Light-induced charge separation in anatase TiO2 particles

Ultraviolet light-induced electron-hole pair excitations in anatase TiO2 powders were studied by a combination of electron paramagnetic resonance and infrared spectroscopy measurements. During continuous UV irradiation in the mW.cm(-2) range, photogenerated electrons are either trapped at localized sites, giving paramagnetic Ti3+ centers, or remain in the conduction band as EPR silent species which may be observed by their IR absorption. Using low temperatures (90 K) to reduce the rate of the electron-hole recombination processes, trapped electrons and conduction band electrons exhibit lifetimes of hours. The EPR-detected holes produced by photoexcitation are O- species, produced from lattice O2- ions. It is found that under high vacuum conditions, the major fraction of photoexcited electrons remains in the conduction band. At 298 K, all stable hole and electron states are lost from TiO2. Defect sites produced by oxygen removal during annealing of anatase TiO2 are found to produce a Ti3+ EPR spectrum identical to that of trapped electrons, which originate from photoexcitation of oxidized TiO2. Efficient electron scavenging by adsorbed O-2 at 140 K is found to produce two long-lived O2- surface species associated with different cation surface sites. Reduced TiO2, produced by annealing in vacuum, has been shown to be less efficient in hole trapping than oxidized TiO2.