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 x 10(-3) M), H2PtCl6 (1.2 x 10(-3) M), citric acid (4 x 10(-4) M) and NaBH4 (1.2 x 10(-2) M). The silica-coating was performed in aqueous/ethanolic 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 alloy nanoparticles. TEM observation and X-ray diffraction measurements revealed that the alloy nanoparticles were coated with silica and consisted of fee 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 degrees C. Annealing above 300 degrees 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. (c) 2005 Elsevier B.V. All rights reserved.