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J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3315-3322 3315 Time-resolved Microwave Conductivity

Summary: J. CHEM. SOC. FARADAY TRANS., 1994, 90(21), 3315-3322 3315
Time-resolved Microwave Conductivity
Part 1.-TiO, Photoreactivityand Size Quantization
Scot T. Martin, Hartmut Herrmann,?Wonyong Choi and Michael R. Hoffmann"
W. M.Keck Laboratories, California lnsiituie of Technology, Pasadena, CA 91125,USA
~ ~ ~~
Charge-carrier recombination dynamics after laser excitation are investigated by time-resolved microwave con-
ductivity (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 recom-
bination 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 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
charge-order recombination.
When the crystallite dimension of a semiconductor particle
falls below a critical radius of ca. 10 nm, the charge carriers


Source: Adolphs, Ralph - Psychology and Neuroscience, California Institute of Technology


Collections: Biology and Medicine