**Aluminum Anodic Oxide AAO as a Template for Formation of Metal Nanostructures**

Piotr Tomassi and Zofia Buczko

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/61263

#### **Abstract**

The aim of the chapter is to describe the applications of AAO as a template in metal nanostructures formation and to present the experimental results obtained by authors in this field. The basic mechanism of the process of anodic oxidation of aluminum was described. The influence of oxidation parameters on the AAO structure was dis‐ cussed. The processes of electrochemical metal deposition in AAO were described. The main present as well as future applications of metal nanostructures formed were listed.

**Keywords:** anodic oxidation of aluminum (AAO), metal nanostructures

## **1. Introduction**

The aim of this chapter is to describe the applications of AAO as a template in metal nano‐ structures formation and to present the experimental results obtained by authors in this field.

Metals in a state of high dispersion currently play an important role in technology. Their chemical and physical macroscopic properties, such as the rate of their reaction with other substances, colours and mechanical properties are significantly different from the bulk metals. Fundamental research and technology development over the last decade have resulted in wider use and implementation of metal-containing materials in a state of high dispersion into the industrial practice.

Solid nanoparticles are a system of unstable thermodynamic state, so even precious metals are not found in this form in nature. However, in the case of metals having a high cohesive energy, and thus a high melting point, it is possible to obtain and maintain them in the form of nanodispersion for a long period of time. This is the case of high activation energy of the

agglomeration process of particles, which is a crucial parameter. This applies in particular to transition metals. Nanodispersions of these metals are most often applied in practice.

The basic properties of highly dispersed metals are described in monographs of Romanowski [1] and also of Feldheim and Foss [2]. The authors point out that the systems containing metal nanoparticles are particularly interesting because their synthesis and chemical modification are simple. The main directions, according to the authors, of current basic research are given in the following text.

#### **1.1. Investigation of optical properties**

Metal particles often exhibit strong plasmon resonance extinction bands in the visible spec‐ trum. While the spectra of molecules can be understood only in terms of quantum mechanics, the plasmon resonance bands of nanoscopic metal particles can often be described in terms of classical free-electron theory and electrostatic models for particle polarizability. In contrast to molecular systems, the linear optical properties of metal nanoparticle composites can be changed significantly without a change in essential chemical composition.

Optical properties of metal nanoparticles are usually described by Mia theory. Material testing is mostly done by optical spectroscopy. Deviation from the theoretical and measured values is unavoidable due to uneven sizes and shapes of metal particles in real systems and the diversity of their distribution in a material. Therefore, theoretical analytical methods are not enough to describe the real particle systems and numerical methods are often implemented. Investigation results of extinction, absorption, scattering, and optical interaction of metal nanoparticles can be found, among others, in the work of Lin et al. [3] and Noguez et al. [4].

#### **1.2. Investigation of electrical properties**

Metal particles and bulk materials are similar taking into account their electrical properties. Simple classical charging expressions and RC equivalent circuit diagrams can be used in both cases to describe surface charging and electron transport processes. Description of electric properties of metal nanoparticles requires knowledge of their size and dielectric properties of the surrounding medium.

Examples of research in this field are given in the work of Wu et al. [5] on study of metal clusters of silver, gold, and copper. Static electric polarizability and absorption spectrum have been measured. Density functional theory (DFT) has been implemented for description of investi‐ gated systems. A competition of charge transfer and electron cloud distortion was proposed for explaining the spacing dependence of both electric and optical properties.

#### **1.3. Investigation of magnetic properties**

Small, single-domain ferromagnetic particles, containing a few tens or a few hundred atoms have a large total magnetic moment. When these ferromagnetic particles are separated by comparatively large distances, their magnetic moments do not interact strongly and they form a system of magnetic dipoles, whose relative orientation can be randomized by thermal motion even in relatively low temperatures. The overall magnetization of the system is therefore similar to that of a paramagnetic material, expressed by the Langevin equation.

The study of ferromagnetic nanoparticles systems are mainly targeted to the specific use of new nanomaterials. In particular, researches are provided to receive improved materials for high-density data storage, for the construction of high-speed computing components, and for a new-generation high-performance display.

The test results of the magnetic properties of metal nanoparticles have been published, among others, by Kim et al. [6]. The authors investigated Ni-Co, Ni-Fe, and Co-Pt alloy nanoparticles. Magnetic hysteresis loops were presented and values of magnetic susceptibility, the saturation magnetization, and coercivity were calculated.

Implementation of porous alumina, obtained by anodic oxidation of aluminum, as a template for producing new nanostructured materials is a quickly developing part of nanotechnology in recent years. Introducing a metal by electrodeposition inside the AAO pores, it is possible to obtain a durable material, characterized by a uniform structure of nanoparticles of a single metal or alloy in a matrix of dielectric amorphous alumina. The basis of manufacturing processes of composite materials of this type is presented in the following sections. References are listed in Table 1.


