Dispersions and inks based on copper nanoparticles have raised extensive interest for printed electronics as copper holds promise for attaining high electric conductivity at low cost. Here, we use the decomposition of copper formate in oleylamine to produce a nanocolloid consisting of ∼4 nm copper nanocrystals, a size that is ideal to study the binding of ligands to nanocopper. Using solution 1 H NMR spectroscopy, we demonstrate that oleylamine binds to the surface of as-synthesized copper nanocrystals, thus stabilizing the dispersion by steric hindrance. We find that addition of a carboxylic acid to a purified nanocolloid induces an exchange between the originally bound oleylamine and the carboxylic acid as the surface-bound ligand. We provide evidence that the carboxylic acid dissociates upon binding to the copper nanocrystals. As such a process requires an amphoteric surface, a characteristic of a metal oxide but not of an elementary metal, we argue that ligand binding is determined by residual surface oxides and not by the pristine copper surface. Finally, we demonstrate that stable copper nanocolloids can be obtained in a variety of polar solvents by replacing oleylamine as a ligand by 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEAA). The inevitable oxidation of the small copper nanocrystals used here can be undone by mild thermal annealing, which especially in the case of MEEAA-stabilized nanocopper leads to significant grain growth. In this way, we turn an assynthesized dispersion of colloidal copper nanocrystals into a nano-ink that can be formulated to produce metallic copper strips by screen or inkjet printing.

We demonstrate the synthesis of copper nanocolloids by the thermal decomposition of copper formate in oleylamine under ambient conditions. By progressively increasing the loading of copper formate in the reaction mixture and imposing sufficiently high conversion rates, we demonstrate the formation of nanocrystals that are more than 97% pure copper without using an inert atmosphere. We attribute this result to the excess of copper formate relative to initially dissolved oxygen, and to the suppression of oxygen influx in the reactor. By adjusting the precursor and ligand concentrations, we obtain copper nanocrystals with sizes ranging from 10 to 200 nm. In view of applications, we show that the reaction can be upscaled to a 1 L reaction volume to produce over 1 50 grams of copper nanocrystals. Moreover, we formulate a conductive ink based on the copper nanocolloids obtained here with which we printed copper films exhibiting a resistivity of 23 µΩ • cm after thermal sintering under N 2 . We conclude that the approach presented here consititutes a next step towards the cost-effective production of metallic copper nanocrystals for printed electronics.