Inkjet printing of a pH sensitive palladium catalyst patterns of ITO glass for electroless copper

• The pH-sensitive copolymer was used to self-reduce Pd nanoparticles.
• Cu pattern is deposited by ink-jet printing and electroless Cu deposition.
• The Cu wire was successfully fabricated, which may greatly apply to solar cell.

In this work, pH-sensitive chitosan (CTS)-g-polyvinyl acetate (PVAc) copolymer was used to self-reduce Pd nanoparticles (PdNPs). By heating the aqueous solution in the presence of metal ions and CTS-g-PVAc, the metal nuclei reduced within the core. Since the amino group in chitosan has a pKa value of ~ 6.5, thus, chitosan is positively charged (NH3 +) in acidic or neutral solution, whereas it turns to neutral in an alkaline solution. The pH-sensitive CTS chains not only function as stabilizing agent for noble metal nanoparticles in ink solution but also enhance the adhesion between PdNPs and substrate during the electroless deposition of Cu lines. Therefore, the Cu line with dramatically enhanced adhesion is formed on the surface of indium tin oxide (ITO) glass substrate without special pretreatment step before electroless deposition of Cu lines. The obtained pattern could be effectively controlled by adjusting the parameters including printing droplet spacing, operating voltages, substrate temperature, electroless plating time and temperature. Optical microscope observation showed that a continuous pattern was formed with a printing voltage of 37 V, space of drop was about 30 μm and the substrate temperature was 50 °C. When eletroless plating with plating solution at 60 °C and at pH of 12.5, the smallest metallic line formed in this study is about 40 μm.