Silver halide colloid precursors for the synthesis of monolayer-protected clusters.

A new method for the synthesis of monolayer-protected silver clusters (MPCs) based on the two-phase reduction of a stable negatively charged silver bromide sol is described. Phase transfer of the colloid to toluene is accomplished using tetra-n-octylammonium bromide as the phase transfer reagent. The advantage of this synthesis is to uncouple the formation of the silver halide colloid from its transfer and reduction in the organic phase, thus allowing control over each reaction step. The silver colloid in toluene was reduced with aqueous borohydride in the presence of 4-bromobenzenethiol as the passivating agent. The UV-visible absorption spectra indicate the intermediate formation of Ag(core)AgBr(shell) clusters during reduction. The resulting MPCs have been characterized by optical and transmission electron microscopy, energy-dispersive X-ray analysis, thermogravimetry, and UV-vis absorption spectroscopy. The formation of spiral cracks in the nanoparticulate agglomerates on solvent evaporation was observed. The spectra of thin films obtained by solvent evaporation have been analyzed using an effective medium theory.