Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation

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

Gold–Copper Nano-Alloy, “<i>Tumbaga</i>”, in the Era of Nano: Phase Diagram and Segregation

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

CuS₂‑Passivated Au-Core, Au₃Cu-Shell Nanoparticles Analyzed by Atomistic-Resolution Cs-Corrected STEM

Au-core, Au₃Cu-alloyed shell nanoparticles passivated with CuS₂ were fabricated by the polyol method, and characterized by Cs-corrected scanning transmission electron microscopy. The analysis of the high-resolution micrographs reveals that these nanoparticles have decahedral structure with shell periodicity, and that each of the particles is composed by Au core and Au₃Cu alloyed shell surrounded by CuS₂ surface layer. X-ray diffraction measurements and results from numerical simulations confirm these findings. From the atomic resolution micrographs, we identified edge dislocations at the twin boundaries of the particles, as well as evidence of the diffusion of Cu atoms into the Au region, and the reordering of the lattice on the surface, close to the vertices of the particle. These defects will impact the atomic and electronic structures, thereby changing the physical and chemical properties of the nanoparticles. On the other hand, we show for the first time the formation of an ordered superlattice of Au₃Cu and a self-capping layer made using one of the alloy metals. This has significant consequences on the physical mechanism that form multicomponent nanoparticles

ESI-MS Identification of Abundant Copper–Gold Clusters Exhibiting High Plasmonic Character

The protected noble-metal structures comprising 145 metal-atom sites and 60 ligands are among the frequently identified larger metal-cluster systems exploited in many avenues of research. Herein we report a comparative electrospray ionization-mass spectrometry (ESI-MS) investigation of the 60-fold thiolated Au₁₄₄ and CuAu₁₄₄ clusters, in various positive charge-states, in conjunction with a density-functional theoretical (DFT) analysis based upon the icosahedral Pd₁₄₅-structure-type applicable to these systems. Samples rich in the hexanethiolate-protected CuAu₁₄₄ clusters are obtained via a single-phase reduction process. The predicted electronic structure of the vacancy-centered Au₁₄₄(SR)₆₀ system provided a simple rationale for the limiting [4+] charge-state observed of Au₁₄₄, whereas the maximal [3+] charge detected on the CuAu₁₄₄(SR)₆₀ cluster can be explained if the 145th atom occupies the central site. Occupancy of the center-site stabilizes the superatomic 3S-orbital, and thereby shifts the shell-closing count from 82 to 84 free electrons. The DFT-calculated energetics also predicts a strong (0.65 eV) preference for placing the smaller Cu ion in this central site. Remarkably, the optical absorption spectra of dilute tetrahydrofuran (THF) solutions feature a broad band centered near 2.3 eV, in contrast to the previously reported “nonplasmonic” response of sub-2.0-nm all-gold or -copper clusters. Other methods (matrix-assisted laser desorption ionization mass spectrometry and high-resolution electron microscopy) were used to investigate whether aggregation phenomena might account for this observed plasmon emergence. This unusual result points to the need to obtain highly purified samples of copper-doped gold clusters of ca. 145 atoms total