**Table 1.** Basic studies on AAO as a template for formation of metal nanostructures

## **2. Anodic oxidation of aluminum**

agglomeration process of particles, which is a crucial parameter. This applies in particular to

The basic properties of highly dispersed metals are described in monographs of Romanowski [1] and also of Feldheim and Foss [2]. The authors point out that the systems containing metal nanoparticles are particularly interesting because their synthesis and chemical modification are simple. The main directions, according to the authors, of current basic research are given

Metal particles often exhibit strong plasmon resonance extinction bands in the visible spec‐ trum. While the spectra of molecules can be understood only in terms of quantum mechanics, the plasmon resonance bands of nanoscopic metal particles can often be described in terms of classical free-electron theory and electrostatic models for particle polarizability. In contrast to molecular systems, the linear optical properties of metal nanoparticle composites can be

Optical properties of metal nanoparticles are usually described by Mia theory. Material testing is mostly done by optical spectroscopy. Deviation from the theoretical and measured values is unavoidable due to uneven sizes and shapes of metal particles in real systems and the diversity of their distribution in a material. Therefore, theoretical analytical methods are not enough to describe the real particle systems and numerical methods are often implemented. Investigation results of extinction, absorption, scattering, and optical interaction of metal nanoparticles can be found, among others, in the work of Lin et al. [3] and Noguez et al. [4].

Metal particles and bulk materials are similar taking into account their electrical properties. Simple classical charging expressions and RC equivalent circuit diagrams can be used in both cases to describe surface charging and electron transport processes. Description of electric properties of metal nanoparticles requires knowledge of their size and dielectric properties of

Examples of research in this field are given in the work of Wu et al. [5] on study of metal clusters of silver, gold, and copper. Static electric polarizability and absorption spectrum have been measured. Density functional theory (DFT) has been implemented for description of investi‐ gated systems. A competition of charge transfer and electron cloud distortion was proposed

Small, single-domain ferromagnetic particles, containing a few tens or a few hundred atoms have a large total magnetic moment. When these ferromagnetic particles are separated by comparatively large distances, their magnetic moments do not interact strongly and they form a system of magnetic dipoles, whose relative orientation can be randomized by thermal motion

for explaining the spacing dependence of both electric and optical properties.

changed significantly without a change in essential chemical composition.

transition metals. Nanodispersions of these metals are most often applied in practice.

in the following text.

76 Electroplating of Nanostructures

**1.1. Investigation of optical properties**

**1.2. Investigation of electrical properties**

**1.3. Investigation of magnetic properties**

the surrounding medium.

Passivation phenomenon of aluminum spontaneously occurs in contact with oxygen and thin film of alumina on the surface of the metal is formed. Thicker oxide coatings can be produced by the use of electrochemical anodic polarization in selected solutions. This surface finishing of aluminum work pieces is widely applied for corrosion and abrasion protection and for decorative purposes.

In carrying out the anodizing process of aluminum in an electrolyte, which slightly dissolves alumina, an oxide layer is formed with a unique porous structure on the metal surface. Highly ordered pores are perpendicularto the base and pass through almost the entire thickness of the oxide layer. Directly on the metal surface nonporous barrier layer is grown and it usually has a thickness of several tens of nanometers. When an aluminum substrate with high purity and uniform structure is used and the parameters of the operation of the electrochemical oxida‐ tion are carefully controlled, homogeneous distribution of pores as a hexagonal network can be obtained. The pore diameters are uniform; it depends on the oxidation conditions and can be controlled in the range from 10 to several hundred nanometers. The total thickness of the oxide layer is determined by the oxidation time. It is usually in the range of tenths of a micron to tens of microns. The schematic drawing of the AAO structure is presented in Figure 1.

**Figure 1.** (a) Schematic drawing of AAO structure prepared by electrochemical oxidation of Al. (b) Summary of selfordering voltage and corresponding interpore distance of AAO produced within three well-known regimes of electro‐ lytes (sulfuric, oxalic, and phosphoric). (c) (Top) SEM cross-sectional view of AAO membrane formed by MA (0.3 M H2C2O4, 1OC, 40 V) and (bottom) by HA (at 140 V) for 2 h (insets: SEM top view of pore structures). Permission Elsevier [61].

The mechanism of the process of anodic oxidation of aluminum and forming the porous oxide layer is still under investigation. There are some basic theories, like the Keller, Hunter, and Robinson geometric model [7] and the colloidal one of Murphy and Michelson [8]. It is believed that the porous structure is a result of two competitive reactions: Al2O3 forming and dissolution of the oxide. These reactions are stimulated by the electric field that is distributed inside the barrier layer. The electric field intensity inside the barrier layer is of the order of 1 V/nm during the oxidation process. Such high field is needed to excite the ionic current inside the oxide. This field also stimulates the dissolution reaction ("field-assisted dissolution"). Also, due to the tunneling effect, additional electron current may occur. The role of that current in the anodic oxidation of aluminum was described by Palibroda [9-10].

