Trapping of photogenerated charges in oxide nanoparticles

Light induced charge separation was studied in situ by electron paramagnetic resonance spectroscopy on MgO and TiO2 nanocrystals. For both materials, polychromatic excitation (1.2 < E < 6.2 eV) produces well-separated electron and hole centers. On MgO, where only surface features such as 3- and 4-coordinated anions are excited with energies < 6.2 eV, very different time evolutions were observed for the EPR signals of trapped electron and hole centers which is explained by secondary reactions stimulated by the low energy part of the radiation spectrum. As in the case of MgO, in TiO2 supra band gap excitation (E > 3.2 eV) generates hole centers which form O− radicals when the holes get trapped at oxygen anions. Only a limited fraction of trapped electrons, however, is localized as Ti3+ states, the majority remains in the conduction band. Different to MgO, this state of charge separation can be kept constant only at temperatures below 200 K, because of thermally driven charge carrier annihilation. The performance of dynamic excitation experiments under high-vacuum conditions and low temperatures allows one to track the process of light induced charge separation on a time scale of seconds and minutes.