Characterization and magnetic properties of carbon-coated cobalt nanocapsules synthesized by the chemical vapor-condensation process

摘要

Carbon-coated cobalt nanocapsules were synthesized by the chemical vapor-condensation process with cobalt carbonyl (Co_2(CO)_8) used as precursor and carbon monoxide (CO) as carrier gas. The characterization and magnetic properties of carbon-coated cobalt nanocapsules were investigated systematically. The transmission electron microscope (TEM) images showed that the as-prepared nanoparticles consist of a metal core and an amorphous carbon shell. X-ray diffraction and TEM selected area diffraction revealed the presence of f.c.c. Co phase, h.c.p. Co phase, and minority Co_2C, Co_3C phases. The saturation magnetization at room temperature of the nanocapsules is 146.9 Am~2 kg~(-1), which is 90 percent of the bulk ferromagnetic element counterpart. The coercive force at room temperature of the nanocapsules is 0.12 T, while the ratio of remnant to saturation magnetization M_r/M_s is about 0.4. The saturation magnetization and the coercive force increase with increasing the decomposition temperature, mainly due to the increase of the size of the magnetic particles. The decomposition of the cobalt carbonyl (Co_2(CO)_8) and CO gas can decrease efficiently the oxygen content in nanocapsules. The metallic Co nanoparticles completely coated by carbon can resist the dilute acid erosion as well as the oxidation. The thermal stability of the Co nanocapsules is also studied.