Revealing the Facet-Selective Electrochemistry That Drives Metal Nanostructure Growth
Authors: Kim Myung Jun , Cruz Mutya A. , Chen Zihao , Brown Micah , Fichthorn Kristen , Wiley Benjamin J. 
Published: May 2020

Since about 20 years ago, researchers have developed a variety of syntheses to control the morphology of metal nanostructures grown in solution.1 Control over nanostructure morphology has enabled the optimization of their properties for applications including electrocatalysis, solution-coatable transparent conducting films, conductive composites and plasmonic biosensors.1 To explain how anisotropic nanostructures can grow from metals with symmetric cubic crystal structures, researchers have often invoked the capping agent hypothesis, which proposes that the organic additives added to nanostructure syntheses are facet-selective in that they block atomic addition to certain facets but not others. However, direct evidence of such facet-selective blocking by organic additives has been lacking.
This talk will discuss efforts to use electrochemical measurements on single crystals to test the capping agent hypothesis. For a copper nanowire synthesis in which ethylenediamine was thought to act as a capping agent for {100} facets, single crystal measurements showed it in fact acted as a facet-selective promoter of Cu nanowire growth by keeping the {111} facets on the ends of a nanowire relatively free of surface oxidation.2 In another synthesis in which hexadecylamine is thought to serve as a capping agent for {100} facets, single crystal measurements showed that in fact hexadecylamine passivates both Cu(111) and Cu(100) equally.3 The presence of chloride ions in the synthesis selectively disrupts the alkylamine monolayer on the {111} facets on the ends of the nanowires, enabling anisotropic growth.
Addition of iodide ions to the same copper nanowire synthesis results in the formation of Cu microplates instead of nanowires. Single-crystal electrochemistry confirmed that a 75 micromolar concentration of iodide resulted in passivation of {111} facets on the top and bottom surface of microplates due to the displacement of chloride by iodide. At the same time, iodide activated the {100} facets on the sides of the microplates by disrupting the hexadecylamine monolayer on those facets. Results from density functional theory calculations indicate the stronger binding affinity of iodide causes it to displace both chloride and the hexadecylamine monolayer from copper surfaces. The binding of iodide to copper leads to a lower surface energy for {111} facets than {100} facets, resulting in the preferential atomic addition of Cu to the {100} facets on the sides of microplates.
These examples illustrate that organic additives do not always act as facet-selective capping agents, and that halide ions can play a dominant in driving the anisotropic growth of copper nanostructures.

1. Huo, D.; Kim, M.J.; Lyu, Z.; Shi, Y.; Wiley, B.J.; Xia, Y. One-Dimensional Metal Nanostructures: From Colloidal Syntheses to Applications. Chem. Rev., 2019, 119, 8972-9073.

2. Kim, M.J.; Flowers, P.F.; Stewart, I.E.; Ye, S.; Baek, S.; Kim, J.J.; Wiley, B.J. Ethylenediamine Promotes Cu Nanowire Growth by Inhibiting Oxidation of Cu (111). J. Am. Chem. Soc. 2017, 39, 277-284.

3. Kim, M.J.; Alvarez, S.; Chen, Z.; Fichthorn, K.A.; Wiley, B.J. Single-Crystal Electrochemistry Reveals Why Metal Nanowires Grow. J. Am. Chem. Soc., 2018, 140, 14740-14746.