Oxygen Reduction Allows Morphology-Tunable Copper Nanoparticle Electrodeposition from Aqueous Nanodroplets

Expanding the tunability of metallic nanoparticles in simple and cost-effective manners is essential for developing heterogeneous catalysts needed for the energy conversion systems of the future. Many current methods of switching between different nanoparticle morphologies and compositions include the use of surfactants, pH adjustments or other coreactants. One relatively unexplored and new route to tuning these nanoparticle properties involves taking advantage of the organic phase surrounding the aqueous droplets used in nanodroplet mediated electrodeposition techniques. These aqueous nanodroplets contain metal precursor salts that electrodeposit nanoparticles when they collide with a sufficiently biased electrode. Organic solvents such as 1,2-dichloroethane, known to have relatively high dioxygen solubilities compared to water, may provide an oxygen rich environment at the droplet interface, promoting heterogeneous oxygen reduction. In this work, the oxygen reduction reaction is used in the electrodeposition of copper to tune the resulting nanoparticle morphologies and compositions. These effects are also compared to those in bulk aqueous electrodeposition. The properties of the nanoparticles and the role of oxygen reduction in their synthesis are probed through electrochemical techniques, electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. When only reducing copper at the electrode, the resulting nanoparticles possess a range of cubic and spherical morphologies and multiple copper oxidation states indicative of zerovalent copper and copper oxide nanoparticles. When reducing both copper and oxygen, the electrodeposited nanoparticles possess a distinctive rod-like morphology with oxidation states and atomic ratios indicative of copper hydroxide. The latter nanoparticle morphology and composition was not attainable when copper was electrodeposited from a bulk aqueous solution at the same applied reducing potential. Our results show that one can take advantage of the fundamental electrochemistry taking place at the aqueous|organic|electrode interface to tune key properties of copper nanoparticles.