TIME-RESOLVED MICROWAVE CONDUCTIVITY .1. TIO2 PHOTOREACTIVITY AND SIZE QUANTIZATION

Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave conductivity (TRMC) measurements of quantum-sized (Q-) TiO2, Fe(III)-doped Q-TiO2, ZnO and CdS, and several commercial bulk-sized TiO2 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 conduction-band 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 TiO2. The quantum efficiencies are 0.4% for Q-TiO2, 1.6% for Degussa P25, and 2.0% for Fe(III)-doped Q-TiO2. The lower quantum efficiencies for Q-TiO2 are consistent with the relative interfacial electron-transfer rates observed by TRMC for Q-TiO2 and Degussa P25. The increased quantum efficiencies of Fe(III)-doped Q-TiO2 and the observed TRMC decays are consistent with a mechanism involving fast trapping of valence-band holes as Fe(IV) and inhibition of charge-order recombination.