title: Gold-copper nano-alloy, "Tumbaga", in the era of nano: phase diagram and segregation.
abstract: Gold-copper (Au-Cu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of core-shell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations.
authors.affiliation: Department of Physics & Astronomy, University of Texas at San Antonio , One UTSA Circle, San Antonio, Texas 78249, United States.
authors.forename: Grégory
authors.initials: G
authors.lastname: Guisbiers
authors.forename: Sergio
authors.initials: S
authors.lastname: Mejia-Rosales
authors.forename: Subarna
authors.initials: S
authors.lastname: Khanal
authors.forename: Francisco
authors.initials: F
authors.lastname: Ruiz-Zepeda
authors.forename: Robert L
authors.initials: RL
authors.lastname: Whetten
authors.forename: Miguel
authors.initials: M
authors.lastname: José-Yacaman
mesh_terms: D003300:Copper / Q000737:chemistry*; D006047:Gold Alloys / Q000737:chemistry*; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure; D056004:Molecular Dynamics Simulation; D036103:Nanotechnology; D010316:Particle Size; D044367:Phase Transition; D013816:Thermodynamics
chemical_list: D006047:Gold Alloys; D003300:Copper
keywords: binary phase diagram; congruent melting; nanoalloy; nanothermodynamics; polyhedra; size effect