.-TiO, Photoreactivity and Size Quantization

Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave conductivity (TRMC) measurements of quantum-sized (Q-) TiO, , Fell'-doped Q-TiO, , ZnO and CdS, and several commercial bulk-sized TiO, samples. After pulsed laser excitation of charge carriers, holes that escape recombination react with sorbed trans-decalin within ns while the measured conductivity signal is due to conductionband electrons remaining in the semiconductor lattice. The charge-carrier recombination lifetime and the interfacial electron-transfer rate constant that are derived from the TRMC measurements correlate with the CW photo-oxidation quantum efficiency obtained for aqueous chloroform in the presence of TiO,. The quantum efficiencies are 0.4% for Q-TiO, , 1.6% for Degussa P25, and 2.0% for Fell'-doped Q-TiO, . The lower quantum efficiencies for Q-TiO, are consistent with the relative interfacial electron-transfer rates observed by TRMC for Q-TiO, and Degussa P25. The increased quantum efficiencies of Fell'-doped Q-TiO, and the observed TRMC decays are consistent with a mechanism involving fast trapping of valence-band holes as Fe'" and inhibition of c harg e-o rde r recom bi nation.