Reversible Redox Cycling of Well-Defined, Ultrasmall Cu/Cu₂O Nanoparticles

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.