Nonaqueous synthesis of magnetite nanoparticles via oxidation of tetrachloroferrate anions by pyridine-N-oxide

Fe₃O₄ nanoparticles were prepared from a salt comprising a tetrachloroferrate anion and a methyltrioctylammonium cation in toluene using ethylenediamine as a reductant and pyridine-N-oxide as an oxygen donor and an oxidant. The X-ray diffraction (XRD)analysis, X-ray photoelectron spectroscopy (XPS)and Raman spectroscopy showed that the product was Fe₃O₄. Water content measurement with a Karl Fischer moisture meter showed the presence of only a small amount of water in the present system, indicating a limited contribution of water to the formation of Fe₃O₄ nanoparticles. The Fe₃O₄ nanoparticle size based on transmission electron microscopy (TEM)observation was approximately 10–30 nm, a result consistent with the crystallite diameter estimated by Scherrer's equation (15.7 nm). A possible reaction mechanism involves the reduction of Fe³⁺ to Fe²⁺ by ethylenediamine, coordination of both ethylenediamine and pyridine-N-oxide to Fe²⁺, and oxidation of a part of Fe²⁺, leading to a mixed-valence iron-oxygen network, which was a precursor of Fe₃O₄ nanoparticles. As concerns magnetic properties, saturation magnetization of the product was 57 emu g⁻¹. Both the coercivity and remanent magnetization were nearly zero and the similar decreases in magnetization were observed above the blocking temperature in the zero-field-cooled and field-cooled curves, results indicating the formation of superparamagnetic Fe₃O₄ nanoparticles.