Inter-Cluster and Intra-Cluster Dynamics of Thiolate-Protected Gold-Silver Alloys Revealed

Metal clusters are becoming profoundly useful in the health, energy, and environment sectors and are used in several products from fuel cells to effective medicines to molecular sensors. This unique functionality of clusters forms due to the variability in type and size.

Presently, researchers led by Professor Yuichi Negishi, of the Department of Applied Chemistry at Tokyo University of Science, add to this continuing story by illuminating the dynamics of the metal cluster, thiolate-protected gold-silver alloy, in solution; this helps in appreciating the geometry, stability, and tenability of these clusters for their applications.

Metal clusters form when a group of metal atoms come together to develop into clumps, anywhere between the size of a molecule and that of a bulk solid. Lately, these clusters have garnered much attention due to their diverse chemical capabilities that rely on their size and composition.

In contrast to the closed, set, and stable packing seen in bulk metal lattices, the geometry of these clusters, which normally manages their chemical reactivity too, is based on special atomic arrangements that decrease the energy. Moreover, their functionalities differ based on the number of basic atoms in the cluster.

As these micro-level factors rule the ultimate macro-level activity of the clusters, appreciating the cluster dynamics at the atomic scale is vital. Latest exploration in the area of such metal clusters has enabled the classification of these clumps as compounds of defined chemical compositions.

One such fascinating metal cluster with luminescence and catalytic properties is the thiolate-protected gold-silver alloy cluster. These metal clusters are formed when thiolate-protected individual silver and gold clusters are placed together in a solution. The individual pure clusters experience metal exchange, similar to a chemical "barter": a gold for a silver atom.

While the cluster-metal complex reaction (CMCR) technique is extensively used, the real dynamics of it and the energy incentive managing such processes are not comprehended. This became the seed of interest for Prof. Negishi's team, as they state, "the dynamic behavior of these clusters in solution must be taken into consideration to understand the origins of the catalytic activity and luminescence properties of gold-silver alloys clusters in addition to the geometric structure."

To understand the metal exchange behavior between the pure clusters after synthesis, the researchers came up with an experiment based on reverse-phase chromatography. They identified this arrangement because it distinguishes molecules based on electronic features, that is, whether the molecule is polar (with a concurrent positive and negative center) or non-polar (without charge separation).

Using this arrangement proved beneficial as the team stated that, in fact, the individual structural isomers (different geometrical and spatial distribution for a particular cluster) alter in solution even though the mass of the cluster stays unaffected. This showed that there was intra-cluster exchange of metal atoms, which altered the cluster’s electronic state even though the mass stayed unchanged. They also informed that after the synthesis, in the course of time, the concentration of various structural types of gold-silver alloys in the solution altered. This showed that there was also an inter-cluster metal exchange behind the scenes. Finally, the scientists also noticed that the inter-cluster metal exchange happens a lot more regularly after synthesis and in due course decelerates after standing for a long time. They assigned this to the variance in stability and energy among the diverse structures.

The metastable geometries formed initially likely convert to thermodynamically stable geometries through inter-cluster (and intra-cluster) metal exchange in solution.

Yuichi Negishi, Professor, Department of Applied Chemistry, Tokyo University of Science

The researchers proved their claims about the observed dynamics of the cluster metal complex reaction (CMCR) by conducting a comparative study with the alternate synthesis process. Since traditional procedures (Co-Reduction of Metal Ions) create alloys under extreme conditions, only the thermodynamically and energetically fortunate structures see the light of day. Therefore, largely stable structures are formed, showing that metal exchange is comparatively suppressed. This stood in opposition to the clusters produced by the CMCR where signatures for several species are observed at first. As time goes by, similar to all things in nature, the unstable species attempt to reorder arrange themselves into stable ones. They achieve this through metal exchange.

To encapsulate, Prof. Negishi states, "These results demonstrate that gold-silver alloy clusters have different geometric structures (and distributions) immediately after synthesis, depending on the synthesis method. Thereby, their dynamic behavior in solution also depends on the synthesis method."

The research of clusters with variable core sizes and compositions is stimulating as it offers thrilling opportunities to harness novel chemical and physical properties. However, that is not all. It also offers an insight into their structure-property interactions, more or less like peeking into the "social life" of atoms.

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