X-ray Absorption Spectroscopy on Atomically Precise Metal Clusters

Metal clusters show novel and size-specific properties due to unique geometric and quantized electronic structures. State-of-the art synthetic methods allow us to control with atomic precision the size and compositions of clusters stabilized with polymers, protected by ligands, and immobilized on supports. The geometric structure is key information for understanding the origin of the specific and novel properties and for rationally designing their functions. Single-crystal X-ray diffraction analysis provides direct and atomic-level structural information on ligand-protected metal clusters that can be crystallized, but cannot be applied to polymer-stabilized and supported clusters even though their size and composition are precisely defined. X-ray absorption spectroscopy (XAS) is a versatile tool for determining the local structure and electronic state of a specific element within the clusters regardless of their environment. In addition to static structures, dynamic changes in electronic and geometric structures can be probed by a time-resolved measurement. Simultaneous measurement of XAS with other spectroscopies provides further insight into the reaction mechanism. This article summarizes our XAS studies on the size and atomic packing of metal clusters, location of dopant in the clusters, interfacial structures between the clusters and the surroundings, thermal properties of the clusters, and structural and electronic dynamics during the reactions.