FEMTOSECOND DIFFUSE-REFLECTANCE SPECTROSCOPY OF TIO2 POWDERS

The charge carrier dynamics of several TiO2 powders are investigated u sing femtosecond time-resolved diffuse-reflectance spectroscopy. Ultra fast pump-probe measurements of transparent solutions of 2-nm TiO2 nan oclusters 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 sol utions of TiO2 to trapped electrons. The conclusions are further suppo rted by an intensity-dependent study of the electron-hole recombinatio n kinetics of TiO2 powder which was consistent with a second-order pro cess. The sensitivity of the technique is demonstrated by comparison o f 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.