Formation of air-stable copper-silver core-shell nanoparticles for inkjet printing

Copper nanoparticles can be utilized as a low-cost replacement for silver and gold nanoparticles which are currently used in inkjet printing of conductive patterns. However, the main obstacle for using copper nanoparticles is their spontaneous oxidation at ambient conditions. Here we describe the synthesis of nonoxidizable copper nanoparticles by coating them with a silver shell, and inkjet printing of these particles. The formation of these core-shell nanoparticles is driven by a transmetalation reaction on the surface of copper nanoparticles, where the copper atoms present on the particles' surface are used as the reducing agent for the silver. This process results in formation of solely copper-silver core-shell nanoparticles, with no individual silver particles. It was found that coating 40 nm copper nanoparticles with a 2 nm layer of silver prevents oxidation of the copper core and preserves its metallic characteristic. Characterization of these nanoparticles by HR-TEM, SEM, EDS, XRD, spectrophotometry and XPS confirm the core-shell structure and their stability to oxidation. Inkjet printing of concentrated aqueous dispersions of these copper-silver nanoparticles was done on various substrates, and it was found that conductive and decorative patterns with metallic appearance, stable to oxidation (up to 150 degrees C) are formed.