Abstract
The present work investigates the photo-electrochemical behavior of different types of self-organized layers of TiO2 nanotubes vertically oriented along one dimension and compares it to three-dimensional nanoparticle layers of similar thicknesses. In nanotubes, the electron diffusion length is dependent on tube order and wall smoothness by way of our investigations finding that there are much greater electron diffusion lengths for nanotubes made in organic solutions compared to the nanotubes made in aqueous solutions. These superior diffusion lengths are approximately 30 times greater than those of nanoparticle layers and are accompanied by majority charge-carrier transportation times of the order of seconds. However, the conversion efficiency of the nanotubes is not adversely affected by the length of the nanotubes for layers that have a thickness of less than the electron diffusion length. This is of high significance for photo-electrochemical applications of TiO2 that rely on electron transport processes, where the nanotube layers could provide a very high effective surface area and thickness without significant losses.