Synthesis and Electrocatalytic HER Studies of Carbene-Ligated Cu_3–xP Nanocrystals

Abstract

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 applications 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 carbenestabilized 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 carbene ligand and Cu +. To study the effect of the NHC surface ligands on catalytic performance, we tested the electrocatalytic hydrogen evolving ability of the NHC-ligated Cu3-xP nanocrystals and found 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 likely contributes to the superior performance of NHC-ligated Cu3-xP catalysts as compared to oleylamine-ligated Cu3-xP catalysts. 2 Cu3-xP nanocrystal synthesis, characterization, and electrocatalytic HER studies will be discussed in detail, and density functional theory (DFT) calculations were carried out to elucidate the electronic effects of NHC ligands on catalysis. EXPERIMENTAL SECTION Materials and General Procedures. Reagents and solvents were purchased from commercial sources and used as received, unless otherwise noted. Benzimidazole (C7H6N2, 99%), 1-bromotetradecane (C14H29Br), copper(I) oxide (99.9%), potassium carbonate (K2CO3, anhydrous, 99%), and 1,4-dioxane were purchased from Alfa Aesar. Copper(I) bromide (98%) was purchased from Strem Chemicals. 1-octadecene (ODE, technical grade, 90%), oleylamine (technical grade, 70%), tris(trimethylsilyl)phosphine ((TMS)3P, 95%) and a phosphorus ICP standard (1000 ppm in 2% aqueous nitric acid) were purchased from Sigma-Aldrich. The copper ICP standard (1000 ppm in 2% aqueous nitric acid) was purchased from Perkin Elmer. Reactions involving air-or moisture-sensitive compounds were conducted under a nitrogen atmosphere by using standard Schlenk techniques. 1-Octadecene and oleylamine were degassed for 4 h at 105 °C and then overnight at room temperature prior to use.

Colloidal Nanocrystals as Electrocatalysts with Tunable Activity and Selectivity

“…As a second example, Tappan et al recently made use of an NHC ligand to greatly improve the HER catalysis of Cu 3−x P NCs, where the strong electron donation from the ligand reduced the electrostatic repulsion between Cu + and H + . 304 In another study, the HER activity of Ru NCs was demonstrated to increase in the presence of phenylpyridine ligands, in both acidic and basic media. 305 This result was attributed to the moderate metal−ligand binding strength, which permitted a larger number of H atoms to adsorb to the surface.…”

“…Specifically, the most electron-donating ligand induced the lowest overpotentials, smallest Tafel slopes, and charge-transfer resistances; the overpotential was regulated by 0.5 V just by tuning the ligands, thus demonstrating the huge potential of such approaches. As a second example, Tappan et al recently made use of an NHC ligand to greatly improve the HER catalysis of Cu 3–x P NCs, where the strong electron donation from the ligand reduced the electrostatic repulsion between Cu + and H + . In another study, the HER activity of Ru NCs was demonstrated to increase in the presence of phenylpyridine ligands, in both acidic and basic media .…”

Photoluminescence properties of cuprous phosphide prepared through phosphating copper with a native oxide layer

“…However, as a p-type semiconductor material, its electrical and optical properties and applications have been neglected. [12][13][14][15][16][17][18][19] Mu et al synthesized self-doped colloidal copper phosphide (Cu 3ÀX P) nanocrystals in solution using a "one-pot method" and found that Cu 3ÀX P emitted uorescence at 550-650 nm. 20 The above reports indicate that Cu 3ÀX P is a kind of uorescent material, which has potential uses in uorescence emission and photodetectors.…”

Probing the Ligand Exchange of N-Heterocyclic Carbene-Capped Ag2S Nanocrystals with Amines and Carboxylic Acids

“…Comparatively little work has been done on non-metal NHC-stabilized nanocrystals (e.g., binary metal chalcogenide or phosphides). We were the first to report the synthesis of NHC-stabilized coinage metal chalcogenide (i.e., Ag 2 S, Ag 2 Se, Cu 2– x S, CuS, Cu 2– x Se, and CuSe) and phosphide (i.e., Cu 3– x P) nanocrystals starting from NHC-MBr (M = Ag or Cu) precursors in the absence of any additional supporting ligands. ,, We observed large differences in the colloidal stability of NHC-Ag nanocrystals (∼1 week) versus NHC-Ag 2 S nanocrystals (>6 months) . Density functional theory (DFT) calculations revealed significantly higher binding affinities between the NHC and the Ag 2 S nanocrystal surface over the Ag nanocrystal surface, with NHC binding energies ( E b ) of 0.57–0.70 and <0.30 eV, respectively.…”