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 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.