Adsorption of a quaternary ammonium surfactant on Au(100)

Electrochemical investigations of metal nanostructure growth with single crystals

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.

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 1. Au(111)

The coadsorption of the anionic and cationic components of a model quaternary ammonium bromide surfactant on Au(111) has been measured using the thermodynamics of an ideally polarized electrode. The results indicate that both bromide and trimethyloctylammonium (OTA(+)) ions are coadsorbed over a broad range of the electrical state of the gold surface. At negative polarizations, the Gibbs surface excess of the cationic surfactant is largely unperturbed by the presence of bromide ions in solution. However, when the Au(111) surface is weakly charged the existence of a low-coverage, gaslike phase of adsorbed halide induces an appreciable (~25%) enhancement of the interfacial concentration of the cationic surfactant ion. At more positive polarizations, the coadsorbed OTA(+)/Br(-) layer undergoes at least one phase transition which appears to be concomitant with the lifting of the Au(111) reconstruction and the formation of a densely packed bromide adlayer. In the absence of coadsorbed halide, the OTA(+) ions are completely desorbed from the Au(111) surface at the most positive electrode polarizations studied. However, with NaBr present in the electrolyte, a high surface excess of bromide species leads to the stabilization of adsorbed OTA(+) at such positive potentials (or equivalent charge densities).

Quaternary ammonium bromide surfactant adsorption on low-index surfaces of gold. 2. Au(100) and the role of crystallographic-dependent adsorption in the formation of anisotropic nanoparticles

A qualitative and quantitative description of the coadsorption of a quaternary ammonium bromide surfactant on Au(100) has been determined using electrochemical techniques. Cyclic voltammetry reveals that both the cationic surfactant ion and its halide counterion are adsorbed on the surface of unreconstructed Au(100) over a wide range of electrode potentials or charge densities. The relative Gibbs excesses of the cationic and anionic components of octyltrimethylammonium (OTA(+)) bromide have been determined using the thermodynamics of ideally polarized electrodes. Coadsorbed OTA(+) does not strongly affect the behavior of bromide layers on Au(100) with low-coverage films being replaced by commensurate overlayers at positive electrode charge densities. The presence of surface bromide allows for the stabilization of adsorbed OTA(+) at positive polarizations. Furthermore, charge-induced phase changes in the bromide layer lead to subtle but appreciable changes in the surface excesses of OTA(+) ions which is consistent with a hierarchical model of surfactant adsorbed upon a halide-modified Au(100) surface. A comparison of the OTA(+) adsorption isotherms on Au(100) and Au(111) reveals that the presence of coadsorbed bromide does not lead to preferential accumulation of cationic surfactant ions on a particular crystal facet. These results are inconsistent with explanations of anisotropic nanoparticle formation that invoke a thermodynamic argument of preferred surfactant adsorption on different crystal facets of an embryonic nanoparticle seed crystal.