Chemical durability of metallic copper nanoparticles in silica thin films synthesized by sol-gel

In this study, chemical durability of metallic copper nanoparticles dispersed in sol-gel silica thin films was investigated by exposing the films to air after a reduction process. At first, heat treatment in air for 1 h produced silica films containing crystalline cupric oxide nanoparticles agglomerated on the film surface. Subsequently, reduction of the oxidized films in a reducing environment of N(2)-H(2) for another 1 h at temperatures of 400, 500 and 600 degrees C resulted in the formation of crystalline metallic Cu nanoparticles diffused in the silica matrix. The time evolution of the surface plasmon resonance absorption peak of the reduced Cu nanoparticles was studied after the reduction processes at different temperatures. By fitting the optical absorption spectra with the Mie model, the conversion of Cu into CuO in the silica films exposed to air was examined as a function of the elapsing time. It was found that increasing the reducing temperature resulted in greater diffusion of the reduced Cu nanoparticles into the substrate, and also, in a decrease in the water content of the silica film. Diffusion of the nanoparticles decreased the number of particles exposed to air, and further, the decrease in the water content densified the silica film surrounding the diffused nanoparticles. While after the reduction process of the films at 400 degrees C, the presence of water in the film and considerable copper on the surface resulted in conversion of 94% of the reduced Cu into CuO in just 24 h, by reducing the film at the high temperature of 600 degrees C, no water and small copper concentration could be detected on the silica film so that only 8% of the Cu nanoparticles converted to CuO in as much as 12 months.