Solvent-Free Mechanochemical Synthesis of Ultrasmall Nickel Phosphide Nanoparticles and Their Application as a Catalyst for the Hydrogen Evolution Reaction (HER)

The photocatalytic (PC) splitting of water into oxygen and hydrogen has attracted considerable attention in the past decade. Despite the promise of hydrogen gas as a new energy resource, the environmentally friendly design of viable catalysts with good morphological and size control remains a continual challenge. Of the many classes of available catalysts, metal phosphides are a low-cost and potentially accessible option as catalysts for this reaction, in comparison to the traditionally used precious-metal catalysts. However, the synthesis of metal phosphide nanomaterials currently involves the use of highly reactive phosphorus sources at high temperatures in organic solvents. Herein, we demonstrate the application of sodium phosphide as an excellent solid-state phosphorus source for the synthesis of nickel phosphide nanoparticles below 3 nm in diameter, a size range previously unheard of for the mechanochemical synthesis of metal phosphides. These nanoparticles in turn showed success for the hydrogen evolution reaction, using graphitic carbon nitride as a photocatalytic support, generating 233.9 mu mol g(-1) h(-1) of hydrogen, using broad spectrum light at room temperature after only 3 h, and being readily recyclable and reusable, without any decrease in reactivity. This mechanochemical method shows a mass intensity (MI) value over 2.5 times lower than those of traditionally used solution-based methods, even after workup and washing of the product. Upon scaling up the reaction to a 2.5 g scale, we were able to further improve the MI to 3 times lower than those of traditional solvent-based methods, while the energy demand was reduced over 18-fold.