The effect of [Fe³⁺]/[Fe²⁺] molar ratio and iron salts concentration on the properties of superparamagnetic iron oxide nanoparticles in the water/ethanol/toluene system

In this study, we developed a convenient one-pot method with sodium oleate as both the surfactant and precipitant to synthesize pure magnetite nanoparticles in the water/ethanol/toluene system. The initial molar ratio of [Fe³⁺]/[Fe²⁺] and the concentration of iron salts were changed in order to systematically investigate their influences on the chemical and physical properties of nanoparticles, such as the crystal structure, morphology, particle sizes, dispersion and magnetism. Samples were determined by XRD, XPS, FTIR, DLS, and VSM. The oleate coating steadily existed on the surface of the nanoparticles to profit them of excellent monodispersibility and stability in non-polar solvents with very narrow size distribution and extremely approximate mean diameters of ~7 nm. Particles consisted mainly of magnetite with a little or no maghemite phase with the molar ratio of [Fe³⁺]/[Fe²⁺] decreasing from 2:1 to 1:1, but they all exhibited superparamagnetism at room temperature. After the optimization, pure magnetite nanoparticles could be prepared with the saturation magnetization successfully increasing to 75 emu/g(Fe), when the molar ratio of [Fe³⁺]/[Fe²⁺] was 1.5:1 and the concentration of iron precursors was 95 mM.