Nickel Boride (Ni_xB) Nanocrystals: From Solid-State Synthesis to Highly Colloidally Stable Inks

Metal borides, a class of materials intensively used in industry as superconductors, magnetic materials, or hot cathodes, remain largely unexplored at the nanoscale mainly due to the difficulty in synthesizing single-phase nanocrystals. Recent works have shown that synthetic methods at lower temperatures (<400 °C) yield amorphous polydisperse nanoparticles, while phase purity is an issue at higher temperatures. Among all the metal-rich borides, nickel borides (Ni_xB) could be a potential catalyst for a broad range of applications (hydrogenations, electrochemical hydrogen, and oxygen evolution reactions) under challenging conditions (such as high pH or high temperatures). Here, we report a novel solid-state method to synthesize Ni_xB nanopowders (with a diameter of approximately 45 nm) and their conversion into colloidal suspensions (inks) through treatment of the nanocrystal surface. For the solid-state synthesis, we used commercially available salts and explored the reaction between the Ni and B sources while varying the synthetic parameters under mild and solvent-free reaction conditions. We show that pure phase Ni₃B and Ni₂B NCs can be obtained with high yield in the pure phase using as precursors NiCl₂ and Ni, respectively. Through extensive mechanistic studies, we show that Ni nanoclusters (1-2 nm) are an intermediate in the boriding process, while the metal co-reactant lowers the decomposition temperature of NaBH₄ (used as a reducing agent and B source). Size control can instead be exerted through reaction mediators, as seen from the differential nucleation and growth of Ni (clusters) or Ni_xB NCs when employing L- (amine, phosphine) and X-type (carboxylate) mediators. Applying surface engineering methods to our Ni_xB NCs, we stabilized them with inorganic (NOBF₄) or organic (borane tert-butyl amine, oleylamine) ligands in the appropriate solvent (DMSO, hexane). With this method, we produce stable inks for further solution processing applications. Our results provide tools for further development of catalysts based on Ni_xB NCs and pave the way for synthesizing other metal boride colloidal nanostructures.