An rrde investigation of light-induced cathodic currents at polycrystalline TIO 2 electrodes in the presence of dioxygen : Part I. The light intensity dependence and the cathodic action spectra

Abstract

A detailed investigation of the photo-induced anodic and cathodic currents observed at polycrystalline TiO 2 electrodes prepared by thermal oxidation of titanium has been made. The anodic action spectra, the cathodic action spectra and the current-voltage characterization of the electrode are given. At sufficiently negative potentials, light-induced cathodic currents are observed. The presence of an oxidant such as O 2 is required for the appearance of photostationary cathodic currents. ‘Cathodic quantum efficiencies’ (or ‘gains’) close to 200 were measured. The latter observations show that the electrode at negative potentials works in a photoconductive mode. At all wavelengths in the interval 250 – 420 nm, the photostationary cathodic currents which were registered close to or negative of the flatband potential show a logarithmic dependence on the logarithm of the photon flow. Doping the electrode surface region by electrochemical reduction decreases the photo-induced cathodic effect. The anodic action spectrum is shifted towards shorter wavelengths, and a marked increase in the quantum efficiency at shorter wavelengths is registered, while in the cathodic action spectrum a slight shift towards longer wavelengths and a decrease in quantum efficiency at shorter wavelengths is observed. On the basis of the experimental results, it is proposed that the photocathodic effect is a consequence of a low-doped top layer with low conductivity near the electrode/electrolyte interface. This layer is formed during the oxidation procedure. The sub-bandgap response in the action spectra of the photo-induced cathodic current indicates the presence ofintra-bandgap states. When excited, these states produce electrons and holes. The holes are trapped. The Fermi level is shifted upwards towards the conduction band and the conductivity is increased, especially in the low-doped top layer. Thus photoenhanced cathodic currents are registered.