Silica coating of Co-Pt alloy nanoparticles prepared in the presence of poly(vinylpyrrolidone)

This article describes a method for silica coating of Co-Pt alloy nanoparticles prepared in the presence of poly(vinylpyrrolidone) (PVP) as a stabilizer. The Co-Pt nanoparticles were prepared in an aqueous solution at 25-80 °C from CoCl₂ (3.0 × 10^-4 M), H ₂PtCl₆ (3.0 × 10^-4 M), PVP (0-10 g/L), and NaBH₄ (4.8 × 10^-3-2.4 × 10^-2 M). The silica coating was performed for the Co-Pt nanoparticle colloid containing the PVP ([Co] = [Pt] = 3.0 × 10^-5 M) at 25 °C in (1/4) (v/v) water/ethanol solution with tetraethoxyorthosilicate (TEOS) (7.2 × 10^-5-7.2 × 10^-3 M) and ammonia (0.1-1.0 M). Silica particles, which had an average size of 43 nm and contained multiple cores of Co-Pt nanoparticles with a size of ca. 8 nm, were produced at 1.4 × 10^-3 M TEOS and 0.5 M ammonia after the preparation of Co-Pt nanoparticles at 80 °C, 5 g/L PVP, and 2.4 × 10^-2 M NaBH₄. Their core particles were fcc Co-Pt alloy crystallites. Their saturation magnetization was 2.0-emu/g sample, and their coercive field was 12 Oe.