TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

In this paper we address several key aspects to the formation mechanism of self-organized oxide nanotube layers grown by anodization of valve metals and their alloys in fluoride ion containing electrolytes. We suggest that ͑i͒ the self-organized structure is produced as a result of an autocatalytic reaction, in which electrochemical oxidation and chemical dissolution of oxide accelerate each other; ͑ii͒ in the initial growth stage the competition for oxidizable area between neighboring initial growth spots is a key element in self-organization; and ͑iii͒ the diameter of the nanotubes on different materials and as a function of anodization voltage is strongly related with the anodic growth factor ͑nm/V͒ of the valve metal oxides. Additionally, for multilayer pore structure growth the present work provides insight into the sites of highest reactivity in repeated anodization experiments ͑bottom of the pores, in between pores͒.

The present work demonstrates that uniform and highly ordered arrays of TiO(2)-WO(3) nanotubes can be grown by anodization of Ti alloys in an ethylene glycol/fluoride based electrolyte under selected electrochemical conditions. These aligned mixed oxide nanotube structures are highly suitable for enhanced electrochromic reactions; in particular we show that already small amounts of WO(3) (such as 0.2 at%) present in the tube oxide drastically improve the electrochromic properties (contrast, onset potential, cycling stability) of nanotube layer based devices.

This work reports on the behavior of mesenchymal stem cells on anodic ZrO(2) nanotube layers grown by a self-ordering process on zirconium with defined diameters between 10 and 50 nm. It is demonstrated that mesenchymal stem cells show a size-specific reaction to these nanoscale patterned surfaces. We compare the behavior on these ZrO(2) nanotubes to findings on TiO(2) nanotubes of different diameters. For both nanotube materials, TiO(2) and ZrO(2), cell adhesion and spreading are enhanced for nanotube diameters of approximately 15-30 nm, while a strong decay in cell activity is observed for diameters >50 nm. Focal complex formation on adherent cells is selectively modulated by the specific nanoscale. Moreover, even if the surface chemistry of the nanotubes is completely modified with a dense AuPd coating onto the formed nanotube layers, or the length of the nanotubes is varied, the observed nano size effects still prevail. This demonstrates how strong the pure geometric diameter dependence in the range between 15 and 100 nm dominates over other possible effects on cell activity.

Self-organized porous structures of WO3 were grown on tungsten by an anodic oxidation, and their photoelectrochemical properties were characterized. The porous WO3 layers show a regular morphology with average pore sizes of approximately 70nm and a pore wall thickness of approximately 10nm. As formed layers show an amorphous structure but the layers can be altered to a crystalline monoclinic structure by thermal annealing. The annealed porous WO3 layers show a very high specific photocurrent conversion efficiency.

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Using a sputtering-deposited titanium substrate, incorporating six equally spaced nanolayers of Ti-W alloy, the volume and composition changes accompanying the formation of porous anodic films on titanium in 0.5 wt % NH 4 F in glycerol are investigated. The findings reveal amorphous films with nanotubes of TiO 2 , containing fluoride ions and possibly glycerol derivatives. Tungsten and titanium species are lost to the electrolyte at differing rates during anodizing, leading to an enrichment of tungsten in the film relative to the composition of the substrate. The spacing of tungsten-containing bands in the film is ϳ2.3 that of the original alloy layers during growth of the major pores. The generation of the nanotubes can be explained either by field-assisted flow of film material within the barrier layer to the pore walls, with cation and anion transport numbers of anodic titania in the barrier layer region similar to those of barrier films and with field-assisted ejection of Ti 4+ ions to the electrolyte, or by fieldassisted dissolution, but with a reduction in cation transport number.

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