Transforming Cobalt Nanospheres into Co2P Nanorods: the Key Roles of Oleylamine and Organophosphorus Ligands in a Co(I) Precursor Uncovered Through X-ray Photoelectron Spectroscopy Analysis

This research offers fresh insights into the growth mechanism of cobalt nanoparticles in oleylamine from a precursor with nominal stoichiometry CoCl(PPh3)(3), prompting a reevaluation of the model delineated in '' The five Shades of Oleylamine '' by Moisset et al. (Nanoscale 2021, 13, 11289-11297) that explicated the cobalt sphere-to-rod morphological transition. While the crucial role of oleylamine is reconfirmed, X-ray photoelectron spectroscopy (XPS) discloses the significant involvement of the organophosphorus ligand of the Co(I) precursor. By scrutinizing aliquots from a tetradecane:oleylamine solution, XPS substantiates the disproportionation of Co(I) into Co(II) and Co degrees during the sphere growth stage in line with the tenets of Five Shades. Pure Co spheres are formed, with phosphorus-containing species disappearing from the XPS probed layers. However, further inspection of washed nanoparticles corroborates the formation of Co-P bonds, with the Co/P atomic ratio nearing 2 for the nanorods. In response to these findings, previously published data and electron diffraction patterns of the nanorods structure are reassessed, conclusively demonstrating that the nanorods are composed of Co2P, contrary to earlier assumptions of pure cobalt content. This leads to a revised depiction of the sphere-to-rod transition, placing emphasis on the role of organophosphorus ligands (triphenylphosphine and, possibly, its oxidation products) that were previously overlooked.