Ferromagnetic Co/SiO2 core/shell structured nanoparticles prepared by a novel aqueous solution method

Magnetic Co nanoparticles coated with silica were successfully synthesized using a novel aqueous solution method, and their structures and properties were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The experimental results showed that the size and the shape of the particles varied with the molar content of SiO2, reducing time and temperature. There was an optimum reducing time matching with SiO2 content at a certain reducing temperature. With the increase of SiO2 content, the reducing time should be increased too. The insulating layer of SiO2 can hinder nanoparticles from growing up. Two kinds of structures, f.c.c. Co and h.c.p. Co, could be found in the particles. The smaller the Co particles, the easier to obtain f.c.c. Co structure. And with increasing the reducing time and particles' growing up, h.c.p. Co structure appeared. The spherical-shaped particles with average size about 10-150 nm were dominant in these samples. However, some needle-shaped particles, which were about 70 nm long and 10 nm in diameter, also appeared in the samples with high SiO2 content. A relatively high saturation magnetization, 110-130 emu/g or so, was obtained when the molar content of SiO2 was from 3 to 15 mol%. The least coercivity of Co/SiO2 nanoparticles appeared in the sample with 8 mol% SiO2, herein, the structure of particles was f.c.c. and the shape was spherical. With the increase of h.c.p. structure in the particles, the coercivity of nanoparticles increased.