol-gel

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Ti-SBA-15. *Chem. Mater.*, vol. 14, 2002, pp. 1657-1664.

method. *Mat. Lett.*, vol. 59, 2005, pp. 4013-4015.

Mesopor. Mat., vol. 84, 2005, pp. 218-222.

Catalytic Properties in Alkene Epoxidation of Hydrothermally stable Mesoporous

hollow mesoporous silica spheres and advanced storage property. Micropor.

Recently, titanium compounds with one-dimensional nanostructures, such as nanotubes and nanofibers, have recently attracted much attention. Among these 1-D compounds, nanotubes composed of titanium dioxide and titanate are now being studied actively. Titanium dioxide nanotubes can be synthesized using porous anodic alumina membranes (Imai et al., 1999; Yamanaka et al., 2004), organic molecules (Jung et al., 2002), or polycarbonate membranes (Shin et al., 2004) as templates, or methods involving anodization of titanium metals (Macak et al., 2005). Since the interesting reports by Kasuga et al. (Kasuga et al., 1998; Kasuga et al., 1999) and Chen et al. (Chen et al., 2002), titanate and titanium dioxide nanotubes synthesized using the hydrothermal method have found a wide range of potential uses in photocatalysis (Tokudome et al., 2004; Jiang et al., 2008), dye sensitizing solar batteries (Uchida et al., 2002), hydrogen storage (Bavykin et al., 2005), electrochromism (Tokudome et al., 2005), bonelike apatite formation (Kubota et al., 2004), proton conductors (Thorne et al., 2005), electron field emission characteristic (Miyauchi et al., 2006), photoinduced hydrophilicity (Tokudome et al., 2004), etc.

In order to maximize the characteristics of the nanotube and to use them efficiently, preventing their excessive aggregation and arrangement at larger than micrometer or centimeter size are considered important. Especially, it is important to fabricate thin films composed of nanotubes. Kasuga et al. (Kasuga et al., 2003) reported the fabrication of titanate nanotube thin films by coating a titanate nanotube dispersion liquid to a substrate, and then calcinating the substrate. Tokudome et al. (Tokudome et al., 2004) and Ma et al. (Ma et al., 2004) also reported the fabrication of titanate nanotube thin films using a layerby-layer method. However, neither study had transformed titanate nanotube thin films into titanium dioxide thin films. Kim et al. (Kim et al., 2007) used electrophoretic deposition (EPD) to fabricate 2-μm-thick titanate nanotube thin films, and they transformed the titanate nanotube thin films into titanium dioxide nanotube thin films by calcination. However, these methods involve complicated processes, including (1) synthesis of nanotubes, (2) preparation of a liquid in which the synthesized nanotubes are dispersed, (3) coating of the nanotubes onto a substrate using the prepared liquid, and (4) fixation of the coated nanotubes onto the substrate surface by calcination. Since it is generally difficult to prepare

Synthesis of Titanate and Titanium Dioxide

Nanotube Thin Films and Their Applications to Biomaterials 43

preventing infections, antibiotics are administered even in operation rooms with few pathogens. However, this does not prevent every infection. Therefore, imparting antibacterial properties to implants is currently under investigation. There have been reports of the use of apatite coating containing silver on implants by sputtering (Chen et al., 2006), silver-plated implants (Hardes et al., 2007), and gentamicin–hydroxyapatite coating for cementless joints (Alt et al., 2006), all of which have shown antibacterial properties. However, these methods have drawbacks such as the need for expensive instruments and the use of antibiotics that may cause the emergence of resistant bacteria. Therefore, further research is required. In this study, in order to develop more convenient and inexpensive antibacterial implants, silver ions are studied as an antibacterial component along with titanate nanotube formed on the surface of titanium. Silver is one of the most common antibacterial elements and is considered highly safe with high antibacterial activity. Sodium titanates are composed of a titanate framework with a negative electric charge and Na+ ions with a positive electric charge. Since they have a cation exchange property, Na+ ions can be exchanged with several cations (Kim et al., 1997; Chen et al., 2002; Sun et al., 2003; Bavykin et al., 20006). Therefore, it is considered that sodium titanate can be transformed into silver titanate by exchange of Na+ in sodium titanate with Ag+, and the in vivo elution of silver ions from the titanates would be promising for application to antibacterial implants. In addition, it is suggested that the titanate nanotube thin film would be able to possess a larger amount of silver and allow the amount of silver to be controlled more widely as compared with the titanate thin film previously reported (Kim et al., 1996). In this chapter, we will describe the synthesis and characterization of titanate nanotube thin films with silver and the behavior of silver ion elution of the thin films in vitro. We will also describe the antibacterial properties against methicillin-resistant Staphylococcus aureus (MRSA) with a biofilm-forming gene, which is a major concern in actual infections, to investigate the

possibility of using synthesized thin films as antibacterial implants.

promise for use as implant materials.

Finally, we will describe the apatite-forming abilities of titanium compound nanotube thin films by comparing the apatite deposition behaviors of a sodium titanate nanotube thin film (Yada et al., 2007), a titanium dioxide nanotube thin film (Inoue et al., 2010), and a silver nanoparticle/silver titanate nanotube nanocomposite thin film (Inoue et al., 2010), in simulated body fluid (SBF) (Yada et al., 2010). In evaluating the in vivo apatite-forming ability or the osteoconductive property of a material, researchers commonly perform experiments in SBF (Kokubo et al., 2006)). Kim et al. (Kim et al., 1996) first reported the formation of a sodium titanate thin film with a porous network structure on a titanium metal plate by alkali and heat treatment and demonstrated the osteoconductive property of the obtained sodium titanate thin film. Since then, researchers have actively performed many studies on the applications of sodium titanate thin films in implants (Kokubo et al., 1996; Kim et al., 1997; Kim et al., 1997; Nishiguchi et al., 1999; Jonášová et al., 2003; Kim et al., 2003; Muramatsu et al., 2003; Wang et al., 2007; Wang et al., 2008). Similar studies have also been performed on calcium titanate thin films (Hanawa et al., 1997; Hamada et al., 2002; Nakagawa et al., 2005; Kon et al., 2007; Ohtsu et al., 2008), titanium dioxide thin films (Ohtsuki et al., 1997; Wang et al., 2001; Wang et al., 2003; Byon et al., 2007), and a nanohydroxyapatite thin film (Xiong et al., 2010), and the excellent biocompatibilities of these films have been reported. Therefore, titanium compound thin films show tremendous

a liquid in which nanotubes are uniformly dispersed and that partial aggregation is inevitable, the homogeneity of thin films thus formed is questionable. Moreover, permanent fixation of the thin films onto the substrates is also doubtful. On the other hand, titanate and titanium dioxide nanotube thin films can also be formed on titanium metal by immersing titanium metal as a raw material into NaOH aqueous solution and then performing hydrothermal treatment (Miyauchi et al., 2006; Tian et al., 2003; Chi et al., 2007; Yada et al., 2007; Guo et al., 2007). The fabrication of titanate nanotube thin films using titanium metal plates was first reported by Tian et al. (Tian et al., 2003). The thin (~10 μm) films were detached from the titanium metal plates by hydrothermal reaction for 20 h. In contrast, thin films obtained by a short (6 h) hydrothermal reaction strongly adhered to the titanium metal plate. Miyauchi et al. (Miyauchi et al. 2006) obtained a titanium dioxide nanotube thin film by hydrothermal treatment on titanium metal, followed by acid treatment and calcination. Although this thin film was fixed onto the substrate, its thickness was only a few hundred nanometers. Therefore, it is clear that titanate and titanium dioxide nanotube thin films tend to detach from the substrates when they become too thick. Chi et al. also reported the fabrication of a sodium titanate nanotube thin film (Chi et al., 2007). However, the thickness of the film was not mentioned in their report, and the sodium titanate nanotubes were not transformed into titanium dioxide nanotubes.

In this chapter, first, we will report the synthesis and organization of sodium titanate nanotube (hereafter referred to as Na-TNT) of size larger than a micrometer, using various titanium metals with controlled shapes of a micrometer size including plate, wire with a diameter of a micrometer, mesh woven from the titanium wire, microspheres, and microtube (Yada et al., 2007). The titanium metal acts as a template for the organization as well as a titanium source. Therefore, the originality of our study is to use titanium metal as a morphology-directing material. In addition, we will report a novel procedure for fixation of Na-TNT thin film on titanium metal (Yada et al., 2007). As a result, the thickness of the sodium titanate nanotube thin film can be adjusted by changing the duration of the hydrothermal reaction and the obtained films are thicker than those reported in previous studies (Miyauchi et al., 2006; Tian et al., 2003). Furthermore, we will also introduce a novel "hydrothermal transcription method" for forming Na-TNT films on various substrates such as Co-Cr alloy and SUS316L (Yada et al., 2008). Transformation of Na-TNT thin films into thin films consisting of anatase nanotube, anatase nanowires, anatase nanoparticles, and rhomboid-shaped anatase nanoparticles are also introduced (Inoue et al. 2010). To obtain an anatase nanotube thin film, it is necessary to slightly modify previously reported methods for synthesizing titanium dioxide nanotube particles.

Next, in this chapter, obtained titanate and titanium dioxide nanotube thin films will be applied to antibacterial biomaterials (Inoue et al., 2010). The nanotube thin film has several advantages: it can be formed on titanium, titanium alloy, Co–Cr alloy, and SUS316L, which are useful for manufacturing surgical instruments and implants such as artificial joints; its thickness can be controlled up to 20 μm or more, in contrast to only 1 μm for the thickness of the previously reported sodium titanate thin film with a porous network structure; and medicines can be incorporated into the nanotube. It is well known that bacterial infection may occur during surgery because of several factors. For example, during hip-replacement arthroplasty, bacterial infections occur in 1% to 2% of operations and usually cause physical and economic burdens for patients, such as re-implantation. As a conventional method for

a liquid in which nanotubes are uniformly dispersed and that partial aggregation is inevitable, the homogeneity of thin films thus formed is questionable. Moreover, permanent fixation of the thin films onto the substrates is also doubtful. On the other hand, titanate and titanium dioxide nanotube thin films can also be formed on titanium metal by immersing titanium metal as a raw material into NaOH aqueous solution and then performing hydrothermal treatment (Miyauchi et al., 2006; Tian et al., 2003; Chi et al., 2007; Yada et al., 2007; Guo et al., 2007). The fabrication of titanate nanotube thin films using titanium metal plates was first reported by Tian et al. (Tian et al., 2003). The thin (~10 μm) films were detached from the titanium metal plates by hydrothermal reaction for 20 h. In contrast, thin films obtained by a short (6 h) hydrothermal reaction strongly adhered to the titanium metal plate. Miyauchi et al. (Miyauchi et al. 2006) obtained a titanium dioxide nanotube thin film by hydrothermal treatment on titanium metal, followed by acid treatment and calcination. Although this thin film was fixed onto the substrate, its thickness was only a few hundred nanometers. Therefore, it is clear that titanate and titanium dioxide nanotube thin films tend to detach from the substrates when they become too thick. Chi et al. also reported the fabrication of a sodium titanate nanotube thin film (Chi et al., 2007). However, the thickness of the film was not mentioned in their report, and the sodium titanate nanotubes were not

In this chapter, first, we will report the synthesis and organization of sodium titanate nanotube (hereafter referred to as Na-TNT) of size larger than a micrometer, using various titanium metals with controlled shapes of a micrometer size including plate, wire with a diameter of a micrometer, mesh woven from the titanium wire, microspheres, and microtube (Yada et al., 2007). The titanium metal acts as a template for the organization as well as a titanium source. Therefore, the originality of our study is to use titanium metal as a morphology-directing material. In addition, we will report a novel procedure for fixation of Na-TNT thin film on titanium metal (Yada et al., 2007). As a result, the thickness of the sodium titanate nanotube thin film can be adjusted by changing the duration of the hydrothermal reaction and the obtained films are thicker than those reported in previous studies (Miyauchi et al., 2006; Tian et al., 2003). Furthermore, we will also introduce a novel "hydrothermal transcription method" for forming Na-TNT films on various substrates such as Co-Cr alloy and SUS316L (Yada et al., 2008). Transformation of Na-TNT thin films into thin films consisting of anatase nanotube, anatase nanowires, anatase nanoparticles, and rhomboid-shaped anatase nanoparticles are also introduced (Inoue et al. 2010). To obtain an anatase nanotube thin film, it is necessary to slightly modify previously reported methods

Next, in this chapter, obtained titanate and titanium dioxide nanotube thin films will be applied to antibacterial biomaterials (Inoue et al., 2010). The nanotube thin film has several advantages: it can be formed on titanium, titanium alloy, Co–Cr alloy, and SUS316L, which are useful for manufacturing surgical instruments and implants such as artificial joints; its thickness can be controlled up to 20 μm or more, in contrast to only 1 μm for the thickness of the previously reported sodium titanate thin film with a porous network structure; and medicines can be incorporated into the nanotube. It is well known that bacterial infection may occur during surgery because of several factors. For example, during hip-replacement arthroplasty, bacterial infections occur in 1% to 2% of operations and usually cause physical and economic burdens for patients, such as re-implantation. As a conventional method for

transformed into titanium dioxide nanotubes.

for synthesizing titanium dioxide nanotube particles.

preventing infections, antibiotics are administered even in operation rooms with few pathogens. However, this does not prevent every infection. Therefore, imparting antibacterial properties to implants is currently under investigation. There have been reports of the use of apatite coating containing silver on implants by sputtering (Chen et al., 2006), silver-plated implants (Hardes et al., 2007), and gentamicin–hydroxyapatite coating for cementless joints (Alt et al., 2006), all of which have shown antibacterial properties. However, these methods have drawbacks such as the need for expensive instruments and the use of antibiotics that may cause the emergence of resistant bacteria. Therefore, further research is required. In this study, in order to develop more convenient and inexpensive antibacterial implants, silver ions are studied as an antibacterial component along with titanate nanotube formed on the surface of titanium. Silver is one of the most common antibacterial elements and is considered highly safe with high antibacterial activity. Sodium titanates are composed of a titanate framework with a negative electric charge and Na+ ions with a positive electric charge. Since they have a cation exchange property, Na+ ions can be exchanged with several cations (Kim et al., 1997; Chen et al., 2002; Sun et al., 2003; Bavykin et al., 20006). Therefore, it is considered that sodium titanate can be transformed into silver titanate by exchange of Na+ in sodium titanate with Ag+, and the in vivo elution of silver ions from the titanates would be promising for application to antibacterial implants. In addition, it is suggested that the titanate nanotube thin film would be able to possess a larger amount of silver and allow the amount of silver to be controlled more widely as compared with the titanate thin film previously reported (Kim et al., 1996). In this chapter, we will describe the synthesis and characterization of titanate nanotube thin films with silver and the behavior of silver ion elution of the thin films in vitro. We will also describe the antibacterial properties against methicillin-resistant Staphylococcus aureus (MRSA) with a biofilm-forming gene, which is a major concern in actual infections, to investigate the possibility of using synthesized thin films as antibacterial implants.

Finally, we will describe the apatite-forming abilities of titanium compound nanotube thin films by comparing the apatite deposition behaviors of a sodium titanate nanotube thin film (Yada et al., 2007), a titanium dioxide nanotube thin film (Inoue et al., 2010), and a silver nanoparticle/silver titanate nanotube nanocomposite thin film (Inoue et al., 2010), in simulated body fluid (SBF) (Yada et al., 2010). In evaluating the in vivo apatite-forming ability or the osteoconductive property of a material, researchers commonly perform experiments in SBF (Kokubo et al., 2006)). Kim et al. (Kim et al., 1996) first reported the formation of a sodium titanate thin film with a porous network structure on a titanium metal plate by alkali and heat treatment and demonstrated the osteoconductive property of the obtained sodium titanate thin film. Since then, researchers have actively performed many studies on the applications of sodium titanate thin films in implants (Kokubo et al., 1996; Kim et al., 1997; Kim et al., 1997; Nishiguchi et al., 1999; Jonášová et al., 2003; Kim et al., 2003; Muramatsu et al., 2003; Wang et al., 2007; Wang et al., 2008). Similar studies have also been performed on calcium titanate thin films (Hanawa et al., 1997; Hamada et al., 2002; Nakagawa et al., 2005; Kon et al., 2007; Ohtsu et al., 2008), titanium dioxide thin films (Ohtsuki et al., 1997; Wang et al., 2001; Wang et al., 2003; Byon et al., 2007), and a nanohydroxyapatite thin film (Xiong et al., 2010), and the excellent biocompatibilities of these films have been reported. Therefore, titanium compound thin films show tremendous promise for use as implant materials.

Synthesis of Titanate and Titanium Dioxide

Nanotube Thin Films and Their Applications to Biomaterials 45

a b c

d e f

300 ºC calcined (d-f) products obtained after the 20 h reaction.

300 °C.

Fig. 1. Photograph (a, d), TEM (b), and SEM (c, e, f) images for the as-grown (a-c) and the

to the plate. When the plate was washed with water after heat treatment at 300°C for 1 h in air, although the NaOH crystals dissolved, the thin Na-TNT film still adhered to the plate firmly and no detachment was observed as shown in Fig. 1d. Na-TNT formation was confirmed by the fibrous morphologies observed on the surface of the thin film in an enlarged SEM image (Figs. 1e, f) and nanotubes observed in a TEM image of the thin film. Moreover, in an XRD pattern of the thin film (Fig. 2), only diffraction peaks characteristic of Na-TNT (Chen et al., 2002) were observed along with peaks assigned to titanium metal. The reason for this stable coating is probably because polycondensation of hydroxyl groups in the interface area between the titanium plate and the Na-TNT-free sodium titanate phase occurred by the heat treatment at 300°C, and Na-TNT being firmly fixed on the plate. The slow drying process is also considered to be important for the fixation of Na-TNT onto the titanium plate, since the thin film detached from the titanium plate by drying at 60°C. The formation and fixation of the Na-TNT thin film were also observed in the reaction after 3 h. Nanotubular structures similar to those of the 20 h product were also observed. The thickness of approximately 5 μm for the film obtained after the 3 h reaction was smaller than that of 20.2 μm for the film obtained after the 20 h reaction. The thickness of the film is thus controllable by the reaction time. On the other hand, when an as-synthesized Na-TNT thin film obtained by hydrothermal reaction in 10 mol/L NaOH solution at 160 °C for 1 h was washed with large amounts of water, the Na-TNT thin film do not detach from the substrates and remains as thin as approximately 1 μm. Therefore, it is clear that sodium titanate nanotube thin films tend to detach from the substrates when they become too thick, but remain firmly fixed on substrates when the obtained samples are dried (without washing with water) and subsequently calcined at

## **2. Synthesis and characterization of titanate and titanium dioxide nanotube thin films**

#### **2.1 Sodium titanate nanotube thin films formed on various shaped titanium metal templates**

#### **2.1.1 Sodium titanate nanotube thin film formed a titanium plate**

First, the growth and fixation of Na-TNT on titanium plate were investigated. A titanium plate (20 mm 20 mm 2 mm) was immersed in 20 ml of 10 mol/l aqueous NaOH solution in a Teflon container and reactions were carried out at 160°C. After hydrothermal treatment for 20 h, the surface of the plate changed to pale, indicating the formation of a thin film on the titanium plate. In order to wash out NaOH and the particles that adhered to the surface of the thin film, the plate was washed with water after the reaction. The thin film immediately exfoliated as shown in Fig. 1a, and then a surface with a metallic luster similar to that of titanium metal appeared on the surface of the plate. The greater part of the film was posited to consist of nanotubes with an outer diameter of approximately 8 nm (Fig. 1b). Through EDX analysis, the mol fraction of Na/Ti/O for the obtained film was determined to be 1:1.947:4.943. The film was thus assumed to be Na2Ti4O9·H2O, though some titanate structures, such as A2Ti2O5·H2O (Yang et al., 2003), A2Ti3O7 (Chen et al., 2002), H2Ti4O9·H2O (Nakahira et al., 2004), and lepidocrocite titanates (Ma et al., 2003), have been assigned as nanotube constituents (A=Na and /or H) as summarized by Tasi et al (Tasi et al., 2006). Moreover, by detailed SEM observation of the cross-section of this film, the film thickness was determined to be approximately 20.2 μm, as shown in Fig. 1c. The thickness of the Na-TNT phase was determined to be approximately 19.2 μm, and the thickness of the dense sodium titanate phase without Na-TNT was determined to be approximately 1.0 μm. Although fibrous morphologies were observed on the surface of the film, the back of the film was flat with no visible fibers. Therefore, the film exfoliation was considered to occur at the interface between the titanium metal phase and the sodium titanate phase without Na-TNT. Based on the above results, the formation of the Na-TNT thin film can be explained as follows: (1) titanium dissolves into titanium ions (Ti4+) by oxidizers, H+ and/or O2; (2) dissolved Ti4+ ions immediately form titanium species (Wu et al., 2006) such as TiO32−, TiO2(OH)22−, and TinO2n+m2m− and the concentration of titanium species in the reaction solution increases as the dissolution of titanium is accelerated; (2) titanium species are reprecipitated as sodium titanate with an increase in the concentration of titanium species in the reaction solution; (3) since the concentration of the titanium species in the reaction solution is expected to increase with time, the sodium titanate phase without Na-TNT is formed when the concentration of titanium species is low and the Na-TNT phase is formed after the concentration becomes sufficiently high. The Na-TNT-free sodium titanate phase formed at low concentrations of titanium species may be amorphous sodium titanate. Since the concentration of titanium species in the reaction solution is considered to affect the type of sodium titanate cluster and the formation rate of the sodium titanate phase, the concentration of titanium species, together with temperature and other concentrations, are also considered to be factors in determining the type of phase formed.

Moreover, in order to prevent detachment of the thin Na-TNT film, the as-synthesized plate was slowly dried at room temperature after the hydrothermal treatment without washing it with water. Although NaOH crystals were observed on the plate, the thin film still adhered

**2. Synthesis and characterization of titanate and titanium dioxide nanotube** 

**2.1 Sodium titanate nanotube thin films formed on various shaped titanium metal** 

First, the growth and fixation of Na-TNT on titanium plate were investigated. A titanium plate (20 mm 20 mm 2 mm) was immersed in 20 ml of 10 mol/l aqueous NaOH solution in a Teflon container and reactions were carried out at 160°C. After hydrothermal treatment for 20 h, the surface of the plate changed to pale, indicating the formation of a thin film on the titanium plate. In order to wash out NaOH and the particles that adhered to the surface of the thin film, the plate was washed with water after the reaction. The thin film immediately exfoliated as shown in Fig. 1a, and then a surface with a metallic luster similar to that of titanium metal appeared on the surface of the plate. The greater part of the film was posited to consist of nanotubes with an outer diameter of approximately 8 nm (Fig. 1b). Through EDX analysis, the mol fraction of Na/Ti/O for the obtained film was determined to be 1:1.947:4.943. The film was thus assumed to be Na2Ti4O9·H2O, though some titanate structures, such as A2Ti2O5·H2O (Yang et al., 2003), A2Ti3O7 (Chen et al., 2002), H2Ti4O9·H2O (Nakahira et al., 2004), and lepidocrocite titanates (Ma et al., 2003), have been assigned as nanotube constituents (A=Na and /or H) as summarized by Tasi et al (Tasi et al., 2006). Moreover, by detailed SEM observation of the cross-section of this film, the film thickness was determined to be approximately 20.2 μm, as shown in Fig. 1c. The thickness of the Na-TNT phase was determined to be approximately 19.2 μm, and the thickness of the dense sodium titanate phase without Na-TNT was determined to be approximately 1.0 μm. Although fibrous morphologies were observed on the surface of the film, the back of the film was flat with no visible fibers. Therefore, the film exfoliation was considered to occur at the interface between the titanium metal phase and the sodium titanate phase without Na-TNT. Based on the above results, the formation of the Na-TNT thin film can be explained as follows: (1) titanium dissolves into titanium ions (Ti4+) by oxidizers, H+ and/or O2; (2) dissolved Ti4+ ions immediately form titanium species (Wu et al., 2006) such as TiO32−, TiO2(OH)22−, and TinO2n+m2m− and the concentration of titanium species in the reaction solution increases as the dissolution of titanium is accelerated; (2) titanium species are reprecipitated as sodium titanate with an increase in the concentration of titanium species in the reaction solution; (3) since the concentration of the titanium species in the reaction solution is expected to increase with time, the sodium titanate phase without Na-TNT is formed when the concentration of titanium species is low and the Na-TNT phase is formed after the concentration becomes sufficiently high. The Na-TNT-free sodium titanate phase formed at low concentrations of titanium species may be amorphous sodium titanate. Since the concentration of titanium species in the reaction solution is considered to affect the type of sodium titanate cluster and the formation rate of the sodium titanate phase, the concentration of titanium species, together with temperature and other concentrations, are

**2.1.1 Sodium titanate nanotube thin film formed a titanium plate** 

also considered to be factors in determining the type of phase formed.

Moreover, in order to prevent detachment of the thin Na-TNT film, the as-synthesized plate was slowly dried at room temperature after the hydrothermal treatment without washing it with water. Although NaOH crystals were observed on the plate, the thin film still adhered

**thin films** 

**templates** 

Fig. 1. Photograph (a, d), TEM (b), and SEM (c, e, f) images for the as-grown (a-c) and the 300 ºC calcined (d-f) products obtained after the 20 h reaction.

to the plate. When the plate was washed with water after heat treatment at 300°C for 1 h in air, although the NaOH crystals dissolved, the thin Na-TNT film still adhered to the plate firmly and no detachment was observed as shown in Fig. 1d. Na-TNT formation was confirmed by the fibrous morphologies observed on the surface of the thin film in an enlarged SEM image (Figs. 1e, f) and nanotubes observed in a TEM image of the thin film. Moreover, in an XRD pattern of the thin film (Fig. 2), only diffraction peaks characteristic of Na-TNT (Chen et al., 2002) were observed along with peaks assigned to titanium metal. The reason for this stable coating is probably because polycondensation of hydroxyl groups in the interface area between the titanium plate and the Na-TNT-free sodium titanate phase occurred by the heat treatment at 300°C, and Na-TNT being firmly fixed on the plate. The slow drying process is also considered to be important for the fixation of Na-TNT onto the titanium plate, since the thin film detached from the titanium plate by drying at 60°C. The formation and fixation of the Na-TNT thin film were also observed in the reaction after 3 h. Nanotubular structures similar to those of the 20 h product were also observed. The thickness of approximately 5 μm for the film obtained after the 3 h reaction was smaller than that of 20.2 μm for the film obtained after the 20 h reaction. The thickness of the film is thus controllable by the reaction time. On the other hand, when an as-synthesized Na-TNT thin film obtained by hydrothermal reaction in 10 mol/L NaOH solution at 160 °C for 1 h was washed with large amounts of water, the Na-TNT thin film do not detach from the substrates and remains as thin as approximately 1 μm. Therefore, it is clear that sodium titanate nanotube thin films tend to detach from the substrates when they become too thick, but remain firmly fixed on substrates when the obtained samples are dried (without washing with water) and subsequently calcined at 300 °C.

Synthesis of Titanate and Titanium Dioxide

Nanotube Thin Films and Their Applications to Biomaterials 47

a b c

d e f

(b), microtube (c), and wire (d-f) obtained after the 3 h reaction.

Fig. 3. SEM (a, b, d, e), Photograph (c), and TEM (f) images for the mesh (a), micro-sphere

explained as follows. Surface area and surface texture strongly affect the concentration of titanium species in the reaction solution. The amount of titanium species in the reaction solution increases with an increase in the diameter of the wire, since the surface area of the wire increases with an increase in the diameter. Additionally, the difference in the surface texture of the wires also affects the concentration of titanium species near their surfaces. Detailed SEM observations of the original titanium wires confirmed that the surfaces of the wire of diameter 104.4 μm were porous, but the surface of the wire of diameter 53.4 μm was relatively smooth. The concentration of titanium species would be higher near the wire and lower as the distance from the wire increases. In particular, the concentration of titanium species near the porous surface would be higher than that near the smooth surface. Therefore, in the experiments using the 104.4 μm diameter wire, the amount of titanium species formed per unit of time and the concentration of the titanium species would be large due to their larger diameters and porous surfaces, and consequently the concentration of titanium species would be sufficiently high for the formation of Na-TNT as the dissolution of titanium proceeded. On the other hand, since the surface area of the 53.4 μm diameter wire was predicted to be smaller than the 104.4 μm diameter wire due to its diameter and smooth surface, the concentration of titanium species formed per unit of time would also be small. Therefore, the concentration of titanium species would be too low for the formation of Na-TNT. Consequently, sodium titanate with irregular morphology was formed without Na-TNT at a reaction time of 3 h and the original wire completely dissolved at a reaction time of 20 h. Taking into consideration the above discussion, a similar hydrothermal and fixing treatment was performed using a wire of diameter 53.4 μm and length 24 cm wound onto the above mentioned titanium plate, which could act as a source of titanium species.

Fig. 2. XRD pattern for the plate obtained after the 20 h reaction. Peak assignment: ○ titanium metal, ● sodium titanate nanotube.

#### **2.1.2 Sodium titanate nanotube thin films formed on titanium wire, titanium mesh, titanium sphere, and titanium microtube**

Titanium wire (lengths: 5 cm, 24 cm, and diameters: 53.4 μm, 104.4 μm, 203.7 μm), titanium mesh (woven from the titanium wire with a diameter of 104.4 μm, 20 mm 20 mm), titanium tube (inner diameter: 800 μm, outer diameter: 1 mm, length: 1 cm), and titanium spheres (diameter: 850–1180 μm, weight: 0.21-0.24 g) were used as metal titanium sources instead of a titanium plate.

After the hydrothermal treatments for 3 h and 20 h, the surfaces of titanium mesh and titanium sphere were completely covered with Na-TNT thin film. Both outer and inner surfaces of the microtube were also covered with uniform nanotubes with an average diameter of 8 nm. Typical digital camera and SEM images are shown in Figs. 3a-c.

On the other hand, the formation of Na-TNT thin film on a titanium wire requires special procedures which are different from those for plate, mesh, sphere, and microtube. First, the synthesis and fixation of Na-TNT were investigated using titanium metal wires of diameters 53.4 and 104.4 μm and length 5 cm as titanium sources. As a result, after the hydrothermal treatment for 3 h, sodium titanate with an irregular morphology was formed on the surface of the titanium wires, and only small amount of nanotubes was observed in the product synthesized using the titanium wire of diameter 104.4 μm. The diameters decreased from 53.4 and 104.4 μm for the original wires to 36.3 and 93.8 μm for the wires after a reaction time of 3 h, respectively. Moreover, after the hydrothermal treatment for 20 h, both wires completely dissolved. In addition, in the experiment using a wire of diameter 53.4 μm and length 24 cm, no nanotubes were observed on the surface of the obtained wire at a reaction time of 3 h and the wire completely dissolved at a reaction time of 20 h. On the other hand, in the experiment using a wire of diameter 104.4 μm and length 24 cm, the amount of Na-TNT formed increased at 3 h reaction time, and the surface of the wire was completely covered with Na-TNT thin film at the 20 h reaction. The reason for the complete dissolution of the original wires is because the dissolution rates of titanium species from the wires were faster than the redeposition rate of sodium titanate on the surface of the wires. On the other hand, the reason for the complete coverage of Na-TNT on the wire without dissolution is that the redeposition rate of sodium titanate nanotubes on the wire's surface became faster than the dissolution rate of titanium species from the wire with an increase in its diameter and length. These differences depending on the diameters of the original wires are

Fig. 2. XRD pattern for the plate obtained after the 20 h reaction. Peak assignment: ○

**2.1.2 Sodium titanate nanotube thin films formed on titanium wire, titanium mesh,** 

Titanium wire (lengths: 5 cm, 24 cm, and diameters: 53.4 μm, 104.4 μm, 203.7 μm), titanium mesh (woven from the titanium wire with a diameter of 104.4 μm, 20 mm 20 mm), titanium tube (inner diameter: 800 μm, outer diameter: 1 mm, length: 1 cm), and titanium spheres (diameter: 850–1180 μm, weight: 0.21-0.24 g) were used as metal titanium sources

After the hydrothermal treatments for 3 h and 20 h, the surfaces of titanium mesh and titanium sphere were completely covered with Na-TNT thin film. Both outer and inner surfaces of the microtube were also covered with uniform nanotubes with an average

On the other hand, the formation of Na-TNT thin film on a titanium wire requires special procedures which are different from those for plate, mesh, sphere, and microtube. First, the synthesis and fixation of Na-TNT were investigated using titanium metal wires of diameters 53.4 and 104.4 μm and length 5 cm as titanium sources. As a result, after the hydrothermal treatment for 3 h, sodium titanate with an irregular morphology was formed on the surface of the titanium wires, and only small amount of nanotubes was observed in the product synthesized using the titanium wire of diameter 104.4 μm. The diameters decreased from 53.4 and 104.4 μm for the original wires to 36.3 and 93.8 μm for the wires after a reaction time of 3 h, respectively. Moreover, after the hydrothermal treatment for 20 h, both wires completely dissolved. In addition, in the experiment using a wire of diameter 53.4 μm and length 24 cm, no nanotubes were observed on the surface of the obtained wire at a reaction time of 3 h and the wire completely dissolved at a reaction time of 20 h. On the other hand, in the experiment using a wire of diameter 104.4 μm and length 24 cm, the amount of Na-TNT formed increased at 3 h reaction time, and the surface of the wire was completely covered with Na-TNT thin film at the 20 h reaction. The reason for the complete dissolution of the original wires is because the dissolution rates of titanium species from the wires were faster than the redeposition rate of sodium titanate on the surface of the wires. On the other hand, the reason for the complete coverage of Na-TNT on the wire without dissolution is that the redeposition rate of sodium titanate nanotubes on the wire's surface became faster than the dissolution rate of titanium species from the wire with an increase in its diameter and length. These differences depending on the diameters of the original wires are

diameter of 8 nm. Typical digital camera and SEM images are shown in Figs. 3a-c.

titanium metal, ● sodium titanate nanotube.

**titanium sphere, and titanium microtube** 

instead of a titanium plate.

a b c

Fig. 3. SEM (a, b, d, e), Photograph (c), and TEM (f) images for the mesh (a), micro-sphere (b), microtube (c), and wire (d-f) obtained after the 3 h reaction.

explained as follows. Surface area and surface texture strongly affect the concentration of titanium species in the reaction solution. The amount of titanium species in the reaction solution increases with an increase in the diameter of the wire, since the surface area of the wire increases with an increase in the diameter. Additionally, the difference in the surface texture of the wires also affects the concentration of titanium species near their surfaces. Detailed SEM observations of the original titanium wires confirmed that the surfaces of the wire of diameter 104.4 μm were porous, but the surface of the wire of diameter 53.4 μm was relatively smooth. The concentration of titanium species would be higher near the wire and lower as the distance from the wire increases. In particular, the concentration of titanium species near the porous surface would be higher than that near the smooth surface. Therefore, in the experiments using the 104.4 μm diameter wire, the amount of titanium species formed per unit of time and the concentration of the titanium species would be large due to their larger diameters and porous surfaces, and consequently the concentration of titanium species would be sufficiently high for the formation of Na-TNT as the dissolution of titanium proceeded. On the other hand, since the surface area of the 53.4 μm diameter wire was predicted to be smaller than the 104.4 μm diameter wire due to its diameter and smooth surface, the concentration of titanium species formed per unit of time would also be small. Therefore, the concentration of titanium species would be too low for the formation of Na-TNT. Consequently, sodium titanate with irregular morphology was formed without Na-TNT at a reaction time of 3 h and the original wire completely dissolved at a reaction time of 20 h. Taking into consideration the above discussion, a similar hydrothermal and fixing treatment was performed using a wire of diameter 53.4 μm and length 24 cm wound onto the above mentioned titanium plate, which could act as a source of titanium species.

Synthesis of Titanate and Titanium Dioxide

be made on heterogeneous substrates.

2.2 cm

Nanotube Thin Films and Their Applications to Biomaterials 49

5d). The thickness of this film is about 5 µm. This thickness was less than the 20 µm thickness of the Na-TNT film formed on the titanium plate when reacted singly (Yada et al., 2007). As the XRD pattern of the Co–Cr alloy surface countered to the titanium plate, a diffraction peak characteristic to titanate nanotube near 2θ = 10° as well as the peaks attributed to the Co–Cr alloy of the raw material were observed. From a EDX analysis, it was found that the film contains Na, Ti, O, Co, Cr, and Si, and the molar ratio for the film was Na:Ti:O:Co:Cr:Si=0.322:1:2.401:0.112:0.052:0.045. Sodium titanate nanotube film is thus thought to be formed on the Co–Cr alloy disk. The elements of Co, Cr, and Si would dissolve from the Co–Cr alloy disk and would be incorporated into the titanate framework and/or the interlayer spacing of the titanate. Furthermore, as observed above, the white Na-TNT film reflecting the square form of the titanium plate was observed on the Co–Cr alloy disk countered to the titanium plate (Fig. 5a). Thus, the titanium species capable of forming Na-TNT were present near the surface of titanium, and it can be considered that Na-TNT patterning reflecting the form of the titanium plate was made on the surface of the Co–Cr alloy disk countered to the titanium plate. The above results suggest that by using several forms of the titanium plate as the titanium source, several forms of Na-TNT patterning can

a b c d

on the surface of Co-Cr alloy countered to titanium metal.

Fig. 5. Digital camera (a), SEM (b, c), and TEM (d) images for the obtained thin film formed

When the same experiment was conducted with SUS316L plate instead of the Co–Cr alloy disk, it was found that as in the case of Co–Cr alloy, diluted white Na-TNT thin film was formed on the surface of SUS316L plate countered to the titanium plate. On the other hand, when the same reaction was performed using the SUS430 plate instead of Co–Cr alloy disk, brown and black iron compounds were formed on the SUS430 plate, although white Na-TNT film was formed in part. SUS430 is an industrial grade stainless alloy, whereas SUS316L is a stainless alloy used in implants and has exceptionally high corrosion resistance, and the results reflecting this corrosion resistance were obtained. When the same reaction was performed with the tantalum plate instead of the Co–Cr alloy plate, copious amounts of white products were produced on the tantalum plate, and particles other than nanotubes were observed. In addition, when the same experiment was performed using a silicon plate instead of the Co–Cr alloy disk, the silicon plate was completely dissolved by the hydrothermal reaction. On considering the differences in the responsiveness of substrates, it is thought that the dissolution rate of the titanium plate and substrates and redeposition rate of chemical species that arose from the dissolved titanium plate and substrates should be considered. Particularly, in this experiment system, it is considered that the dissolution rate of substrates has a large effect on the results of the experiments. The dissolution rate of using metals as substrates, as in this study, can be explained by the

Wired morphologies remained for 3 h (Fig. 3d) and 20 h reactions, respectively, and the surfaces of both wires were completely covered with uniform Na-TNT thin films (Figs. 3e, f). It is considered that since the amount of titanium species reprecipitated on the wire, supplied by the dissolution from the titanium plate, was larger than the amount of titanium species dissolved from the wire, the surface of the wire was covered with Na-TNT. These results also indicate that dense concentration of titanium species near the titanium surface is required for the formation of Na-TNT on the titanium wire. Based on the above results, Na-TNT applications can be largely extended by the hydrothermal treatment of a cloth woven with titanium wires and by weaving a cloth with Na-TNT/Ti wires.

## **2.2 Sodium titanate nanotube thin films formed on Co–Cr alloy and SUS316L plates**

We devised a "hydrothermal transcription method" for forming Na-TNT films on various substrates, as shown in Fig. 4. In this method, Na-TNT would be produced by re-depositing or transcribing the titanium species such as TiO32−, TiO2(OH)2 <sup>2</sup>−, and TinO2n+m2m<sup>−</sup> formed near the surface of the titanium plate by hydrothermal treatment in aqueous NaOH solution on other substrate as Na-TNT, and grown to form dense films on several substrates as well as on the titanium plate. As shown in Fig. 4, under the conditions where a titanium metal plate and a substrate were adjacently placed, the titanium metal plate and substrate were spaced uniformly (about 200 µm) and fixed using titanium wires or SUS316 wires. For the substrate, Co–Cr alloy disk, SUS316L plate, SUS430 plate, tantalum plate, and silicon plate were used. These were immersed in 10 mol/l NaOH aqueous solution and reacted hydrothermally for 20 h at 160 °C. After the reaction, the samples were removed from the container and dried. Then, by washing in water following heat treatment at 300 °C, excessive NaOH adhered on the substrate was removed.

Fig. 4. Schematic representation of a reaction process by the hydrothermal transcription method.

Firstly, the Co–Cr alloy disk was used as a substrate. As shown in Fig. 5a, after the reaction, the formation of a white film whose base is the color of Co–Cr alloy along the square form of counter titanium plate on only the face countered to the titanium plate was observed. This white film strongly adhered to the Co–Cr alloy plate. By SEM images (Figs. 5b, c), the uniform and dense formation of fibrous substances was identified. Also by TEM observation of fibrous substances, nanotubes with an outer diameter of about 8 mm were identified (Fig.

Wired morphologies remained for 3 h (Fig. 3d) and 20 h reactions, respectively, and the surfaces of both wires were completely covered with uniform Na-TNT thin films (Figs. 3e, f). It is considered that since the amount of titanium species reprecipitated on the wire, supplied by the dissolution from the titanium plate, was larger than the amount of titanium species dissolved from the wire, the surface of the wire was covered with Na-TNT. These results also indicate that dense concentration of titanium species near the titanium surface is required for the formation of Na-TNT on the titanium wire. Based on the above results, Na-TNT applications can be largely extended by the hydrothermal treatment of a cloth woven

**2.2 Sodium titanate nanotube thin films formed on Co–Cr alloy and SUS316L plates**  We devised a "hydrothermal transcription method" for forming Na-TNT films on various substrates, as shown in Fig. 4. In this method, Na-TNT would be produced by re-depositing or transcribing the titanium species such as TiO32−, TiO2(OH)22−, and TinO2n+m2m<sup>−</sup> formed near the surface of the titanium plate by hydrothermal treatment in aqueous NaOH solution on other substrate as Na-TNT, and grown to form dense films on several substrates as well as on the titanium plate. As shown in Fig. 4, under the conditions where a titanium metal plate and a substrate were adjacently placed, the titanium metal plate and substrate were spaced uniformly (about 200 µm) and fixed using titanium wires or SUS316 wires. For the substrate, Co–Cr alloy disk, SUS316L plate, SUS430 plate, tantalum plate, and silicon plate were used. These were immersed in 10 mol/l NaOH aqueous solution and reacted hydrothermally for 20 h at 160 °C. After the reaction, the samples were removed from the container and dried. Then, by washing in water following heat treatment at 300 °C,

Substrate

Film Reflecting Form of Ti Plate

Ti Plate

Deposition of TNT Formation of TNT

Substrate

Ti Plate

with titanium wires and by weaving a cloth with Na-TNT/Ti wires.

excessive NaOH adhered on the substrate was removed.

Ti Plate

Formation of Ti Species

Substrate

Hydrothermal Reaction in NaOH Aqueous Solution

Fig. 4. Schematic representation of a reaction process by the hydrothermal transcription

Firstly, the Co–Cr alloy disk was used as a substrate. As shown in Fig. 5a, after the reaction, the formation of a white film whose base is the color of Co–Cr alloy along the square form of counter titanium plate on only the face countered to the titanium plate was observed. This white film strongly adhered to the Co–Cr alloy plate. By SEM images (Figs. 5b, c), the uniform and dense formation of fibrous substances was identified. Also by TEM observation of fibrous substances, nanotubes with an outer diameter of about 8 mm were identified (Fig.

Ti Plate Substrate

method.

200 mμ

5d). The thickness of this film is about 5 µm. This thickness was less than the 20 µm thickness of the Na-TNT film formed on the titanium plate when reacted singly (Yada et al., 2007). As the XRD pattern of the Co–Cr alloy surface countered to the titanium plate, a diffraction peak characteristic to titanate nanotube near 2θ = 10° as well as the peaks attributed to the Co–Cr alloy of the raw material were observed. From a EDX analysis, it was found that the film contains Na, Ti, O, Co, Cr, and Si, and the molar ratio for the film was Na:Ti:O:Co:Cr:Si=0.322:1:2.401:0.112:0.052:0.045. Sodium titanate nanotube film is thus thought to be formed on the Co–Cr alloy disk. The elements of Co, Cr, and Si would dissolve from the Co–Cr alloy disk and would be incorporated into the titanate framework and/or the interlayer spacing of the titanate. Furthermore, as observed above, the white Na-TNT film reflecting the square form of the titanium plate was observed on the Co–Cr alloy disk countered to the titanium plate (Fig. 5a). Thus, the titanium species capable of forming Na-TNT were present near the surface of titanium, and it can be considered that Na-TNT patterning reflecting the form of the titanium plate was made on the surface of the Co–Cr alloy disk countered to the titanium plate. The above results suggest that by using several forms of the titanium plate as the titanium source, several forms of Na-TNT patterning can be made on heterogeneous substrates.

Fig. 5. Digital camera (a), SEM (b, c), and TEM (d) images for the obtained thin film formed on the surface of Co-Cr alloy countered to titanium metal.

When the same experiment was conducted with SUS316L plate instead of the Co–Cr alloy disk, it was found that as in the case of Co–Cr alloy, diluted white Na-TNT thin film was formed on the surface of SUS316L plate countered to the titanium plate. On the other hand, when the same reaction was performed using the SUS430 plate instead of Co–Cr alloy disk, brown and black iron compounds were formed on the SUS430 plate, although white Na-TNT film was formed in part. SUS430 is an industrial grade stainless alloy, whereas SUS316L is a stainless alloy used in implants and has exceptionally high corrosion resistance, and the results reflecting this corrosion resistance were obtained. When the same reaction was performed with the tantalum plate instead of the Co–Cr alloy plate, copious amounts of white products were produced on the tantalum plate, and particles other than nanotubes were observed. In addition, when the same experiment was performed using a silicon plate instead of the Co–Cr alloy disk, the silicon plate was completely dissolved by the hydrothermal reaction. On considering the differences in the responsiveness of substrates, it is thought that the dissolution rate of the titanium plate and substrates and redeposition rate of chemical species that arose from the dissolved titanium plate and substrates should be considered. Particularly, in this experiment system, it is considered that the dissolution rate of substrates has a large effect on the results of the experiments. The dissolution rate of using metals as substrates, as in this study, can be explained by the

Synthesis of Titanate and Titanium Dioxide

a b

Intensity (a.u.)

Fig. 6. TEM images of the ion-exchange-treated thin films at 90 °C (a) and 140 °C (b).

0 10 20 30 40 50 60 70 2 / deg. θ

Fig. 7. XRD patterns of the as-grown (a) and the ion-exchange-treated thin films at 40 °C (b),

90 °C (c), and 140 °C (d). Peak assignment: ■, α-titanium; ○, anatase; ▲, titanate.

a

b

c

d

redeposition reactions.

Nanotube Thin Films and Their Applications to Biomaterials 51

are obtained at 80 °C and aggregates of nanocrystals are obtained at 120 °C. Our results also suggest that the change in the crystal structure change of titanate compounds to anatase is determined not only by pH but also by the temperature of the ion-exchange treatment. We suggest that the high temperature (140 °C) of the ion-exchange treatment is responsible for the change in the crystal structure of hydrogen titanate to the anatase structure, with a high degree of crystallization, and that this change occurs due to polycondensation and dissolution–

ionization tendency, i.e., oxidation–reduction potential. It is considered that titanium dissolves into titanium ions (Ti4+) by oxidizers, H+ and/or O2, and these dissolved ions immediately form chemical species (Wu et al., 2006) such as TiO3 <sup>2</sup>−, TiO2(OH)22−, and TinO2n+m2m− which are re-deposited as Na-TNT. When a substrate whose ionization tendency is smaller than titanium, especially materials such as SUS316L and Co–Cr alloy, is hydrothermally reacted with titanium simultaneously, the dissolution rate of titanium is higher than that of the substrate. In this reaction, titanium species are immediately produced following the dissolution of titanium and spread and re-deposited on the substrate as Na-TNT film, which predominates the dissolving reaction of the substrates. As a result, the surface of the substrate is covered by Na-TNT film, and the dissolution of the substrate was further minimally suppressed. On the other hand, it is considered that when the substrates with ionization tendencies larger than titanium, i.e., substrates such as silicon and tantalum, and titanium were hydrothermally reacted at the same time, the dissolution reaction of substrates predominate the dissolution reaction of titanium. Na-TNT film was not thus obtained on the substrates.

#### **2.3 Hydrogen titanate and anatase-type titanium dioxide nanotube thin films**

H+ ion-exchange treatment for the sodium titanate nanotube thin film and the subsequent calcination can produce an anatase-type titanium dioxide nanotube thin film.

