Energy positions of oxide semiconductors and photocatalysis with iron complex oxides

Energy position, bandgap, band structure, and their relationships were reviewed for various oxide semiconductors, especially iron oxides, in photoelectrochemistry and photocatalysis, and the photocatalytic reduction of CO2 on some iron complex oxides was demonstrated. A linear relationship between bandgap and band edge was obtained for almost all the semiconductor oxides. It was pointed out that a bandgap energy higher than about 2.46 eV is necessary for water photolysis without bias voltage. It was found that the energy positions of the band edge can be controlled by the electronegativity of the metal elements constituting the iron complex oxides. The relationship between the band structure and the charge transfer site was also examined. The photocatalytic reduction of CO2 to CH3OH was demonstrated for CaFe2O4 and Fe-Bi-Sr-Pb-O complex oxides. The photocatalytic activity of the latter oxide with a layer structure increased with increased Pb content. The catalytic mechanism was discussed from the point of view of the redox mechanism as well as the energy position of the band edge. (C) 1996 Academic Press, Inc.