Preparation of Monodisperse ε-Fe2O3 Nanoparticles by Crystal Structural Transformation

Monodisperse iron oxide nanoparticles in the epsilon-Fe2O3 phase, which is the rare orthorhombic phase among the four crystal systems of Fe2O3, are prepared via crystal structural transformation induced by heat treatment of SiO2-coated monodisperse FeO nanoparticles. The mean diameter of the resulting epsilon-Fe2O3 nanoparticles is 41.0 +/- 4.2 nm. Compared with the as-synthesized 40.9 +/- 3.8 nm-FeO nanoparticles, which are used as the precursor, the size and size distribution of the nanoparticles remain almost unchanged after heat treatment. By optimizing the thickness of the SiO2 shell, coalescence of the nanoparticles during heat treatment can be prevented. XRD measurements do not detect the alpha-Fe2O3 phase and gamma-Fe2O3 phase in the obtained sample. The resulting epsilon-Fe2O3 nanoparticles exhibit a high coercivity of 23.4 kOe at room temperature.