Dendritic Ni3C1-x, nanoparticles (NPs) with controlled branching have been synthesized through the thermolysis (230 degrees C) of nickel acetylacetonate using oleylamine as a reducing agent and 1-octadecene (ODE) as the solvent. Addition of trioctylphosphine (TOP) as a ligand inhibits formation of dendritic shapes and prevents incorporation of C, resulting in spherical Ni NPs. In comparison, when using octadecane (ODA) or trioctylphosphine oxide (TOPO) as the solvent, Ni NPs are obtained at 230 degrees C that have fewer, larger branches than when using ODE. Higher temperatures are required for incorporation of C from ODA or TOPO into Ni NPs, resulting in Ni3C1-x NPs. Therefore, the allyl group in ODE facilitates formation of Ni3C1-x NPs at lower temperatures. Conversion of dendritic Ni3C1-x NPs into Ni12P5 NPs after adding TOP and heating to 300 degrees C results in the formation of multiple voids in the branches, rather than yolk-in-shell structures or unfilled single voids observed for spherical NPs.