TEA–NaOAC Synergistic System for Hydrothermal Synthesis of Fe3O4 with Tunable Morphologies from a Single Iron Source

Achieving tunable morphologies of Fe3O4 using a single iron source remains challenging, mainly due to the oxidation of Fe2+ and the difficulty of regulating anisotropic crystal growth. In this study, Fe3O4 was synthesized via a one-step hydrothermal method using FeSO4·7H2O as a single iron source in a TEA–NaOAC synergistic system. The effects of hydrothermal temperature, additive ratio, and dosage were systematically investigated. Time-dependent and TEA dosage-dependent experiments were also designed to elucidate the morphological evolution mechanism. The results show that pure-phase Fe3O4 can be obtained with TEA alone, as TEA controls the release rate of Fe2+ and inhibits its oxidation. However, the synergistic addition of NaOAC provides a mild alkaline environment that not only maintains phase purity but also further promotes anisotropic crystal growth and enables a broader morphological tunability. By tuning the reaction conditions, a systematic morphological evolution from flower-like to cubic, regular octahedral, and polyhedral structures was achieved. Time-dependent experiments reveal a complete dissolution–recrystallization pathway from flower-like to cubic structures. TEM and SAED confirm that the polyhedral particles are micrometer-sized single crystals. Under optimized conditions (160 °C, TEA 3 mL, NaOAC 26 mmol), the polyhedral Fe3O4 exhibits a saturation magnetization of 91.4 emu/g, approaching the bulk theoretical value (92 emu/g), and a coercivity of approximately 100 Oe. This study provides new experimental evidence for morphology regulation of Fe3O4 using a single iron source, achieving high saturation magnetization close to the bulk theoretical value and moderate coercivity suitable for non-biomedical applications.