A summary of recent interpretations of the reaction mechanism of anodic oxidation of aluminum can be found in the Brace monograph [11] and the Wielage et al. work [12]. The total chemical reaction is as follows:

$$2\text{Al} + 3\text{H}\_2\text{O} \rightarrow \text{Al}\_2\text{O}\_3 + 6\text{H}^+ + 6\text{e}^- \tag{1}$$

It consists of two partial reactions:

ordered pores are perpendicularto the base and pass through almost the entire thickness of the oxide layer. Directly on the metal surface nonporous barrier layer is grown and it usually has a thickness of several tens of nanometers. When an aluminum substrate with high purity and uniform structure is used and the parameters of the operation of the electrochemical oxida‐ tion are carefully controlled, homogeneous distribution of pores as a hexagonal network can be obtained. The pore diameters are uniform; it depends on the oxidation conditions and can be controlled in the range from 10 to several hundred nanometers. The total thickness of the oxide layer is determined by the oxidation time. It is usually in the range of tenths of a micron to tens of microns. The schematic drawing of the AAO structure is presented in Figure 1.

**Figure 1.** (a) Schematic drawing of AAO structure prepared by electrochemical oxidation of Al. (b) Summary of selfordering voltage and corresponding interpore distance of AAO produced within three well-known regimes of electro‐ lytes (sulfuric, oxalic, and phosphoric). (c) (Top) SEM cross-sectional view of AAO membrane formed by MA (0.3 M H2C2O4, 1OC, 40 V) and (bottom) by HA (at 140 V) for 2 h (insets: SEM top view of pore structures). Permission Elsevier

The mechanism of the process of anodic oxidation of aluminum and forming the porous oxide layer is still under investigation. There are some basic theories, like the Keller, Hunter, and Robinson geometric model [7] and the colloidal one of Murphy and Michelson [8]. It is believed that the porous structure is a result of two competitive reactions: Al2O3 forming and dissolution of the oxide. These reactions are stimulated by the electric field that is distributed inside the

[61].

78 Electroplating of Nanostructures

$$2\text{Al} \rightarrow 2\text{Al}^{3+} + 6\text{e}^{\cdot} \tag{2}$$

$$\text{CH}\_2\text{O} \to 6\text{H}^+ + 3\text{O}^{2-} \tag{3}$$

The oxide forms due to migration of Al3+ ions from basic metal into the solution, while the movement of ions O2- is in the opposite direction. Under the influence of a high electric field, there is ionic conductivity in the oxide layer where the aluminum and oxygen ions are charge carriers.

A characteristic porous structure of the alumina layer is formed as a result of the chemical and electrochemical dissolution of the oxide. Dissolution reaction is promoted by local increase in hydrogen ion concentration (reaction 1) and high electric field inside the barrier layer ("fieldassisted dissolution"). Due to the high electrical resistance of the barrier layer, the Joule heat is given off during a flow of electric charge, which causes local increase in temperature and also enhances the dissolution reaction of the oxide.

Defects and impurities, which are always present in the metal substrate, are precursors of pores. Due to the contact with an aqueous solution to the oxide layer surface, there are always various intermediate forms of hydrated oxide, including aluminum hydroxide. In addition, due to the adsorption of anions from the solution, the oxide material is enriched with other compounds, like for example sulfates in the case of processing in sulfuric acid.

Oxide layer with porous structure is obtained using solutions of dibasic and tribasic acids, in which the oxide is dissolved. When the critical thickness of the barrier layer is obtained, pores are initiated in flows, cracks, and impurities spots. Only some part of embryo pores develops up to form final porous structure with the hexagonal arrangement. It is determined by the reactions' activation energy of oxide formation and dissolution, also by the distribution of electric field in the barrier layer

Attempts have been made to create modeling and mathematical description of the electro‐ chemical processes of formation of porous aluminum oxide layers [13-18]. Parkhutik and Shershulsky [13] are the first to prove that the potential distribution inside the oxide layer can be calculated with the use of Laplace equation. Calculations with the adequate assumptions of the local moving rate of the solution–oxide and oxide–metal phase boundaries have been made for a number of cases in the two-dimensional system. As the results pictures of the development of porous structures have been obtained similar to those observed on the SEM images of experimental samples. Also, numerical approximations are often used for the mathematical modeling [19].

Beside the above-described mechanism of aluminum anodic oxidation process and its mathematical modeling, the authors point to additional important factors that have an impact on the process of forming a porous oxide structure:


On the basis of these assumptions, Wu et al. [17] attempted to demonstrate that the chemical and electrochemical reactions of oxide dissolution do not have significant effect on the process of forming a porous oxide structure. This statement, however, is not consistent with the conclusions of other authors.

## **3. Preparation of AAO template**

A porous oxide coating can be obtained by anodic oxidation of aluminum using a number of different types of electrolyte solutions. Sulfuric, oxalic, phosphoric, or chromic acids solutions are typically implemented. The porous anodic layer can also be obtained from solutions of many organic acids, especially the polybasic acids, such as tartaric, citric, sulfosalicylic, maleic, and succinic acids.

