New Insights into the Bonding Properties of [Ag25(SR)18]- Nanoclusters from X-ray Absorption Spectroscopy

Atomically precise metal nanoclusters have attracted significant interest due to their molecule-like properties. [Ag25(SR)18]- is one of the Ag nanoclusters having a unique structure similar to its Au counterpart but different from most other Ag nanoclusters. In this study, a new five-shell fitting method was developed to analyze the extended X-ray absorption fine structure (EXAFS) spectra of [Ag25(SR)18]- to provide more insights into its bonding properties. This new method was successfully applied to compare the bond lengths as the temperature changed from 300 K to 90 K. Interestingly, the metal core of [Ag25(SR)18]- shows negative thermal expansion behaviour that is not observed for Au25(SR)18. These unique bonding properties of [Ag25(SR)18]- could be related to the Ag4 tetrahedral units found in the metal core, which are absent in Au25(SR)18. These new findings about its bonding properties can provide a better understanding of the structure-property relationship of [Ag25(SR)18]-. This new EXAFS analysis method could be applied to gain insights into the bonding properties of other metal nanoclusters.