Preparation and properties of silica-coated metallic nickel particles

A simple method that suppresses the oxidation and aggregation of metallic particles has been required to be developed. In the present work, metallic nickel (Ni) particles were used as a control, and silica-coating of them was performed for the suppression. Metallic Ni particles were synthesized in water exposed to air, in which nickel(II) acetate tetrahydrate, sodium borohydride, and citric acid monohydrate (Cit) were used as the Ni source, reducing reagent, and stabilizer, respectively. The metallic Ni particles with the size of ca. 33 nm were coated with silica shells with thicknesses of 14–40 nm by adding tetraethylorthosilicate (TEOS)/ethanol to the metallic Ni particle colloid solution (Ni/SiO₂). In this solution, TEOS was the silica source, and ethanol was the solvent. The morphology of Ni/SiO₂ particles was dependent on the concentrations of Cit and TEOS. Both aggregation of the metallic Ni particles and their oxidation even with annealing in air were controlled by the silica shells due to their function as a physical barrier. In addition, crystallization of the cores to metallic Ni with annealing in air was exhibited. The annealed Ni/SiO₂ particles showed paramagnetic behavior because of their small size, and the Ni/SiO₂ particles annealed at 600 °C had a saturation magnetization of 46.5 emu/g-Ni roughly comparable to that of bulk metallic Ni, which also supported that the aggregation and oxidation of metallic Ni particles were successfully controlled with the silica coating.