TiO2 nanocolumn arrays are prepared via porous-anodic-alumina-assisted anodizing of Al/Ti or Al/TiNx layers in an oxalic acid electrolyte. The upper Al layer is anodized at 40 V to form a nanoporous anodic film; then the Ti or TiNx underlayer is re-anodized to 100 V. This leads to the growth of anodic TiO2 or N-containing TiO2 nanocolumns within the alumina pores, which are approx. 40 nm wide and 160 nm long. The crystallinity and doping degree of the nanocolumns are modified by the annealing in air or vacuum at 600°C. Cyclic voltammetry, electrochemical impedance spectroscopy, and Mott-Schottky analysis revealed n-type semiconducting properties of the N-doped nanocolumns whereas most of the undoped TiO2 nanocolumns exhibited dielectric behavior. The calculated doping concentration and the flat-band potential vary with the annealing conditions. This allows for controlled alteration of the depletion layer thickness in order to enhance the photoelectrochemical water-splitting ability of the films by improving photogenerated charge carrier separation and band-gap tuning. The N-containing nanocolumns appeared to possess trap states, as manifested by Fermi level pining, which may provide additional benefits for heterogeneous charge-carrier transport for water oxidation.Keywords: Porous anodic alumina, anodizing, Mott-Schottky analysis, photoelectrochemical water splitting
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