Semiconducting oxide anodes in photoassisted electrolysis of water

The electrochemical properties of semiconducting anodes of TiO2, SrTiO3, BaTiO3, Fe2O3, CdO, CdFe2O4, WO3, PbFe12O19, Pb2Ti1.5W0.5O6.5, Hg2Ta2O7, and Hg2Nb2O7 in photoassisted electrolysis of water were determined. All of these oxides formed a rectifying junction with the electrolyte and anodic photocurrents were generated only with larger-than-band-gap illumination. For Fe2O3, the optical absorption spectrum was different from the photoelectrochemical spectrum due to crystal field transitions. These oxides were found to be stable over certain range of pH. In a given electrolyte, the flatband potential Vfb varied linearly with the band gap. A good correlation was obtained between Vfb and the heat of formation of the oxide per metal atom per metal-oxygen bond, but not between Vfb and the calculated Fermi energy of the oxide. This suggests that a semiconductor-electrolyte interface may be approximated by a semiconductor-metal junction where the barrier height is determined by the heat of formation of the metal-semiconductor compound. The practical aspect of using a photoassisted electrolysis cell for solar energy conversion is discussed.