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+

Ti O 2 nanotube layers were grown on titanium by a self-organized anodic oxidation. The layers consist of arrays of individual tubes with a length of ∼2μm, a diameter of ∼100nm, and a wall thickness of ∼10nm. These layers can be annealed to an anatase structure which strongly increases the photocurrent efficiency. Moreover, the nanotube layers can—under certain conditions—exhibit a drastically enhanced photocurrent compared to compact anatase layers. These strong changes in the photoresponse are attributed to the characteristics of the space charge layer within the tube wall.

The anodic formation of self-organized porous TiO 2 on titanium was investigated in 1 M ͑NH 4 ͒ 2 SO 4 electrolytes containing 0.5 wt % NH 4 F by potential sweeps to 20 V SCE . By a combination of electrochemical, morphological, and compositional information we show that the sweep rate has a significant influence on the initiation and growth of the porous structures. In the first phase of the anodization process, a precursor barrier type of oxide film is formed; underneath this film pores then start growing first randomly and then self-organize. High-aspect-ratio TiO 2 nanostructures can be obtained under optimized electrochemical conditions. These nanotubular oxide layers have single-pore diameter ranging from 90 to 110 nm, average spacing of 150 nm, and porosity in the order of 37-42%. The current work indicates that the nature of the initial barrier-type layer has a strong influence on establishing optimized pore growth conditions.

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