Chemistry of Ag n aggregates in aqueous solution: non-metallic oligomeric clusters and metallic particles

Abstract

Since the discovery of long-lived oligomeric silver clusters in aqueous polyanion solutions it has become possible to investigate the chemical reactivity of silver aggregates over a wide size range, especially in the range where the transition from the atom to the metal takes place. Both oligomeric and metallic particles react with nucleophilic reagents such as NH₃, CN^– and SH^–. The reactivity is attributed to the coordinative unsaturation of silver surface atoms. In the case of the metallic particles, the changes in the shape of the surface plasmon absorption band can be used to detect the interaction of nucleophilic reagents with surface atoms. A mechanism is proposed for oxidative corrosion catalysed by chemisorbed molecules. Oligomeric clusters are strong reducing agents which can transfer an electron to O₂, nitro-compounds, carbon tetrachloride etc. The standard potential of the silver microelectrode, Ag_n⇄ Ag_n–1+ Ag⁺+ e^–, as a function of the agglomeration number n is also discussed to explain the stability of the clusters. Electron acceptors often catalyse the conversion of oligomeric clusters into larger metallic particles. Electrons are readily photo-emitted from oligomeric clusters. As the transition to the metal occurs with increasing size, the quantum yield of emission strongly decreases. The emitted electrons appear as e^– _aq in the aqueous solvent.