Preparation and characterization of core-shell structured Co/SiO2 nanosphere

The Co/SiO2 nanospheres have been prepared by the improved sol-gel method combined with hydrogen reduction. The size, the phase and the shape of the particles varied with the molar ratio of SiO2, reduction temperature and drying temperature. The initial temperature of the phase transformation from hcp-Co to fcc-Co in core-shell structured Co/SiO2 nanospheres is lower than that in the bulk case due to the lower surface energy of nanoscale particles. Result of the thermogravimetric analysis (TGA) indicates that the thermal stability of Co/SiO2 nanoparticles is much better than that of pure Co particles. The value of saturation magnetization decreases with the increase of SiO2 content due to the presence of the nonmagnetic silica coating.