#### **2.3.1 H+ ion-exchange treatment for sodium titanate nanotube thin films**

We performed H+ ion-exchange treatment for Na-TNT thin film obtained after the 3 h reaction using 0.01 mol/l hydrochloric acid solution at 90 and 140 °C for 3 h. The thin films resulting from treatment at these two temperatures remain attached over the entire surface of each sample. EDX analysis reveal that because the molar ratio of Na/Ti decrease from 0.48 before treatment to 0 after treatment at 90 and 140 °C, Na+ ions between titanate layers are confirmed to be completely exchanged for H+ ions. We observe nanotubes with an average outer diameter of 8.3 nm and inner diameter of 3.3 nm in the ion-exchange-treated sample at 90 °C using 0.01 mol/l hydrochloric acid solution (Fig. 6a). No change is observed in the porous structure of the thin film before or after treatment. The XRD pattern for the H+ ion-exchanged sample at 90 °C shows four diffraction peaks (near 2*θ* = 9, 24, 29, and 48°) attributed to titanate together with peaks attributed to α-titanium, similar to those for the as-grown sample, as shown in Fig. 7. We therefore believe that the H+ ion-exchange treatment at 90 °C transforms sodium titanate nanotubes into hydrogen titanate nanotubes while maintaining the crystal structure of titanate, nanotubular morphology, and porous thin-film structure. In contrast, the H+ ion-exchange treatment at 140 °C replaces the fibrous morphology with rhomboid-shaped particles (average diameter 21 nm) (Fig. 6b) and pores (~ 45 nm diameter) in the interstitial gaps. The XRD pattern of this sample (Fig. 7) shows peaks attributed to anatase. Therefore, a porous thin film consisting of rhomboid-shaped anatase is confirmed to be formed on the titanium metal plate. Change in the crystal structure of sodium titanate nanotubes to anatase nanotubes by acid treatment have been described previously by Tsai et al. (Tsai et al., 2006). They reported that although a nanotube form is maintained by acid treatment at pH 1.6, only irregular-shaped anatase particles are formed by acid treatment at pH 0.38. Zhu et al. (Zhu et al., 2005) reported that hydrogen titanate nanofiber transforms into anatase nanocrystals in dilute (0.05 mol/L) HNO3 at 80–120 °C. They stated that monodispersed anatase nanocrystals

ionization tendency, i.e., oxidation–reduction potential. It is considered that titanium dissolves into titanium ions (Ti4+) by oxidizers, H+ and/or O2, and these dissolved ions immediately form chemical species (Wu et al., 2006) such as TiO32−, TiO2(OH)22−, and TinO2n+m2m− which are re-deposited as Na-TNT. When a substrate whose ionization tendency is smaller than titanium, especially materials such as SUS316L and Co–Cr alloy, is hydrothermally reacted with titanium simultaneously, the dissolution rate of titanium is higher than that of the substrate. In this reaction, titanium species are immediately produced following the dissolution of titanium and spread and re-deposited on the substrate as Na-TNT film, which predominates the dissolving reaction of the substrates. As a result, the surface of the substrate is covered by Na-TNT film, and the dissolution of the substrate was further minimally suppressed. On the other hand, it is considered that when the substrates with ionization tendencies larger than titanium, i.e., substrates such as silicon and tantalum, and titanium were hydrothermally reacted at the same time, the dissolution reaction of substrates predominate the dissolution reaction of titanium. Na-TNT film was

**2.3 Hydrogen titanate and anatase-type titanium dioxide nanotube thin films** 

 **ion-exchange treatment for sodium titanate nanotube thin films** 

calcination can produce an anatase-type titanium dioxide nanotube thin film.

H+ ion-exchange treatment for the sodium titanate nanotube thin film and the subsequent

We performed H+ ion-exchange treatment for Na-TNT thin film obtained after the 3 h reaction using 0.01 mol/l hydrochloric acid solution at 90 and 140 °C for 3 h. The thin films resulting from treatment at these two temperatures remain attached over the entire surface of each sample. EDX analysis reveal that because the molar ratio of Na/Ti decrease from 0.48 before treatment to 0 after treatment at 90 and 140 °C, Na+ ions between titanate layers are confirmed to be completely exchanged for H+ ions. We observe nanotubes with an average outer diameter of 8.3 nm and inner diameter of 3.3 nm in the ion-exchange-treated sample at 90 °C using 0.01 mol/l hydrochloric acid solution (Fig. 6a). No change is observed in the porous structure of the thin film before or after treatment. The XRD pattern for the H+ ion-exchanged sample at 90 °C shows four diffraction peaks (near 2*θ* = 9, 24, 29, and 48°) attributed to titanate together with peaks attributed to α-titanium, similar to those for the as-grown sample, as shown in Fig. 7. We therefore believe that the H+ ion-exchange treatment at 90 °C transforms sodium titanate nanotubes into hydrogen titanate nanotubes while maintaining the crystal structure of titanate, nanotubular morphology, and porous thin-film structure. In contrast, the H+ ion-exchange treatment at 140 °C replaces the fibrous morphology with rhomboid-shaped particles (average diameter 21 nm) (Fig. 6b) and pores (~ 45 nm diameter) in the interstitial gaps. The XRD pattern of this sample (Fig. 7) shows peaks attributed to anatase. Therefore, a porous thin film consisting of rhomboid-shaped anatase is confirmed to be formed on the titanium metal plate. Change in the crystal structure of sodium titanate nanotubes to anatase nanotubes by acid treatment have been described previously by Tsai et al. (Tsai et al., 2006). They reported that although a nanotube form is maintained by acid treatment at pH 1.6, only irregular-shaped anatase particles are formed by acid treatment at pH 0.38. Zhu et al. (Zhu et al., 2005) reported that hydrogen titanate nanofiber transforms into anatase nanocrystals in dilute (0.05 mol/L) HNO3 at 80–120 °C. They stated that monodispersed anatase nanocrystals

not thus obtained on the substrates.

**2.3.1 H+**

are obtained at 80 °C and aggregates of nanocrystals are obtained at 120 °C. Our results also suggest that the change in the crystal structure change of titanate compounds to anatase is determined not only by pH but also by the temperature of the ion-exchange treatment. We suggest that the high temperature (140 °C) of the ion-exchange treatment is responsible for the change in the crystal structure of hydrogen titanate to the anatase structure, with a high degree of crystallization, and that this change occurs due to polycondensation and dissolution– redeposition reactions.

Fig. 6. TEM images of the ion-exchange-treated thin films at 90 °C (a) and 140 °C (b).

Fig. 7. XRD patterns of the as-grown (a) and the ion-exchange-treated thin films at 40 °C (b), 90 °C (c), and 140 °C (d). Peak assignment: ■, α-titanium; ○, anatase; ▲, titanate.

Synthesis of Titanate and Titanium Dioxide

Nanotube Thin Films and Their Applications to Biomaterials 53

calcination at 900 °C yields a dense rutile thin film because of the densification and phase

10 20 30 40 50 60 2 / deg. θ

Fig. 9. XRD patterns of the 90 °C ion-exchange-treated (a) and the 90 °C ion-exchangetreated thin films calcined at 300 °C (b), 450 °C (c), 600 °C (d), 750 °C (e), and 900 °C (f). Peak

assignment: ■, α-titanium; ○, anatase; ●, rutile; △, hydrogen titanate; \*, distorted

obtained by the silver ion-exchange treatment of Na-TNT-TF was called Ag-TNT-TF.

Na-TNT thin film obtained after the 3 h reaction with dimensions of 20 mm × 20 mm × 2 mm (hereafter referred to as Na-TNT-TF) was immersed in 12 mL of 0.05 M silver acetate solution at 40 °C for 3 h, then repeatedly washed with distilled water and dried in a cool dark place, to exchange the Na+ in the sodium titanate with Ag+. Hereafter, the sample

The EDX spectra of the samples before and after the silver ion-exchange treatment were then compared. Since the peaks attributed to Na, observed in the samples before the silver ion-

**2.4 Silver nanoparticle / silver titanate nanotube nanocomposite thin film** 

c

b

a

d

e

f

0

titanium.

Intensity (a.u.)

transition caused by the sintering of anatase nanoparticles (Fig. 9f).

The temperature required for the complete H+ ion-exchange reaction would be higher than that previously reported (Kasuga et al., 1998; Kasuga et al., 1999; Tokudome et al., 2004; Uchida et al., 2002; Tokudome et al., 2005; Thorne et al., 2005; Miyauchi et al., 2006; Tokudome et al., 2004; Kasuga et al., 2003), because the H+ ion-exchange treatments at 40 °C using 0.01, 0.1, and 1 mol/l hydrochloric acid solutions were unsuccessful. H+ ion-exchange treatment at 40 °C for 3 h using 0.01 and 0.1 mol/l hydrochloric acid solutions resulted in nanotube films respectively. However, substantial amounts of Na+ ions remained in the samples after treatment. It is considered that elevated temperature assists the diffusion of ions, allowing ion-exchange to occur within the deepest regions of the film. After H+ ionexchange treatment at 40 °C using 1.0 mol/l hydrochloric acid solution, the nanotube thin films detached from the titanium metal plate.

## **2.3.2 Calcination of hydrogen titanate nanotube thin film**

The hydrogen titanate nanotube thin films obtained by H+ ion-exchange treatment at 90 °C using a 0.01 mol/L solution of hydrochloric acid were calcined at 300–900 °C for 3 h in air to transform them into titanium dioxide nanotube thin films. A uniform thin film formed on each sample surface, similar to the sample before calcination.

TEM images (Fig. 8) and XRD patterns (Fig. 9) show that calcination at 300 and 450 °C yields anatase nanotubes. Although the average inner diameter of 3.3 nm for nanotubes synthesized by calcination at 450 °C is similar to that of the as-grown sodium titanate nanotubes, the average outer diameter of the nanotubes decreased from 8.3 nm for the asgrown thin film to 8.1 nm for the anatase nanotubes synthesized by calcination at 450 °C. This slight decrease in the average outer diameter may be due to a phase transition from titanate into anatase. An cross-section image of the thin film calcined at 450 °C is similar to that of the as-grown sodium titanate nanotube thin film. Although a dense phase is observed at the bottom of the thin film (i.e., at the interface between the nanotube phase and the titanium metal), the porous structure composed of fibrous particles is observed in the film itself. Calcination at 600 °C yields anatase nanofibers approximately 11 nm thick, but not nanotubes (Figs. 8c and 9d). The porous structure consisting of fibrous particles are maintained until calcination at 600 °C. Calcination at 750 °C changes the thin film into a porous thin film consisting of particles (with 50-nm average diameter) and interstitial pores (with 79-nm average size) as shown in Fig. 8d. We attribute these changes in morphology to a progressive sintering reaction caused by the high calcination temperature. Furthermore,

Fig. 8. TEM images of the 90 °C ion-exchange-treated thin films calcined at 300 °C (a), 450 °C (b), 600 °C (c), and 750 °C (d).

The temperature required for the complete H+ ion-exchange reaction would be higher than that previously reported (Kasuga et al., 1998; Kasuga et al., 1999; Tokudome et al., 2004; Uchida et al., 2002; Tokudome et al., 2005; Thorne et al., 2005; Miyauchi et al., 2006; Tokudome et al., 2004; Kasuga et al., 2003), because the H+ ion-exchange treatments at 40 °C using 0.01, 0.1, and 1 mol/l hydrochloric acid solutions were unsuccessful. H+ ion-exchange treatment at 40 °C for 3 h using 0.01 and 0.1 mol/l hydrochloric acid solutions resulted in nanotube films respectively. However, substantial amounts of Na+ ions remained in the samples after treatment. It is considered that elevated temperature assists the diffusion of ions, allowing ion-exchange to occur within the deepest regions of the film. After H+ ionexchange treatment at 40 °C using 1.0 mol/l hydrochloric acid solution, the nanotube thin

The hydrogen titanate nanotube thin films obtained by H+ ion-exchange treatment at 90 °C using a 0.01 mol/L solution of hydrochloric acid were calcined at 300–900 °C for 3 h in air to transform them into titanium dioxide nanotube thin films. A uniform thin film formed on

TEM images (Fig. 8) and XRD patterns (Fig. 9) show that calcination at 300 and 450 °C yields anatase nanotubes. Although the average inner diameter of 3.3 nm for nanotubes synthesized by calcination at 450 °C is similar to that of the as-grown sodium titanate nanotubes, the average outer diameter of the nanotubes decreased from 8.3 nm for the asgrown thin film to 8.1 nm for the anatase nanotubes synthesized by calcination at 450 °C. This slight decrease in the average outer diameter may be due to a phase transition from titanate into anatase. An cross-section image of the thin film calcined at 450 °C is similar to that of the as-grown sodium titanate nanotube thin film. Although a dense phase is observed at the bottom of the thin film (i.e., at the interface between the nanotube phase and the titanium metal), the porous structure composed of fibrous particles is observed in the film itself. Calcination at 600 °C yields anatase nanofibers approximately 11 nm thick, but not nanotubes (Figs. 8c and 9d). The porous structure consisting of fibrous particles are maintained until calcination at 600 °C. Calcination at 750 °C changes the thin film into a porous thin film consisting of particles (with 50-nm average diameter) and interstitial pores (with 79-nm average size) as shown in Fig. 8d. We attribute these changes in morphology to a progressive sintering reaction caused by the high calcination temperature. Furthermore,

a b c d

(b), 600 °C (c), and 750 °C (d).

Fig. 8. TEM images of the 90 °C ion-exchange-treated thin films calcined at 300 °C (a), 450 °C

films detached from the titanium metal plate.

**2.3.2 Calcination of hydrogen titanate nanotube thin film** 

each sample surface, similar to the sample before calcination.

calcination at 900 °C yields a dense rutile thin film because of the densification and phase transition caused by the sintering of anatase nanoparticles (Fig. 9f).

Fig. 9. XRD patterns of the 90 °C ion-exchange-treated (a) and the 90 °C ion-exchangetreated thin films calcined at 300 °C (b), 450 °C (c), 600 °C (d), 750 °C (e), and 900 °C (f). Peak assignment: ■, α-titanium; ○, anatase; ●, rutile; △, hydrogen titanate; \*, distorted titanium.

#### **2.4 Silver nanoparticle / silver titanate nanotube nanocomposite thin film**

Na-TNT thin film obtained after the 3 h reaction with dimensions of 20 mm × 20 mm × 2 mm (hereafter referred to as Na-TNT-TF) was immersed in 12 mL of 0.05 M silver acetate solution at 40 °C for 3 h, then repeatedly washed with distilled water and dried in a cool dark place, to exchange the Na+ in the sodium titanate with Ag+. Hereafter, the sample obtained by the silver ion-exchange treatment of Na-TNT-TF was called Ag-TNT-TF.

The EDX spectra of the samples before and after the silver ion-exchange treatment were then compared. Since the peaks attributed to Na, observed in the samples before the silver ion-

Synthesis of Titanate and Titanium Dioxide

sodium titanate; ●, silver titanate.

**nanotube nanocomposite thin film** 

Nanotube Thin Films and Their Applications to Biomaterials 55

Fig. 11. TF-XRD patterns of sodium titanate nanotube thin film (a) and silver nanoparticle/ silver titanate nanotube nanocomposite thin film (b). Peak assignment: □, titanium; ○,

**3.1 Elution properties of silver ions from the silver nanoparticle / silver titanate** 

The elution properties of silver ions from the samples were examined in various solutions to determine the behavior of silver in MRSA environment or in the body. Ag-TNT-TF with dimensions of 20 mm × 20 mm × 2 mm was immersed in 15 mL physiological saline, phosphate buffered saline (+) (PBS(+)), phosphate buffered saline (−) (PBS(−)), and fetal bovine serum solution, maintained at 37 °C for 24 h. Then, the eluates were collected, centrifuged, and filtrated through a 0.22-µm filter. After filtration, Ag concentration in the eluates was determined by inductively coupled plasma mass spectrometry (ICP-MS). In physiological saline, PBS(+), and PBS(−), almost the same average concentration of eluted silver was measured—300, 320, and 440 ppb, respectively. The eluted silver ions may originate from metallic silver and silver titanate. Since the solubility of metallic silver is known to be very small, the large portion of eluted silver was eluted by exchanging silver ions in titanates with Na+, K+, and H+ in the solutions. On the other hand, in fetal bovine serum, the average eluted silver concentration was measured in large amounts—82000 ppb for Ag-TNT-TF. This was because a large quantity of a compound composed of silver and a protein was formed together with AgCl, since the protein that exists in fetal bovine serum has very high affinity with Ag+ through the –SH group or –NH group in the protein, and the amount of exchangeable cations in fetal bovine serum was larger than that in physiological saline and PBS. Moreover, when the silver elution test in fetal bovine serum was performed for a silver metal plate under similar conditions as that for Ag-TNT-TF, silver of 7900 ppb was eluted. This amount was also significantly smaller than that for of Ag-TNT-TF. In the

**3. Antibacterial activities of titanate nanotube thin films** 

exchange treatment (Na-TNT-TF), disappeared in the samples after the silver ion-exchange treatment (Ag-TNT-TF), and the peaks attributed to Ag appeared after the silver ion-exchange treatment, Na+ in the sodium titanate seemed to be exchanged with Ag+ during the silver ionexchange treatment. However, for the composition calculated from these spectra, the molar ratio of Ag/Ti was 0.67 for Ag-TNT-TF, while the molar ratio of Na/Ti was 0.50 for Na-TNT-TF. This confirmed presence of Ag in an excess compared with the exchangeable cations in the sample. SEM observation at the micrometer scale did not show changes in the morphologies before and after the silver ion-exchange treatment. However, the TEM observations of Ag-TNT-TF (Fig. 10) show particles with sizes ranging from several nanometers to a few dozen nanometers, which were not observable before the ion-exchange treatment. These are considered to be silver nanoparticles, since the color of Ag-TNT-TF was slightly yellow, indicating the formation of silver nanoparticles. The silver nanoparticles were deposited on titanates by the photoreduction of silver ions that were adsorbed on the titanate surface. The excess silver determined through the exchangeable mass of ions observed via EDX analysis is thus attributed to these silver nanoparticles. In other word, in the silver ion-exchange treatment, Ag+ ion was not only incorporated into the titanate by ion exchange with Na+ ion, but also deposited on the outer surface of titanate as silver nanoparticles.

Fig. 10. TEM image of silver nanoparticle/silver titanate nanotube nanocomposite thin film.

Furthermore, TF-XRD patterns of Na-TNT-TF and Ag-TNT-TF shown in Fig. 11 also indicate the transformation of sodium titanate thin film into silver titanate thin film. When silver ionexchange treatment was performed for Na-TNT-TF, a diffraction peak expressing the interlayer distance of 10 Å that was observed in Na-TNT-TF, disappeared in Ag-TNT-TF. The disappearance of the diffraction peak expressing the interlayer distance of 10 Å is considered to be due to the insertion of Ag+ ions into an interlayer of titanate and disappearance of the layered structure of titanate. A further reason is a strong and peculiar interaction between the inserted Ag+ ions and the titanate layer, which would cause a structural change of the layered structure into a three-dimensional structure. This structural change can also be confirmed, as the diffraction peaks in Na-TNT-TF due to the crystal structure of titanate, observed at 2θ = 24.2° and 28.3°, disappeared in Ag-TNT-TF, concomitant with the appearance of a new diffraction peak at 2θ = 29.3° in Ag-TNT-TF. These results indicate the formation of silver titanate nanotube.

exchange treatment (Na-TNT-TF), disappeared in the samples after the silver ion-exchange treatment (Ag-TNT-TF), and the peaks attributed to Ag appeared after the silver ion-exchange treatment, Na+ in the sodium titanate seemed to be exchanged with Ag+ during the silver ionexchange treatment. However, for the composition calculated from these spectra, the molar ratio of Ag/Ti was 0.67 for Ag-TNT-TF, while the molar ratio of Na/Ti was 0.50 for Na-TNT-TF. This confirmed presence of Ag in an excess compared with the exchangeable cations in the sample. SEM observation at the micrometer scale did not show changes in the morphologies before and after the silver ion-exchange treatment. However, the TEM observations of Ag-TNT-TF (Fig. 10) show particles with sizes ranging from several nanometers to a few dozen nanometers, which were not observable before the ion-exchange treatment. These are considered to be silver nanoparticles, since the color of Ag-TNT-TF was slightly yellow, indicating the formation of silver nanoparticles. The silver nanoparticles were deposited on titanates by the photoreduction of silver ions that were adsorbed on the titanate surface. The excess silver determined through the exchangeable mass of ions observed via EDX analysis is thus attributed to these silver nanoparticles. In other word, in the silver ion-exchange treatment, Ag+ ion was not only incorporated into the titanate by ion exchange with Na+ ion,

Fig. 10. TEM image of silver nanoparticle/silver titanate nanotube nanocomposite thin film.

Furthermore, TF-XRD patterns of Na-TNT-TF and Ag-TNT-TF shown in Fig. 11 also indicate the transformation of sodium titanate thin film into silver titanate thin film. When silver ionexchange treatment was performed for Na-TNT-TF, a diffraction peak expressing the interlayer distance of 10 Å that was observed in Na-TNT-TF, disappeared in Ag-TNT-TF. The disappearance of the diffraction peak expressing the interlayer distance of 10 Å is considered to be due to the insertion of Ag+ ions into an interlayer of titanate and disappearance of the layered structure of titanate. A further reason is a strong and peculiar interaction between the inserted Ag+ ions and the titanate layer, which would cause a structural change of the layered structure into a three-dimensional structure. This structural change can also be confirmed, as the diffraction peaks in Na-TNT-TF due to the crystal structure of titanate, observed at 2θ = 24.2° and 28.3°, disappeared in Ag-TNT-TF, concomitant with the appearance of a new diffraction peak at 2θ = 29.3° in Ag-TNT-TF.

but also deposited on the outer surface of titanate as silver nanoparticles.

These results indicate the formation of silver titanate nanotube.

Fig. 11. TF-XRD patterns of sodium titanate nanotube thin film (a) and silver nanoparticle/ silver titanate nanotube nanocomposite thin film (b). Peak assignment: □, titanium; ○, sodium titanate; ●, silver titanate.

## **3. Antibacterial activities of titanate nanotube thin films**

#### **3.1 Elution properties of silver ions from the silver nanoparticle / silver titanate nanotube nanocomposite thin film**

The elution properties of silver ions from the samples were examined in various solutions to determine the behavior of silver in MRSA environment or in the body. Ag-TNT-TF with dimensions of 20 mm × 20 mm × 2 mm was immersed in 15 mL physiological saline, phosphate buffered saline (+) (PBS(+)), phosphate buffered saline (−) (PBS(−)), and fetal bovine serum solution, maintained at 37 °C for 24 h. Then, the eluates were collected, centrifuged, and filtrated through a 0.22-µm filter. After filtration, Ag concentration in the eluates was determined by inductively coupled plasma mass spectrometry (ICP-MS). In physiological saline, PBS(+), and PBS(−), almost the same average concentration of eluted silver was measured—300, 320, and 440 ppb, respectively. The eluted silver ions may originate from metallic silver and silver titanate. Since the solubility of metallic silver is known to be very small, the large portion of eluted silver was eluted by exchanging silver ions in titanates with Na+, K+, and H+ in the solutions. On the other hand, in fetal bovine serum, the average eluted silver concentration was measured in large amounts—82000 ppb for Ag-TNT-TF. This was because a large quantity of a compound composed of silver and a protein was formed together with AgCl, since the protein that exists in fetal bovine serum has very high affinity with Ag+ through the –SH group or –NH group in the protein, and the amount of exchangeable cations in fetal bovine serum was larger than that in physiological saline and PBS. Moreover, when the silver elution test in fetal bovine serum was performed for a silver metal plate under similar conditions as that for Ag-TNT-TF, silver of 7900 ppb was eluted. This amount was also significantly smaller than that for of Ag-TNT-TF. In the

Synthesis of Titanate and Titanium Dioxide

nanocomposite thin film.

**nanocomposite thin film** 

Nanotube Thin Films and Their Applications to Biomaterials 57

Fig. 12. Repeated silver ion elution test of silver nanoparticle/silver titanate nanotube

The modified Japanese Industrial Standard test (JIS Z 2801) was performed as an antibacterial test as follows. To approximate an infection environment within an actual organism, an inactivated bovine serum (0.4 mL) was used as a solvent of bacterial suspension to create a eutrophic condition, and the antibacterial test was conducted using MRSA with a biofilm-forming gene. A bacterial suspension (0.4 mL) was dropped on a 50 mm × 50 mm × 2 mm test piece, covered with a 40 mm × 40 mm polyethylene film (Elmex Corp.), and cultured at 37 °C for 24 h. The test piece was washed, and the viable MRSA count was determined. The antibacterial test was performed thrice for each of the samples of Na-TNT-TF and Ag-TNT-TF, to obtain averaged values of viable MRSA counts. The

*R* = {log(*B*/*A*) − log(*C*/*A*)} = log(*B*/*C*) Here, *A*, *B*, and *C* are the average viable MRSA counts just after inoculation, after 24 h for a blank and after 24 h for a sample, respectively. The viable MRSA count just after the inoculation was 2.1 × 105 CFU/sample, and for a blank, the average viable MRSA count after 24 h increased to 5.9 × 108 CFU/sample. In Na-TNT-TF, the average viable MRSA count after 24 h was slightly less than that of the blank: 1.1 × 107 CFU/sample. On the other hand, in Ag-TNT-TF, the average viable MRSA count after 24 h was markedly small: 3.3 × 102 CFU/sample. *R* increased from 1.7 for Na-TNT-TF to 6.3 for Ag-TNT-TF through the silver ion-exchange treatment. These results indicate that the silver ion-exchanged titanate thin films display high antibacterial activity against MRSA. It was also revealed that although the crystal structure of titanate itself does not have a large antibacterial effect, higher antibacterial activity arises in the silver in the titanate. The conversion of sodium titanates into antibacterial materials through the silver ion-exchange treatment can apply to other nanostructured sodium titanates. For example, by the same silver ion-exchange treatment, porous sodium titanate film calcined at 600 °C reported by Kim et al. (Kim et al., 1997) can also be converted into silver nanoparticle / silver titanate nanocomposite thin film

**3.2 Antibacterial property of silver nanoparticle / silver titanate nanotube** 

antibacterial activity value (*R*) for the sample was calculated as follows.

with high antibacterial activity for MRSA of *R*=6.7.

silver elution test of Ag-TNT-TF, although silver is eluted from silver nanoparticles deposited on the surface of titanate, its elution amount is thus considered small. Consequently, in the XRD and TF-XRD patterns of the sample obtained by the silver elution test of Ag-TNT-TF, diffraction peaks appeared near 2θ = 10°, 24° and 28°, which were not observed in Ag-TNT-TF. These diffraction peaks of the sample obtained by the silver elution test of Ag-TNT-TF appeared at the same locations as the diffraction peaks observed in Na-TNT-TF. This indicates that the crystal structure of the sample obtained by the silver elution test of Ag-TNT-TF is similar to that of Na-TNT-TF. The likely reasons are as follows: (1) the layered structure of titanate of Na-TNT-TF transformed into a three-dimensional structure because Ag+ ions were inserted into the interlayer of titanate by the silver ion-exchange treatment to form silver titanate, (2) Ag+ ions were eluted from the silver titanate and Na+ ions were reinserted into the titanate during the silver elution test; and (3) the threedimensional structure of titanate returned to the original condition as in Na-TNT-TF. Thus, it is clearly demonstrated that insertion (intercalation) and elimination (deintercalation) of Ag+ ions occurs during the silver ion-exchange treatment and the silver elution test, respectively. Therefore, this experiment indicated that Ag+ ions in silver titanate greatly contributed to the elution of Ag+. Diffraction peaks attributable to AgCl were also observed in the sample obtained by the silver elution test of Ag-TNT-TF, because AgCl particles were formed by the reaction between eluted Ag+ and Cl in fetal bovine serum. Since fetal bovine serum solution is considered as the system most similar to MRSA environment, silver elution tests in fetal bovine serum solution were repeated (Fig. 12). In Ag-TNT-TF, the elution concentration slowly decreased from 94000 ppb for the first test to 11000 ppb for the tenth, indicating that a large amount of eluted silver was measured in the tenth test for Ag-TNT-TF. The 2-step elution curve was obtained from Ag-TNT-TF. A rapid elution of a large amount of silver at the initial stage of the repeated elution test (between the first and third time) was considered to be mainly due to Ag+ ion elution from the silver titanate based on the ion-exchange reaction. Since the elution reaction (ion-exchange reaction) of Ag+ from silver titanate is rapid, the elution of Ag+ is considered to be almost completed at the initial stage of the repeated elution test. These discussions are also supported by the above described TF-XRD data, indicating that crystal structure of the sample obtained by the silver elution test of Ag-TNT-TF is similar to that of Na-TNT-TF. Therefore, a slow elution of a small amount of silver after the forth repetition of the elution test was considered to be mainly because of silver elution from the silver nanoparticles. This two-step elution curve is difficult to explain if it is considered that only silver nanoparticles are formed in the thin film, but it is reasonably explained if two types of Ag (silver nanoparticles and silver titanate) exhibiting different elution behaviors are present in Ag-TNT-TF. The silver titanates loading silver nanoparticles would be promising as a novel antibacterial material, because they have two silver sources. The silver-ion elution property of silver titanate would be different from that of the silver nanoparticles, i.e., the elution speed of silver ions from silver titanate would be greater than that from silver nanoparticles. Therefore, silver titanate would be effective for short-term bacterial killing, and silver nanoparticles would be effective for long-term antibacterial action. Since we have already found that a thicker (i.e., 20 µm thick) titanate nanotube film can be formed after a longer reaction time or 20 h in NaOH solution, it would be possible to prolong the elution period of silver ions and to increase the amount of eluted silver ions or the duration.

silver elution test of Ag-TNT-TF, although silver is eluted from silver nanoparticles deposited on the surface of titanate, its elution amount is thus considered small. Consequently, in the XRD and TF-XRD patterns of the sample obtained by the silver elution test of Ag-TNT-TF, diffraction peaks appeared near 2θ = 10°, 24° and 28°, which were not observed in Ag-TNT-TF. These diffraction peaks of the sample obtained by the silver elution test of Ag-TNT-TF appeared at the same locations as the diffraction peaks observed in Na-TNT-TF. This indicates that the crystal structure of the sample obtained by the silver elution test of Ag-TNT-TF is similar to that of Na-TNT-TF. The likely reasons are as follows: (1) the layered structure of titanate of Na-TNT-TF transformed into a three-dimensional structure because Ag+ ions were inserted into the interlayer of titanate by the silver ion-exchange treatment to form silver titanate, (2) Ag+ ions were eluted from the silver titanate and Na+ ions were reinserted into the titanate during the silver elution test; and (3) the threedimensional structure of titanate returned to the original condition as in Na-TNT-TF. Thus, it is clearly demonstrated that insertion (intercalation) and elimination (deintercalation) of Ag+ ions occurs during the silver ion-exchange treatment and the silver elution test, respectively. Therefore, this experiment indicated that Ag+ ions in silver titanate greatly contributed to the elution of Ag+. Diffraction peaks attributable to AgCl were also observed in the sample obtained by the silver elution test of Ag-TNT-TF, because AgCl particles were

serum solution is considered as the system most similar to MRSA environment, silver elution tests in fetal bovine serum solution were repeated (Fig. 12). In Ag-TNT-TF, the elution concentration slowly decreased from 94000 ppb for the first test to 11000 ppb for the tenth, indicating that a large amount of eluted silver was measured in the tenth test for Ag-TNT-TF. The 2-step elution curve was obtained from Ag-TNT-TF. A rapid elution of a large amount of silver at the initial stage of the repeated elution test (between the first and third time) was considered to be mainly due to Ag+ ion elution from the silver titanate based on the ion-exchange reaction. Since the elution reaction (ion-exchange reaction) of Ag+ from silver titanate is rapid, the elution of Ag+ is considered to be almost completed at the initial stage of the repeated elution test. These discussions are also supported by the above described TF-XRD data, indicating that crystal structure of the sample obtained by the silver elution test of Ag-TNT-TF is similar to that of Na-TNT-TF. Therefore, a slow elution of a small amount of silver after the forth repetition of the elution test was considered to be mainly because of silver elution from the silver nanoparticles. This two-step elution curve is difficult to explain if it is considered that only silver nanoparticles are formed in the thin film, but it is reasonably explained if two types of Ag (silver nanoparticles and silver titanate) exhibiting different elution behaviors are present in Ag-TNT-TF. The silver titanates loading silver nanoparticles would be promising as a novel antibacterial material, because they have two silver sources. The silver-ion elution property of silver titanate would be different from that of the silver nanoparticles, i.e., the elution speed of silver ions from silver titanate would be greater than that from silver nanoparticles. Therefore, silver titanate would be effective for short-term bacterial killing, and silver nanoparticles would be effective for long-term antibacterial action. Since we have already found that a thicker (i.e., 20 µm thick) titanate nanotube film can be formed after a longer reaction time or 20 h in NaOH solution, it would be possible to prolong the elution period of silver ions and to

in fetal bovine serum. Since fetal bovine

formed by the reaction between eluted Ag+ and Cl-

increase the amount of eluted silver ions or the duration.

Fig. 12. Repeated silver ion elution test of silver nanoparticle/silver titanate nanotube nanocomposite thin film.

#### **3.2 Antibacterial property of silver nanoparticle / silver titanate nanotube nanocomposite thin film**

The modified Japanese Industrial Standard test (JIS Z 2801) was performed as an antibacterial test as follows. To approximate an infection environment within an actual organism, an inactivated bovine serum (0.4 mL) was used as a solvent of bacterial suspension to create a eutrophic condition, and the antibacterial test was conducted using MRSA with a biofilm-forming gene. A bacterial suspension (0.4 mL) was dropped on a 50 mm × 50 mm × 2 mm test piece, covered with a 40 mm × 40 mm polyethylene film (Elmex Corp.), and cultured at 37 °C for 24 h. The test piece was washed, and the viable MRSA count was determined. The antibacterial test was performed thrice for each of the samples of Na-TNT-TF and Ag-TNT-TF, to obtain averaged values of viable MRSA counts. The antibacterial activity value (*R*) for the sample was calculated as follows.

$$R = \{\log(B/A) - \log(C/A)\} = \log(B/C)$$

Here, *A*, *B*, and *C* are the average viable MRSA counts just after inoculation, after 24 h for a blank and after 24 h for a sample, respectively. The viable MRSA count just after the inoculation was 2.1 × 105 CFU/sample, and for a blank, the average viable MRSA count after 24 h increased to 5.9 × 108 CFU/sample. In Na-TNT-TF, the average viable MRSA count after 24 h was slightly less than that of the blank: 1.1 × 107 CFU/sample. On the other hand, in Ag-TNT-TF, the average viable MRSA count after 24 h was markedly small: 3.3 × 102 CFU/sample. *R* increased from 1.7 for Na-TNT-TF to 6.3 for Ag-TNT-TF through the silver ion-exchange treatment. These results indicate that the silver ion-exchanged titanate thin films display high antibacterial activity against MRSA. It was also revealed that although the crystal structure of titanate itself does not have a large antibacterial effect, higher antibacterial activity arises in the silver in the titanate. The conversion of sodium titanates into antibacterial materials through the silver ion-exchange treatment can apply to other nanostructured sodium titanates. For example, by the same silver ion-exchange treatment, porous sodium titanate film calcined at 600 °C reported by Kim et al. (Kim et al., 1997) can also be converted into silver nanoparticle / silver titanate nanocomposite thin film with high antibacterial activity for MRSA of *R*=6.7.

Synthesis of Titanate and Titanium Dioxide

TiO2-NT-FT.

Nanotube Thin Films and Their Applications to Biomaterials 59

films for 4 days, the SEM images showed a stretch of the dome-shaped form consisting of foliaceous particles peculiar to apatite (Figs. 13c, d). XRD patterns of the two films showed diffraction peaks attributable to apatite, respectively. These results indicate that the surfaces of these two thin films are completely covered with apatite and that the apatite-forming ability of the two films is greater than that of Na-TNT-TF having layered structure. In contrast, for TiO2-NT-FT after immersing the film in SBF for 2 days, the surface is thinly covered with apatite. However immersing Ag-TNT-TF for 2 days, the surface is almost covered with a dome-shaped form consisting of foliaceous particles peculiar to the apatite (Figs. 13e, f). The apatite-forming ability of Ag-TNT-TF is, thus, slightly higher than that of

We then investigated the newly observed high apatite-forming ability of silver nanoparticle/silver titanate nanocomposite thin film (Ag-TNT-TF). After immersing the film in SBF for 4 days, we observed bulky particles of a few micrometers in diameter together with apatite. XRD pattern shows diffraction peaks attributable to silver chloride. EDX element mapping analysis shows the bulky particles to be composed of Ag and Cl (Fig. 14) and, therefore, to be silver chloride particles. Therefore, Ag+ ions are eluted from silver titanate mainly by ion-exchange reaction with cations, resulting in deposition of silver chloride particles. After immersing the film in SBF for 1 day, SEM images reveal only bulky AgCl particles on the film surface, however immersing the film for 2 days, the surface is almost covered with a dome-shaped form consisting of foliaceous particles peculiar to the apatite (Figs. 13e, f), as mentioned above. We clarified whether silver nanoparticle or silver titanate contributes more to the apatite formation by investigating the apatite-forming ability of a silver metal plate. After immersing the plate in SBF for 4 days, no apatite formation was evident; thus, the silver titanate nanotubes are responsible for the high apatite-forming ability. Researchers have reported that the effects of crystal structure (Uchida et al., 2003) and surface hydroxyl groups such as Ti-OH (Kasuga et al., 2002) influence the apatite formation on the titanium compounds immersed in SBF. Kokubo et al. reported that the apatite-forming ability is improved by the crystal structure transformation of sodium titanate into titanium dioxide (Fujibayashi et al., 2001; Uchida et al., 2002; Takemoto et al., 2006). Although the detailed crystal structure of silver titanate is not yet known, we speculate that the surface atomic arrangement and surface functional groups of silver titanate might be suitable for rapid apatite formation. We further investigated the high apatite forming ability by considering –OH groups that influence apatite formation using the FT-IR measurements. As shown in Fig. 15, Na-TNT-TF and TiO2-NT-FT exhibited similar absorption spectra in a wide range of 3000−3700 cm−1. These absorption spectra are considered to be mainly due to water molecules adsorbed on the inner and outer surfaces of nanotubes and partially due to –OH groups on the surface. Unlike Na-TNT-TF and TiO2- NT-FT, strong absorption was observed at 3000−3400 cm−1 in addition to 3400−3700 cm−1 in Ag-TNT-TF. This absorption at 3000−3400 cm−1 is considered to indicate the existence of surface –OH groups due to silver titanate. A surface atomic arrangement peculiar to silver titanate would arise and a large number of –OH groups would be generated on the

nanotube surfaces, which would stimulate apatite formation.

Oh et al. (Oh et al., 2005) and Tsuchiya et al. (Tsuchiya et al., 2006) reported anatase-type titanium dioxide nanotube thin films synthesized by anodization and heat treatment of the titanium metal. We compared the apatite-forming ability of these nanotube thin films with

## **4. Apatite-forming ability of titanate and titanium dioxide nanotube thin films**

The three thin films (Na-TNT-TF, TiO2-NT-FT (anatase type titanium dioxide nanotube thin film formed by the H+ ion-exchange treatment and calcination at 450 °C of Na-TNT-TF), Ag-TNT-TF) formed on a titanium metal were immersed in simulated body fluid (SBF) and monitored the development of apatite formation on their surfaces. In accordance with ISO 23317, "Implants for surgery-*In vitro* evaluation for apatite-forming ability of implant materials," we evaluated the apatite-forming ability on the surface of the coating in SBF. A plate was placed in 96.0 mL SBF at 36.5°C. After 2, 4, and 14 days, the plate was removed and gently rinsed with water. The surface of the plate was dried in air.

For Na-TNT-TF, after immersing the film for 4 days, the SEM images showed only nanotubes and no substances with foliaceous morphology peculiar to apatite; the XRD patterns remained unchanged. However, when the SBF immersion was extended to 14 days, the SEM images showed the surface of the film to be completely covered with a domeshaped form consisting of foliaceous particles peculiar to apatite (Figs. 13a, b); the XRD pattern showed diffraction peaks attributable to apatite. Thus, apatite is confirmed to be formed on the sodium titanate thin films. In addition, after immersing the hydrogen titanate nanotube thin film formed by exchanging Na+ ions between the layers of the layered sodium titanate for H+ ions in SBF for 4 days, no apatite was evident. This lack of apatite indicates that ions (Na+ and H+) between the titanate layers do not particularly contribute to the acceleration of the apatite formation. In contrast, for TiO2-NT-FT formed by the 450°C calcination of the hydrogen titanate nanotube thin film and Ag-TNT-TF, after immersing the

Fig. 13. Low magnification (a, c, e) and high magnification (b, d, f) SEM images of Na-TNT-TF (a, b), TiO2-NT-FT (c, d), and Ag-TNT-TF (e, f) after immersions in SBF.

**4. Apatite-forming ability of titanate and titanium dioxide nanotube thin films**  The three thin films (Na-TNT-TF, TiO2-NT-FT (anatase type titanium dioxide nanotube thin film formed by the H+ ion-exchange treatment and calcination at 450 °C of Na-TNT-TF), Ag-TNT-TF) formed on a titanium metal were immersed in simulated body fluid (SBF) and monitored the development of apatite formation on their surfaces. In accordance with ISO 23317, "Implants for surgery-*In vitro* evaluation for apatite-forming ability of implant materials," we evaluated the apatite-forming ability on the surface of the coating in SBF. A plate was placed in 96.0 mL SBF at 36.5°C. After 2, 4, and 14 days, the plate was removed

For Na-TNT-TF, after immersing the film for 4 days, the SEM images showed only nanotubes and no substances with foliaceous morphology peculiar to apatite; the XRD patterns remained unchanged. However, when the SBF immersion was extended to 14 days, the SEM images showed the surface of the film to be completely covered with a domeshaped form consisting of foliaceous particles peculiar to apatite (Figs. 13a, b); the XRD pattern showed diffraction peaks attributable to apatite. Thus, apatite is confirmed to be formed on the sodium titanate thin films. In addition, after immersing the hydrogen titanate nanotube thin film formed by exchanging Na+ ions between the layers of the layered sodium titanate for H+ ions in SBF for 4 days, no apatite was evident. This lack of apatite indicates that ions (Na+ and H+) between the titanate layers do not particularly contribute to the acceleration of the apatite formation. In contrast, for TiO2-NT-FT formed by the 450°C calcination of the hydrogen titanate nanotube thin film and Ag-TNT-TF, after immersing the

and gently rinsed with water. The surface of the plate was dried in air.

a c e

b d f

TF (a, b), TiO2-NT-FT (c, d), and Ag-TNT-TF (e, f) after immersions in SBF.

Fig. 13. Low magnification (a, c, e) and high magnification (b, d, f) SEM images of Na-TNT-

films for 4 days, the SEM images showed a stretch of the dome-shaped form consisting of foliaceous particles peculiar to apatite (Figs. 13c, d). XRD patterns of the two films showed diffraction peaks attributable to apatite, respectively. These results indicate that the surfaces of these two thin films are completely covered with apatite and that the apatite-forming ability of the two films is greater than that of Na-TNT-TF having layered structure. In contrast, for TiO2-NT-FT after immersing the film in SBF for 2 days, the surface is thinly covered with apatite. However immersing Ag-TNT-TF for 2 days, the surface is almost covered with a dome-shaped form consisting of foliaceous particles peculiar to the apatite (Figs. 13e, f). The apatite-forming ability of Ag-TNT-TF is, thus, slightly higher than that of TiO2-NT-FT.

We then investigated the newly observed high apatite-forming ability of silver nanoparticle/silver titanate nanocomposite thin film (Ag-TNT-TF). After immersing the film in SBF for 4 days, we observed bulky particles of a few micrometers in diameter together with apatite. XRD pattern shows diffraction peaks attributable to silver chloride. EDX element mapping analysis shows the bulky particles to be composed of Ag and Cl (Fig. 14) and, therefore, to be silver chloride particles. Therefore, Ag+ ions are eluted from silver titanate mainly by ion-exchange reaction with cations, resulting in deposition of silver chloride particles. After immersing the film in SBF for 1 day, SEM images reveal only bulky AgCl particles on the film surface, however immersing the film for 2 days, the surface is almost covered with a dome-shaped form consisting of foliaceous particles peculiar to the apatite (Figs. 13e, f), as mentioned above. We clarified whether silver nanoparticle or silver titanate contributes more to the apatite formation by investigating the apatite-forming ability of a silver metal plate. After immersing the plate in SBF for 4 days, no apatite formation was evident; thus, the silver titanate nanotubes are responsible for the high apatite-forming ability. Researchers have reported that the effects of crystal structure (Uchida et al., 2003) and surface hydroxyl groups such as Ti-OH (Kasuga et al., 2002) influence the apatite formation on the titanium compounds immersed in SBF. Kokubo et al. reported that the apatite-forming ability is improved by the crystal structure transformation of sodium titanate into titanium dioxide (Fujibayashi et al., 2001; Uchida et al., 2002; Takemoto et al., 2006). Although the detailed crystal structure of silver titanate is not yet known, we speculate that the surface atomic arrangement and surface functional groups of silver titanate might be suitable for rapid apatite formation. We further investigated the high apatite forming ability by considering –OH groups that influence apatite formation using the FT-IR measurements. As shown in Fig. 15, Na-TNT-TF and TiO2-NT-FT exhibited similar absorption spectra in a wide range of 3000−3700 cm−1. These absorption spectra are considered to be mainly due to water molecules adsorbed on the inner and outer surfaces of nanotubes and partially due to –OH groups on the surface. Unlike Na-TNT-TF and TiO2- NT-FT, strong absorption was observed at 3000−3400 cm−1 in addition to 3400−3700 cm−1 in Ag-TNT-TF. This absorption at 3000−3400 cm−1 is considered to indicate the existence of surface –OH groups due to silver titanate. A surface atomic arrangement peculiar to silver titanate would arise and a large number of –OH groups would be generated on the nanotube surfaces, which would stimulate apatite formation.

Oh et al. (Oh et al., 2005) and Tsuchiya et al. (Tsuchiya et al., 2006) reported anatase-type titanium dioxide nanotube thin films synthesized by anodization and heat treatment of the titanium metal. We compared the apatite-forming ability of these nanotube thin films with

Synthesis of Titanate and Titanium Dioxide

**5. Conclusion** 

Nanotube Thin Films and Their Applications to Biomaterials 61

to that of the thin film with 2-μm long nanotube synthesized by Tsuchiya et al. (Tsuchiya et al., 2006) and clearly superior to that of the nanotube thin film synthesized by Oh et al. (Oh et al., 2005) and the thin film with 500-nm long nanotube synthesized by Tsuchiya et al. (Tsuchiya et al., 2006). While comparing the ratio of the void parts to the anatase-type titanium dioxide part on the surface using SEM images, we found the proportion consisting anatase-type titanium dioxide to be larger in the surface of our synthesized thin film as compared to that in the surfaces of the thin films reported by Oh et al. and Tsuchiya et al.

In this study, novel procedure of synthesis and fixation of Na-TNT onto titanium metals with various morphologies such as plate, wire, mesh, tube, and sphere was reported. Especially, since the Na-TNT/Ti composite wires have softness and flexibility peculiar to metal titanium because of the existence of titanium metal in its core part, this wire can be fabricated into various shapes of cloth, fiber, etc. with centimeter or meter size by using conventional spinning techniques. The Na-TNT thin films can be transformed into anatasetype titanium dioxide nanotube thin films. Another advantage of the proposed procedure is that the thickness of the thin films produced is greater than that of the thin films reported by other researchers. Therefore, Na-TNT's applications mentioned in the introduction would be remarkably expanded. In addition, the novel growth of the Na-TNT film on substrates such as Co–Cr alloy and SUS316L and simple patterning of the Na-TNT phase by the hydrothermal transcription method were also reported. As these substrates including titanium metal, Co-Cr alloy, and SUS316L have superior mechanical properties and corrosion resistance, they are frequently used as implants such as artificial joints. Generally, the coating of films to implants with complex shapes requires thin films with uniform and controlled thickness, and a high fixing strength to the implants. Because of the direct growth of the nanotubes from the substrate, our proposed method is very simple and the fixing strength to the substrate is expected to be higher. Therefore, the method proposed in the

Next, through a silver ion-exchange treatment, Na+ ions in sodium titanate nanotube were exchanged with Ag+ ions in silver acetate solution, along with the loading of silver nanoparticles on the titanate surfaces, and the layered structure of titanate transformed into a new three-dimensional crystal structure. Results of silver ion elution tests of the obtained thin films in fetal bovine serum solution indicate that the release period and the number of silver ions released from the silver titanate thin films can be controlled. The silver ionexchanged titanate thin films showed high antibacterial activity against MRSA. It was also revealed that although the crystal structure of titanate itself has no large antibacterial effect, higher antibacterial activity mainly arises from the silver ions held in the titanate. The samples coated with apatite containing silver and silver plate have already been reported as possessing antibacterial properties through metallic silver with low solubility. In contrast, in this study, the antibacterial properties were mainly caused by the elution of silver ions from a titanate with an ion-exchange property. Since the thin film obtained by this study has a higher silver-ion elution speed, greater and more rapid antibacterial effects than in metallic silver can be expected. Since we have also revealed that the morphology, thickness, and crystal structure of the titanate phase can be controlled, we think that this can also promise

Hence, the apatite-forming ability of our film is also correspondingly higher.

present study has excellent potential for these biomedical applications.

Fig. 14. Elemental mapping performed on Ag-TNT-TF after immersion in SBF for 4 days using EDX analysis.

Fig. 15. FT-IR spectra of Na-TNT-TF (a), TiO2-NT-FT (b), and Ag-TNT-TF (c).

that of TiO2-NT-FT obtained in this study by immersing the film in SBF for 2 days. SEM images show not only a small amount of the dome-shaped form consisting of foliaceous particles peculiar to apatite, but also several slightly swelled and whitish areas. EDX element mapping on this thin film revealed that titanium dioxide nanotubes exist in the blackish areas and apatite exists in the whitish areas. At this point, after 2 days of immersion, the apatite phase has grown slightly but not yet achieved its dome-shaped form. Therefore, the apatite-forming ability of TiO2-NT-FT is slightly superior though still similar to that of the thin film with 2-μm long nanotube synthesized by Tsuchiya et al. (Tsuchiya et al., 2006) and clearly superior to that of the nanotube thin film synthesized by Oh et al. (Oh et al., 2005) and the thin film with 500-nm long nanotube synthesized by Tsuchiya et al. (Tsuchiya et al., 2006). While comparing the ratio of the void parts to the anatase-type titanium dioxide part on the surface using SEM images, we found the proportion consisting anatase-type titanium dioxide to be larger in the surface of our synthesized thin film as compared to that in the surfaces of the thin films reported by Oh et al. and Tsuchiya et al. Hence, the apatite-forming ability of our film is also correspondingly higher.

## **5. Conclusion**

60 Smart Nanoparticles Technology

Ca

O

Fig. 14. Elemental mapping performed on Ag-TNT-TF after immersion in SBF for 4 days

Ag Cl

Fig. 15. FT-IR spectra of Na-TNT-TF (a), TiO2-NT-FT (b), and Ag-TNT-TF (c).

that of TiO2-NT-FT obtained in this study by immersing the film in SBF for 2 days. SEM images show not only a small amount of the dome-shaped form consisting of foliaceous particles peculiar to apatite, but also several slightly swelled and whitish areas. EDX element mapping on this thin film revealed that titanium dioxide nanotubes exist in the blackish areas and apatite exists in the whitish areas. At this point, after 2 days of immersion, the apatite phase has grown slightly but not yet achieved its dome-shaped form. Therefore, the apatite-forming ability of TiO2-NT-FT is slightly superior though still similar

P

using EDX analysis.

