Engineering the Solid State Synthesis and Processing of Nickel Boride with Enhanced Functionality

Nickel boride NixBy has shown potential as an efficacious catalyst for a broad range of systems (hydrogenations, hydrogen, and oxygen evolution reactions) under challenging conditions (such as high pH or high temperatures). Preparing the nanoscale analogues of nickel borides in an attractive prospect as the increased surface area and creation of more active sites will enhance their catalytic activity. However, the nanostructure of NixBy remains underdeveloped mainly due to the notoriously difficult and energy demanging synthetic process which is not industrially compatible.[1-3] Recent works have shown that synthetic methods at lower temperatures (<400 ºC) yield amorphous polydisperse nanocrystals (NCs), while phase purity remains an issue at higher temperatures. Here, a simple, scalable synthesis is demonstrated to obtain a phase-pure nanocrystalline NixBy. Through the solid-state reaction of either metallic Ni0 or NiCl2 with NaBH4 at a relatively low temperature (400oC) under atmospheric pressure, crystalline nanosized Ni2B and Ni3B could be obtained with high yield in pure phase and with narrow size distribution (15-30 nm). Through extensive mechanistic studies, we show that Ni nanoclusters (approx. 1 nm) are intermediate in the boriding process, while the halide precursors lower the decomposition temperature of the NaBH4 (used as a reducing agent and B source). We further explore the size control using reaction mediators, and we probe the differential nucleation and growth of Ni (clusters) or NixBy NCs while using L (amine, phosphine) and X-type (carboxylate) mediators. The synthesized Ni3B nanopowder can undergo surface functionalization with inorganic ligands in polar solvents, forming a stable ink. Furthermore, the Ni3B nanocrystals ink can be ligand-exchanged with organic ligands in a non-polar low-boiling point solvent. This work opens the door for the large-scale production of NixBy nanocrystals solution-processable inks to make it a commercially viable alternative catalyst to noble-metals and pave the way for other metal borides colloidal nanostructures