The synthesis of zinc ferrite spinel: Determination of pH value in the co-precipitation step

Abstract Zinc ferrite (ZnFe2O4) nanomaterial can be facilely and conveniently synthesized via the co-precipitation method from aqueous Zn2+ and Fe3+ salt solutions using sodium hydroxide as the precipitant. However, zinc hydroxide is amphoteric and can be soluble at low and high pH values; therefore, the study on pH for simultaneous precipitation of Fe3+ and Zn2+ to achieve pure ZnFe2O4 spinel is essential. In the present study, we elaborate a theoretical equation expressing the relationship between the zinc concentration in complex formation and pH value in the co-precipitation step. Since then, we synthesized ZnFe2O4 spinel and analyzed the products via various techniques such as thermoanalysis (TG-DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Vis spectra to investigate the appropriate pH for simultaneous precipitation of Fe3+ and Zn2+. The results indicate that in a wide range of pH values, from 7 to 12, ZnFe2O4 single-phase spinel ferrite structure is formed after calcination of the precipitate at 800 °C, the average particle sizes (SEM, nm) of ZnFe2O4 samples are in the range of 59.5 nm–85.5 nm (but is non-linear change), and their bandgap values are similar, around 1.96 eV. However, the unwanted appearance of α-Fe2O3 phase, the shifting in peak position and the decrease in intensity in the XRD pattern of the spinel products synthesized at pH of 6 and 13, respectively, reflect that the stoichiometric ratio of Zn2+ to Fe3+ in the spinel products is not achieved to 1:2. Such results provide a fundamental basis for choosing the appropriate pH in synthesizing ZnFe2O4 nanoparticles with orientationally structural and optical characteristics.