Synthesis, growth mechanism and magnetic properties of SiO2-coated Co nanocapsules

The growth mechanism and magnetic properties of SiO2-coated Co nanocapsules synthesized by the arc-discharge method were investigated. Most of the as-prepared products were nanocapsules with a core/shell structure consisting of a Co core and a SiO2 shell, while some of them displayed a rod-like configuration. The formation of the SiO2 nanorods was governed by an extended vapor–liquid–solid (VLS) mechanism and Co nanoparticles acted as a catalyst for the growth of the nanorods. The Co/SiO2 nanocapsules were annealed at 300, 400 and 500 °C, respectively, for 30 min in air, in order to observe their oxidation behavior. The crystalline grains of the Co/SiO2 nanocapsules clearly grew during annealing. After annealing at 500 °C, the ferrimagnetic Co3O4 phase appeared. The grain size effect, oxidation behavior and magnetic properties are discussed in detail for the nanocapsules.