Effects of crystal structure, size, shape and surface structural differences on photo-induced electron transport in TiO2 mesoporous electrodes

Various kinds of mesoporous electrodes composed of TiO2 nanocrystallites with different crystal structures (rutile or anatase), crystalline shapes, crystalline sizes and crystallinity have been prepared. The electron diffusion coefficients of these TiO2 mesoporous electrodes have been determined by time-resolved photocurrent measurements. The electron diffusion coefficients of the rutile electrodes were one order of magnitude smaller than those of the anatase ones. The electrode prepared using a high crystalline anatase TiO2 needed a higher annealing temperature to obtain an increased diffusion coefficient, which was explained as being due to the effective necking at higher annealing temperature. The diffusion coefficient of the rutile electrode was enhanced four times by the treatment with aq. TiCl4 solution, while no significant changes were observed in the case of the anatase film electrodes. New layers of rutile TiO2 with a piled-needle-like shape were produced on the surface of the rutile nanocrystallites after the treatment with aq. TiCl4 solution. These newly formed rutile layers on the surface and their interface should promote the necking between the nanocrystallites at annealing, increasing the diffusion coefficient of the rutile film electrode.