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The charge carrier dynamics of several T1O2 powders are investigated using femtosecond time-resolved diffusereflectance spectroscopy. Ultrafast pump-probe measurements of transparent solutions of 2-nm T1O2 nanoclusters and the transient diffuse reflection observed in dry powders of Ti02 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 T1O2 to trapped electrons. The conclusions are further supported by an intensitydependent study of the electron-hole recombination kinetics of Ti02 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 Ti02 materials.

Pump-probe diffuse reflectance measurements were carried out on particulate titanium(IV) oxide photocatalysts in the form of aqueous suspension and dry powder. A linear relation between second-order rate constants of electron-hole recombination for the suspension and powder systems was seen by analyses of decay profile of charge carriers. Presence of air gave negligible effect on the recombination kinetics, but loading of platinum (Pt) significantly reduced the rate constant.

The dynamics of charge carrier trapping and recombination are measured as a function of ZnO cluster diameter by ultrafast pump–probe absorption spectroscopy. A finite spherical potential well model which shows good agreement with previous experimental work is employed to predict ZnO cluster diameters from absorption onsets. The rate of electron trapping is measured for clusters of 3.2 and 6.2 nm, and is found to increase with increasing cluster size. This increase in trapping rate for increasing cluster size is not consistent with either a diffusional or quantum mechanical picture of electron trapping. A mechanism for electron trapping involving trap-to-trap hopping is discussed whereby the number density of optically accessible deep traps must increase with increasing cluster size. Differences in the dynamics and in the ratio of interior to exterior atoms on the cluster are correlated and discussed. The time-resolved absorption data of the subsequent electron–hole recombination shows the appearance of an early time signal which increases as the cluster size grows. The early time species decays away within the first 50 ps to a diameter-independent plateau value via second-order recombination, and is assigned to electrons trapped in the interior of the cluster. The electron–hole recombination is found to occur faster and to a greater extent in the largest nanoclusters.

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