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Journal Articles Nanoscale Year : 2022

The Delicate Balance of Phase Speciation in Bimetallic Nickel Cobalt Nanoparticles

Abstract

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 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 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.
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Dates and versions

hal-03631254 , version 1 (05-04-2022)

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Alberto Palazzolo, Cyprien Poucin, Alexy Freitas, Anthony Ropp, Corinne Bouillet, et al.. The Delicate Balance of Phase Speciation in Bimetallic Nickel Cobalt Nanoparticles. Nanoscale, In press, ⟨10.1039/D2NR00917J⟩. ⟨hal-03631254⟩
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