Structure and Bonding Properties of a 20-Gold-Atom Nanocluster Studied by Theoretical X-ray Absorption Spectroscopy

摘要

Gold nanoclusters with precisely controlled atomic composition have emerged as promising materials for applications in nanotechnology because of their unique optical, electronic and catalytic properties. The recent discovery of a 20-gold-atom nanocluster protected by 16 organothiolate molecules, Au_(20)(SR)_(16), is the smallest member in a surprising series of small gold-thiolate nanoclusters with a face-centered cubic (FCC) ordered core structures. A fundamental challenge facing gold nanocluster research is being able to understand the composition-dependent properties from a site-specific perspective in order to confidently establish structure-property relationships. A step in this direction is to examine the influence of various structural features (core geometry and thiolate-gold bonding motifs) on the bonding properties of gold-thiolate nanoclusters. In this work, ab initio simulations were conducted to systematically study the local structure and electronic properties of Au_(20)(SR)_(16) from each unique Au and S atomic site using Au L_3-edge extended X-ray absorption fine structure (EXAFS), projected density of states (l-DOS) and S K-edge X-ray absorption near edge structure (XANES) spectra. Two larger FCC-like gold-thiolate nanoclusters (Au_(28)(SR)_(20) and Au_(36)(SR)_(24)) were used for a comparative study with Au_(20)(SR)_(16), providing further predictions about the cluster size effect on the bonding properties of gold-thiolate nanoclusters with FCC-like core structures. Through this comparison, the smaller core size of Au_(20)(SR)_(16) produces an EXAFS scattering signature that is non-FCC-like but shows very similar electronic properties with a larger FCC-like gold-thiolate nanocluster.