Principles and Examples

In the past, most research in the area of titanium anodising was focussed on the preparation of the non-porous, durable and corrosion-resistant film of TiO2. However, the recent developments in nanotechnology and the demand for new nanomaterials have stimulated the development of methods for preparing porous TiO2 films. It is well known that the addition of fluoride ions to an aqueous electrolyte solution can significantly lower the corrosion resistance of titanium and a TiO2 coating, due to the formation of pitting channels. Based on this effect, a new method to produce nanoporous TiO2 films has been established using fluoride-containing electrolytes.

In early 2001, Grimes et al. reported the preparation of self-organised TiO2 nanotube arrays by direct anodising of titanium foil in a H2O-HF electrolyte64 at room temperature. The nanotubes were all oriented in the same direction, perpendicular to the surface of the electrode, forming a continuous film. The thickness of this film (or the length of the tubes) was only 200 nm (see Table 2.2). The average internal diameter of nanotubes exceeded 50 nm (see Figure 2.11). One end of the nanotubes (facing the electrolyte) was always open, while the other end (which was in contact with the titanium electrode) was always closed (Figure 2.11c). The thickness of the walls of the nanotubes was approximately 10 nm. Usually the walls were thinner at the end of the nanotubes facing the electrolyte, and thicker at the end in contact with the titanium electrode (Figure 2.11d). In this preparation, the wall of nanotubes consists of amorphous TiO2, which can be converted to polycrystalline anatase or an anatase-rutile mixture by heat treatments at temperatures above 500 °C. The surface of the

Table 2.2 Morphological properties of TiO2 nanotubes produced by anodis-

ing Ti foil in various electrolytes at 25 °C.

Table 2.2 Morphological properties of TiO2 nanotubes produced by anodis-

ing Ti foil in various electrolytes at 25 °C.

Electrolyte composition

Electrode potential vs SCE/V

Internal diameter/

nm

Length / mm

Ref.

0.5-3.5 wt% HF in H2O

3-20

25-65

0.2

64

0.5 wt% NH4F in 1 mol dm"3 (NH4)2SO4

20

90-110

0.5-0.8

65

4 wt% HF in 48 wt% DMSO, 48 wt% ethanol

20

60

2.3

66

0.5 wt% NH4F in 1 mol dm"3 (NH4)H2PO4, + 1 mol dm-3 H3PO4

20

40-100

0.1-4

67

0.5 wt% NH4F in CH3COOH

10-120

20

0.1-0.5

68

0.1-1 wt% NaF in 0.1-2 1 mol dm"3 Na2SO4

20

100

2.4

69

0.2 wt% H2O in ethylene glycol with 0.2 mol dm"3 HF

120

70-200

260

70

H2O-glycerol (from 50:50 to 0:100 vol.%), 0.27 mol dm"3 NH4F

2-40

20-300

0.15-3

89

0.5 mol dm"3 HCl and 0.5moldm"3 H2O2 in H2O + ethylene glycol

10-23

15

0.86

71

nanotube walls can be corrugated or smooth, depending on the electrolyte composition.

More recently, a number of fluoride ion-containing electrolytes, including: a NH4F-(NH4)2SO4 mixture,65 HF in a dimethyl sulfoxide (DMSO)-ethanol mixture,66 phosphate,67 acetate,68 a non-acidic Na2SO4-NaF mixture69 and an electrolyte containing ethylene glycol,70 have been used. Some attempts have also been made to use a fluoride-free electrolyte, based on a mixture of HCl with H2O2.71 The morphological properties of the resultant nanotubes are summarised in Table 2.2. Several major review papers, which consider the fabrication, properties and various applications of anodised ordered TiO2 nanotubular coatings have been published.72 74

In contrast to techniques using AAO as a template, TiO2 nanotubes prepared by direct anodising are not usually separated from each other in a regular manner and do not have well-developed cavities between the tubes (see Figures 2.3. and 2.11). Usually, in such cases, the walls of aligned nanotubes are mutually connected by small bridges.

Starting from titanium alloys (titanium combined with other valve metals) instead of pure titanium, it is possible to prepare nanotube arrays of mixed oxide composition using the method of anodising in fluoride ion-containing electrolytes. Such composite oxide nanotubes can increase drastically the potential functionality of the tubes (e.g. the incorporation of doping species on

Figure 2.11 Typical SEM images of TiO2 nanotube array films prepared by anodic oxidation of titanium in fluoride-containing electrolytes: a) top view and b) side view of the wall structure, and c) and d) bottom views. (Images are reproduced with kind permission as follows: a) and c) from ref. 72 b) from ref. 67 and d) from ref. 70).

Figure 2.11 Typical SEM images of TiO2 nanotube array films prepared by anodic oxidation of titanium in fluoride-containing electrolytes: a) top view and b) side view of the wall structure, and c) and d) bottom views. (Images are reproduced with kind permission as follows: a) and c) from ref. 72 b) from ref. 67 and d) from ref. 70).

the oxide structure), hence expanding the potential for industrial applications. Furthermore, novel nanostructured composites may also have interesting properties. A range of valve metal oxide nanotube arrays have been reported, including: binary TiAl (ref. 75), TiNb (ref. 76) and TiZr (ref. 77,78), as well as complex Ti6Al7Nb (ref. 79) and Ti29Nb13Ta4 6Zr.(ref. 80). The morphological properties of mixed oxide nanotube arrays are similar to those for TiO2 nanotubes. The range of metals which are capable of forming mixed oxide nanotubes via anodising of the corresponding titanium alloy is limited, due to the difference in solubility of each alloy component in the electrolyte, resulting in a selective dissolution of the least stable element and different reaction rates for the different alloy phases.74

The advantage of TiO2 nanotubes produced by anodising is that they have been effectively immobilised on a titanium surface during preparation. As the result, these nanotubes have several possible applications. It has recently been found that the electroconductivity of TiO2 nanotube films increases by several orders of magnitude in the presence of gaseous hydrogen at 290 °C (ref. 81), making this a promising material for hydrogen sensing. Similar TiO2 nanotubes have also shown promise for use as photocatalytic, self-cleaning surfaces,82 as photoanodes for water splitting83 or in dye-sensitised solar cells84 86 (where the efficiency of the photoanodic response depends on the nanotube wall thickness, and the current collection efficiency is a function of the quality of the nano-tube-electrode contact).

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