Phase directing ability of an ionic liquid solvent for the synthesis of colloidal Ni_2P nanocrystals

Abstract

In the past decade, nickel phosphide (Ni_2P) has garnered considerable interest due to the earth abundance of its constituent elements, its stability in alk. and acidic media, and its comparable catalytic activity to precious metal catalysts for the hydrodeoxygenation of biomass, hydrodesulfurization of petrol, and the hydrogen evolution reaction. However, current methods of synthesizing high-quality colloidal Ni_2P nanocrystals typically use flammable org. solvents. Ionic liqs. (ILs) are attractive candidates for use as solvents as a result of their nonflammable nature and high thermal and chem. stability. In addn., the low interfacial tension possessed by ILs tends to result in fast nucleation rates in the context of nanocrystal syntheses, which generally produces smaller particles with higher surface-area-to-vol. ratios. Herein, we report the high-temp. synthesis of Ni_2P nanocrystals using a 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-Tf_2N) IL solvent in combination with triphenylphosphine (PPh_3) as a low-cost and less-reactive phosphine source, yielding smaller, phase-pure Ni_2P nanocrystals in shorter times and lower temps. compared to the std. reported methods. When the IL solvent was replaced with the prototypical high-boiling org. solvent 1-octadecene under otherwise identical reaction conditions, mixed-phased products contg. the undesired nickel-rich Ni_(12)P_5 and the target Ni_2P were obtained. We will discuss the viability of the resulting Ni_2P nanocrystals synthesized in IL for the hydrogen evolution reaction, and extension of the synthesis method to the prepn. of Ni_(2-xCox)P nanocrystals.