Synthesis and properties of Co-Pt alloy silica core-shell particles

This paper describes a method for fabrication of silica-coated Co-Pt alloy nanoparticles in a liquid phase process. The Co-Pt nanoparticles were prepared from CoCl2 (4.2 x 10(-5) M), H2PtCl6 (1.8 x 10(-5) M), citric acid (4 x 10(-4) M) and NaBH4 (1.2 x 10(-2) M) with a Co:Pt mole ratio of 7:3. The silica coating was performed in water/ethanol solution with a silane coupling agent, 3-aminopropyltrimethoxysilane (8 x 10(-5) M), and a silica source, tetraethoxyorthosilicate (7.2 x 10(-4) M) in the presence of the Co-Pt nanoparticles. Observations with a transmittance electron microscope and a scanning transmission electron microscope revealed that the Co-rich and Pt-rich nanoparticles were coated with silica. According to X-ray diffraction measurements, core particles were crystallized to metallic Co crystallites and fcc Co-Pt alloy crystallites with annealing in air at 300-500 degrees C. Magnetic properties of the silica-coated particles were strongly dependent on annealing temperature. Maximum values of 11.4 emu/g-sample for saturation magnetization and 365 Oe for coercive field were obtained for the particles annealed at 300 and 500 degrees C, respectively. Annealing at a temperature as high as 700 degrees C destroyed the coating structures because of crystallization of silica shell, resulting in reduction in saturation magnetization and coercive field.