Stabilization of copper-based colloidal particles in sol-gel SiO2 thin films

SiO2 amorphous thin films containing two different concentrations of copper -based colloidal particles were prepared by the sol-gel method. Thermal tre atments in air at 350 degrees C result in the segregation of cupric oxide p articles. Subsequent reducing treatments in a CO atmosphere at 300 and 500 degrees C for one hour partially reduces the cupric oxide into particles wi th metallic copper as the main component. The optical properties of the red uced films were characterized measuring the optical absorption spectra as a function of the elapsed time after the reduction process. The absorption d ata were analyzed in terms of the Mie model for colloidal systems. The fitt ing of this model to the experimental data provides the temporal evolution of the volume fractions of the three components of the colloidal particles, that is; Cu, Cu2O and CuO. It is found that the exposure to air of the fil ms treated at 300 degrees C under the reducing atmosphere, results in a gra dual reoxidation of the colloidal particles modifying their chemical compos ition and therefore the optical properties of the composite him. On the oth er hand, the reduction treatment at 500 degrees C results in the stabilizat ion of the composition of the colloidal particles, producing stable films w ith a high optical density. (C) 2000 Elsevier Science S.A. All rights reser ved.