ESR Study on the Reversible Electron Transfer from O22- to Ti4+ on TiO2 Nanoparticles Induced by Visible-Light Illumination

Photoinduced electron transfer reactions of O-2 molecules adsorbed on the H-2-reduced surface of TiO2 (rutile) nanoparticles have been studied at 77 K by in situ ESR spectroscopy. Upon visible-light illumination, ESR signals due to O-2(-) (g(z) = 2.020) and Ti3+ appeared and then returned to the original ones by raising the temperature in the dark. The numbers of O-2(-) and Ti3+ ions formed by illumination were equal, suggesting that a reversible electron transfer between peroxo O-2(2-) species and the adjacent Ti4+ ion occurs by sub-band gap illumination of TiO2.