SEM image

0.5 m μ

In this study, novel procedure of synthesis and fixation of Na-TNT onto titanium metals with various morphologies such as plate, wire, mesh, tube, and sphere was reported. Especially, since the Na-TNT/Ti composite wires have softness and flexibility peculiar to metal titanium because of the existence of titanium metal in its core part, this wire can be fabricated into various shapes of cloth, fiber, etc. with centimeter or meter size by using conventional spinning techniques. The Na-TNT thin films can be transformed into anatasetype titanium dioxide nanotube thin films. Another advantage of the proposed procedure is that the thickness of the thin films produced is greater than that of the thin films reported by other researchers. Therefore, Na-TNT's applications mentioned in the introduction would be remarkably expanded. In addition, the novel growth of the Na-TNT film on substrates such as Co–Cr alloy and SUS316L and simple patterning of the Na-TNT phase by the hydrothermal transcription method were also reported. As these substrates including titanium metal, Co-Cr alloy, and SUS316L have superior mechanical properties and corrosion resistance, they are frequently used as implants such as artificial joints. Generally, the coating of films to implants with complex shapes requires thin films with uniform and controlled thickness, and a high fixing strength to the implants. Because of the direct growth of the nanotubes from the substrate, our proposed method is very simple and the fixing strength to the substrate is expected to be higher. Therefore, the method proposed in the present study has excellent potential for these biomedical applications.

Next, through a silver ion-exchange treatment, Na+ ions in sodium titanate nanotube were exchanged with Ag+ ions in silver acetate solution, along with the loading of silver nanoparticles on the titanate surfaces, and the layered structure of titanate transformed into a new three-dimensional crystal structure. Results of silver ion elution tests of the obtained thin films in fetal bovine serum solution indicate that the release period and the number of silver ions released from the silver titanate thin films can be controlled. The silver ionexchanged titanate thin films showed high antibacterial activity against MRSA. It was also revealed that although the crystal structure of titanate itself has no large antibacterial effect, higher antibacterial activity mainly arises from the silver ions held in the titanate. The samples coated with apatite containing silver and silver plate have already been reported as possessing antibacterial properties through metallic silver with low solubility. In contrast, in this study, the antibacterial properties were mainly caused by the elution of silver ions from a titanate with an ion-exchange property. Since the thin film obtained by this study has a higher silver-ion elution speed, greater and more rapid antibacterial effects than in metallic silver can be expected. Since we have also revealed that the morphology, thickness, and crystal structure of the titanate phase can be controlled, we think that this can also promise

Synthesis of Titanate and Titanium Dioxide

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Furthermore, the present study compares the apatite-forming ability of a sodium titanate nanotube thin film, an anatase-type titanium dioxide nanotube thin film, and a silver nanoparticle/silver titanate nanotube nanocomposite thin film. Of these, the apatite-forming ability of the silver titanate nanotube was higher than that of the titanium dioxide nanotubes or the sodium titanate nanotubes, in that order. This superior apatite-forming ability of the silver nanoparticle/silver titanate nanotube nanocomposite thin film is a novel phenomenon and is presumably due to the surface atomic arrangement of silver titanate, the large amount of Ti-OH formed on the nanotube surface, or both. In conclusion, the silver nanoparticle/silver titanate nanotube nanocomposite thin film, which have the antibacterial property and the ability to form bone-like apatite, i.e., the osteoconductive property, may have bright prospects for future use in implant materials such as artificial joints.

## **6. Acknowledgment**

This research was partially supported by KAKENHI (16685021, 19750172) and Saga University Dean's Grant 2010 For Promising Young Researchers. Figures are reproduced with permission from American Chemical Society, Elsevier, and John Wiley & Sons.

#### **7. References**


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**4** 

*Japan* 

**Self-Organization of Silver-Core Bimetallic** 

Metal nanoparticles have received much attentions as a building block of advanced materials for nanoscience and nanotechnology (Bönnemann & Richards, 2001). Their optical, (Fukumi et al., 1994; Lu et al., 1999; Link et al., 1999; Shipway et al., 2000), magnetic (Sun et al., 1999; Teranishi & Miyake, 1999), and catalytic (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Harriman, 1990; Lee et al., 1995; Toshima et al., 1995; Bonilla et al., 2000; Siepen et al., 2000) properties have been reported with great interests. The character of metal nanoparticle can be altered by the addition of other metals. Bimetallic nanoparticles, composed of two different metallic elements, have been reported to show outstanding characters different from the corresponding monometallic nanoparticles (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1997, 1999; Toshima & Wang, 1994; Lee et al., 1995). For example, catalytic activities of gold (Au)-core structured bimetallic nanoparticles, gold/platinum (Au/Pt) (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1999), gold/palladium (Au/Pd) (Toshima & Hirakawa, 1999; Lee et al., 1995), and gold/rhodium (Au/Rh) (Toshima & Hirakawa, 1999), for hydrogenation and/or water reduction are higher than platinum (Pt), palladium (Pd), and rhodium (Rh) monometallic nanoparticles, respectively. Surprisingly, in some cases, a physical mixture of monometallic nanoparticles such as Pt and ruthenium (Ru) nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition (Toshima et al., 1995; Toshima & Hirakawa, 1997). This suggests that an interaction between two kinds of monometallic nanoparticles can produce novel nanoparticles. Further, it has been reported that physical mixture of silver (Ag) and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles

In this chapter, the simple method of the preparation of core-shell structured bimetallic nanoparticles by the physical mixing and the application of the formed novel metal nanoparticles for catalytic reaction are described. The topics of the catalytic reaction presented in this chapter are the visible light induced hydrogen generation (Toshima &

demonstrate superior character for certain catalytic reactions.

**1. Introduction** 

**Nanoparticles and Their Application** 

**for Catalytic Reaction** 

*Faculty of Engineering, Shizuoka University* 

Kazutaka Hirakawa


## **Self-Organization of Silver-Core Bimetallic Nanoparticles and Their Application for Catalytic Reaction**

Kazutaka Hirakawa *Faculty of Engineering, Shizuoka University Japan* 

## **1. Introduction**

66 Smart Nanoparticles Technology

Wang, F.; Liao, Y.; Wang, M.; Gong, P.; Li, X.; Tang, H.; Man, Y.; Yuan, Q.; Wei, N.; Tan, Z. &

Wang, X. X.; Hayakawa, S.; Tsuru, K. & Osaka, A. (2001). A comparative study of in vitro

Wang, X. X.; Yan, W.; Hayakawa, S.; Tsuru, K. & Osaka, A. (2003). Apatite deposition on

Wu, D.; Liu, J.; Zhao, X.; Li, A.; Chen, Y. & Ming, N. (2006). Sequence of events for the

Xiong, J.; Li, Y.; Hodgson, P. D. & Wen, V. (2010). Nanohydroxyapatite coating on a

Yada, M.; Inoue, Y.; Uota, M.; Torikai, T.; Watari, T.; Noda, I. & Hotokebuchi, T. (2007).

Nanotubes on a Titanium Metal Template. *Langmuir*, Vol. 23, pp. 2815-2823. Yada, M.; Inoue, Y.; Uota, M.; Torikai, T.; Watari, T.; Noda, I. & Hotokebuchi, T. (2008).

Yada, M.; Inoue, Y.; Gyoutoku, A.; Noda, I.; Torikai, T.; Watari, T. & Hotokebuchi, T. (2010).

Yamanaka, S.; Hamaguchi, T.; Muta, H.; Kurosaki, K. & Uno, M. (2004). Fabrication of Oxide

Yang, J.; Jin, Z.; Wang, X.; Li, W.; Zhang, S.; Guo, X. & Zhang, Z. (2003). Study on

Zhu, H. Y.; Lan, Y.; Gao, X. P.; Ringer, S., P.; Zheng, Z. F.; Song, D. Y. & Zhao, J. C. (2005).

*of the Chemical Society Dalton Transactions*, Vol. 3, pp. 3898-3901.

Method In Vitro. *Key Engineering Materials*, Vol. 330-332, pp. 777-780. Wang, X. J.; Li, Y. C.; Lin, J. G.; Yamada, Y.; Hodgson, P. D. & Wen, C. E. (2008). In vitro

morphologies. *Acta Biomaterialia*, Vol. 4, pp. 1530-1535.

*Biomedical Materials Research*, Vol. 54, pp. 172-178.

*Biomaterials*, Vol. 24, pp. 4631-4637.

*Chemistry of Materials*, Vol. 20, pp. 364-366.

*Materials*, Vol. 18, pp. 547-553.

1584-1590.

Vol. 80, pp. 116-124.

pp. 6730-6736.

*Compounds*, Vol. 373, pp. 312-315.

Ban, Y. (2007). Evaluation of Sodium Titanate Coating on Titanium by Sol-Gel

bioactivity evaluation of titanium and niobium metals with different surface

apatite deposition on heat-, H2O2-, and NaOH- treated titanium surfaces. *Journal of* 

thermally and anodically oxidized titanium surfaces in a simulated body fluid.

formation of titanate nanotubes, nanofibers, nanowires, and nanobelts. *Chemistry of* 

titanium–niobium alloy by a hydrothermal process. *Acta Biomaterialia*, Vol. 6, pp.

Plate, Wire, Mesh, Microsphere, and Microtube Composed of Sodium Titanate

Formation of Sodium Titanate Nanotube Films by Hydrothermal Transcription.

Apatite-forming ability of titanium compound nanotube thin films formed on a titanium metal plate in a simulated body fluid. *Colloids and Surfaces B: Biointerfaces*,

Nanohole Arrays by a Liquid Phase Deposition Method. *Journal of Alloys and* 

composition, structure and formation process of nanotube Na2Ti2O4(OH)2. *Journal* 

Phase Transition between Nanostructures of Titanate and Titanium Dioxides via Simple Wet-Chemical Reactions. *Journal of the American Ceramic Society*, Vol. 127, Metal nanoparticles have received much attentions as a building block of advanced materials for nanoscience and nanotechnology (Bönnemann & Richards, 2001). Their optical, (Fukumi et al., 1994; Lu et al., 1999; Link et al., 1999; Shipway et al., 2000), magnetic (Sun et al., 1999; Teranishi & Miyake, 1999), and catalytic (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Harriman, 1990; Lee et al., 1995; Toshima et al., 1995; Bonilla et al., 2000; Siepen et al., 2000) properties have been reported with great interests. The character of metal nanoparticle can be altered by the addition of other metals. Bimetallic nanoparticles, composed of two different metallic elements, have been reported to show outstanding characters different from the corresponding monometallic nanoparticles (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1997, 1999; Toshima & Wang, 1994; Lee et al., 1995). For example, catalytic activities of gold (Au)-core structured bimetallic nanoparticles, gold/platinum (Au/Pt) (Harriman, 1990; Yonezawa & Toshima, 1993; Toshima & Hirakawa, 1999), gold/palladium (Au/Pd) (Toshima & Hirakawa, 1999; Lee et al., 1995), and gold/rhodium (Au/Rh) (Toshima & Hirakawa, 1999), for hydrogenation and/or water reduction are higher than platinum (Pt), palladium (Pd), and rhodium (Rh) monometallic nanoparticles, respectively. Surprisingly, in some cases, a physical mixture of monometallic nanoparticles such as Pt and ruthenium (Ru) nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition (Toshima et al., 1995; Toshima & Hirakawa, 1997). This suggests that an interaction between two kinds of monometallic nanoparticles can produce novel nanoparticles. Further, it has been reported that physical mixture of silver (Ag) and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles demonstrate superior character for certain catalytic reactions.

In this chapter, the simple method of the preparation of core-shell structured bimetallic nanoparticles by the physical mixing and the application of the formed novel metal nanoparticles for catalytic reaction are described. The topics of the catalytic reaction presented in this chapter are the visible light induced hydrogen generation (Toshima &

Self-Organization of Silver-Core Bimetallic

0

L-1, 50 mL) nanoparticles were mixed.

after indicated periods.

0.5

1

Absorbance

1.5

2

absorption band completely.

Nanoparticles and Their Application for Catalytic Reaction 69

extinguished within 10 min, suggesting that influences of Rh nanopartilces on Ag nanoparticles depend on the size of the Ag nanoparticles. When relatively smaller molar quantity of Rh to Ag was added, the plasmon absorption was not completely extinguished. More than 40 atom-mol% of Rh against to Ag was required to extinguish the plasmon

> 0 min 5 min 10 min 20 min 30 min 24 h

300 400 500 600

Wavelength / nm

nanoparticles. The aqueous solutions of Ag (1 atom-mmol L-1, 50 mL) and Rh (1 atom-mmol

Figure 3 shows transmission electron microscopy (TEM) photographs of the physical mixtures of Ag and Rh monometallic nanoparticles. The samples for TEM measurement were prepared by drying the aqueous dispersions of the physical mixtures of Ag and Rh nanoparticles under vacuum in 0, 10, and 30 min, and 24 h, respectively, after mixing. Relatively large particles are attributed to Ag nanoparticles, and rather small ones are Rh nanoparticles. The TEM photographs showed that Rh particles gathered around Ag particle to surround within several minutes, comparable period of the extinction of plasmon absorption. Interestingly, these aggregated particles changed into homogeneous small particles (average diameter =2.7 nm) after 24 h. Preliminary study has shown that the

0 min 10 min 24 h Fig. 3. TEM photographs of the physical mixtures of Ag and Rh nanoparticles. The aqueous solutions of Ag and Rh nanoparticles (1/1, atom-mol/atom-mol) were mixed, and dried

Fig. 2. UV-Vis spectral change of the physical mixtures of dispersions of Ag and Rh

**2.1.2 Transmission electron microgram of the siver-core bimetallic nanoparticles** 

Hirakawa, 2003), the removal of reactive oxygen species (Hirakawa & Sano, 2009), and its application to the chemoprevention of ultraviolet induced biomolecules damage (Hirakwa et al., 2008, 2009).

## **2. Spontaneous formation of silver-core bimetallic nanoparticles**

Much attention has been paid to bimetallic nanoparticles, especially those having a core/shell structure (Toshima et al., 2007). From the view point of Au catalysts, bimetallic nanoparticles have received much attention recently. On the other hand, a physical mixture of monometallic nanoparticles such as Pt and Ru nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition (Toshima et al., 1995; Toshima & Hirakawa, 1997). Further, it has been reported that physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution (Figure 1). In this section, the spontaneous formation of the Ag-core bimetallic nanoparticles is reviewed.

## **2.1 Siver-core/rhodium-shell bimetallic nanoparticles**

The interaction between Ag and Rh monometallic nanoparticles in solution by physical mixing was reported. The main reason for using Ag and Rh nanoparticles is the reported prominent characteristics of Rh nanoparticles as a catalyst (Toshima & Hirakawa, 1999), and the expected electronic effect of Ag similar to Au upon enhancement of the catalytic activity of Rh. Furthermore, Ag is inexpensive metal compared with Au. The colloidal dispersions of Ag and Rh monometallic nanoparticles protected by poly(*N*-vinyl-2-pyrrolidone) (PVP), a water soluble polymer, were prepared by an alcohol reduction method (Hirai et al., 1979). Average diameters of Ag and Rh monometallic nanoparticles were 7.5 nm and 2.2 nm, respectively.

### **2.1.1 Surface plasmon absorption of siver-core bimetallic nanoparticles**

Colloidal sol of Ag nanoparticles shows characteristic plasmon absorption aeound 400 nm (Henglein, 1979). The plasmon absorption band of Ag nanoparticles decreased by addition of Rh nanoparticles and was almost completely extinguished within 30 min after mixing (Figure 2). The parts of plasmon absorption in larger wavelength region were preferentially

Hirakawa, 2003), the removal of reactive oxygen species (Hirakawa & Sano, 2009), and its application to the chemoprevention of ultraviolet induced biomolecules damage (Hirakwa

Much attention has been paid to bimetallic nanoparticles, especially those having a core/shell structure (Toshima et al., 2007). From the view point of Au catalysts, bimetallic nanoparticles have received much attention recently. On the other hand, a physical mixture of monometallic nanoparticles such as Pt and Ru nanoparticles in solution shows higher catalytic activity than the corresponding monometallic nanoparticles under a certain condition (Toshima et al., 1995; Toshima & Hirakawa, 1997). Further, it has been reported that physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution (Figure 1). In this section, the spontaneous formation of the Ag-core bimetallic

Fig. 1. Schematic diagram of the spontaneous formation of Ag-core bimetallic nanoparticles

Self-aggregation <sup>X</sup>

Ag-core/X-shell nanoparticles

The interaction between Ag and Rh monometallic nanoparticles in solution by physical mixing was reported. The main reason for using Ag and Rh nanoparticles is the reported prominent characteristics of Rh nanoparticles as a catalyst (Toshima & Hirakawa, 1999), and the expected electronic effect of Ag similar to Au upon enhancement of the catalytic activity of Rh. Furthermore, Ag is inexpensive metal compared with Au. The colloidal dispersions of Ag and Rh monometallic nanoparticles protected by poly(*N*-vinyl-2-pyrrolidone) (PVP), a water soluble polymer, were prepared by an alcohol reduction method (Hirai et al., 1979). Average diameters of Ag and Rh monometallic nanoparticles were 7.5 nm and 2.2 nm,

Colloidal sol of Ag nanoparticles shows characteristic plasmon absorption aeound 400 nm (Henglein, 1979). The plasmon absorption band of Ag nanoparticles decreased by addition of Rh nanoparticles and was almost completely extinguished within 30 min after mixing (Figure 2). The parts of plasmon absorption in larger wavelength region were preferentially

**2.1.1 Surface plasmon absorption of siver-core bimetallic nanoparticles** 

**2.1 Siver-core/rhodium-shell bimetallic nanoparticles** 

**Ag**

X: for example Rh, Pt, Pd

**X** 

**2. Spontaneous formation of silver-core bimetallic nanoparticles** 

et al., 2008, 2009).

nanoparticles is reviewed.

respectively.

extinguished within 10 min, suggesting that influences of Rh nanopartilces on Ag nanoparticles depend on the size of the Ag nanoparticles. When relatively smaller molar quantity of Rh to Ag was added, the plasmon absorption was not completely extinguished. More than 40 atom-mol% of Rh against to Ag was required to extinguish the plasmon absorption band completely.

Fig. 2. UV-Vis spectral change of the physical mixtures of dispersions of Ag and Rh nanoparticles. The aqueous solutions of Ag (1 atom-mmol L-1, 50 mL) and Rh (1 atom-mmol L-1, 50 mL) nanoparticles were mixed.

## **2.1.2 Transmission electron microgram of the siver-core bimetallic nanoparticles**

Figure 3 shows transmission electron microscopy (TEM) photographs of the physical mixtures of Ag and Rh monometallic nanoparticles. The samples for TEM measurement were prepared by drying the aqueous dispersions of the physical mixtures of Ag and Rh nanoparticles under vacuum in 0, 10, and 30 min, and 24 h, respectively, after mixing. Relatively large particles are attributed to Ag nanoparticles, and rather small ones are Rh nanoparticles. The TEM photographs showed that Rh particles gathered around Ag particle to surround within several minutes, comparable period of the extinction of plasmon absorption. Interestingly, these aggregated particles changed into homogeneous small particles (average diameter =2.7 nm) after 24 h. Preliminary study has shown that the

Fig. 3. TEM photographs of the physical mixtures of Ag and Rh nanoparticles. The aqueous solutions of Ag and Rh nanoparticles (1/1, atom-mol/atom-mol) were mixed, and dried after indicated periods.

Self-Organization of Silver-Core Bimetallic

**2.2 Silver-core/noble metal-shell bimatallic nanoparticles** 

**2.3 Application to the preparation of trimetallic nanoparticles** 

the order of Rh/Ag > Pd/Ag > Pt/Ag.

nanoparticles.

photocatalyst.

Nanoparticles and Their Application for Catalytic Reaction 71

required to completely extinguish the plasmon absorption, which is reasonably supporting the above assumption. These observations suggest that the physical mixture of Ag and Rh nanoparticles spontaneously generates Ag/Rh bimetallic nanoparticles with an Agcore/Rh-shell structure. The disappearance of the XRD peak of Ag nanoparticles suggests that the core of this bimetallic nanoparticles is not complete Ag, but possibly has a partial Ag/Rh alloy structure. The driving force of the formation of this Ag/Rh bimetallic nanoparticles may be due to the larger binding energy between Ag and Rh atoms than between Rh atoms (Peiner & Kopitzki, 1998). Reduction of diameter of the nanoparticle increases not only its surface energy but also number of the binding sites between Ag and Rh atoms, which stabilizes the total energy. Therefore, the shrinking of Ag/Rh bimetallic nanoparticles might be explained by the balance between the binding energy and the surface energy. The size and the rate of formation of the bimetallic nanoparticles can be controlled by the kind and concentration of protective agents. The self-assembling formation of bimetallic nanoparticle using Ag nanoparticle is applicable to construction of novel

The above mentioned procedure can be used to prepare the Ag-core/noble metal shell nanoparticles, other than Ag-core/Rh-shell nanoparticles. The physical mixing of Ag and other metal nanoparticles, such as Au, Pt, Rh, and Pd particles, produces Ag-core bimetallic particles. The interaction rate between Ag and other metal nanoparticles was determined by the extinction of the surface plasmon absorption of Ag nanoparticle. The initial step of this reaction was investigated by isothermal titration calorimetry (Toshima et al., 2005). This study revealed that the strength of the interaction between Ag and other metals increases in

The formed Ag-core/Pt-shell nanoparticle catalyzed the decomposition of hydrogen peroxide (described later). On the other hand, Au and Au/Ag nanoparticles showed an activity of photocatalytic decomposition of methylene blue (Hirakawa, 2007), although their activities were significantly smaller than that of well-known titanium dioxide photocatalyst (Fujishima et al., 2000, 2008). The physical mixing method is simple and useful to prepare novel bimetallic nanoparticles. These nanoparticles may be used as catalyst and

This method can be applied to the preparation of trimetallic nanoparticles (Toshima et al., 2007, 2011). It has been reported that the synthesis of trimetallic nanoparticles having a Aucore structure by a combination of the preparation of bimetallic nanoparticles by coreduction with the formation of core/shell-structured bimetallic nanoparticles by selforganization in physical mixture (Figure 5). The formation of trimetallic nanoparticles has been suggested by UV–Vis spectral change, TEM image change, FT-IR spectra of adsorbed carbon monoxide, XPS spectra and calorimetric studies. The catalytic activity of trimetallic nanoparticles in the molar ratio of Au/Pd/Rh = 1/4/20 was higher than the corresponding monometallic and bimetallic nanoparticles for hydrogenation of methyl acrylate. This high catalytic activity can be understood by sequential electronic charge transfer from surface Rh

atoms to interlayered Pt atoms and then to core Au atoms (Toshima et al., 2011).

increase of Rh/Ag molar ratio reduces the average diameter and the size distribution of the nanoparticles. The elemental analysis using characteristic X-ray in high-resolution TEM measurement has shown that the particles produced from their physical mixtures in 24 h are composed of Ag and Rh.

#### **2.1.3 X-ray diffraction of the of siver-core bimetallic nanoparticles**

Figure 4 shows X-ray diffraction (XRD) patterns of poly(*N*-vinyl-2-pyrrolidone)-protected Ag and Rh monometallic nanoparticles, and their physical mixture. The sample of the physical mixture of Ag and Rh nanoparticles was prepared by drying the mixtures of their aqueous solutions under vacuum for 24 h after mixing. The XRD pattern of the mixtures of Ag and Rh nanoparticles was similar to that of Rh nanoparticle, suggesting that the surface of the particle produced by mixing Ag and Rh nanoparticles is composed of Rh. Similarly, Au-core/Pt-shell and Au-core/Pd-shell structured nanoparticles have shown the XRD pattern quite similar to that of their surface metals (Yonezawa & Toshima, 1995). These findings suggest that the aggregation of Rh particles around the Ag particle is involved in the extinction of the plasmon absorption.

#### **2.1.4 Mechanism of the formation of the siver-core/rhodium shell bimetallic nanoparticles**

Henglein *et al*. reported that lead (Pb) atoms transfer from Pb colloidal particle onto the surface of Ag colloidal particle in physical mixing of Ag and Pb colloidal sols (Henglein et al., 1992). If the extinction of the plasmon absorption is due to coating of the surface of Ag particle by Rh atoms transferred from Rh nanoparticle, at least 28 mol% of Rh to Ag is required assuming that a Ag particle (average diameter = 7.5 nm) is uniformly coated by Rh atoms in a one-atom layer. In the present experiments about 40 atom-mol% of Rh to Ag was

Fig. 4. XRD patterns of Ag and Rh monometallic nanoparticles, and their physical mixture (1/1, atom-mol/atom-mol)

increase of Rh/Ag molar ratio reduces the average diameter and the size distribution of the nanoparticles. The elemental analysis using characteristic X-ray in high-resolution TEM measurement has shown that the particles produced from their physical mixtures in 24 h are

Figure 4 shows X-ray diffraction (XRD) patterns of poly(*N*-vinyl-2-pyrrolidone)-protected Ag and Rh monometallic nanoparticles, and their physical mixture. The sample of the physical mixture of Ag and Rh nanoparticles was prepared by drying the mixtures of their aqueous solutions under vacuum for 24 h after mixing. The XRD pattern of the mixtures of Ag and Rh nanoparticles was similar to that of Rh nanoparticle, suggesting that the surface of the particle produced by mixing Ag and Rh nanoparticles is composed of Rh. Similarly, Au-core/Pt-shell and Au-core/Pd-shell structured nanoparticles have shown the XRD pattern quite similar to that of their surface metals (Yonezawa & Toshima, 1995). These findings suggest that the aggregation of Rh particles around the Ag particle is involved in

**2.1.3 X-ray diffraction of the of siver-core bimetallic nanoparticles** 

**2.1.4 Mechanism of the formation of the siver-core/rhodium shell bimetallic** 

Henglein *et al*. reported that lead (Pb) atoms transfer from Pb colloidal particle onto the surface of Ag colloidal particle in physical mixing of Ag and Pb colloidal sols (Henglein et al., 1992). If the extinction of the plasmon absorption is due to coating of the surface of Ag particle by Rh atoms transferred from Rh nanoparticle, at least 28 mol% of Rh to Ag is required assuming that a Ag particle (average diameter = 7.5 nm) is uniformly coated by Rh atoms in a one-atom layer. In the present experiments about 40 atom-mol% of Rh to Ag was

37.9˚

(1 1 1) surface of hcp structure 40.7˚

Ag

Rh

Ag + Rh

(1/1, atom-mol/atom-mol)

(1 1 1) surface of hcp structure

47.6˚

(2 1 1) surface of hcp structure

30.00 40.00 50.00 60.00 2 / ˚

Fig. 4. XRD patterns of Ag and Rh monometallic nanoparticles, and their physical mixture

composed of Ag and Rh.

**nanoparticles** 

the extinction of the plasmon absorption.

required to completely extinguish the plasmon absorption, which is reasonably supporting the above assumption. These observations suggest that the physical mixture of Ag and Rh nanoparticles spontaneously generates Ag/Rh bimetallic nanoparticles with an Agcore/Rh-shell structure. The disappearance of the XRD peak of Ag nanoparticles suggests that the core of this bimetallic nanoparticles is not complete Ag, but possibly has a partial Ag/Rh alloy structure. The driving force of the formation of this Ag/Rh bimetallic nanoparticles may be due to the larger binding energy between Ag and Rh atoms than between Rh atoms (Peiner & Kopitzki, 1998). Reduction of diameter of the nanoparticle increases not only its surface energy but also number of the binding sites between Ag and Rh atoms, which stabilizes the total energy. Therefore, the shrinking of Ag/Rh bimetallic nanoparticles might be explained by the balance between the binding energy and the surface energy. The size and the rate of formation of the bimetallic nanoparticles can be controlled by the kind and concentration of protective agents. The self-assembling formation of bimetallic nanoparticle using Ag nanoparticle is applicable to construction of novel nanoparticles.

## **2.2 Silver-core/noble metal-shell bimatallic nanoparticles**

The above mentioned procedure can be used to prepare the Ag-core/noble metal shell nanoparticles, other than Ag-core/Rh-shell nanoparticles. The physical mixing of Ag and other metal nanoparticles, such as Au, Pt, Rh, and Pd particles, produces Ag-core bimetallic particles. The interaction rate between Ag and other metal nanoparticles was determined by the extinction of the surface plasmon absorption of Ag nanoparticle. The initial step of this reaction was investigated by isothermal titration calorimetry (Toshima et al., 2005). This study revealed that the strength of the interaction between Ag and other metals increases in the order of Rh/Ag > Pd/Ag > Pt/Ag.

The formed Ag-core/Pt-shell nanoparticle catalyzed the decomposition of hydrogen peroxide (described later). On the other hand, Au and Au/Ag nanoparticles showed an activity of photocatalytic decomposition of methylene blue (Hirakawa, 2007), although their activities were significantly smaller than that of well-known titanium dioxide photocatalyst (Fujishima et al., 2000, 2008). The physical mixing method is simple and useful to prepare novel bimetallic nanoparticles. These nanoparticles may be used as catalyst and photocatalyst.

## **2.3 Application to the preparation of trimetallic nanoparticles**

This method can be applied to the preparation of trimetallic nanoparticles (Toshima et al., 2007, 2011). It has been reported that the synthesis of trimetallic nanoparticles having a Aucore structure by a combination of the preparation of bimetallic nanoparticles by coreduction with the formation of core/shell-structured bimetallic nanoparticles by selforganization in physical mixture (Figure 5). The formation of trimetallic nanoparticles has been suggested by UV–Vis spectral change, TEM image change, FT-IR spectra of adsorbed carbon monoxide, XPS spectra and calorimetric studies. The catalytic activity of trimetallic nanoparticles in the molar ratio of Au/Pd/Rh = 1/4/20 was higher than the corresponding monometallic and bimetallic nanoparticles for hydrogenation of methyl acrylate. This high catalytic activity can be understood by sequential electronic charge transfer from surface Rh atoms to interlayered Pt atoms and then to core Au atoms (Toshima et al., 2011).

Self-Organization of Silver-Core Bimetallic

ratio of Au/Pt = 2/3 is the most active catalyst.

Similar results reported on the other catalytic reactions.

**3.2 Application of siver-core/rhodium-shell bimetallic nanoparticles** 

0

0.2

*k*H2 / s-1

mixture of Ag and Rh monometallic nanoparticles.

**metal nanoparticle** 

(Toshima et al., 1995) (Figure 8).

0.4

0.6

Nanoparticles and Their Application for Catalytic Reaction 73

the presence of water-soluble polymers and non-ionic surfactant-micelles, respectively. The UV-Vis spectra and the transmission electron micrographs suggest that the polymerprotected Au/Pt bimetallic systems are composed of bimetallic alloy clusters, but the micelle-protected ones are mostly composed of the mixtures of the monometallic Au and Pt particles. The *in-situ* UV-Vis spectra during the reductions can elucidate the formation processes of the bimetallic dispersions which are different from each other depending on the protective reagent. The Au/Pt bimetallic systems can be used as the catalyst for visible lightinduced hydrogen generation. The bimetallic system stabilized by the polymer at a molar

It has been reported that the catalytic activity of the Ag/Rh bimetallic nanoparticles for visible-light-induced hydrogen generation (Toshima & Hirakawa, 1999) in an aqueous solution composed of ethylenediaminetetraacetic acid, tris(bipyridine)ruthenium(II), methyl viologen, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles and alloy-structured Ag/Rh nanoparticles, suggesting that the Ag-core shows an electronic effect on the surface Rh as in the case of the Au-core (Yonezawa & Toshima, 1993) and enhances the catalytic activity of the surface Rh. The highest catalytic activity was observed at 1:9 ratio of Ag and Rh atoms (Figure 7).

> 0 20 40 60 80 100 Molar ratio of Ag (%)

Fig. 7. Hydrogen generation rate coefficient (*k*H2) depending on the molar ratio of Ag of Ag/Rh bimetallic nanoparticles. The *k*H2 indicates the number of generated H2 molecules on a surface metal atom per one second. The average is the calculated activity of the simple

**3.3 Carbon dioxide reduction by visible-light-induced electron transfer system using** 

A photochemical reduction of CO2 can be applied to a novel energy storage process for the utilization of solar energy in the future. The above mentioned catalytic system can be applied to CO2 reduction. The strategy is the catalytic reduction of CO2 using electrons gathered by an electron transfer system (Willner et al., 1987, Toshima et al., 1995). It has been reported that nanoparticles catalyzes the reduction of CO2 and the generation of methane

Average

Random Alloy structure

Core-Shell structure

Fig. 5. Schematic diagram of the formation of the trimetallic Au/Pt/Rh nanoparticles

## **3. Visible-light-induced hydrogen generation by metal nanoparticle catalytic system**

Metal nanoparticles are very important materials for nanoscience and nanotechnology (Fukumi et al., 1994; Lu et al., 1999; Link et al., 1999; Sun et al., 1999; Teranishi & Miyake, 1999; Akinaga, 2002). A particularly large number of reports have been published on their applications to catalysts (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Widegren & Finke 2003; Willner et al., 1987; Toshima et al., 1995; Yonezawa & Toshima, 1993). As the catalyst in the homogeneous system, the colloidal dispersions of metal nanoparticles have the advantage that they are soluble or homogeneous in an aqueous solution and transparent to visible light (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Widegren & Finke 2003; Willner et al., 1987). Thus, colloidal metal nanoparticles are useful for photocatalytic reaction systems. For example, colloidal metal nanoparticles catalyze the water reduction in the visible-lightinduced electron transfer system composed of ethylenediaminetetraacetic acid disodium salt (EDTA), tris(bipyridine)ruthenium(II) dichloride ([Ru(bpy)3]2+), and 1,1'-dimethyl-4,4' bipyridium dichloride (methyl viologen, MV2+) (Yonezawa & Toshima, 1993) (Figure 6).

Fig. 6. Schematic diagram of the visible-light induced hydrogen generation using the electron transfer system and metal nanoparticle catalyst

#### **3.1 Catalytic activity of gold-core/platinum-shell bimetallic nanoparticles**

The bimetallization of metal nanoparticle can improve the catalytic activity of surface metal. Especially, core-shell structured nanoparticles are important. Several study demonstrated the Au-core/Pt shell metal nanoparticles show higher catalytic activity for the visible-lightinduced hydrogen generation than Pt monometallic nanoparticles. The following study is an example of the hydrogen generation using Au/Pt nanoparticle catalyst (Yonezawa & Toshima, 1993). In this study, the Au/Pt bimetallic systems stabilized by polymer and micelle were obtained by alcohol- and photo-reduction of the corresponding metal ions in

HAuCl4 + H2PtCl6 gold ion platinum ion

rhodium ion

**system** 

Electron Donor

Oxidized Product

*h* **Rh**

RhCl3 *d* = 2.9 nm

**3. Visible-light-induced hydrogen generation by metal nanoparticle catalytic** 

Metal nanoparticles are very important materials for nanoscience and nanotechnology (Fukumi et al., 1994; Lu et al., 1999; Link et al., 1999; Sun et al., 1999; Teranishi & Miyake, 1999; Akinaga, 2002). A particularly large number of reports have been published on their applications to catalysts (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Widegren & Finke 2003; Willner et al., 1987; Toshima et al., 1995; Yonezawa & Toshima, 1993). As the catalyst in the homogeneous system, the colloidal dispersions of metal nanoparticles have the advantage that they are soluble or homogeneous in an aqueous solution and transparent to visible light (Kiely et al., 1998; Pileni, 1998; Bradley, 1994; Widegren & Finke 2003; Willner et al., 1987). Thus, colloidal metal nanoparticles are useful for photocatalytic reaction systems. For example, colloidal metal nanoparticles catalyze the water reduction in the visible-lightinduced electron transfer system composed of ethylenediaminetetraacetic acid disodium salt (EDTA), tris(bipyridine)ruthenium(II) dichloride ([Ru(bpy)3]2+), and 1,1'-dimethyl-4,4' bipyridium dichloride (methyl viologen, MV2+) (Yonezawa & Toshima, 1993) (Figure 6).

MV2+

MV•+

**Metal**

2H+

H2

Chemical reduction

Fig. 5. Schematic diagram of the formation of the trimetallic Au/Pt/Rh nanoparticles

[Ru(bpy)3]2+

[Ru(bpy)3]3+

electron transfer system and metal nanoparticle catalyst

Fig. 6. Schematic diagram of the visible-light induced hydrogen generation using the

The bimetallization of metal nanoparticle can improve the catalytic activity of surface metal. Especially, core-shell structured nanoparticles are important. Several study demonstrated the Au-core/Pt shell metal nanoparticles show higher catalytic activity for the visible-lightinduced hydrogen generation than Pt monometallic nanoparticles. The following study is an example of the hydrogen generation using Au/Pt nanoparticle catalyst (Yonezawa & Toshima, 1993). In this study, the Au/Pt bimetallic systems stabilized by polymer and micelle were obtained by alcohol- and photo-reduction of the corresponding metal ions in

**3.1 Catalytic activity of gold-core/platinum-shell bimetallic nanoparticles** 

*d* = 2.3 nm

**Au Pt**

**Rh** *<sup>d</sup>* = 3.6 nm

Physical mixing

**Au**

**Pt**

the presence of water-soluble polymers and non-ionic surfactant-micelles, respectively. The UV-Vis spectra and the transmission electron micrographs suggest that the polymerprotected Au/Pt bimetallic systems are composed of bimetallic alloy clusters, but the micelle-protected ones are mostly composed of the mixtures of the monometallic Au and Pt particles. The *in-situ* UV-Vis spectra during the reductions can elucidate the formation processes of the bimetallic dispersions which are different from each other depending on the protective reagent. The Au/Pt bimetallic systems can be used as the catalyst for visible lightinduced hydrogen generation. The bimetallic system stabilized by the polymer at a molar ratio of Au/Pt = 2/3 is the most active catalyst.

## **3.2 Application of siver-core/rhodium-shell bimetallic nanoparticles**

It has been reported that the catalytic activity of the Ag/Rh bimetallic nanoparticles for visible-light-induced hydrogen generation (Toshima & Hirakawa, 1999) in an aqueous solution composed of ethylenediaminetetraacetic acid, tris(bipyridine)ruthenium(II), methyl viologen, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles and alloy-structured Ag/Rh nanoparticles, suggesting that the Ag-core shows an electronic effect on the surface Rh as in the case of the Au-core (Yonezawa & Toshima, 1993) and enhances the catalytic activity of the surface Rh. The highest catalytic activity was observed at 1:9 ratio of Ag and Rh atoms (Figure 7). Similar results reported on the other catalytic reactions.

Fig. 7. Hydrogen generation rate coefficient (*k*H2) depending on the molar ratio of Ag of Ag/Rh bimetallic nanoparticles. The *k*H2 indicates the number of generated H2 molecules on a surface metal atom per one second. The average is the calculated activity of the simple mixture of Ag and Rh monometallic nanoparticles.

#### **3.3 Carbon dioxide reduction by visible-light-induced electron transfer system using metal nanoparticle**

A photochemical reduction of CO2 can be applied to a novel energy storage process for the utilization of solar energy in the future. The above mentioned catalytic system can be applied to CO2 reduction. The strategy is the catalytic reduction of CO2 using electrons gathered by an electron transfer system (Willner et al., 1987, Toshima et al., 1995). It has been reported that nanoparticles catalyzes the reduction of CO2 and the generation of methane (Toshima et al., 1995) (Figure 8).

Self-Organization of Silver-Core Bimetallic

reduction of 12CO2 originated from EDTA.

is not miscible with liposome in water.

reduction.

**3.3.3 Liposome-protected metal nanoparticle catalyst** 

**3.3.2 Methane generation from carbon dioxide reduction** 

Nanoparticles and Their Application for Catalytic Reaction 75

The formation of methane was then clearly detected by gas chromatography (about 19 nmol in the case of the liposome protected Pt nanoparticles system). In order to confirm the methane generation from CO2, isotope experiments were carried out using NaH13CO3 as a CO2 source and analyzed by a gas chromatograph mass-spectrometer. Since NaHCO3 is equilibrated with CO2 in solution and easily treated, it was a good source of CO2 in the present experiments. In this experiment, 13CH4 was clearly detected, though the produced methane was not pure 13CH4 and it did contain 12CH4. In the same experiment, the mole ratio of 13CO2 to 12CO2 in the gas phase was about 57:43, which is nearly the same as the isotopic ratio of the generated methane. EDTA works as an electron donor in the system and is known to decompose into CO2. Therefore, 12CH4 generation possibly occurs through the reduction of 12CO2 generated from EDTA. The effect of EDTA on methane generation was examined in the Pt-liposome system. Methane was detected on visible-light irradiation of the system involving EDTA without CO2 or NaHCO3 but could not be detected in the absence of EDTA. These results suggest that the detected 12CH4 is generated by the

Liposome was better than other protective-colloid of Pt nanoparticles for methane generation. This is probably explained by assuming that liposome can form a larger and stronger hydrophobic region to concentrate CO2 around a Pt nanoparticle than C12EO micelle and poly(*N*-vinyl-2-pyrrolidone). In addition, Ru-C12EO showed higher catalytic activity than Pt-C12EO. Thus, Ru-liposome was considered to be an active catalyst for methane generation in the system tested here. The synthesis of Ru-liposome was tried in a way similar to that of Pt-liposome, but the suspension of the Ru-liposome was not active as a catalyst. The resulting Ru-liposome was not as homogeneous, probably because the Ru ion

**3.3.4 Summary of the carbon dioxide photo-reduction by metal nanoparticle catalyst**  The Pt and Ru nanoparticle catalysts, which were prepared by a photoreduction method of metal salt in water without ethanol, successfully generated methane from CO2. The methane generation suggests that the eight-electron reduction of CO2 easily proceeds on metal nanoparticles possibly due to a thermodynamic advantage. This is different from an electrochemical CO2 reduction using Pt electrodes, on which CO2 is reduced to CO with adsorbed hydrogen atoms. In the present system using metal nanoparticles, the competition reaction, i.e., the kinetically favorably hydrogen generation, inhibits the methane generation. An increase of CO2 concentration, the electron supply rate, or both may enhance CO2

**4. Catalytic decomposition of hydrogen peroxide by metal nanoparticle** 

The modification of biomacromolecules upon exposure to reactive oxygen species, including hydrogen peroxide (H2O2), dioxide(1-) (superoxide O2•-), hydroxyl radical (HO•), and singlet oxygen (1O2), is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents (Kawanishi et al. 2001; Cadet et al., 2003;

Fig. 8. Schematic diagram of the visible-light induced CO2 reduction using the electron transfer system and metal nanoparticle catalyst

The possible reaction scheme of the CO2 reduction is as follows:

$$\rm{CO\_2 + 8H^+ + 8e^- \to CH\_4 + 2H\_2O} \tag{1}.$$

This eight-electron reduction of CO2 is advantageous process compared with other possible CO2 reduction process from the thermodynamic point of view. Although it is not a study using the silver–core bimetallic nanoprticles, this topic is closely related to the applications of bimetallic nanoparticles to catalytic reaction. Thus, the topic of the CO2 reduction using metal nanoparticle catalyst is presented here.

Typical reactions were performed by the similar manner to the hydrogen generation. A 20 cm3 Pyrex Schlenk tube was charged with a 10 cm3 aqueous solution, containing EDTA (a sacrificial electron donor), [Ru(bpy)3]2+ (a photosensitizer), MV2+ (an electron mediator), NaHCO3 (a pH adjuster and a CO2 source), and colloidal dispersion of metal nanoparticles. The mixtures were degassed by freeze-thaw cycles and the tubes were then filled with 1 atm of CO2. The photo-irradiation was carried out for 3 or 4 h with a 500 W super-high-pressure mercury lamp through a UV cut filter (> 390 nm) in a water bath maintained at 30 C. About 100 μmol of methane was detected in this system (Toshima et al., 1995). However, it has not been confirmed that methane was actually the reduction product of CO2.

#### **3.3.1 Strategy for the demonstration of the methane generation from carbon dioxide**

In a heterogeneous system, photoreduction of CO2 was confirmed by experiments using an isotope (Ishitani et al. 1993). To our knowledge, however, the isotopic method has not been applied to the confirmation of the photoreduction of CO2 in a homogeneous system using the colloidal dispersion of metal nanoparticles. To confirm the above mentioned methane generation, the following study was carried out. In this study, photoreduction of CO2 was carried out in a similar system to one reported previously (Toshima et al., 1995), and the generation of methane from CO2 was confirmed by isotopic experiments. As the catalysts, novel metal nanoparticles, *i.e*., liposome-protected Pt nanoparticles, were prepared and used in the present system. Colloidal dispersions of Pt and Ru nanoparticles were prepared by photoreduction without using ethanol (Yamaji et al. 1995). Preparation of nanoparticles without ethanol is required, because the coexisting ethanol is decomposed during the photochemical reaction, leading to the formation methane. This methane formation cannot be distinguished from the actual methane generation from CO2. Protective agents used for the metal nanoparticles were poly(*N*-vinyl-2-pyrrolidone), C12EO, and liposome. The products in the gas phase were analyzed with a gas chromatograph. The characterization of gaseous products was carried out with a gas chromatograph mass-spectrometer.

MV2+

MV•+

**Metal**

CO2 + H+

CH4 + H2

[Ru(bpy)3]

*h*

Electron Donor

Oxidized Product

[Ru(bpy)3]

The possible reaction scheme of the CO2 reduction is as follows:

transfer system and metal nanoparticle catalyst

metal nanoparticle catalyst is presented here.

2+

3+

been confirmed that methane was actually the reduction product of CO2.

Fig. 8. Schematic diagram of the visible-light induced CO2 reduction using the electron

 CO2 + 8H+ + 8e- → CH4 + 2H2O (1). This eight-electron reduction of CO2 is advantageous process compared with other possible CO2 reduction process from the thermodynamic point of view. Although it is not a study using the silver–core bimetallic nanoprticles, this topic is closely related to the applications of bimetallic nanoparticles to catalytic reaction. Thus, the topic of the CO2 reduction using

Typical reactions were performed by the similar manner to the hydrogen generation. A 20 cm3 Pyrex Schlenk tube was charged with a 10 cm3 aqueous solution, containing EDTA (a sacrificial electron donor), [Ru(bpy)3]2+ (a photosensitizer), MV2+ (an electron mediator), NaHCO3 (a pH adjuster and a CO2 source), and colloidal dispersion of metal nanoparticles. The mixtures were degassed by freeze-thaw cycles and the tubes were then filled with 1 atm of CO2. The photo-irradiation was carried out for 3 or 4 h with a 500 W super-high-pressure mercury lamp through a UV cut filter (> 390 nm) in a water bath maintained at 30 C. About 100 μmol of methane was detected in this system (Toshima et al., 1995). However, it has not

**3.3.1 Strategy for the demonstration of the methane generation from carbon dioxide**  In a heterogeneous system, photoreduction of CO2 was confirmed by experiments using an isotope (Ishitani et al. 1993). To our knowledge, however, the isotopic method has not been applied to the confirmation of the photoreduction of CO2 in a homogeneous system using the colloidal dispersion of metal nanoparticles. To confirm the above mentioned methane generation, the following study was carried out. In this study, photoreduction of CO2 was carried out in a similar system to one reported previously (Toshima et al., 1995), and the generation of methane from CO2 was confirmed by isotopic experiments. As the catalysts, novel metal nanoparticles, *i.e*., liposome-protected Pt nanoparticles, were prepared and used in the present system. Colloidal dispersions of Pt and Ru nanoparticles were prepared by photoreduction without using ethanol (Yamaji et al. 1995). Preparation of nanoparticles without ethanol is required, because the coexisting ethanol is decomposed during the photochemical reaction, leading to the formation methane. This methane formation cannot be distinguished from the actual methane generation from CO2. Protective agents used for the metal nanoparticles were poly(*N*-vinyl-2-pyrrolidone), C12EO, and liposome. The products in the gas phase were analyzed with a gas chromatograph. The characterization of

gaseous products was carried out with a gas chromatograph mass-spectrometer.

## **3.3.2 Methane generation from carbon dioxide reduction**

The formation of methane was then clearly detected by gas chromatography (about 19 nmol in the case of the liposome protected Pt nanoparticles system). In order to confirm the methane generation from CO2, isotope experiments were carried out using NaH13CO3 as a CO2 source and analyzed by a gas chromatograph mass-spectrometer. Since NaHCO3 is equilibrated with CO2 in solution and easily treated, it was a good source of CO2 in the present experiments. In this experiment, 13CH4 was clearly detected, though the produced methane was not pure 13CH4 and it did contain 12CH4. In the same experiment, the mole ratio of 13CO2 to 12CO2 in the gas phase was about 57:43, which is nearly the same as the isotopic ratio of the generated methane. EDTA works as an electron donor in the system and is known to decompose into CO2. Therefore, 12CH4 generation possibly occurs through the reduction of 12CO2 generated from EDTA. The effect of EDTA on methane generation was examined in the Pt-liposome system. Methane was detected on visible-light irradiation of the system involving EDTA without CO2 or NaHCO3 but could not be detected in the absence of EDTA. These results suggest that the detected 12CH4 is generated by the reduction of 12CO2 originated from EDTA.

## **3.3.3 Liposome-protected metal nanoparticle catalyst**

Liposome was better than other protective-colloid of Pt nanoparticles for methane generation. This is probably explained by assuming that liposome can form a larger and stronger hydrophobic region to concentrate CO2 around a Pt nanoparticle than C12EO micelle and poly(*N*-vinyl-2-pyrrolidone). In addition, Ru-C12EO showed higher catalytic activity than Pt-C12EO. Thus, Ru-liposome was considered to be an active catalyst for methane generation in the system tested here. The synthesis of Ru-liposome was tried in a way similar to that of Pt-liposome, but the suspension of the Ru-liposome was not active as a catalyst. The resulting Ru-liposome was not as homogeneous, probably because the Ru ion is not miscible with liposome in water.

