Solution synthesis of nanocrystalline core-shell structured NiCo particles

Synthesis of nanometer size core-shell structured NiCo particles via the modified polyol process is described. Cobalt particles of about 20 nm with hcp crystal structure was synthesized by the use of appropriate type of glycol, hydroxyl ions, adsorbing ions, nucleating agents and other reaction conditions, which otherwise were few microns in diameter and fcc in structure. It is proposed that the suitable control of those factors permitted the enhancement of the metal forming reaction rate. The room temperature (RT) saturation magnetization (M₅) and coercivity (H_c) of the fcc-cobalt particles of few hundred nanometer in diameter ranged from 135 to 150 emu/g and 35 to 161 Oe, respectively. However, the 20 nm hcp-Co particles reported a M_s of 67 emu/g and H_c, 535 Oe. On the other hand, the nickel metal particles of few tens of nanometer in diameter were synthesized by modifying the ethylene glycol EG-Ni(II) solution chemistry, which otherwise were sub micron in size and had plate-like morphology. The nickel particles were polycrystalline with fcc structure. The M_s and H_c at RT ranged from 38 to 55 emu/g and 100 to 170 Oe, respectively. Though nickel was reduced easily than the cobalt in the absence of hydroxyl ions in EG, the reaction kinetics was reversed in the presence of hydroxyl ions. Thus, the unconstrained control of the reaction kinetics through the manipulation of the reaction parameters permitted the co-reduction of Ni and Co ions and facilitated the synthesis of nanocrystalline core-shell structured NiCo particles. The TEM-EDX analysis revealed that the core was rich in Co and the shell was rich in Ni. The magnetic properties of the particles depended on the Ni to Co ratio and the M_s of these particles ranged between 50 and 150 emu/g.