Nucleation and Growth of Magnetic Ni-Co (Core-Shell) Nanoparticles in a One-Pot Reaction under Microwave Irradiation

We have successfully prepared face-centered cubic (fcc) Ni-Co (core shell) nanoparticles using both nickel(II) acetate and cobalt(II) formate complexes with oleylamine in a one-pot reaction under microwave irradiation. Observation using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) technique has shown that the nanostructure was composed of a Co-rich shell and a Ni-rich core. Ni50Co50 nanoparticles with an average particle size of 71.0 nm were composed of a Ni-core with a diameter of ca. 46.9 nm, a ca. 10.0 nm-thick Co shell, and a ca. 4.0 nm-thick interlayer of mixed Ni Co alloy in between. Co shells were overgrown on the Ni-core. The crystalline shapes of the Ni-Co nanoparticles were easily controlled using different nickel precursors (acetate or formate complexes) as starting materials. The crystalline shape of the Ni-core played a key role in determining the final shape of a Ni-Co (core shell) nanocrystal. A formation mechanism for Ni-Co (core shell) nanoparticles was proposed on the basis of the results of several synthetic routes. First, Ni-core nanoparticles were produced at around 498 K through the redox reaction between oleylamine and Ni2+ in a mixture of nickel(II) acetate and cobalt(II) formate complexes with oleylamine. Then, Co2+ on the surface of the Ni-core was easily reduced at 498 K, because Ni nanoparticles could act as a catalyst for the redox reaction between oleylamine and Co2+.