Raman study of copper and iron oxide particles embedded in an SiO2 matrix

The structure of heat-treated sol–gel-made SiO2 samples containing copper or iron in various amounts was studied using Raman spectroscopy and x-ray diffraction. The treatments were in air in the temperature range 200–800 °C. Copper and iron concentrations were in the range 7–55 wt%. It was found that in the samples containing copper, the metal is mainly dissolved in the SiO2 network when the thermal treatment is at low temperatures. For higher annealing temperatures, first a copper silicate is formed and then most of the copper forms cupric oxide particles at temperatures of about 800 °C. In samples containing iron, the formation of particles with nanometric size of the γ-Fe2O3 (maghemite) phase is observed after annealing at 500 °C. At 800 °C the maghemite phase is transformed into the stable hematite (α-Fe2O3) phase. Copyright © 1999 John Wiley & Sons, Ltd.