Such coating parameters as the thickness of the barrier layer, pore diameter, the distance between the pores and their surface density considerably vary depending on the type of processing solutions, as is shown in Tables 2 and 3 [20]. For a given type of solution, the geometric parameters of the net of pores can be controlled by varying the voltage during anodic oxidation. The thickness of the barrier layer and the diameter of the pores are proportional to the applied voltage (Table 2). The pore diameter is usually from 10 to a few tens of nanometers. Pores having higher values of diameter can be obtained from solutions of phosphoric acid or organic acids at high voltages. Li et al. [21] give the equation that correlates distance between the pores and the voltage and pH value for processing solutions. Uniform grid of unusually high pore diameters of up to 200–500 nm have been successfully obtained with properly selected oxidation conditions in solutions containing phosphoric acid and/or organic acids with high voltage current in [22-26]. In the solution of citric acid using high current voltage, Mozalev et al. [27] obtained the interpore distance 1.1 μm, barrier layer thickness 0.5 μm, and pore diameter 0.23 μm. Also, lowering the temperature of anodic oxidation, thicker coatings can be easier to obtain; their porosity is lower and hardness is higher. The examples of SEM images of AAO coatings are presented in Figures 2 and 3.



Attempts have been made to create modeling and mathematical description of the electro‐ chemical processes of formation of porous aluminum oxide layers [13-18]. Parkhutik and Shershulsky [13] are the first to prove that the potential distribution inside the oxide layer can be calculated with the use of Laplace equation. Calculations with the adequate assumptions of the local moving rate of the solution–oxide and oxide–metal phase boundaries have been made for a number of cases in the two-dimensional system. As the results pictures of the development of porous structures have been obtained similar to those observed on the SEM images of experimental samples. Also, numerical approximations are often used for the

Beside the above-described mechanism of aluminum anodic oxidation process and its mathematical modeling, the authors point to additional important factors that have an impact

**1.** The volume of the oxide is higher than the metal consumed and therefore there are strong strains in oxide layer that cause mechanical stress fractures (cracks). These cracks are often

**2.** The process of oxidation may be affected by local differences in the wettability of the

**3.** Part of the aluminum ions Al3+ is ejected from the metal to the solution without binding to the oxide structure. This phenomenon reduces the current efficiency of anodic oxidation

**4.** In the presence of phases containing foreign elements in aluminum substrate, it is possible, as polarization is anodic, side reaction of evolution of oxygen gas to occur. Like the

On the basis of these assumptions, Wu et al. [17] attempted to demonstrate that the chemical and electrochemical reactions of oxide dissolution do not have significant effect on the process of forming a porous oxide structure. This statement, however, is not consistent with the

A porous oxide coating can be obtained by anodic oxidation of aluminum using a number of different types of electrolyte solutions. Sulfuric, oxalic, phosphoric, or chromic acids solutions are typically implemented. The porous anodic layer can also be obtained from solutions of many organic acids, especially the polybasic acids, such as tartaric, citric, sulfosalicylic, maleic,

Such coating parameters as the thickness of the barrier layer, pore diameter, the distance between the pores and their surface density considerably vary depending on the type of

previous phenomenon, it reduces the current efficiency of the oxidation reaction. **5.** Barrier layer thickness, the distance between the pores and their diameters are propor‐

tional to the applied voltage, with the other process parameters being fixed.

mathematical modeling [19].

80 Electroplating of Nanostructures

the beginning of a pore.

surface of the oxide.

conclusions of other authors.

and succinic acids.

**3. Preparation of AAO template**

reaction.

on the process of forming a porous oxide structure:


**Table 3.** Pore density in oxide coatings [20]

**Figure 2.** SEM image of AAO coating obtained in solution of sulfosalicylic acid 50 g/dm3 , oxalic acid 10 g/dm3 , and sulfuric acid 5 g/dm3 , 3 A/dm2 , 20OC, 30 min. Permission Librant [114].

**Figure 3.** SEM image of AAO coating obtained in solution of sulfosalicylic acid 50 g/dm3 , oxalic acid 10 g/dm3 , and sulfuric acid 5 g/dm3 , 3 A/dm2 , 20OC, 30 min. Permission Librant [114].

For many applications of Al2O3 coatings in nanotechnology, including membranes for nanofiltration, it is necessary to separate oxide layers from aluminum substrates. The following techniques are most frequently used:


For applications in nanotechnology, it is very important to obtain the pore distribution, which is homogeneous and uniform over the entire surface in the oxide layer. In order to obtain the highest possible uniformity of the pore distribution, the following techniques are applied:

	- **•** Prepatterning using a tip of the scanning probe microscope (SPM) or atomic force microscope (AFM)
	- **•** Focused ion beam lithography
	- **•** Holographic lithography

**Figure 2.** SEM image of AAO coating obtained in solution of sulfosalicylic acid 50 g/dm3

**Figure 3.** SEM image of AAO coating obtained in solution of sulfosalicylic acid 50 g/dm3

, 20OC, 30 min. Permission Librant [114].

