FEMTOSECOND DIFFUSE-REFLECTANCE SPECTROSCOPY OF TIO2 POWDERS

The charge carrier dynamics of several TiO2 powders are investigated using femtosecond time-resolved diffuse-reflectance spectroscopy. Ultrafast pump-probe measurements of transparent solutions of 2-nm TiO2 nanoclusters and the transient diffuse reflection observed in dry powders of TiO2 are found to be qualitatively similar. The comparison allows (i) the decay kinetics observed by time-resolved diffuse reflection in powders to be attributed to electron-hole recombination and (ii) the unequivocal assignment of the long-lived absorption in nanocluster solutions of TiO2 to trapped electrons. The conclusions are further supported by an intensity-dependent study of the electron-hole recombination kinetics of TiO2 powder which was consistent with a second-order process. The sensitivity of the technique is demonstrated by comparison of the recombination dynamics of powders of differing size and crystal structure and with Fe(III) dopants. The results are discussed in terms of the photoreactivities of the various TiO2 materials.