“Fish-eye” particles consisting of metal clusters (Ag, Cu) a few nanometers in diameter encapsulated within a thin layer (-1 nm) of silica are produced using aerosol synthesis procedures. We present a method for predicting stable “fish-eye” nanostructures and describe synthesis techniques for producing significant quantities of silica-encapsulated metal nanoparticles.
For many metal/oxide pairs, gas phase formation of oxide encapsulated metal particles is thermodynamically favorable. Using known surface free energies and binary phase diagrams, it is possible to predict whether SiO2-encapsulated metal clusters will form in the gas phase. Two conditions which must be satisfied are: 1) that the surface free energy of the metal is higher than that of Si; and 2) that the metal composition in the particle is greater than the eutectic composition in the metal/Si phase diagram. Ag-SiO2 and Cu-SiO2 are two examples of systems which readily form “fish-eye” structures.
Two types of gas phase cluster sources are used at Purdue for producing encapsulated metal nanoparticles. The Multiple Expansion Cluster Source (MECS) is a well established apparatus which produces small quantities (- 50 mg/hr) of very uniform materials using resistive heating for evaporation. The new Arc Cluster Evaporation Source (ACES) offers much higher production rates (>1 g/hr) using DC arc evaporation. These two cluster sources make possible the study of a unique class of materials.