Ligand Binding to Copper Nanocrystals: Amines and Carboxylic Acids and the Role of Surface Oxides

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 1H 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 as-synthesized dispersion of colloidal copper nanocrystals into a nano-ink that can be formulated to produce metallic copper strips by screen or inkjet printing.