Direct transformation from goethite to magnetite nanoparticles by mechanochemical reduction

We present a novel method for synthesizing highly crystalline superparamagnetic magnetite (Fe₃O₄) nanoparticles (particle size about 15 nm) with relatively high saturation magnetization by direct transformation via ball-milling treatment from an amorphous goethite precipitate in a water system at room temperature. The obtained product was characterized by transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy; the particle size was measured by dynamic light scattering particle size analysis, and magnetic properties were measured by superconducting quantum interference device magnetometer. The mechanochemical effect induced by ball-milling treatment generates hydrogen gas, which contributes in reduction of part of the goethite, without addition of reducing agents, to give ferrous hydroxide. The mechanochemical effect also promotes solid-phase reaction between ferrous hydroxide and goethite to give magnetite, simultaneously crystallizing the formed magnetite nanoparticles while inhibiting particle growth with addition of neither heat nor additives such as surfactants and organic solvents. Thus, mechanochemical reduction provides an easy route for the synthesis of crystalline magnetite nanoparticles from ferric ion solution.