## **3.3.4 Summary of the carbon dioxide photo-reduction by metal nanoparticle catalyst**

The Pt and Ru nanoparticle catalysts, which were prepared by a photoreduction method of metal salt in water without ethanol, successfully generated methane from CO2. The methane generation suggests that the eight-electron reduction of CO2 easily proceeds on metal nanoparticles possibly due to a thermodynamic advantage. This is different from an electrochemical CO2 reduction using Pt electrodes, on which CO2 is reduced to CO with adsorbed hydrogen atoms. In the present system using metal nanoparticles, the competition reaction, i.e., the kinetically favorably hydrogen generation, inhibits the methane generation. An increase of CO2 concentration, the electron supply rate, or both may enhance CO2 reduction.

## **4. Catalytic decomposition of hydrogen peroxide by metal nanoparticle**

The modification of biomacromolecules upon exposure to reactive oxygen species, including hydrogen peroxide (H2O2), dioxide(1-) (superoxide O2•-), hydroxyl radical (HO•), and singlet oxygen (1O2), is the likely initial event involved in the induction of the mutagenic and lethal effects of various oxidative stress agents (Kawanishi et al. 2001; Cadet et al., 2003;

Self-Organization of Silver-Core Bimetallic

*σ*= 2.0 nm).

oxygen as follows:

colloidal dispersion was purified with an ultra-filter.

Reactive Oxygen Species

**4.2.2 Method of the detection of hydrogen peroxide** 

Folic acid H2O2

Fig. 9. Fluorometry of reactive oxygen species (hydrogen peroxide) using folic acid

measured using a fluorescence spectrophotometer with 350-nm excitation.

**4.2.3 Platinum nanoparticles effectively scavenge hydrogen peroxide** 

The generated H2O2 was measured by a previously reported method using folic acid (Hirakawa, 2006). This assay is based on the fluorescence enhancement of less-fluorescent folic acid via oxidative decomposition by H2O2 and copper(II) ion into strong-fluorescent 2 amino-4-oxo-3*H*-pterine-6-carboxylic acid (Figure 9). The concentration of H2O2 ([H2O2]) can be determined using a calibration curve. A reaction mixture containing folic acid, copper(II) chloride, and the H2O2 sample (or H2O2 generator 4) with or without the metal nanoparticle in a sodium phosphate buffer (pH 7.6) was incubated in a microtube for 30 min. After incubation at 37 C, the fluorescence intensity of the reaction mixture at 450 nm was

Platinum nanoparticles effectively scavenged H2O2 in a dose-dependent manner and showed the highest activity among the metal nanoparticles used in this study (Figure 10). A sample solution of 5 μM/atom Pt nanoparticles, among which 1 μg Pt metal is included, exhibits comparable activity for H2O2 decomposition to that of 10 units of catalase. One unit of catalase can remove 1.0 μmol H2O2 per min in water (pH 7.0, 25 C). Poly(*N*-vinyl-2 pyrrolidone) itself did not scavenge H2O2. This experiment confirmed that poly(*N*-vinyl-2 pyrrolidone)-protected Pt nanoparticles can remove H2O2. The mechanism of H2O2 removal by Pt nanoparticles can be explained by catalytic decomposition into water and molecular

Nanoparticles and Their Application for Catalytic Reaction 77

nanoparticles (Shiraishi & Toshima, 1999) was prepared from reduction of 1 mM AgNO3 with NaBH4 in the presence of 40 mM poly(*N*-vinyl-2-pyrrolidone). The obtained Ag

These poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles formed water-soluble sols. The average diameters (*d*) and standard deviations (*σ*) of monometallic nanoparticles determined by TEM measurement were as follows: Pt (*d* = 2.2 nm, *σ*= 1.0 nm), Pd (*d* = 2.0 nm, *σ*= 0.9 nm), Rh (*d* = 2.2 nm, *σ*= 1.0 nm), Ag (*d* = 10.0 nm, *σ*=1.9 nm), and Au (*d* = 10.2 nm,

**OFF**

Wavelength / nm

350 450 550

**ON**

Intensity

0

100

200

300

400

Fluorescence

Drechsel & Patel, 2008). Therefore, the activity of reactive oxygen species generation by various chemical compounds is closely related to their toxicity, carcinogenicity, or both. For example, hydroquinone, a metabolite of carcinogenic benzene, causes DNA damage via H2O2 generation (Hirakawa et al., 2002). Many studies have addressed the role of antioxidants, such as vitamins (Slaga, 1995; Sohmiya et al., 2004) and catechins (Weyant et al., 2001), in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al. 1991; Omenn et al., 1996). Solovieva et al. reported that antioxidants, ascorbic acid (Solovieva et al., 2007) and dithiothreitol (Solovieva et al., 2008), exhibit cytotoxicity via H2O2 generation. Relevantly, it has been reported that vitamins A (Murata & Kawanishi, 2000) and E (Yamashita et al., 1998) and catechins (Oikawa et al., 2003) induce DNA oxidation through H2O2 generation during their oxidation. H2O2 is a long-lived reactive oxygen species which plays an important role in biomacromolecular damage induced by various chemical compounds (Kawanishi et al., 2001; Hirakawa et al., 2002).

## **4.1 Metal catalyzes decomposition of hydrogen peroxide**

Various studies have demonstrated the catalytic decomposition of H2O2 by noble metals such as Pt (Keating et al., 1965; McKee, 1969; Bianchi et al., 1962), Pd (Keating et al., 1965; McKee, 1969; Bianchi et al., 1962; Eley & Macmahon, 1972) Ag (Baumgartner et al., 1963; Goszner et al., 1972; Goszner & Bischof, 1974), and Au (Eley & Macmahon, 1972; Goszner & Bischof, 1974). These metals themselves are hardly oxidized by reactive oxygen species, however, it is difficult to use metal powder or foils as anti-oxidative drugs. Recently, Kajita et al. reported that Pt nanoparticles catalyze the decomposition of reactive oxygen species (Kajita et al., 2007). These nanoparticles can be dispersed in water and used as homogenous solutions. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel chemopreventive agents against reactive oxygen species-mediated biomolecules damage. In this section, the examination of the removal of H2O2 generated from a chemical compound, hydroquione, using water-soluble polymer-protected Pt and Ag/Pt nanoparticles are reviewed.

## **4.2 Catalytic activity of monometallic nanoparticles**

## **4.2.1 Preparation of metal nanoparticles for reactive oxygen scavenger**

Colloidal dispersions of poly(*N*-vinyl-2-pyrrolidone)-protected Pt, Pd, Rh, and Au nanoparticles were prepared using an alcohol reduction method (Hirai et al., 1979). 50 mL of water/ethanol (1/1, v/v) solution containing 1 mM metal salts and 40 mM poly(*N*-vinyl-2 pyrrolidone) (monomer unit) was refluxed for 2 h, resulting in the formation of typical colored sols of metal nanoparticles. The solvent was removed by vacuum evaporation, and the nanoparticles were dispersed into water to prepare 1 mM/atom (atomic concentration) metal colloidal sols. An aqueous solution of poly(*N*-vinyl-2-pyrrolidone)-protected Ag

Drechsel & Patel, 2008). Therefore, the activity of reactive oxygen species generation by various chemical compounds is closely related to their toxicity, carcinogenicity, or both. For example, hydroquinone, a metabolite of carcinogenic benzene, causes DNA damage via H2O2 generation (Hirakawa et al., 2002). Many studies have addressed the role of antioxidants, such as vitamins (Slaga, 1995; Sohmiya et al., 2004) and catechins (Weyant et al., 2001), in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al. 1991; Omenn et al., 1996). Solovieva et al. reported that antioxidants, ascorbic acid (Solovieva et al., 2007) and dithiothreitol (Solovieva et al., 2008), exhibit cytotoxicity via H2O2 generation. Relevantly, it has been reported that vitamins A (Murata & Kawanishi, 2000) and E (Yamashita et al., 1998) and catechins (Oikawa et al., 2003) induce DNA oxidation through H2O2 generation during their oxidation. H2O2 is a long-lived reactive oxygen species which plays an important role in biomacromolecular damage induced by

various chemical compounds (Kawanishi et al., 2001; Hirakawa et al., 2002).

Various studies have demonstrated the catalytic decomposition of H2O2 by noble metals such as Pt (Keating et al., 1965; McKee, 1969; Bianchi et al., 1962), Pd (Keating et al., 1965; McKee, 1969; Bianchi et al., 1962; Eley & Macmahon, 1972) Ag (Baumgartner et al., 1963; Goszner et al., 1972; Goszner & Bischof, 1974), and Au (Eley & Macmahon, 1972; Goszner & Bischof, 1974). These metals themselves are hardly oxidized by reactive oxygen species, however, it is difficult to use metal powder or foils as anti-oxidative drugs. Recently, Kajita et al. reported that Pt nanoparticles catalyze the decomposition of reactive oxygen species (Kajita et al., 2007). These nanoparticles can be dispersed in water and used as homogenous solutions. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel chemopreventive agents against reactive oxygen species-mediated biomolecules damage. In this section, the examination of the removal of H2O2 generated from a chemical compound, hydroquione, using water-soluble polymer-protected Pt and Ag/Pt nanoparticles are

**4.1 Metal catalyzes decomposition of hydrogen peroxide** 

**4.2 Catalytic activity of monometallic nanoparticles** 

**4.2.1 Preparation of metal nanoparticles for reactive oxygen scavenger** 

Colloidal dispersions of poly(*N*-vinyl-2-pyrrolidone)-protected Pt, Pd, Rh, and Au nanoparticles were prepared using an alcohol reduction method (Hirai et al., 1979). 50 mL of water/ethanol (1/1, v/v) solution containing 1 mM metal salts and 40 mM poly(*N*-vinyl-2 pyrrolidone) (monomer unit) was refluxed for 2 h, resulting in the formation of typical colored sols of metal nanoparticles. The solvent was removed by vacuum evaporation, and the nanoparticles were dispersed into water to prepare 1 mM/atom (atomic concentration) metal colloidal sols. An aqueous solution of poly(*N*-vinyl-2-pyrrolidone)-protected Ag

reviewed.

nanoparticles (Shiraishi & Toshima, 1999) was prepared from reduction of 1 mM AgNO3 with NaBH4 in the presence of 40 mM poly(*N*-vinyl-2-pyrrolidone). The obtained Ag colloidal dispersion was purified with an ultra-filter.

These poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles formed water-soluble sols. The average diameters (*d*) and standard deviations (*σ*) of monometallic nanoparticles determined by TEM measurement were as follows: Pt (*d* = 2.2 nm, *σ*= 1.0 nm), Pd (*d* = 2.0 nm, *σ*= 0.9 nm), Rh (*d* = 2.2 nm, *σ*= 1.0 nm), Ag (*d* = 10.0 nm, *σ*=1.9 nm), and Au (*d* = 10.2 nm, *σ*= 2.0 nm).

Fig. 9. Fluorometry of reactive oxygen species (hydrogen peroxide) using folic acid

## **4.2.2 Method of the detection of hydrogen peroxide**

The generated H2O2 was measured by a previously reported method using folic acid (Hirakawa, 2006). This assay is based on the fluorescence enhancement of less-fluorescent folic acid via oxidative decomposition by H2O2 and copper(II) ion into strong-fluorescent 2 amino-4-oxo-3*H*-pterine-6-carboxylic acid (Figure 9). The concentration of H2O2 ([H2O2]) can be determined using a calibration curve. A reaction mixture containing folic acid, copper(II) chloride, and the H2O2 sample (or H2O2 generator 4) with or without the metal nanoparticle in a sodium phosphate buffer (pH 7.6) was incubated in a microtube for 30 min. After incubation at 37 C, the fluorescence intensity of the reaction mixture at 450 nm was measured using a fluorescence spectrophotometer with 350-nm excitation.

## **4.2.3 Platinum nanoparticles effectively scavenge hydrogen peroxide**

Platinum nanoparticles effectively scavenged H2O2 in a dose-dependent manner and showed the highest activity among the metal nanoparticles used in this study (Figure 10). A sample solution of 5 μM/atom Pt nanoparticles, among which 1 μg Pt metal is included, exhibits comparable activity for H2O2 decomposition to that of 10 units of catalase. One unit of catalase can remove 1.0 μmol H2O2 per min in water (pH 7.0, 25 C). Poly(*N*-vinyl-2 pyrrolidone) itself did not scavenge H2O2. This experiment confirmed that poly(*N*-vinyl-2 pyrrolidone)-protected Pt nanoparticles can remove H2O2. The mechanism of H2O2 removal by Pt nanoparticles can be explained by catalytic decomposition into water and molecular oxygen as follows:

Self-Organization of Silver-Core Bimetallic

0

1

2

Absorbance

Average diameter: *d* = 2.2 nm Standard Deviation:

nanoparticles 24 hour after mixing.

catalytic activity per unit atom.

**nanoparticles** 

= 1.0 nm

3

4

2007).

Nanoparticles and Their Application for Catalytic Reaction 79

nanopaticles (Toshima & Hirakawa, 2003). These findings suggest the formation of selforganized Ag/Pt bimetallic nanoparticles. These metal nanoparticles are stable in water for several months. The Ag/Pt (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticles were prepared using a self-organization method to mix Pt and Ag monometallic nanoparticles according to previous reports (Toshima & Hirakawa, 2003; Toshima et al., 2002, 2005; Matsushita et al.,

> 300 400 500 600 Wavelength / nm

50nm 50nm 50nm

*d* = 10.0 nm = 1.9 nm

*d* = 2.6 nm = 1.6 nm

Fig. 11. Absorption spectral change of the physical mixture of dispersions of Ag and Pt nanoparticles. The aqueous solutions of Ag (1 mM/atom, 10 mL) and Pt (1 mM/atom, 10 mL) nanoparticles were mixed and measured at 0, 10, 20, and 30 min, and 24 h after mixing.

Fig. 12. TEM photographs of metal nanoparticles. The sample of Ag/Pt nanoparticles was prepared by drying the mixtures of the aqueous solutions of Pt and Ag monometallic

**4.3.2 Hydrogen peroxide formation from hydroquinone and its removal by metal** 

Hydroquinone, which is a metabolite of carcinogenic benzene, was used as H2O2 source. This compound can generate H2O2 through autooxidation (Figure 13) (Hirakwa et al., 2002). Under these experimental conditions, hydroquinone generated H2O2 in a dose-dependent manner (Figure 14). Twenty units/mL catalase effectively removed H2O2 generated from this system, and 10 μM/atom (2 μg/mL) Pt nanoparticles exhibited a comparable activity to that of this catalase. Silver nanoparticles showed apparently weaker activity for H2O2 removal than Pt nanoparticles. The bimetallization of Pt with Ag apparently suppressed the

(A) Pt (B) Ag (C) Ag/Pt

Time 0 min 10 min 20 min 30 min 24 h

$$\text{H}\_2\text{O}\_2 \to \text{H}\_2\text{O} + 1/2\,\text{O}\_2\tag{2}$$

The generation of O2 gas through the H2O2 decomposition was confirmed with a gas-burette as following procedure. The 10 mL of aqueous solution containing 0.1 M H2O2 was treated by 10 μg Pt nanoparticles and generated O2 gas was measured with a gas-burette. The volume of detected gas coincided with that of the theoretically calculated value of O2 generation from the decomposition of H2O2 in the sample solution.

Fig. 10. Removal of H2O2 by metal nanoparticles and catalase. The 1 mL of sample solution containing 100 μM H2O2, 10 μM folic acid, 20 μM copper(II) chloride, and indicated concentration of metal nanoparticles or catalase was incubated for 30 min. The concentration of H2O2 was estimated from the fluorescence measurement.

#### **4.3 Application of siver-core/platinum-shell bimetallic nanoparticles to catalytic decomposition of hydrogen peroxide generated by chemical compound**

#### **4.3.1 Preparation of silver-core bimetallic nanoparticles for hydrogen peroxide scavenger**

The catalytic activity of Pt and its modified particles with Ag (Ag/Pt) on the decomposition of H2O2 generated from chemical compounds was evaluated, since Pt showed the highest activity. The Ag/Pt nanoparticles were prepared from the following procedure. The absorption spectrum of the sol of Pt nanoparticles is a flat curve (Figure 11), indicating the formation of homogenous particles. Ag nanoparticles exhibited a typical yellow color due to surface plasmon absorption around 400 nm. It has been reported that a physical mixture of Ag and Pt nanoparticles spontaneously forms bimetallic nanoparticles, possibly Ag-core/Ptshell structured particles (Toshima et al., 2005). The time-course of the absorption spectra of this physical mixture showed the extinction of Ag surface plasmon absorption, and the absorption was completely extinguished within 24 h (Figure 11), suggesting that the surface of the formed bimetallic nanoparticles is composed of Pt atoms. Typical TEM images showed the formation of relatively small particles of Pt and large particles of Ag (Figure 12). TEM photographs showed that the large Ag particles disappeared through interaction with Pt particles, resulting in the formation of bimetallic particles smaller than the parent Ag particles (Figure 12). A similar result has been observed in the case of Ag/Rh bimetallic

 H2O2 → H2O + 1/2 O2 (2). The generation of O2 gas through the H2O2 decomposition was confirmed with a gas-burette as following procedure. The 10 mL of aqueous solution containing 0.1 M H2O2 was treated by 10 μg Pt nanoparticles and generated O2 gas was measured with a gas-burette. The volume of detected gas coincided with that of the theoretically calculated value of O2

0 20 40 60 80 100

[Catalase] / units mL-1

0 2 4 6 8 10

[Nanoparticle] / M/atom

Fig. 10. Removal of H2O2 by metal nanoparticles and catalase. The 1 mL of sample solution containing 100 μM H2O2, 10 μM folic acid, 20 μM copper(II) chloride, and indicated

concentration of metal nanoparticles or catalase was incubated for 30 min. The concentration

The catalytic activity of Pt and its modified particles with Ag (Ag/Pt) on the decomposition of H2O2 generated from chemical compounds was evaluated, since Pt showed the highest activity. The Ag/Pt nanoparticles were prepared from the following procedure. The absorption spectrum of the sol of Pt nanoparticles is a flat curve (Figure 11), indicating the formation of homogenous particles. Ag nanoparticles exhibited a typical yellow color due to surface plasmon absorption around 400 nm. It has been reported that a physical mixture of Ag and Pt nanoparticles spontaneously forms bimetallic nanoparticles, possibly Ag-core/Ptshell structured particles (Toshima et al., 2005). The time-course of the absorption spectra of this physical mixture showed the extinction of Ag surface plasmon absorption, and the absorption was completely extinguished within 24 h (Figure 11), suggesting that the surface of the formed bimetallic nanoparticles is composed of Pt atoms. Typical TEM images showed the formation of relatively small particles of Pt and large particles of Ag (Figure 12). TEM photographs showed that the large Ag particles disappeared through interaction with Pt particles, resulting in the formation of bimetallic particles smaller than the parent Ag particles (Figure 12). A similar result has been observed in the case of Ag/Rh bimetallic

**4.3 Application of siver-core/platinum-shell bimetallic nanoparticles to catalytic** 

**4.3.1 Preparation of silver-core bimetallic nanoparticles for hydrogen peroxide** 

**decomposition of hydrogen peroxide generated by chemical compound** 

0

20

40

60

80

Ag Rh Au Pd Pt Catalase 100

generation from the decomposition of H2O2 in the sample solution.

0

of H2O2 was estimated from the fluorescence measurement.

20

40

[H

**scavenger** 

O2 2] / M

60

80

100

nanopaticles (Toshima & Hirakawa, 2003). These findings suggest the formation of selforganized Ag/Pt bimetallic nanoparticles. These metal nanoparticles are stable in water for several months. The Ag/Pt (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticles were prepared using a self-organization method to mix Pt and Ag monometallic nanoparticles according to previous reports (Toshima & Hirakawa, 2003; Toshima et al., 2002, 2005; Matsushita et al., 2007).

Fig. 11. Absorption spectral change of the physical mixture of dispersions of Ag and Pt nanoparticles. The aqueous solutions of Ag (1 mM/atom, 10 mL) and Pt (1 mM/atom, 10 mL) nanoparticles were mixed and measured at 0, 10, 20, and 30 min, and 24 h after mixing.

Fig. 12. TEM photographs of metal nanoparticles. The sample of Ag/Pt nanoparticles was prepared by drying the mixtures of the aqueous solutions of Pt and Ag monometallic nanoparticles 24 hour after mixing.

## **4.3.2 Hydrogen peroxide formation from hydroquinone and its removal by metal nanoparticles**

Hydroquinone, which is a metabolite of carcinogenic benzene, was used as H2O2 source. This compound can generate H2O2 through autooxidation (Figure 13) (Hirakwa et al., 2002). Under these experimental conditions, hydroquinone generated H2O2 in a dose-dependent manner (Figure 14). Twenty units/mL catalase effectively removed H2O2 generated from this system, and 10 μM/atom (2 μg/mL) Pt nanoparticles exhibited a comparable activity to that of this catalase. Silver nanoparticles showed apparently weaker activity for H2O2 removal than Pt nanoparticles. The bimetallization of Pt with Ag apparently suppressed the catalytic activity per unit atom.

Self-Organization of Silver-Core Bimetallic

0

20

40 60

[H

estimated from the fluorescence measurement.

O2 2] / % 80

100

hydroquinone.

**nanoparticles** 

modification with Ag.

Nanoparticles and Their Application for Catalytic Reaction 81

absorbance reader. The oxidized form of hydroquinone can be reduced into the parent hydroquinone by NADH (Hirakwa et al., 2002). The concentration of NADH was gradually decreased through the redox of hydroquinone and Pt nanoparticles hardly inhibited NADH consumption (data not shown). This result indicated that Pt nanoparticles do not inhibit the H2O2 generation itself, because H2O2 is produced through the autooxidation of

0 20 40 60 80 100

[Nanoparticle] / M/atom

Fig. 15. Removal of H2O2 generated through the autooxidation of hydroquinone by metal nanoparticles and catalase. The 1 mL of sample solution containing 10 μM folic acid, 20 μM

**4.4 Summary and possible mechanism of hydrogen peroxide decomposition by metal** 

Poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular Pt nanoparticles, exhibited a removal effect on H2O2 generated through autooxidation of hydroquinone (Figure 16). The removal of H2O2 by these metal nanoparticles can be explained by a catalytic reaction similar to that by catalase, which decomposes H2O2 into H2O and O2. The formation of H2O2 during autooxidation of hydroquinone is through O2•-, which is generated from a reduction of O2 by hydroquinone (Hirakawa et al., 2002). Because the lifetime of O2•-, which dismutates into H2O2 through reaction with H+, is short (~ 0.1 ms), the scavenging of O2•- by a metal nanoparticle can be negligible. The H2O2 removal activity per metal atom of these metal nanoparticles occurred in the following order: Pt > Ag ≈ Ag/Pt. The activities of H2O2 decomposition per metal atom consisting of these metal nanoparticles (μM-H2O2/μM-nanometal) have been estimated, and the resulting values are 4.2, 12.2, and 3.8 for Ag, Pt, and Ag/Pt, respectively. Further, the activity on the surface area of the Ag/Pt nanoparticles (17 μM-H2O2/cm2-nanometal) was also smaller than that of Pt (49 μM-H2O2/cm2-nanometal). These findings showed that the Pt nanoparticles have the highest catalytic activity for H2O2 decomposition in the metal nanoparticles used in this experiment and the activity of Pt nanoparticles is suppressed by

copper(II) chloride, 50 μM hydroquinone, and indicated concentration of metal nanoparticles or catalase was incubated for 30 min. The concentration of H2O2 was

0 20 40 60 80 100

[Catalase] / units mL-1

Ag/Pt Ag Catalase Pt

Fig. 13. Schematic diagram of hydrogen peroxide formation by the autooxidation of hydroquinone

Fig. 14. H2O2 generation through autooxidation of hydroquinone in the absence or presence of metal nanoparticles and catalase. The 1 mL of sample solution containing 10 μM folic acid, 20 μM copper(II) chloride, and indicated concentration of hydroquinone with or without 10 μM/atom metal nanoparticles or 20 units/mL catalase was incubated for 30 min. The concentration of generated H2O2 was estimated from the fluorescence measurement.

#### **4.3.3 Activity of silver-core/platinum-shell nanoparticles on hydrogen peroxide decomposition**

Figure 15 shows the removal activity of H2O2 generated from a high concentration of hydroquinone (50 μM) by metal nanoparticles. These metal nanoparticles and catalase scavenged H2O2 in a dose-dependent manner. The activity of the 10 μM/atom (2 μg/mL) Pt nanoparticles was comparable to that of 20 units/mL catalase, and Pt completely scavenged H2O2 over 20 μM/atom (4 μg/mL). The activity per atom of the Ag/Pt bimetallic nanoparticles was almost the same as that of the Ag monometallic nanoparticles.

To investigate the effect of Pt nanoparticles on H2O2 generation through the autooxidation of hydroquinone, NADH consumption during this autooxidation was measured. The consumption of NADH during the autooxidation of hydroquinone was measured by a previously reported method (Oikawa et al., 2003). A sample solution containing 100 µM NADH, 50 µM hydroquinone, and 20 µM copper(II) chloride was incubated at 37 C in the absence or presence of 20 µM/atom Pt nanoparticles. The concentration of NADH was determined by the measurement of absorbance of NADH at 340 nm using a microplate

O

O

O

+ Ag

+ Ag/Pt

+ Pt + catalase

OH

0 20 40 60 80 100 [Hydroquinone] / M

Fig. 14. H2O2 generation through autooxidation of hydroquinone in the absence or presence of metal nanoparticles and catalase. The 1 mL of sample solution containing 10 μM folic acid, 20 μM copper(II) chloride, and indicated concentration of hydroquinone with or without 10 μM/atom metal nanoparticles or 20 units/mL catalase was incubated for 30 min. The concentration of generated H2O2 was estimated from the fluorescence measurement.

**4.3.3 Activity of silver-core/platinum-shell nanoparticles on hydrogen peroxide** 

nanoparticles was almost the same as that of the Ag monometallic nanoparticles.

Figure 15 shows the removal activity of H2O2 generated from a high concentration of hydroquinone (50 μM) by metal nanoparticles. These metal nanoparticles and catalase scavenged H2O2 in a dose-dependent manner. The activity of the 10 μM/atom (2 μg/mL) Pt nanoparticles was comparable to that of 20 units/mL catalase, and Pt completely scavenged H2O2 over 20 μM/atom (4 μg/mL). The activity per atom of the Ag/Pt bimetallic

To investigate the effect of Pt nanoparticles on H2O2 generation through the autooxidation of hydroquinone, NADH consumption during this autooxidation was measured. The consumption of NADH during the autooxidation of hydroquinone was measured by a previously reported method (Oikawa et al., 2003). A sample solution containing 100 µM NADH, 50 µM hydroquinone, and 20 µM copper(II) chloride was incubated at 37 C in the absence or presence of 20 µM/atom Pt nanoparticles. The concentration of NADH was determined by the measurement of absorbance of NADH at 340 nm using a microplate

O2

2O2 - + 2H+ H2O2 + O2

O2 - + H+

OH

OH

0

50

[H

O2

2] / M

100

hydroquinone

**decomposition** 

O2

Cu2+

O2 -

Cu+ + H+

Fig. 13. Schematic diagram of hydrogen peroxide formation by the autooxidation of

absorbance reader. The oxidized form of hydroquinone can be reduced into the parent hydroquinone by NADH (Hirakwa et al., 2002). The concentration of NADH was gradually decreased through the redox of hydroquinone and Pt nanoparticles hardly inhibited NADH consumption (data not shown). This result indicated that Pt nanoparticles do not inhibit the H2O2 generation itself, because H2O2 is produced through the autooxidation of hydroquinone.

Fig. 15. Removal of H2O2 generated through the autooxidation of hydroquinone by metal nanoparticles and catalase. The 1 mL of sample solution containing 10 μM folic acid, 20 μM copper(II) chloride, 50 μM hydroquinone, and indicated concentration of metal nanoparticles or catalase was incubated for 30 min. The concentration of H2O2 was estimated from the fluorescence measurement.

### **4.4 Summary and possible mechanism of hydrogen peroxide decomposition by metal nanoparticles**

Poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular Pt nanoparticles, exhibited a removal effect on H2O2 generated through autooxidation of hydroquinone (Figure 16). The removal of H2O2 by these metal nanoparticles can be explained by a catalytic reaction similar to that by catalase, which decomposes H2O2 into H2O and O2. The formation of H2O2 during autooxidation of hydroquinone is through O2•-, which is generated from a reduction of O2 by hydroquinone (Hirakawa et al., 2002). Because the lifetime of O2•-, which dismutates into H2O2 through reaction with H+, is short (~ 0.1 ms), the scavenging of O2•- by a metal nanoparticle can be negligible. The H2O2 removal activity per metal atom of these metal nanoparticles occurred in the following order: Pt > Ag ≈ Ag/Pt. The activities of H2O2 decomposition per metal atom consisting of these metal nanoparticles (μM-H2O2/μM-nanometal) have been estimated, and the resulting values are 4.2, 12.2, and 3.8 for Ag, Pt, and Ag/Pt, respectively. Further, the activity on the surface area of the Ag/Pt nanoparticles (17 μM-H2O2/cm2-nanometal) was also smaller than that of Pt (49 μM-H2O2/cm2-nanometal). These findings showed that the Pt nanoparticles have the highest catalytic activity for H2O2 decomposition in the metal nanoparticles used in this experiment and the activity of Pt nanoparticles is suppressed by modification with Ag.

Self-Organization of Silver-Core Bimetallic

induced by various chemical compounds.

activity of other metal nanoparticles.

through catalytic decomposition.

**radiation and its problem** 

**molecules** 

Nanoparticles and Their Application for Catalytic Reaction 83

mentioned above, the application of metal nanoparticles to scavenge reactive oxygen species

**5.1 Traditional methods of chemoprevention to biomolecules damage by ultraviolet** 

**5.2 Preventive action of metal nanoparticles on ultraviolet-sensitized oxidation of** 

chemopreventive agents against UVA-induced biomolecules damage.

**5.2.1 Preparation of metal nanoparticles for ultraviolet protection** 

As mentioned above, metal nanoparticles catalyze the decomposition of reactive oxygen species. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel

Recently, it has been reported that the removal of reactive oxygen species generated from a photocatalytic reaction of titanium dioxide (TiO2) particles using water-soluble polymerprotected Pt, Rh, and Pt/Ag bimetallic nanoparticles. Silver, a relatively inexpensive noble metal, is also used as a food additive, and bimetallization with Ag may improve the catalytic

The colloidal dispersions of poly(*N*-vinyl-2-pyrrolidone)-protected Pt and Rh nanoparticles were prepared from an alcohol reduction. The size (particle diameter) of these nanoparticles is about 2 nm. The aqueous solution of poly(*N*-vinyl-2-pyrrolidone)-protected Ag nanoparticle was prepared from a reduction of silver nitrate by sodium borohydride in the presence of poly(*N*-vinyl-2-pyrrolidone). The Ag-core/Pt-shell (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticle was prepared using a physical method to mix Pt and Ag

TiO2 (anatase) and methylene blue were used as a model of the UVA-induced reaction. The sample solution containing methylene blue and TiO2 dispersion in sodium phosphate buffer (pH 7.6) with or without metal nanoparticle was irradiated with a UVA lamp (365 nm, 1

monometallic nanoparticles according to the previous reports (Toshima et al., 2005).

**5.2.2 Evaluation model for the biomolecules damage by ultraviolet radiation** 

Many studies have addressed the role of antioxidants, such as vitamins and catechins, in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is, in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer. It has been reported that antioxidants, ascorbic acid and dithiothreitol, exhibit cytotoxicity via H2O2 generation, and their toxic effects are significantly enhanced by vitamin B12. H2O2 is a longlived reactive oxygen species which plays an important role in biomacromolecules damage

Fig. 16. Hydrogen peroxide generation from an autooxidation of chemical compound and its catalytic decomposition by metal nanoparticle

H2O2 is a long-lived reactive oxygen species and plays an important role in DNA damage (Kawanishi et al., 2001, Hirakawa et al., 2002). Indeed, various chemical compounds, including carcinogens, generate H2O2 during redox reaction (Kawanishi et al., 2001, Hirakawa et al., 2002). Molecular oxygen is easily reduced by various compounds, leading to the formation of O2•-. Formed O2•- is rapidly dismutated into H2O2. Although H2O2 itself is not a strong reactive species, it can generate highly reactive HO• through a Fenton reaction or a Haber-Weiss reaction. Furthermore, H2O2 can penetrate a cytoplasm membrane and be incorporated into the cell nucleus. Therefore, H2O2 is considered to be one of the most important reactive species or a precursor participating in carcinogenesis. The removal of H2O2 is an effective method for cancer chemoprevention. Furthermore, protective agents against H2O2 are important to treat *acatalasemia,* a genetic deficiency of erythrocyte catalase inherited as an autosomal recessive trait. Antioxidants, such as vitamins A and E, are effective protective agents. However, the oxidized products of antioxidants or these molecules themselves promote the formation of secondary H2O2 (Yamashita et al., 1998; Murata & Kawanishi, 2000). Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al., 1991; Omenn et al., 1996). A catalyst consisting of an inorganic stable material is not oxidized and does not generate secondary reactive oxygen species. Water-soluble nanoparticles of noble metal may become novel protective agents against reactive oxygen species.

In summary, Pt, Ag, and Ag/Pt nanoparticles effectively scavenge H2O2 generated from autooxidation of a highly concentrated hydroquinone. Platinum nanoparticles exhibited the highest catalytic activity among these nanoparticles. Pt is a very stable metal against various chemical compounds and permitted as a food additives. The noble metal nanoparticles may be used as novel chemopreventive agents for cancer or other non-malignant conditions induced by chemical compounds through H2O2 generation.

### **5. Application of metal nanoparticles to prevention of ultraviolet radiation induced biomolecules damage**

Exposure to solar ultraviolet radiation is undoubtedly linked to skin carcinogenesis and phototoxic effect. Photosensitized reaction by ultraviolet radiation, especially ultraviolet-A (UVA) radiation (320~400 nm), is considered to cause toxic effect through oxidative biomolecules damage including DNA damage (Hiraku et al., 2007). Photosensitized formation of reactive oxygen species, such as hydrogen peroxide, superoxide, hydroxyl radicals, and singlet oxygen, is involved in UVA-induced biomolecules damage. As

H2O + O <sup>2</sup> 1 2

> Oxidized product

Fig. 16. Hydrogen peroxide generation from an autooxidation of chemical compound and its

H2O2 is a long-lived reactive oxygen species and plays an important role in DNA damage (Kawanishi et al., 2001, Hirakawa et al., 2002). Indeed, various chemical compounds, including carcinogens, generate H2O2 during redox reaction (Kawanishi et al., 2001, Hirakawa et al., 2002). Molecular oxygen is easily reduced by various compounds, leading

is not a strong reactive species, it can generate highly reactive HO• through a Fenton reaction or a Haber-Weiss reaction. Furthermore, H2O2 can penetrate a cytoplasm membrane and be incorporated into the cell nucleus. Therefore, H2O2 is considered to be one of the most important reactive species or a precursor participating in carcinogenesis. The removal of H2O2 is an effective method for cancer chemoprevention. Furthermore, protective agents against H2O2 are important to treat *acatalasemia,* a genetic deficiency of erythrocyte catalase inherited as an autosomal recessive trait. Antioxidants, such as vitamins A and E, are effective protective agents. However, the oxidized products of antioxidants or these molecules themselves promote the formation of secondary H2O2 (Yamashita et al., 1998; Murata & Kawanishi, 2000). Indeed, an excess of these antioxidants elevates the incidence of cancer (Nitta et al., 1991; Omenn et al., 1996). A catalyst consisting of an inorganic stable material is not oxidized and does not generate secondary reactive oxygen species. Water-soluble nanoparticles of noble metal may become novel protective agents

In summary, Pt, Ag, and Ag/Pt nanoparticles effectively scavenge H2O2 generated from autooxidation of a highly concentrated hydroquinone. Platinum nanoparticles exhibited the highest catalytic activity among these nanoparticles. Pt is a very stable metal against various chemical compounds and permitted as a food additives. The noble metal nanoparticles may be used as novel chemopreventive agents for cancer or other non-malignant conditions

**5. Application of metal nanoparticles to prevention of ultraviolet radiation** 

Exposure to solar ultraviolet radiation is undoubtedly linked to skin carcinogenesis and phototoxic effect. Photosensitized reaction by ultraviolet radiation, especially ultraviolet-A (UVA) radiation (320~400 nm), is considered to cause toxic effect through oxidative biomolecules damage including DNA damage (Hiraku et al., 2007). Photosensitized formation of reactive oxygen species, such as hydrogen peroxide, superoxide, hydroxyl radicals, and singlet oxygen, is involved in UVA-induced biomolecules damage. As

O2 •- H+

Chemical compound

catalytic decomposition by metal nanoparticle

to the formation of O2•-. Formed O2

against reactive oxygen species.

**induced biomolecules damage** 

induced by chemical compounds through H2O2 generation.

O2

H2O2

**M**

•- is rapidly dismutated into H2O2. Although H2O2 itself

mentioned above, the application of metal nanoparticles to scavenge reactive oxygen species through catalytic decomposition.

## **5.1 Traditional methods of chemoprevention to biomolecules damage by ultraviolet radiation and its problem**

Many studies have addressed the role of antioxidants, such as vitamins and catechins, in protection against cancers and cardiovascular diseases. These antioxidants can scavenge reactive oxygen species and protect against cancer occurrence. On the other hand, every antioxidant is, in fact, a redox agent, protecting against reactive oxygen species in some circumstances and promoting free radical or secondary reactive oxygen species generation in others. Indeed, an excess of these antioxidants elevates the incidence of cancer. It has been reported that antioxidants, ascorbic acid and dithiothreitol, exhibit cytotoxicity via H2O2 generation, and their toxic effects are significantly enhanced by vitamin B12. H2O2 is a longlived reactive oxygen species which plays an important role in biomacromolecules damage induced by various chemical compounds.

## **5.2 Preventive action of metal nanoparticles on ultraviolet-sensitized oxidation of molecules**

As mentioned above, metal nanoparticles catalyze the decomposition of reactive oxygen species. Because this removal mechanism is catalytic decomposition, no oxidized product is formed through this reaction. Platinum metal is used as a food additive and is not considered to be a toxic material. This result led us to the idea that inorganic materials, in particular noble metals, rather than organic antioxidants, can be used as novel chemopreventive agents against UVA-induced biomolecules damage.

Recently, it has been reported that the removal of reactive oxygen species generated from a photocatalytic reaction of titanium dioxide (TiO2) particles using water-soluble polymerprotected Pt, Rh, and Pt/Ag bimetallic nanoparticles. Silver, a relatively inexpensive noble metal, is also used as a food additive, and bimetallization with Ag may improve the catalytic activity of other metal nanoparticles.

## **5.2.1 Preparation of metal nanoparticles for ultraviolet protection**

The colloidal dispersions of poly(*N*-vinyl-2-pyrrolidone)-protected Pt and Rh nanoparticles were prepared from an alcohol reduction. The size (particle diameter) of these nanoparticles is about 2 nm. The aqueous solution of poly(*N*-vinyl-2-pyrrolidone)-protected Ag nanoparticle was prepared from a reduction of silver nitrate by sodium borohydride in the presence of poly(*N*-vinyl-2-pyrrolidone). The Ag-core/Pt-shell (Ag-atom/Pt-atom, 1/1) bimetallic nanoparticle was prepared using a physical method to mix Pt and Ag monometallic nanoparticles according to the previous reports (Toshima et al., 2005).

## **5.2.2 Evaluation model for the biomolecules damage by ultraviolet radiation**

TiO2 (anatase) and methylene blue were used as a model of the UVA-induced reaction. The sample solution containing methylene blue and TiO2 dispersion in sodium phosphate buffer (pH 7.6) with or without metal nanoparticle was irradiated with a UVA lamp (365 nm, 1

Self-Organization of Silver-Core Bimetallic

catalyst, these effects can be negligible.

0

(Ex = 365 nm, 1 mW cm-2) for 30 min.

generation.

**6. Conclusion** 

20

40

[Methylene

blue] (%)

60

80

100

Nanoparticles and Their Application for Catalytic Reaction 85

The UV-Vis absorption spectra of these metal nanoparticles were hardly changed by the photocatalytic reaction, suggesting that the noble metal nanoparticles are stable for reactive oxygen species and UVA irradiation. Organic antioxidant undergoes oxidation in the removal process of reactive oxygen species, leading to the formation of various oxidized products and may produce secondary reactive oxygen species. In the case of noble metal

> 0 200 400 600 800 1000 [TiO2 ] / g mL-1

Pt, Rh, and Pt/Ag nanoparticles effectively inhibited the methylene blue decomposition photocatalyzed by TiO2. TiO2 photocatalytic system was used as a UVA-induced reactive oxygen species generation. The most important reactive oxygen species in this photocatalytic reaction is H2O2, because of its long lifetime in aqueous solution. This inhibitory effect of metal nanoparticle can be explained by the removal of H2O2. Unexpectedly, the activity of Pt nanoparticle was not improved by the bimetallization with Ag. Platinum is a very stable metal against various chemical compounds and is used as food additive. A poly(*N*-vinyl-2-pyrrolidone)-protected Pt nanoparticle may be used as a novel preventive agent for UVA-induced biomolecules damage through reactive oxygen species

Physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. These monometallic nanoparticles can be easily prepared from an alcohol reduction of the corresponding metal ions in the presence of water-soluble polymer such

Fig. 18. Inhibitory effect of metal nanoparticles on methylene blue decomposition photocatalyzed by TiO2. The sample solution containing 20 μg mL-1 metal nanoparticle, TiO2, and 10 μM methylene blue in 10 mM sodium phosphate buffer (pH 7.6) was irradiated

**5.3 Summary of the ultraviolet protection by metal nanoparticles** 

without metal

Pt Rh Pt / Ag

mW cm-2). The decomposition of methylene blue was evaluated by absorption measurement at 659 nm. TiO2 is a well-known photocatalyst (Fujishima et al., 2000, 2008). When exposing to UVA light, the reduction-oxidation activity of TiO2 has a significant biological impact, as is exemplified by its bactericidal activity. Photo-irradiated TiO2 effectively decomposed methylene blue (Figure 17). Various reactive oxygen species contribute to the photocatalytic reaction of TiO2. Especially, hydrogen peroxide is long-lived reactive oxygen species and plays an important role in oxidative biomolecules damage. Molecular oxygen is reduced by photoexcited materials, leading to the formation of superoxide. Formed superoxide is rapidly dismutated into hydrogen peroxide. Although hydrogen peroxide itself is not a strong reactive species, it can generate highly reactive hydroxyl radicals through a Fenton reaction or a Haber-Weiss reaction. Furthermore, hydrogen peroxide can penetrate a cytoplasm membrane and be incorporated into the cell nucleous. Therefore, hydrogen peroxide is considered to be one of the most important reactive oxygen species participating in UVA carcinogenesis and phototoxicity. Since other reactive oxygen species, such as directly produced hydroxyl radicals (Hirakawa et al., 2004) and singlet oxygen (Hirakawa & Hirano, 2006), rapidly quenched in aqueous solution, hydrogen peroxide should be key reactive species in this experiment. The TiO2 and methylene blue could be used as a simple model of UVA-induced oxidation.

Fig. 17. UV-Vis absorption spectra of methylene blue photocatalyzed by TiO2. The sample solution containing 10 μM methylene blue and indicated concentration of TiO2 in 10 mM sodium phosphate buffer (pH 7.6) was irradiated (Ex = 365 nm, 1 mW cm-2) for 30 min.

#### **5.2.3 Preventive action of metal nanoparticles on ultraviolet radiation induced biomolecules damage**

Poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular, the Pt nanoparticle, inhibited the methylene blue decomposition photocatalyzed by TiO2 (Figure 18). Poly(*N*-vinyl-2-pyrrolidone) itself did not inhibit the methylene blue decomposition. This inhibitory effect can be explained by the catalytic decomposition of H2O2 generated through TiO2 photocatalysis. These nanoparticles decomposed H2O2 into H2O and O2 similar to catalase. In the case of H2O2 decomposition, the Pt nanoparticle showed the highest catalytic activity per unit atom. The activity of a 1 μg Pt nanoparticle was comparable to that of 5 units of catalase. One unit of catalase can remove 1.0 μmol H2O2 per 1 min in water (pH 7.0, 25C). Unexpectedly, the bimetallization with Ag did not show improvement effect and rather decreased the inhibitory effect of Pt nanoparticle on the decomposition of methylene blue.

mW cm-2). The decomposition of methylene blue was evaluated by absorption measurement at 659 nm. TiO2 is a well-known photocatalyst (Fujishima et al., 2000, 2008). When exposing to UVA light, the reduction-oxidation activity of TiO2 has a significant biological impact, as is exemplified by its bactericidal activity. Photo-irradiated TiO2 effectively decomposed methylene blue (Figure 17). Various reactive oxygen species contribute to the photocatalytic reaction of TiO2. Especially, hydrogen peroxide is long-lived reactive oxygen species and plays an important role in oxidative biomolecules damage. Molecular oxygen is reduced by photoexcited materials, leading to the formation of superoxide. Formed superoxide is rapidly dismutated into hydrogen peroxide. Although hydrogen peroxide itself is not a strong reactive species, it can generate highly reactive hydroxyl radicals through a Fenton reaction or a Haber-Weiss reaction. Furthermore, hydrogen peroxide can penetrate a cytoplasm membrane and be incorporated into the cell nucleous. Therefore, hydrogen peroxide is considered to be one of the most important reactive oxygen species participating in UVA carcinogenesis and phototoxicity. Since other reactive oxygen species, such as directly produced hydroxyl radicals (Hirakawa et al., 2004) and singlet oxygen (Hirakawa & Hirano, 2006), rapidly quenched in aqueous solution, hydrogen peroxide should be key reactive species in this experiment. The TiO2 and methylene blue could be used as a simple

model of UVA-induced oxidation.

Absorbance

**biomolecules damage** 

decomposition of methylene blue.

0 0.5 1 1.5 2 2.5

> 250 350 450 550 650 750 Wavelength / nm

Fig. 17. UV-Vis absorption spectra of methylene blue photocatalyzed by TiO2. The sample solution containing 10 μM methylene blue and indicated concentration of TiO2 in 10 mM sodium phosphate buffer (pH 7.6) was irradiated (Ex = 365 nm, 1 mW cm-2) for 30 min.

Poly(*N*-vinyl-2-pyrrolidone)-protected metal nanoparticles, in particular, the Pt nanoparticle, inhibited the methylene blue decomposition photocatalyzed by TiO2 (Figure 18). Poly(*N*-vinyl-2-pyrrolidone) itself did not inhibit the methylene blue decomposition. This inhibitory effect can be explained by the catalytic decomposition of H2O2 generated through TiO2 photocatalysis. These nanoparticles decomposed H2O2 into H2O and O2 similar to catalase. In the case of H2O2 decomposition, the Pt nanoparticle showed the highest catalytic activity per unit atom. The activity of a 1 μg Pt nanoparticle was comparable to that of 5 units of catalase. One unit of catalase can remove 1.0 μmol H2O2 per 1 min in water (pH 7.0, 25C). Unexpectedly, the bimetallization with Ag did not show improvement effect and rather decreased the inhibitory effect of Pt nanoparticle on the

**5.2.3 Preventive action of metal nanoparticles on ultraviolet radiation induced** 

<sup>80</sup> Methylene blue

[TiO2] / g mL-1 0 8 16 40

The UV-Vis absorption spectra of these metal nanoparticles were hardly changed by the photocatalytic reaction, suggesting that the noble metal nanoparticles are stable for reactive oxygen species and UVA irradiation. Organic antioxidant undergoes oxidation in the removal process of reactive oxygen species, leading to the formation of various oxidized products and may produce secondary reactive oxygen species. In the case of noble metal catalyst, these effects can be negligible.

Fig. 18. Inhibitory effect of metal nanoparticles on methylene blue decomposition photocatalyzed by TiO2. The sample solution containing 20 μg mL-1 metal nanoparticle, TiO2, and 10 μM methylene blue in 10 mM sodium phosphate buffer (pH 7.6) was irradiated (Ex = 365 nm, 1 mW cm-2) for 30 min.

## **5.3 Summary of the ultraviolet protection by metal nanoparticles**

Pt, Rh, and Pt/Ag nanoparticles effectively inhibited the methylene blue decomposition photocatalyzed by TiO2. TiO2 photocatalytic system was used as a UVA-induced reactive oxygen species generation. The most important reactive oxygen species in this photocatalytic reaction is H2O2, because of its long lifetime in aqueous solution. This inhibitory effect of metal nanoparticle can be explained by the removal of H2O2. Unexpectedly, the activity of Pt nanoparticle was not improved by the bimetallization with Ag. Platinum is a very stable metal against various chemical compounds and is used as food additive. A poly(*N*-vinyl-2-pyrrolidone)-protected Pt nanoparticle may be used as a novel preventive agent for UVA-induced biomolecules damage through reactive oxygen species generation.

## **6. Conclusion**

Physical mixture of Ag and other metal nanoparticles, such as Pt, Rh, and Pd, spontaneously forms the bimetallic nanoparticles with Ag-core structure in aqueous solution. These monometallic nanoparticles can be easily prepared from an alcohol reduction of the corresponding metal ions in the presence of water-soluble polymer such

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as poly(*N*-vinyl-2-pyrrolidone), a protective colloid. Aqueous sol of Ag nanoparticles exhibits the surface plasmon absorption around 400 nm. The surface plasmon absorption was diminished through interaction with other metal nanoparticle in the physical mixture of these nanoparticles. This phenomenon was explained by that the Ag nanoparticle was coated by other metal. The transmission electron micrograph and X-ray diffraction measurement confirmed the formation of the Ag-core bimetallic nanoparticles. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles act superior character for certain catalytic reactions. The catalytic activity of the silver/rhodium bimetallic nanoparticles for visible-light-induced hydrogen generation in an aqueous solution was examined. This system composed of an electron source, a photosensitizer, an electron relay, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles, suggesting that the silver-core enhances the catalytic activity of the surface rhodium. On the other hand, the catalytic activity of the decomposition of hydrogen peroxide was decreased by this bimetallization. Platinum nanoparticle effectively catalyzes hydrogen peroxide decomposition. The Ag-core/platinum shell bimetallic nanoparticle, which was prepared by the physical mixing of Ag and Pt nanoparticles, demonstrated lower activity of the decomposition of hydrogen peroxide than the monometallic Pt nanoparticle. Metal nanoparticles can be applied to various catalytic reactions. The bimetallic and trimetallic nanoparticles demonstrate superior activity in the certain reaction. The self-assembly formation of Ag-cored nanoparticle may be convenient method to prepare novel metal nanoparticle catalyst.

