Electrum, the Gold-Silver Alloy, from the Bulk Scale to the Nanoscale: Synthesis, Properties, and Segregation Rules

Abstract

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.

Keywords: aberration corrected electron microscopy; nanothermodynamics; noble metals; optical properties; phase diagram; polyhedra; surface segregation; thermal properties.

Figure 1

Bulk phase diagram of the gold–silver alloy. Experimental points are taken from ref (63). The liquidus–solidus curves are plotted using the regular solution model (eq 1).

Figure 2

Nanophase diagrams of gold–silver for various shapes of polyhedra: (a) tetrahedron, (b) octahedron, (c) decahedron, (d) dodecahedron, (e) icosahedron, (f) cube, (g) truncated octahedron, (h) cuboctahedron, and (i) rhombic dodecahedron. The black, red, and blue curves indicate the bulk, 10 and 4 nm behavior of the alloy, respectively.

Figure 3

Nanophase diagrams showing the segregated and nonsegregated liquidus–solidus curves at a size equal to 10 nm for various shapes: (a) tetrahedron, (b) octahedron, (c) decahedron, (d) dodecahedron, (e) icosahedron, (f) cube, (g) truncated octahedron, (h) cuboctahedron, and (i) rhombic dodecahedron.

Figure 4

Nanophase diagrams showing the segregated and nonsegregated liquidus-solidus curves at a size equal to 4 nm for various shapes: (a) tetrahedron, (b) octahedron, (c) decahedron, (d) dodecahedron, (e) icosahedron, (f) cube, (g) truncated octahedron, (h) cuboctahedron, and (i) rhombic dodecahedron.

Figure 5

Nanophase diagram of a cuboctahedron at 4 nm calculated with different ΔH_sub/kT values. In all cases, silver is always predicted to be found preferentially at the surface.

Figure 6

(a) HAADF-STEM image of an icosahedral Au₅₀Ag₅₀ nanoparticle having a size ∼8 nm. The blue arrow indicates the scan direction. (b) EDX line scan across this particle revealing the silver surface enrichment.

Figure 7

(a) HAADF-STEM image of a decahedron Au₅₀Ag₅₀ nanoparticle having a size ∼9 nm. The blue arrow indicates the scan direction. (b–d) EDX elemental chemical maps of Au, Ag, and overlay, respectively. Yellow regions in the EDX map indicate the presence of gold, whereas gray regions mark the presence of silver. The overlay mapping reveals the silver surface enrichment.

Figure 8

Extinction efficiency of gold–silver nanoparticles at a size equal to 10 nm for various shapes: (a) tetrahedron, (b) octahedron, (c) decahedron, (d) dodecahedron, (e) icosahedron, (f) cube, (g) truncated octahedron, (h) cuboctahedron, and (i) rhombic dodecahedron.

Figure 9

Extinction efficiency of gold–silver nanoparticles at a size equal to 4 nm for various shapes: (a) tetrahedron, (b) octahedron, (c) decahedron, (d) dodecahedron, (e) icosahedron, (f) cube, (g) truncated octahedron, (h) cuboctahedron, and (i) rhombic dodecahedron.

Figure 10

Extinction efficiency of a gold–silver cuboctahedron (length side =10 nm) (a) with different chemical orderings in hexane, (b) with different surrounding environments, (c) with different compositions in hexane, (d) experimental UV–vis spectrum.