Facile synthesis of porous magnetite microspheres and solvent-induced phase transition

Uniform porous magnetite microspheres have been synthesized in a large scale by a dodecyltrimethylammonium bromide (DTAB)-assisted solvothermal route. The as-obtained products were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analyzer, Brunauer-Emmett-Teller gas sorptometry and vibrating sample magnetometer. The factors influencing the phase and morphology of the products were investigated. It demonstrates that the ratio of water and ethylene glycol (EG) plays a crucial role for the phase structure and morphology of the products. When the ratio of water and EG does not exceed 1:3, the as-obtained products belong to magnetite phase. Nevertheless, the as-obtained products begin to convert to hematite phase with the ratio increase to 3:5. The reason could be ascribed by the increased water volume, which effectively weakens the reductive activity of EG. Followed by the change of the ratio of water and EG, the morphologies of as-obtained products undergo a series of evolution, such as from flower-like magnetite microspheres to polyhedral hematite nanoplates. On the basis of experimental investigation and analysis, a possible formation mechanism was proposed. The magnetic study demonstrated the porous magnetite microspheres with high saturation magnetization (Ms) and the decreased Ms caused by the phase transition.