Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on gamma-Fe2O3 4 +/- 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of gamma-Fe2O3 phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that gamma-Fe2O3 nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that epsilon-Fe2O3 is an intermediate phase between gamma-Fe2O3 and alpha-Fe2O3 and a series of distinct Raman vibrational bands were identified with the epsilon-Fe2O3 phase. The structural transformation of gamma-Fe2O3 into alpha-Fe2O3 occurs either directly or via epsilon-Fe2O3, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.