## **7. Acknowledgments**

The author wish to thank Professor Naoki Toshima (Tokyo University of Science, Yamaguchi) for his helpful discussion and Professor Kenji Murakami (Research Institute of Electronics, Shizuoka University) for his helpful advice on TEM measurement. These works were supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese Government.

### **8. References**


as poly(*N*-vinyl-2-pyrrolidone), a protective colloid. Aqueous sol of Ag nanoparticles exhibits the surface plasmon absorption around 400 nm. The surface plasmon absorption was diminished through interaction with other metal nanoparticle in the physical mixture of these nanoparticles. This phenomenon was explained by that the Ag nanoparticle was coated by other metal. The transmission electron micrograph and X-ray diffraction measurement confirmed the formation of the Ag-core bimetallic nanoparticles. This reaction can be used to construct the core-shell structured novel bimetallic nanoparticles. The formed nanoparticles act superior character for certain catalytic reactions. The catalytic activity of the silver/rhodium bimetallic nanoparticles for visible-light-induced hydrogen generation in an aqueous solution was examined. This system composed of an electron source, a photosensitizer, an electron relay, and metal nanoparticle catalyst. The activity is clearly higher than the corresponding monometallic nanoparticles, suggesting that the silver-core enhances the catalytic activity of the surface rhodium. On the other hand, the catalytic activity of the decomposition of hydrogen peroxide was decreased by this bimetallization. Platinum nanoparticle effectively catalyzes hydrogen peroxide decomposition. The Ag-core/platinum shell bimetallic nanoparticle, which was prepared by the physical mixing of Ag and Pt nanoparticles, demonstrated lower activity of the decomposition of hydrogen peroxide than the monometallic Pt nanoparticle. Metal nanoparticles can be applied to various catalytic reactions. The bimetallic and trimetallic nanoparticles demonstrate superior activity in the certain reaction. The self-assembly formation of Ag-cored nanoparticle may be convenient method to prepare novel metal

The author wish to thank Professor Naoki Toshima (Tokyo University of Science, Yamaguchi) for his helpful discussion and Professor Kenji Murakami (Research Institute of Electronics, Shizuoka University) for his helpful advice on TEM measurement. These works were supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of the Japanese

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**5** 

 *Japan* 

Motonari Adachi et al.\*

**Utilization of Nanoparticles Produced** 

**1-D TiO2 for Dye-Sensitized Solar Cells** 

*Fuji Chemical Co., Ltd., 1-35-1 Deyashikinishi-Machi, Hirakata,* 

**by Aqueous-Solution Methods – Formation** 

**of Acid Sites on CeO2-TiO2 Composite and** 

Nanoparticles with well-defined nanostructures with unique physical properties are assembled into optoelectronic (Colvin et al. 1994), and nano electronic (Fuhrer et al. 2000) devices and other functional materials (Morris et al. 1999). Highly crystallized nanoparticles can be produced by aqueous-solution methods which provide low cost and ease of

In this chapter two utilizations of nanoparticles are presented. First one is formation of acid sites on CeO2-TiO2 composite. Cerium dioxide has an unusual ability to shift easily between the reduced and oxidized states (Ce3+ ⇄ Ce4+). This ability coupled with a high oxygen transport capacity gives a unique property of catalysis. Based on the remarkable properties of cerium dioxide, catalytic activity of nanoscale composite of CeO2-TiO2 was studied with variation in composition and formation temperature, which brought change in the number

The second one is 1-D TiO2 for dye-sensitized solar cells (DSSCs). We succeeded in the preparation of titania nanorods (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains. All cells composed of these highly crystallized 1-dimensional titania nanoscale materials (1DTNM) show high power

Keizo Nakagawa2, Yusuke Murata3, Masahiro Kishida4, Masahiko Hiro5, Kenzo Susa6,

*3Toyo Tanso Co., Ltd., 5-7-12 Takeshima, Nishiyodogawa-ku, Osaka, Japan 4Graduate School of Engineering, Kyushu University,744 Motooka, Nishi-ku, Fukuoka, Japan 5Hitachi Chemical Co., Ltd., 2-1-1 Nishishinjuku, Shinjuku, Tokyo, Japan 6Trial Corporation., 2-195 Asahi, Kitamoto, Japan* 

*7National Instituite of Biomedical Innovation, 7-6-8 Asagi Saito, Ibaraki, Japan* 

*1Fuji Chemical Co., Ltd., 1-35-1 Deyashikinishi-machi, Hirakata, Japan 2Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima,* 

*8The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Japan* 

of Lewis acid site together with morphological changes.

Jun Adachi7, Jinting Jiu8 and Fumio Uchida1

*Minami-josanjima, Tokushima, Japan* 

**1. Introduction** 

fabrication.

 \*

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## **Utilization of Nanoparticles Produced by Aqueous-Solution Methods – Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells**

 Motonari Adachi et al.\* *Fuji Chemical Co., Ltd., 1-35-1 Deyashikinishi-Machi, Hirakata, Japan* 

## **1. Introduction**

92 Smart Nanoparticles Technology

Yonezawa, T. & Toshima, N. (1995). Mechanistic Consideration of Formation of Polymer-

*Transactions*, Vol.91, No.22, (November 1995), pp.4111-4119, ISSN 0956-5000

ISSN 1381-1169

Methods. *Journal of Molecular Catalysis*, Vol.83, No.1-2, (July 1993), pp.167-181,

protected Nanoscopic Bimetallic Clusters. *Journal of the Chemical Society, Faraday* 

Nanoparticles with well-defined nanostructures with unique physical properties are assembled into optoelectronic (Colvin et al. 1994), and nano electronic (Fuhrer et al. 2000) devices and other functional materials (Morris et al. 1999). Highly crystallized nanoparticles can be produced by aqueous-solution methods which provide low cost and ease of fabrication.

In this chapter two utilizations of nanoparticles are presented. First one is formation of acid sites on CeO2-TiO2 composite. Cerium dioxide has an unusual ability to shift easily between the reduced and oxidized states (Ce3+ ⇄ Ce4+). This ability coupled with a high oxygen transport capacity gives a unique property of catalysis. Based on the remarkable properties of cerium dioxide, catalytic activity of nanoscale composite of CeO2-TiO2 was studied with variation in composition and formation temperature, which brought change in the number of Lewis acid site together with morphological changes.

The second one is 1-D TiO2 for dye-sensitized solar cells (DSSCs). We succeeded in the preparation of titania nanorods (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains. All cells composed of these highly crystallized 1-dimensional titania nanoscale materials (1DTNM) show high power

<sup>\*</sup> Keizo Nakagawa2, Yusuke Murata3, Masahiro Kishida4, Masahiko Hiro5, Kenzo Susa6, Jun Adachi7, Jinting Jiu8 and Fumio Uchida1

*<sup>1</sup>Fuji Chemical Co., Ltd., 1-35-1 Deyashikinishi-machi, Hirakata, Japan 2Department of Advanced Materials, Institute of Technology and Science, The University of Tokushima, Minami-josanjima, Tokushima, Japan* 

*<sup>3</sup>Toyo Tanso Co., Ltd., 5-7-12 Takeshima, Nishiyodogawa-ku, Osaka, Japan 4Graduate School of Engineering, Kyushu University,744 Motooka, Nishi-ku, Fukuoka, Japan 5Hitachi Chemical Co., Ltd., 2-1-1 Nishishinjuku, Shinjuku, Tokyo, Japan 6Trial Corporation., 2-195 Asahi, Kitamoto, Japan* 

*<sup>7</sup>National Instituite of Biomedical Innovation, 7-6-8 Asagi Saito, Ibaraki, Japan* 

*<sup>8</sup>The Institute of Scientific and Industrial Research (ISIR), Osaka University, 8-1 Mihogaoka, Ibaraki, Japan* 

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

alkoxides and by changing the calcination temperature.

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 95

composite nanostructures are influenced by changing the mole ratio of cerium/titanium

The preparation method of CeO2 nanoparticles and CeO2-TiO2 composite are based on the aqueous solution system including metal alkoxides and amine surfactant molecules. The experimental procedure has been described in detail in our previous papers (Murata & Adachi, 2004; Nakagawa et al., 2007). The typical synthesis was as follows: first, laurylamine hydrochloride (LAHC) was dissolved in distilled water. Cerium tri-isopropoxide (CTIP) or cerium *n*-butoxide (CeBu) was used as a cerium source. Tetraisopropyl orthotitanate (TIPT) was used as a titanium source. In the synthesis of CeO2 nanoparticles, CTIP or CeBu was mixed with acetylacetone (ACA) in a beaker and immediately added to an aqueous LAHC solution at pH 4.6. In the case of the synthesis of CeO2–TiO2 composite nanostructures, the mole ratio of CeBu to TIPT (CeBu/TIPT) was changed to 100/0, 75/25, 25/75 and 0/100. Each mixed alkoxide solution was mixed with acetylacetone. In all cases, the mole ratio of metal alkoxides to ACA and metal alkoxides to LAHC were 1 and 4, respectively. After stirring at room temperature for 1 h, the reaction temperature was then changed to 353 K. When the two solutions were mixed, precipitation occurred immediately. After 1 week, the precipitates were separated by centrifugation. After washing with 2-propanol and successive centrifugation, the obtained products were dried through a combination of

**2.1 Preparation of CeO2 nanoparticles and CeO2-TiO2 composite nanostructure** 

freeze-drying and vacuum drying, and calcined in air at different temperatures.

The formation yield of CeO2 particles for the surfactant assisted-process was 100% approximately. The structure of CeO2 nanoparticles was studied by TEM image of CeO2 sample in a dried state. During the formation process, we observed systematic changes in color of the precipitated particles. After mixing of the solution of CTIP or CeBu modified with ACA with the aqueous solution of LAHC, the brown transparent original solution immediately became dark brown. The white colloidal suspension was formed after stirring for 1 h at room temperature. A brown and clear supernatant was formed after the precipitation. Further color change of the precipitate was observed. First, the color of the CeO2 particles changed from white to dark blue in about 1 day at 353 K. Subsequently, the color of the precipitate gradually turned into pale purple for 1 week, but the color change was slower than the first change. Moreover the wet centrifuged precipitate appeared dark blue, and the freeze-dried powders were gray. But final CeO2 particles calcined at 673 K was light yellow. These changes in color were observed in the cases that the particles were synthesized in LAHC surfactant aqueous solution at pH 4.2. On the other hand, there was no color change of the precipitate without LAHC surfactant. These color changes are related to the valence state of the Ce; most likely purple corresponds to Ce3+ and yellow corresponds to Ce4+. Therefore, it is clear that the Ce3+ oxide is stabilized by existence of

We succeeded in the preparation of CeO2 nanoparticles with cubic structures and 1D, 2D or 3D CeO2 nanostructures by assembling the cubic-shape CeO2 nanoparticle building blocks (Murata & Adachi, 2004, Nakagawa et al., 2007) as shown in Fig. 1, 2 and 3. It is evident

**2.2 Cubic CeO2 nanoparticles and their assembled structures** 

LAHC in aqueous solution.

conversion efficiency about 9 %. We also present necessity of 1DTNM for attainment high efficiency theoretically based on the consideration of electron transport processes in the titania electrode and then present that it is indispensable to use highly crystallized 1DTNM for attainment of higher efficient DSSCs based on the analysis of experimental results obtained by electrochemical impedance spectroscopy (EIS) and I-V measurements.

## **2. CeO2-TiO2 composite as a catalyst**

Ceria-based materials are major compounds of the rare earth family, and these have been extensively studied and found application as ultraviolet absorbers (Masui et al., 1997, 2000), solid electrolytes (Inaba & Tagawa, 1996), so-called three-way catalysts for automotive exhaust catalysts (Bekyarova et al., 1998), and soot oxidation catalysts (Pisarello et al., 2002; Aneggi et al., 2006). Nanocrystalline ceria materials have received much attention owing to their physical and chemical properties, which are markedly different from those of the bulk materials. Of particularly interest, the electronic conductivity of CeO2 can be enhanced four orders of magnitude when its microstructure is changed from the micro- to nanocrystalline region (Chiang et al., 1996). Various aqueous solution-based methods for synthesizing crystallized CeO2 nanoparticles (Masui et al., 2002a; Hirano et al., 2000; Li et al., 2001; Wu et al., 2002; Zhou et al., 2003; Bumajdad et al., 2004) and 1D, 2D and 3D CeO2 nanostructures with different morphologies (Vantomme et al., 2005, Zhou et al., 2005, Kuiry et al., 2005, Ho et al., 2005; Han et al., 2005; Sun et al., 2006; Zhong et al., 2007) have been investigated. Some of the properties of these materials, such as the dispersibility of the particles (Masui et al., 2002a) and their catalytic properties (Masui et al., 1997; Sun et al., 2006; Zhong et al., 2007) have also been studied.

The features of CeO2 in these applications are mainly due to the unique combination of its elevated oxygen transport capacity, coupled with its ability to shift easily between the reduced and oxidized states (Ce3+↔Ce4+). To increase the temperature stability and ability of ceria to store and release oxygen, other transition and non transition metal ions (such as Al3+, Si4+, Ti4+ and Zr4+) are normally introduced into the ceria cubic structure (Reddy et al., 2003, 2005; Rynkowski et al., 2000; Masui et al., 2002b). The redox and catalytic properties of CeO2 are strongly influenced when it is combined with other transition metals. In addition, when the particle size is decreased below 100 nm, the materials become nanophasic, where the density of defects increases, such that up to half (50%) of the atoms are situated in the cores of the defects, promoting fast catalyst activation and reaction kinetics (Reddy et al., 2005). Thus, a study of the synthesis and reaction characteristics of nano-sized ceria-based mixed oxides is very important for utilizing the oxygen transport capacity and redox properties. One of the main disadvantages of ceria-based nanoparticles prepared in aqueous solution, however, is the resultant hard agglomeration of the fine particles, which has posed a major challenge to the realization of the full potential of nanocrystalline CeO2 powders.

In this section, first we present the preparation of cubic CeO2 nanoparticles using an alkoxide-primary amine surfactant in an aqueous solution and the existence of a clear potential to make 1D, 2D or 3D CeO2 materials by assembling cubic-shape CeO2 nanoparticle building blocks. Amine surfactant works as a colloidal stabilizer through the adsorption on the CeO2 nanoparticles. Second, the preparation of CeO2-TiO2 nanocomposite nanostructures is presented. The morphologies and redox reactivities of CeO2-TiO2

conversion efficiency about 9 %. We also present necessity of 1DTNM for attainment high efficiency theoretically based on the consideration of electron transport processes in the titania electrode and then present that it is indispensable to use highly crystallized 1DTNM for attainment of higher efficient DSSCs based on the analysis of experimental results

Ceria-based materials are major compounds of the rare earth family, and these have been extensively studied and found application as ultraviolet absorbers (Masui et al., 1997, 2000), solid electrolytes (Inaba & Tagawa, 1996), so-called three-way catalysts for automotive exhaust catalysts (Bekyarova et al., 1998), and soot oxidation catalysts (Pisarello et al., 2002; Aneggi et al., 2006). Nanocrystalline ceria materials have received much attention owing to their physical and chemical properties, which are markedly different from those of the bulk materials. Of particularly interest, the electronic conductivity of CeO2 can be enhanced four orders of magnitude when its microstructure is changed from the micro- to nanocrystalline region (Chiang et al., 1996). Various aqueous solution-based methods for synthesizing crystallized CeO2 nanoparticles (Masui et al., 2002a; Hirano et al., 2000; Li et al., 2001; Wu et al., 2002; Zhou et al., 2003; Bumajdad et al., 2004) and 1D, 2D and 3D CeO2 nanostructures with different morphologies (Vantomme et al., 2005, Zhou et al., 2005, Kuiry et al., 2005, Ho et al., 2005; Han et al., 2005; Sun et al., 2006; Zhong et al., 2007) have been investigated. Some of the properties of these materials, such as the dispersibility of the particles (Masui et al., 2002a) and their catalytic properties (Masui et al., 1997; Sun et al., 2006; Zhong et al., 2007)

The features of CeO2 in these applications are mainly due to the unique combination of its elevated oxygen transport capacity, coupled with its ability to shift easily between the reduced and oxidized states (Ce3+↔Ce4+). To increase the temperature stability and ability of ceria to store and release oxygen, other transition and non transition metal ions (such as Al3+, Si4+, Ti4+ and Zr4+) are normally introduced into the ceria cubic structure (Reddy et al., 2003, 2005; Rynkowski et al., 2000; Masui et al., 2002b). The redox and catalytic properties of CeO2 are strongly influenced when it is combined with other transition metals. In addition, when the particle size is decreased below 100 nm, the materials become nanophasic, where the density of defects increases, such that up to half (50%) of the atoms are situated in the cores of the defects, promoting fast catalyst activation and reaction kinetics (Reddy et al., 2005). Thus, a study of the synthesis and reaction characteristics of nano-sized ceria-based mixed oxides is very important for utilizing the oxygen transport capacity and redox properties. One of the main disadvantages of ceria-based nanoparticles prepared in aqueous solution, however, is the resultant hard agglomeration of the fine particles, which has posed a major challenge to the realization of the full potential of nanocrystalline CeO2 powders.

In this section, first we present the preparation of cubic CeO2 nanoparticles using an alkoxide-primary amine surfactant in an aqueous solution and the existence of a clear potential to make 1D, 2D or 3D CeO2 materials by assembling cubic-shape CeO2 nanoparticle building blocks. Amine surfactant works as a colloidal stabilizer through the adsorption on the CeO2 nanoparticles. Second, the preparation of CeO2-TiO2 nanocomposite nanostructures is presented. The morphologies and redox reactivities of CeO2-TiO2

obtained by electrochemical impedance spectroscopy (EIS) and I-V measurements.

**2. CeO2-TiO2 composite as a catalyst** 

have also been studied.

composite nanostructures are influenced by changing the mole ratio of cerium/titanium alkoxides and by changing the calcination temperature.

## **2.1 Preparation of CeO2 nanoparticles and CeO2-TiO2 composite nanostructure**

The preparation method of CeO2 nanoparticles and CeO2-TiO2 composite are based on the aqueous solution system including metal alkoxides and amine surfactant molecules. The experimental procedure has been described in detail in our previous papers (Murata & Adachi, 2004; Nakagawa et al., 2007). The typical synthesis was as follows: first, laurylamine hydrochloride (LAHC) was dissolved in distilled water. Cerium tri-isopropoxide (CTIP) or cerium *n*-butoxide (CeBu) was used as a cerium source. Tetraisopropyl orthotitanate (TIPT) was used as a titanium source. In the synthesis of CeO2 nanoparticles, CTIP or CeBu was mixed with acetylacetone (ACA) in a beaker and immediately added to an aqueous LAHC solution at pH 4.6. In the case of the synthesis of CeO2–TiO2 composite nanostructures, the mole ratio of CeBu to TIPT (CeBu/TIPT) was changed to 100/0, 75/25, 25/75 and 0/100. Each mixed alkoxide solution was mixed with acetylacetone. In all cases, the mole ratio of metal alkoxides to ACA and metal alkoxides to LAHC were 1 and 4, respectively. After stirring at room temperature for 1 h, the reaction temperature was then changed to 353 K. When the two solutions were mixed, precipitation occurred immediately. After 1 week, the precipitates were separated by centrifugation. After washing with 2-propanol and successive centrifugation, the obtained products were dried through a combination of freeze-drying and vacuum drying, and calcined in air at different temperatures.

## **2.2 Cubic CeO2 nanoparticles and their assembled structures**

The formation yield of CeO2 particles for the surfactant assisted-process was 100% approximately. The structure of CeO2 nanoparticles was studied by TEM image of CeO2 sample in a dried state. During the formation process, we observed systematic changes in color of the precipitated particles. After mixing of the solution of CTIP or CeBu modified with ACA with the aqueous solution of LAHC, the brown transparent original solution immediately became dark brown. The white colloidal suspension was formed after stirring for 1 h at room temperature. A brown and clear supernatant was formed after the precipitation. Further color change of the precipitate was observed. First, the color of the CeO2 particles changed from white to dark blue in about 1 day at 353 K. Subsequently, the color of the precipitate gradually turned into pale purple for 1 week, but the color change was slower than the first change. Moreover the wet centrifuged precipitate appeared dark blue, and the freeze-dried powders were gray. But final CeO2 particles calcined at 673 K was light yellow. These changes in color were observed in the cases that the particles were synthesized in LAHC surfactant aqueous solution at pH 4.2. On the other hand, there was no color change of the precipitate without LAHC surfactant. These color changes are related to the valence state of the Ce; most likely purple corresponds to Ce3+ and yellow corresponds to Ce4+. Therefore, it is clear that the Ce3+ oxide is stabilized by existence of LAHC in aqueous solution.

We succeeded in the preparation of CeO2 nanoparticles with cubic structures and 1D, 2D or 3D CeO2 nanostructures by assembling the cubic-shape CeO2 nanoparticle building blocks (Murata & Adachi, 2004, Nakagawa et al., 2007) as shown in Fig. 1, 2 and 3. It is evident

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

demonstrates the superlattice-like assembled CeO2 nanoparticles.

alkoxides are affected by the positive partial charge

structure.

charge

on the surface of the particles.

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 97

surface energy, which is attained by the association of the cubic CeO2 with a face-to-face

Fig. 3. TEM image of CeO2 nanocrystals self-assembled into a superlattice-like arrangement with dimensions of the order on the nano-scale, Inset a: the FFT pattern confirms the orientational order of the superlattice-like structure, Inset b: the model structure

The thermodynamics of hydrolysis and condensation depend on the strength of the entering nucleophile and electrophilicity of the metal, and on the partial charge. Transition metals are very electropositive, and the hydrolysis and condensation kinetics of the transition metal

0.75, titanium alkoxides: 0.63, and silicon alkoxides: 0.32. Since a large positive partial charge corresponds to a rapid reaction rate, the precursor for the complex formed from cerium alkoxide was not generated gradually, but the nano-sized particles were formed by the rapid hydrolysis and condensation reactions. In our systems using LAHC and CTIP (or CeBu) modified with ACA, the resulting suspensions of CeO2 nanoparticles were exceptionally mono-dispersive without aggregation, demonstrating the high power of LAHC as a colloidal stabilizer through the adsorption of LAHC on the surface of the CeO2 nanoparticles, in accordance with the results of Sugimoto et al. who reported the effect of primary amines as shape controllers for the synthesis of TiO2 (Sugimoto et al., 2003). Since the shape of the CeO2 particles is nearly cubic even if the cubic shape has somewhat rounded edges and corners, the LAHC would control the morphology of the CeO2 particles. For hydrous oxides in aqueous solution systems, the charge-determing ions are H+ and OH-

which establish the charge on the particles by protonating or deprotonating the MOH bonds

M-OH + H+ → M-OH2

The ease of protonation and deprotonation on the surface of the oxide depends on the metal atom. The pH at which the particle is neutrally charged is called the point of zero charge (PZC). At pH > PZC, Eq. 2 predominates, and the particle is negatively charged, whereas at pH < PZC, Eq.1 makes the particle positive. Value of the PZC for CeO2 particles is 8.1 (De Faria and Trasatti, 1994). The magnitude of the surface potential depends on the departure

M-OH + OH- → M-O-

for metals in various alkoxides have been reported; for example, cerium alkoxide:

(Livage et al., 1988). Positive partial

+ (1)

+ H2O (2)

,

from this figure that the particle shape was square, and the particle size was calculated to be 2.7-3.8 nm. Furthermore, it seems that the particles were aligned. TEM image of Figure 1a clearly showed the mono-dispersed CeO2 nanoparticles. The inset picture shows the SAED pattern and Debye-Scherrer rings of the nanoparticles, which can be indexed as those of cerium oxide with the cubic fluorite structure. The HRTEM images and FFT pattern as shown in Figure 1b show that the CeO2 cubic nanoparticles had a single crystalline structure and high crystallinity; these lattice images were observed for many particles. The main lattice spacing of the crystalline structure was calculated to be 3.11 Å according to FFT analysis. This lattice spacing corresponds to the (111) planes of CeO2 with a cubic phase, which coincides with the SAED analysis.

Fig. 1. (a) Low-magnification TEM images of the freeze-dried CeO2 nanoparticles prepared at 353 K for 1 week. Inset: SAED pattern. (b) High-Resolution TEM images of the aggregated CeO2. The lattice images were observed. Inset: FFT pattern obtained from HRTEM.

1D rod-like CeO2 structures are obtained after calcination at 673 K. Rod-like CeO2 with diameters of 30 nm and lengths of 180 nm are observed although the majority of CeO2 samples were assembled into aggregates as shown in Fig.2. The HRTEM image show that the principal axis of the crystal growth of CeO2 was aligned along the rod axis.

Fig. 2. (a) TEM images of CeO2 calcined at 673 K for 4 h. (b) HRTEM image of rod-like CeO2 with a clearly lattice image of (111) planes (d = 3.11 Å).

An ordered structure (2D or 3D superlattice-like structure) are also obtained from the freezedried CeO2 nanoparticles. Figure 3 shows an array of cubic nanocrystals with a mean interparticle (center-to-center) distance of 2.9 nm, as determined from direct imaging and the FFT pattern. We believe this assembly with an ordered structure is formed to minimize the total

from this figure that the particle shape was square, and the particle size was calculated to be 2.7-3.8 nm. Furthermore, it seems that the particles were aligned. TEM image of Figure 1a clearly showed the mono-dispersed CeO2 nanoparticles. The inset picture shows the SAED pattern and Debye-Scherrer rings of the nanoparticles, which can be indexed as those of cerium oxide with the cubic fluorite structure. The HRTEM images and FFT pattern as shown in Figure 1b show that the CeO2 cubic nanoparticles had a single crystalline structure and high crystallinity; these lattice images were observed for many particles. The main lattice spacing of the crystalline structure was calculated to be 3.11 Å according to FFT analysis. This lattice spacing corresponds to the (111) planes of CeO2 with a cubic phase,

Fig. 1. (a) Low-magnification TEM images of the freeze-dried CeO2 nanoparticles prepared at 353 K for 1 week. Inset: SAED pattern. (b) High-Resolution TEM images of the aggregated

1D rod-like CeO2 structures are obtained after calcination at 673 K. Rod-like CeO2 with diameters of 30 nm and lengths of 180 nm are observed although the majority of CeO2 samples were assembled into aggregates as shown in Fig.2. The HRTEM image show that

Fig. 2. (a) TEM images of CeO2 calcined at 673 K for 4 h. (b) HRTEM image of rod-like CeO2

An ordered structure (2D or 3D superlattice-like structure) are also obtained from the freezedried CeO2 nanoparticles. Figure 3 shows an array of cubic nanocrystals with a mean interparticle (center-to-center) distance of 2.9 nm, as determined from direct imaging and the FFT pattern. We believe this assembly with an ordered structure is formed to minimize the total

with a clearly lattice image of (111) planes (d = 3.11 Å).

CeO2. The lattice images were observed. Inset: FFT pattern obtained from HRTEM.

the principal axis of the crystal growth of CeO2 was aligned along the rod axis.

which coincides with the SAED analysis.

surface energy, which is attained by the association of the cubic CeO2 with a face-to-face structure.

Fig. 3. TEM image of CeO2 nanocrystals self-assembled into a superlattice-like arrangement with dimensions of the order on the nano-scale, Inset a: the FFT pattern confirms the orientational order of the superlattice-like structure, Inset b: the model structure demonstrates the superlattice-like assembled CeO2 nanoparticles.

The thermodynamics of hydrolysis and condensation depend on the strength of the entering nucleophile and electrophilicity of the metal, and on the partial charge. Transition metals are very electropositive, and the hydrolysis and condensation kinetics of the transition metal alkoxides are affected by the positive partial charge (Livage et al., 1988). Positive partial charge for metals in various alkoxides have been reported; for example, cerium alkoxide: 0.75, titanium alkoxides: 0.63, and silicon alkoxides: 0.32. Since a large positive partial charge corresponds to a rapid reaction rate, the precursor for the complex formed from cerium alkoxide was not generated gradually, but the nano-sized particles were formed by the rapid hydrolysis and condensation reactions. In our systems using LAHC and CTIP (or CeBu) modified with ACA, the resulting suspensions of CeO2 nanoparticles were exceptionally mono-dispersive without aggregation, demonstrating the high power of LAHC as a colloidal stabilizer through the adsorption of LAHC on the surface of the CeO2 nanoparticles, in accordance with the results of Sugimoto et al. who reported the effect of primary amines as shape controllers for the synthesis of TiO2 (Sugimoto et al., 2003). Since the shape of the CeO2 particles is nearly cubic even if the cubic shape has somewhat rounded edges and corners, the LAHC would control the morphology of the CeO2 particles.

For hydrous oxides in aqueous solution systems, the charge-determing ions are H+ and OH- , which establish the charge on the particles by protonating or deprotonating the MOH bonds on the surface of the particles.

$$\rm M-OH + H^{\*} \rightarrow M-OH 2^{\*} \tag{1}$$

$$\text{M-OH} + \text{OH} \cdot \rightarrow \text{M-O} + \text{H}\_2\text{O} \tag{2}$$

The ease of protonation and deprotonation on the surface of the oxide depends on the metal atom. The pH at which the particle is neutrally charged is called the point of zero charge (PZC). At pH > PZC, Eq. 2 predominates, and the particle is negatively charged, whereas at pH < PZC, Eq.1 makes the particle positive. Value of the PZC for CeO2 particles is 8.1 (De Faria and Trasatti, 1994). The magnitude of the surface potential depends on the departure

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 99

Fig. 4. (a) TEM and (b) HRTEM image of CeO2–TiO2 composite nanostructures (CeBu/TIPT = 75/25) after reaction at 353 K for 1 week, inset: SAED patterns.

Fig. 5. (a) TEM and (b) HRTEM image of CeO2–TiO2 composite nanostructures (CeBu/TIPT = 25/75) after reaction at 353 K for 1 week, inset: SAED patterns.

also included, because the reaction rate of CeBu is faster than TIPT.

Since the positive partial charge

from that of pure CeO2. No TiO2 anatase peaks were observed. Therefore, the formed materials under CeBu/TIPT = 75/25 constitute the composite materials of CeO2 and TiO2, i.e., the formed materials are not a simple mixture of pure CeO2 and TiO2. As a characteristic of our reaction system, the initial solution, including the two metal alkoxides is uniformly well mixed on a molecular scale, easily leading to the formation of composite materials.

alkoxide, as mentioned above, it is inferred that the reaction rate of CeBu is faster than TIPT. Moreover, the content of cerium is much higher than titanium. From these facts, the crystalline structure of the composite materials is inferred as a CeO2 cubic fluorite structure, which is different from that of pure CeO2. The different crystalline structure creates a new morphology, i.e., a nano-network structure, which also leads to the formation of Lewis acid sites, as described later. The XRD patterns at CeBu/TIPT = 25/75 show mainly broad peaks of the TiO2 anatase phase and also show a broad peak of CeO2 around 2*θ* = 30. The broad peaks indicate the formation of composite materials, which lead to a nanorod structure. Since the content of titanium is much higher than cerium, the main crystalline structure corresponds to the TiO2 anatase phase, but a small amount of CeO2 crystalline structure is

of cerium alkoxide is larger than that of titanium

of the pH from the PZC, and that potential attracts oppositely charged ions that present in the solution. Therefore, at pH 4.2, the hydrolyzed and condensed CeO2 particle is positively charged. LAHC molecules also have a positively charged amine group under acidic condition. Hence, there seems to be no driving force for adsorption by electrostatic attraction. However, chloride ion (Cl- ) mediates the interaction between the laurylamine surfactant and charged CeO2 by weak H-bonding forces, and CeO2 particles are covered by surfactant molecules, resulting in the formation of cube crystals. Since the adsorption of LAHC takes place to a specific crystal face, anisotropic structures such as cubes would be formed.

### **2.3 Morphology of CeO2-TiO2 composite**

A few studies on CeO2–TiO2 composite nanoparticles (Reddy et al., 2003, 2005; Rynkowski et al., 2000; Masui et al., 2002b) have been reported. Reddy et al. obtained CeO2–TiO2 composites comprised of relatively larger nanocrystals of CeO2 and TiO2 (anatase), and some overlapped regions (Reddy et al., 2005). Rynkowski et al. studied the redox properties of CeO2–TiO2 composites (Rynkowski et al., 2000), and stated the existence of the CeO2– TiO2 composite. Masui et al. also synthesized CeO2–TiO2 composite nanoparticles, and reported the deactivation of the thermal and photocatalytic properties of this species by the formation of the CeO2–TiO2 composite (Masui et al., 2002b).

We also studied the preparation of CeO2-TiO2 composite nanostructures by changing the mole ratio of cerium/titanium alkoxides and found the effective redox reactivities of CeO2-TiO2 composite nanostructures (Nakagawa et al., 2007). During the synthesis of CeO2-TiO2 composite, the reaction behavior of each solution was observed. When the surfactant solution and metal alkoxide solutions were mixed, precipitation occurred immediately. When the mole ratio of CeBu/TIPT = 75/25 and 25/75, dark brown-gels and dark purple-precipitates formed, while purple-precipitates with a transparent liquid layer were observed at the mole ratios of CeBu/TIPT = 100/0, that was the same behavior using CTIP. The morphology and crystalline structure of the CeO2-TiO2 composite nanostructures varied according to the change in the mole ratio of CeBu to TIPT. When CeBu/TIPT was 75/25, the nano-network structure with a diameter of 3–9 nm was observed and the SAED pattern indicated a cubic fluorite structure (Figure 4). Whereas, when CeBu/TIPT was 25/75, aggregate structures of rod-like morphology with an average diameter of 20 nm and length of 80 nm were observed, and the SAED pattern showed several spots corresponding to the lattice plane of the anatase phase of TiO2 (Figure 5). In the case of the synthesis with only TIPT, a TiO2 nano-network structure of connecting nanowires with diameter of 5–15 nm formed by an oriented attachment mechanism (Adachi et al., 2004, Nakagawa et al., 2005).

Figure 6 shows the variation in XRD patterns of the CeO2-TiO2 composite calcined at 673 K for 4 h (Nakagawa et al., 2007). The peaks at CeBu/TIPT = 100/0 are sharp and can be indexed to a CeO2 cubic fluorite structure. When CeBu/TIPT was 75/25, the XRD peaks were indexed to a CeO2 cubic fluorite structure, although the peaks became very broad. The reason for the broad peak is due to the formation of composite materials. In the HRTEM image shown in Figure 4b, the lattice image of the (111) plane of the cubic fluorite structure could be observed. These observations indicate that the crystalline structure of the nanonetwork at CeBu/TIPT = 75/25 consists of a CeO2 cubic fluorite structure, which is different

of the pH from the PZC, and that potential attracts oppositely charged ions that present in the solution. Therefore, at pH 4.2, the hydrolyzed and condensed CeO2 particle is positively charged. LAHC molecules also have a positively charged amine group under acidic condition. Hence, there seems to be no driving force for adsorption by electrostatic

surfactant and charged CeO2 by weak H-bonding forces, and CeO2 particles are covered by surfactant molecules, resulting in the formation of cube crystals. Since the adsorption of LAHC takes place to a specific crystal face, anisotropic structures such as cubes would be

A few studies on CeO2–TiO2 composite nanoparticles (Reddy et al., 2003, 2005; Rynkowski et al., 2000; Masui et al., 2002b) have been reported. Reddy et al. obtained CeO2–TiO2 composites comprised of relatively larger nanocrystals of CeO2 and TiO2 (anatase), and some overlapped regions (Reddy et al., 2005). Rynkowski et al. studied the redox properties of CeO2–TiO2 composites (Rynkowski et al., 2000), and stated the existence of the CeO2– TiO2 composite. Masui et al. also synthesized CeO2–TiO2 composite nanoparticles, and reported the deactivation of the thermal and photocatalytic properties of this species by the

We also studied the preparation of CeO2-TiO2 composite nanostructures by changing the mole ratio of cerium/titanium alkoxides and found the effective redox reactivities of CeO2-TiO2 composite nanostructures (Nakagawa et al., 2007). During the synthesis of CeO2-TiO2 composite, the reaction behavior of each solution was observed. When the surfactant solution and metal alkoxide solutions were mixed, precipitation occurred immediately. When the mole ratio of CeBu/TIPT = 75/25 and 25/75, dark brown-gels and dark purple-precipitates formed, while purple-precipitates with a transparent liquid layer were observed at the mole ratios of CeBu/TIPT = 100/0, that was the same behavior using CTIP. The morphology and crystalline structure of the CeO2-TiO2 composite nanostructures varied according to the change in the mole ratio of CeBu to TIPT. When CeBu/TIPT was 75/25, the nano-network structure with a diameter of 3–9 nm was observed and the SAED pattern indicated a cubic fluorite structure (Figure 4). Whereas, when CeBu/TIPT was 25/75, aggregate structures of rod-like morphology with an average diameter of 20 nm and length of 80 nm were observed, and the SAED pattern showed several spots corresponding to the lattice plane of the anatase phase of TiO2 (Figure 5). In the case of the synthesis with only TIPT, a TiO2 nano-network structure of connecting nanowires with diameter of 5–15 nm formed by an oriented attachment

Figure 6 shows the variation in XRD patterns of the CeO2-TiO2 composite calcined at 673 K for 4 h (Nakagawa et al., 2007). The peaks at CeBu/TIPT = 100/0 are sharp and can be indexed to a CeO2 cubic fluorite structure. When CeBu/TIPT was 75/25, the XRD peaks were indexed to a CeO2 cubic fluorite structure, although the peaks became very broad. The reason for the broad peak is due to the formation of composite materials. In the HRTEM image shown in Figure 4b, the lattice image of the (111) plane of the cubic fluorite structure could be observed. These observations indicate that the crystalline structure of the nanonetwork at CeBu/TIPT = 75/25 consists of a CeO2 cubic fluorite structure, which is different

) mediates the interaction between the laurylamine

attraction. However, chloride ion (Cl-

**2.3 Morphology of CeO2-TiO2 composite** 

formation of the CeO2–TiO2 composite (Masui et al., 2002b).

mechanism (Adachi et al., 2004, Nakagawa et al., 2005).

formed.

Fig. 4. (a) TEM and (b) HRTEM image of CeO2–TiO2 composite nanostructures (CeBu/TIPT = 75/25) after reaction at 353 K for 1 week, inset: SAED patterns.

Fig. 5. (a) TEM and (b) HRTEM image of CeO2–TiO2 composite nanostructures (CeBu/TIPT = 25/75) after reaction at 353 K for 1 week, inset: SAED patterns.

from that of pure CeO2. No TiO2 anatase peaks were observed. Therefore, the formed materials under CeBu/TIPT = 75/25 constitute the composite materials of CeO2 and TiO2, i.e., the formed materials are not a simple mixture of pure CeO2 and TiO2. As a characteristic of our reaction system, the initial solution, including the two metal alkoxides is uniformly well mixed on a molecular scale, easily leading to the formation of composite materials. Since the positive partial charge of cerium alkoxide is larger than that of titanium alkoxide, as mentioned above, it is inferred that the reaction rate of CeBu is faster than TIPT. Moreover, the content of cerium is much higher than titanium. From these facts, the crystalline structure of the composite materials is inferred as a CeO2 cubic fluorite structure, which is different from that of pure CeO2. The different crystalline structure creates a new morphology, i.e., a nano-network structure, which also leads to the formation of Lewis acid sites, as described later. The XRD patterns at CeBu/TIPT = 25/75 show mainly broad peaks of the TiO2 anatase phase and also show a broad peak of CeO2 around 2*θ* = 30. The broad peaks indicate the formation of composite materials, which lead to a nanorod structure. Since the content of titanium is much higher than cerium, the main crystalline structure corresponds to the TiO2 anatase phase, but a small amount of CeO2 crystalline structure is also included, because the reaction rate of CeBu is faster than TIPT.

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

**Ce/Ti = 0/100 Ce/Ti = 25/75 Ce/Ti = 75/25 Ce/Ti = 100/0**

**Reaction time / min**

**0 100 200 300**

**0**

CeBu/TIPT = 75/25.

robs the electron of I-

**2.5 Conclusions of 2nd section** 

**0.5**

**Concentration of I3**

**-** ×

**10-4 / M**

**1**

**(a)**

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 101

**0**

Fig. 7. The variation of concentration of I3- with reaction time, (a) the effect of the mole ratio of CeBu/TIPT calcined at 673 K, (b) the effect of calcination temperature at the mole ratio of

Ce2O3 under oxidizing and reducing conditions, respectively. Another is its structure: the stable structure of cerium oxide at room temperature under atmospheric pressure is the cubic fluorite structure in which oxygen ions do not have a close-packed structure. Owing to this structure, cerium oxide can easily form many oxygen vacancies while maintaining the basic crystal structure (Reddy et al., 2003). Cerium has a family of related mixed-valency binary oxides, which are anion-deficient and fluorite-related Ce2O2*<sup>n</sup>*-2*<sup>m</sup>* between Ce2O3 and CeO2 at lower temperatures (Kang & Eyring, 1997). It is considered that many vacant oxygen sites exist in cerium oxide; the cerium cation (Cen+) acts as the Lewis acid site and

changing the composition of the CeO2-TiO2 composite, because mixed oxides, e.g., SiO2-TiO2 composites, have been frequently reported to exhibit higher catalytic activity than the pure metal oxide (Méndez-Román & Cardona-Martínez, 1998; Hu et al., 2003). As pointed out above, the uniformly mixed solution of the metal alkoxides led to homogeneously mixed

We confirmed the formation and number of Lewis acid sites from the pyridine adsorption on the surface of the CeO2-TiO2 composite nanostructure (Nakagawa et al., 2007) as shown in Figure 8. In the results of IR spectra, two peaks at 1620 and 1350 cm-1 were assigned to the antisymmetric and symmetric stretching vibrations of the carboxyl group, respectively. A peak at 1595 cm-1 and two peaks at 1480 and 1440 cm-1 were observed, and these peaks were assignable to hydrogen-bonded pyridine and pyridine bonded to a Lewis site, respectively (Zaki et al., 1989, 2001). It was found that Lewis acid sites evidently exist in the CeO2-TiO2 composite nanostructures and these results show a good correlation between the reaction activity (Figure 7a) and the peak area as determined from the Lewis acid sites (Figure 8).

1. The preparation method of cubic CeO2 nanoparticles using an alkoxide-primary amine surfactant in an aqueous solution was presented. In additoion, a clear potential to make

composite oxides on the atomic scale in our preparation method.

. Additionally, the number of Lewis acid sites could be altered by

**0 100 200 300**

**Reaction time / min**

**723K 673K 623K**

**0.5**

**1**

**1.5**

**2**

**(b)**

Fig. 6. XRD patterns of the CeO2–TiO2 composite nanostructures at the mole ratio of CeBu/TIPT = 100/0, 75/25, 25/75 and 0/100 after calcination at 673K for 4 h.

#### **2.4 Surface properties of CeO2-TiO2 composites**

The reaction activity of CeO2-TiO2 composite nanostructures was investigated through the formation rate of I3- , formed due to the oxidation of I- to I2 in excess KI aqueous solution (Nakagawa et al., 2007). Nanostructured CeO2-TiO2 (10 mg) was suspended by magnetic stirring in 10 ml of 0.2 M KI aqueous solution without light irradiation. After initiation of the reaction, 0.3 ml of the reaction solution was taken, and the concentration diluted to one tenth. The concentration of I3 - was measured using a Shimadzu UV-2450 spectrometer from the absorbance at 288 nm. Figure 7 shows the I3 - formation results of CeO2-TiO2 composite nanostructures after calcination at 673 K. It was found that CeO2 nanoparticles and CeO2- TiO2 composite nanostructures have the ability to oxidize I- to I2 although the TiO2 nanostructure shows little activity. The activity of the CeO2-TiO2 composite nanostructure reaches a maximum at CeBu/TIPT = 75/25 at 623 K.

It is known that cerium oxide shows a high oxidation ability and oxygen storage capacity, and the appearance of these functions is attributed to the following two reasons. One is the redox couple Ce3+/Ce4+, which shows the ability of cerium oxide to shift between CeO2 and

Fig. 7. The variation of concentration of I3- with reaction time, (a) the effect of the mole ratio of CeBu/TIPT calcined at 673 K, (b) the effect of calcination temperature at the mole ratio of CeBu/TIPT = 75/25.

Ce2O3 under oxidizing and reducing conditions, respectively. Another is its structure: the stable structure of cerium oxide at room temperature under atmospheric pressure is the cubic fluorite structure in which oxygen ions do not have a close-packed structure. Owing to this structure, cerium oxide can easily form many oxygen vacancies while maintaining the basic crystal structure (Reddy et al., 2003). Cerium has a family of related mixed-valency binary oxides, which are anion-deficient and fluorite-related Ce2O2*<sup>n</sup>*-2*<sup>m</sup>* between Ce2O3 and CeO2 at lower temperatures (Kang & Eyring, 1997). It is considered that many vacant oxygen sites exist in cerium oxide; the cerium cation (Cen+) acts as the Lewis acid site and robs the electron of I- . Additionally, the number of Lewis acid sites could be altered by changing the composition of the CeO2-TiO2 composite, because mixed oxides, e.g., SiO2-TiO2 composites, have been frequently reported to exhibit higher catalytic activity than the pure metal oxide (Méndez-Román & Cardona-Martínez, 1998; Hu et al., 2003). As pointed out above, the uniformly mixed solution of the metal alkoxides led to homogeneously mixed composite oxides on the atomic scale in our preparation method.

We confirmed the formation and number of Lewis acid sites from the pyridine adsorption on the surface of the CeO2-TiO2 composite nanostructure (Nakagawa et al., 2007) as shown in Figure 8. In the results of IR spectra, two peaks at 1620 and 1350 cm-1 were assigned to the antisymmetric and symmetric stretching vibrations of the carboxyl group, respectively. A peak at 1595 cm-1 and two peaks at 1480 and 1440 cm-1 were observed, and these peaks were assignable to hydrogen-bonded pyridine and pyridine bonded to a Lewis site, respectively (Zaki et al., 1989, 2001). It was found that Lewis acid sites evidently exist in the CeO2-TiO2 composite nanostructures and these results show a good correlation between the reaction activity (Figure 7a) and the peak area as determined from the Lewis acid sites (Figure 8).

## **2.5 Conclusions of 2nd section**

100 Smart Nanoparticles Technology

**(200)**

**(105)**

**(211)**

**(004) (112)**

**(103)**

**CeO2, JCPDS 4-0593**

**CeBu/TIPT = 100/0**

**CeBu/TIPT = 75/25**

**CeBu/TIPT = 25/75**

**TiO2, JCPDS 21-1272 anatase**

**(116)**

**(220)**

**(107)**

**(215)**

**(301)**

**(204)**

**CeBu/TIPT = 0/100**

**(213)**

**(400)**

**(311)**

**(420)**

**20 40 60 80**

**(220)**

**(311)**

**(222)**

, formed due to the oxidation of I- to I2 in excess KI aqueous solution


**2**θ **/ degree**

The reaction activity of CeO2-TiO2 composite nanostructures was investigated through the

(Nakagawa et al., 2007). Nanostructured CeO2-TiO2 (10 mg) was suspended by magnetic stirring in 10 ml of 0.2 M KI aqueous solution without light irradiation. After initiation of the reaction, 0.3 ml of the reaction solution was taken, and the concentration diluted to one

the absorbance at 288 nm. Figure 7 shows the I3- formation results of CeO2-TiO2 composite nanostructures after calcination at 673 K. It was found that CeO2 nanoparticles and CeO2- TiO2 composite nanostructures have the ability to oxidize I- to I2 although the TiO2 nanostructure shows little activity. The activity of the CeO2-TiO2 composite nanostructure

It is known that cerium oxide shows a high oxidation ability and oxygen storage capacity, and the appearance of these functions is attributed to the following two reasons. One is the redox couple Ce3+/Ce4+, which shows the ability of cerium oxide to shift between CeO2 and

Fig. 6. XRD patterns of the CeO2–TiO2 composite nanostructures at the mole ratio of CeBu/TIPT = 100/0, 75/25, 25/75 and 0/100 after calcination at 673K for 4 h.

**(111)**

**2.4 Surface properties of CeO2-TiO2 composites** 

reaches a maximum at CeBu/TIPT = 75/25 at 623 K.

formation rate of I3-

tenth. The concentration of I3

**Intensity**

 **/ a.u.** **(101)**

**(200)**

1. The preparation method of cubic CeO2 nanoparticles using an alkoxide-primary amine surfactant in an aqueous solution was presented. In additoion, a clear potential to make

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

evidences verifying the consideration.

electricity conversion efficiency around 9%.

