Electrochemical Evaluation of Surfactants Relevant to Ligand Stabilized Nanoparticles

Abstract

A very good understanding of nanoparticle-ligand interactions is a key step toward obtaining mechanistic insight into the formation of ligand protected metal nanoparticles and their stability. Since a direct investigation of the processes happening at the nanoparticle/solution interface is practically very difficult, an alternative approach is desirable. The use of interfacial electrochemistry is very promising in this direction as the ligand interactions on an electrode surface is analogous to those occurring on a nanoparticle surface and, importantly, they can be quantified using the thermodynamics of ideally polarized electrodes. An attempt to explain certain phenomena in the nanoparticle domain with the help of interfacial electrochemistry is the underlying theme of this research. This research is primarily focused on two main projects; (1) Halide induced aggregation of 4-dimethylaminopyridine (DMAP) stabilized gold nanoparticles and (2) Addressing the popular perception concerning the growth mechanism of quaternary ammonium surfactant stabilized gold nanorods.

Halide induced aggregation of DMAP monolayer protected gold nanoparticles is investigated by studying the electrochemical adsorption of DMAP and halide ions on analogous gold electrode surfaces. A quantitative evaluation of the adsorbed species on the electrode surface is provided using the thermodynamics of ideally polarized electrodes and the results explain observations made in the nanoparticle domain. Additionally, a quantitative evaluation of the pH dependent adsorption of dimethylaminopyridine on Au(111) surface is discussed to provide a much better understanding of the adsorption behavior of this molecule on gold surfaces.

The current perception on the mechanism of gold nanorod growth is that anisotropy results from the preferential adsorption of quaternary ammonium bromide surfactant on different facets of the nanoparticle seed crystal. A systematic evaluation of quaternary ammonium bromide adsorption on different crystal planes of gold is provided to evaluate this popular postulate. As the low index (100) and (111) crystal planes are the most pertinent to these nanorod growth mechanisms, single crystal electrodes with these crystallographic surfaces have been prepared for this investigation. The quaternary ammonium surfactant chosen is octyltrimethylammonium with a nonspecifically adsorbing triflate counterion. An electrochemical evaluation of the adsorption behavior of this surfactant on the respective crystal planes in the absence of any other specifically adsorbing species is provided as an initial survey. Subsequently, a quantitative evaluation of quaternary ammonium bromide adsorption on these crystal planes is also discussed.