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 degrees 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 degrees C. In samples containing iron, the formation of particles with nanometric size of the gamma-Fe2O3 (maghemite) phase is observed after annealing at 500 degrees C. At 800 degrees C the maghemite phase is transformed into the stable hematite (alpha-Fe2O3) phase. Copyright (C) 1999 John Wiley & Sons, Ltd.