Controlled synthesis of hollow magnetic Fe3O4 nanospheres: Effect of the cooling rate

The controlled synthesis of hollow magnetite (Fe₃O₄) nanospheres of varying sizes and structures was successfully obtained via a facile solvothermal process and varying cooling processes. The Fe₃O₄ nanospheres were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and superconducting quantum interference device magnetometry. The diameters of the as-synthesized nanospheres were controlled at around 500–700 nm by simply changing the cooling rate, which had an obvious influence on the morphology and magnetic properties of these Fe₃O₄ nanospheres. While a low cooling rate triggered the formation and extension of the cracks present in the Fe₃O₄ nanospheres, a sudden drop of temperature tended to favor multi-site nucleation of the crystals as well as the formation of compact and smooth hollow nanospheres with superior crystallinity and high saturation magnetization. The growth mechanism of hollow magnetite oxide nanospheres was proposed and the correlation between the structure and the magnetic properties of the hollow nanospheres was discussed, which promises the potential of the hollow nanospheres in various applications such as drug delivery and cell separation.