For many applications of Al2O3 coatings in nanotechnology, including membranes for nanofiltration, it is necessary to separate oxide layers from aluminum substrates. The following

, 20OC, 30 min. Permission Librant [114].

sulfuric acid 5 g/dm3

82 Electroplating of Nanostructures

sulfuric acid 5 g/dm3

, 3 A/dm2

techniques are most frequently used:

, 3 A/dm2

, oxalic acid 10 g/dm3

, oxalic acid 10 g/dm3

, and

, and


Asoh et al. [46] obtained square layout of cells with centrally located pores after the initial shaping of a surface using the imprinting process. This square system is fundamentally different from the natural hexagonal distribution of pores.

Choi et al. [47] implemented a similar technique and got a uniform distribution of pores having a triangular cross section.

As can be seen there is a great range of possibilities of appropriate selection techniques for producing the porous aluminum oxide layer in order to achieve a certain distribution and pore geometry for use in nanotechnology. For example, a typical technique used to obtain a template for the production of metallic nanostructures is shown in Table 4 [29].

Various tools can be used for evaluation of the uniformity of pore distribution, e.g., Voronoi diagrams, radial distribution function (RDF) [48], fast Fourier transform (FFT) images, Delaunay triangulations (defects map), pair distribution functions (PDF), or angular distribu‐ tion functions (ADF) [44].


**Table 4.** The example of procedure of AAO template formation [29]

## **4. Metal deposition**

**•** Using stamps (molds) with regular array of convexes prepared lithographically

Asoh et al. [46] obtained square layout of cells with centrally located pores after the initial shaping of a surface using the imprinting process. This square system is fundamentally

Choi et al. [47] implemented a similar technique and got a uniform distribution of pores having

As can be seen there is a great range of possibilities of appropriate selection techniques for producing the porous aluminum oxide layer in order to achieve a certain distribution and pore geometry for use in nanotechnology. For example, a typical technique used to obtain a template

Various tools can be used for evaluation of the uniformity of pore distribution, e.g., Voronoi diagrams, radial distribution function (RDF) [48], fast Fourier transform (FFT) images, Delaunay triangulations (defects map), pair distribution functions (PDF), or angular distribu‐

**•** Optical diffraction grating

a triangular cross section.

84 Electroplating of Nanostructures

tion functions (ADF) [44].

**Step no. Process**

2.

3.

4.

5.

**•** Nanosphere lithography (NSL)

different from the natural hexagonal distribution of pores.

1. Degreasing of AA 1050 alloy in ethanol and acetone

Electrochemical polishing

for the production of metallic nanostructures is shown in Table 4 [29].

Perchloric acid (60 wt.%) and ethanol (1:4 vol) Constant potential 20 V for 1 min at 10OC

Alumina layer removal by chemical dissolution

Constant potential 45V for 1, 2, 4, or 8 h at 20OC

The samples with different coating thicknesses were obtained

Anodic oxidation in 0.3M oxalic acid Constant potential 45V for 60 min at 20OC

6 wt.% H3PO4 + 1.8 wt. % H2Cr2O4 Time 12 h, temperature 45OC

Anodic oxidation in 0.3M oxalic acid

6. Removal of aluminum substrate by chemical dissolution Saturated HgCl2 solution

7. Chemical etching of alumina barrier layer 5 wt.% H3PO4 at 25OC

**Table 4.** The example of procedure of AAO template formation [29]

The composite systems AAO–metal can be formed by inexpensive and simple method of electrochemical metal deposition. It does not require complex equipment as in the case of CVD or PVD methods.

The first attempts of metals electrodeposition inside the pores of the anodic oxide layer on aluminum AAO were associated with the development of electrochemical coloring technolo‐ gy. Metal nanoparticles can be deposited into the pores of Al2O3 layer by transferring a freshly prepared sample to the solution of the salt of easily reducible metal and then use of cathodic polarization. This causes color effect as a result of optical phenomena like absorption and scattering of light by the deposited metal particles inside the pores. In this way, the oxide layer which was initially colorless can be converted to durably colored; usually there are shades of brown or black.

Electrochemical technologies of coloring anodized aluminum were developed in the 1970s and it is associated with the names of researchers such as Caboni, Langbein, Pfanhauser, Assad, and Sheasby [49]. Herrmann [50] gives 93 examples of electrochemical coloring processes using solutions of salts of the following metals: Pb, Cd, Cr, Fe, Au, Co, Cu, Mn, Ni, Se, Ag, Te, Zn, Sn. If an additional modification of the shape of the pores is introduced before the metal deposition process, it is possible to get a new color effect due to the interference effect [51].

Electrochemical coloring method of anodized aluminum is currently widely used as durable, decorative surface treatment for aluminum components, particularly for construction and architecture applications. Usually, tin nanoparticles are deposited in the pores of AAO using alternating AC or pulse current. When only cathodic polarization is implemented in that process, there is a risk of damage to the oxide layer.

The mechanism of electrodeposition of metals in the pores of the oxide layer was investigated by Skominas et al. [52] and Zemanova et al. [53] and lately by Bograchev et al. [54]. The effects of the reduction reaction of hydrogen ions and of the current frequency were studied.

Wider research in this field was provided by Tomassi [55].

