Determining the Facet-Selective Electrochemistry That Drives Anisotropic Growth of Cu Nanowires
Authors: Wiley Benjamin J. , Kim Myung Jun , Alvarez Samuel 
Published: February 2020

Cu nanowires are being studied for use in many applications, including transparent/flexible/stretchable conductors, electrochemical sensors, electrocatalysts, and batteries.1-3 Solution-phase synthesis of Cu nanowires could enable the low-cost production of Cu nanowires at large scales under mild reaction conditions. Despite many articles on the synthesis of Cu nanowires, the driving force for anisotropic growth of nanowires and the rate limiting step in nanowire growth is not well understood.
The solution-phase synthesis of Cu nanowires is based on two electrochemical reactions: the reduction of Cu ion complexes, and the oxidation of reducing agents. In addition, organic additives are necessary for inducing the anisotropic growth of Cu nanowires; such additives are typically regarded as facet-selective capping agents.4 It is thought that capping agents selectively adsorb on the side of nanowires and suppress deposition of Cu while leaving the ends of growing nanowires active for Cu deposition.4 The ends and sides of Cu nanowires consist of (111) and (100) surfaces, respectively. Although the anisotropic growth of Cu nanowires is hypothesized to be the result of facet-dependent electrochemical reactions, there has been very few electrochemical tests of the capping agent hypothesis.
Cu nanowires can be produced with (I) ethylenediamine (EDA) in an alkaline solution5,6 or with (II) an alkylamine (e.g., hexadecylamine) in a neutral solution.7,8 In synthesis I, hydrazine (N2H4) is used as the reducing agent for the reduction of a hydroxocuprate complex to metallic Cu. It has been hypothesized that EDA acts as a capping agent, but we recently proved by electrochemical analyses with Cu(111) and (100) single crystals that EDA is actually a facet-dependent promotor for Cu deposition.6 EDA prevents surface oxidation of Cu(111) (i.e. the ends of Cu nanowires) more effectively than Cu(100) surface, which enables anisotropic deposition of Cu onto the ends of growing nanowires. The growth of Cu nanowires in this alkaline synthesis is mass-transport limited.5,6

In synthesis II, alkylamines act as capping agents, driving anisotropic growth by selectively adsorbing on the sides of Cu nanowires. Glucose or ascorbic acid have been reported as reducing agents in this reaction,7,8 however it was found that glucose cannot act as the reducing agent.9 Instead, the Malliard reaction between glucose and alkylamines produces reductones (such as ascorbic acid), which can reduce Cu-alkylamine complexes to metallic Cu. Interestingly, it was also found that chloride ions, such as in the CuCl2 precursor, are necessary for anisotropic growth (Figure 1). Without chloride ions, only Cu nanoparticles were obtained. Thus, it seems the presence of chloride is necessary for the facet-selective adsorption of alkylamine, but more work is necessary to understand the role of chloride and alkylamine in the Cu nanowire synthesis.
This presentation will discuss the fundamental chemistry behind the alkylamine-mediated growth of Cu nanowires. New evidence will be presented to (1) show alkylamine-mediated growth of Cu nanowires is kinetically limited due to surface adsorption of alkyamines, and (2) clarify the role halide ions play in the anisotropic growth of Cu nanowires.
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Figure 1. Cu nanostructures synthesized with (a) CuCl2, (b) Cu(NO3)2, and (c) Cu(NO3)2+NaCl. Hexadecylamine and glucose were used as capping and reducing agents.

Figure 1