Geometrical and electronic structures of gold, silver, and gold-silver binary clusters

Origins of ductility of gold and gold-silver alloy formation

Han Myoung Lee, Maofa Ge, B. R. Sahu, Tarakeshwar Pilarisetty, Kwang S. Kim

Research output: Contribution to journalArticle

247 Citations (Scopus)

Abstract

The structures of pure gold and silver clusters (Auk, Agk, k = 1-13) and neutral and anionic gold-silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination in contrast to that of Au in Auk which tends to be small in 1-D or 2-D coordination. This trend is critical in determining the cluster structures. The calculated electronic properties and dissociation energy of both pure and binary clusters are in good agreement with the available experimental data. Since the Ag-5s orbital is much higher in energy than the Au-6s orbital energy, the partial charge transfer from Au to Ag takes place in gold-silver binary clusters. Au atoms tend to be negatively charged, while Ag atoms tend to be positively charged. Combined with the trend that Au atoms favor the surface, edges, or vertices with smaller Nc, the outer part of the cluster tends to be negatively charged, while Ag atoms favor the inside with larger Nc, and so the inner part tends to be positively charged. The partial charge transfer in the binary system results in electrostatic energy gain for the binary AumAgn cluster over pure Auk and Agk clusters, which is responsible for the formation of alloys. In a neutral alloy, the equivalent mixing is favored, and the even numbered k tends to be more stable due to the electron spin pairing, whereas in an anionic alloy the odd numbered k tends to be more stable.

Original languageEnglish (US)
Pages (from-to)9994-10005
Number of pages12
JournalJournal of Physical Chemistry B
Volume107
Issue number37
StatePublished - Sep 18 2003
Externally publishedYes

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Gold Alloys
Silver alloys
Gold alloys
silver alloys
gold alloys
ductility
Silver
Gold
Electronic structure
Ductility
silver
gold
electronic structure
Atoms
Charge transfer
Static Electricity
orbitals
Electronic properties
Density functional theory
Electrostatics

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

Cite this

Geometrical and electronic structures of gold, silver, and gold-silver binary clusters : Origins of ductility of gold and gold-silver alloy formation. / Lee, Han Myoung; Ge, Maofa; Sahu, B. R.; Pilarisetty, Tarakeshwar; Kim, Kwang S.

In: Journal of Physical Chemistry B, Vol. 107, No. 37, 18.09.2003, p. 9994-10005.

Research output: Contribution to journalArticle

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N2 - The structures of pure gold and silver clusters (Auk, Agk, k = 1-13) and neutral and anionic gold-silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination in contrast to that of Au in Auk which tends to be small in 1-D or 2-D coordination. This trend is critical in determining the cluster structures. The calculated electronic properties and dissociation energy of both pure and binary clusters are in good agreement with the available experimental data. Since the Ag-5s orbital is much higher in energy than the Au-6s orbital energy, the partial charge transfer from Au to Ag takes place in gold-silver binary clusters. Au atoms tend to be negatively charged, while Ag atoms tend to be positively charged. Combined with the trend that Au atoms favor the surface, edges, or vertices with smaller Nc, the outer part of the cluster tends to be negatively charged, while Ag atoms favor the inside with larger Nc, and so the inner part tends to be positively charged. The partial charge transfer in the binary system results in electrostatic energy gain for the binary AumAgn cluster over pure Auk and Agk clusters, which is responsible for the formation of alloys. In a neutral alloy, the equivalent mixing is favored, and the even numbered k tends to be more stable due to the electron spin pairing, whereas in an anionic alloy the odd numbered k tends to be more stable.

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