**3.1 Experimental procedure** 

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 103

Titania dioxide is the most promising material for the electrode of DSSCs. Many investigators have improved the anodic electrode over 10 years (Kim et al. 2009, Ito et al. 2008, Grinis et al. 2008, Hamann et al. 2008, Chen, D. et al. 2009, Miyashita et al. 2008, Wang, M. et al. 2009, Youngblood et al. 2009). One-dimensional titania nanoscale materials (1DTNM) have been investigated for attainment of highly efficient solar cells (Colodrero et al. 2009, Kar et al. 2009, Shankar et al. 2009, Wang, D. et al. 2009, Kang, T-S. et al. 2009, Kuang, D. et al. 2008, Shankar et al. 2008, Adachi et al. 2004, Jiu et al. 2006). In this section we present the clear reason for necessity of 1DTNM for attainment of higher efficient dyesensitized solar cells through theoretical consideration and based on the experimental

First we present necessity of 1DTNM theoretically based on the consideration of the electron transport processes obtained from electrochemical impedance spectroscopy (EIS), together with I-V measurement of the same cell. We present then experimentally that it is indispensable to use highly crystallized 1DTNM for attainment of higher efficient DSSCs based on the analysis of experimental results obtained by EIS and I-V measurements. Also we present that all electrodes composed of our three kinds of 1DTNM showed high light-to-

In order to elucidate the relationship between the composition of titania thin film electrode and performance of the electrode, we made three kinds of electrodes, i.e., an electrode made of P-25 only, an electrode made of P-25 with polyethylene glycol (PEG) and an electrode made of network structure of titania nanowires (TNW) mixed with P-25 with PEG (TNW 28%) first. Since the electrode containing TNW was the best one, we made electrodes made of various amount of TNW mixed with the mixture of P-25 and PEG. The percentage of TNW to (TNW + P-25) in Ti atom content was varied from 0 % to 100 %. Furthermore, we

The procedure of TiO2 single crystalline nanowires with network structure has been reported in our previous paper (Adachi et al. 2004). The synthesis procedure of highly crystallized titania nanorods has been described in our previous paper (Jiu et al. 2006). The procedure of titania nanochains is almost the same as that of titania nanorods, except usage of HCl instead of ethylenediamine (EDA) to adjust pH values to 1.3 to 5. Titania nanochains can be synthesized using P123 (triblock copolymer of (poly(ethylene oxide)20 poly(propylene oxide) 70-poly(ethylene oxide)20 ) instead of F127 (triblock copolymer of

We synthesized highly crystallized titania nanoparticles (TNP) with diameter of 3-5 nm other than 1DTNM mentioned above (Jiu et al. 2004, Jiu et al. 2007). Titania electrodes with thin film were made by applying titania samples on an electric conducting glass plate. Fluorine doped tin oxide (FTO) was used as an electric conducting oxide. Dilute solution of

made DSSCs with electrodes composed of all our three kinds of 1DTNM.

(poly(ethylene oxide)106-poly(propylene oxide) 70-poly(ethylene oxide)106)).

**3.1.2 Preparation of titania electrods and dye-sensitized solar cells** 

**3.1.1 Synthesis of highly crystallized TiO2 nanoscale materials** 

1D, 2D or 3D CeO2 materials by assembling cubic-shape CeO2 nanoparticle building blocks was also revealed.

2. CeO2-TiO2 composite nanostructures could be prepared by changing the mole ratio of cerium/titanium alkoxides. The morphology and crystalline structure of the CeO2-TiO2 composite nanostructures were influenced with the mole ratio of the metal alkoxides. These composite nanostructures showed effective reaction activity to oxidize I- to I2 because of the formation of the Lewis acid sites.

Fig. 8. Pyridine adsorption results (at room temperature) of the CeO2-TiO2 composite nanostructure at the different mole ratio of CeBu/TIPT calcined at 673 K.

## **3. 1-D TiO2 for dye-sensitized solar cells**

Dye-sensitized solar cells (DSSCs) have attracted much attention as they offer the possibility of extremely inexpensive and efficient solar energy conversion, because light from the sun is the ideal source of energy, and the supply of energy is gigantic, i.e., 3×1024 J/year or about 104 times more than what mankind consumes currently. In 1991, O'Regan and Grätzel (O'Regan & Grätzel 1991) published a remarkable report, and the Grätzel group attained 10 % efficiency in 1993 (Nazeeruddin et al. 1993). The system already reached conversion efficiency 11.5 % (Chen, C-Y. et al. 2009), and recently even 12.3 % was reported by Grätzel in Hybrid Organic Photovoltaics Conference in Valencia Spain. These conversion efficiencies exceed the level to supply electricity at the rate of home use, i.e., 10 %. Nevertheless, the energy conversion efficiency of the cells for commercial devices has not yet reached the level, which provides lower cost than that of conventional methods of electricity generation using fossil fuel. Therefore, attainment of higher efficient cells is still one of the most important challenges for the dye-sensitized solar cells.

Titania dioxide is the most promising material for the electrode of DSSCs. Many investigators have improved the anodic electrode over 10 years (Kim et al. 2009, Ito et al. 2008, Grinis et al. 2008, Hamann et al. 2008, Chen, D. et al. 2009, Miyashita et al. 2008, Wang, M. et al. 2009, Youngblood et al. 2009). One-dimensional titania nanoscale materials (1DTNM) have been investigated for attainment of highly efficient solar cells (Colodrero et al. 2009, Kar et al. 2009, Shankar et al. 2009, Wang, D. et al. 2009, Kang, T-S. et al. 2009, Kuang, D. et al. 2008, Shankar et al. 2008, Adachi et al. 2004, Jiu et al. 2006). In this section we present the clear reason for necessity of 1DTNM for attainment of higher efficient dyesensitized solar cells through theoretical consideration and based on the experimental evidences verifying the consideration.

First we present necessity of 1DTNM theoretically based on the consideration of the electron transport processes obtained from electrochemical impedance spectroscopy (EIS), together with I-V measurement of the same cell. We present then experimentally that it is indispensable to use highly crystallized 1DTNM for attainment of higher efficient DSSCs based on the analysis of experimental results obtained by EIS and I-V measurements. Also we present that all electrodes composed of our three kinds of 1DTNM showed high light-toelectricity conversion efficiency around 9%.

## **3.1 Experimental procedure**

102 Smart Nanoparticles Technology

2. CeO2-TiO2 composite nanostructures could be prepared by changing the mole ratio of cerium/titanium alkoxides. The morphology and crystalline structure of the CeO2-TiO2 composite nanostructures were influenced with the mole ratio of the metal alkoxides. These composite nanostructures showed effective reaction activity to oxidize I- to I2

**1350**

blocks was also revealed.

**1620**

**0**

**0.1**

**Absorbance**

**0.2**

**1595**

because of the formation of the Lewis acid sites.

**1480**

**1440**

**1600 1400 1200**

Fig. 8. Pyridine adsorption results (at room temperature) of the CeO2-TiO2 composite

Dye-sensitized solar cells (DSSCs) have attracted much attention as they offer the possibility of extremely inexpensive and efficient solar energy conversion, because light from the sun is the ideal source of energy, and the supply of energy is gigantic, i.e., 3×1024 J/year or about 104 times more than what mankind consumes currently. In 1991, O'Regan and Grätzel (O'Regan & Grätzel 1991) published a remarkable report, and the Grätzel group attained 10 % efficiency in 1993 (Nazeeruddin et al. 1993). The system already reached conversion efficiency 11.5 % (Chen, C-Y. et al. 2009), and recently even 12.3 % was reported by Grätzel in Hybrid Organic Photovoltaics Conference in Valencia Spain. These conversion efficiencies exceed the level to supply electricity at the rate of home use, i.e., 10 %. Nevertheless, the energy conversion efficiency of the cells for commercial devices has not yet reached the level, which provides lower cost than that of conventional methods of electricity generation using fossil fuel. Therefore, attainment of higher efficient cells is still one of the most

**Wavenumber / cm-1**

nanostructure at the different mole ratio of CeBu/TIPT calcined at 673 K.

**3. 1-D TiO2 for dye-sensitized solar cells** 

important challenges for the dye-sensitized solar cells.

1D, 2D or 3D CeO2 materials by assembling cubic-shape CeO2 nanoparticle building

**Ce/Ti=100/0 Ce/Ti=75/25 Ce/Ti=25/75 Ce/Ti=0/100**

In order to elucidate the relationship between the composition of titania thin film electrode and performance of the electrode, we made three kinds of electrodes, i.e., an electrode made of P-25 only, an electrode made of P-25 with polyethylene glycol (PEG) and an electrode made of network structure of titania nanowires (TNW) mixed with P-25 with PEG (TNW 28%) first. Since the electrode containing TNW was the best one, we made electrodes made of various amount of TNW mixed with the mixture of P-25 and PEG. The percentage of TNW to (TNW + P-25) in Ti atom content was varied from 0 % to 100 %. Furthermore, we made DSSCs with electrodes composed of all our three kinds of 1DTNM.

## **3.1.1 Synthesis of highly crystallized TiO2 nanoscale materials**

The procedure of TiO2 single crystalline nanowires with network structure has been reported in our previous paper (Adachi et al. 2004). The synthesis procedure of highly crystallized titania nanorods has been described in our previous paper (Jiu et al. 2006). The procedure of titania nanochains is almost the same as that of titania nanorods, except usage of HCl instead of ethylenediamine (EDA) to adjust pH values to 1.3 to 5. Titania nanochains can be synthesized using P123 (triblock copolymer of (poly(ethylene oxide)20 poly(propylene oxide) 70-poly(ethylene oxide)20 ) instead of F127 (triblock copolymer of (poly(ethylene oxide)106-poly(propylene oxide) 70-poly(ethylene oxide)106)).

## **3.1.2 Preparation of titania electrods and dye-sensitized solar cells**

We synthesized highly crystallized titania nanoparticles (TNP) with diameter of 3-5 nm other than 1DTNM mentioned above (Jiu et al. 2004, Jiu et al. 2007). Titania electrodes with thin film were made by applying titania samples on an electric conducting glass plate. Fluorine doped tin oxide (FTO) was used as an electric conducting oxide. Dilute solution of

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

**Experimental Calculation**

Fig. 9. (a) Typical Nyquist plot obtained by EIS, (b) I-V curve for the same cell

recombination reactions between electrons in the titania electrode and I3-

However, the largest arc of around 10 Hz in Fig. 9a represents the resistance of

electrolyte. Small total dc resistance means small resistance for recombination reactions, indicating rapid reaction rate of recombination. Thus, small total dc resistance seems an obstacle for attainment of highly efficient solar cells. But, whether electrons in the titania electrode are properly collected by the transparent conducting glass electrode or react with

 ions in the electrolyte by recombination reactions is determined by the ratio of the resistance for the transport rate to the conducting glass electrode against the resistance for the recombination reactions. When the resistance for the transport rate to the conducting glass electrode is much smaller than that of the recombination reactions, almost all electrons are properly collected by the conducting glass electrode. This means that the transport rate of electrons in the titania electrode should be very rapid, indicating that we need nice titania materials with high electron transport rate, i.e., highly crystallized one-dimensional

> **0 0.2 0.4 0.6 0.8 1 -Voltage [V]**

Fig. 10. Reproduction of I-V curve by total dc resistances at various bias voltages.

**0 5 10 15 20 25 30 35 Z'(real)**

**10 Hz 0.7 Hz**

**27.3** Ω **6300 Hz**

nanoscale TiO2 materials are needed.

**Current density [mA/cm2**

**]**

small.

I3-

**a**

**-Z''(imaginary)**

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 105

resistance is given by the length from 0 to the point at =0 on the real axis as shown by Fig. 9a. This fact is confirmed later by reproduction of I-V curve using measured total dc resistances at various bias voltages as shown in Fig. 10. Total dc resistance is also obtained from the slope of the tangent line at the point of Voc. (Fig. 9b) When the total dc resistance becomes small, the slope becomes steep, and the fill factor becomes larger, resulting in a high light-to-electricity conversion efficiency. Thus, the total dc resistance should be

> **0 0.001 0.002 0.003 0.004**

**b**

**Current [A]**

**0 0.2 0.4 0.6 0.8 1 -Voltage [V]**

> Experiment Calculation

**27.3** Ω

ions in the

TNP with diameter 3-5 nm was applied on the surface of FTO as a blocking layer. The three kinds of electrodes made of P-25 only, P-25 with PEG and titania nanowire network (TNW) mixed with P-25 with PEG were prepared by coating each gel solution containing these titania materials on the FTO glass by doctor blade method. The gel solution of P-25 only was made by dissolving P-25 powder into water. The aqueous gel solution of P-25 with PEG was made after the procedure reported by Grätzel's group (Nazeeruddin et al. 1993). The gel solution of TNW mixed with P-25 with PEG was made by mixing the gel solution of P-25 with PEG with the reaction products TNW after centrifugation and washing by 2-propanol.

The higher efficient cells constituted with 1DTNM were fabricated as follows. First, the gel solution of TNP with diameter of 3-5 nm was coated three times by doctor blade method on a FTO glass, making 3 layers of TNP. In the case of cells made of TNW, the gel solution of TNW mixed with P-25 with PEG was coated by 8-10 times. The ratio of TNW to P-25 in Ti atom content was around 0.3. In the case of cells made of titania nanorods, the reaction products after centrifugation was mixed with the two gel solutions of P-25 with PEG and the solution of TNP. The mixed gel solution was coated 7-10 times. In the case of titania nanochains, the procedure was the same as the case of titania nanorods.

After each coating, the sample was calcined at 773 K for 10 min. The last calcination was made at 773 K for 30 min. Dye was introduced to the titania thin films by soaking the film 1-3 days in 3×10-4 M solution of ruthenium dye in the mixed solvent of tert-butanol and acetnitryl. Cis-di(thiocyanate) bis(2,2'-bipyridyl-4,4'-di-carboxylate)-ruthenium(II) bis-tetrabutyl-ammonium (N719) (Solaronix SA) produced by Grätzel's group (Nazeeruddin et al. 1993) was used as the dye.

The DSSCs were comprised of a titania thin film electrode on a conducting glass plate, and a platinum electrode made by sputtering on the conducting glass and electrolyte between the titania thin film and the platinum. The composition of the used electrolyte was 0.1 M Guanidium thiocyanate, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, and 0.5 M TBP (4-tert-butyl pyridine) in the mixed solvent of acetonitrile + n-valeronitrile (volume 85 : 15).

## **3.1.3 Characterization of titania materials and solar cells**

Characterization of the produced materials was made by X-ray diffraction (XRD) (Rigaku Goniometer PMG-A2, CN2155D2), transmission electron microscopy (TEM) (JEOL 200 CX and JEM-2100F), fast Fourier transform (FFT), selected-area electron diffraction (SAED), scanning electron microscopy (SEM) (JEOL JSM 7500FA) and isotherm of nitrogen adsorption (BEL SORP 18 PLUS). The photo-current-voltage characteristics were measured using an AM 1.5 solar simulator (YSS-E40, Yamashita Denso) and in which the light intensity is 100 mW/cm2 calibrated with a secondary reference solar cell standardized by JET (Japan Electrical Safety & Environmental Technology Laboratories). Electron transport processes were measured by electrochemical impedance spectroscopy (EIS) (Solartron 1255B). The cell size was 0.25 cm2.

## **3.2 Necessity of highly crystallized titania nanoscale materials**

First, let us consider the reason why highly crystallized one-dimensional titania materials are needed. Fig. 9a shows a typical Nyquist plot obtained by EIS. Total direct current (dc)

TNP with diameter 3-5 nm was applied on the surface of FTO as a blocking layer. The three kinds of electrodes made of P-25 only, P-25 with PEG and titania nanowire network (TNW) mixed with P-25 with PEG were prepared by coating each gel solution containing these titania materials on the FTO glass by doctor blade method. The gel solution of P-25 only was made by dissolving P-25 powder into water. The aqueous gel solution of P-25 with PEG was made after the procedure reported by Grätzel's group (Nazeeruddin et al. 1993). The gel solution of TNW mixed with P-25 with PEG was made by mixing the gel solution of P-25 with PEG with the reaction products TNW after centrifugation and washing by 2-propanol. The higher efficient cells constituted with 1DTNM were fabricated as follows. First, the gel solution of TNP with diameter of 3-5 nm was coated three times by doctor blade method on a FTO glass, making 3 layers of TNP. In the case of cells made of TNW, the gel solution of TNW mixed with P-25 with PEG was coated by 8-10 times. The ratio of TNW to P-25 in Ti atom content was around 0.3. In the case of cells made of titania nanorods, the reaction products after centrifugation was mixed with the two gel solutions of P-25 with PEG and the solution of TNP. The mixed gel solution was coated 7-10 times. In the case of titania

After each coating, the sample was calcined at 773 K for 10 min. The last calcination was made at 773 K for 30 min. Dye was introduced to the titania thin films by soaking the film 1-3 days in 3×10-4 M solution of ruthenium dye in the mixed solvent of tert-butanol and acetnitryl. Cis-di(thiocyanate) bis(2,2'-bipyridyl-4,4'-di-carboxylate)-ruthenium(II) bis-tetrabutyl-ammonium (N719) (Solaronix SA) produced by Grätzel's group (Nazeeruddin et al.

The DSSCs were comprised of a titania thin film electrode on a conducting glass plate, and a platinum electrode made by sputtering on the conducting glass and electrolyte between the titania thin film and the platinum. The composition of the used electrolyte was 0.1 M Guanidium thiocyanate, 0.6 M 1-butyl-3-methylimidazolium iodide, 0.03 M I2, and 0.5 M TBP (4-tert-butyl pyridine) in the mixed solvent of acetonitrile + n-valeronitrile (volume 85 : 15).

Characterization of the produced materials was made by X-ray diffraction (XRD) (Rigaku Goniometer PMG-A2, CN2155D2), transmission electron microscopy (TEM) (JEOL 200 CX and JEM-2100F), fast Fourier transform (FFT), selected-area electron diffraction (SAED), scanning electron microscopy (SEM) (JEOL JSM 7500FA) and isotherm of nitrogen adsorption (BEL SORP 18 PLUS). The photo-current-voltage characteristics were measured using an AM 1.5 solar simulator (YSS-E40, Yamashita Denso) and in which the light intensity is 100 mW/cm2 calibrated with a secondary reference solar cell standardized by JET (Japan Electrical Safety & Environmental Technology Laboratories). Electron transport processes were measured by electrochemical impedance spectroscopy (EIS) (Solartron

First, let us consider the reason why highly crystallized one-dimensional titania materials are needed. Fig. 9a shows a typical Nyquist plot obtained by EIS. Total direct current (dc)

nanochains, the procedure was the same as the case of titania nanorods.

**3.1.3 Characterization of titania materials and solar cells** 

**3.2 Necessity of highly crystallized titania nanoscale materials** 

1993) was used as the dye.

1255B). The cell size was 0.25 cm2.

resistance is given by the length from 0 to the point at =0 on the real axis as shown by Fig. 9a. This fact is confirmed later by reproduction of I-V curve using measured total dc resistances at various bias voltages as shown in Fig. 10. Total dc resistance is also obtained from the slope of the tangent line at the point of Voc. (Fig. 9b) When the total dc resistance becomes small, the slope becomes steep, and the fill factor becomes larger, resulting in a high light-to-electricity conversion efficiency. Thus, the total dc resistance should be small.

Fig. 9. (a) Typical Nyquist plot obtained by EIS, (b) I-V curve for the same cell

However, the largest arc of around 10 Hz in Fig. 9a represents the resistance of recombination reactions between electrons in the titania electrode and I3- ions in the electrolyte. Small total dc resistance means small resistance for recombination reactions, indicating rapid reaction rate of recombination. Thus, small total dc resistance seems an obstacle for attainment of highly efficient solar cells. But, whether electrons in the titania electrode are properly collected by the transparent conducting glass electrode or react with I3- ions in the electrolyte by recombination reactions is determined by the ratio of the resistance for the transport rate to the conducting glass electrode against the resistance for the recombination reactions. When the resistance for the transport rate to the conducting glass electrode is much smaller than that of the recombination reactions, almost all electrons are properly collected by the conducting glass electrode. This means that the transport rate of electrons in the titania electrode should be very rapid, indicating that we need nice titania materials with high electron transport rate, i.e., highly crystallized one-dimensional nanoscale TiO2 materials are needed.

Fig. 10. Reproduction of I-V curve by total dc resistances at various bias voltages.

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

shoulder like increase because of the strong scattering of TNW.

Fig. 12. Results of IPCE for three kinds of cells.

TNW+(P25+PEG)

0 5 10 15 20 25 30 Z'(real)

Fig. 13. Nyquist plot of three kinds of cells under open circuit conditions.

1

2 , *eff*

 , *k eff* 

*D L*

*d*

*d k*

Z''(imaginary)

2006).

where,

Z''(imaginary)

**IPCE [%]**

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 107

of the largest amount of dye adsorption. Also IPCE in the range of 600 nm to 700 nm shows

**P-25+P mixture**

**P25+PEG**

**TNW**+(**P25+PEG)**

**P25 only**

P25 only (no PEG)

0 10 20 30 40 50 Z'(real)

(4)

1/2

*k*

*R Con*

*eff*

*Lk* (5)

 

Z''(imaginary)

**400 500 600 700 800 900**

**wavelength [nm]**

Fig. 13 shows Nyquist plots of the three kinds of cells under open circuit conditions. The total resistance of each cell was obtained as 49 Ω for P-25 only cell, 32 Ω for P-25+PEG cell and 27 Ω for TNW+(P-25+PEG) cell, respectively. Since total resistance corresponds to the slope of the tangent line at *V*oc, the slope of the tangent line in I-V curves in Fig. 11 became steeper with decreasing of the total resistance of the cell. The plotted squares in Fig. 13 represent experimental results and the solid curves show the calculated spectra from equations (4) to (7) using parameters shown in Table 2 for each cell (Adachi et al.

P25+PEG

0 5 10 15 20 25 30 35 Z'(real)

Impedance equations for electron transport processes are given as follows (Adachi et al.

1/2

<sup>1</sup> coth 1

*s eff eff*

*q An D D* <sup>1</sup>

*k d k*

2006). For the impedance concerning with titania electrode, equation (4) was derived:

*<sup>k</sup> <sup>w</sup>*

 

*<sup>i</sup> Z R i*

*<sup>k</sup>* <sup>2</sup> , *<sup>B</sup> <sup>w</sup>*

*kT L L <sup>R</sup> Con*

Solid line in Fig. 10 shows experimentally obtained I-V curve under illumination. The square keys show calculated curve based on the observed total dc resistances at various bias voltages by EIS and the following relationship between current density and voltage,

$$d\dot{\mathbf{i}} = \frac{dV}{R\_t} \tag{3}$$

where *Rt* stands for total dc resistance. The calculated curve reproduces experimentally obtained I-V curve very well, confirming that the total dc resistances can be determined accurately from Nyquist plot of EIS analysis.

#### **3.3 Comparison of three kinds of electrodes (P-25 only, P-25 with PEG and TNW mixed with mixture of P-25 and PEG)**

Fig. 11 shows I-V curves of the cells made of three kinds of electrodes, i.e., (a) P-25 only, (b) P-25 with PEG and (c) titania nanowire network (TNW) mixed with mixture of P-25 and PEG. The cell made of P-25 only showed the lowest power conversion efficiency (PCE) 4.02 %. PCE of 6.86 % was obtained for the cell made of P-25 with PEG. The highest PCE 8.64 % was obtained for the cell made of TNW mixed with mixture of P-25 and PEG, in which the percentage of titanium atoms of TNW was 28 % for the total titanium atoms, i.e., TNW + P-25. Table 1 shows the current density *J*sc, open circuit voltage *V*oc, fill factor *FF* and power conversion efficiency of the three kinds of cells.

Fig. 11. I-V curves of the cells made of three kinds of electrodes, i.e., (a) P-25 only, (b) P-25 with PEG and (c) titania nanowire network (TNW) mixed with mixture of P-25 and PEG.


Table 1. Current density *J*sc,, open circuit voltage *V*oc, fill factor *FF* and power conversion efficiency of the three kinds of cells.

The results of incident photon to current efficiency (IPCE) for the three kinds of cells are shown in Fig. 12. IPCE of the cell made of P-25 only was lowest because of the small amount of dye adsorption. The cell made of TNW with P-25 with PEG showed highest IPCE because of the largest amount of dye adsorption. Also IPCE in the range of 600 nm to 700 nm shows shoulder like increase because of the strong scattering of TNW.

Fig. 12. Results of IPCE for three kinds of cells.

Fig. 13 shows Nyquist plots of the three kinds of cells under open circuit conditions. The total resistance of each cell was obtained as 49 Ω for P-25 only cell, 32 Ω for P-25+PEG cell and 27 Ω for TNW+(P-25+PEG) cell, respectively. Since total resistance corresponds to the slope of the tangent line at *V*oc, the slope of the tangent line in I-V curves in Fig. 11 became steeper with decreasing of the total resistance of the cell. The plotted squares in Fig. 13 represent experimental results and the solid curves show the calculated spectra from equations (4) to (7) using parameters shown in Table 2 for each cell (Adachi et al. 2006).

Fig. 13. Nyquist plot of three kinds of cells under open circuit conditions.

Impedance equations for electron transport processes are given as follows (Adachi et al. 2006). For the impedance concerning with titania electrode, equation (4) was derived:

$$Z = R\_w \left[ \frac{1}{\left(\frac{o\nu\_k}{o\nu\_d}\right) \left(1 + \frac{i\nu}{o\nu\_k}\right)}\right]^{1/2} \coth\left[\left(\frac{o\nu\_k}{o\nu\_d}\right) \left(1 + \frac{i\nu}{o\nu\_k}\right)\right]^{1/2} \tag{4}$$

where,

106 Smart Nanoparticles Technology

Solid line in Fig. 10 shows experimentally obtained I-V curve under illumination. The square keys show calculated curve based on the observed total dc resistances at various bias

> *t dV di*

where *Rt* stands for total dc resistance. The calculated curve reproduces experimentally obtained I-V curve very well, confirming that the total dc resistances can be determined

Fig. 11 shows I-V curves of the cells made of three kinds of electrodes, i.e., (a) P-25 only, (b) P-25 with PEG and (c) titania nanowire network (TNW) mixed with mixture of P-25 and PEG. The cell made of P-25 only showed the lowest power conversion efficiency (PCE) 4.02 %. PCE of 6.86 % was obtained for the cell made of P-25 with PEG. The highest PCE 8.64 % was obtained for the cell made of TNW mixed with mixture of P-25 and PEG, in which the percentage of titanium atoms of TNW was 28 % for the total titanium atoms, i.e., TNW + P-25. Table 1 shows the current density *J*sc, open circuit voltage *V*oc, fill factor *FF* and power

> **0 0.2 0.4 0.6 0.8 1 -Voltage [V]**

> > Voc [V]

FF [%]

**c**

**a b**

Fig. 11. I-V curves of the cells made of three kinds of electrodes, i.e., (a) P-25 only, (b) P-25 with PEG and (c) titania nanowire network (TNW) mixed with mixture of P-25 and PEG.

**3.3 Comparison of three kinds of electrodes (P-25 only, P-25 with PEG and TNW** 

of the three kinds of cells.

J

sc [mA/cm2]

 P-25 6.73 0.84 0.72 4.02 P-25 + PEG 11.38 0.83 0.73 6.86 TNW + (P-25 + PEG) 14.56 0.82 0.72 8.64

Table 1. Current density *J*sc,, open circuit voltage *V*oc, fill factor *FF* and power conversion

The results of incident photon to current efficiency (IPCE) for the three kinds of cells are shown in Fig. 12. IPCE of the cell made of P-25 only was lowest because of the small amount of dye adsorption. The cell made of TNW with P-25 with PEG showed highest IPCE because

*<sup>R</sup>* (3)

voltages by EIS and the following relationship between current density and voltage,

accurately from Nyquist plot of EIS analysis.

**Current densit**

of the three kinds of cells.

**[mA/cm2]**

**y**

**mixed with mixture of P-25 and PEG)** 

conversion efficiency

efficiency

$$\alpha\_d = \frac{D\_{\text{eff}}}{L^2}, \ \alpha\_k = k\_{\text{eff}}, \ R\_w = \frac{k\_B T}{q^2 A n\_s} \frac{L}{D\_{\text{eff}}} = \text{Con} \frac{L}{D\_{\text{eff}}}, \ R\_k = \text{Con} \frac{1}{L k\_{\text{eff}}} \tag{5}$$

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

]

*J*sc

[V]

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 109

[mA/cm [m] <sup>2</sup>

[%] *FF <sup>V</sup>*oc

thickness

100wt% 10.16 0.84 0.70 5.97 11 100wt% 9.93 0.82 0.71 5.75 9 100wt% 11.94 0.84 0.73 7.28 5 50wt% 15.18 0.80 0.70 8.51 22 50wt% 13.39 0.85 0.71 8.02 14 28wt% 13.09 0.83 0.74 8.04 27 28wt% 14.88 0.82 0.71 8.66 24 10wt% 12.68 0.84 0.72 7.64 20 10wt% 12.84 0.84 0.73 7.87 18 5wt% 10.96 0.81 0.73 6.50 32 5wt% 11.98 0.85 0.73 7.48 19 0wt% 10.92 0.80 0.71 6.20 35 0wt% 11.82 0.82 0.71 6.87 26 0wt% 10.93 0.85 0.74 6.85 10

electron density, becomes smaller with increasing conversion efficiency, i.e., Con value increases in the order of P-25 only > (P-25+PEG) > TNW+(P-25+PEG). Therefore the electron density *n* increases with increasing conversion efficiency. So, the characteristics of highly efficient cells are high electron density, small resistance for the electron transport to the conducting glass electrode, and large ratio of the resistances *R*k/*R*w with small rate constant

Since the cells containing TNW gave high conversion efficiencies, we examined the effects of content of TNW in the electrode composed of TNW and P-25 upon the conversion efficiency with variation in TNW from 0 % to 100 %. Content of TNW was defined as percentage of titanium atoms of TNW in the total titanium atoms included in the titania electrode. Table 3 shows performance of DSSCs with various TNW content, i.e., *J*sc, *V*oc, *FF* and *η*, together

PCE of the cells including TNW are higher than those cells without TNW, indicating that TNW is useful to attain high efficiency except 100% TNW case. When the film thickness of 100% TNW cells increased larger than 5 m, peel off of the films with cracks was observed by SEM images as shown in Fig. 15, resulting that less than 6% of PCE were observed as shown in Table 3 and Fig. 14. Thus, mixing of TNW with P-25 nanoparticles is important to

Since the amount of adsorbed dyes is another important factor to affect PCE, the amounts of adsorbed dyes for the cells with various TNW contents are shown against film thickness in Fig. 16. The amount of adsorbed dye in the cells containing TNW from 0 % to 50 % locates in the same straight line regardless of the difference in TNW contents, except 100 % TNW

Table 3. Performance of dye sensitized solar cells with various TNW content.

**3.4 Effects of content of TNW on the properties of dye-sensitized solar cells** 

with film thickness. Effect of TNW content on PCE is shown in Fig. 14.

for recombination reactions.

make robust films.

which shows higher adsorbed amounts.

For the impedance concerning with platinum electrode, equation (6) was assumed:

$$Z\_{Pt} = \frac{1}{1 + i\alpha r\_{pt}C\_{pt}}\tag{6}$$

where, *r*pt and *C*pt represent the resistance at the Pt surface and the capacitance at the Pt surface, respectively. For the impedance concerning with tri-iodide diffusion, finite Warburg impedance equation, i.e., equation (7) was assumed:

$$Z\_N = R\_{13-} \frac{1}{\sqrt{\frac{i\alpha}{\left(D\_{13-} \;/\,\delta^2\right)}}} \tanh\sqrt{\frac{i\alpha}{\left(D\_{13-} \;/\,\delta^2\right)}}\tag{7}$$

The calculated solid curves in Fig. 13 agree quite well with the plotted experimental data. The characteristics shown in Table 2 are following three points which show strong tendency for the highly efficient cells. 1) The resistance for the electron transport from the titania electrode to the conducting glass electrode *R*w becomes smaller with increasing conversion efficiency. 2) The ratio of the resistance for the recombination reactions against the resistance for the transport rate to the conducting glass electrode (*R*k/*R*w) becomes large, and the rate constant of recombination reactions *k*eff becomes smaller with increasing conversion efficiency. 3) The values of Con, which represents constant inversely proportional to the


Where, Con=*k*B*T*/*qAn* [cms-1], where *k*B [JK-1] represents Boltzmann constant, *T* [K] is absolute temperature, *q* [C] is elementary charge, *A* [cm2] is area of the cell and *n* [cm-3] is electron density. Con: constant inversely proportional to the electron density, *D*eff [cm2s-1]: diffusion coefficient of electron, *L* [cm]: film thickness of TiO2 electrode, *k*eff [s-1]: reaction rat constant of recombination reactions, *R*I3- []: diffusion resistance of I3- , *D*I3- [cm2s-1]: diffusion coefficient of I3- , *δ* [cm]: thickness of the electrolyte phase, *R*Pt []: resistance of Pt electrode, *C*Pt [F]: capacity of Pt electrode, *R*sub []: resistance of substrate, *R*w[]: resistance for electron transport in the TiO2 electrode, *R*k []: resistance for recombination reaction.

Table 2. Determined parameters concerning with electron transport by impedance spectroscopy for three kinds of cells

1

where, *r*pt and *C*pt represent the resistance at the Pt surface and the capacitance at the Pt surface, respectively. For the impedance concerning with tri-iodide diffusion, finite Warburg

*i rC* 

<sup>3</sup> <sup>2</sup>

P25 only P25+PEG TNW+(P25+PEG)

*i D*

2

The calculated solid curves in Fig. 13 agree quite well with the plotted experimental data. The characteristics shown in Table 2 are following three points which show strong tendency for the highly efficient cells. 1) The resistance for the electron transport from the titania electrode to the conducting glass electrode *R*w becomes smaller with increasing conversion efficiency. 2) The ratio of the resistance for the recombination reactions against the resistance for the transport rate to the conducting glass electrode (*R*k/*R*w) becomes large, and the rate constant of recombination reactions *k*eff becomes smaller with increasing conversion efficiency. 3) The values of Con, which represents constant inversely proportional to the

Con= 2.65 0.28 0.163

= 0.0006 0.00014 0.00008

= 0.0042 0.0025 0.002

= 23 13.8 10

I3-= 5.1 7 7.5

I3-= 0.000015 0.000003 0.000005

= 0.005 0.005 0.005

Pt= 1.5 4.7 3.65

Pt= 0.00005 0.00007 0.00005

sub= 7.9 10 6.7

w= 14.8 5 4.08

k= 27.4 8.11 8.15

= 2.42×1017 2.29×1018 3.94×1018

Where, Con=*k*B*T*/*qAn* [cms-1], where *k*B [JK-1] represents Boltzmann constant, *T* [K] is absolute temperature, *q* [C] is elementary charge, *A* [cm2] is area of the cell and *n* [cm-3] is electron density. Con: constant inversely proportional to the electron density, *D*eff [cm2s-1]: diffusion coefficient of electron, *L* [cm]: film thickness of TiO2 electrode, *k*eff [s-1]: reaction rat constant of recombination

the electrolyte phase, *R*Pt []: resistance of Pt electrode, *C*Pt [F]: capacity of Pt electrode, *R*sub []: resistance of substrate, *R*w[]: resistance for electron transport in the TiO2 electrode,

Table 2. Determined parameters concerning with electron transport by impedance

w= 1.48 1.62 2

, *D*I3- [cm2s-1]: diffusion coefficient of I3-

, *δ* [cm]: thickness of

<sup>1</sup> tanh

3

*I*

/

(7)

*pt pt*

(6)

For the impedance concerning with platinum electrode, equation (6) was assumed:

1 *Pt*

3

*I*

*D*

/

*<sup>i</sup> Z R*

*Z*

impedance equation, i.e., equation (7) was assumed:

Deff

L

keff

R

D

R

C

R

Rk/R

R

R

n

reactions, *R*I3- []: diffusion resistance of I3-

*R*k []: resistance for recombination reaction.

spectroscopy for three kinds of cells

*N I*


Table 3. Performance of dye sensitized solar cells with various TNW content.

electron density, becomes smaller with increasing conversion efficiency, i.e., Con value increases in the order of P-25 only > (P-25+PEG) > TNW+(P-25+PEG). Therefore the electron density *n* increases with increasing conversion efficiency. So, the characteristics of highly efficient cells are high electron density, small resistance for the electron transport to the conducting glass electrode, and large ratio of the resistances *R*k/*R*w with small rate constant for recombination reactions.

## **3.4 Effects of content of TNW on the properties of dye-sensitized solar cells**

Since the cells containing TNW gave high conversion efficiencies, we examined the effects of content of TNW in the electrode composed of TNW and P-25 upon the conversion efficiency with variation in TNW from 0 % to 100 %. Content of TNW was defined as percentage of titanium atoms of TNW in the total titanium atoms included in the titania electrode. Table 3 shows performance of DSSCs with various TNW content, i.e., *J*sc, *V*oc, *FF* and *η*, together with film thickness. Effect of TNW content on PCE is shown in Fig. 14.

PCE of the cells including TNW are higher than those cells without TNW, indicating that TNW is useful to attain high efficiency except 100% TNW case. When the film thickness of 100% TNW cells increased larger than 5 m, peel off of the films with cracks was observed by SEM images as shown in Fig. 15, resulting that less than 6% of PCE were observed as shown in Table 3 and Fig. 14. Thus, mixing of TNW with P-25 nanoparticles is important to make robust films.

Since the amount of adsorbed dyes is another important factor to affect PCE, the amounts of adsorbed dyes for the cells with various TNW contents are shown against film thickness in Fig. 16. The amount of adsorbed dye in the cells containing TNW from 0 % to 50 % locates in the same straight line regardless of the difference in TNW contents, except 100 % TNW which shows higher adsorbed amounts.

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

δ

nanoparticles.

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 111

TNW [%] 0 0 055 5 10 10 10 10 Con= 0.309 0.397 0.4 0.316 0.257 0.33 0.22 0.224 0.277 0.23 *D* eff = 0.000038 0.000093 0.00024 0.0000747 0.000095 0.00025 0.00009 0.000085 0.00022 0.00015 *L* = 0.001 0.00264 0.0035 0.00191 0.00191 0.0032 0.0018 0.0018 0.002 0.002 *k* eff = 13.8 7.67 13.8 7.67 7.67 13.8 7.67 7.67 16.3 13.8 *R* I3- = 7.7 7 7 8 7.6 6.5 9.1 8.1 5.1 6.4 *D* I3- = 0.00001 0.000004 0.00000295 0.000007 0.0000068 0.00000328 0.0000027 0.0000057 0.0000045 0.0000045

 = 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 *R* pt = 3.5 6.7 4.8 15 5.8 3.5 4.25 3.4 7 4.6 *C* pt = 0.00006 0.000046 0.000055 0.0000313 0.0000675 0.00005 0.00005 0.00006 0.00004 0.000065 *R* sub = 7.96 8.9 9.8 12 11.79 10.4 8.69 8.92 8.5 9.8 *R* k/*R* w= 2.75 1.74 1.42 2.67 3.4 1.77 3.62 3.42 3.37 2.72 *R* w= 8.13 11.3 5.83 8.08 5.17 4.22 4.4 4.74 2.52 3.07 *R* k= 22.4 19.6 8.28 21.6 17.5 7.47 15.9 16.2 8.5 8.33 *n* = 2.17×1018 1.69×1018 1.67×1018 2.11×1018 2.6×1018 2.03×1018 3.04×1018 2.99×1018 2.42×1018 2.91×1018 TNW [%] 28 28 50 50 50 50 100 100 100 Con= 0.165 0.225 0.206 0.217 0.251 0.15 0.161 0.154 0.185 *D* eff = 0.0001 0.00012 0.00014 0.00018 0.00015 0.000104 0.000072 0.000023 0.00015 *L* = 0.002 0.002 0.00139 0.00139 0.0022 0.0022 0.000925 0.000505 0.0009 *k* eff = 10 12.5 18.65 19.9 7.67 5 8 10.32 18 *R* I3- = 5.6 5.8 6.9 6.9 9.5 11.5 14.5 13.4 11 *D* I3- = 0.000007 0.0000055 0.000005 0.0000053 0.000003 0.0000035 0.000011 0.0000096 3.52E-06 *δ* = 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 0.005 *R* pt = 2 2.8 3 9.85 5 3.88 4 5.8 5.7 *C* pt = 0.0001 0.00006 0.000038 0.00006 0.0000577 0.000045 0.000052 0.00005 0.0000433 *R* sub = 5.6 5.8 9.12 10.1 8.21 8.07 8 10.7 11 *R* k/*R* w= 2.5 2.4 3.89 4.68 4.04 4.3 10.5 8.74 10.3 *R* w= 3.3 3.75 2.04 1.68 3.68 3.17 2.07 3.38 1.11 *R* k= 8.25 9 7.94 7.84 14.9 13.6 21.8 29.5 11.4 *n* = 3.89×1018 2.85×1018 3.25×1018 3.08×1018 2.67×1018 4.46×1018 4.16×1018 4.35×1018 3.62×1018 Table 4. Parameters determined by EIS analysis for the cells with various TNW contents.

This higher adsorption of 100 % TNW is attributed to the smaller diameter of TNW of 3-7 nm, which is much smaller than the diameter of P-25 of 23 nm. The specific surface area of P-25 and the mixture of 28 % TNW with P-25 after calcinations at 773 K for 30 min. were 45 m2/g and 48 m2/g, respectively. These values of specific surface area are much smaller than that of 100 % TNW which is 78 m2/g after calcinations. This difference in specific surface area between 28 % TNW with P-25 and pure 100 % TNW corresponds well to the difference in adsorbed dye amount between from 0 % to 50 % TNW with P-25 and pure 100 % TNW. These findings suggest some interesting structural change in the surface of the mixture of TNW and P-25. However, the reason why the cells containing different TNW content from 0% to 50 % locates in the same straight line in Fig. 16 is not well understood at present.

Resistance for electron transport from titania electrode to the transparent conducting glass electrode *R*w are plotted against TNW content in Fig. 17a. *R*w values decrease steeply up to 10 % and become gradual decrease after 20 % of TNW content. This decrease indicates clearly that electron transport in the titania electrode is improved by mixing TNW with P-25

The ratios of *R*k representing the resistance for the recombination reactions between electrons in the titania electrode and I3- in the electrolyte to *R*w are plotted against TNW

Fig. 14. Effect of content of TNW on power conversion efficiency

Fig. 15. Top views of 100% TNW films. Left: 5 μm, right: 11μm thickness.

Fig. 16. Relationship between film thickness and the amount of dye.


**0% 5% 10% 28% 50% 100%**

0 % 5 % 10% 28% 50% 100%

**[m]**

**0 10 20 30 40**

**0 10 20 30 40**

**Filn thoickness**

F ilm thickness [μm」

**[m]**

**Film thickness**

Fig. 14. Effect of content of TNW on power conversion efficiency

Fig. 15. Top views of 100% TNW films. Left: 5 μm, right: 11μm thickness.

**0.E+00**

Fig. 16. Relationship between film thickness and the amount of dye.

**5.E-08**

**Adsorbed**

**amount**

 **of**

f

d

y

e

A

d

s

o

r

e

a

m

o

u

n

t

o

b

d

**dye [mol/cm2**

m[

ol/c

m2]

**]**

**1.E-07**

**2.E-07**

**2.E-07**

**3.E-07**

**3.E-07**

**5 5.5 6 6.5 7 7.5 8 8.5 9**

**Power conversion**

**efficiency [%]**


Table 4. Parameters determined by EIS analysis for the cells with various TNW contents.

This higher adsorption of 100 % TNW is attributed to the smaller diameter of TNW of 3-7 nm, which is much smaller than the diameter of P-25 of 23 nm. The specific surface area of P-25 and the mixture of 28 % TNW with P-25 after calcinations at 773 K for 30 min. were 45 m2/g and 48 m2/g, respectively. These values of specific surface area are much smaller than that of 100 % TNW which is 78 m2/g after calcinations. This difference in specific surface area between 28 % TNW with P-25 and pure 100 % TNW corresponds well to the difference in adsorbed dye amount between from 0 % to 50 % TNW with P-25 and pure 100 % TNW. These findings suggest some interesting structural change in the surface of the mixture of TNW and P-25. However, the reason why the cells containing different TNW content from 0% to 50 % locates in the same straight line in Fig. 16 is not well understood at present.

Resistance for electron transport from titania electrode to the transparent conducting glass electrode *R*w are plotted against TNW content in Fig. 17a. *R*w values decrease steeply up to 10 % and become gradual decrease after 20 % of TNW content. This decrease indicates clearly that electron transport in the titania electrode is improved by mixing TNW with P-25 nanoparticles.

The ratios of *R*k representing the resistance for the recombination reactions between electrons in the titania electrode and I3- in the electrolyte to *R*w are plotted against TNW

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

thiocyanate decreased redox potential of I-/I3-

efficiency was obtained.

2008).

**Current density [mA/cm**

**-3]**

(Fig. 19)

– Formation of Acid Sites on CeO2-TiO2 Composite and 1-D TiO2 for Dye-Sensitized Solar Cells 113

efficiency with complex titania electrode made of titania nanowires and P-25. Recently, we attained the same conversion efficiency 9.33 % using different electrolyte, i.e., 0.6M 1-butyl 3-methyl imidazolium iodide, 0.1M guanidium thiocyanate, 0.05M I2, 0.5M tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85:15) for a complex titania electrode made of titania nanowires, titania nanoparticles (3-5 nm in diameter) and P-25.

**0 0.2 0.4 0.6**

Fig. 19. I-V curve obtained for a cell with a complex electrode composed of network structure of single-crystal-like titania nanowires, titania nanoparticles and P-25

In our previous paper (Adachi et al. 2004), we used an electrolyte composed of 0.1 M of LiI, 0.6 M of1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M of I2, 1 M of 4-tert-butylpyridine in methoxyacetonitrile and got 9.33 % conversion efficiency with short circuit current density *J*sc=19.2 mA/cm2, open circuit voltage *V*oc=0.72 V and fill factor 0.675. In the recent results, *V*oc value 0.8 V is larger than that of previous one 0.72 V, because guanidium

short circuit current density *J*sc=16.8 mA/cm2 than that of our previous one, and the same

Highly crystallized titania nanorods (TNR) have been synthesized by hydrothermal process using blockcopolymer (F127) and surfactant cetyltrimethylammonium bromide (CTAB) as a mixed template (Jiu et al. 2006). TNR with 100-300 nm in length and 20-30 nm in diameter was obtained. A high-resolution TEM (HRTEM) image of single TNR shows that titanium atoms align perfectly in titania anatase crystalline structure with no lattice defect, and the surface of TNR is facetted with the TiO2 anatase {101} faces (Yoshida et al. 2008). The fringes are {101} planes of anatase TiO2 with a lattice spacing of about 0.351 nm, which agrees with the value recorded in JCPDS card. The highly crystallized titania nanorods prepared successfully were used to fabricate a titania electrode of DSSCs. The complex electrodes were made by the repetitive coating-calcining process: 3 layers of titania nanoparticles (3-5 nm in diameter) were first coated on FTO conducting glass, followed by 8 layers of mixed gel composed of titania nanorods and titania nanoparticles. A high light-to-electricity conversion efficiency of 8.93 % was achieved (Yoshida et al.

*J*sc=16.8 mA/cm2 *V*oc=-0.800 V FF=0.694 efficiency=9.33%

**-Voltage[V]**

**0.8**

in the electrolyte. Unfortunately, we got lower

content in Fig. 17b. The ratio of *R*k/ *R*w increases with increase in TNW content. This shows that TNW restrains the recombination reactions between electrons and I3 - and contributes to collect electrons properly to the transparent conducting glass electrode. The findings shown in Fig. 17 a, b bring the high electron density in the titania electrode as shown in Fig. 17c.

Fig. 17. a) Relationship between *R*w and TNW content, b) relationship between *R*k/*R*w and TNW cointent and c) relationship between electron density and TNW content.

Thus, the conclusion deduced from the experiments of three kinds of cells, i.e., small resistance for the electron transport to the conducting glass electrode, large value of resistance ratio *R*k/*R*w, and high electron density in the titania electrode as the characteristics of highly efficient cells, was confirmed again by the experiments of variation in TNW content.

#### **3.5 Some examples of our highly crystallized one-dimensional TiO2 nanoscale materials for fabricating highly efficient dye-sensitized solar cells**

We succeeded in the preparation of titania nanorods (TNR) (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains (see Fig. 18 ), which have been newly synthesized. We applied these materials for DSSCs.

Fig. 18. TEM image of titania nanochains.

We present highly crystallized one-dimensional titania nanoscale materials are effective to attain high light-to-electricity conversion yield. As shown in our previous paper (Adachi et al. 2004), network structure of single crystal-like titania nanowires can be synthesized successfully by the oriented attachment mechanism. We attained 9.33 % conversion

content in Fig. 17b. The ratio of *R*k/ *R*w increases with increase in TNW content. This shows

collect electrons properly to the transparent conducting glass electrode. The findings shown in Fig. 17 a, b bring the high electron density in the titania electrode as shown in Fig. 17c.

> **0 20 40 60 80 100 TNW content [%]**

Fig. 17. a) Relationship between *R*w and TNW content, b) relationship between *R*k/*R*w and

Thus, the conclusion deduced from the experiments of three kinds of cells, i.e., small resistance for the electron transport to the conducting glass electrode, large value of resistance ratio *R*k/*R*w, and high electron density in the titania electrode as the characteristics of highly efficient cells, was confirmed again by the experiments of variation

We succeeded in the preparation of titania nanorods (TNR) (Jiu et al. 2006), network structure of titania nanowires (Adachi et al. 2004) and one-dimensional titania nanochains (see Fig. 18 ), which have been newly synthesized. We applied these materials for DSSCs.

We present highly crystallized one-dimensional titania nanoscale materials are effective to attain high light-to-electricity conversion yield. As shown in our previous paper (Adachi et al. 2004), network structure of single crystal-like titania nanowires can be synthesized successfully by the oriented attachment mechanism. We attained 9.33 % conversion

and contributes to

**0 20 40 60 80 100 TNW content [%]**

**0.E+00 1.E+18 2.E+18 3.E+18 4.E+18 5.E+18**

*n* **[cm-3]**

that TNW restrains the recombination reactions between electrons and I3-

a b c

TNW cointent and c) relationship between electron density and TNW content.

**3.5 Some examples of our highly crystallized one-dimensional TiO2 nanoscale** 

**materials for fabricating highly efficient dye-sensitized solar cells** 

R k/R w **[-]**

in TNW content.

