SURFACE-CHEMISTRY OF COLLOIDAL SILVER - REDUCTION OF ADSORBED CD2+ IONS AND ACCOMPANYING OPTICAL EFFECTS

Cadmium ions can be reduced by 1-hydroxyethylmethyl radicals in solutions containing colloidal silver particles (about 40 angstrom diameter). The mechanism is postulated to consist of the transfer of electrons from the radicals to the colloidal particles followed by the reduction of adsorbed Cd2+ ions. The Cd atoms formed cause a blue shift of the surface plasmon absorption band of silver. This is explained by free electron donation from adsorbed Cd into the silver particles. At a coverage of 3 monolayers, the surface plasmon band is shifted to 260 nm, where pure cadmium absorbs. Adsorbed Cd atoms at a coverage of less than 0.4 monolayers cannot be reoxidized by methyl viologen; at higher coverages this oxidation is possible and can be used to determine the amount of reduced cadmium. A mechanism is proposed in which methyl viologen does not react directly with an adsorbed Cd atom but with the silver-cadmium particle as a whole. UV illumination leads to anodic dissolution of adsorbed cadmium.