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 °C by X-ray, FTIR and Micro-Raman spectroscopy. Depending on the nanoparticle content, the silica matrix provides extra stabilization of γ-Fe 2O 3 nanoparticles against transformation into α-Fe 2O 3 and ɛ-Fe 2O 3. In nanocomposite samples presenting low nanoparticle concentration, γ-Fe 2O 3 was the only iron oxide phase found up to 900 °C. However, above 900 °C, ɛ-Fe 2O 3 was the dominant phase. For high nanoparticle contents, mixtures of α-Fe 2O 3 and γ-Fe 2O 3 phases were identified at 500 and 700 °C, though α-Fe 2O 3 and ɛ-Fe 2O 3 phases were found at higher temperatures.