Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu3-xP Nanocrystals.

N-heterocyclic carbenes (NHCs) are an important class of ligands capable of making strong carbon-metal bonds. Recently, there has been a growing interest in the study of carbene-ligated nanocrystals, primarily coinage metal nanocrystals, which have found application as catalysts for numerous reactions. The general ability of NHC ligands to positively affect the catalytic properties of other types of nanocrystal catalysts remains unknown. Herein, we present the first carbene-stabilized Cu3-xP nanocrystals. Inquiries into the mechanism of formation of NHC-ligated Cu3-xP nanocrystals suggest that crystalline Cu3-xP forms directly as a result of a high-temperature metathesis reaction between a tris(trimethylsilyl)phosphine precursor and an NHC-CuBr precursor, the latter of which behaves as a source of both the carbene ligand and Cu+. To study the effect of the NHC surface ligands on the catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu3-xP nanocrystals and found that they possess superior activity to analogous oleylamine-ligated Cu3-xP nanocrystals. Density functional theory calculations suggest that the NHC ligands minimize unfavorable electrostatic interactions between the copper phosphide surface and H+ during the first step of the hydrogen evolution reaction, which contributes to the superior performance of NHC-ligated Cu3-xP catalysts as compared to oleylamine-ligated Cu3-xP catalysts.