Understanding shape control in gold nanoparticles from molecular dynamics simulations
Physical Chemistry of Solids and Surfaces, Material Characterisation
Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Final Report Abstract
Gold nanoparticles are widely used in many areas such as photothermal cancer therapy, biochemical sensing and medical imaging due to their size and shapedependent optical properties. Directly manipulating and controlling the size and shape of metal nanoparticles is, therefore, a key step for their tailored applications. We use atomistic molecular dynamics simulations in order to understand the microscopic origin of the asymmetric growth mechanism in gold nanorods. The different factors influencing the growth are selectively included in the models in order to unravel the role of the surfactants and ions. In particular, both infinite planes models, representing the mature stage of the growth, and nanoseed models in the size of a few nanometers are used to understand how asymmetry between the different facets of the nanorods builds up. We find that on all the investigated surfaces, cetyltrimethylammonium bromide forms a layer of distorted cylindrical micelles where channels among micelles provide direct ion access to the surface. When the low index facets are examined, a lower surface density of surfactant is found on the Au(111) facets, with respect to the Au(100) and Au(110) facets. In addition an higher electrostatic potential difference is measured between the gold surface and the bulk solution at the Au(111) interface, which would provide a stronger driving force for the negatively charged AuCl2- species, which are reduced at the gold surface. The two factors together would result into higher diffusion flux of the gold reactant toward the Au(111) facets and could result into a preferential growth of the Au(111) surfaces. In order to investigate if the anisotropy is preserved at the nanoscale, we also investigate penta-twinned decahedral seeds and cuboctahedral seeds, whose dimensions are comparable to those of the cetyltrimethylammonium bromide micelle. We find that the asymmetry in adsorption behavior between the different low-index facets, which characterized the infinite planes, shows up even more dramatically on the nanoseeds. Indeed, if on the (100) and (110) facets of both the cuboctahedral and penta-twinned seeds the surfactant layer presents a structure similar to that observed on the infinite plane, basically no micellar adsorption is found on the small Au(111) facets, which e.g. form the tips of the penta-twinned nanoseeds. This huge difference in the coverage of the early stage seeds would then promote a symmetry breaking in the penta-twinned seeds and therefore an anisotropic growth of nanocrystals. Simulations also provide a microscopic understanding of the role of halides in controlling the anisotropic growth. In particular we find that bromide adsorption in on the gold nanorods is not only responsible for surface passivation, but also acts as the driving force for the micelle adsorption and stabilization on the gold surface in a facet-dependent way. Partial replacement of bromide by chloride decreases the difference between facets and the surfactant density. Finally if only chloride is present in the growing solution, no halides or micellar structure protect the gold surface and further gold reduction should be uniformly possible.
Publications
- Understanding the microscopic origin of gold nanoparticles anisotropic growth from molecular dynamics simulations. Langmuir 2013
S.K. Meena and M. Sulpizi
(See online at https://doi.org/10.1021/la403843n) - From Gold Nanoseeds to Nanorods: The Microscopic Origin of the Anisotropic Growth. Angewandte Chemie International Edition 2016
S.K. Meena and M. Sulpizi
(See online at https://doi.org/10.1002/anie.201604594) - The role of halide ions in the anisotropic growth of gold nanoparticles: a microscopic, atomistic perspective. Phys. Chem. Chem. Phys., 2016, 18, 13246-13254
S. K. Meena, S. Celiksoy, P. Schäfer, A. Henkel, C. Sönnichsen, M. Sulpizi
(See online at https://doi.org/10.1039/c6cp01076h) - Nanophase Segregation of Self-Assembled Monolayers on Gold Nanoparticles. ACS Nano, 2017
S.K. Meena, C. Goldmann, D. Nassoko, M. Seydou, T. Marchandier, S. Moldovan, O. Ersen, F. Ribot, C. Chanéac, C. Sanchez, D. Portehault, F. Tielens, and M. Sulpizi
(See online at https://doi.org/10.1021/acsnano.7b03616)
