Semiconducting oxide anodes in photoassisted electrolysis of water

The electrochemical properties of semiconducting anodes of TiO₂, SrTiO₃, BaTiO₃, Fe₂O₃, CdO, CdFe₂O₄, WO₃, PbFe₁₂O₁₉, Pb₂Ti_1.5W_0.5O_6.5, Hg ₂Ta₂O₇, and Hg₂Nb₂O ₇ 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 V_fb varied linearly with the band gap. A good correlation was obtained between V_fb and the heat of formation of the oxide per metal atom per metal-oxygen bond, but not between V_fb 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.