THE PHOTOELECTROCHEMISTRY OF THIN PASSIVE LAYERS - INVESTIGATION OF ANODIC OXIDE-FILMS ON TITANIUM METAL

A photoelectrochemical investigation has been performed on thin TiO2 films grown anodically in 0.5 M H2SO4 solution at high growth rates. The shape of the photocurrent vs. potential curves under monochromatic irradiation (photocharacteristics) depends on the photon energy of the incident light at energies above the optical band gap of the films (3.25 +/- 0.05eV). This finding has been explained by considering the presence of geminate recombination of the photogenerated electron-hole pairs. In order to fit the experimental photocharacteristics, an expression for the photocurrent is proposed which takes into account the low drift range of photocarriers and possible recombination in the space-charge region. Moreover, an exponential potential distribution in the film was assumed from the theory of the amorphous semiconductor (a-SC) Schottky barrier. The results of theoretical simulations suggest that the behaviour of TiO2 passive films can be explained in the frame of the theory of a-SC, taking into account the influence of the photon energy on the thermalization length of the photocarriers.