Preparation of silica-coated Co-Pt alloy nanoparticles

A previously proposed method for silica-coating of Co nanoparticles was extended to silica-coating of Co-Pt alloy nanoparticles. The alloy nanoparticles were prepared from CoCl₂ (1.2 × 10^- 3 M), H₂PtCl₆ (1.2 × 10^- 3 M), citric acid (4 × 10^- 4 M) and NaBH₄ (1.2 × 10^- 2 M). The silica-coating was performed in aqueous/ethanolic 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 alloy nanoparticles. TEM observation and X-ray diffraction measurements revealed that the alloy nanoparticles were coated with silica and consisted of fcc Co-Pt alloy crystallites with crystal sizes of 4.6-10.7 nm. Magnetic properties of the silica-coated particles were strongly dependent on annealing temperature. Maximum values of 1.4 emu/g-sample for saturation magnetization and 450 Oe for coercive field were obtained for the particles annealed at 300 °C. Annealing above 300 °C transformed the crystallite from the alloy to pure Pt probably through oxidation of cobalt in the alloy, resulting in reductions in saturation magnetization and coercive field.