Photoelectrochemical microscopy of oxide films on metals: Ti/TiO₂ interface

Thin electrochemically grown oxide films on polycrystalline titanium substrates were examined by photoelectrochemical and electron diffraction techniques. Photoelectrochemical microscopy, based on illumination by a focused laser beam, was used to measure the local photoresponse of the oxide films. For oxide films grown slowly (1 nm/h)if, by ramping the electrode potential at 0.1 mV/s, the oxide photoresponse was found to map the grain structure of the underlying substrate. As the oxide growth rate was increased the photoresponse became more homogeneous and the average photocurrent decreased. The photoresponse heterogeneities were associated with variations in the structure of the oxide film. Electron diffraction studies showed that for slowly grown films, an ordered oxide structure was present with a preferential growth direction of [110] occurring on some substrate grains. As the oxide growth rate was increased the film structure became more homogeneous and the average oxide crystalline size decreased. The decrease in oxide crystallinity with increase in oxide growth rate was also observed in photospectroscopy measurements as a loss of the 3.7 eV direct bandgap. For all oxide films studied here, rutile was the only identifiable phase.