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

• Published in: Journal of sol-gel science and technology 2008-07, Vol.47 (1), p.16-22
• Main Authors: Kobayashi, Y., Kakinuma, H., Nagao, D., Ando, Y., Miyazaki, T., Konno, M.
• Format: Article
• Language: English
• Subjects: Annealing; Ceramics; Chemistry; Chemistry and Materials Science; Citric acid; Coated particles; Coating; Cobalt; Coercivity; Colloidal gels. Colloidal sols; Colloidal state and disperse state; Composites; Core-shell particles; Coupling agents; Crystallites; Crystallization; Electron microscopes; Ethanol; Exact sciences and technology; General and physical chemistry; Glass; Inorganic Chemistry; Liquid phases; Magnetic properties; Magnetic saturation; Magnetization; Materials Science; Nanoalloys; Nanoparticles; Nanotechnology; Natural Materials; Optical and Electronic Materials; Original Paper; Platinum; Platinum base alloys; Silicon dioxide; Temperature dependence; X-ray diffraction
• ISSN: 0928-0707; 1573-4846
• DOI: 10.1007/s10971-008-1740-1

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 CoCl 2 (4.2 × 10 −5  M), H 2 PtCl 6 (1.8 × 10 −5  M), citric acid (4 × 10 −4  M) and NaBH 4 (1.2 × 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 × 10 −5  M), and a silica source, tetraethoxyorthosilicate (7.2 × 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 °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 °C, respectively. Annealing at a temperature as high as 700 °C destroyed the coating structures because of crystallization of silica shell, resulting in reduction in saturation magnetization and coercive field.