*R***w [**Ω**]**

> **0 20 40 60 80 100 TNW content [%]**

Fig. 18. TEM image of titania nanochains.

efficiency with complex titania electrode made of titania nanowires and P-25. Recently, we attained the same conversion efficiency 9.33 % using different electrolyte, i.e., 0.6M 1-butyl 3-methyl imidazolium iodide, 0.1M guanidium thiocyanate, 0.05M I2, 0.5M tertbutylpyridine in a mixture of acetonitrile and valeronitrile (85:15) for a complex titania electrode made of titania nanowires, titania nanoparticles (3-5 nm in diameter) and P-25. (Fig. 19)

Fig. 19. I-V curve obtained for a cell with a complex electrode composed of network structure of single-crystal-like titania nanowires, titania nanoparticles and P-25

In our previous paper (Adachi et al. 2004), we used an electrolyte composed of 0.1 M of LiI, 0.6 M of1,2-dimethyl-3-n-propylimidazolium iodide, 0.05 M of I2, 1 M of 4-tert-butylpyridine in methoxyacetonitrile and got 9.33 % conversion efficiency with short circuit current density *J*sc=19.2 mA/cm2, open circuit voltage *V*oc=0.72 V and fill factor 0.675. In the recent results, *V*oc value 0.8 V is larger than that of previous one 0.72 V, because guanidium thiocyanate decreased redox potential of I-/I3 in the electrolyte. Unfortunately, we got lower short circuit current density *J*sc=16.8 mA/cm2 than that of our previous one, and the same efficiency was obtained.

Highly crystallized titania nanorods (TNR) have been synthesized by hydrothermal process using blockcopolymer (F127) and surfactant cetyltrimethylammonium bromide (CTAB) as a mixed template (Jiu et al. 2006). TNR with 100-300 nm in length and 20-30 nm in diameter was obtained. A high-resolution TEM (HRTEM) image of single TNR shows that titanium atoms align perfectly in titania anatase crystalline structure with no lattice defect, and the surface of TNR is facetted with the TiO2 anatase {101} faces (Yoshida et al. 2008). The fringes are {101} planes of anatase TiO2 with a lattice spacing of about 0.351 nm, which agrees with the value recorded in JCPDS card. The highly crystallized titania nanorods prepared successfully were used to fabricate a titania electrode of DSSCs. The complex electrodes were made by the repetitive coating-calcining process: 3 layers of titania nanoparticles (3-5 nm in diameter) were first coated on FTO conducting glass, followed by 8 layers of mixed gel composed of titania nanorods and titania nanoparticles. A high light-to-electricity conversion efficiency of 8.93 % was achieved (Yoshida et al. 2008).

Utilization of Nanoparticles Produced by Aqueous-Solution Methods

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Fig. 20. I-V curve obtained for the cell composed of one-dimensional chains of titania nanoparticles mixed with fine titania nanoparticles (3 - 5 nm in diameter).

We have newly synthesized titania nanochains as shown in Fig. 18. Highly crystallized titania nanoparticles with diameter of around 10 nm combine with each other and make chains. The obtained white solid product was mixed with spherical titania nanoparticles (3-5 nm in diameter) synthesized using F127 reported in our previous paper (Jiu et al. 2004, Jiu et al. 2007) to fabricate titania film electrodes. The I-V curve of the cell is shown in Fig. 20. The obtained light-to-electricity conversion yield of the cell was 9.2%.

All three kinds of one-dimensional titania nanoscale materials mentioned above show high light-to-electricity conversion yield around 9%, suggesting strongly that highly crystallized one-dimensional titania materials are essentially important for attainment of high efficient dye-sensitized solar cells.

#### **3.6 Conclusions of 3rd section**


## **4. References**

114 Smart Nanoparticles Technology

*J*sc **= 19.8 mA/cm2**

*V*oc **= 0.8 V** *FF* **= 0.704**

= 9.19 %

Fig. 20. I-V curve obtained for the cell composed of one-dimensional chains of titania

η

nanoparticles mixed with fine titania nanoparticles (3 - 5 nm in diameter).

The obtained light-to-electricity conversion yield of the cell was 9.2%.

i.e., very small *R*w is indispensable for the highly efficient cells.

is large. 3) Electron density *n* in the titania electrode is high.

nanochains, show high power conversion efficiency about 9 %.

**0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7**

**-Voltage [V]**

We have newly synthesized titania nanochains as shown in Fig. 18. Highly crystallized titania nanoparticles with diameter of around 10 nm combine with each other and make chains. The obtained white solid product was mixed with spherical titania nanoparticles (3-5 nm in diameter) synthesized using F127 reported in our previous paper (Jiu et al. 2004, Jiu et al. 2007) to fabricate titania film electrodes. The I-V curve of the cell is shown in Fig. 20.

All three kinds of one-dimensional titania nanoscale materials mentioned above show high light-to-electricity conversion yield around 9%, suggesting strongly that highly crystallized one-dimensional titania materials are essentially important for attainment of high efficient

1. Many researchers familiar with EIS measurement know that highly efficient dyesensitized solar cells show small total resistance of the cell, i.e., small Nyquist spectrum. They also know that largest arc of Nyquist plot represents the resistance for recombination reactions *R*k. This apparent conflict is solved clearly by theoretical consideration through recognition that the large value of the ratio *R*k/*R*w is essentially important for the highly efficient cells, and the absolute value of *R*k is not important,

2. The experimental results of I-V and EIS measurements of the three kinds of cells made of P-25 only, P-25+PEG, and TNW+P-25+PEG and also cells made of various content of TNW with P-25+PEG clearly showed the following three points as characteristics of highly crystallized 1-dimensional titania nanoscale material TNW. 1) Resistance of electron transport in the titania electrode *R*w is small. 2) The ratio of resistance *R*k/*R*w

3. All cells composed of three kinds of highly crystallized 1-dimensional titania nanoscale materials, i. e., network structure of titania nanowires, titania nanorods, and titania

**0**

**5**

**10**

**Current density [mA/cm2]**

dye-sensitized solar cells.

**3.6 Conclusions of 3rd section** 

**15**

**20**


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**6** 

 *Australia* 

**Experimental and Theoretical Study of** 

, Yusuf Valentino Kaneti and Aibing Yu

**Low-Dimensional Iron Oxide Nanostructures** 

*School of Materials Science and Engineering, University of New South Wales, Sydney,* 

Iron oxide has many phases, including 16 pure phases (e.g., FeO, Fe3O4), 5 polymorphs of FeOOH (e.g., -FeOOH, -FeOOH) and 4 kinds of Fe2O3 (e.g., -Fe2O3, -Fe2O3). Because of their unique properties (optical, electronic, magnetic), they have found many applications in the areas of catalysts, magnetic recording, sorbents, pigments, flocculants, coatings, gas sensors, lubrications, and biomedical applications (e.g., magnetic resonance imaging, drug

Many efforts have been made in the synthesis (co-precipitation, hydrothermal, microemulsion, and sol-gel method), structural characterization, and functional exploration, as well as fundamental understandings of iron oxide nanostructures. Despite some success, several challenges still exist regarding the synthesis, strcture, properties, and fundamental understanding of the iron oxides. the grand challenge is how to efficiently synthesize iron oxides with controlled morphology, size and functionality, and how to fundamentally understand the formation and growth mechanisms, structure, and interaction forces. Therefore, the development of simple but effective experimental and theoretical strategies to

To fundamentally understand the nanoscale system, theoretical methods should exist. Computational modeling is one of the most important enabling techniques in nanotechnology and material research. It can increase the pace of discovery across the entire scientific scope, and reduce the cost in the development and commercialization of technologies and materials. Various computational approaches have been developed and used to predict the materials properties (e.g., electronic, magnetic, optical) at different length and time scales. For example, at an atomic scale, density functional theory (DFT) is widely used for binding energy calculation, while at a microscopic scale, molecular dynamics (MD)

This Chapter will give a brief overview of the experimental and theoretical methods conducted on iron oxide nanostructures, particularly for low-dimensional iron oxide nanoparticles. This includes: (i) several representative methods for iron oxides nanomaterials in Sections 2 and 3; (ii) surface modified iron oxide nanostructures by

Jeffrey Yue, Xuchuan Jiang

**1. Introduction** 

delivery and therapy).

 

Corresponding Author

overcome the challenges is still imperative.

are able to provide insights into atomic/molecular systems.

nanotube and nanowire arrays for oxidative photoelectrochemistry. *J. Phys. Chem. C*, 113, pp. 6327-6359


## **Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures**

Jeffrey Yue, Xuchuan Jiang , Yusuf Valentino Kaneti and Aibing Yu *School of Materials Science and Engineering, University of New South Wales, Sydney, Australia* 

## **1. Introduction**

118 Smart Nanoparticles Technology

Shankar, K., Bandara, J., Paulose, M., Wietasch, H., Varghese, O. K., Mor, G. K., LaTempa, T.

Sugimoto, T.; Zhou, X. & Muramatsu, A. (2003) Synthesis of uniform anatase TiO2

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Yoshida, K. Jiu, J. Nagamatsu, D. Nemoto, T. Kurata, H. Adachi, M. & Isoda, S. (2008).

Youngblood, J. W., Lee, S-H. A., Kobayashi, Y., Hernandez-Pagan, E. A., Hoertz, P. G.,

Zaki, M. I.; M Hussein, G. A.; Mansour, S. A. A. & El-Ammawy, H. A. (1989) Adsorption

Zaki, M. I.; Hasan, M. A. & Pasupulety, L. (2001) Surface Reactions of Acetone on Al2O3,

Zhou, X. -D.; Huebner, W. & Anderson, H. U. (2003) Processing of Nanometer-Scale CeO2

Zhou, K.; Wang, X.; Peng, Q. & Li, Y. (2005) Enhanced catalytic activity of ceria nanorods

Zhong, L-S.; Hu, J-S.; Cao, A-M.; Liu, Q.; Song, W-G.; & Wan, L-J. (2007) 3D Flowerlike Ceria

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sensitized solar cells. *ChemPhysChem*, 10, pp. 290-299

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Acid−Base Properties. *Langmuir.,* 17, pp. 768-774.

Particles. *Chem. Mater.* 15, pp. 378-382.

Removal. *Chem. Mater.* 19, pp. 1648-1655.

53-61.

nanotube and nanowire arrays for oxidative photoelectrochemistry. *J. Phys. Chem.* 

J., Thelakkat, M., & Grimes, C. A. (2008) Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass:

nanoparticles by gel–sol method: 4. Shape control. *J. Colloid Interface Sci.,* 259, pp.

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morphology, diameter, and length: Synthesis and photo-electrical/catalytic

influence of charge transport and recombination on the performance of dye-

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and surface reactions of pyridine on pure and doped ceria catalysts as studied by

TiO2, ZrO2, and CeO2: IR Spectroscopic Assessment of Impacts of the Surface

Micro/Nanocomposite Structure and Its Application for Water Treatment and CO

Iron oxide has many phases, including 16 pure phases (e.g., FeO, Fe3O4), 5 polymorphs of FeOOH (e.g., -FeOOH, -FeOOH) and 4 kinds of Fe2O3 (e.g., -Fe2O3, -Fe2O3). Because of their unique properties (optical, electronic, magnetic), they have found many applications in the areas of catalysts, magnetic recording, sorbents, pigments, flocculants, coatings, gas sensors, lubrications, and biomedical applications (e.g., magnetic resonance imaging, drug delivery and therapy).

Many efforts have been made in the synthesis (co-precipitation, hydrothermal, microemulsion, and sol-gel method), structural characterization, and functional exploration, as well as fundamental understandings of iron oxide nanostructures. Despite some success, several challenges still exist regarding the synthesis, strcture, properties, and fundamental understanding of the iron oxides. the grand challenge is how to efficiently synthesize iron oxides with controlled morphology, size and functionality, and how to fundamentally understand the formation and growth mechanisms, structure, and interaction forces. Therefore, the development of simple but effective experimental and theoretical strategies to overcome the challenges is still imperative.

To fundamentally understand the nanoscale system, theoretical methods should exist. Computational modeling is one of the most important enabling techniques in nanotechnology and material research. It can increase the pace of discovery across the entire scientific scope, and reduce the cost in the development and commercialization of technologies and materials. Various computational approaches have been developed and used to predict the materials properties (e.g., electronic, magnetic, optical) at different length and time scales. For example, at an atomic scale, density functional theory (DFT) is widely used for binding energy calculation, while at a microscopic scale, molecular dynamics (MD) are able to provide insights into atomic/molecular systems.

This Chapter will give a brief overview of the experimental and theoretical methods conducted on iron oxide nanostructures, particularly for low-dimensional iron oxide nanoparticles. This includes: (i) several representative methods for iron oxides nanomaterials in Sections 2 and 3; (ii) surface modified iron oxide nanostructures by

Corresponding Author

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 121

**Crystallographic structural features References** 

Cornell and Schwertmann,

Cudennec and Lecerf, 2005

Cornell and Schuwertmann,

Cornell and Schuwertmann,

Cornell and Schuwertmann,

Mohapatra and Anand,2010

Cornell and Schuwertmann,

Weckler and Lutz, 1998

Cornell and Schuwertmann,

Mohapatra and Anand,2010

1991;

1991

1991;

1991; Garcia et al.,

2009

1991

1991

3D-structure built up with FeO3(OH) octahedra spreading along the (010) direction, with each octahedron linked to eight neighbouring octahedral by four edges and three vertices. Oxygen atoms are in tetrahedral surroundings,

either OFe3H or OFe3H (bond).

Double chains of edge linked Fe(O, OH) octahedral that share corners to form a framework containing large tunnels with square cross sections.

Arrays of close cubic -packed anions (O2-/OH-) stacked along the [150] direction with Fe3+ ions occupying the

Disordered hexagonally close-packed

Stacking of sheets of octahedrally (sixfold) coordinated Fe3+ ions. Between two close-packed layers of oxygen ions. Each oxygen ion is bonded to

Each cell of maghemite contains 32 O2 ions, 21(1/3) Fe3+ ions and 2 (1/3) O vacancies. Eight cations occupy tetrahedral sites and the remaining cations are distributed over the octahedral sites. The vacancies are confined to octahedral sites.

Inverse spinel structure with a facecentered cubic cell based on 32 O2 ions, regularly close-cubic packed along [111], with Fe2+ ions and half of the Fe3+ occupying the octahedral sites

and the other half of Fe3+ ions, occupying the tetrahedral sites.

array of anions with Fe3+ ions distributed over half the octahedral

sites in an orderly manner.

only two Fe ions.

octahedral interstices.

**Iron oxides Crystallographic system** 

> Orthorhombica = 0.9956 nm, b = 0.30215 nm and c = 0.4608 nm

Monoclinic a = 1.056 nm, b = 0.3031 nm and c = 1.0483 nm

Orthorhombic a = 0.3071 nm, b = 1.2520 nm and c = 0.3873 nm

Hexagonal a = 0.293 nm and c = 0.456 nm

Hexagonal a= 0.5035 nm and c = 1.375 nm

Cubic

Cubic

a = 0.8396 nm

Table 1. Complicated phases and polymorphs of iron oxides in nature

a= 0.83474 nm

Goethite (-FeOOH)

Akaganéite (β-FeOOH)

Lepidocrocite (γ-FeOOH)

Feroxyhyte (δ'-FeOOH)

Hematite (α-Fe2O3)

Maghemite (γ-Fe2O3)

Magnetite (Fe3O4)

surfactants, polymers, silica or metals in Section 4; (iii) functional properties of such nanomaterials in gas sensing, catalysis, and biotechnology in Section 5. Moreover, in Section 6, the discussion will be extended to the theoretical modeling and simulation methods that can predict the formation and performance of nanomaterials, such as MD and DFT methods.

## **2. Iron oxide materials**

Iron is the fourth most abundant element in the Earth's crust, and iron oxides are commonly found in nature and have become the most plentiful transition metal oxides (Morrissey and Guerinot, 2009; Ilani et al., 1999). The complicated phases and features of iron oxides have been listed in **Table 1**.

Some crystalline phases of iron oxides are not very stable and can convert into others. Much work has been conducted to convert akaganéite to hematite and/or magnetite phases to pursuit good performance in catalysis and gas sensing applications. Magnetite nanorods can be produced by the conversion of iron oxyhydroxides into a thermally stable structure of hematite by heating above 400 ºC in air, or magnetite in a mixture of H2 and Ar gas (Bomati-Miguel et al., 2008). Recently, our group has simplified the phase conversion procedures among the iron oxides. The iron oxyhydroxides can directly convert into magnetite by using hydrazine as a reducing agent, and the morphology was maintained. Using this method, magnetite nanorods could be directly synthesized from akaganéite rather than using hematite as an intermediate (Yue et al., 2010).

By using hydrazine, iron(III) ions can be reduced to iron(II). The change in coordination number to the iron atom will therefore transfer from FeOH to FeO following dehydration. The structure change caused by loss of H2O will create pores or holes within the nanorod framework. Continuous reaction with hydrazine can form larger defects in the 1-D nanostructure, leading to the collapse of the framework. At the same time, the FeO6 units will reconstruct into other crystals, and the broken fractions could fuse with neighboring particles to form larger ones. However, this does not happen to hematite because of its thermally stable structure under the considered conditions. The nature of such a conversion needs further investigation. Nevertheless, the proposed approach could be used for a controlled conversion of akaganéite to magnetite nanostructures (**Fig. 1**) without hightemperature treatment. These porous magnetic structures would find more applications in electronic and magnetic areas (Yue et al., 2011).

## **3. Synthesis methods**

A variety of methods have been reported to synthesize iron oxide nanoparticles, including solid-state, liquid-phase, and gas-phase syntheses, as listed in **Table 2**. Among the synthesis approaches, liquid-phase synthesis is the most popular. The iron salts are highly soluble in water and different additives can be used in conjunction to modify the structure of the nanoparticles. Moreover, the liquid-phase synthesis is convenient for understanding ageing, recrystallization, and evolution into other shapes and sizes. It is also available for controlling experimental conditions in liquid (e.g., concentration, salt precursor, pH, temperature, surface modifiers). A few representative synthesis methods are briefly introduced in this Section, such as co-precipitation, hydrothermal and microemulsion.

surfactants, polymers, silica or metals in Section 4; (iii) functional properties of such nanomaterials in gas sensing, catalysis, and biotechnology in Section 5. Moreover, in Section 6, the discussion will be extended to the theoretical modeling and simulation methods that can predict the formation and performance of nanomaterials, such as MD

Iron is the fourth most abundant element in the Earth's crust, and iron oxides are commonly found in nature and have become the most plentiful transition metal oxides (Morrissey and Guerinot, 2009; Ilani et al., 1999). The complicated phases and features of iron oxides have

Some crystalline phases of iron oxides are not very stable and can convert into others. Much work has been conducted to convert akaganéite to hematite and/or magnetite phases to pursuit good performance in catalysis and gas sensing applications. Magnetite nanorods can be produced by the conversion of iron oxyhydroxides into a thermally stable structure of hematite by heating above 400 ºC in air, or magnetite in a mixture of H2 and Ar gas (Bomati-Miguel et al., 2008). Recently, our group has simplified the phase conversion procedures among the iron oxides. The iron oxyhydroxides can directly convert into magnetite by using hydrazine as a reducing agent, and the morphology was maintained. Using this method, magnetite nanorods could be directly synthesized from akaganéite rather than using

By using hydrazine, iron(III) ions can be reduced to iron(II). The change in coordination number to the iron atom will therefore transfer from FeOH to FeO following dehydration. The structure change caused by loss of H2O will create pores or holes within the nanorod framework. Continuous reaction with hydrazine can form larger defects in the 1-D nanostructure, leading to the collapse of the framework. At the same time, the FeO6 units will reconstruct into other crystals, and the broken fractions could fuse with neighboring particles to form larger ones. However, this does not happen to hematite because of its thermally stable structure under the considered conditions. The nature of such a conversion needs further investigation. Nevertheless, the proposed approach could be used for a controlled conversion of akaganéite to magnetite nanostructures (**Fig. 1**) without hightemperature treatment. These porous magnetic structures would find more applications in

A variety of methods have been reported to synthesize iron oxide nanoparticles, including solid-state, liquid-phase, and gas-phase syntheses, as listed in **Table 2**. Among the synthesis approaches, liquid-phase synthesis is the most popular. The iron salts are highly soluble in water and different additives can be used in conjunction to modify the structure of the nanoparticles. Moreover, the liquid-phase synthesis is convenient for understanding ageing, recrystallization, and evolution into other shapes and sizes. It is also available for controlling experimental conditions in liquid (e.g., concentration, salt precursor, pH, temperature, surface modifiers). A few representative synthesis methods are briefly introduced in this

and DFT methods.

**2. Iron oxide materials** 

been listed in **Table 1**.

hematite as an intermediate (Yue et al., 2010).

electronic and magnetic areas (Yue et al., 2011).

Section, such as co-precipitation, hydrothermal and microemulsion.

**3. Synthesis methods** 


Table 1. Complicated phases and polymorphs of iron oxides in nature

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 123

**Particle shape/size**

Spheres D= 2.9-3.6 nm

Nanospheres (d = 30 - 80 nm)

Nanorods (l: 400-600 nm, w =20-30 nm) Nanodiscs (d = 50 nm, thickness = 6.5

nm)

Nanospheres (d = 16 nm)

Nanoparticles (24-52 nm)

Nanoparticles (3-5 nm)

Nanorods 10-80 nm in diameter

(d= 3-20 nm)

Hollow nanospheres, (d =300 nm)

Nanowires (30 nm 1-5

Nanobelts (100 nm 7 µm)

µm)

**Features References**

Lu et al. (2007)

et al. (2007)

Li et al. (2006); Yue et al. (2010, 2011); Jiang et al. (2009)

Dong and Zhu

Vidal-Vidal et al. (2006)

Vijayakumar et al. (2000, 2001)

Zhang et al (2007); Pascal et al (1999)

González-Carreño et al. (1993)

Morber et al. (2006)

Liu

Sun et al. (2004)

(2004)

Mechanical energy to smash.

Low

Hightemperature decomposition of iron organic precursors.

Dissolve, condensation, and calcinations of alkoxides.

Reaction in two immiscible phases (water and oil).

Ultrasound to promote chemical reaction.

Electrons act as reactant with no pollution.

Spraying, aerosol evaporation, condensation, drying, and thermolysis.

Heating of a gaseous mixture of iron precursor.

Ageing of ferric and ferrous salts in a basic medium.

temperature, reaction, commonly conducted in autoclaves, and high efficiency.

**Common products**

δ-Fe2O3, Fe3O4

Fe2O3, Fe3O4

δ-Fe2O3, Fe3O4 (-FeOOH β-FeOOH

α-Fe2O3, δ-Fe2O3, Fe3O4

Fe2O3, Fe3O4

Fe2O3, Fe3O4

Fe3O4

Fe3O4

α-Fe2O3, δ-Fe2O3, ε-Fe2O3 Fe3O4

Electrochemical -Fe2O3 Nanoparticles

Table 2. Several typical synthesis methods for iron oxides

Co-precipitation α-Fe2O3, δ-

**Synthesis media** 

Liquid state

**Synthesis methods**

milling

Hydrothermal α-Fe2O3,

decomposition

Sol-gel α-Fe2O3, δ-

Microemulsion α-Fe2O3, δ-

Sonochemical Fe2O3,

Gas state Spray pyrolysis δ-Fe2O3,

Laser pyrolysis or deposition

Solid state Mechanical

Thermal

Fig. 1. TEM images showing the conversion of nanorods: (A) β-FeOOH nanorods, (B) β-FeOOH nanorods calcined at 300 ºC, (C) β-FeOOH nanorods reduced with N2H4 at 80 ºC; and (D-F) the corresponding HRTEM images with labeled lattice spacing and crystal planes. Reprinted with permission from (Yue et al., 2011).

## **3.1 Co-precipitation**

One simple and efficient way is to use co-precipitation technique in solution. By this approach, iron(II) and/or iron(III) salts are first dissolved in aqueous solution, and then one alkaline media (e.g., NaOH, Na2CO3) solution is added to form precipitate. The prepared particles can be tuned to be uniform in size, shape as well as pure in its composition. Various crystalline phases of iron oxides can be produced using this method, which is controlled by experimental parameters such as types of iron salts (e.g., chloride, sulphate and nitrate), alkaline media, concentration, temperature, and pH (Iida et al., 2007).

Moreover, the phase of iron oxide(s) formed through the co-precipitate approach is often reported as goethite or hematite if iron(III) salt is used. However, the initially precipitated material is usually found as ferrihydrite, which is a thermodynamically unstable phase. The precipitate can further convert into other phases (e.g., hematite, magnetite) depending on the pH, ionic medium, and temperature. For example, Varada et al. (2002) prepared monodispersed acicular goethite particles by precipitating Fe(III) using sodium carbonate. If sodium hydroxide was used, the axial ratio of particles will increase from 60 to 230 nm. It was proposed that different bases have different ability to maintain the solution at a constant pH, where other pH levels would produce polydispersed and hematite particles. The mechanism of the growth of spherical hematite nanoparticles has been explored by Liu et al.(2007). The variation in the final pH of the solution plays a key role in the formation of hematite at different sizes. They found that the particles with diameter of 60-80 nm were obtained at pH 7, while reduced to 30-40 nm in diameter at pH 9.

Fig. 1. TEM images showing the conversion of nanorods: (A) β-FeOOH nanorods, (B) β-FeOOH nanorods calcined at 300 ºC, (C) β-FeOOH nanorods reduced with N2H4 at 80 ºC; and (D-F) the corresponding HRTEM images with labeled lattice spacing and

One simple and efficient way is to use co-precipitation technique in solution. By this approach, iron(II) and/or iron(III) salts are first dissolved in aqueous solution, and then one alkaline media (e.g., NaOH, Na2CO3) solution is added to form precipitate. The prepared particles can be tuned to be uniform in size, shape as well as pure in its composition. Various crystalline phases of iron oxides can be produced using this method, which is controlled by experimental parameters such as types of iron salts (e.g., chloride, sulphate and nitrate), alkaline media, concentration, temperature, and pH (Iida

Moreover, the phase of iron oxide(s) formed through the co-precipitate approach is often reported as goethite or hematite if iron(III) salt is used. However, the initially precipitated material is usually found as ferrihydrite, which is a thermodynamically unstable phase. The precipitate can further convert into other phases (e.g., hematite, magnetite) depending on the pH, ionic medium, and temperature. For example, Varada et al. (2002) prepared monodispersed acicular goethite particles by precipitating Fe(III) using sodium carbonate. If sodium hydroxide was used, the axial ratio of particles will increase from 60 to 230 nm. It was proposed that different bases have different ability to maintain the solution at a constant pH, where other pH levels would produce polydispersed and hematite particles. The mechanism of the growth of spherical hematite nanoparticles has been explored by Liu et al.(2007). The variation in the final pH of the solution plays a key role in the formation of hematite at different sizes. They found that the particles with diameter of 60-80 nm were obtained at pH 7, while reduced to 30-40 nm in diameter at

crystal planes. Reprinted with permission from (Yue et al., 2011).

**3.1 Co-precipitation** 

et al., 2007).

pH 9.


Table 2. Several typical synthesis methods for iron oxides

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 125

Microemulsion method (surfactant-stabilized water/oil (W/O) microemulsion) has been widely used to prepare shape- and size-controlled iron oxide nanoparticles. Generally, a microemulsion is transparent, isotropic and thermodynamically stable dispersion of two immiscible phases (e.g., water and oil). When a surfactant is present in W/O system, the surfactant molecules may form a monolayer at the interface of oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the oil phase and the hydrophilic head groups in the aqueous phase (Wu et al., 2008). In a binary system such as water/surfactant or oil/surfactant, a variety of self-assembled structures can be formed, ranging from spherical and cylindrical micelles to lamellar phases or bi-continuous microemulsions depending on the molar ratio of water, oil and surfactant(s). This will be

For example, magnetite nanoparticles ~4 nm in diameter have been prepared by the controlled hydrolysis of ammonium hydroxide with FeCl2 and FeCl3 aqueous solution within the reverse micelles nanocavities generated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and heptane as a continuous oil phase (López-Quintela and Rivas, 1993). Lee and co-workers (2005) have successfully synthesized uniform and highly crystalline magnetite nanoparticles in microemulsion nanoreactors. The particle size of the prepared magnetite nanoparticles could be adjusted from 2-10 nm by varying the relative concentrations of iron salt, surfactant, and solvent. Li et al (2009) demonstrated the effect of volumetric ratios of aqueous FeCl3 solution to 1,2-propanediamine on the formation of magnetic particles, as shown in **Fig. 3**. Chin and Yaacob (2007) reported the synthesis of

Fig. 3. SEM images of the products obtained at different volume ratios of aqueous FeCl3 solution to 1,2 propanediamine: (a) without 1,2 propanediamine, (b) 3:1, (c) 1:1, (d) 1:2,

(e) 1:4, and (f) 1:5. Reprinted with permission from (Li et al., 2009).

useful for the generation of nanoparticles with different shapes and sizes.

**3.3 Microemulsion** 

## **3.2 Hydrothermal and thermal decomposition methods**

Hydrothermal technique is defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under a high pressure and a temperature (6-10 atm, 100- 200 C). A hydrothermal reaction requires the iron(III) salt (e.g., iron chloride, nitrate, or sulphate), which can be dissolved in solution followed by reaction with water. This is different from the thermal decomposition reaction that generally takes place for those iron organic precursors (Fe(CO)5, Fe(acac)3, and Fe(cup)3)in an organic solvent at high temperatures (Hyeon et al., 2001; Li et al., 2004; Rockenberger et al., 1999). Both hydrothermal and thermal decomposition methods are commonly used for the synthesis of iron oxide nanoparticles.

The hydrothermal method is often performed in an autoclave, where the reaction system can exceed the boiling point of liquid(s) at normal atmospheric pressure (Jia et al., 2005). The temperature can alter the system in such a way that disrupts the thermodynamics of a material, which is governed by enthalpy (ΔH) and entropy (ΔS), and hence Gibbs free energy (ΔG). The essential role of a fluid under high temperatures is that it changes the vapor pressure of the fluid. This is also beneficial for diverse choices of solvents (polar and non-polar). The morphology and crystalline phase of iron oxides produced through this approach can vary by simply tuning reaction temperature, concentration, and additive(s) (Almeida et al., 2009; Jiang et al., 2010).

The synthesis of iron oxide nanoparticles via a hydrothermal approach can be conducted with or without the use of surfactant(s). Hematite nanoparticles have been prepared by Sahu et al*.* (1997) under conditions of pH (3-10) and 180 °C in autoclaves. In this study, the average particle size of hematite nanoparticles was found to decrease with an increase of pH. In our recent work (Jiang et al*.* 2010), we reported a facile hydrothermal route for the synthesis of monodispersed hematite nanodiscs with diameters of ~ 50 nm and thickness of ~6.5 nm in the absence of any surfactants in water at around 90 C (**Fig. 2**). The nanodiscs exhibited interesting paramagnetic property at a low temperature (20 K), but ferromagnetic at room temperature (~300 K). In addition, the hematite nanodiscs also showed lowtemperature catalytic activity in CO oxidation to CO2.

Fig. 2. A) TEM image of α-Fe2O3 nanodiscs with overlapping as pointed by arrows; B) HRTEM image showing the lattice fringe of {110} plates with spacing between two adjacent planes of 0.411 nm. Reprinted with permission from (Jiang et al. 2010).

## **3.3 Microemulsion**

124 Smart Nanoparticles Technology

Hydrothermal technique is defined as any heterogeneous reaction in the presence of aqueous solvents or mineralizers under a high pressure and a temperature (6-10 atm, 100- 200 C). A hydrothermal reaction requires the iron(III) salt (e.g., iron chloride, nitrate, or sulphate), which can be dissolved in solution followed by reaction with water. This is different from the thermal decomposition reaction that generally takes place for those iron organic precursors (Fe(CO)5, Fe(acac)3, and Fe(cup)3)in an organic solvent at high temperatures (Hyeon et al., 2001; Li et al., 2004; Rockenberger et al., 1999). Both hydrothermal and thermal decomposition methods are commonly used for the synthesis of

The hydrothermal method is often performed in an autoclave, where the reaction system can exceed the boiling point of liquid(s) at normal atmospheric pressure (Jia et al., 2005). The temperature can alter the system in such a way that disrupts the thermodynamics of a material, which is governed by enthalpy (ΔH) and entropy (ΔS), and hence Gibbs free energy (ΔG). The essential role of a fluid under high temperatures is that it changes the vapor pressure of the fluid. This is also beneficial for diverse choices of solvents (polar and non-polar). The morphology and crystalline phase of iron oxides produced through this approach can vary by simply tuning reaction temperature, concentration, and additive(s)

The synthesis of iron oxide nanoparticles via a hydrothermal approach can be conducted with or without the use of surfactant(s). Hematite nanoparticles have been prepared by Sahu et al*.* (1997) under conditions of pH (3-10) and 180 °C in autoclaves. In this study, the average particle size of hematite nanoparticles was found to decrease with an increase of pH. In our recent work (Jiang et al*.* 2010), we reported a facile hydrothermal route for the synthesis of monodispersed hematite nanodiscs with diameters of ~ 50 nm and thickness of ~6.5 nm in the absence of any surfactants in water at around 90 C (**Fig. 2**). The nanodiscs exhibited interesting paramagnetic property at a low temperature (20 K), but ferromagnetic at room temperature (~300 K). In addition, the hematite nanodiscs also showed low-

Fig. 2. A) TEM image of α-Fe2O3 nanodiscs with overlapping as pointed by arrows; B) HRTEM image showing the lattice fringe of {110} plates with spacing between two adjacent planes of 0.411 nm. Reprinted with permission from (Jiang et al. 2010).

**3.2 Hydrothermal and thermal decomposition methods** 

iron oxide nanoparticles.

(Almeida et al., 2009; Jiang et al., 2010).

temperature catalytic activity in CO oxidation to CO2.

Microemulsion method (surfactant-stabilized water/oil (W/O) microemulsion) has been widely used to prepare shape- and size-controlled iron oxide nanoparticles. Generally, a microemulsion is transparent, isotropic and thermodynamically stable dispersion of two immiscible phases (e.g., water and oil). When a surfactant is present in W/O system, the surfactant molecules may form a monolayer at the interface of oil and water, with the hydrophobic tails of the surfactant molecules dissolved in the oil phase and the hydrophilic head groups in the aqueous phase (Wu et al., 2008). In a binary system such as water/surfactant or oil/surfactant, a variety of self-assembled structures can be formed, ranging from spherical and cylindrical micelles to lamellar phases or bi-continuous microemulsions depending on the molar ratio of water, oil and surfactant(s). This will be useful for the generation of nanoparticles with different shapes and sizes.

For example, magnetite nanoparticles ~4 nm in diameter have been prepared by the controlled hydrolysis of ammonium hydroxide with FeCl2 and FeCl3 aqueous solution within the reverse micelles nanocavities generated by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant and heptane as a continuous oil phase (López-Quintela and Rivas, 1993). Lee and co-workers (2005) have successfully synthesized uniform and highly crystalline magnetite nanoparticles in microemulsion nanoreactors. The particle size of the prepared magnetite nanoparticles could be adjusted from 2-10 nm by varying the relative concentrations of iron salt, surfactant, and solvent. Li et al (2009) demonstrated the effect of volumetric ratios of aqueous FeCl3 solution to 1,2-propanediamine on the formation of magnetic particles, as shown in **Fig. 3**. Chin and Yaacob (2007) reported the synthesis of

Fig. 3. SEM images of the products obtained at different volume ratios of aqueous FeCl3 solution to 1,2 propanediamine: (a) without 1,2 propanediamine, (b) 3:1, (c) 1:1, (d) 1:2, (e) 1:4, and (f) 1:5. Reprinted with permission from (Li et al., 2009).

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 127

2008). It has shown that through careful choice of the passivating and activating polymers and/or reaction conditions, polymer-stabilized iron oxide nanoparticles with tailored and

The iron oxide particles by ionic properties can be modified with functional polymer groups with COOH, NH2 (Chibowski et al., 2009; Kandori et al., 2005; Li et al., 2004). The polymer coated particles can be synthesized by the *ex situ* method, i.e. dispersion of the nanoparticles in a polymeric solution, or *in situ* method, i.e. monomer polymerization in the

Polymeric coating materials can be classified into two main classes: natural (e.g., dextran, starch, gelatin, chitosan) and synthetic (e.g., polyethylene glycol, PEG; polymethylmethacrylate, PMMA; polyacrylic acid, PAA). However, the saturation magnetization value of iron oxide nanoparticles will decrease after polymer-fictionalization. Dextran is often utilized as a coating polymer because of its stability and biocompatibility (Laurent et al. 2008). Molday and Mackenzie (Molday and Mackenzie, 1982) have reported the formation of Fe3O4 in the presence of dextran with molecular weight (MW) of 40,000. In the synthesis of dextran-coated ultra-small superparamagnetic iron oxides (USPIO), the reduction of the terminal glucose of dextran was found to be significant for controlling particle size, stability, and magnetic properties. For low molecular weight dextrans (MW,

Polyvinyl alcohol (PVA) is a hydrophilic and biocompatible polymer that can be used for particle surface modification to prevent particle agglomeration (Laurent et al. 2008). Lee et al. (1996) have modified the surface of magnetite nanoparticles with PVA by precipitation of iron salts at a high pH (13.8) to form stable magnetite colloidal dispersions, and particle size is around 4 nm. The investigators noted that the crystallinity of the magnetite nanoparticles decreased with PVA concentration increasing, although morphology and particle sizes remained. When PVA is introduced, it reacts with the surface through hydrogen bonding between polar functional groups of the polymer and hydroxylated and/or protonated surface of the iron oxide. In addition to the polymer-surface interactions, PVA is known for its hydrogen bonding interaction, resulting in hydrogel structure embedding the nanoparticles. When the PVA concentration is over the critical saturation value,

Polymerized tetraethoxysilane (TEOS) network is often used as a surface coating material for iron oxide nanoparticles as this coating can prevent aggregation in solution, improve the chemical stability, and provide better protection against toxicity (Laurent et al. 2008). Additionally, polymerized silica-coated iron oxide nanoparticles exhibited good biocompatibility and solubility in water. Silica coating can stabilize the magnetite nanoparticles in two different ways: one is by shielding the magnetic dipole interaction with the silica shell, and another one is by enhancing the coulomb repulsion of the magnetic nanoparticles. Such a silica coating increases the size of the particles and decreases the

presence of the synthesized nanoparticles (Mammeri et al., 2005; Guo et al., 2007).

<10,000), it is difficult to obtain nanoparticles with a small size of <20 nm.

agglomeration may occur for PVA-coated particles via bridging interactions.

desired properties can be synthesized.

**4.3 Polymerized amorphous silica** 

saturation magnetization value.

magnetic iron oxide nanoparticles with an average particle size of <10 nm by mixing two microemulsion systems, one containing Fe2+ ions and the other containing OH– ions. The study reveals that the nanoparticles prepared by the microemulsion technique were smaller in size and higher in saturation magnetization than those nanoparticles prepared by Massart's procedure (Massart et al., 1981).

Despite some success, this microemulsion approach has some drawbacks, such as the difficulty in scale-up production, the adverse effect of residual surfactants on the properties of the nanoparticles, and the aggregation of the produced nanoparticles. Repeated wash processes and further stabilization treatment are usually required for such a reaction approach (Wu et al., 2008).

## **4. Surface modifications**

The surface modifications of nanoparticles have attracted much more attention, which can improve the surface-related properties like hydrophobic or hydrophilic. This can be achieved by using surfactants, polymers, and inorganic materials (silica).

## **4.1 Surfactants**

Surface modification with surfactant(s) is widely used for altering surface properties such as hydrophobic or hydrophilic. The use of surfactant molecules, such as oleic acid, oleylamine, or thiols (Wang et al., 2005), can easily functionalize iron oxide nanoparticles to be hydrophobic surfaces. These molecules can covalently bond to the iron atoms or clusters against particle degradation (Soler et al., 2007).

Many researches focus on the synthesis of water-soluble iron oxide nanoparticles with biocompatibility and biodegradability for biological applications. For example, one is to directly introduce the biocompatible organic molecules, e.g., amino acid (Sousa et al., 2001), vitamin (Mornet et al., 2004), and citric acid (Morais et al., 2003). Despite some advantages, the instability of small organic molecules in alkaline or acidic environment may result in agglomeration of the functionalized iron oxide nanoparticles.

Another alternative technique is to transform the oil-soluble type into water-soluble one via a ligand exchange reaction (Chen et al., 2008). The ligand exchange involves the addition of an excess of ligand(s) to nanoparticle suspension, which has stronger interaction with the nanoparticles than the original ones. Sun et al. (2003) converted the synthesized hydrophobic maghemite nanoparticles into hydrophilic ones by mixing with bipolar surfactants such as tetramethylammonium 11-aminoudecanoate. Lattuada and Hatton (2006) reported that the oleic groups initially present on the surface of magnetite nanoparticles were replaced by various capping agents containing reactive hydroxyl moieties. They also tuned the particle size in the range of 6-11 nm by varying the heating rate.

## **4.2 Polymers**

Polymer-functionalized iron oxide nanoparticles have gained much more attention due to the benefits offered by polymeric coating, which may increase repulsive forces to balance the magnetic and van der Waals attractive forces acting on the nanoparticles (Wu et al.

magnetic iron oxide nanoparticles with an average particle size of <10 nm by mixing two microemulsion systems, one containing Fe2+ ions and the other containing OH– ions. The study reveals that the nanoparticles prepared by the microemulsion technique were smaller in size and higher in saturation magnetization than those nanoparticles prepared by

Despite some success, this microemulsion approach has some drawbacks, such as the difficulty in scale-up production, the adverse effect of residual surfactants on the properties of the nanoparticles, and the aggregation of the produced nanoparticles. Repeated wash processes and further stabilization treatment are usually required for such a reaction

The surface modifications of nanoparticles have attracted much more attention, which can improve the surface-related properties like hydrophobic or hydrophilic. This can be

Surface modification with surfactant(s) is widely used for altering surface properties such as hydrophobic or hydrophilic. The use of surfactant molecules, such as oleic acid, oleylamine, or thiols (Wang et al., 2005), can easily functionalize iron oxide nanoparticles to be hydrophobic surfaces. These molecules can covalently bond to the iron atoms or clusters

Many researches focus on the synthesis of water-soluble iron oxide nanoparticles with biocompatibility and biodegradability for biological applications. For example, one is to directly introduce the biocompatible organic molecules, e.g., amino acid (Sousa et al., 2001), vitamin (Mornet et al., 2004), and citric acid (Morais et al., 2003). Despite some advantages, the instability of small organic molecules in alkaline or acidic environment may result in

Another alternative technique is to transform the oil-soluble type into water-soluble one via a ligand exchange reaction (Chen et al., 2008). The ligand exchange involves the addition of an excess of ligand(s) to nanoparticle suspension, which has stronger interaction with the nanoparticles than the original ones. Sun et al. (2003) converted the synthesized hydrophobic maghemite nanoparticles into hydrophilic ones by mixing with bipolar surfactants such as tetramethylammonium 11-aminoudecanoate. Lattuada and Hatton (2006) reported that the oleic groups initially present on the surface of magnetite nanoparticles were replaced by various capping agents containing reactive hydroxyl moieties. They also tuned the particle size in the range of 6-11 nm by varying the heating

Polymer-functionalized iron oxide nanoparticles have gained much more attention due to the benefits offered by polymeric coating, which may increase repulsive forces to balance the magnetic and van der Waals attractive forces acting on the nanoparticles (Wu et al.

achieved by using surfactants, polymers, and inorganic materials (silica).

Massart's procedure (Massart et al., 1981).

against particle degradation (Soler et al., 2007).

agglomeration of the functionalized iron oxide nanoparticles.

approach (Wu et al., 2008).

**4. Surface modifications** 

**4.1 Surfactants** 

rate.

**4.2 Polymers** 

2008). It has shown that through careful choice of the passivating and activating polymers and/or reaction conditions, polymer-stabilized iron oxide nanoparticles with tailored and desired properties can be synthesized.

The iron oxide particles by ionic properties can be modified with functional polymer groups with COOH, NH2 (Chibowski et al., 2009; Kandori et al., 2005; Li et al., 2004). The polymer coated particles can be synthesized by the *ex situ* method, i.e. dispersion of the nanoparticles in a polymeric solution, or *in situ* method, i.e. monomer polymerization in the presence of the synthesized nanoparticles (Mammeri et al., 2005; Guo et al., 2007).

Polymeric coating materials can be classified into two main classes: natural (e.g., dextran, starch, gelatin, chitosan) and synthetic (e.g., polyethylene glycol, PEG; polymethylmethacrylate, PMMA; polyacrylic acid, PAA). However, the saturation magnetization value of iron oxide nanoparticles will decrease after polymer-fictionalization.

Dextran is often utilized as a coating polymer because of its stability and biocompatibility (Laurent et al. 2008). Molday and Mackenzie (Molday and Mackenzie, 1982) have reported the formation of Fe3O4 in the presence of dextran with molecular weight (MW) of 40,000. In the synthesis of dextran-coated ultra-small superparamagnetic iron oxides (USPIO), the reduction of the terminal glucose of dextran was found to be significant for controlling particle size, stability, and magnetic properties. For low molecular weight dextrans (MW, <10,000), it is difficult to obtain nanoparticles with a small size of <20 nm.

Polyvinyl alcohol (PVA) is a hydrophilic and biocompatible polymer that can be used for particle surface modification to prevent particle agglomeration (Laurent et al. 2008). Lee et al. (1996) have modified the surface of magnetite nanoparticles with PVA by precipitation of iron salts at a high pH (13.8) to form stable magnetite colloidal dispersions, and particle size is around 4 nm. The investigators noted that the crystallinity of the magnetite nanoparticles decreased with PVA concentration increasing, although morphology and particle sizes remained. When PVA is introduced, it reacts with the surface through hydrogen bonding between polar functional groups of the polymer and hydroxylated and/or protonated surface of the iron oxide. In addition to the polymer-surface interactions, PVA is known for its hydrogen bonding interaction, resulting in hydrogel structure embedding the nanoparticles. When the PVA concentration is over the critical saturation value, agglomeration may occur for PVA-coated particles via bridging interactions.

## **4.3 Polymerized amorphous silica**

Polymerized tetraethoxysilane (TEOS) network is often used as a surface coating material for iron oxide nanoparticles as this coating can prevent aggregation in solution, improve the chemical stability, and provide better protection against toxicity (Laurent et al. 2008). Additionally, polymerized silica-coated iron oxide nanoparticles exhibited good biocompatibility and solubility in water. Silica coating can stabilize the magnetite nanoparticles in two different ways: one is by shielding the magnetic dipole interaction with the silica shell, and another one is by enhancing the coulomb repulsion of the magnetic nanoparticles. Such a silica coating increases the size of the particles and decreases the saturation magnetization value.

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 129

Fig. 4. (A) Schematics of the hematite-gold core-shell nanorice particles. SEM (left) and TEM (right) of (B) hematite core, (C) seed particles, (D) nanorice with thin shells, and (E) nanorice

Carbon has been widely studied since its poly-morphologies as active carbon, graphite, graphene, carbon nanotubes, and fullerene bucky ball structures. They have exhibited extraordinary tensile strengths and electrical conductivity due to their covalent *sp*2 hybridized network structure. The combination of semi-conductive iron oxides and carbon may therefore enhance the electrical properties of the nanocomposite material. The method to coat carbon on the surface of iron oxide is often performed by the decomposition of a carbon source (i.e., hydrocarbons, polymer or glucose) at high temperatures under oxygen-

Carbon coated iron oxide particles have attracted much more attention. Zhang et al. (2008) demonstrated that carbon coated magnetite nanorods can be synthesized through a series of procedures. In this process, hematite nanorods were firstly synthesized by a hydrothermal method as previously mentioned. Secondly, glucose was coated onto the hematite nanorods by pyrolysis under hydrothermal conditions. Finally, the product was heated at 600 ºC under N2 to carbonize glucose and reduce hematite into magnetite simultaneously. Boguslavsky et al.(2008) reported a similar procedure, in which polydivinylbenzene (PDVB) was used as the carbon source. The PDVB coating was formed by emulsion polymerization of DVB in the presence of γ-Fe2O3, followed by annealing of the powder in a quartz tube at 1050 ºC under flowing Ar gas for 2 hours. The decomposition of the polymer in this case reduced γ-Fe2O3 to metallic Fe, which finally forms carbon coated iron (Fe/C) nanoparticles.

free environments (Tristão et al., 2010; Tristão et al., 2009; Zhang et al., 2008, 2010).

with thick shells. Reprinted with permission from (Wang et al., 2006).

**4.5 Carbon** 

A commonly used method to coat iron oxide nanoparticles with silica is the well-known Stőber method, in which silica is formed *in situ* via hydrolysis and condensation of a sol-gel precursor such as TEOS. For example, Im et al*.*(2005) have reported the synthesis of silica colloids loaded with superparamagnetic iron oxide nanoparticles, which revealed that the final size of silica colloids depended upon the concentration of iron oxide nanoparticles because the size of silica was closely related to the number of seeds (emulsion drops). The lower concentration the iron oxide nanoparticles in alcohol, the larger size the obtained colloids.

Another one is aerosol-pyrolysis method, in which silica-coated magnetic nanoparticles were prepared by pyrolysis of a mixed precursor of silicon alkoxides and metal compound in a flame environment (Deng et al. 2005). Tartaj *et al.*(2001) synthesized silica-coated γ-Fe2O3 hollow spheres with size of 150 ± 100 nm by aerosol pyrolysis of methanol solution containing iron ammonium citrate and silicon ethoxide.

## **4.4 Metals**

Noble metals (e.g., Au, Ag, Pt, and Pd), possessing unique electronic and catalytic properties, can be utilized to improve the physicochemical properties of magnetic nanoparticles and applications in biomedicine. The coating of iron oxide nanoparticles with noble metals can be helpful to improve stability from aggregation, however, decrease the saturation magnetization value in some cases (Wu et al. 2008).

