Silver nanowires (AgNWs) hold promise as the conductive material in conductive inks suitable for printing processes to create conductive patterns thanks to their three combined characteristics including high conductivity, transparency, and mechanical flexibility. The cost associated with raw material production for this application is among the most important factors in material selection and AgNWs can be synthesized simply in small scale batch process. However, rapid synthesis of high aspect ratio of AgNWs with high concentration is still a challenge that makes a huge gap in their large-scale practical applications. Moreover, generally a mixture of AgNWs and other silver nanostructures in organic solvent are obtained that need to be separated which makes the cost of purification and recovery of synthesized AgNWs a further barrier towards their practical applications. Application of a millifluidic reactor is a method to overcome the challenges in batch AgNWs synthesis through control of a uniform chemical and thermal environment in a small reaction volume to produce high quality corresponding to high aspect ratio, and high yield or purity. Their chemical stability is also critical in practical applications as their oxidation can not only weaken mechanical stability but also increase the sheet resistance of transparent conductive films (TCFs). To eliminate or mitigate the oxidation concern, the surface of AgNWs can be alloyed or coated with graphene, graphene oxide, titanium oxide, zinc oxide, and noble metals such as gold and palladium. The formation of high reduction potential alloys on the surface of AgNWs is a promising technique to improve the performance and stability of AgNW-based TCFs. It has been shown that the chemical stability of AgNWs towards oxidation can be achieved with very small amount of Pd as the noble metal modifier, which is promising for improving the chemical stability of the synthesized AgNWs.
In this study, the AgNWs are synthesized using millifluidic polyol process. The galvanic replacement reaction using sugar substitutes as reducing agent at room temperature is used to modify the surface of synthesized AgNWs continuously. The outlet polytetrafluoroethylene (PTEF) tubing of the millifluidic reactor of polyol process is coiled and replaced in ice bath to quench and cool down the synthesized AgNWs suspension. Another PTEF tubing is connected to the flow of cooled synthesized AgNWs suspension through a T-junction. This PTEF tubing is connected to two millifluidic tubes and syringe pumps that inject the Pd precursor solution (Pd(NO3)2.2H2O aqueous solution) and reducing agent (raw Maui Turbinado brown sugar aqueous solution) to the flow of cooled synthesized AgNWs suspension. After the completion of the galvanic replacement reaction, the Pd treated AgNWs are washed and separated and redispersed in deionized water for further TEM, SEM, and EDX characterizations to illustrate the formation of PdNPs on the surface of AgNWs.
A novel low-cost synthesis of modified AgNWs with high chemical stability against oxidation ready to be coupled with conductive ink formulation and writing machine to create TCFs is illustrated, which is capable to be scaled up for large-scale industry-relevant demand production. This study is supported by National Science Foundation.