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 phy sical properties of metal colloids. As such, they are being developed as ad vanced sensor, catalytic, and electrocatalytic materials. We have prepared colloidal gold-silica aerogels containing gold colloids ranging in size fro m 5 to 100 nm. The results presented herein focus on 5- and 28-nm Au-contai ning silica aerogels for the initial characterization of the interaction be tween the metal colloid and the silica matrix. A blue-shift of the Au plasm on resonance for silica-immobilized Au colloids (relative to the same collo ids in a native Au sol) indicates an interaction between the Au colloid and the nanoscale silica network. Transmission electron microscopy measurement s have been used to determine the average size and distribution of the coll oidal Au particles, as well as to image the nanoscale silica environment su pporting an immobilized Au colloid. Small-angle neutron scattering measurem ents show no significant changes in the three-dimensional structures of eit her 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 (re lative 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 t hat the Au colloid occludes pore space) while it increases for 5-nm colloid al Au-silica aerogel. The accessibility of the Au surface in colloidal Au-s ilica aerogels to species introduced from solution is demonstrated by direc t adsorption of the dye methyl orange to the Au surface.