Anodic oxidation of 99.5% Al samples was conducted in 1.75 M H2SO4 solution in temperature 20o C with voltage 16 V. Thickness of oxide layer obtained was 15 μm. The oxide layers from phosphoric acid solutions were also obtained. The electrodeposition of nickel, copper, tin, or silver was performed in temperature 25o C in the following solutions:

> 4 4 4 3 3 3 2 4 2 4 2 4 a NiSO 0,1 M b CuSO 0,1 M c SnSO 0,1 M d AgNO 0.01 M H BO 0,5 M H SO 0,05 M H SO 0,2 M H ) SO 0 ) ) ) ,1 M

The maximum voltage of alternating current was 16 V (Ni) or 14 V (Cu, Sn, Ag). Usually, the frequency 50 Hz was used, but wide range from 0.1 to 1000 Hz was checked.

Electrochemical investigations have been performed using transient curves method, polari‐ zation methods, and impedance spectroscopy.

The uniform composite layers were obtained in the frequency region 10 – 100 Hz. With frequencies below 10 Hz the defects in oxide layer appear. In the high frequency region under 500 Hz the electrodeposition does not occur and high capacity current is observed.

Chemical analyses of obtained composite layers as well as structure, magnetic, microbiological properties, gravimetric and electrochemical investigations have been performed.

The X-ray analysis has confirmed that the metal deposition starts at the bottom of the pores close to the aluminum surface and the volume occupied by metal increases.

The investigations performed by transmission microscopy have shown that all pores are filled by metal deposit (Figure 4). The diameter of metal particles depends on pore diameter and has been found to be from 5 to 30 nm.

**Figure 4.** TEM image of AAO obtained in 1.75 M sulfuric acid, 20OC, 16 V, 30 min with nickel nanoparticles, AC nickel deposition, 2 min.

The role of alternating polarization in metal deposition process can be clarified as a result of the investigations. In the cathodic cycles, the hydrogen ions' reduction occurs simultaneously to metal electrodeposition. The diffusion of hydrogen ions inside the oxide layer leads to formation of hydrogen bonds and hydrated regions of higher conductivity (active sites) are created. In these regions, the probability of electron transport is higher than in the other sites, where the current is mainly of ionic character. In the cathodic cycles, the new active sites are formed and filled by metal deposit.

In the anodic cycles, the repassivation of active sites occurs. This process ensures that the barrier layer is not damaged. The metal electrodeposition can then be effectively conducted with the formation of a uniform oxide–metal composite coating.

Electrochemical investigations have been performed using transient curves method, polari‐

The uniform composite layers were obtained in the frequency region 10 – 100 Hz. With frequencies below 10 Hz the defects in oxide layer appear. In the high frequency region under

Chemical analyses of obtained composite layers as well as structure, magnetic, microbiological

The X-ray analysis has confirmed that the metal deposition starts at the bottom of the pores

The investigations performed by transmission microscopy have shown that all pores are filled by metal deposit (Figure 4). The diameter of metal particles depends on pore diameter and has

**Figure 4.** TEM image of AAO obtained in 1.75 M sulfuric acid, 20OC, 16 V, 30 min with nickel nanoparticles, AC nickel

The role of alternating polarization in metal deposition process can be clarified as a result of the investigations. In the cathodic cycles, the hydrogen ions' reduction occurs simultaneously to metal electrodeposition. The diffusion of hydrogen ions inside the oxide layer leads to formation of hydrogen bonds and hydrated regions of higher conductivity (active sites) are created. In these regions, the probability of electron transport is higher than in the other sites, where the current is mainly of ionic character. In the cathodic cycles, the new active sites are

500 Hz the electrodeposition does not occur and high capacity current is observed.

properties, gravimetric and electrochemical investigations have been performed.

close to the aluminum surface and the volume occupied by metal increases.

zation methods, and impedance spectroscopy.

86 Electroplating of Nanostructures

been found to be from 5 to 30 nm.

deposition, 2 min.

formed and filled by metal deposit.

The optimum current frequencies are in good correlation with a time of charge carriers formation determined for aluminum oxide layers by Ebling [56], Hassel, and others [57-58]. Authors of this chapter have obtained composite layers consisting of the following metals: Ni, Cu, Sn, Fe, Co, Zn, Cd, Au, Pd, Ag.

Metal nanoparticles deposited with the method described above may create durable system with an oxide matrix characterized with interesting scientific and operation properties. Metal nanoparticles can also be separated from the matrix, after, for example, a selective dissolution of alumina, and then examined individually. Nanowires, nanotubes, or nanodots can be similarly obtained. The TEM image of AAO layer with nickel nanoparticles is shown in Figure 4. The bigger diameter of metal nanostructures can be obtained when the oxide matrix is formed in phosphoric acid solution. The AAO formed in phosphoric acid is presented alone in Figure 5 and with nickel nanostructures in Figures 6 and 7.

