Magnetic, thermal and martensitic phase transitions in Ni-Co nanoparticles

The delicate balance of phase speciation in bimetallic nickel cobalt nanoparticles

Bimetallic nickel-cobalt nanoparticles are highly sought for their potential as catalytic and magnetic nanoparticles. These are typically prepared in organic solvents in the presence of strong stabilizing ligands such as tri-n-octylphosphine (TOP). Due to the variety of cobalt crystallographic phases and to the strong interaction of the ligands with the metallic surfaces, forming fcc nanoparticles rather than a phase mixture is a challenging endeavor. Here, using a two-step synthesis strategy that aims at a core-shell nickel-cobalt morphology, we demonstrated that many parameters have to be adjusted: concentration of the metal precursors, stoichiometry of TOP, and heating program from room temperature to 180 °C. We found optimized conditions to form size-controlled fcc NiCo nanoparticles from preformed Ni nanoparticles, and the phase attribution was confirmed with a combination of X-Ray diffraction on powder and X-Ray absorption spectroscopy at the Co K edge. We then investigated the early stages of Co nucleation on the nickel using a lower stoichiometry of Co, down to 0.05 equiv. vs. Ni. Using X-ray photoelectron spectroscopy and scanning transmission electron microscopy coupled to energy-dispersive X-Ray spectroscopy and electron energy loss spectroscopy, we showed that cobalt reacts first on the nickel nanoparticles but easily forms cobalt-rich larger aggregates in the further steps of the reaction.

The impact of alloying on defect-free nanoparticles exhibiting softer but tougher behavior

The classic paradigm of physical metallurgy is that the addition of alloying elements to metals increases their strength. It is less known if the solution-hardening can occur in nano-scale objects, and it is totally unknown how alloying can impact the strength of defect-free faceted nanoparticles. Purely metallic defect-free nanoparticles exhibit an ultra-high strength approaching the theoretical limit. Tested in compression, they deform elastically until the nucleation of the first dislocation, after which they collapse into a pancake shape. Here, we show by experiments and atomistic simulations that the alloying of Ni nanoparticles with Co reduces their ultimate strength. This counter-intuitive solution-softening effect is explained by solute-induced local spatial variations of the resolved shear stress, causing premature dislocation nucleation. The subsequent particle deformation requires more work, making it tougher. The emerging compromise between strength and toughness makes alloy nanoparticles promising candidates for applications.