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

Abstract

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 O₂ into H₂O₂ via two-electron reduction. Therefore, the acceleration in the reduction of O₂ with doping Ru and grafting Fe ions allows a larger number of holes to oxidize organic compounds, resulting in the higher photocatalytic activity