文摘
We report the intriguing bonding behavior of selenolate-protected Au25 nanoclusters (Au25(SeR)18), revealed by temperature-dependent X-ray absorption spectroscopy (XAS) from both the metal (Au L3-edge) and ligand (Se K-edge) perspectives. The structure of Au25(SeR)18 was first analyzed in great detail using a site-specific, multishell (i.e., Au鈥揂u core/surface/staple, Au鈥揝e/Se-Au and Se鈥揅 shells) approach. It was found that the Au13 core of Au25(SeR)18 remains relatively unchanged at low temperature while aurophilic interactions on the surface are significantly longer in distance compared with their thiolate-protected counterpart, Au25(SR)18. Remarkably, temperature-dependent studies showed a significant thermal contraction of the Au鈥揂u framework in Au25(SeR)18, which is absent in the Au25(SR)18 system. This unusual bonding behavior of Au25(SeR)18 is proposed to be induced by the dimeric staple-like motif (鈭扐u鈥揝e鈥揂u鈥揝e鈥揂u鈭? surface structures, where aurophilic bond distances and Au鈥揝e鈥揂u bond angles are sensitive toward temperature change. Density functional theory and molecular dynamics (DFT-MD) simulations were conducted to confirm this mechanism and provide further insight into the bonding behavior of the Au25(SeR)18 nanocluster. Finally, we use near-edge XAS results to demonstrate that the thermal contraction effect induces a change to the electronic properties of both the Au and Se and consistently accounted for using ab initio simulations of the near-edge and valence band structure.