Gold cluster

Gold clusters in cluster chemistry can be either discrete molecules or larger colloidal particles. Both types are described as nanoparticles, with diameters of less than one micrometer. A nanocluster is a collective group made up of a specific number of atoms or molecules held together by some interaction mechanism.[1] Gold nanoclusters have potential applications in optoelectronics[2] and catalysis.[3]

Bare gold clusters

Bare gold clusters, i.e., clusters without stabilizing ligand shells can be synthesized and studied in vacuum using molecular beam techniques. Their structures have been experimentally studied using, e.g., anion photoelectron spectroscopy,[4] far-infrared spectroscopy,[5] as well as measurements of their ion mobility and electron diffraction studies[6] in conjunction with quantum chemical calculations. The structures of such clusters differ strongly from those of the ligand-stabilized ones, indicating an pivotal influence of the chemical environment on the cluster structure. A notable example is Au20 which forms a perfect tetrahedron in which the Au atom packing closely resembles the atomic arrangement in the fcc bulk structure of metallic gold.[4][5] Evidence has been presented for the existence of hollow golden cages with the partial formula Aun− with n = 16 to 18.[7] These clusters, with diameter of 550 picometres, are generated by laser vaporization and characterized by photoelectron spectroscopy.

Structure of ligand-stabilized Au clusters
Construction of the Au13 icosahedron.

Bulk gold exhibits a face-centered cubic (fcc) structure. As gold particle size decreases the fcc structure of gold transforms into a centered-icosahedral structure illustrated by Au13.[1] It can be shown that the fcc structure can be extended by a half unit cell in order to make it look like a cuboctahedral structure. The cuboctahedral structure maintains the cubic-closed pack and symmetry of fcc. This can be thought of as redefining the unit cell into a more complicated cell. Each edge of the cuboctahedron represents a peripheral Au–Au bond. The cuboctahedron has 24 edges while the icosahedron has 30 edges; the transition from cuboctahedron to icosahedron is favored since the increase in bonds contributes to the overall stability of the icosahedron structure.[1]

The centered icosahedral cluster Au13 is the basis of constructing large gold nanoclusters. Au13 is the endpoint of atom-by-atom growth. In other words, starting with one gold atom up to Au12, each successful cluster is created by adding one additional atom. The icosahedral motif is found in many gold clusters through vertex sharing (Au25 and Au36), face-fusion (Au23 and Au29), and interpenetrating biicosahedrons (Au19, Au23, Au26, and Au29).[1] Large gold nanoclusters can essentially be reduced to a series of icosahedrons connecting, overlapping, and/or surrounding each other. The crystallization process of gold nanoclusters involves the formation of surface segments that grow towards the center of the cluster. The cluster assumes an icosahedral structure because of the associated surface energy reduction.[8]

Discrete gold clusters

Well-defined, molecular clusters are known, invariably containing organic ligands on their exteriors. Two examples are [Au6C(P(C6H5)3)6]2+ and [Au9(P(C6H5)3)8]3+.[9] In order to generate naked gold clusters for catalytic applications, the ligands must be removed, which is typically done via a high-temperature (200 °C/392 °F or higher) calcination process,[10] but can also be achieved chemically at low temperatures (below 100 °C/212 °F), e.g. using a peroxide-assisted route.[11]

Colloidal clusters

Gold clusters can be obtained in colloid form. Such colloids often occur with a surface coating of alkanethiols or proteins. Such clusters can be used in immunohistochemical staining.[12] Gold metal nanoparticles (NPs) are characterized by an intense absorption in the visible region, which enhances the utility of these species for the development of completely optical devices. The wavelength of this surface plasmon resonance (SPR) band depends on the size and shape of the nanoparticles as well as their interactions with the surrounding medium. The presence of this band enhances the utility of gold nanoparticle as building blocks for devices for data storage, ultrafast switching, and gas sensors. Whilst plasmonic gold nanoparticles only exhibit electric moments, clusters of such particles can exhibit magnetic moments making them of great interest for use in optical metamaterials [13]

Catalysis
Main article: Heterogeneous gold catalysis

When implanted on a FeOOH surface, gold clusters catalyze oxidation of CO at ambient temperatures.[14] Similarly gold clusters implanted on TiO2 can oxidize CO at temperatures as low as 40K.[15] Catalytic activity correlated with the structure of gold nanoclusters. A strong relationship between energetic and electronic properties with size and structure of gold nanoclusters.[16][17]

See also

References
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Further reading
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