The motivation for visualizing growth of nanostructures is to (1) determine what species is added to a growing nanowire (e.g. atoms vs. particles) and (2) determine the rate-limiting step for nanowire growth. Although visualization of nanostructure growth with liquid-cell transmission electron microscopy has shed light on the processes of atomic assembly and nanostructure growth, this technique is limited to relatively small nanoparticles because the thickness of the liquid cell is <150 nm. The Wiley lab was the first to show dark-field optical microscopy could serve as a powerful and inexpensive tool for measuring rates of nanostructure growth. Prior to these visualizations, it was not clear whether the species adding to the nanowires consisted of metal ions, reduced metal atoms, or metal clusters. In addition, it was not clear whether nanowire growth was limited by reaction kinetics or mass transport.
Video 1 below shows the growth of nanowires in a solution consisting of CuNO3, hydrazine, NaOH, and ethylenediamine in water. By analyzing videos like these we discovered the growth of nanowires in this reaction was limited by diffusion of Cu(OH)2− to the nanowire surface (Nano Lett., 2014). We have also used this method to demonstrate the effect of additives on nanowire growth rates and aspect ratio (Chem. Commun. 2014), as well as show nanowires can grow photocatalytically from copper oxide seeds (Chem. Mater. 2015).
Video 1. Growth of copper nanowires from copper nitrate with ethylene diamine, hydrazine, and alot (15 M) of NaOH
We continue to use in situ visualization to provide new insights into nanostructure growth. Most recently, we have used in situ visualization to show the growth of copper nanowires in an aqueous solution of ascorbic acid, CuCl2, and alkylamines is charge transfer limited (Chem. Mater. 2018), and thus has a completely different growth mechanism from the ethylenediamine-mediated synthesis of copper nanowires. Video 2 below shows that rather than growing from a spherical seed as in the ethylenediamine-mediated synthesis of nanowires, growth in the presence of tetradecylamine takes place from the center out. Increasing the alkylamine chain length from 14 to 18 carbons decreased the nanowire growth rate from 69 to 5 nm s-1, and decreased the current for the reduction of CuCl2 onto a copper surface. Molecular dynamics simulations provided by Kristin Ficthorn’s group (Chem. Eng., Penn State) show the greater passivation provided by longer alkylamines can be ascribed to the greater amount of energy required to remove a longer alkylamine from its monolayer on a copper surface. The more effective inhibition of electron transfer provided by longer alkylamines translates into slower nanowire growth rates, reduced yields, and shorter lengths. By relating the length of alkylamines to the nanowire growth rate, yield and aspect ratio, this work provides a novel link between the adsorption energy of molecules within a self-assembled monolayer and the resulting nanostructure morphology.
Video 2. Growth of copper nanowires from copper chloride with tetradecylamine and ascorbic acid
Ye, S.; Rathmell, A.R.; Stewart, I.E.; Ha, Y.-C.; Wilson, A.R.; Chen, Z.; Wiley, B.J. A Rapid Synthesis of High Aspect Ratio Copper Nanowires for High-Performance Transparent Conducting Films. Chem. Commun. 2014, 50, 2562-2564.
Ye, S.; Chen, Z.; Ha, Y.-C.; Wiley, B.J. Real-Time Visualization of Diffusion-Controlled Nanowire Growth in Solution. Nano Lett., 2014, 14, 4671–4676.
Alvarez, S.; Ye, S.; Flowers, P.F.; Wiley, B.J. Photocatalytic Growth of Copper Nanowires from Cu2O Seeds. Chem. Mater., 2015, 27, 570-573.
Kim, M.J.; Alvarez, S.; Tianyu, Y.; Tadepalli, V.; Fichthorn, K. A.; Wiley, B.J. Modulating the Growth Rate, Aspect Ratio, and Yield of Copper Nanowires with Alkylamines. Chem. Mater. 2018, 30, 2809-2818.