Structural and Magnetic Properties of Oxidatively Stable Cobalt Nanoparticles Encapsulated in Graphite Shells

Oxidatively stable magnetic cobalt nanoparticles were prepared by annealing cobalt nanoparticles coated with poly(styrene- b -4-vinylphenoxyphthalonitrile) block copolymers. An oxygen-impermeable graphitic coating around the cobalt nanoparticles was created during thermal treatment at 700 °C owing to the thermal decomposition/cross-linking of a polymer with a highly aromatic character. A variety of analytical techniques were used to elucidate the physical properties of the pre-heat-treated and heat-treated samples. The cobalt-specific saturation magnetization (σ s ) increased from 81 to 173 emu g - 1 Co upon heating of the polymer-coated cobalt nanoparticles. The pre-heat-treated sample readily oxidized, as revealed by low-temperature magnetic susceptometry studies, whereas the heat-treated sample showed oxidative stability for over 1 year. The mean cobalt particle size increased from 18.5 to 36.1 nm during thermal treatment. Electron diffraction and X-ray diffraction revealed that the pre-heat-treated particles were weakly crystalline, while the heat-treated particles were strongly crystalline. The dominant phase of the heat-treated sample was determined to be face-centered cubic with other minor phases present (hexagonal close packed and epsilon).