Molecular oxygen species formed on the surface of partially reduced TiO 2 (rutile) nanoparticles have been studied by in situ electron spin resonance (ESR) and diffuse-reflectance spectroscopies. ESR signals due to O 2 -(g z ) 2.020) and Ti 3+ appeared upon visible-light illumination at 77 K and vanished by raising the temperature in the dark. The numbers of O 2 and Ti 3+ radicals formed by sub-band-gap illumination were equal, suggesting a reversible electron transfer between peroxo O 2 2species and the adjacent Ti 4+ ion at an oxygen vacancy site on the TiO 2 surface:The ESR intensity was saturated by a prolonged illumination and a surface coverage of O 2 molecules adsorbed at the oxygen vacancy site was evaluated as 1.3 × 10 13 sites cm -2 . The spectral response for the generation rate of O 2 exhibited a broad peak at around 480 nm, in agreement with the absorption band observed by the diffuse-reflectance measurements. It was concluded that F-type color centers generated in subsurface layers of TiO 2 absorb the visible light to induce indirectly the electron-transfer reaction from O 2 2to Ti 4+ at the surface oxygen vacancy site.

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Photoinduced electron transfer reactions of O 2 molecules adsorbed on the H 2 -reduced surface of TiO 2 (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 Ti 3+ appeared and then returned to the original ones by raising the temperature in the dark. The numbers of O 2 -and Ti 3+ ions formed by illumination were equal, suggesting that a reversible electron transfer between peroxo O 2 2-species and the adjacent Ti 4+ ion occurs by sub-band gap illumination of TiO 2 .

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