**Figure 5.** SEM image of AAO obtained in 500 g/dm2 phosphoric acid, 30OC, 30 V, 20 min.

**Figure 6.** SEM image of AAO obtained in phosphoric acid with nickel particles. AC nickel deposition, 30 V, 30 s.

**Figure 7.** SEM image of AAO obtained in phosphoric acid with nickel particles. AC nickel deposition, 40 V, 15 s.

Oxide layers can be of different porosity with branches when anodic oxidation process is unsteady. The pore branching is clearly visible in the SEM photograph of the oxide layer obtained in a solution of sulfosalicylic acid (Figure 8). After deposition of the metal within the pores of oxide layer, the structure of metal nanoparticles is complex, e.g., in the form of branches [59-60] as is shown in Figure 9. Anodic oxidation was performed in oxalic acid solution; various barrier layer thinning procedures (BLT) were used before metal deposition. Metal structures deposited with DC are usually compact, but when AC pulse current is implemented, they may be of grained structure.

**Figure 6.** SEM image of AAO obtained in phosphoric acid with nickel particles. AC nickel deposition, 30 V, 30 s.

88 Electroplating of Nanostructures

**Figure 7.** SEM image of AAO obtained in phosphoric acid with nickel particles. AC nickel deposition, 40 V, 15 s.

Oxide layers can be of different porosity with branches when anodic oxidation process is unsteady. The pore branching is clearly visible in the SEM photograph of the oxide layer obtained in a solution of sulfosalicylic acid (Figure 8). After deposition of the metal within the pores of oxide layer, the structure of metal nanoparticles is complex, e.g., in the form of

**Figure 8.** SEM image of AAO coating obtained in solution of sulfosalicylic acid 50 g/dm3 , oxalic acid 10 g/dm3 and sul‐ furic acid 5 g/dm3 , 3 A/dm2 , 20OC, 30 min [114].

**Figure 9.** Nickel branches obtained by anodic oxidation in oxalic acid, 20OC, 45 V, 1 h, and nickel pulse electrodeposi‐ tion from Watt's solution with various BLT procedures (Permission Elsevier [59]).

In the processes for preparing metallic nanostructures with the use of AAO layers, sometimes additional operations are applied in order to obtain geometrically complex systems, e.g., PVD methods, photochemical techniques, and other methods. An excellent overview of these methods can be found in the monograph of Jani et al. [61]. Some examples of complex manufacturing of anodic oxide layer and forming of the metal nanostructures are given in Table 5.


**Table 5.** Examples of complex methods of preparation of oxide templates and metal nanostructures

## **5. Emerging applications**

Electrochemically obtained porous layers of aluminum oxide are widely used in nanotech‐ nology, not only in scientific research but also in various applications. Porous oxide layer can be separated from the substrate. After removing the barrier layer membranes are obtained. They can be applied for example in nanofiltration for removing bacteria from medical preparations (cold sterilization). They are also used to prepare the solutions of test samples for liquid chromatography. Revised filtration properties of these membranes can be found in Lee et al. [69].

Alumina is insoluble, nontoxic, and characterizes excellent biocompatibility. AAO porous layer has a highly developed surface area – about 2 to 3 orders of magnitude higher than the surface of the aluminum substrate. Hence, importance of these materials is growing and they are more widely used in chemical catalysis, in bioreactors, and in sensors. Porous anodic alumina is also an excellent substrate for culturing bacteria in discrete growth compartments. These cultures are investigated with the use of fluorescence microscopy. A broad overview of the use of the porous AAO in biotechnology is presented by Ingham et al. [70]. They present the list of fields of AAO applications as follows:


The unique properties of porous AAO are used in sensors and biosensors as already men‐ tioned. A review article was published by Santos et al. [71] in which authors distinguished two types of AAO sensors: optical and electrochemical. The phenomena and techniques used in both types of sensors are summarized as follows:

Optical Biosensors

methods, photochemical techniques, and other methods. An excellent overview of these methods can be found in the monograph of Jani et al. [61]. Some examples of complex manufacturing of anodic oxide layer and forming of the metal nanostructures are given in

[62]

[63]

[64]

[65]

[66]

[67]

[68]

**No. Process References**

using a lithographic process the regular patterns of porous oxide

polyethylene glycol hydrogel microstructures on alumina membranes

hydrothermal reaction, the Ag nanoflakes were obtained on the oxide

1050 alloy followed by removal of Al and pore opening/widening procedure. Au conducting layer was sputtered on one side of AAO membrane. The dense arrays of Ag, Au, and Sn nanowires were fabricated by a DC electrochemical deposition process.

fabricated by evaporating Au on the surface of porous anodic

by sputtering and replica processing techniques using nanoporous

Electrochemically obtained porous layers of aluminum oxide are widely used in nanotech‐ nology, not only in scientific research but also in various applications. Porous oxide layer can be separated from the substrate. After removing the barrier layer membranes are obtained. They can be applied for example in nanofiltration for removing bacteria from medical preparations (cold sterilization). They are also used to prepare the solutions of test samples for liquid chromatography. Revised filtration properties of these membranes can be found in

**Table 5.** Examples of complex methods of preparation of oxide templates and metal nanostructures

1. Evaporation of aluminum 0.3–1 μm layer on oxide surface. After

with 200 nm nanopores using photolitography.

