Grain size dependence of physico-optical properties of nanometallic silver in silica aerogel matrix

Synthesis of silver nanoparticles in silica aerogel matrix by sub-critical drying technique is reported in the present article. Physical characterization of silver/silica aerogel nanocomposites with 1, 5 and 25 wt% of silver has been discussed. Physico-optical properties of the composites have been evaluated as a function of the silver particle size crystallized within the silica matrix. The maximum size of the silver grains that could be accommodated in the amorphous matrix was observed to be 25 nm. Silver particles of diameter larger than 25 nm were found segregated out of the silica matrix; such silver particles were single crystalline with dendritic morphology. Optical absorption analysis confirmed the presence of both the oligomeric and nanometallic silver in the samples heat-treated up to 500 °C. With decreasing silver particle size, the surface plasmon resonance was found first to shift towards blue followed then by a red shift. The blue shift is attributed to the chemisorption occurring between the metallic core and the silica matrix. With larger grain size of 25-80 nm in 25 wt% Ag-silica aerogel sample, the diffuseness of electron cloud outside the potential well was observed to cause a red shift in the surface plasmon resonance.