The charge carrier dynamics of opaque, aqueous suspensions of Degussa P-25 TiO2 are probed with femtosecond time-resolved diffuse reflectance spectroscopy. Comparison of ultrafast pump−probe diffuse reflectance measurements of P-25 suspensions with dry P-25 powder and the transient absorption of transparent, aqueous Q-TiO2 solutions allows the observed kinetics to be assigned to charge carrier recombination. The electron−hole recombination kinetics are consistent with a second-order process as demonstrated by a laser fluence dependence study. Interfacial hole transfer dynamics of the P-25 TiO2/SCN- complex are probed as a function of thiocyanate ion concentration. A dramatic increase in the population of trapped charge carriers is observed within the first few picoseconds, demonstrating that interfacial charge transfer of an electron from the SCN- to a hole on the photoexcited TiO2 effectively competes with electron−hole recombination on an ultrafast time scale. The experimental dependence of the charge carrier dynamics are shown to be consistent with a kinetic model of competing second-order processes. The implications of the results on the use of nanoscale TiO2 for photocatalysis are discussed.

Femtosecond experiments on 15 nm diameter SnO2 nanoclusters measure the elementary charge carrier reactions of electron trapping and electron−hole recombination. From the early time transient absorption data, an electron-trapping time of 200 ± 20 fs is determined. In addition, an experimental scheme to determine the effect of electron thermalization on the relaxation of photoexcited electrons is presented. Excess excitation energy above the conduction band increases the decay time to 500 ± 50 fs indicating that thermalization plays an important role in the electron-trapping kinetics. The dynamics of charge carrier recombination are investigated by an ultraviolet pump intensity study. A second-order rate constant of (1.0 ± 0.3) × 10-10 cm3/s is found to fit all of the decays. The early time decay kinetics in metal oxide nanoclusters do not agree with a recently proposed fractal kinetic study but are consistent with trapped electron/free hole recombination. The assignment of the early time transient absorption at 620 nm to trapped electrons is supported by comparing the transient absorption kinetics to ground state recovery results in both SnO2 and TiO2 nanoclusters.