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.

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The packing of spheres is a subject that has drawn the attention of mathematicians and philosophers for centuries, and that currently attracts the interest of the scientific community in several fields. At the nanoscale, the packing of atoms affect the chemical and structural properties of the material, and hence, its potential applications. This report describes the experimental formation of five-fold nanostructures by the packing of interpenetrated icosahedral and decahedral units. These nanowires, formed by the reaction of a mixture of metal salts (Au and Ag) in the presence of oleylamine, are obtained when the chemical composition is specifically Ag/Au=3/1. The experimental images of the icosahedral nanowires have a high likelihood with simulated electron micrographs of structures formed by two or three Boerdijk-Coxeter-Bernal helices roped on a single structure, whereas for the decahedral wires, simulations using a model of adjacent decahedra match the experimental structures. To our knowledge, this is the first report of the synthesis of nanowires formed by the packing of structures with five-fold symmetry. These icosahedral nanowire structures remind those of quasicrystals that can only be formed if at least two atomic species are present and in which icosahedral and decahedral packing has been found for bulk crystals.

Nickel-based bimetallic nanoalloys (nickel–palladium, nickel–platinum, nickel–rhodium, and nickel–iridium) play an important role in catalysis, electrocatalysis, and magnetic applications. To improve the performance of those materials at the nanoscale, the knowledge of their phase diagrams is critically needed. However, such knowledge is still lacking because calorimetry experiments are extremely challenging to perform at the nanoscale. Then, a smart and necessary alternative to those challenging and time-consuming experiments is to obtain this knowledge from theoretical predictions by using nanothermodynamics. The phase diagrams at the nanoscale for the considered alloys are therefore predicted for various polyhedral shapes, while the nature of the surface segregated element is established by using two segregation rules. Finally, the theoretical results are supported by advanced transmission electron microscopy characterization.

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