Growth Mechanism and Stability of Magnetite Nanoparticles Synthesized by the Hydrothermal Method

Magnetite (Fe3O4) nanoparticles were synthesized using FeCl3, FeSO4 (initial molar ratio Fe2+/Fe3+ is fixed at 1/1) and KOH as raw materials by the hydrothermal method. The crystallographic and microstructures, thermal and magnetic properties of Fe3O4 were characterized by X-ray powder diffraction, scanning electron microscopy, thermogravimetric analyses and vibrating sample magnetometer. The results show that by controlling the reaction temperature and time, Fe3O4 nanocrystals with homogeneous morphology and nanometer grain sizes can be obtained. The Fe3O4 nanocrystals are stable up to temperature of 115 degrees C in air then undergo allotropic transformation to maghetite and hematite. The kinetics of the grain growth of Fe3O4 nanocrystals during the hydrothermal process were investigated. Based on the experimental data, the radius of the initial Fe3O4 particles in the hydrothermal synthesis was found to be 23 nm. The average particle radius increases linearly with the reaction time in agreement with the Lifshitz-Slyozov-Wagner model for coarsening and the rate constant for the coarsening increases with the temperature of the hydrothermal reaction.