4. AAO membranes were immersed in AgNO3 solution. As a result of

5. The AAO membranes were fabricated by a two-step anodizing of AA

6. Thin Au films with highly ordered arrays of hemispherical dots were

7. Highly ordered arrays of Ni nanoholes and FeNi dots were prepared

alumina membranes as a template.

3. Electroless deposition of nickel–boron nanoparticle structures on

2. Micropatterned substrates were prepared by fabrication of

were obtained.

AAO membrane.

membrane surface.

alumina template.

**5. Emerging applications**

Lee et al. [69].

Table 5.

90 Electroplating of Nanostructures


Electrochemical Sensors and Biosensors


Sensors and biosensors with AAO are used for the detection of gases, vapors, organic mole‐ cules, biomolecules, DNA, proteins, antibodies and cells, viruses, bacteria, cancer cells, which are in air, water, and biological environments.

As described in the previous chapter, complex composite systems produced by deposition inside the pores of AAO additional phases of metal, carbon, inorganic or organic compounds have been widely used in basic research and applications. This is especially due to their unique features like electrical, optical, catalytic, biological, and other properties. Major applications and references to the literature are given in Table 6.


**Table 6.** Main applications of AAO layers in science, technology, and nanotechnology

## **6. Summary**

**No. Application Foreign phases in AAO**

5. Formation of dots and nanodots InAs, SiGe, Si, CdTe, GaAs,

9. Examination of optical systems and models Ni, Co [64,89,90] 10. Examination of magnetic systems and models Fe, Ni, Co, alloys [91-93] 11. Catalysts of chemical reactions Ni, Au, Pd, C [81,94,95]

4. Formation of metal nanoparticles and

6. Formation of carbon structures, carbon nanotubes, graphene

7 Formation of inorganic compounds

8. Formation of organic compounds nanoparticles

12. Electronic components, capacitors, semiconductors

13. Luminescence, electroluminescence, and photoluminescence devices

14. Magnetic systems, data storage Fe, Ni, Co, alloys

15. Sliding joints MoS2, Sn [106] 16. Solar collectors Ni, Sn [20] 17. Solar batteries CdS [107,108] 18. Lithium-ion batteries LiMn2O4, CdS, CoSb [109] 19. Antibacterial materials Ag [110] 20. Photonic crystals [111,112] 21. Fuel cells Solid acid [113]

**Table 6.** Main applications of AAO layers in science, technology, and nanotechnology

nanoparticles

nanowires

92 Electroplating of Nanostructures

1. Membranes for ultra- and nanofiltration [69,73] 2. Sensors and biosensors [71,72] 3. Decorative coatings on aluminum Sn, Ni, Cu [20]

**matrix**

Fe, Ag

copolymer

In, Ga

Pt

Fe, Ni, Co, Au, Ag, Cu, Bi, Pt, Pd, Sn, W, Zn, Sb, Se, alloys

C [78-84]

GaN, ZnO, CdS, AgI, PbS [85,86]

Polypyrrole, polyaniline, DNA, polythiophene, ethylene vinyl acetate

C, CdS, ZnO, compounds of

Cu2O, ZnS, In2O3 [96,99-102]

**References**

[66,74,75]

[67,68,76,77]

[87,88]

[80,96-98]

[103-105]

The structure of AAO layers can be precisely controlled with operation parameters of electro‐ chemical oxidation. Complicated and expensive apparatus is not required for their prepara‐ tion. It is for this reason that these layers are so widely used in nanotechnology.

Using a variety of techniques, in particular electrochemical methods, various composite materials of alumina and the foreign phase can be prepared with new or improved physical or chemical properties. AAO template having a uniform porous structure allows obtaining sustainable systems of nanoparticles, nanowires, nanotubes, and nanodots with an unlimited number of materials. AAO has become in recent years one of the most important materials for nanotechnology development, both in research as well as in applications.

The significant number of publications in this field now exceeds the size of one hundred items per year. Further applications of the AAO as nanofiltration membranes are anticipated, including drug delivery, gas permeation, and hemodialysis. Flat membranes are already commercially available and now efforts have been made to prepare tubular membranes [69].

In biomedical applications, AAO has been confirmed as a good substrate for the study of biomolecules, cells, and bacteria and their detection by means of optical imaging methods (SERS, optical waveguides), and fluorescence-based detection methods [70].

In the field of sensors and biosensors, AAO applications allow to broaden choice and minia‐ turization of sensors, especially those with optical measurements. The design of implantable biosensors with the ability to monitor biological systems in vivo and real time is promising for the application of AAO immunosensors [71-72].

It seems that in order to further the development of manufacturing methods and applications of metal nanostructures, it is advisable to develop further research in the following areas:

	- **•** Multistage oxidation
	- **•** Use programmable waveform pulse currents
	- **•** The importance of the pretreatment of the surface of aluminum substrate (alloy type, its structure, impurities)
	- **•** Effect of heat pretreatment of the substrate and the final thermal treatment of AAO layer

## **Author details**

Piotr Tomassi\* and Zofia Buczko

\*Address all correspondence to: piotr.tomassi@gmail.com

Institute of Precision Mechanics, Warsaw, Poland

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