Understanding shape control in gold nanoparticles from molecular dynamics simulations
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Abstract
Gold nanoparticles are widely used in many areas such as photothermal
cancer therapy, biochemical sensing and medical imaging due to their size
and shape-dependent optical properties. Directly manipulating and control-
ling the size and shape of gold nanoparticles is, therefore, a key step for
their tailored applications.
We use molecular dynamics simulations in order to understand the mi-
croscopic 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 under-
stand 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, a 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 diffusion of 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 a cuboctahedral seed,
whose dimensions are comparable to those of the cetyltrimethylammoniumbromide micelle. We find that the asymmetry in adsorption behavior be-
tween the different low index facets, which characterized the infinite planes,
shows up even more dramatically on the nanoseeds. Indeed, the Au(100)
and Au(110) facets show structures similar to the ones observed on the in-
finite planes in both the cuboctahedral as well as penta-twinned seeds. The
(111) facets, which e.g. form the tips of the penta-twinned nanoseeds, on
the other hand show basically no micellar adsorption. 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 provides a microscopic understanding of the role of
halides in controlling the anisotropic growth. In particular, we find that bro-
mide adsorption on the gold nanorods is not only responsible for surface
passivation, but also acts as the driving force for micelle adsorption and
stabilization on the gold surface in a facet-dependent way. Partial replace-
ment of bromide by chloride decreases the difference between facets and the
surfactant density. Finally, while only chloride is present in the growing so-
lution, no halides or micellar structure protect the gold surface and further
gold reduction should be uniformly possible.
Finally, we also address the role of the silver ions. We find that silver
ions have a strong propensity to adsorb on the gold surface where they
form AgBr islands with different specific geometry depending on the sur-
face plane. Although the structure of the micellar layer is not qualitatively
modified by the addition of silver, silver substantially increases the Br- con-
centration at the interface, resulting into an increased surface passivation.
Overall the asymmetry between facets is maintained, with a lower Br- sur-
face density at the Au(111) interface with respect to that at the Au(100) and
Au(110) interfaces.