Femtosecond study of the intensity dependence of electron-hole dynamics in TiO2 nanoclusters

The dynamics of charge carrier trapping and recombination as a function of pump fluence of 20 Å diameter TiO2 particles in aqueous solutions is measured. Trapping of photogenerated conduction band electrons occurs in less than 500 fs. The subsequent fluence-dependent decay of trapped electrons has both a fast and a slow component. The early time recombination results are interpreted by second-order kinetics. A global fit to the data predicts a bulk second-order recombination rate constant of (1.8 ± 0.7) × 10−10 cm3 s−1 for trapped electrons with holes. The signal at long times is attributed to electrons ejected from the nanocluster and/or deeply trapped electrons produced via third-order kinetics, consistent with an Auger process.