**1. Introduction**

Searching for new and effective substrates for surface enhanced Raman Spectroscopy (SERS) applications is a subject of research at many scientific centers. This is because SERS spectroscopy is used to quickly characterize many functional materials important in advanced technologies, analytical chemistry and biology [1]. An important practical problem when carrying out analytical SERS measurements is to obtain suitable substrates containing a sufficient number of electromagnetic nanoresonators. The SERS effect is strongly dependent on an enhancement of the Raman scattering intensity by molecules adsorbed on a nanostructured metallic surface. The SERS enhancement factor is related to the size and shape of the nanostructures, which causes electromagnetic enhancement [2, 3]. Usually, the average value of SERS enhancement is around 105 –106 , but localized enhancement may reach values of 1010 at certain, highly efficient sub-wavelength regions of the surface [4]. It is therefore desirable for the resulting substrates to render a very large SERS spectra enhancement factor, where the enhancement factor thus obtained does not depend on where the measurement is made (the same enhancement factor should be present at different locations on the sample surface [5]).

A promising substrate for SERS measurements that meets these two most important practical requirements is regular TiO2 nanotubes loaded with metal nanoparticles, which support surface plasmon resonance. One way of modifying the nanotubular layer and attaining high SERS activity is to enhance the surface with a small quantity of Ag or other plasmonic metal (e.g. Au or Cu) nanoparticles (plasmonic nanoparticles) [1, 6]. Such plasmonic metals themselves are known for their high SERS activity, which results only after a proper roughening of their surfaces [7]. The dielectric constant for these metals consists of a negative real part and a small positive imaginary part. When a nanoparticle made of these metals interacts with electromagnetic radiation, collective oscillations of the surface plasmons are induced [8]. These oscillations result in an enhanced electromagnetic field in close proximity to the surface of the nanoparticles. The surface of these metals, when properly roughened, provides suitable slits and cavities which serve as surface plasmon resonators that strongly enhance the intensity of the electromagnetic field. For Raman bands with a small Raman shift, the increase in the efficiency of Raman scattering is roughly proportional to the fourth power of the field enhancement [2, 9]. A detailed description of the SERS effect can be found in literature [2, 9–12].

It is well known that the enhancement factor (EF ) of SERS spectra depends on the metal morphology/topography. Only a suitable surface roughness of the SERS substrates can produce a stronger Raman signal. The optimum value of the size and shape of the noble metal particles may lead to a maximum EF (which also depends on the nature of the metal, the excitation laser wavelength, and special experimental conditions) [13]. A highly active substrate provides superior conditions for measuring the SERS spectra of an adsorbate. Therefore, to gain some insight into the substrate–adsorbate interactions at the molecular level, and to detect different kinds of organic adsorbates, detailed knowledge of how to fabricate a highly sensitive and reproducible SERS substrate is of considerable importance [1, 5].

nanotubes (i.e. the metal nanoparticle systems are close to each other on the side walls of TiO2 nanotubes that are perpendicular to the macroscopic Ti surface). As shown experimentally, the narrow gaps between the metal nanoparticles supporting surface plasmon resonance pro-

nanoparticles deposited by different PVD methods (which provide precise control over the amount of silver sputtered onto the nanotubular substrate) on the SERS activity of the substrates prepared. We focus on the geometrical effects on SERS activity: nanotube diameter, size and

larly suitable for such investigations because the strictly controlled electrochemical procedures make it possible to produce well-formed, nanotubular substrates that are "homogeneous," statistically having the same nanotube diameter and nanotube wall thickness (see **Figure 2**).

nanoporous structure with Ag

nanotube layer and functionalization process for SERS application.

Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity…

http://dx.doi.org/10.5772/intechopen.72739

39

nanotubes are particu-

duce the largest SERS signal enhancement factors [17–19] (see **Figure 1**).

distribution of Ag-n, and morphology of the Ag-n agglomerates. TiO2

In this chapter we discuss in more detail the effect of a TiO<sup>2</sup>

**Figure 1.** Schematic presentation of fabrication of TiO2

The high regularity of the TiO2 nanotubular structures obtained ensures that the enhancement factors are high reproducible [14–16]. The specific morphology of the resulting structures (the large side wall surfaces of the nanotubes grown perpendicular to the substrate) makes it possible to prepare metallic nanograin systems with a large number of gaps between the Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity… http://dx.doi.org/10.5772/intechopen.72739 39

