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

Abstract

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.