Formation of long-lived silver clusters in aqueous solution by anodic dispersion

It was found that long-lived, subnanometer-sized silver clusters can be pre pared by anodic dispersion of a silver electrode in aqueous NaOH solutions (pH 10.5-12), free of stabilizing polymers. The method, described by Nouhuy s in 1938, is based upon the application of a de voltage (10-65 V) between two silver electrodes and yields a range of different clusters, colloids, a nd oxides. The large colloids and oxides can be removed by centrifugation l eaving small colloids and clusters in the sols as revealed by the strong ab sorption of light at wavelengths between 210 and 400 nm. Absorption spectra of the sols remain fairly unchanged fur weeks, implying the stabilization of the clusters by hydroxyl ions. Significant formation of the clusters is only observed between pH similar to 10.5 and 12 and is explained by Joule h eat generated in the Ag2O surface layer at the anode. At pH above 12, a pas sivating AgO layer forms, preventing the dispersion of clusters while no si gnificant amounts of clusters form either at pH below 10.5 due to the absen ce of Ag2O at the surface.