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 degrees 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 degrees C, the ferrimagnetic Co3O4 phase appeared. The grain size effect, oxidation behavior and magnetic properties are discussed in detail for the nanocapsules. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.