Bridging hcp-Ni and Ni₃C via a Ni₃C _1-x solid solution: Tunable composition and magnetism in colloidal nickel carbide nanoparticles

Nanoparticles of elemental nickel underpin a large number of magnetic and catalytic applications, and the possibility of tuning these properties via the formation of different allotropes is intriguing. While bulk elemental nickel adopts a face centered cubic (fcc) structure, a growing number of reports suggest that colloidal nickel nanoparticles can crystallize in the metastable hexagonal close packed (hcp) structure. However, there is some disagreement in the literature concerning the formation of hcp-Ni, particularly with respect to the crystallographically-related Ni₃C phase. Most notable is a range of lattice constants and magnetic properties that have been attributed to hcp-Ni. Here, we show that reaction time can be used to tune the carbon content of a Ni₃C_1-x solid solution. Importantly, colloidal nanoparticles of Ni₃C_1-x can help to experimentally rationalize the range of lattice constants and magnetic properties reported for hcp-Ni and Ni₃C, effectively bridging these two end-member systems. All samples, including those isolated immediately upon reduction of Ni ²⁺ to Ni⁰, contained some carbon, as evidenced by XRD, XPS, TGA, DSC, TEM, and SQUID magnetometry. As reaction time increases, the average carbon content increases, and this correlates with a systematic increase in unit cell volume and a systematic decrease in saturation magnetization. These results also provide a straightforward pathway for tuning the magnetic properties of isomorphous Ni nanoparticles.