ELECTROCHEMICAL AND RUTHERFORD BACKSCATTERING STUDY OF THERMALLY CRYSTALLIZED TUNGSTEN TRIOXIDE ANODIC FILMS

Thermally crystallized tungsten trioxide anodic films 65-110 nm thick were investigated using electrochemical methods (cyclic voltammetry, impedance analysis, photocurrent spectroscopy) and Rutherford backscattering spectroscopy. It is shown that the stoichiometric composition of these films varies gradually with depth and the width of the homogeneous surface layer does not exceed 50% of the total thickness of the film. Non-uniform density distribution of ionized donors is observed in the surface layer since heating the amorphous anodic oxide leads to the formation of both a moderately doped phase (approximately 10(19) donors cm-3) and a heavily doped phase. This fact determines, in many respects, the eletrochemical behaviour of crystallized tungsten trioxide films, because it makes possible the localization of dark electrochemical reactions on electrocatalytically active heavily doped surface sites, while the photogeneration process is concentrated in the moderately doped oxide phase. The heavily doped phase on the electrode surface can be blocked by eletrodeposition of poly-o-phenylenediamine.