Characterization of sol-gel glasses with different copper concentrations treated under oxidizing and reducing conditions

Using the sol-gel process SiO2 coating and powdered samples doped with different concentrations of copper were prepared. Two sets of samples were made: one was thermally treated in air atmosphere, and the other, was annealed under a CO atmosphere. Both sets of samples were characterized using X-ray diffraction, optical transmission spectroscopy, photoacoustic absorption spectroscopy (PAS) in the visible and near infrared region and electron paramagnetic resonance (EPR). It is found that the chemical state of the copper depends on the type of treatment, amount of copper and the temperature of the treatment. The formations of CuO or Cu colloidal particles occur after the thermal treatments in air or in the CO atmosphere, respectively. In both cases, copper is found in the Cu2+ state in interstitial positions and probably substituting the Si atoms in the SiO2 matrix. The evolution of the optical absorption spectrum as function of the Cu content is a consequence of the changes in the composition and size of the colloidal particles. The optical and the PAS measurements are found to be complementary tools to elucidate the origin of the blue band in the absorption spectrum of these doped samples. (C) 1999 Elsevier Science Ltd, All rights reserved.