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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+

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Anodically grown TiO2 nanotubes have a double‐walled structure. The two shells can either be separated or fused together using different thermal annealing treatments. The morphology is determined by the heating rate. Highly regular and robust TiO2 membranes can be obtained with a crystal structure that can be adjusted to an anatase or an anatase/rutile mixture.

In the present work we show a simple and robust fabrication process of a dense and free-standing membrane consisting of vertically oriented, both-side-open TiO2 nanotubes. This membrane structure allows direct, size-selective, flow-through photocatalytic reactions with a very high efficiency.