Copper nanoparticles: aqueous phase synthesis and conductive films fabrication at low sintering temperature

Conductive copper nanoinks can be used as a low-cost replacement for silver and gold nanoinks that are used in inkjet printing of conductive patterns. We describe a high-throughput, simple, and convenient method for the preparation of copper nanoparticles in aqueous solution at room temperature. Copper acetate is used as the precursor, hydrazine as the reducing agent, and short chain carboxylic acids as capping agents. The concentration of the carboxylic acid plays a key role in the preparation of such copper nanoparticles. Stable copper nanoparticles with a diameter of less than 10 nm and a narrow size distribution were prepared when high concentrations of lactic acid, citric acid, or alanine were used. Thermogravimetric analysis results showed that any lactic acid or glycolic acid adsorbed on the surface of the copper nanoparticles can be removed at a relatively low temperature, especially, glycolic acid, which can be removed from the surface at about 125 °C. Highly conductive copper films prepared using lactic acid and glycolic acid as capping agents were obtained by drop coating a copper nanoparticle paste onto a glass slide followed by low temperature sintering. The electrical resistivity of the copper film using glycolic acid as the capping agent was 25.5 ± 8.0 and 34.8 ± 9.0 μΩ·cm after annealing at 150 and 200 °C for 60 min under nitrogen, respectively. When lactic acid was used as the capping agent, the electrical resistivity of the copper films was 21.0 ± 7.0 and 9.1 ± 2.0 μΩ·cm after annealing at 150 and 200 °C for 60 min under nitrogen, respectively, with the latter being about five times greater than the resistivity of bulk copper (1.7 μΩ·cm).