Electrical and Point Defect Properties of TiO2 Nanotubes Fabricated by Electrochemical Anodization

Pure and N-doped titanium oxide nanotubes (TiO 2 nt) were manufactured by anodization of sputtered Ti thin films on a Si (100) substrate. Both solid state contacts and electrolyte contacts were used to investigate the properties of TiO 2 nt. Mott−Schottky analysis gave a flat-band potential E fb = −0.57 V vs Ag/AgCl for pure TiO 2 nt and E fb = −0.22 V vs Ag/AgCl for N-doped TiO 2 nt. The charge carrier density was N D = 6.7 × 10 20 cm −3 for pure TiO 2 nt and N D = 3.9 × 10 20 for N-doped TiO 2 nt. This corresponds to 1.1% oxide ion vacancies in pure TiO 2 nt. In nitrogen-doped TiO 2 nt, the decrease of donor density would correspond to 0.47% of vacancies being occupied by nitrogen acceptors. This investigation also allowed estimation of the apparent diffusion coefficient of H + in TiO 2 nt. Following the Randles−Sevcik method, the effective proton diffusion coefficient in TiO 2 nt is (2 ± 1)·10 −11 cm 2 s −1 while in N-doped TiO 2 nt it is (4 ± 1)·10 −11 cm 2 s −1 . Using the Warburg diffusion element determined by electrochemical impedance spectroscopy, the proton diffusion coefficient is (2 ± 1)·10 −11 cm 2 s −1 for pure TiO 2 nt while for nitrogen-doped TiO 2 nt a value of (7 ± 3)·10 −11 cm 2 s −1 is found. These values are consistent with those of the Randles−Sevcik method.