Nickel Boride (NiXB) 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 degrees C) yield amorphous polydisperse nanoparticles, while phase purity is an issue at higher temperatures. Among all the metal-rich borides, nickel borides (NixB) 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 NixB 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 NixB NCs when employing L-(amine, phosphine) and X-type (carboxylate) mediators. Applying surface engineering methods to our NixB 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 NixB NCs and pave the way for synthesizing other metal boride colloidal nanostructures.

Automated summary

Metal borides, such as nickel borides (NixB), are widely used materials in industry. However, their exploration at the nanoscale has been limited due to synthesis difficulties. In this study, a solid-state method was developed to synthesize NixB nanopowders and convert them into colloidal suspensions. The synthesis was achieved using commercially available salts and adjusting the synthetic parameters under mild and solvent-free conditions. Through mechanistic studies, it was found that Ni nanoclusters are intermediates in the boriding process, and size control can be achieved through reaction mediators. The stabilized NixB nanocrystals were then converted into stable inks for further solution processing applications, providing tools for the development of NixB-based catalysts and other metal boride colloidal nanostructures.