Core Geometry Effect on the Bonding Properties of Gold–Thiolate Nanoclusters: The Case of Hexagonal-Close-Packed Au₃₀(SR)₁₈

Diversity of core geometry for thiolate-protected gold nanoclusters (AuSR NCs) is a fascinating phenomenon, and thus, understanding the core geometry effect on the bonding properties of AuSR NCs is of significant interest. Herein, we report extended X-ray absorption fine structure (EXAFS) studies on the bonding properties of hexagonal-close-packed (HCP) Au₃₀(SR)₁₈ NCs. To illustrate the core geometry effects, Au₂₅(SR)₁₈ with icosahedral core and Au₃₆(SR)₂₄ with face-centered-cubic (FCC) core are selected for a comparative study. Temperature-dependent EXAFS experiments show considerable difference in the thermally induced core contraction/expansion behavior of the NCs. The HCP Au₃₀(SR)₁₈ exhibits a significant negative thermal expansion (NTE) of 3% for the metal core, whereas the BCC Au₃₆(SR)₂₄ shows only a 0.5% NTE. In contrast, the icosahedral Au₂₅(SR)₁₈ shows a positive thermal expansion. These experimental findings can be accounted for by considering the metal/molecule-like behavior and the defects of metallic bonds of the NC cores, and highlight the important role of the core structure on the bonding properties of AuSR NCs. Comparison of solid and solution phase Au₃₀ EXAFS results shows no change in bond distance, in contrast to the Au₂₅ NCs which show a decrease in size of the metal core when it changed from solid to liquid. These results arise from the difference in thiolate ligands present in the two NCs.