The alloy Au–Ag system is
an important noble bimetallic
phase, both historically (as “Electrum”) and now especially
in nanotechnology, as it is applied in catalysis and nanomedicine.
To comprehend the structural characteristics and the thermodynamic
stability of this alloy, a knowledge of its phase diagram is required
that considers explicitly its size and shape (morphology) dependence.
However, as the experimental determination remains quite challenging
at the nanoscale, theoretical guidance can provide significant advantages.
Using a regular solution model within a nanothermodynamic approach
to evaluate the size effect on all the parameters (melting temperature,
melting enthalpy, and interaction parameters in both phases), the
nanophase diagram is predicted. Besides an overall shift downward,
there is a “tilting” effect on the solidus–liquidus
curves for some particular shapes exposing the (100) and (110) facets
(cube, rhombic dodecahedron, and cuboctahedron). The segregation calculation
reveals the preferential presence of silver at the surface for all
the polyhedral shapes considered, in excellent agreement with the
latest transmission electron microscopy observations and energy dispersive
spectroscopy analysis. By reviewing the nature of the surface segregated
element of different bimetallic nanoalloys, two surface segregation
rules, based on the melting temperatures and surface energies, are
deduced. Finally, the optical properties of Au–Ag nanoparticles,
calculated within the discrete dipole approximation, show the control
that can be achieved in the tuning of the local surface plasmon resonance,
depending of the alloy content, the chemical ordering, the morphology,
the size of the nanoparticle, and the nature of the surrounding environment.
We report on energetic surface reconstruction phenomena observed on bimetallic nanoparticle systems of AuPd and AuCu, similar to a resolidification effect observed during the cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in [100] facets on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence so far. We demonstrate experimentally as well as by computational simulations that this new eutectic structure is favored in such nanoalloy systems.
Several series of molecular dynamics runs were performed to simulate the melting transition of bimetallic cuboctahedral nanoparticles of gold-palladium at different relative concentrations to study their structural properties before, in, and after the transition. The simulations were made in the canonical ensemble, each series covering a range of temperatures from 300 to 980 K, using the Rafii-Tabar version of the Sutton and Chen interatomic potential for metallic alloys. We found that the melting transition temperature has a strong dependence on the relative concentrations of the atomic species. We also found that, previous to the melting transition, the outer layer of the nanoparticle gets disordered in what can be thought as a premelting stage, where Au atoms near the surface migrate to the surface and remain there after the particle melts as a whole. The melting of the surface below T m is consistent with studies of the interaction of a TEM electron beam with Au and Au-Pd nanoparticles.
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