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.

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We generated, on titanium surfaces, self-assembled layers of vertically oriented TiO2 nanotubes with defined diameters between 15 and 100 nm and show that adhesion, spreading, growth, and differentiation of mesenchymal stem cells are critically dependent on the tube diameter. A spacing less than 30 nm with a maximum at 15 nm provided an effective length scale for accelerated integrin clustering/focal contact formation and strongly enhances cellular activities compared to smooth TiO2 surfaces. Cell adhesion and spreading were severely impaired on nanotube layers with a tube diameter larger than 50 nm, resulting in dramatically reduced cellular activity and a high extent of programmed cell death. Thus, on a TiO2 nanotube surface, a lateral spacing geometry with openings of 30-50 nm represents a critical borderline for cell fate.

In 1999 Zwilling et al. reported on the electrochemical formation of self-assembled TiO 2 nanotubes (p-TiO 2 ) by the anodization of Ti, [1] and other reports followed soon thereafter. [2,3] A factor limiting the application of this first generation of nanotubes was their production in hydrofluoric acid based electrolytes. As a result of the high rate of chemical dissolution of TiO 2 in these solutions, the nanotubes could be grown only up to a length of 500 nm. Recently, we have shown [4][5][6][7] and explained [4] how a second generation of nanotubes with lengths up to several micrometers and aspect ratios up to 50 can be formed by adjusting the pH gradient within the growing nanotube. Common to all these anodic approaches is that the side walls of the tubes show strong irregularities, that is, the side-wall profiles show considerable thickness variations (ripples) as shown in Figure 1 a. In the present work we demonstrate how TiO 2 nanotubes with extremely smooth homogenous walls and aspect ratios up to 175 can be grown (as shown in Figure 1 b). This third generation of nanotubes is formed by suppressing local concentration fluctuations and pH bursts during anodization by using highly viscous glycerol electrolytes.In previous work it has been established that the length of the nanotubes is essentially the result of a steady-state situation between electrochemical formation of TiO 2 at the pore tip and the chemical dissolution of formed TiO 2 by fluorides from the electrolyte. [3,5] We showed how the pH at the pore tip is lowered by the hydrolysis reactions of the Ti 4+

4.1 Doping of anodic TiO 2 nanotubes 4.2 Doping of hydrothermal TiO 2 nanotubes 4.3 Self-doping/Magneli phases/black titanium 4.4 Conversion of tubes 4.5 Particle decoration, heterojunctions, charge transfer catalysis 4.6 Self-assembled monolayers (SAMs) 5. Applications of TiO 2 nanotubes 5.1 Solar cells 5.2 Photocatalysis 5.3 Ion-intercalation (insertion) devices

Among all one dimensional nanostructures other than carbon, titania nanotubes have gained increasingly more scientific interest due to a successful combination of functional material properties with a well controllable nano-architecture. For self-organized TiO(2) nanotube arrays not only the simple increase in the specific surface area but also their self-aligned nature leads to a significant enhancement of the performance when used in photoelectrochemistry, photocatalysis, dye-sensitized solar cells, or electrochromic devices. In addition to this, these ordered and size-controlled nanostructured TiO(2) surfaces also have material-specific advantages, for example in superhydrophobic/superhydrophilic and biomedical applications. The formation of these vertically oriented nanotube arrays can be achieved by a simple one-step electrochemical self-assembly process. By adjusting the anodization parameters, the geometry such as the tube length or diameter can easily be controlled. The present review addresses the formation, properties and applications not only of TiO(2) nanotubes but also of related transition metal oxides.

Dye-sensitized solar cells fabricated using ordered arrays of titania nanotubes (tube lengths 5, 10, and 20 microm) grown on titanium have been characterized by a range of experimental methods. The collection efficiency for photoinjected electrons in the cells is close to 100% under short circuit conditions, even for a 20 microm thick nanotube array. Transport, trapping, and back transfer of electrons in the nanotube cells have been studied in detail by a range of complementary experimental techniques. Analysis of the experimental results has shown that the electron diffusion length (which depends on the diffusion coefficient and lifetime of the photoinjected electrons) is of the order of 100 microm in the titania nanotube cells. This is consistent with the observation that the collection efficiency for electrons is close to 100%, even for the thickest (20 microm) nanotube films used in the study. The study revealed a substantial discrepancy between the shapes of the electron trap distributions measured experimentally using charge extraction techniques and those inferred indirectly from transient current and voltage measurements. The discrepancy is resolved by introduction of a numerical factor to account for non-ideal thermodynamic behavior of free electrons in the nanostructured titania.

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