One-step synthesis of MFe2O4 (M = Fe, Co) hollow spheres by template-free solvothermal method

Monodispersed magnetic MFe₂O₄ (M = Fe, Co) hollow spheres were synthesized by simple template free solvothermal method in ethylene glycol (EG) solution. The hollow spheres were in the same size with an average diameter of about 360 nm and the shells of these spheres were about 80 nm, consisted of closely packed nanocrystallines due to Ostwald ripening. EG plays the key role in the synthesis of hollow spheres in contrast with octahedral crystals synthesized in aqueous solution. The products synthesized in aqueous solution were calcined at 800 °C and 1000 °C. The amount of spinel ferrite products increased monotonically with the increase of temperature and appeared as a single phase at 1000 °C. The saturation magnetization (M_s), remanent magnetization (M_r) and coercivity (H_c) for Fe₃O₄ hollow spheres was 74.47 emu/g, 2.59 emu/g and 32.503 Oe respectively whereas the reading of the same indicators for CoFe₂O₄ hollow spheres was 69.07 emu/g, 14.46 emu/g and 242.79 Oe, respectively. The magnetic variation between Fe₃O₄ and CoFe₂O₄ hollow spheres was caused by the radius difference of Fe²⁺ (3d⁶) and Co²⁺ (3d⁷) ions and it was also relevant with nanocrystal sizes of the spin disorder of crystal surface.