Synthesis of Cu Nanowires Using Ascorbic Acid in Aqueous Solution and Their Applications for Flexible Transparent Conductive Films

Abstract

Cu nanowires (NWs) have been extensively studied to produce flexible, low-cost and high-conductivity transparent films because of their flexibility, low cost and high conductivity. Most synthetic procedures are based on chemical reduction, which often requires high temperature, organic solvents, and toxic reagents such as N2H4. It is desirable to develop a cost-efficient and environmentally friendly synthetic procedure for practical use. Furthermore, Cu NWs with high aspect ratios are required to improve performances of the transparent conductive films. Previously, we synthesized Cu nanoparticles based on chemical aqueous reduction employing ascorbic acid (vitamin C) as a reducing agent instead of commonly used toxic reducing agents. The key to the wire-like morphologies is the adsorption of surface ligands to metal crystal facets during the nucleation of NWs. Surface ligands selectively adsorbs onto {100} facets and consequently the anisotropic growth along the <110> direction occurs. In addition, reaction speed control and additives are also important for syntheses of NWs with high aspect ratios. In this study, we examined to synthesize Cu NWs by chemically reducing aqueous Cu chloride complexes using ascorbic acid as a mild reducing agent instead of the commonly employed toxic reducing agent. We chose polyvinylpyrrolidone (PVP) and NaCl as a surface ligand and an additive, respectively. In addition, we controlled aspect ratios of Cu NWs by changing type of capping agent, optimizing additives and atmosphere. Cu chloride complexes were reduced to Cu metal through CuCl by ascorbic acid. The shape of the Cu metal particles highly depended on pH in the presence of PVP. Cu NWs were synthesized in the pH range of 3 to 4 while Cu nanoparticles were synthesized above pH 4. The reduction speed accelerated with increasing pH because the reduction capability of ascorbic acid increased with increasing pH. In fast reduction, all Cu chloride complexes were reduced to Cu particles before the anisotropic growth. In slow reduction, PVP effectively stabilized {110} facets of Cu rather than {111} facets, resulting in Cu NWs. The length and diameter of the Cu NWs were affected by atmosphere rather than additives and type of PVP. In the case of using ascorbic acid in aqueous solution, dissolved oxygen reacted with ascorbic acid to form hydrogen peroxide. Subsequently, hydrogen peroxide oxidized Cu metal surface and consequently prevented the anisotropic growth, resulting in Cu NWs with low aspect rations. By changing atmosphere from air to N2, we synthesized Cu NWs (length: 36 µm, diameter: 124 nm) from the reaction using ascorbic acid in aqueous solution. In our presentation, detailed results and discussion about the synthesis mechanism and performances as transparent conductive films will be introduced.