Sensitive X-ray Absorption Near Edge Structure Analysis on the Bonding Properties of Au₃₀(SR)₁₈ Nanoclusters

Au nanoclusters (NCs) with organothiolate protecting ligands are a field of great interest and X-ray absorption spectroscopy is a useful tool for the structure and property studies of these Au NCs. However, the Au NCs normally show broad and low-intensity features in the gold X-ray absorption near-edge structure (XANES) region, lowering the sensitivity of the technique and making it difficult to use for the analysis of Au NCs. In this work we report a sensitive gold L₃-edge XANES study on the bonding properties of the newly discovered Au₃₀(SR)₁₈ NCs utilizing a combined approach of the first derivative XANES spectra and quantum simulations. First derivative XANES spectra are compared with the well-studied Au₂₅(SR)₁₈ with the aim of determining the unique features of Au₃₀(SR)₁₈. It is found that the early XANES region of the Au NCs is significantly influenced by the gold-gold bonding environment in the surface sites, as the varying surface Au-Au bond lengths in Au₂₅(SR)₁₈ and Au₃₀(SR)₁₈ result in pronounced difference in the first derivative XANES. These findings can be consistently explained using site-selective quantum simulations of the XANES spectra based on the Au NC structural models. The XANES method presented in this work offers a useful tool for the sensitive analysis on structure and bonding properties of Au NCs.