(26b) Optimized Continuous Millifluidic Surface Modification of Synthesized Silver Nanowires with Palladium

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: 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, 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 aspect ratio and high yield of AgNW. Millifluidic reactors are an improvement over batch reactors, but optimization is still required to produce 100% yield of the desired morphology. In addition, AgNWs chemical stability is 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 (Pd). 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, Design of Experiments (DOE) is used to optimize the millifluidic AgNW synthesis. The goal is to find the optimized conditions that produce close to 100% yield of synthesized AgNWs. The factors being examined are temperature, flowrate, and reagents concentration. Once the optimized reaction conditions are established, the AgNWs are synthesized using a millifluidic polyol process. The galvanic replacement reaction is then used to modify the surface of synthesized AgNWs. Based on previous studies, the desired amount of Pd that is galvanically replaced on a single AgNW is around 5%. Again, DOE is used to optimize the atomic weight percentage of Pd deposited on the AgNWs. The factors being examined are pH, flowrate, and reagents concentration. Once optimized conditions are set for both reactions, they are connected for continuous synthesis and surface modification. The outlet polytetrafluoroethylene (PTEF) tubing of the optimized AgNW polyol process is submerged in an ultrasonic bath to break up agglomeration and to cool down the suspension. Once the outlet PTEF tubing exits the ultrasonic bath, it is connected to the optimized galvanic surface modification millifluidic reactor via a cross junction. The AgNW suspension in the outlet PTEF tubing reacts with two other millifluidic tubes containing the Pd precursor solution (Pd(NO₃)₂.2H₂O aqueous solution) and stabilizing solution (ascorbic acid and polyvinylpyrrolidone (PVP) aqueous solution). After the completion of the galvanic replacement reaction, the Pd treated AgNWs are washed and separated, and redispersed in deionized water for further Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDX), and Mass Spectrometry (MS) characterizations to illustrate the formation of desired amount 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 (NSF Award ID 1939018).