High‐Aspect‐Ratio TiO₂ Nanotubes by Anodization of Titanium

Abstract

Nanotubular material surfaces produced by the electrochemical formation of self-organized porous structures on materials such as aluminum [1,2] and silicon [3,4] 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 [5,6] or controlled catalysis, [7] exploit the optical properties in photonic crystals, [8] waveguides, [9] or in 3D arranged Bragg-stack type of reflectors.[10]The highly organized structures may be used indirectly as templates [11] for the deposition of other materials such as metals, [12] semiconductors, [13] or polymers. [14] Over the past few years, nanoporous TiO 2 structures have also been formed by electrochemical anodization of titanium. [15][16][17] Although several applications have been proposed, [18,19] 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, TiO 2 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 (NH 4 ) 2 SO 4 electrolyte containing 0.5 wt. % NH 4 F. 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 TiO 2 .The key to achieve high-aspect-ratio growth is to adjust the dissolution rate of TiO 2 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 [15,20] it was established that the thickness of the porous layer is essentially the result of an equilibrium between electrochemical formation of TiO 2 at the pore bottom and the chemical dissolution of this TiO 2 in an F 2À , 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 TiO 2 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 (suc...

Porous Aluminum Oxide Formed by Anodizing in Various Electrolyte Species

“…It is important to choose an appropriate electrolyte and anodizing conditions for ordered porous aluminum oxide fabrication. Recently, porous oxide technology including nanotube structures extends not only to aluminum but also to other metals such as titanium [171,172], zirconium [173], hafnium [174], niobium [175], tantalum [176], and tungsten [177]. Moreover, a third generation anodic oxide, different from barrier and porous oxide, anodic alumina nanofibers fabricated by novel anodizing has been investigated by us.…”

Aligned Fe2TiO5-containing nanotube arrays with low onset potential for visible-light water oxidation

“…Although some semiconductor photoanodes such as Fe 2 O 3 were reported to be able to absorb visible lights, their photocarrier utilization for oxidizing water is relatively low and their quantum efficiencies are usually lower than 10% at 1.23 V versus reversible hydrogen electrode (RHE) 7,8 . The architecture of vertically aligned nanotube arrays has shown considerate improvements to the quantum efficiencies of TiO 2 and Fe 2 O 3 photoanodes [9][10][11][12][13][14] , but the total conversion efficiency of visible lights is still not satisfactory. Therefore, it is highly desirable yet challenging to find an efficient photoanode semiconductor that is capable of using the abundant visible lights.…”

TiO2 nanotubes with ultrathin walls for enhanced water splitting

“…However, the thin wall thickness would prevent the formation of a large depletion region, and consequently there would not be enough potential drop across the wall, leading to less separation of the photogenerated charge carriers. 7,22 When the wall thickness is less than the diffusion length in titania, which is thought to be about 10 nm, only in this case, there is no electric diffusion layer needed to separate electrons and holes, as the holes simply diffuse through the tube walls to the semiconductor/ electrolyte interface, 7 see Scheme 1.…”