In this presentation work investigating the photoelectrochemical behaviour of different types of self-organised layers of vertically oriented TiO₂ nanotubes will be presented and comparison to three-dimensional nanoparticle layers of similar thickness will be made. It will be shown that the conversion efficiency of bulk anatase TiO₂ can be improved by nanostructuring the electrode by a single electrochemical step followed by annealing. The role of the order and wall smoothness of the nanotubes produced by this electrochemical step on their photoelectrochemical behaviour will be discussed in the light of comparison between two different electrolytes used to produce nanotubes: i) aqueous electrolytes, where very rough nanotube walls are formed and ii) organic electrolytes, where nanotubes with much smoother walls are grown. The obtained results suggest a similar electron diffusion mechanism, under complete depletion conditions, for electrodes constructed from nanotubes or nanoparticles when illuminated with UV light. We propose a model pointing out the differences and analogies between nanoparticle and nanotube films in contact with an electrolyte, when illuminated with UV light. The lower number of grain boundaries through which the electrons have to pass in the case of nanotubes allows electron diffusion lengths of 24 µm, which are 30 times higher than for nanoparticles measured under the same conditions. However, the energetic states present in the band gap make the movement of the majority charge carriers, along the nanotube length, extremely slow: i.e. in the order of seconds. Such extended transportation times are not necessarily a loss mechanism. Indeed, the conversion efficiency of the nanotubes is not adversely affected by the length of the nanotubes for layer depths less than the electron diffusion length. These findings show the possibility of using nanotubes with a length of 20 µm in photo-electrochemical cells without having detrimental losses to efficiency due to electron-hole recombination. This is of high significance for photoelectrochemical applications of TiO₂ that rely on electron transport processes (such as dye sensitized solar cells, where the length of the nanotube layer defines the amount of the dye absorbed), providing a very high effective surface area and thickness, without significant losses.

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