Aqueous electrophoretic deposition of citric-acid-stabilized copper nanoparticles

Citric-acid-stabilized copper (Cu) nanoparticles (NPs) were deposited on indium tin oxide (ITO) substrates using aqueous electrophoretic deposition (EPD). Usually, Cu NPs are unstable in aqueous solution because they oxidize and aggregate easily, which is why aqueous EPD has not been used before to deposit them on substrates. However, we showed that treating Cu NPs using citric acid in aqueous solution removed surface oxides and stabilized the surfaces sufficiently. After the treatment, the Cu NPs remained highly stable in aqueous solution even after one week, and this stability was sustained under an electric field in aqueous solution. The Cu NPs were deposited rapidly on ITO substrates by aqueous EPD at a deposition rate that increased with current density (0.25, 0.46, and 0.96 mg/cm²/min at 0.10, 0.20, and 0.30 mA/cm², respectively). The Cu NP films obtained by the rapid deposition at 0.30 mA/cm² contained too much water, leading to cracked and low density films. Instead, we demonstrated that a current density of 0.20 mA/cm² resulted in uniform and high density Cu NP films.