Photoelectrochemical characterization of nanocrystalline TiO2 films on titanium substrates

This paper deals with the photo-electrochemical characterization of nanocrystalline TiO2 films deposited on titanium substrates in 0.1 M NaCl solutions. These films were confirmed to contain mainly anatase crystallites by X-ray diffraction (XRD) and have n-type properties by UV-photoelectron spectroscopy (UPS). Experiments were also pet-formed on thermal and electrochemical TiO2 oxides formed in air or the same electrolyte, respectively, to delineate the role in the electrochemical behavior of the nanocrystalline films and differentiate their properties from the titanium substrate and its spontaneously formed oxide. Systematic enhancement of the photocurrent was observed for nanocrystalline TiO2 films compared with the thermal oxide. This effect arises principally from the area factor but, as it was observed by UPS, capacitance and cyclic voltammetry measurements, it was not the only factor affecting the photocurrent response. It was found that the shape of the photocurrent vs. wavelength curves depend on the electrode potential when the electrode is irradiated at energies above the optical bandgap of the films (3.2 eV for anatase single crystal), whereas at energies below the bandgap, it remains almost potential independent. The capacitance measurements of the nanocrystalline Ti/TiO2 electrodes in the dark and under illumination conditions did not show substantial changes under our experimental conditions. This fact is indicating the pinning of semiconductor bands still during illumination. The origin of this effect was related with a high rate of surface trap filling. The absorption coefficient for nanocrystalline oxide films was calculated from i(ph)(2) vs. V plots and a value of 1 x 10(2) cm(-1) was obtained. The flat band potential (VFB) calculated from photocurrent plots was not in good correlation with the Nernstian behavior obtained from impedance measurements (Mott-Schottky plots). (C) 2002 Elsevier Science B.V. All rights reserved.