The Role of Light and Alkylamines in Controlling the Growth of Copper Nanowires

The synthesis of various anisotropically assembled metal structures has increased at a rapid pace in the past two decades in order produce nanomaterials with unique properties that can meet the demands of future electronics and materials. However, the growth of many nanomaterials is still poorly understood. A key insight for the future of current anisotropic nanomaterials is to understand both their mechanism of growth as well as how to manipulate their dimensions, which controls their properties, for full utilization in different applications. One nanomaterial of interest are copper nanowires because of their favorable material characteristics for various applications.

Here I investigate the growth of copper nanowires in an EDA-NaOH and an alkylamine driven synthesis, the two most common types of solution phase syntheses of copper nanowires. The first synthesis I investigated was a seeded EDA-NaOH synthesis of copper nanowires where the nanomaterials were grown photocatalytically using visible light. We discovered that when exposed to visible light with an energy greater than the band gap of Cu2O, electrons excited from the valence band to the conduction band within the Cu2O octahedra seed reduce Cu(OH)2- onto the octahedra surface to form copper nanowires. This phenomenon was used to turn nanowire growth on and off with visible light and control the length of the copper nanowires. The phenomenon was also used to pattern the growth of nanowires on a substrate.

The second synthesis I investigated was the alkylamine driven copper nanowires synthesis, where I examined the role of alkylamine chain length on the dimensions and yield of the copper nanowires in the synthesis. Testing of 10 alkylamines (C6H15N-C18H35N) revealed that only those with 12–18 carbons induce anisotropic growth of copper nanowires. Tetradecylamine produced the highest yield of nanowires (54%) and octadecylamine the lowest (6%). The length of the Cu nanowires generally increased with decreasing alkylamine chain length and decreasing alkylamine concentration, whereas the diameter remained generally the same. To explain these phenomena, we explored the effect of the alkylamine chain length on the generation of reducing agent and the reduction of Cu. Alkylamine chain length has a negligible effect on the conversion of glucose to reductones, i.e. the reducing agents. In situ visualization of nanowire growth and electrochemical measurements indicate the growth of nanowires is kinetically-limited rather than mass transport-limited. In situ visualization of nanowire growth rates corroborate these measurements, with nanowires growing 14 times faster with tetradecylamine relative to octadecylamine. Alkylamine chain length also affects the adsorption of alkylamines on the Cu surface and the rate of Cu oxidation. These results indicate that the binding of alkylamines to Cu ions and their adsorption on Cu surfaces both likely play a role in modulating the rate of nanowire growth, and thus the yield and aspect ratio of nanowires.