(Nanowerk Spotlight) The Spaniards called it Tumbaga – an alloy of gold and copper which they found in widespread use by the pre-Columbian cultures in Central America. The popularity of this alloy comes from its congruent melting point, i.e. at a particular composition, the alloy behaves like a pure element (i.e. it melts at a definite temperature rather than over a range) and also the melting point of the alloy is reduced as compared to the two pure elements. For bulk Au-Cu, the congruent melting point occurs at 44% copper composition and 910°C, well below
the gold melting point (1064°C) and the copper melting point (1084°C).
"This alloy exhibits novel physical and chemical properties at the nanoscale," Dr. Grégory Guisbiers from the Department of Physics & Astronomy at the University of Texas at San Antonio (UTSA), tells Nanowerk. "Although the Au-Cu alloy has been extensively studied in the literature both at the bulk and nano-scales, the prediction of phase diagrams at the nanoscale is still missing."
The team from Prof. Miguel Jose Yacaman's group used a thermodynamic approach that gives some interesting insights concerning the behavior of several polyhedral Au-Cu nanoalloys.
"Our results illustrate how the dynamic and structural behavior of the alloy as calculated at the special sizes 10 and 4 nm, evolves from the gold sequence to the copper sequence as the copper composition increases" notes Guisbiers. "Whatever the composition and the size of the nano-alloy, the preferred shapes remain the dodecahedron, truncated octahedron and the icosahedron. Shapes with a Cu-rich core/Au-rich surface are the most stable structures."
Predicted sequence of preferred shapes (left to right) for a) gold, b) copper and c) copper-gold alloy at 10nm and 4 nm where 1, 2, 3, 4, 5 and 6 represent different range of composition. (Reprinted with permission by RSC Publishing) (click on image to enlarge)
The paper highlights the size and shape effects on the congruent melting point, showing its copper enrichment when size decreases: For each shape investigated, by decreasing the size, the congruent melting point is shifted to lower temperature and higher copper composition.
Previous work has shown that among bimetallic systems built with elements of the same group in the periodic table, the trend to be in the core is higher for the element with the smaller core electron density (copper in this case) i.e. the element with the higher core electron density (gold in this case) will be located at the surface. The structures predicted thermodynamically in this work are in good agreement with these observations.
"The trend observed from our approach can also be used as a starting point for ab initio density functional theory (DFT) methods to predict the behavior of smaller Au-Cu clusters," concludes Guisbiers.