Photochemistry of Colloidal Metal Sulfides. 5. Fluorescence and Chemical Reactions of ZnS and ZnS/CdS Co‐Colloids

Small colloidal particles of ZnS were obtained by precipitation of Zn 2+ with SH − in the presence of colloidal silicon dioxide. Co‐colloids were prepared by simultaneously precipitating ZnS and CdS. Colloidal ZnS fluoresces at 425 nm. 90% of the fluorescence decays within 30 ns. The fluorescence is partly quenched by oxygen, chloroethanol and chloralhydrate. It can be completely quenched by methyl viologen and sulfide anions at low concentrations. The quenching actions are attributed to the scavenging of electrons or positive holes which are photo‐generated in ZnS. The onset of the absorption and the maximum of the fluorescence band are shifted towards shorter wavelengths with increasing Zn/Cd ratio of the co‐colloid. – ZnS in deaerated solution undergoes a slow photo‐anodic dissolution (quantum yield ø = 2 · 10 −3 ZnS molecules/photon), the products being H 2 , Zn 2+ (aq), sulfur and SO (aq). In the presence of air, a rapid decomposition with ø = 0.32 ZnS/photon takes place. In deaerated solutions, chloroorganic compounds promote the dissolution of ZnS. The chloroorganic compounds are decomposed, too. In the presence of sulfite, zinc metal is photo‐deposited onto the colloidal particles until hydrogen production takes over with ø = 0.18 H 2 /photon. In the presence of both sulfite and carbon dioxide, formic acid is produced with ø = 0.2 HCOOH/photon.