The role of pH and effect of calcination temperature on polymorphs and properties of iron oxide nanoparticles

Iron oxide nanoparticles were successfully synthesised by a facile solution approach employing iron (III) chloride (FeCl3) as starting precursor and sodium hydroxide (NaOH) as reducing agent, followed by calcination in air at different temperatures viz. 400, 600 and 800°C for three hours. The range of calcination temperature has been chosen from thermogravimetry analysis. X-ray diffraction patterns clearly confirmed the structural transformation of cubic Fe3O4 to hexagonal α-Fe2O3 upon calcination. FTIR vibrational bands corresponding to Fe-O bondings in the tetrahedral and octahedral sites and their shift upon calcination confirmed the structural transitions. Scanning electron microscopy observations revealed agglomers meanwhile energy dispersive spectroscopy analysis confirm the elemental composition. Optical band gaps estimated from Tauc plots was found to vary with structural transformation. Electrical conductivity measurements with temperature were explained on the basis of fluctuation of ions in the B sites of Fe3O4 and deviation from stoichiometry in the case of α-Fe2O3.