Chemical Properties of Gold Clusters
Gold nanoparticles are of great interest in a wide range of research fields from novel composite materials and quantum dots to biomedical and environmental sensors and much more besides. However, stepping down from the current fixation with particles that exist on the nanoscale at several dozen nanometer diameters, are, of course, the superficially simpler metal clusters. These entities were the original bridge between the atomic realm and the macroscopic, bulk realm before nanoparticles became the fashionable focus of research and are more akin to what chemists might consider metallic molecules than species that might contain hundreds of atoms.
Chemist Mikael Johansson of the University of Helsinki in Finland and colleagues Ingolf Warnke, Alexander Le and Filipp Furche of the University of California, Irvine, USA, are investigating an intriguing property of gold clusters – at what size do the flat, planar gold clusters become three-dimensional and essentially solid structures? They point out that despite gold metal itself being rather chemically inert, a phenomenon that gives rise to its long-term value as a proxy for wealth, in fact, gold clusters containing a few handfuls of atoms and the aforementioned nanoparticles do have chemical properties of interest. In particular, gold clusters have surprisingly high activity in heterogeneous catalytic reactions. The team explains that anions containing clusters of gold atoms number eight to twelve are two dimensional, and this is fairly well known. Indeed, dimensionality is thought to be critical to catalytic activity.
Transition from 2D to 3D
Understanding the notional transition from the smaller two-dimensional cluster to the bigger three-dimensional counterparts is vital to the development of new chemical technologies that exploit the intriguing activity of these clusters. However, until recently there was detailed data only on the smallest of clusters containing either two or three atoms. About six years ago, a major step forward was taken in a gas-phase study of gold clusters tagged with the noble gas krypton and revelations concerning seven-, nineteen- and twenty-gold atom clusters was published . Work with xenon too was possible, all of which pinned down clusters of seven or eight gold atoms in the planar world . The place at which the crossover from the 2D to the 3D remained elusive.
“While the dimensionality crossover for the charged species, cations and anions of gold, has been established experimentally some time ago, theoretical methods have until now failed to reproduce these experiments, sometimes spectacularly,” Johansson told ChemistryViews.org. He adds that, “Neutral gold clusters are experimentally very difficult to investigate, only recently, and only for a few specific cluster sizes, have André Fielicke and co-workers at the Fritz-Haber-Institut der Max-Planck-Gesellschaft been able to (in some impressive work) deduce the structures of neutral clusters. The cluster size when 2D becomes 3D has so far not been observed experimentally, and previous theoretical studies disagree wildly, predicting the crossover to take place anywhere between seven and fourteen atoms.”
3D Gold Clusters
As such, Johansson and colleagues have now used global structure optimizations, a genetic algorithm analysis and meta-generalized density functional theory (DFT) of neutral gold clusters with nine to thirteen atoms to investigate the relative energies of the lowest-lying isomers of these putative clusters. They included various factors, for example thermal, scalar relativistic and spin-orbit effects in their calculations and then looked for a correlation with data from cross section and electron diffraction measurements. This points to the crossover from flat to solid in the world of gold clusters as occurring with the surprisingly large number of eleven gold atoms. Clusters with more than eleven gold atoms are manifestly three-dimensional, but it is rather unusual that a cluster of up to eleven atoms can maintain itself in a stable and flat state.
The 2D and 3D isomers of 11-atom gold clusters are almost isoenergetic with a slight bias towards the three-dimensional. For clusters with twelve or more gold atoms, 3D structure is clearly favored, everything smaller is flat, the team reports. “Neutral gold clusters turn 3D at a larger size than cations, which become 3D at a cluster size of eight atoms,” the team says.
“It is finally possible to correctly model all the important components of the aurophilic Au-Au attraction, that is, electron correlation, dispersion, and relativistic effects,” Johansson told ChemistryViews.org. “With this, we proceed to establish the crossover point from planarity to compactness at eleven gold atoms, an unusually large size for metal clusters, and a unique property of gold.”
He adds that, “Looking forward, having established a reliable and computationally affordable quantum chemical modeling procedure of these experimentally very challenging clusters should prove important for elucidating, for example, their catalytic behavior.”
“I believe this work is important because this ‘2D or not 2D’ question provides a stringent test of how well quantum chemistry (theory) can handle the chemical bonding in this important class of systems,” Robert Whetten of the University of Texas at San Antonio told ChemistryViews.org. “It is worth pointing out that the (supported) gold-cluster catalysts are most active when the ‘nano particles’ are actually clusters in this very size-range (< 20 atoms),” he adds. “Also that the 2D/not-2D question is related closely to the special hybridization (5d – 6s orbitals, but specifically the d–z2 orbital aligned perpendicular to the plane) that is strongest in gold, thanks to the maximum in the relativistic effects occurring at that atomic number Z = 79.”
Whetten also points out that, “An enormous amount of experimental work has already gone into determining the structures of the gold clusters, usually (±)-charged, in this size-range. The charge-neutral clusters are somewhat more difficult to investigate, but have also been important historically and in the newer experimental work.” He adds that, “In particular, I could give reasons why establishing the planarity of the neutral Au9 and Au10 clusters will be greeted with relief.”
- At What Size Do Neutral Gold Clusters Turn Three-Dimensional?
Mikael P. Johansson, Ingolf Warnke, Alexander Le, Filipp Furche
J. Phys. Chem. C, 2014
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