Controlled Method for Silica Coating of Silver Colloids. Influence of Coating on the Rate of Chemical Reactions

Abstract

The preparation and chemical reactivity of Ag@SiO 2 particles have been investigated using UV−vis spectroscopy, laser Doppler electrophoresis, and electron microscopy. The factors governing the deposition of silica onto silane-primed silver particles have been investigated and the deposition conditions (pH and reagent concentrations) optimized. It is demonstrated that the thin silica shells deposited from aqueous/ethanolic sodium silicate solutions are porous. Silver cores dissolve in alkaline cyanide solutions, with the rate depending on the shell thickness. Ag 2 S@SiO 2 can be prepared following exposure to sulfide ion, while silica-coated alloys of silver and gold can be prepared by reaction of Ag@SiO 2 with AuCl 4 - . The kinetics of these core reactions with solution reagents is governed by their rate of diffusion through the shell. A diffusion coefficient of 10 - 12 cm 2 s - 1 is estimated for cyanide ion diffusing through non-heat-treated silica shells.