Controlled method for silica coating of silver colloids. Influence of coating on the rate of chemical reactions

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.