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 TiO2 (Fe/TiO2) and Fe(III)-grafted Ru-doped TiO2 (Fe/Ru:TiO2). ESR spectroscopy revealed that the visible-light response of the Fe/TiO2 photocatalyst resulted in the direct charge transfer from the valence band of TiO2 to the grafted Fe ions. For the Fe/Ru:TiO2 photocatalyst, acceptor levels were formed by doping Ru ions in the lattice of TiO2, 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:TiO2 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/TiO2. Moreover, chemiluminescence photometry confirmed that the grafted Fe ions function as a promoter to reduce O-2 into H2O2 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.