**1. Introduction**

38 Raman Spectroscopy

SERS enhancement is around 105

requirements is regular TiO2

may lead to a maximum EF

The high regularity of the TiO2

Searching for new and effective substrates for surface enhanced Raman Spectroscopy (SERS) applications is a subject of research at many scientific centers. This is because SERS spectroscopy is used to quickly characterize many functional materials important in advanced technologies, analytical chemistry and biology [1]. An important practical problem when carrying out analytical SERS measurements is to obtain suitable substrates containing a sufficient number of electromagnetic nanoresonators. The SERS effect is strongly dependent on an enhancement of the Raman scattering intensity by molecules adsorbed on a nanostructured metallic surface. The SERS enhancement factor is related to the size and shape of the nanostructures, which causes electromagnetic enhancement [2, 3]. Usually, the average value of

certain, highly efficient sub-wavelength regions of the surface [4]. It is therefore desirable for the resulting substrates to render a very large SERS spectra enhancement factor, where the enhancement factor thus obtained does not depend on where the measurement is made (the same enhancement factor should be present at different locations on the sample surface [5]). A promising substrate for SERS measurements that meets these two most important practical

face plasmon resonance. One way of modifying the nanotubular layer and attaining high SERS activity is to enhance the surface with a small quantity of Ag or other plasmonic metal (e.g. Au or Cu) nanoparticles (plasmonic nanoparticles) [1, 6]. Such plasmonic metals themselves are known for their high SERS activity, which results only after a proper roughening of their surfaces [7]. The dielectric constant for these metals consists of a negative real part and a small positive imaginary part. When a nanoparticle made of these metals interacts with electromagnetic radiation, collective oscillations of the surface plasmons are induced [8]. These oscillations result in an enhanced electromagnetic field in close proximity to the surface of the nanoparticles. The surface of these metals, when properly roughened, provides suitable slits and cavities which serve as surface plasmon resonators that strongly enhance the intensity of the electromagnetic field. For Raman bands with a small Raman shift, the increase in the efficiency of Raman scattering is roughly proportional to the fourth power of the field enhancement [2, 9].

phology/topography. Only a suitable surface roughness of the SERS substrates can produce a stronger Raman signal. The optimum value of the size and shape of the noble metal particles

wavelength, and special experimental conditions) [13]. A highly active substrate provides superior conditions for measuring the SERS spectra of an adsorbate. Therefore, to gain some insight into the substrate–adsorbate interactions at the molecular level, and to detect different kinds of organic adsorbates, detailed knowledge of how to fabricate a highly sensitive and

factors are high reproducible [14–16]. The specific morphology of the resulting structures (the large side wall surfaces of the nanotubes grown perpendicular to the substrate) makes it possible to prepare metallic nanograin systems with a large number of gaps between the

, but localized enhancement may reach values of 1010 at

) of SERS spectra depends on the metal mor-

nanotubes loaded with metal nanoparticles, which support sur-

(which also depends on the nature of the metal, the excitation laser

nanotubular structures obtained ensures that the enhancement

–106

A detailed description of the SERS effect can be found in literature [2, 9–12].

reproducible SERS substrate is of considerable importance [1, 5].

It is well known that the enhancement factor (EF

**Figure 1.** Schematic presentation of fabrication of TiO2 nanotube layer and functionalization process for SERS application.

nanotubes (i.e. the metal nanoparticle systems are close to each other on the side walls of TiO2 nanotubes that are perpendicular to the macroscopic Ti surface). As shown experimentally, the narrow gaps between the metal nanoparticles supporting surface plasmon resonance produce the largest SERS signal enhancement factors [17–19] (see **Figure 1**).

