Abstract
Exceptionally small and well-defined copper (Cu) and cuprite (Cu2O) nanoparticles (NPs) are synthesized by the reaction of mesitylcopper(I) with either H2 or air, respectively. In the presence of substoichiometric quantities of ligands, namely, stearic or di(octyl)phosphinic acid (0.1-0.2 equiv vs Cu), ultrasmall nanoparticles are prepared with diameters as low as ∼2 nm, soluble in a range of solvents. The solutions of Cu NPs undergo quantitative oxidation, on exposure to air, to form Cu2O NPs. The Cu2O NPs can be reduced back to Cu(0) NPs using accessible temperatures and low pressures of hydrogen (135 °C, 3 bar H2). This striking reversible redox cycling of the discrete, solubilized Cu/Cu(I) colloids was successfully repeated over 10 cycles, representing 19 separate reactions. The ligands influence the evolution of both composition and size of the nanoparticles, during synthesis and redox cycling, as explored in detail using vacuum-transfer aberration-corrected transmission electron microscopy, X-ray photoelectron spectroscopy, and visible spectroscopy.
Highlights
Copper nanoparticles (Cu NPs) have wide applications ranging from photonic materials for surface-enhanced spectroscopies and imaging techniques[1] to conductive inks for microelectronics[2−4] and as catalysts.[5−10] Ultrasmall NPs are of particular interest due to their very high surface areas and small internal volumes, which lead to modified intrinsic properties compared to larger NPs.[4,11] Copper NPs could be an attractive replacement for the heavier congeners, silver and gold, for reasons of the elemental abundance and relatively low cost
The smaller size measured by X-ray diffraction (XRD) compared to transmission electron microscopy (TEM) is consistent with the presence of twin or grain boundaries within the nanocrystals, as can often be identified by high-resolution (HR) TEM (Figure S8).[24]
Visible spectroscopy of Cu@L NP solutions showed surface plasmon resonance (SPR) peaks centered at 566−574 nm, consistent with the formation of small Cu NPs (Figure S9).[15−18] The Fourier transform infrared (FTIR) spectrum of a carefully dried sample of Cu@ Str, handled using anaerobic techniques, showed two signals for the carboxylate ligand
Summary
Copper nanoparticles (Cu NPs) have wide applications ranging from photonic materials for surface-enhanced spectroscopies and imaging techniques[1] to conductive inks for microelectronics[2−4] and as catalysts.[5−10] Ultrasmall NPs (i.e., particles in the size regime of 1−3 nm) are of particular interest due to their very high surface areas and small internal volumes, which lead to modified intrinsic properties compared to larger NPs.[4,11] Copper NPs could be an attractive replacement for the heavier congeners, silver and gold, for reasons of the elemental abundance and relatively low cost Their rapid oxidation, upon exposure to air, remains a major practical limitation. The organo-copper(I) reagent, mesitylcopper(I) (CuMes), is a promising precursor for Cu NPs;[53] reduction using hydrogen at moderate temperatures produces clean products without coordinating organic byproducts, allowing independent control over ligating additives.[17,54] Excess neutral amine ligands can stabilize such Cu
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