L. Qiao, M.T. Swihart
University at Buffalo (SUNY),
Keywords: nanoparticle, nickel, nickel carbide, branched structure
Summary:Transition metal carbides (TMCs), e.g., nickel carbide (Ni3C), molybdenum carbide (Mo2C), and tungsten carbide (WC), can exhibit metallic conductivity with outstanding thermal and mechanical stability, making them a popular category of substrate for noble metal catalysts, e.g., platinum (Pt), gold (Au), palladium(Pd), and alloys of such elements. In addition, the TMCs themselves are excellent catalysts with performance challenging noble metals in some applications. Solution phase synthesis is widely used for preparing complex nanostructures. However, for Ni3C, only a few cases have been reported, including morphologies such as dots, spheres and multipods. Accessible surface area, along with intrinsic catalytic activity, determines the performance of materials in catalytic applications. Here we report a hierarchical structure of Ni3C with significantly increased specific surface area. The nanostructure is synthesized via the thermal decomposition of an organometallic nickel precursor. The NCs initially form as a Ni3C1-x solid solution. As the reaction proceeds, HCP nickel is gradually carburized into rhombohedral Ni3C. A typical hierarchical nanocrystal (HNC) consists of one core, many radial branches, and hexagonal platelet “caps” on the end of most branches. We conclude that the displacement causing stacking faults is in (0 0 1) planes in each branch, which is common for hexagonal lattices. To optimize the synthesis of HNCs and provide insights into the steps producing the HNCs, we conducted time-dependent growth experiments. These show a sequence of events in which NCs evolve from aggregates of primary particles, from which “buds”, then branches grow. Finally hexagonal “caps” grow on the end of the branches producing a hierarchical structure on a dense quasi-spherical core. In short, we developed a facile formula for creating a hierarchical nickel carbide nanostructure. The unique HNC has complex structure and consequently large specific area. The structure was fully characterized by a number of techniques. 1. Gogotsi, Y., Chemical vapour deposition: Transition metal carbides go 2D. Nat Mater, 2015. 14(11): p. 1079-1080. 2. Schaefer, Z.L., et al., Bridging hcp-Ni and Ni3C via a Ni3C1-x Solid Solution: Tunable Composition and Magnetism in Colloidal Nickel Carbide Nanoparticles. Chemistry of Materials, 2011. 23(9): p. 2475-2480.