Characterization of nanocrystalline γ-Fe₂O₃ prepared by wet chemical method

Homogeneous maghemite (γ-Fe₂O₃) nanoparticles with an average crystal size around 5 nm were synthesized by successive hydrolysis, oxidation, and dehydration of tetrapyridino-ferrous chloride. Morphological, thermal, and structural properties were investigated by transmission electron microscopy (TEM), differential scanning calorimetry (DSC), and x-ray diffraction (XRD) techniques. Rietveld refinement indicated a cubic cell. The superstructure reflections, related to the ordering of cation lattice vacancies, were not detected in the diffraction pattern. Kinetics of the solid-state phase transition of nanocrystalline maghemite to hematite (α-Fe₂O₃), investigated by energy dispersive x-ray diffraction (EDXRD), indicates that direct transformation from nanocrystalline maghemite to microcrystalline hematite takes place during isothermal treatment at 385°C. This temperature is lower than that observed both for microcrystalline maghemite and for nanocrystalline maghemite supported on silica.