Temperature-driven structural thermodynamic analysis of cubic spinel iron oxide nanoparticles synthesis via ascorbic acid-assisted co-precipitation

Abstract

Iron oxide nanoparticles possess remarkable properties including high surface area, strong magnetic response and good biocompatibility.  The co-precipitation method is widely used because of its simplicity, mild operating conditions and high scalability. Using ascorbic acid as reducing agent helps maintain the Fe²⁺/Fe³⁺ balance, which improves dispersion stability and supports the formation of magnetite. In this study, iron oxide nanoparticles were synthesized at reaction temperatures from 30 °C to 90 °C. X-ray diffraction (XRD) patterns, field emission scanning electron microscope (FESEM) images and vibrating sample magnetometer (VSM) curves were evaluated to determine the phase evolution, crystallite size, full width at half maximum (FWHM), particle morphology, and magnetic properties. Standard thermodynamic parameters were calculated to link with the experimentally observed structural quality magnetic transformations. The results show that 70 °C is the minimum temperature required to form a stable cubic spinel iron oxide phase. The lower investigated temperature at 50 °C forms only hematite (α-Fe₂O₃) due to accelerated oxidation and insufficient thermal energy. The reaction temperature at 80 °C produced a uniform agglomerated particulates due to their highly magnetic behaviour as evaluated by VSM. The best structural quality, with the largest crystallite size of 7.42 nm and the narrowest FWHM of 0.0205 rad was recorded at 80 °C. Reaction temperatures from 70 °C to 90 °C show exothermic and spontaneous formation of the spinel phase. However, pushing the reaction to 90 °C limits structural and magnetic optimization. At this temperature, the saturation magnetisation (M_s) decreases from 30.6170 emu/g (at 80°C) to 22.6610 emu/g. This behaviour indicates that excessive thermal energy promotes aggressive nucleation, which reduces crystal order and magnetic behaviour. Reaction temperatures from 70 °C to 90 °C also confirm the exothermic, spontaneity and randomness of the reaction. This investigation serves as a crucial preliminary screening and feasibility phase to establish the baseline temperature level required for subsequent Design of Experiment (DOE).