11628
J. Phys. Chem. B 2006, 110, 11628-11634
Low Temperature Kinetics and Energetics of the Electron and Hole Traps in Irradiated TiO2 Nanoparticles as Revealed by EPR Spectroscopy Shyue-Chu Ke,*,† Ting-Chung Wang,† Ming-Show Wong,‡ and Neeruganti O. Gopal† Physics Department and Materials Science & Engineering Department, National Dong Hwa UniVersity, Hualien, Taiwan 974-01 ReceiVed: February 28, 2006; In Final Form: May 1, 2006
We have monitored exclusively the dynamics of photogenerated charge carriers trapping in deep traps and trapped electron-hole recombination in UV irradiated anatase TiO2 powders by electron paramagnetic resonance (EPR) spectroscopy at 10 K. The results reveal that the strategy of using low temperatures contributes to the stabilization of the charged pair states for hours by reducing the rate of electron-hole recombination processes. Since only the localized states such as holes trapped at oxygen anions and electrons trapped at coordinatively unsaturated cations are accessible to EPR spectroscopy, the time-dependent population and depopulation of these EPR signals reflect the kinetics and energetics of these trap states. The data support a model of sequential accumulation of deep trap site populations in which the initial fast direct trapping into a deep trap site is followed by slower carrier trap-to-trap hopping until a deep trap is encountered for both photogenerated electrons and holes. Effective modeling of the subsequent decay of trapped-holes is achieved by employing a first-order kinetics, whereas the decay of either surface- or inner-trapped electrons has both a fast and a slow component. The fast component is attributed to a trapped-electron and a free-hole recombination, and the slow component is attributed to trapped electron-hole recombination. The activation energies for the process of diffusion of trapped electrons from their Ti3+ trapping sites are estimated.
Introduction Nanocrystalline titanium dioxide (TiO2) having an anatase phase is a promising substrate in photocatalytic,1,2 photovoltaic,3-7 and battery8 applications and has been the subject of extensive investigation in the past decade.9,10 Band gap UV radiation on powder anatase TiO2 promotes an electron from the valance band to the conduction band, leaving a positively charged hole in the valence band. While most of the photogenerated electrons are captured in the conduction band as EPR silent species, a fraction of the electrons and holes move to the surface trap sites as EPR detectable species; mainly they are detected as electrons trapped at a surface Ti4+ site and hole trapped at surface OHsite.11-13 The surface trapped electrons and holes either recombine or act as initiators of subsequent surface catalytic reactions. The charge pair recombination accounts, to a large degree, for the reduced photoefficiency and differences in the photoactivity of various catalysts. Recent research efforts have been made in the development of new TiO2-based photocatalysts activated by visible light14-20 and in enhancing the overall quantum efficiency of the photocatalytic process by reducing the charge carrier recombination rates such as in Degussa P25 mixed-phase TiO2.21 Since the photocatalytic efficiencies of TiO2 depend strongly upon the trapping and recombination energetics, numerous chemical and ultrafast kinetic experiments22-32 have been performed to model the early stages of the photocatalytic processes initiated at TiO2 in aqueous solutions. It is generally accepted that the photogenerated electron is promptly (
