Understanding shape control in gold nanoparticles from molecular dynamics simulations

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In this proposal we aim to a microscopic understanding of the initial steps in the gold nanorods growth using an atomistic simulation approach. The innovative aspect here is the atomistic description of the surface of the gold nanoparticle and their interaction with the electrolyte solution where they are dispersed, namely with solvent molecules, surfactants and ions.A fundamental goal in material science is the design and synthesis of materials with tailored shape and size. The last decade has seen a tremendous progress in the synthesis of gold and silver nanoparticles of various sizes and shapes. Nanoscale metallic particles exhibit a variety of functional properties with promising applications in electronics, photonics, biologically inspired nanocomposites, (bio)chemical sensing and imaging and drug delivery.The fundamental step to control the shape of metal nanoparticles, is the microscopic understanding of the crystal growth. The crystallization is a non-equilibrium process and diffusion, activation barriers, surface energies and kinetics of the absorption on a specific crystal, all influence the shape of the the resulting particle providing a still open challenge for physicists and chemists. The particle morphology and crystal structure are controlled in a complex and currently poorly understood process by organic surfactants and metal/halide ions concentration.The growth of a rod-like shape implies that the rod ends grow at a faster rate than the sides. Different growth mechanisms have been proposed to explain the growth of gold nanorods in solution. Among the different proposals is the interplay of sterical chemical factors, including the specif interaction of the metal surfaces with the surfactants. The questions we would like to address here include: - surfacatants micelles formation, with particular emphasis to the the ion loading mechanism and the z-potential which builds up on the micelles;- packing of the surfactant bilayer on the metal surface, in particular role of packing for different facets;- the electrostatic double layer: we would like to understand (i) the counterions distribution; (ii) the potential drop across the interface, comparing in particular the difference in the potential for the different surfaces; - the description of the diffusion mechanism across the bilayer; in particular we want to investigate the interaction between the surfactant double layer and the micelles-bound gold ion.The project will be performed in close collaboration of the experimentalists. Using simultaneous optical spectroscopy and time-resolved small-angle X-ray scattering at a synchrotron X-ray source, they can directly monitor the anisotropic growth kinetics of gold and gold-copper nanorods and extract the growth parameters for both crystal directions (along the rod‘s long and short axes) independently.