In this chapter we discuss in more detail the effect of a TiO<sup>2</sup> nanoporous structure with Ag nanoparticles deposited by different PVD methods (which provide precise control over the amount of silver sputtered onto the nanotubular substrate) on the SERS activity of the substrates prepared. We focus on the geometrical effects on SERS activity: nanotube diameter, size and distribution of Ag-n, and morphology of the Ag-n agglomerates. TiO2 nanotubes are particularly suitable for such investigations because the strictly controlled electrochemical procedures make it possible to produce well-formed, nanotubular substrates that are "homogeneous," statistically having the same nanotube diameter and nanotube wall thickness (see **Figure 2**).

device, and the DC magnetron sputtering technique using a Leica EM MED020 apparatus in a configuration perpendicular to the surface of the samples. More details are given elsewhere

quartz microbalance. One has to consider, however, that the true local amount of the metal deposits may vary substantially from site to site. The highly developed specific surface area of the nanotube arrays and its brush-like morphology may strongly affect Ag local distribution, resulting in considerable non-uniformity. For this type of process, silver targets of 99.9% purity (Kurt J. Lesker Company) were used. To better control the silver sputtering deposition process on the surface of the nanotubes, we applied the thermal evaporation method (0.01 and

Poland). The cell was maintained at a temperature of 900°C during the process of resistive evaporation. Silver (2 mm-diameter wire, 99.999%, Alfa Aesar) was evaporated onto the room

of 0.13 nm/min. The evaporation rate from the silver effusion cell was calibrated and monitored using a TM-400 quartz crystal thickness monitor (Maxtek Inc.). The process of fabricat-

The SERS platforms thus fabricated were characterized using various analytical methods. Electron Microscopy was applied for the morphological and structural characterization of the nanotubes after each preparation stage. The SEM observations (FEI NovaNanoSEM 450, Hitachi S70, Hitachi S-5500) were carried out at an accelerating voltage of 5 or 10 kV and an SE detector to reveal topographic contrast at the tops of the tubes. The thin sample for STEM observations was prepared by using the lift-out technique with a Hitachi NB-5000Focused Ion Beam system. The internal structure of the nanotubes was examined with a dedicated STEM Hitachi HD2700 in BF and atomic mass contrast. Observations at 200 kV revealed a columnar structure of the nanotubes and the transition zone. The surface chemical composition and the chemical state of the surface species (Ti, O, Ag) of the fabricated SERS platforms were examined using XPS spectroscopy. The XPS spectra were measured with a Microlab 350 Thermo Electron spectrometer at 300 W non-monochromatic Al Kα radiation and 1486.6 eV energy. AES spectroscopy was applied to determine the local chemical composition of the SERS substrates obtained. All of the spectra were recorded at an energy of 10 kV. The appropriate standards for AES and XPS reference spectra were also used. Finally, the Raman spectra of 2-mercaptoethanesulfonate (5 × 10−3 M), pyridine (5 × 10−2 M, in a mixture of pyridine and 0.1 M KCl), DBRh (10−4 M, in a mixture of water and ethanol (1:2)), and rhodamine from a 10−7 and 10−9 M aqueous solution were collected with a Horiba Jobin–Yvon Labram HR800 using a He-Ne laser (632.8 nm) as the excitation source. For each sample, 20 measurements were

For our studies, we used an optimized electrolyte based on a mixture of glycerol and water

is perpendicular to the metal substrate, as shown in **Figure 3**. STEM images reveal nanotubes that are hollow in shape, separated from each other, and that feature a characteristic

F. The growth of nanotubes under a constant voltage

ing the SERS substrates is presented schematically in **Figure 1**.

performed locally at various points chosen randomly.

**3. Results and discussion**

(volume ratio 50:50) with 0.27 M NH4

) using an EF 40C1 effusion cell inside an UHV preparation chamber (PREVAC,

Titanium (IV) Oxide Nanotubes in Design of Active SERS Substrates for High Sensitivity…

NT at a pressure of 1–2∙10−6 Pa at a constant evaporation rate

using a

41

http://dx.doi.org/10.5772/intechopen.72739

[18, 23]. We strictly controlled in situ the average amount of metal deposited per cm2

0.05 mg/cm2

temperature-surface of the TiO2

**Figure 2.** Effect of anodic voltage Vmax on average diameter and wall thickness of nanotubes. The inserts show top views of TiO2 nanotubes fabricated at different voltages [21].

As shown in **Figure 2**, the specific morphology of TiO<sup>2</sup> nanotubes can be relatively easily modified by controlling the conditions of anodic polarization (type of electrolyte, voltage and anodization time), because there is a direct linear relationship between anodization voltage and the average diameter of the nanotubes formed. In general, the diameter and wall thickness of the nanotubes increase with anodic voltage [20, 21]. The possibility of preparing nanotubes of different size, shape and wall thickness provides control over the geometrical surface area and specific surface area, which are important parameters when developing new substrates for SERS applications.

