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 x 10(20) cm(-3) for pure TiO(2)nt and N-D = 3.9 x 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.