Single-Crystal Electrochemistry Explains Why Cu Nanoplates Form in the Presence of I-
Authors: Cruz Mutya A. , Kim Myung Jun , Wiley Benjamin J. 
Published: February 2020

Shape-directing agents such as halide ions and organic adsorbates play an important role on the growth of metallic nanocrystals. Halide ions, in particular, have been of interest due to their importance in determining the shape of metallic nanocrystals. A systematic study showed that halide ions determine the final shape of Au nanocrystals such as cubes, plates, and various polyhedrons.1 Additionally, it has been demonstrated that the diameter of Ag nanowires can be tuned by modulating the concentration of Br- in a polyol synthesis.2 Finally, it was reported that varying concentrations of I- impurities found in different brands of CTAB affect the formation of gold nanorods.3 However, although we know that halide ions can dictate the shape of nanocrystals, there is no direct evidence clarifying the role of halide ions and their interaction with other organic shape-directing agents on metal surfaces.
In a recently published article, Kim et al demonstrated the power of single-crystal electrochemistry in elucidating the previously-misunderstood mechanism of Cu nanowire growth.4 Until recently, it was believed that capping agents selectively bind onto {100} facets—which make up the sides of penta-twinned Cu nanowires—and inhibit atomic addition, thereby directing anisotropic growth on the {111} facets that make up the ends of the nanowires.5,6 However, there were limited experimental results that supported this hypothesis. By using Cu(111) and Cu(100) single-crystal electrodes to simulate the crystal facets of Cu nanowires, Kim et al demonstrated that HDA (a shape-directing agent) passivated both the Cu(111) and Cu(100) surfaces equally, unlike previously hypothesized.4 In fact, they found that it was Cl- that selectively facilitated atomic addition on Cu(111) by interrupting the passivating HDA self-assembled monolayer (SAM), leading to anisotropic growth.
Using a similar method, this presentation aims to demonstrate how and why Cu nanoplates grow when I- is introduced to a Cu nanowire growth solution using single-crystal electrochemistry. Synthetic results have shown that when a minimum concentration of NaI (75 µM) is introduced to an HDA-Cl--based synthesis for Cu nanowires, Cu nanoplates can be obtained (Fig. 1). Preliminary electrochemical measurements with a polycrystalline Cu electrode showed that I- was more effective in disrupting the HDA SAM compared to Cl-. In this presentation, we will introduce the interactions between I-, Cl-, and HDA on Cu single-crystal electrodes to understand why the addition of I- ions causes a change in nanocrystal structure from nanowires to nanoplates.
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Figure 1. SEM images of (a) nanowires obtained from the HDA-Cl--based synthesis, and (b) nanoplates resulting from the addition of I- to the same synthesis.

Figure 1