#### **2. Fabrication, surface and structure characterization of SERS substrates based on TiO2 nanotubes**

Innovative SERS active platforms based on TiO2 NT with noble metal deposits (Ag) were used for investigating various organic probe molecules such as pyridine (Py), mercaptobenzoic acid (MBA), organic dye 5-(4-dimethylaminobenzylidene) rhodamine (DBRh) and rhodamine 6G (R6G). For this purpose, simple electrochemical methods were applied: anodic oxidation of Ti foil (0.25 mm-thick, 99.5% purity, Alfa Aesar) in an optimized electrolyte: a glycerol/ water mixture (volume ratio 50:50) with 0.27 M NH4 F under different constant voltages from 10 up to 30 V. They led to the formation of nanoporous titanium oxide structures, and subsequently, to the preparation of specific metal nanostructures on surfaces thereof during PVD processes (magnetron sputtering, evaporation at high and low vacuum). Before the Ag deposition, all of the samples were annealed in air at 650°C for 3 h in order to transform the structure of the TiO2 NT from amorphous to crystalline [19, 22].

Ag nanoparticles (from 0.01 up to 0.03 mg/cm2 ) were deposited using the sputter deposition technique: the evaporation method in a low vacuum (p = 3 × 10−3 Pa) with a JEE-4X JEOL device, and the DC magnetron sputtering technique using a Leica EM MED020 apparatus in a configuration perpendicular to the surface of the samples. More details are given elsewhere [18, 23]. We strictly controlled in situ the average amount of metal deposited per cm2 using a quartz microbalance. One has to consider, however, that the true local amount of the metal deposits may vary substantially from site to site. The highly developed specific surface area of the nanotube arrays and its brush-like morphology may strongly affect Ag local distribution, resulting in considerable non-uniformity. For this type of process, silver targets of 99.9% purity (Kurt J. Lesker Company) were used. To better control the silver sputtering deposition process on the surface of the nanotubes, we applied the thermal evaporation method (0.01 and 0.05 mg/cm2 ) using an EF 40C1 effusion cell inside an UHV preparation chamber (PREVAC, Poland). The cell was maintained at a temperature of 900°C during the process of resistive evaporation. Silver (2 mm-diameter wire, 99.999%, Alfa Aesar) was evaporated onto the room temperature-surface of the TiO2 NT at a pressure of 1–2∙10−6 Pa at a constant evaporation rate of 0.13 nm/min. The evaporation rate from the silver effusion cell was calibrated and monitored using a TM-400 quartz crystal thickness monitor (Maxtek Inc.). The process of fabricating the SERS substrates is presented schematically in **Figure 1**.

The SERS platforms thus fabricated were characterized using various analytical methods. Electron Microscopy was applied for the morphological and structural characterization of the nanotubes after each preparation stage. The SEM observations (FEI NovaNanoSEM 450, Hitachi S70, Hitachi S-5500) were carried out at an accelerating voltage of 5 or 10 kV and an SE detector to reveal topographic contrast at the tops of the tubes. The thin sample for STEM observations was prepared by using the lift-out technique with a Hitachi NB-5000Focused Ion Beam system. The internal structure of the nanotubes was examined with a dedicated STEM Hitachi HD2700 in BF and atomic mass contrast. Observations at 200 kV revealed a columnar structure of the nanotubes and the transition zone. The surface chemical composition and the chemical state of the surface species (Ti, O, Ag) of the fabricated SERS platforms were examined using XPS spectroscopy. The XPS spectra were measured with a Microlab 350 Thermo Electron spectrometer at 300 W non-monochromatic Al Kα radiation and 1486.6 eV energy. AES spectroscopy was applied to determine the local chemical composition of the SERS substrates obtained. All of the spectra were recorded at an energy of 10 kV. The appropriate standards for AES and XPS reference spectra were also used. Finally, the Raman spectra of 2-mercaptoethanesulfonate (5 × 10−3 M), pyridine (5 × 10−2 M, in a mixture of pyridine and 0.1 M KCl), DBRh (10−4 M, in a mixture of water and ethanol (1:2)), and rhodamine from a 10−7 and 10−9 M aqueous solution were collected with a Horiba Jobin–Yvon Labram HR800 using a He-Ne laser (632.8 nm) as the excitation source. For each sample, 20 measurements were performed locally at various points chosen randomly.
