A potential site for trapping photogenerated holes on rutile TiO(2) surface as revealed by EPR spectroscopy: an avenue for enhancing photocatalytic activity.

Rutile TiO(2) nanoparticles with new sites for effectively trapping photogenerated holes have been prepared by reacting the TiO(2) nanoparticles prepared in hydrogen atmosphere with molecular oxygen at elevated temperatures. The observed g values and the occurrence of (47)Ti and (49)Ti octet hyperfine pattern allowed us to assign this EPR active center to surface oxygen centered anion radical with two coordinating titaniums. The effective trapping of photogenerated holes by these new sites inhibits the electron-hole recombination and results in an enhanced photocatalytic activity under visible light by a factor of 2.5 compared with samples prepared parallel in air. Oxidation of reduced TiO(2) apparently is a simple low-cost and promising route for improving the photoactivity of TiO(2).

Chemical state and environment of boron dopant in B,N-codoped anatase TiO2 nanoparticles: an avenue for probing diamagnetic dopants in TiO2 by electron paramagnetic resonance spectroscopy.

Boron is diamagnetic in B-doped anatase TiO2 nanoparticles which exhibit photocatalytic activities in the visible light range. Using N as a paramagnetic probe for the formal oxidation state of boron in N/B-codoped TiO2, with more than 90% unpaired spin density in the N2p orbital, we infer that boron enters the oxygen vacancy substitutionally in the form of B1-. Combination of spin-Hamiltonian analysis and interpretation of light dependent EPR spectra in terms of a charge compensating mechanism supports a model of [N2-B1-]+1 for the new EPR active center which acts as a trap for electrons liberated from [N1-] centers under blue light irradiation. Definite assignment of the boron oxidation state will contribute to the preparation, characterization, and understanding of B-TiO2 photoactivity under visible light which has been the subject of extensive work in the past few years.

Low temperature kinetics and energetics of the electron and hole traps in irradiated TiO2 nanoparticles as revealed by EPR spectroscopy.

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.