Effect of initial particle size on phase transformation temperature of surfactant capped Fe3O4 nanoparticles

We investigate the effect of particle size on reduction temperatures in surfactant capped fine iron oxide (Fe3O4) nanoparticles in size ranging from 7 to 3 nm using in situ high temperature X-ray diffraction (HTXRD). The 7 nm size particles are reduced to metallic alpha-Fe and FeO phase at 400 degrees C and remains stable up to 600 degrees C. On further heating, alpha-Fe phase grows at the expense of FeO and the growth process completes at 800 degrees C. Above 900 degrees C, alpha-Fe is converted to gamma-Fe phase and at 1000 degrees C, a part of gamma-Fe phase is converted to alpha-Fe2O3. As the size is decreased from 7 to 3 nm, the onset of reduction to metallic Fe and FeO is enhanced by 100 to 200 degrees C, due to the increased surface spin disorder. Irrespective of the initial particle size, the final phase obtained after annealing at 1000 degrees C and cooled back to room temperature was a mixed phase of alpha-Fe and alpha-Fe2O3. Thermo Gravimetric Analysis coupled Mass Spectra (TGA-MS) confirm that the evolved carbon from the oleic acid assist the removal of oxygen atom from Fe3O4 lattice, facilitating the reduction of Fe3O4 into alpha-Fe and FeO. The magnetization data of the final products before and after vacuum annealing are consistent with final phases observed in the XRD. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3564964]