Does Interfacial Charge Transfer Compete with Charge Carrier Recombination? A Femtosecond Diffuse Reflectance Investigation of TiO2 Nanoparticles

The charge carrier dynamics of opaque, aqueous suspensions of Degussa P-25 TiO 2 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-TiO 2 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 TiO 2 /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 TiO 2 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 TiO 2 for photocatalysis are discussed.