High-aspect-ratio TiO2 nanotubes by anodization of titanium.

Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum and silicon have attracted significant interest in recent years. While scientific thrust is often directed towards the elucidation of the principles of the self-organization phenomena, technological efforts target applications based on the direct use of the high surface area, for example, for sensing 6] or controlled catalysis, exploit the optical properties in photonic crystals, waveguides, or in 3D arranged Bragg-stack type of reflectors. The highly organized structures may be used indirectly as templates for the deposition of other materials such as metals, semiconductors, or polymers. Over the past few years, nanoporous TiO2 structures have also been formed by electrochemical anodization of titanium. Although several applications have been proposed, a wider impact of these structures has been hampered by the fact that the layers could only been grown to a limiting thickness of a few hundreds of nanometers. Herein we demonstrate for the first time how high-aspectratio, self-organized, TiO2 films can be grown by tailoring the electrochemical conditions during titanium anodization. Figure 1 shows scanning electron microscope (SEM) images of self-organized porous titanium oxide formed to a thickness of approximately 2.5 mm in 1m (NH4)2SO4 electrolyte containing 0.5 wt.% NH4F. From the SEM images it is evident that the self-organized regular porous structure consists of pore arrays with a uniform pore diameter of approximately 100 nm and an average spacing of 150 nm. It is also clear that pore mouths are open on the top of the layer while on the bottom of the structure the tubes are closed by presence of an about 50-nm thick barrier layer of TiO2. The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO2 by localized acidification at the pore bottom while a protective environment is maintained along the pore walls and at the pore mouth. In our previous work in HF and NaF solutions it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO2 at the pore bottom and the chemical dissolution of this TiO2 in an F ion containing solution (Figure 2). The solubility of TiO2 in HF, forming [TiF6] 2 , is essential for pore formation, however, it is also the reason that previous attempts to form porous layers in HF electrolytes always resulted in layer thicknesses in the range of some 100 nm. We tackled the problem by controlling the self-induced acidification of the pore bottom that is caused by the electrochemical dissolution of the metal (Figure 2 a–c). Main reason for the localized acidification is the oxidation and hydrolysis of elemental titanium [Eq. (1), in Figure 2]. The chemical dissolution rate of TiO2 is highly dependent on the pH value (see Figure 2 d). Using a numerical simulation of the relevant ion fluxes we can construct the pH profile in the pore (such as in Figure 2b), in other words, the ideal ion flux for the desired pH profile can then be determined. Furthermore we can tune the dissolution rate by the dissolution current. In other words, using a buffered neutral solution as electrolyte and adjusting the anodic current flow to an ideal value, acid can be created where it is needed, that is, at the pore bottom, while higher pH values are established at the pore mouth as a result of migration and diffusion effects of the pH buffer species (NH4F, (NH4)2SO4). Assuming equilibrium, the flux of dissolving species (leading to acidification at the pore bottom) and the flux of buffering species are equal. The calculations show that for the experimental conditions given in Figure 1 the pH value at the pore bottom is around 2 and increase to about 5 at the pore mouth, this corresponds to a drop in the local chemical etch rate of about 20 times. We used a voltage-sweep technique to achieve a steadystate current and to establish the desired pH profile. The reason to use a voltage-sweep technique rather than a Figure 1. SEM images of porous titanium oxide nanotubes. The crosssectional (a), top (b), and bottom (c) views of a 2.5-mm thick selforganized porous layer. The titanium sample was anodized up to 20 V in 1m (NH4)2SO4 + 0.5 wt. % NH4F using a potential sweep from open-circuit potential to 20 V with sweep rate 0.1 Vs . The average pore diameter is approximately 100 nm and the average pore spacing is approximately 150 nm.