Several procedures have been employed to synthesize such core-shell nanostructures. For example, Mikhaylova *et al.* (2004) have prepared gold-coated superparamagnetic iron oxide nanoparticles (SPION) using a reverse micelle method. In their study, the reverse micelles were formed from surfactant, cetyltrimethylammonium bromide (CTAB), octane (the oil phase), butanol (the co-surfactant), and an aqueous mixture of FeCl3, FeCl2 and HAuCl4 solutions. They found that the Au-coated SPION retained the superparamagnetic properties for a longer period than those of starch-coated and multi-arm polyethylene glycol (MPEG) coated ones. Wang et al*.*(2005) obtained gold coated iron oxide nanoparticles, in which the pre-synthesized Fe3O4 nanoparticles were used as seeds during the reduction of gold precursor, Au(OOCCH3)3. The average size of Fe3O4 nanoparticles increases from 5.2 ± 0.5 nm to 6.7 ± 0.7 nm after coating with gold (**Fig. 4**). Fe3O4/Au and Fe3O4/Au/Ag core/shell nanoparticles with tuneable plasmonic and magnetic properties have been developed by controlling the coating thickness and materials (Xu et al. 2007).

A facile and one-pot synthesis approach has been developed by Zhang et al*.* (Zhang et al., 2010) for generating metal (Au, Pt, Ag and Au-Pt)/Fe2O3 nanocomposites assisted by lysine. Lysine, containing functional groups -NH2 and –COOH, acts as both a linking molecule to the Fe2O3 matrix and a capping agent to stabilize the noble metal nanoparticles for a good dispersion. Jiang et al. (Jiang and Yu, 2009) have demonstrated a facile synthetic method for the preparation of Pd/α-Fe2O3 nanocomposites by adding citric acid into a mixture of iron oxide nanoparticles and palladium precursor, Pd(CH3CN)2Cl2) under a reflux heating at 90°C for 2 hours. The synthesized Pd/α-Fe2O3 nanocomposites inherited the rod-like morphology of the α-Fe2O3 nanoparticles and they exhibited superior catalytic activity in CO oxidation compared with pure α-Fe2O3 nanoparticles. UV-vis measurement of the nanocomposites revealed the presence of two plasma bands centered at around 383 and 552 nm, which can be assigned to the synergistic effect of both Pd and α-Fe2O3 nanoparticles.

Fig. 4. (A) Schematics of the hematite-gold core-shell nanorice particles. SEM (left) and TEM (right) of (B) hematite core, (C) seed particles, (D) nanorice with thin shells, and (E) nanorice with thick shells. Reprinted with permission from (Wang et al., 2006).

## **4.5 Carbon**

128 Smart Nanoparticles Technology

A commonly used method to coat iron oxide nanoparticles with silica is the well-known Stőber method, in which silica is formed *in situ* via hydrolysis and condensation of a sol-gel precursor such as TEOS. For example, Im et al*.*(2005) have reported the synthesis of silica colloids loaded with superparamagnetic iron oxide nanoparticles, which revealed that the final size of silica colloids depended upon the concentration of iron oxide nanoparticles because the size of silica was closely related to the number of seeds (emulsion drops). The lower concentration the iron oxide nanoparticles in alcohol, the larger size the obtained colloids.

Another one is aerosol-pyrolysis method, in which silica-coated magnetic nanoparticles were prepared by pyrolysis of a mixed precursor of silicon alkoxides and metal compound in a flame environment (Deng et al. 2005). Tartaj *et al.*(2001) synthesized silica-coated γ-Fe2O3 hollow spheres with size of 150 ± 100 nm by aerosol pyrolysis of methanol solution

Noble metals (e.g., Au, Ag, Pt, and Pd), possessing unique electronic and catalytic properties, can be utilized to improve the physicochemical properties of magnetic nanoparticles and applications in biomedicine. The coating of iron oxide nanoparticles with noble metals can be helpful to improve stability from aggregation, however, decrease the

Several procedures have been employed to synthesize such core-shell nanostructures. For example, Mikhaylova *et al.* (2004) have prepared gold-coated superparamagnetic iron oxide nanoparticles (SPION) using a reverse micelle method. In their study, the reverse micelles were formed from surfactant, cetyltrimethylammonium bromide (CTAB), octane (the oil phase), butanol (the co-surfactant), and an aqueous mixture of FeCl3, FeCl2 and HAuCl4 solutions. They found that the Au-coated SPION retained the superparamagnetic properties for a longer period than those of starch-coated and multi-arm polyethylene glycol (MPEG) coated ones. Wang et al*.*(2005) obtained gold coated iron oxide nanoparticles, in which the pre-synthesized Fe3O4 nanoparticles were used as seeds during the reduction of gold precursor, Au(OOCCH3)3. The average size of Fe3O4 nanoparticles increases from 5.2 ± 0.5 nm to 6.7 ± 0.7 nm after coating with gold (**Fig. 4**). Fe3O4/Au and Fe3O4/Au/Ag core/shell nanoparticles with tuneable plasmonic and magnetic properties have been developed by

A facile and one-pot synthesis approach has been developed by Zhang et al*.* (Zhang et al., 2010) for generating metal (Au, Pt, Ag and Au-Pt)/Fe2O3 nanocomposites assisted by lysine. Lysine, containing functional groups -NH2 and –COOH, acts as both a linking molecule to the Fe2O3 matrix and a capping agent to stabilize the noble metal nanoparticles for a good dispersion. Jiang et al. (Jiang and Yu, 2009) have demonstrated a facile synthetic method for the preparation of Pd/α-Fe2O3 nanocomposites by adding citric acid into a mixture of iron oxide nanoparticles and palladium precursor, Pd(CH3CN)2Cl2) under a reflux heating at 90°C for 2 hours. The synthesized Pd/α-Fe2O3 nanocomposites inherited the rod-like morphology of the α-Fe2O3 nanoparticles and they exhibited superior catalytic activity in CO oxidation compared with pure α-Fe2O3 nanoparticles. UV-vis measurement of the nanocomposites revealed the presence of two plasma bands centered at around 383 and 552 nm, which can be assigned to the synergistic effect of both Pd and α-Fe2O3 nanoparticles.

containing iron ammonium citrate and silicon ethoxide.

saturation magnetization value in some cases (Wu et al. 2008).

controlling the coating thickness and materials (Xu et al. 2007).

**4.4 Metals** 

Carbon has been widely studied since its poly-morphologies as active carbon, graphite, graphene, carbon nanotubes, and fullerene bucky ball structures. They have exhibited extraordinary tensile strengths and electrical conductivity due to their covalent *sp*2 hybridized network structure. The combination of semi-conductive iron oxides and carbon may therefore enhance the electrical properties of the nanocomposite material. The method to coat carbon on the surface of iron oxide is often performed by the decomposition of a carbon source (i.e., hydrocarbons, polymer or glucose) at high temperatures under oxygenfree environments (Tristão et al., 2010; Tristão et al., 2009; Zhang et al., 2008, 2010).

Carbon coated iron oxide particles have attracted much more attention. Zhang et al. (2008) demonstrated that carbon coated magnetite nanorods can be synthesized through a series of procedures. In this process, hematite nanorods were firstly synthesized by a hydrothermal method as previously mentioned. Secondly, glucose was coated onto the hematite nanorods by pyrolysis under hydrothermal conditions. Finally, the product was heated at 600 ºC under N2 to carbonize glucose and reduce hematite into magnetite simultaneously. Boguslavsky et al.(2008) reported a similar procedure, in which polydivinylbenzene (PDVB) was used as the carbon source. The PDVB coating was formed by emulsion polymerization of DVB in the presence of γ-Fe2O3, followed by annealing of the powder in a quartz tube at 1050 ºC under flowing Ar gas for 2 hours. The decomposition of the polymer in this case reduced γ-Fe2O3 to metallic Fe, which finally forms carbon coated iron (Fe/C) nanoparticles.

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 131

resonance spectroscopy (SERS) for sensing devices (Zhai et al., 2009). This effect is also of importance for bimetallic core/shell nanoparticles. As the ratio of gold to iron oxide increases, the gold character increases and the iron oxide becomes buried beneath and suppresses the dielectric effect. The increasing thickness of the shell structure will therefore

cause blue-shifting in the surface plasmon resonance (Lyon et al. 2004).

Fig. 5. Diagram of different spin arrangements in magnetic nanoparticles:

spins. Reprinted with permission from (Lu et al., 2007).

**5.2 Biomedical applications** 

a) Ferromagnetism (FM), b) Antiferromagnetism (AFM), D = diameter, Dc = critical diameter, c) a combination of two different ferromagnetic phases in permanent magnets, which are materials with high remanence magnetization (Mr) and high coercively (Hc), d) Superparamagnetism (SPM), e) the interaction at the interface between a ferromagnet and an antiferromagnet producing an exchange bias effect, and f) pure anti-ferromagnetic nanoparticles with superparamagnetic relaxation arising from uncompensated surface

Many investigations have been reported the application of nanoparticles for biomedicine, such as magnetic nanoparticles for improving the quality of magnetic resonance imaging (MRI), hyperthermic treatment for malignant cells, site-specific drug delivery, cell labeling, and manipulating cell membranes (Babič et al. 2008; Catherine and Adam, 2003). These

Iron oxide nanocomposites or particle coated with biocompatible polymer(s) have shown some advantages, e.g., reducing aggregation, maintain magnetic stability, slowdown degrading process under physiological conditions, and lower toxicity (Mahmoudi et al. 2009). So far, they have shown promise for monitoring living cells by both MR and

magnetic particles can also be used for diagnosis, imaging, and drug delivery.

fluorescence imaging, as well as for drug delivery (Liong et al., 2008).

In addition, Wang et al*.* (2006) have reported the synthesis of Fe3O4/C nanocomposites by heating the aqueous solution of glucose and oleic acid-stabilized Fe3O4 nanoparticles at 170 °C for 3 hours. The results revealed that without prior surface hydrophobic modification, the magnetite nanoparticles could not be encapsulated by the carbon nanospheres, but instead only bare carbon nanospheres with the size of ~200 nm and Fe3O4 nanoparticles were obtained. The variation of glucose concentration (0.3-0.6 M) and the reaction temperature (160-180 °C) were found to have no significant effect on the morphology of the product, however, both reaction time and the amount of oleic acid-stabilized Fe3O4 nanoparticles showed significant effects. The increase in the concentration of oleic-acid stabilized Fe3O4 nanoparticles from 2.5 to 6 g/L was found to generate a product that has more embedded Fe3O4 nanoparticles increasing from 41 to 63%).

Although carbon-coated iron oxide nanoparticles may offer some advantages, such particles are often obtained as agglomerated clusters due to the lack of effective synthetic control, and lack of proper understanding on the formation mechanism. The synthesis of dispersible carbon-coated nanoparticles in isolated forms still remains a challenge in this field.

Moreover, the surface modification of iron oxide allows the attachment of biomolecules such as proteins and drugs (Mohapatra et al. 2007; Sun et al. 2007). The design of the surface modifications may be determined by factors such as ion energy and ion flux of depositing species, interface volume, crystalline size, coating thickness, surface and interfacial energy (Kim et al. 2003; Pinho et al. 2010).

## **5. Functionalities of iron oxide nanostructures**

## **5.1 Magnetic property**

The magnetic property has been extensively studied since it was discovered and explained through electronic structures of atoms. The magnetic dipole moments generated by the spin and orbital angular momenta of electrons in the Fe atom may vary between each phase of the iron oxide material. In general, magnetic behavior of a material depends on the electron spin vector or the total magnetic dipole moment. One important aspect in iron oxide nanoparticles is the unique form of magnetism called superparamagnetism. At temperature of above the blocking temperature, the magnetization behavior is identical to that of atomic paramagnets. This phenomenon will occur if particles reach below a certain size (10-20 nm), when the particle consists of a single magnetic domain, even though the material is ferro- or ferri-magnetic in bulk form (Ye et al., 2007), as shown in **Fig. 5**. Particles with this type of magnetism show high field irreversibility, high saturation field, extra anisotropy contributions, and shifted loops (Pedro et al., 2003).

For noble gold and silver nanoparticles with unique surface plasmon resonance (SPR) properties, they are often used to modify the iron oxide surfaces for generating coupled or multiple functionalities. At the nanoscale, the metallic electron cloud oscillates on the particle surface and absorbs electromagnetic radiation at a particular energy. The surface geometry of the iron oxide particles such as spheres, cubes, triangles, or rods, can therefore influence the absorption of radiation from the ultra-violet up to the near infrared spectrum (350-1200 nm). Other factors that affect the absorption are the solvent and surface functionalization. They are important contributors that can tune the exact frequency and intensity of the plasmon resonance band, which attracts them to the surface enhanced resonance spectroscopy (SERS) for sensing devices (Zhai et al., 2009). This effect is also of importance for bimetallic core/shell nanoparticles. As the ratio of gold to iron oxide increases, the gold character increases and the iron oxide becomes buried beneath and suppresses the dielectric effect. The increasing thickness of the shell structure will therefore cause blue-shifting in the surface plasmon resonance (Lyon et al. 2004).

Fig. 5. Diagram of different spin arrangements in magnetic nanoparticles: a) Ferromagnetism (FM), b) Antiferromagnetism (AFM), D = diameter, Dc = critical diameter, c) a combination of two different ferromagnetic phases in permanent magnets, which are materials with high remanence magnetization (Mr) and high coercively (Hc), d) Superparamagnetism (SPM), e) the interaction at the interface between a ferromagnet and an antiferromagnet producing an exchange bias effect, and f) pure anti-ferromagnetic nanoparticles with superparamagnetic relaxation arising from uncompensated surface spins. Reprinted with permission from (Lu et al., 2007).

## **5.2 Biomedical applications**

130 Smart Nanoparticles Technology

In addition, Wang et al*.* (2006) have reported the synthesis of Fe3O4/C nanocomposites by heating the aqueous solution of glucose and oleic acid-stabilized Fe3O4 nanoparticles at 170 °C for 3 hours. The results revealed that without prior surface hydrophobic modification, the magnetite nanoparticles could not be encapsulated by the carbon nanospheres, but instead only bare carbon nanospheres with the size of ~200 nm and Fe3O4 nanoparticles were obtained. The variation of glucose concentration (0.3-0.6 M) and the reaction temperature (160-180 °C) were found to have no significant effect on the morphology of the product, however, both reaction time and the amount of oleic acid-stabilized Fe3O4 nanoparticles showed significant effects. The increase in the concentration of oleic-acid stabilized Fe3O4 nanoparticles from 2.5 to 6 g/L was found to generate a product that has

Although carbon-coated iron oxide nanoparticles may offer some advantages, such particles are often obtained as agglomerated clusters due to the lack of effective synthetic control, and lack of proper understanding on the formation mechanism. The synthesis of dispersible

Moreover, the surface modification of iron oxide allows the attachment of biomolecules such as proteins and drugs (Mohapatra et al. 2007; Sun et al. 2007). The design of the surface modifications may be determined by factors such as ion energy and ion flux of depositing species, interface volume, crystalline size, coating thickness, surface and interfacial energy

The magnetic property has been extensively studied since it was discovered and explained through electronic structures of atoms. The magnetic dipole moments generated by the spin and orbital angular momenta of electrons in the Fe atom may vary between each phase of the iron oxide material. In general, magnetic behavior of a material depends on the electron spin vector or the total magnetic dipole moment. One important aspect in iron oxide nanoparticles is the unique form of magnetism called superparamagnetism. At temperature of above the blocking temperature, the magnetization behavior is identical to that of atomic paramagnets. This phenomenon will occur if particles reach below a certain size (10-20 nm), when the particle consists of a single magnetic domain, even though the material is ferro- or ferri-magnetic in bulk form (Ye et al., 2007), as shown in **Fig. 5**. Particles with this type of magnetism show high field irreversibility, high saturation field, extra anisotropy

For noble gold and silver nanoparticles with unique surface plasmon resonance (SPR) properties, they are often used to modify the iron oxide surfaces for generating coupled or multiple functionalities. At the nanoscale, the metallic electron cloud oscillates on the particle surface and absorbs electromagnetic radiation at a particular energy. The surface geometry of the iron oxide particles such as spheres, cubes, triangles, or rods, can therefore influence the absorption of radiation from the ultra-violet up to the near infrared spectrum (350-1200 nm). Other factors that affect the absorption are the solvent and surface functionalization. They are important contributors that can tune the exact frequency and intensity of the plasmon resonance band, which attracts them to the surface enhanced

carbon-coated nanoparticles in isolated forms still remains a challenge in this field.

more embedded Fe3O4 nanoparticles increasing from 41 to 63%).

**5. Functionalities of iron oxide nanostructures** 

contributions, and shifted loops (Pedro et al., 2003).

(Kim et al. 2003; Pinho et al. 2010).

**5.1 Magnetic property** 

Many investigations have been reported the application of nanoparticles for biomedicine, such as magnetic nanoparticles for improving the quality of magnetic resonance imaging (MRI), hyperthermic treatment for malignant cells, site-specific drug delivery, cell labeling, and manipulating cell membranes (Babič et al. 2008; Catherine and Adam, 2003). These magnetic particles can also be used for diagnosis, imaging, and drug delivery.

Iron oxide nanocomposites or particle coated with biocompatible polymer(s) have shown some advantages, e.g., reducing aggregation, maintain magnetic stability, slowdown degrading process under physiological conditions, and lower toxicity (Mahmoudi et al. 2009). So far, they have shown promise for monitoring living cells by both MR and fluorescence imaging, as well as for drug delivery (Liong et al., 2008).

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 133

The sensitivity of iron oxide-based nanosensors can be improved by various doping schemes as well as by changing the sensing material structure. For example, the thin film type sensors tend to exhibit higher sensitivity than bulk material sensor(s) (Mohapatra and Anand, 2010). Tao and co-workers (1999) have studied the sensing characteristics of Y2O3 doped γ-Fe2O3 towards hydrocarbon gases, H2 and CO and found that the addition of Y2O3 to γ-Fe2O3 resulted in a little difference in the sensitivity and selectivity compared with those made of pure γ-Fe2O3. Neri et al*.*(2002) have assessed the gas-sensing properties of Zndoped Fe2O3 thin films prepared by liquid phase deposition method. They observed that the addition of metal Zn can increase the sensitivity of the Fe2O3 thin film to NO2 below 250 °C.

A catalyst can attract atoms and/or molecules, and then change the surface conductivity and other properties. Different from sensing material, the catalyst often converts itself into a different species through a chemical reaction. The iron oxides (hematite and magnetite) have been applied in industry to produce chemicals with high efficiencies, such as ammonia (Haber process) and hydrocarbons (Fischer-Tropsch process) (Teja and Koh, 2009). It is expected that the nanoparticles with high surface areas can perform much better to enhance the chemical reaction rates than that of bulk states. For hematite, its thermal-dynamically

stable structure allows it for high temperature oxidation catalysis (Sivula et al., 2010).

The catalysis effect can also be enhanced by coupling metal nanoparticles on the surface (Jiang and Yu 2009; Zhong et al., 2007). Jiang et al (2009) have reported the synthesis of Pd/α-Fe2O3 nanocomposites at ambient conditions, which displayed superior lowtemperature catalytic activity toward CO oxidation to the pure α-Fe2O3 nanoparticles. It was proposed that the enhanced catalytic activity was due to the reaction between oxygen adsorbed on the reduced sites of the support (Fe2+) and CO adsorbed on Pd at the metal-

By using gold deposited iron oxide materials as a catalyst material, the oxidation and hydrogenation reaction of many organic compounds can be performed at much lower temperatures (Kung et al., 2007; Herzing et al., 2008; Lenz et al., 2009; Scirè et al., 2008). For example, Al-Sayari and co-workers (2007) have shown the dependence of the catalytic performance of Au/Fe2O3 catalyst that the non-calcined Au/Fe2O3 catalyst exhibited a high activity when pH≥ 5, whereas the activity of calcined Au/Fe2O3 catalyst was not influenced by the preparation conditions. Furthermore, the authors also noted that the catalytic activity of Fe2O3 toward CO oxidation was considerably lower than that of the Au/Fe2O3 catalyst. Maghemite and magnetite/carbon composites have been found to be good catalysts for reducing the concentration of undesirable nitrogen in acrylonitrile-butadiene-styrene (ABS) degradation oil (Brebu et al., 2001), whereas hematite can be used as a photocatalyst for the degradation of chlorophenol and azo dyes (Bandara et al., 2007), as well as a support material for gold in catalysts for the oxidation of carbon monoxide (CO) at low temperatures

The challenge of catalysis research being the reaction mechanism for these systems are still yet to be confirmed or explained, especially for the metal oxide/gold systems (Astruc et al., 2005). The reaction can be compared from titanium oxide/gold. The rutile phase of titania provides a support for gold, in which CO will convert mostly along the perimeter between the titania and

**5.4 Catalyst** 

oxide interface, as shown in **Fig. 6**.

(Zhong et al., 2007).

As mentioned previously, iron oxide nanoparticles exhibit paramagnetic or superparamagnetic properties in a limited size range. Particles larger than 50 nm show superparamagnetic iron oxides (SPIO), whereas particles smaller than 50 nm show ultrasmall superparamagnetic property (USP). The smaller ones have the ability to enhance signal detection and increase resolution in the MRI (Foy et al., 2010; Tong et al., 2010). Therefore, the SPIO particles can be used for imaging tumors in the liver and spleen, while superparamagnetic particles for contrast agents for lymphography and angiography. However, the superparamagnetic particles do not retain their magnetism when the external magnetic field is removed, while other magnetic materials will become magnetized and aggregate.

In addition, the problem using magnetite or maghemite nanoparticles in clinic is often limited by the biocompatibility and toxicity of these particles (Martin et al., 2008; Pisanic Ii et al., 2007). This happens from the body's defense system, the reticulo-endothelial system (RES), trying to remove these particles from the bloodstream as they pass through the liver, spleen and lymph nodes. The rapid removal of the iron oxide nanoparticles reduces their life-time. This is why it is necessary to produce nanocomposites with special surface modifications. The surface modification of the particles allows the water-insoluble drugs to be loaded and stored for a long time (Liong et al., 2008; Son et al., 2005). Despite some progress, the challenges in using surface modified magnetic iron oxide nanoparticles still exist. More work needs to be performed in the future.

## **5.3 Gas sensing**

Gas detection with high sensitivity and selectivity is essential for controlling industrial, waste, and vehicle emissions, household activity and environmental monitoring. In the past decades, many sensor devices have been developed for various gases such as CO, CO2, O2, O3, H2, NH3 and SO2, as well as various organic vapors e.g., benzene, methanol, ethanol, amines and isopropanol (Jimenez-Cadena et al. 2007). Although semiconducting oxides have been quite useful as gas sensors, the operation at high temperatures often limits their functionality and applications. This has prompted the exploration of new materials that may offer higher sensing and selective capabilities than traditional ones.

Nanostructured metal oxides are one of the most commonly used materials for gas sensing because of the semiconductors make them possible for the electrical conductivity change when the surrounding atmosphere changes. Additionally, nanosized metal oxides exhibit high ratios of surface to volume, which favors the adsorption of gases on the particle surface, and hence increases the sensitivity in detection.

Iron oxide nanoparticles have shown good sensing capabilities toward hydrocarbon gases, CO and alcohols (Jimenez-Cadena et al., 2007; Han et al., 1996, 1999, and 2001). The studies by Zhang et al*.* (1996) and Tao et al*.* (1999) showed that γ-Fe2O3 nanosensors exhibited good sensitivity and selectivity to a range of hydrocarbon gases such as LPG, petrol and C2H2 at 380 °C, but poor sensitivity to H2 and CO. However, Nakatani and Matsuoka (1983) together with Lee and Choi (1990) reported that the γ-Fe2O3-based sensors exhibited good sensitivity to H2. This suggests that the gas-sensing characteristics of a nanosensor are related to its preparation process.

The sensitivity of iron oxide-based nanosensors can be improved by various doping schemes as well as by changing the sensing material structure. For example, the thin film type sensors tend to exhibit higher sensitivity than bulk material sensor(s) (Mohapatra and Anand, 2010). Tao and co-workers (1999) have studied the sensing characteristics of Y2O3 doped γ-Fe2O3 towards hydrocarbon gases, H2 and CO and found that the addition of Y2O3 to γ-Fe2O3 resulted in a little difference in the sensitivity and selectivity compared with those made of pure γ-Fe2O3. Neri et al*.*(2002) have assessed the gas-sensing properties of Zndoped Fe2O3 thin films prepared by liquid phase deposition method. They observed that the addition of metal Zn can increase the sensitivity of the Fe2O3 thin film to NO2 below 250 °C.

## **5.4 Catalyst**

132 Smart Nanoparticles Technology

As mentioned previously, iron oxide nanoparticles exhibit paramagnetic or superparamagnetic properties in a limited size range. Particles larger than 50 nm show superparamagnetic iron oxides (SPIO), whereas particles smaller than 50 nm show ultrasmall superparamagnetic property (USP). The smaller ones have the ability to enhance signal detection and increase resolution in the MRI (Foy et al., 2010; Tong et al., 2010). Therefore, the SPIO particles can be used for imaging tumors in the liver and spleen, while superparamagnetic particles for contrast agents for lymphography and angiography. However, the superparamagnetic particles do not retain their magnetism when the external magnetic field is removed, while other magnetic materials will become magnetized and

In addition, the problem using magnetite or maghemite nanoparticles in clinic is often limited by the biocompatibility and toxicity of these particles (Martin et al., 2008; Pisanic Ii et al., 2007). This happens from the body's defense system, the reticulo-endothelial system (RES), trying to remove these particles from the bloodstream as they pass through the liver, spleen and lymph nodes. The rapid removal of the iron oxide nanoparticles reduces their life-time. This is why it is necessary to produce nanocomposites with special surface modifications. The surface modification of the particles allows the water-insoluble drugs to be loaded and stored for a long time (Liong et al., 2008; Son et al., 2005). Despite some progress, the challenges in using surface modified magnetic iron oxide nanoparticles still

Gas detection with high sensitivity and selectivity is essential for controlling industrial, waste, and vehicle emissions, household activity and environmental monitoring. In the past decades, many sensor devices have been developed for various gases such as CO, CO2, O2, O3, H2, NH3 and SO2, as well as various organic vapors e.g., benzene, methanol, ethanol, amines and isopropanol (Jimenez-Cadena et al. 2007). Although semiconducting oxides have been quite useful as gas sensors, the operation at high temperatures often limits their functionality and applications. This has prompted the exploration of new materials that may

Nanostructured metal oxides are one of the most commonly used materials for gas sensing because of the semiconductors make them possible for the electrical conductivity change when the surrounding atmosphere changes. Additionally, nanosized metal oxides exhibit high ratios of surface to volume, which favors the adsorption of gases on the particle

Iron oxide nanoparticles have shown good sensing capabilities toward hydrocarbon gases, CO and alcohols (Jimenez-Cadena et al., 2007; Han et al., 1996, 1999, and 2001). The studies by Zhang et al*.* (1996) and Tao et al*.* (1999) showed that γ-Fe2O3 nanosensors exhibited good sensitivity and selectivity to a range of hydrocarbon gases such as LPG, petrol and C2H2 at 380 °C, but poor sensitivity to H2 and CO. However, Nakatani and Matsuoka (1983) together with Lee and Choi (1990) reported that the γ-Fe2O3-based sensors exhibited good sensitivity to H2. This suggests that the gas-sensing characteristics of a nanosensor are related to its

exist. More work needs to be performed in the future.

offer higher sensing and selective capabilities than traditional ones.

surface, and hence increases the sensitivity in detection.

aggregate.

**5.3 Gas sensing** 

preparation process.

A catalyst can attract atoms and/or molecules, and then change the surface conductivity and other properties. Different from sensing material, the catalyst often converts itself into a different species through a chemical reaction. The iron oxides (hematite and magnetite) have been applied in industry to produce chemicals with high efficiencies, such as ammonia (Haber process) and hydrocarbons (Fischer-Tropsch process) (Teja and Koh, 2009). It is expected that the nanoparticles with high surface areas can perform much better to enhance the chemical reaction rates than that of bulk states. For hematite, its thermal-dynamically stable structure allows it for high temperature oxidation catalysis (Sivula et al., 2010).

The catalysis effect can also be enhanced by coupling metal nanoparticles on the surface (Jiang and Yu 2009; Zhong et al., 2007). Jiang et al (2009) have reported the synthesis of Pd/α-Fe2O3 nanocomposites at ambient conditions, which displayed superior lowtemperature catalytic activity toward CO oxidation to the pure α-Fe2O3 nanoparticles. It was proposed that the enhanced catalytic activity was due to the reaction between oxygen adsorbed on the reduced sites of the support (Fe2+) and CO adsorbed on Pd at the metaloxide interface, as shown in **Fig. 6**.

By using gold deposited iron oxide materials as a catalyst material, the oxidation and hydrogenation reaction of many organic compounds can be performed at much lower temperatures (Kung et al., 2007; Herzing et al., 2008; Lenz et al., 2009; Scirè et al., 2008). For example, Al-Sayari and co-workers (2007) have shown the dependence of the catalytic performance of Au/Fe2O3 catalyst that the non-calcined Au/Fe2O3 catalyst exhibited a high activity when pH≥ 5, whereas the activity of calcined Au/Fe2O3 catalyst was not influenced by the preparation conditions. Furthermore, the authors also noted that the catalytic activity of Fe2O3 toward CO oxidation was considerably lower than that of the Au/Fe2O3 catalyst.

Maghemite and magnetite/carbon composites have been found to be good catalysts for reducing the concentration of undesirable nitrogen in acrylonitrile-butadiene-styrene (ABS) degradation oil (Brebu et al., 2001), whereas hematite can be used as a photocatalyst for the degradation of chlorophenol and azo dyes (Bandara et al., 2007), as well as a support material for gold in catalysts for the oxidation of carbon monoxide (CO) at low temperatures (Zhong et al., 2007).

The challenge of catalysis research being the reaction mechanism for these systems are still yet to be confirmed or explained, especially for the metal oxide/gold systems (Astruc et al., 2005). The reaction can be compared from titanium oxide/gold. The rutile phase of titania provides a support for gold, in which CO will convert mostly along the perimeter between the titania and

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 135

and forces. Thus, the macroscopic properties (e.g., pressure, energy, heat capacities) can be

In our recent work, the MD method was used to explain the interactions between various goethite surfaces and surfactants of the nanorods. The simulation results of the side wall (*xy*0) surfaces with six different surfactants have been reported (Yue et al. 2010, 2011). The positively charged surfactants, CTAB (**Fig. 7**) and tetraethylammonium chloride (TEAC), were found to interact greatly with the side wall (*xy*0) of the nanorod, while the polymeric polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) and anionic surfactants (AOT) and Sodium Dodecyl Sulfate (SDS) were not suitable because of the low interaction energies among the surfaces. This is caused by the differences in the active sites on different surfaces (Kim et al. 2007). The ratios of iron and oxygen can vary greatly for different surfaces, in which the packing and exposure of atoms along a particular crystal plane will therefore determine the strength of adsorbed surface molecules. The simulation could provide quantitative information toward the interaction between surfactants and goethite surface(s),

Through a similar MD simulation, the adsorption of minerals has been explored. Kerisit et al. (2006) simulated the interactions for electrolyte solutions to determine the surface properties of monovalent ions, such as NaCl, CsCl, and CsF on the (100) goethite surface. The calculations showed a structured interfacial region is in the first 15 Å on the surface. The structure of the mineral surface will also affect the arrangement and orientation of the water molecules, and hence the diffusive properties and distribution of the ionic species. In comparison, the adsorption of sodium ions is stronger than cesium ions because the former

Fig. 7. MD simulation of CTAB molecular adsorption on the goethite crystal (010) surface at different time: (A) 0 ps, (B) 10 ps, (C) 20 ps, and (D) 50 ps. Reprinted with permission from

Similarly, MD simulation was also employed to explain the growth mechanisms of akaganéite nanorods (Yue et al., 2011), as shown in **Fig. 8**, in which the atomic concentration profiles of various anions on different crystalline surfaces were compared. With the assistance of experimental techniques such as transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD), the role of chloride ions in the lattice

and hence understand the particle formation and growth mechanisms.

can occupy an interstitial site of mineral(s) due to smaller size.

(Yue et al., 2010).

derived by means of statistical mechanics.

gold (Haruta, 2002). In other studies, it was proposed that the nature of the support material has much greater influence on the reactive properties of the deposited nanoparticle, because the active and selective sites are formed by negative gold particles (Milone et al., 2007).

Fig. 6. HRTEM image of Pd particles binding on the surface of iron oxide, in which the lattice distance is ~0.385 nm, corresponding to Pd{1 0 0} planes. Catalytic activity of Pd/α-Fe2O3 nanocomposites showing the dependence of CO oxidation (A); (B) comparison of the catalytic activity of nanoparticles with and without doped palladium. Reprinted with permission from (Jiang and Yu, 2009).

## **6. Theoretical simulations**

Beyond physical phenomena, theoretical methods have been developed and widely used to understand electronic, structure and forces of nanostructures (Cohen et al., 2008; Freund and Pacchioni, 2008; Hafner et al., 2006; Carter, 2008). Specifically, molecular dynamics (MD) method can be used for calculating interaction energies between surface modifiers and the modified matters, density functional theory (DFT) for binding energies, and Monte Carlo (MC) method for equilibrium properties (e.g., free energy, phase equilibrium) of particles. These methods have allowed researchers to understand and explain the growth mechanisms, structure, and functionalities of nanostructures (Hafner et al., 2006).

## **6.1 Molecular dynamics**

MD simulation has been widely used for the study of the molecular behaviours in liquids and solids, examining material properties, and designing new materials, particularly for nanoparticles and nanocomposites. The MD method allows one to predict the time evolution of a system of interacting particles (atoms or molecules) and estimate relevant physicochemical properties. Specifically, it can calculate and simulate the interaction energies among atoms/molecules, which can help understand atomic positions, velocities,

gold (Haruta, 2002). In other studies, it was proposed that the nature of the support material has much greater influence on the reactive properties of the deposited nanoparticle, because

the active and selective sites are formed by negative gold particles (Milone et al., 2007).

Fig. 6. HRTEM image of Pd particles binding on the surface of iron oxide, in which the lattice distance is ~0.385 nm, corresponding to Pd{1 0 0} planes. Catalytic activity of Pd/α-Fe2O3 nanocomposites showing the dependence of CO oxidation (A);

Reprinted with permission from (Jiang and Yu, 2009).

**6. Theoretical simulations** 

**6.1 Molecular dynamics** 

(B) comparison of the catalytic activity of nanoparticles with and without doped palladium.

Beyond physical phenomena, theoretical methods have been developed and widely used to understand electronic, structure and forces of nanostructures (Cohen et al., 2008; Freund and Pacchioni, 2008; Hafner et al., 2006; Carter, 2008). Specifically, molecular dynamics (MD) method can be used for calculating interaction energies between surface modifiers and the modified matters, density functional theory (DFT) for binding energies, and Monte Carlo (MC) method for equilibrium properties (e.g., free energy, phase equilibrium) of particles. These methods have allowed researchers to understand and explain the growth

MD simulation has been widely used for the study of the molecular behaviours in liquids and solids, examining material properties, and designing new materials, particularly for nanoparticles and nanocomposites. The MD method allows one to predict the time evolution of a system of interacting particles (atoms or molecules) and estimate relevant physicochemical properties. Specifically, it can calculate and simulate the interaction energies among atoms/molecules, which can help understand atomic positions, velocities,

mechanisms, structure, and functionalities of nanostructures (Hafner et al., 2006).

and forces. Thus, the macroscopic properties (e.g., pressure, energy, heat capacities) can be derived by means of statistical mechanics.

In our recent work, the MD method was used to explain the interactions between various goethite surfaces and surfactants of the nanorods. The simulation results of the side wall (*xy*0) surfaces with six different surfactants have been reported (Yue et al. 2010, 2011). The positively charged surfactants, CTAB (**Fig. 7**) and tetraethylammonium chloride (TEAC), were found to interact greatly with the side wall (*xy*0) of the nanorod, while the polymeric polyethylene glycol (PEG) and polyvinylpyrrolidone (PVP) and anionic surfactants (AOT) and Sodium Dodecyl Sulfate (SDS) were not suitable because of the low interaction energies among the surfaces. This is caused by the differences in the active sites on different surfaces (Kim et al. 2007). The ratios of iron and oxygen can vary greatly for different surfaces, in which the packing and exposure of atoms along a particular crystal plane will therefore determine the strength of adsorbed surface molecules. The simulation could provide quantitative information toward the interaction between surfactants and goethite surface(s), and hence understand the particle formation and growth mechanisms.

Through a similar MD simulation, the adsorption of minerals has been explored. Kerisit et al. (2006) simulated the interactions for electrolyte solutions to determine the surface properties of monovalent ions, such as NaCl, CsCl, and CsF on the (100) goethite surface. The calculations showed a structured interfacial region is in the first 15 Å on the surface. The structure of the mineral surface will also affect the arrangement and orientation of the water molecules, and hence the diffusive properties and distribution of the ionic species. In comparison, the adsorption of sodium ions is stronger than cesium ions because the former can occupy an interstitial site of mineral(s) due to smaller size.

Fig. 7. MD simulation of CTAB molecular adsorption on the goethite crystal (010) surface at different time: (A) 0 ps, (B) 10 ps, (C) 20 ps, and (D) 50 ps. Reprinted with permission from (Yue et al., 2010).

Similarly, MD simulation was also employed to explain the growth mechanisms of akaganéite nanorods (Yue et al., 2011), as shown in **Fig. 8**, in which the atomic concentration profiles of various anions on different crystalline surfaces were compared. With the assistance of experimental techniques such as transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD), the role of chloride ions in the lattice

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 137

However, MD is a classical simulation method which uses parameterized potentials (or forcefields), which cannot quantify electronic information of nanostructures. This method is limited to its accuracy, although the results can be obtained within a realistic period of time

DFT is another powerful simulation technique for understanding atom/molecular binder energies. The calculation is performed by using approximation method to simplify the

Many DFT studies have emphasized on the structural, electronic, catalytic, and magnetic properties of metal-oxide, such as Fe2O3, and Al2O3 (Alvarez-Ramirez et al., 2004; Ma et al., 2006; Mason et al., 2009; Rohrbach et al., 2004; Rollmann et al., 2004; Zhong et al., 2008; Mason et al., 2010). It has been extended into other systems, e.g., carbon nanotubes or graphene (Li et al., 2010; Chattaraj et al., 2009), transition metals (Cramer et al., 2009),

For example, Wong et al.(2011) demonstrated that the electronic and geometric structure of different metald (M = Au, Pt, Pd, or Ru) bilayers particularly on the α-Fe2O3(0001) support surface (**Fig. 10**). The analysis shows that the synergistic effect depends on the localized electron gain, electron transfer from Fe atoms to the dz2 orbital of the metal bilayer, and interfacial metallic/ionic bonding. These effects were most pronounced for surfaces modified with Pt or Ru, while the Au bilayer is the most stable due to its low α-Fe2O3 lattice deformation and minimal surface of Fe atom spin quenching. Tuning the Ru bilayer can

semiconductors (Jin et al., 2011), and metals (e.g., Pd, Au, Cu) (Yang et al., 2007).

provide an optimal balance of these factors, and hence enhance the catalytic activity.

Fig. 10. Electron density contour maps of M/α-Fe2O3(0001) interfaces, where M = Ru, Pd, Au, and Pt, respectively, and the electron density is in the range 0.0−0.8 eV/Å3. Reprinted

with permission from (Wong et al., 2011).

and larger length scales (Rustad et al., 2003; Zeng et al., 2008).

Schrödinger's equation (Lado-Touriño and Tsobnang, 2000).

**6.2 Density functional theory** 

structure and forming β-FeOOH rodlike structure was determined. The analysis showed that the chloride ions were a small size, as well as having an intermediate interaction on the tunnel structure of the (001) surface, while the tight packing of the (100) and (110) surfaces does not allow interaction with any ions. The information was useful for the development of the simulation model, which explained the filling of the tunnel structure along (001) direction.

Fig. 8. The concentration profiles of various anions on the crystal surface of akaganéite nanorods: (A) (100); (B) (110); and (C) (001) plane. Reprinted with permission from (Yue et al., 2011).

This MD method is used not only for small organic molecules but also for metallic nanoclusters. In our recent work (Yue et al., 2011), the Fe3O4(111) surface modified with various surfactants, polymers, and silica, followed by the deposition of a Au nanoparticle was simulated by MD method). The results show the dynamic motion of the molecules on the Fe3O4(111) surface, followed by the encapsulation of the Au nanoparticle surface. Through an analysis of the concentration profile, it reveals that NH2 groups within the molecule(s) are useful for attracting gold atoms, as shown in **Fig. 9**. Moreover, onedimensional chainlike molecules allow higher flexibility to move toward the Au surface compared with three-dimensional structure (amorphous or polymerized silica)

Fig. 9. Snapshots of PEI coating onto the surface of Fe3O4(111) and the addition of a AuNP at various times. Reprinted with permission from (Yue et al., 2011).

This theoretical method is available for predicting the interaction energies and adsorption sites of molecules on the iron oxides surfaces. Aquino et al.(2006) simulated various molecules such as water, acetic acid, acetate, 2,4-dichlorophenoxyacetic acid, and benzene on the goethite (110) surface. The results show that two OH types, hydroxo and µ-hydroxo, were able to bend and act as proton acceptors, while the third type, µ3-hydroxo, acts only as proton donor due to its more pronounced rigidity.

However, MD is a classical simulation method which uses parameterized potentials (or forcefields), which cannot quantify electronic information of nanostructures. This method is limited to its accuracy, although the results can be obtained within a realistic period of time and larger length scales (Rustad et al., 2003; Zeng et al., 2008).

## **6.2 Density functional theory**

136 Smart Nanoparticles Technology

structure and forming β-FeOOH rodlike structure was determined. The analysis showed that the chloride ions were a small size, as well as having an intermediate interaction on the tunnel structure of the (001) surface, while the tight packing of the (100) and (110) surfaces does not allow interaction with any ions. The information was useful for the development of the simulation model, which explained the filling of the tunnel structure along (001) direction.

Fig. 8. The concentration profiles of various anions on the crystal surface of akaganéite

compared with three-dimensional structure (amorphous or polymerized silica)

This MD method is used not only for small organic molecules but also for metallic nanoclusters. In our recent work (Yue et al., 2011), the Fe3O4(111) surface modified with various surfactants, polymers, and silica, followed by the deposition of a Au nanoparticle was simulated by MD method). The results show the dynamic motion of the molecules on the Fe3O4(111) surface, followed by the encapsulation of the Au nanoparticle surface. Through an analysis of the concentration profile, it reveals that NH2 groups within the molecule(s) are useful for attracting gold atoms, as shown in **Fig. 9**. Moreover, onedimensional chainlike molecules allow higher flexibility to move toward the Au surface

Fig. 9. Snapshots of PEI coating onto the surface of Fe3O4(111) and the addition of a AuNP at

This theoretical method is available for predicting the interaction energies and adsorption sites of molecules on the iron oxides surfaces. Aquino et al.(2006) simulated various molecules such as water, acetic acid, acetate, 2,4-dichlorophenoxyacetic acid, and benzene on the goethite (110) surface. The results show that two OH types, hydroxo and µ-hydroxo, were able to bend and act as proton acceptors, while the third type, µ3-hydroxo, acts only as

various times. Reprinted with permission from (Yue et al., 2011).

proton donor due to its more pronounced rigidity.

nanorods: (A) (100); (B) (110); and (C) (001) plane. Reprinted with permission from (Yue et al., 2011). DFT is another powerful simulation technique for understanding atom/molecular binder energies. The calculation is performed by using approximation method to simplify the Schrödinger's equation (Lado-Touriño and Tsobnang, 2000).

Many DFT studies have emphasized on the structural, electronic, catalytic, and magnetic properties of metal-oxide, such as Fe2O3, and Al2O3 (Alvarez-Ramirez et al., 2004; Ma et al., 2006; Mason et al., 2009; Rohrbach et al., 2004; Rollmann et al., 2004; Zhong et al., 2008; Mason et al., 2010). It has been extended into other systems, e.g., carbon nanotubes or graphene (Li et al., 2010; Chattaraj et al., 2009), transition metals (Cramer et al., 2009), semiconductors (Jin et al., 2011), and metals (e.g., Pd, Au, Cu) (Yang et al., 2007).

For example, Wong et al.(2011) demonstrated that the electronic and geometric structure of different metald (M = Au, Pt, Pd, or Ru) bilayers particularly on the α-Fe2O3(0001) support surface (**Fig. 10**). The analysis shows that the synergistic effect depends on the localized electron gain, electron transfer from Fe atoms to the dz2 orbital of the metal bilayer, and interfacial metallic/ionic bonding. These effects were most pronounced for surfaces modified with Pt or Ru, while the Au bilayer is the most stable due to its low α-Fe2O3 lattice deformation and minimal surface of Fe atom spin quenching. Tuning the Ru bilayer can provide an optimal balance of these factors, and hence enhance the catalytic activity.

Fig. 10. Electron density contour maps of M/α-Fe2O3(0001) interfaces, where M = Ru, Pd, Au, and Pt, respectively, and the electron density is in the range 0.0−0.8 eV/Å3. Reprinted with permission from (Wong et al., 2011).

Experimental and Theoretical Study of Low-Dimensional Iron Oxide Nanostructures 139

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Astruc, D., Lu, F. and Aranzaes, J.R. (2005). Nanoparticles as Recyclable Catalysts: The

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**9. References** 

Despite some success, the DFT method still has limitations in accurately describing the van der Waals interactions, phonon dispersion, spin-and space-degenerate states, strongly conjugated π systems, localization and delocalization errors for band gaps. Moreover, the DFT is difficult to solve the problems related to long range interactions and dispersion forces for complex biological systems. So far, the development of DFT technique is still demanded.

Besides DFT and MD simulations, Monte Carlo (MC) method, a stochastic method, has been employed to generate a statistical or probabilistic model for understanding particular systems. The MC method can be used to predict the crystalline structure of β-FeOOH (Kwon et al. 2006). By combination of quantitative X-ray structural analysis, the MC simulation has been used for characterizing the atomic-scale structure with and without chromium atoms. The results showed that the β-FeOOH particles containing chromium is distorted, while the particles without chromium is similar to its ideal structure. The combination of the experimental and MC simulation method can distinguish the differences between FeO6 and CrO6 octahedral units. However, this MC method can only provide information on equilibrium properties (e.g., free energy, phase equilibrium), but limited to the nonequilibrium systems.

## **7. Summary**

This Chapter briefly overviews some experimental methods (hydrothermal, co-precipitate and microemulsion methods) used for the synthesis and surface modifications of lowdimensional iron oxide nanostructures with desirable functional properties (gas sensing, catalytic, magnetic, and biochemical properties), and a few theoretical simulation techniques (MD, DFT, and MC) for fundamental understandings. However, the challenges still exist. Experimentally, one of the big challenges is how to produce iron oxide nanostructures with desired characteristics (shape, size, and surface properties) for target applications. Theoretically, DFT and MD simulations are limited to the large-scale calculations (e.g., mesoscopic structure with size range of 0.1–10 m) due to the current restraints in computational capability.

To overcome the limitations, the development of simple, cost-saving, and effective strategies for iron oxide and other nanostructures with desirable functional properties is highly demanded. For the computational modelings and simulation methods, much work needs to be performed in two directions: (i) to develop new and improved simulation techniques for large time and length scales; and (ii) to integrate diverse simulation techniques (DFT, MD, MC and others) on different levels together to form a powerful tool for exploring the structural, dynamic, and mechanical properties of nanomaterials and nanosystems. This is crucial to predict process–structure–property relationships in material design, optimization, and manufacturing.

## **8. Acknowledgement**

We gratefully acknowledge the financial support of the Australia Research Council (ARC) the ARC Centres of Excellence for Functional Nanomaterials and ARC projects. The authors acknowledge access to the UNSW node of the Australian Microscopy & Microanalysis Research Facility (AMMRF).

### **9. References**

138 Smart Nanoparticles Technology

Despite some success, the DFT method still has limitations in accurately describing the van der Waals interactions, phonon dispersion, spin-and space-degenerate states, strongly conjugated π systems, localization and delocalization errors for band gaps. Moreover, the DFT is difficult to solve the problems related to long range interactions and dispersion forces for complex biological systems. So far, the development of DFT technique is still

Besides DFT and MD simulations, Monte Carlo (MC) method, a stochastic method, has been employed to generate a statistical or probabilistic model for understanding particular systems. The MC method can be used to predict the crystalline structure of β-FeOOH (Kwon et al. 2006). By combination of quantitative X-ray structural analysis, the MC simulation has been used for characterizing the atomic-scale structure with and without chromium atoms. The results showed that the β-FeOOH particles containing chromium is distorted, while the particles without chromium is similar to its ideal structure. The combination of the experimental and MC simulation method can distinguish the differences between FeO6 and CrO6 octahedral units. However, this MC method can only provide information on equilibrium properties (e.g., free energy, phase equilibrium), but limited to the non-

This Chapter briefly overviews some experimental methods (hydrothermal, co-precipitate and microemulsion methods) used for the synthesis and surface modifications of lowdimensional iron oxide nanostructures with desirable functional properties (gas sensing, catalytic, magnetic, and biochemical properties), and a few theoretical simulation techniques (MD, DFT, and MC) for fundamental understandings. However, the challenges still exist. Experimentally, one of the big challenges is how to produce iron oxide nanostructures with desired characteristics (shape, size, and surface properties) for target applications. Theoretically, DFT and MD simulations are limited to the large-scale calculations (e.g., mesoscopic structure with size range of 0.1–10 m) due to the current restraints in

To overcome the limitations, the development of simple, cost-saving, and effective strategies for iron oxide and other nanostructures with desirable functional properties is highly demanded. For the computational modelings and simulation methods, much work needs to be performed in two directions: (i) to develop new and improved simulation techniques for large time and length scales; and (ii) to integrate diverse simulation techniques (DFT, MD, MC and others) on different levels together to form a powerful tool for exploring the structural, dynamic, and mechanical properties of nanomaterials and nanosystems. This is crucial to predict process–structure–property relationships in material design, optimization,

We gratefully acknowledge the financial support of the Australia Research Council (ARC) the ARC Centres of Excellence for Functional Nanomaterials and ARC projects. The authors acknowledge access to the UNSW node of the Australian Microscopy & Microanalysis

demanded.

equilibrium systems.

computational capability.

and manufacturing.

**8. Acknowledgement** 

Research Facility (AMMRF).

**7. Summary** 


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**Part 2** 

**Testing Technology** 

