Size and Composition Control of CoNi Nanoparticles and Their Conversion into Phosphides

The synthesis of binary rather than unary metal nanoparticles (NPs) introduces challenges in controlling the chemistry and opportunities to tune the properties of the products. Ligand-stabilized CoNi NPs were synthesized by heating mixtures of Ni(acac) 2 and Co(acac) 2 (acac = acetylacetonate), oleylamine, trioctylphosphine, and trioctylphosphine oxide to 240 °C. Varying the amounts of the Co and Ni precursors allows for control over the NP size, giving diameters of 6–18 nm and compositions ( X Co ) of ≤0.7. The products are enriched with Ni, in comparison with the Co:Ni ratio of the precursors. Co and Ni are both dispersed throughout the NPs, while the shells are enriched with Co. The magnetic properties of CoNi NPs are between those of magnetically soft Ni and magnetically harder Co, with additional effects caused by oxidation under ambient atmosphere, which gives rise to exchange bias. When the reaction mixture for synthesizing CoNi NPs is heated to 300 °C, trioctylphosphine decomposes, and conversion into branched Co 2– x Ni x P NPs occurs, where the branches are further enriched with Co. This result suggests a route for synthesizing complex phosphide NPs by adding other metal precursors and trioctylphosphine to presynthesized seed NPs and heating.