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Theoretical investigation of copper–gold particles has given considerable insight into their structures and properties. Ferrando et al. 85 have recently reviewed the area. Theoretical work aims to establish the structure of copper–gold alloy particles and the distribution of gold and copper throughout the nanoparticles. Most studies conclude that the particles are not

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homogeneous alloys, but that there is some degree of segregation of copper into the core and gold towards the mantle (outer part of the particle). Bimetallic particles can often be significantly different to monometallic ones. For example, the effect of exchanging a copper atom into a gold cluster has been investigated.104 Pure copper clusters were found to adopt an icosahedral geometry, whilst pure gold clusters were significantly less symmetrical and in fact, amorphous in some cases. However, substitution of one gold atom by copper was enough to change the structure back to an icosahedral fragment. Clusters with compositions CuAu and CuAu3 were comprised of layered Cu and Au; whereas those with composition

Cu3Au had Cu and Au atoms in a more mixed arrangement. For the icosohedral structures,

Cu was the central atom in the cluster due to its smaller size.

When layered structures are formed, the surfaces of the clusters comprise mainly gold atoms, whereas the copper atoms are located preferentially in the bulk. A trend was observed for the bulk cohesive energies that decreased in the order: Au ~3.81 eV > CuAu3 ~3.75 eV > CuAu

~3.74 eV > Cu3Au ~3.64 eV > Cu ~3.49 eV. Therefore, it was concluded that the atomic

mixing and segregation was determined by a number of factors e.g. minimisation of surface energy, reducing internal strain, atomic packing efficiency and structures that can take advantage of strong Au–Au or Au–Cu interactions. Johnston and co-workers 105 discovered that copper-rich CuAu clusters had more disordered structures. Many geometries were found to be possible for the different 34-atom CuAu clusters. An Au34 cluster was disordered, whilst

perfect core–shell structures are observed for Au28Cu6and Au27Cu7. Joshi and co-workers 106

studied hydrogen peroxide formation from H2 and O2 and investigated the effect of gold–

silver, gold–copper and gold–palladium dimers and trimers. Using DFT methodology, they determined the ground state geometries of the clusters and based the reaction pathway on these to determine the thermodynamics and kinetics of the reaction. They investigated a total of fifteen clusters to observe the thermodynamic and kinetic constraints on H2O2 formation.

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The first H2 addition to form an OOH species was thermodynamically unfavourable for all

the Cu-containing dimers and trimers, except CuAu2, and H2O2 formation was unfavourable

for CuAu2. However, the formation of the OOH and H2O2 species for PdAu and Pd3

kinetically and thermodynamically favourable and was therefore active for H2O2 production,

in line with experimental results for PdAu catalysts. The structure of copper–gold clusters with three different compositions—Cu3Au, CuAu and CuAu3—has been investigated by

simulation of their growth, starting from three different seeds.107 Atoms of copper and gold were added, one-by-one, to the seeds, using molecular dynamics. Interestingly, the structure of the CuAu particles was found to depend on the particle size. For clusters of size 160 atoms or 200 atoms, the CuAu clusters were all icosahedral; whilst for 100 atoms, the CuAu particle was icosohedral, CuAu3 was a double icosohedron and Cu3Au was decahedral. Some

segregation of copper and gold was also observed in the particles. In a separate experiment, copper atoms deposited onto an Au147 core did not migrate into the core. This information is

very applicable to catalysts; for example, core–shell has been reported for both Aucore–Cushell

and Cucore–Aushell compositions, depending on the atmosphere.80 A molecular dynamic study

of the bimetallic nanoparticles AuxCuy108 found the structures to be octahedral, decahedral or

icosahedral. At low cluster sizes, with composition between AuCu3 and Au9Cu1, the AuCu

clusters were stable icosahedra. However, for compositions between AuCu3 and Au3Cu, a

(pseudo) cuboctahedral phase was present. It was determined that the main factors that affected the structural behaviour of the clusters were the cluster size, alloy composition and temperature. It showed the thermal behaviour for different alloy concentrations in a cluster of 561 atoms. The melting temperature was determined as 277 oC and this lowered as the particle size decreased. The density and specific heat, for the undercooled liquid AuCu alloys in a wide composition range (Au, Au3Cu, AuCu, AuCu3 and Cu), have been studied by a

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temperature dependence of the alloy density increased with an increase in the Au concentration, and the specific heat of the alloys increase exponentially with an increase in the copper concentration. The theoretical studies highlighted in this review have concentrated on clusters derived from the three well-known stable, bulk AuCu compositions, i.e. AuCu, Au3Cu and AuCu3. They also focus on the differences between these AuCu clusters and pure

Au and Cu clusters. Only one copper atom doped into a gold cluster is required to cause the structure of the cluster to rearrange. Clearly, the purpose of these studies has been to try to understand the structure, properties and compositional relationships in bimetallic clusters. It is anticipated that these theoretical studies will be informative in catalysis with nanoalloys of AuCu.