Electron-Transfer Reaction of Oxygen Species on TiO2 Nanoparticles Induced by Sub-band-gap Illumination

Molecular oxygen species formed on the surface of partially reduced TiO2 (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 Ti3+ appeared upon visible-light illumination at 77 K and vanished by raising the temperature in the dark. The numbers of O-2(-) and Ti3+ radicals formed by sub-band-gap illumination were equal, suggesting a reversible electron transfer between peroxo O-2(2-) species and the adjacent Ti4+ ion at an oxygen vacancy site on the TiO2 surface: Ti4+center dot center dot center dot O-2(2-)center dot center dot center dot Ti4+ -> Ti3+ + O-2(-)center dot center dot center dot Ti4+ (forward reaction). 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 x 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 TiO2 absorb the visible light to induce indirectly the electron-transfer reaction from O-2(2-) to Ti4+ at the surface oxygen vacancy site.