Formation of Long-Lived Silver Clusters in Aqueous Solution by Anodic Dispersion

It was found that long-lived, subnanometer-sized silver clusters can be prepared by anodic dispersion of a silver electrode in aqueous NaOH solutions (pH 10.5−12), free of stabilizing polymers. The method, described by Nouhuys in 1938, is based upon the application of a dc voltage (10−65 V) between two silver electrodes and yields a range of different clusters, colloids, and oxides. The large colloids and oxides can be removed by centrifugation leaving small colloids and clusters in the sols as revealed by the strong absorption of light at wavelengths between 210 and 400 nm. Absorption spectra of the sols remain fairly unchanged for weeks, implying the stabilization of the clusters by hydroxyl ions. Significant formation of the clusters is only observed between pH ∼10.5 and 12 and is explained by Joule heat generated in the Ag 2 O surface layer at the anode. At pH above 12, a passivating AgO layer forms, preventing the dispersion of clusters while no significant amounts of clusters form either at pH below 10.5 due to the absence of Ag 2 O at the surface.