Light-Induced Charge Separation in Anatase TiO2Particles

Abstract

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 O^2- 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 O₂ 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.