The delicate balance of phase speciation in bimetallic nickel cobalt nanoparticles

• Published in: Nanoscale 2022-05, Vol.14 (2), p.7547-756
• Main Authors: Palazzolo, Alberto, Poucin, Cyprien, Freitas, Alexy P, Ropp, Anthony, Bouillet, Corinne, Ersen, Ovidiu, Carenco, Sophie
• Format: Article
• Language: English
• Subjects: Bimetals; Chemical Sciences; Cobalt; Crystallography; Electron energy loss spectroscopy; Ligands; Material chemistry; Nanoparticles; Nickel; Nucleation; Photoelectrons; Room temperature; Scanning transmission electron microscopy; Speciation; Spectrum analysis; Stoichiometry; Strong interactions (field theory); X ray absorption; X-ray spectroscopy
• ISSN: 2040-3364; 2040-3372
• DOI: 10.1039/d2nr00917j

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. Bimetallic NiCo nanoparticles crystallized in the fcc phase are formed using optimized synthetic parameters.