(474g) Hierarchical Nickel Carbide “Dandelion” Nanostructure: Controlled Synthesis and Potential Applications

Transition metal carbides (TMCs), e.g., Ni₃C, Mo₂C, and WC, can exhibit high electrical and thermal conductivity combined with excellent thermal and mechanical stability due to their mixed covalent-ionic bonding. TMCs can be excellent catalysts with performance comparable to noble metals for some reactions, and can also serve as supports for noble metal catalysts. Rhombohedral nickel carbide, Ni₃C, is a common product of nickel carburization during catalysis using Ni. Research on the transformation of Ni to Ni₃C helps elucidate its formation mechanism. Recently we reported a hierarchical nickel carbide (Ni₃C) nanostructure that resembles a dandelion flower (Qiao et al. Angew. Chemie, 2016). A typical hierarchical nanocrystal (HNC) consists of one core and many radially-arranged branches, most of which are topped by a hexagonal cap. To understand the formation of the Ni₃C HNCs, we conducted time-resolved experiments. Quasi-spherical aggregates of primary particles, referred to as â??coresâ?, were observed after 75 min. The Ni content at this point exceeds that of stoichiometric Ni₃C. After 80 min, the radial arms have grown in one dimension, and the platelets have begun to grow in the perpendicular direction. From the 75^th min to the 80^th min, the cores blossomed into midsize HNCs, while also densifying to become non-porous. Further growth of both the arms and the platelets proceeds through the 90^th min. The atomic composition of the HNCs matched the stoichiometry of Ni₃C, i.e., 75% Ni and 25% C. During evolution from cores to dandelions, the specific surface area (SSA) increased from about 3 m² g^-1 to over 60 m² g^-1. We are presently testing this novel nanomaterial in electrocatalytic applications and will report these results along with the synthesis in this talk. In short, we developed a hierarchical Ni₃C â??dandelionâ? nanostructures with specific surface area much larger than that of dense particles of comparable size. Insights into their growth mechanism can enlighten further research on the controlled synthesis of hierarchical nanostructures in solution. In addition, we believe that our current efforts will demonstrate promising applications of this material.