Colloidal gold aerogels: Preparation, properties, and characterization

Colloidal metal aerogels are composite nanoscale materials that combine the high surface area and porosity of aerogels with the unique optical and physical properties of metal colloids. As such, they are being developed as advanced sensor, catalytic, and electrocatalytic materials. We have prepared colloidal gold-silica aerogels containing gold colloids ranging in size from 5 to 100 nm. The results presented herein focus on 5- and 28-nm Au-containing silica aerogels for the initial characterization of the interaction between the metal colloid and the silica matrix. A blue-shift of the Au plasmon resonance for silica-immobilized Au colloids (relative to the same colloids in a native Au sol) indicates an interaction between the Au colloid and the nanoscale silica network. Transmission electron microscopy measurements have been used to determine the average size and distribution of the colloidal Au particles, as well as to image the nanoscale silica environment supporting an immobilized Au colloid. Small-angle neutron scattering measurements show no significant changes in the three-dimensional structures of either the base- or acid-catalyzed silica aerogels upon incorporation of small amounts (<0.1 vol %) of colloidal Au. However, for base-catalyzed aerogels, nitrogen physisorption measurements reveal that the average pore size (relative to the pure silica aerogel) decreases as the size of the Au colloid is increased above the ca. 10-nm domain size of the silica (which implies that the Au colloid occludes pore space) while it increases for 5-nm colloidal Au-silica aerogel. The accessibility of the Au surface in colloidal Au-silica aerogels to species introduced from solution is demonstrated by direct adsorption of the dye methyl orange to the Au surface.