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 α-Fe and FeO phase at 400°C and remains stable up to 600°C. On further heating, α-Fe phase grows at the expense of FeO and the growth process completes at 800°C. Above 900°C, α-Fe is converted to γ-Fe phase and at 1000°C, a part of γ-Fe phase is converted to α-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°C, due to the increased surface spin disorder. Irrespective of the initial particle size, the final phase obtained after annealing at 1000°C and cooled back to room temperature was a mixed phase of α-Fe and α-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 α-Fe and FeO. The magnetization data of the final products before and after vacuum annealing are consistent with final phases observed in the XRD.