Effect of variation of molar ratio (pH) on the crystallization of iron oxide phases in microwave hydrothermal synthesis

Abstract Microwave hydrothermal (MH) route was employed to synthesize various phases of iron oxide powders by using ferrous sulphate and sodium hydroxide as starting chemicals. All the MH reactions were carried out under the identical MH conditions of 190 °C, 154 psi, 30 min by varying molar ratio (MR) of FeSO 4 /NaOH (i.e. pH variation) from 0.133 to 4.000 in the solution. It was found that the variation of MR of FeSO 4 /NaOH has a profound effect on the crystallization of various phases of iron oxides under identical MH processing conditions. The stoichiometric, submicron-sized (0.15–0.2 μm), spherical agglomerates of Fe 3 O 4 powders were obtained if MR of FeSO 4 /NaOH equal to 0.133 (pH≥10) was maintained. On the other hand, non-stoichiometric Fe 3 O 4 powders were obtained for all the higher MR of FeSO 4 /NaOH between 0.133 and 4.00 (6.6 4 /NaOH was equal to 4.00 (pH≈6.6), a varied distribution of shapes and sizes (1–5 μm) of agglomerates of α-Fe 2 O 3 powders were produced. Various analytical tools such as XRD, Mossbauer spectroscopy, scanning electron microscopy (SEM) and EDAX were used to characterize the MH-derived powders.