CATALYSIS OF PHOTOCHEMICAL-REACTIONS BY COLLOIDAL SEMICONDUCTORS

The absorption spectra of very small CdS or ZnS particles (1-3 nm) show changes which indicated a transition from semiconductor to polymolecular material. The conduction band energy is shifted, and the exciton transition band becomes more pronounced. The fluorescence spectra of small CdS particles contain a band at the onset of absorption and two bands at longer wavelengths. Flash photolysis studies of colloidal TiO//2 gave the absorption spectra of long-lived electrons and positive holes which are trapped in surface states of different energies. Chemical reactions of these charge carriers are also described, including the reduction of tetranitromethane, the oxidation of SCN** minus and OH** minus anions, and of various organic compounds. The electrochemical nature of all these photo-reactions is emphasized.