The thermal stability of maghemite-silica nanocomposites: An investigation using X-ray diffraction and Raman spectroscopy

Maghemite-silica nanocomposites consisting of 5 nm magnetic nanoparticles dispersed in silica xerogel were prepared by the sol-gel technique. The thermal stability of samples with 0.013 and 0.200 Fe/Si molar ratio was investigated in the temperature range of 120-1400 degrees C by X-ray, FTIR and Micro-Raman spectroscopy. Depending on the nanoparticle content, the silica matrix provides extra stabilization of gamma-Fe2O3 nanoparticles against transformation into alpha-Fe2O3 and epsilon-Fe2O3. In nanocomposite samples presenting low nanoparticle concentration, gamma-Fe2O3 was the only iron oxide phase found up to 900 degrees C. However, above 900 degrees C, epsilon-Fe2O3 was the dominant phase. For high nanoparticle contents, mixtures of alpha-Fe2O3 and gamma-Fe2O3 phases were identified at 500 and 700 degrees C, though alpha-Fe2O3 and epsilon-Fe2O3 phases were found at higher temperatures. (C) 2006 Elsevier B.V. All rights reserved.