Photocatalytic Reaction Mechanism of Fe(III)-Grafted TiO₂ Studied by Means of ESR Spectroscopy and Chemiluminescence Photometry

We successfully clarified the mechanisms of visible-light-driven photocatalytic reactions of Fe(III)-grafted TiO 2 (Fe/TiO 2 ) and Fe(III)-grafted Ru-doped TiO 2 (Fe/Ru:TiO 2 ). ESR spectroscopy revealed that the visible-light response of the Fe/TiO 2 photocatalyst resulted in the direct charge transfer from the valence band of TiO 2 to the grafted Fe ions. For the Fe/Ru:TiO 2 photocatalyst, acceptor levels were formed by doping Ru ions in the lattice of TiO 2, and the electrons at the acceptor levels excited on visible-light irradiation readily transfer to Fe ions. Since a longer wavelength light generated the conduction band electrons, we also proposed a two-step electron excitation from valence band to the conduction band through defect levels such as oxygen vacancy. As a result, a part of photogenerated electrons in the conduction band transfer to the grafted Fe ions. Therefore, the Fe/Ru:TiO 2 photocatalyst showed a higher activity because such two kinds of indirect charge transfer to the grafted Fe ions occurred in addition to the direct interfacial charge transfer observed for Fe/TiO 2 . Moreover, chemiluminescence photometry confirmed that the grafted Fe ions function as a promoter to reduce O 2 into H 2 O 2 via two-electron reduction. Therefore, the acceleration in the reduction of O 2 with doping Ru and grafting Fe ions allows a larger number of holes to oxidize organic compounds, resulting in the higher photocatalytic activity.