Redox Reactivity Of Silver Clusters At The Surface Of Agcl Crystallites In Aqueous Solutions

Nanosecond pulse radiolysis experiments have been carried out on AgCl colloidal solutions to investigate the redox properties of silver clusters formed at the surface of AgCl crystallites. Radiolysis of AgCl leads to the formation of silver clusters of few atoms. Experiments in the presence of methylviologen (MV2+) identify an electron-transfer from silver clusters of certain size to MV2+, which is governed by the fixed redox potential of MV2+ E degrees([MV2+/MV+])=-0.41V(NHE)) and the variable, size dependent redox potential of silver clusters. Clusters smaller than a critical size (subcritical clusters) are oxidized by MV2+, because their redox potential values are lower than -0.41V(NHE). This reaction was not observed in previous experiments with free silver clusters in aqueous solutions in the absence of AgCl colloids. In the present investigation, since the clusters are produced on the surface of AgCl crystallites, inter-crystallite processes do not occur. Therefore, oxidation of subcritical clusters by MV2+ could be explained by the AgCl environment and the low coalescence rate of silver atoms. Contrarily, clusters larger than the critical size (supercritical clusters) are developed by MV+ due to their redox potential greater than E degrees([MV2+/MV+]).