Catalysis of photochemical reactions by colloidal semiconductors

Abstract

Illumination of semiconductor colloids such as ZnS, CdS and TiO2, generates electrons and positive holes which may react with the aqueous solvent or dissolved substances. In ZnS and CdS, recombination of the charge carriers is often accompanied by emission of light.The absorption spectra of very small CdS or ZnS particles (1-3 nm) show changes which indicate 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. They are attributed to emission from the shortlived exciton state and from longer-lived states of trapped electrons and holes. The chemical reactions described include the photo-anodic and the photocathodic dissolution of colloidal sulfides, the reduction of water and carbon dioxide, and the oxidation of alcohols and sulfite anions. Two-electron (two-hole) and one-electron (one-hole) transfer mechanisms are discussed.Flash photolysis studies of colloidal TiO2 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 and OH- anions, and of various organic compounds. The electrochemical nature of all these photoreactions is emphasized. © 1984 IUPAC