Synthesis of Oxidation-Resistant Copper Nanoparticles for Fabricating Conductive Patterns in Aqueous Solution
Authors: Suzuki Ippei , Yokoyama Shun , Motomiya Kenichi , Takahashi Hideyuki , Tohji Kazuyuki 
Published: February 2020

Ag and Cu nanoparticles (NPs) inks have attracted considerable interest for formations of conductive patterns on various electronic devices because of their high conductivity and low sintering temperature. Ag NPs have been most extensively studied and utilized due to high stability in air and various solvents. Although Cu NPs have been expected to be one of next generation materials because Cu is much lower cost than Ag, synthesis procedures of Cu NPs and formation methods of conductive patterns are still expensive. Since Cu NPs easily oxidize in aqueous solution, the synthesis and formation methods require organic solvent, resulting in the high cost. Therefore, we have developed a synthesis of Cu NPs based on chemical reduction of Cu complexes in aqueous solution and modification of Cu NPs using hydroxy and amino acids to circumvent the surface oxidation. In addition, we have examined a formation method of a conductive pattern using a Layer-by-Layer (LbL) technique in aqueous solution.
We reported that metal NPs could be synthesized by controlling and reducing metal complexes in the aqueous solution based on metal complex calculations using the critical stability constants[1, 2]. Besides, in this system, complex agent also works as capping agent. We selected hydroxyl and amino acids as complex and capping agent because the acids have potential to prevent surface oxidation of metal NPs surface when the acids absorb on metal NPs surface stably and strongly[3]. Cu NPs are synthesized by using water, hydroxy and amino acids, ascorbic acid used as solvent, complex/capping agent and reducing agent, respectively. The sizes of Cu NPs synthesized by using hydroxyl acids as complex and capping agent is under 100nm whereas Cu NPs with a diameter above 300 nm are synthesized by using amino acids. Cu-hydroxyl acids complexes are suitable for Cu NPs synthesis because the reduction potential of Cu-hydroxyl acids complexes is lower than that of Cu-amino acids. However, hydroxy acids absorbed on the Cu NPs surface are removable in aqueous solution and consequently the Cu NPs oxidize. Thus, ligand exchanges from hydroxy acids to amino acids were examined after the Cu NPs synthesis. Amino acids on the Cu NPs surfaces are stable in aqueous solution and prevent the surface oxidation. We have found that the amino acids modified Cu NPs are suitable in aqueous process.
The LbL technique can assembles positive and negative charged materials alternately on various substrates due to electrostatic attraction. Positive charged polydiallyldimethylammonium chloride (PDDA) and negative charged copper nanoparticles capped by amino acid were employed for a formation method of a conductive pattern on ITO substrates. The substrates were immersed in PDDA aqueous solution to form positively charged PDDA layer on the ITO surface and then transferred in aqueous dispersion of the Cu NPs. The Multilayer film thickness of PDDA/Cu NPs grows to be 1.0 µm by repeating the LbL process 15 times without Cu NPs surface oxidation. This finding creates a path of formation methods of conductive patterns using Cu NPs in aqueous solution.
In our presentation, detailed results about synthesis and resistance to surface oxidation of the Cu NPs using hydroxyl and amino acid and the LBL technique will be introduced. This work was supported by JSPS KAKENHI Grant-in-Aid for Young Scientists (B) 15K16155.
References
[1] H. Takahashi, N. Konishi, H. Ohno, K. Takahashi, Y. Koike, K. Asakura, A. Muramatsu, Applied Catalysis A: General, 392 (2011) 80-85.
[2] S. Yokoyama, H. Takahashi, T. Itoh, K. Motomiya, K. Tohji, Advanced Powder Technology.
[3] D.Y. Deng, Y.X. Jin, Y.R. Cheng, T.K. Qi, F. Xiao, ACS Applied Materials & Interfaces, 5 (2013) 3839-3846.