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The aspect ratio-dependent properties of gold nanorods are used in a variety of applications, but the cause of anisotropic nanorod growth remains unclear. Measurements utilizing single-crystal electrodes were collected to determine what additive(s) in pentatwinned gold nanorod syntheses are responsible for facet-selective atomic addition. With cetyltrimethylammonium in the absence of bromide, the rate of atomic addition to Au(100) and Au(111) single crystals was the same, and isotropic nanoparticles were produced. The addition of increasing concentrations of bromide suppressed the rate of atomic addition to Au(100) relative to Au(111) and increased the aspect ratio of gold nanorods. Bromide was a more effective passivator of Au(100) in the absence of cetyltrimethylammonium, indicating cetyltrimethylammonium does not cause facet-selective atomic addition. Cetyltrimethylammonium surfactant is still necessary for gold nanorod growth because it reduces the rate of gold ion reduction and stabilizes suspended nanoparticles against aggregation. 

Control over the nanoscopic structure of a material allows one to tune its properties for a wide variety of applications. Colloidal synthesis has become a convenient way to produce anisotropic metal nanostructures with a desired set of properties, but in most syntheses, the facet-selective surface chemistry causing anisotropic growth is not well-understood. This review highlights the recent use of electrochemical methods and single-crystal electrodes to investigate the roles of organic and inorganic additives in modulating the rate of atomic addition to different crystal facets. Differential capacitance and chronocoulometric techniques can be used to extract thermodynamic data on how additives selectively adsorb, while mixed potential theory can be used to observe the effect of additives on the rate of atomic addition to a specific facet. Results to date indicate that these experimental methods can provide new insights into the role capping agents and halides play in controlling anisotropic growth.