Structure and Bonding of Gold Metal Clusters, Colloids, and Nanowires Studied by EXAFS, XANES, and WAXS

Abstract

The structure and bonding of a series of gold clusters and gold nanomaterials stabilized by ligands or confined within nanoporous alumina have been investigated using EXAFS, XANES, and WAXS. Two gold clusters stabilized by two different ligands, Au55(PPh3)12Cl6 and Au55(T8-OSS−SH)12Cl6, were confirmed to be of face-centered cubic structure type with metal−metal distances of 2.785 and 2.794 Å, respectively, shorter than in bulk gold. Colloidal gold of 180 Å diameter stabilized by sulfonated phosphine ligands had structural and electronic properties very similar to those of bulk gold but smaller Debye−Waller factors. The cluster Au55(PPh3)12Cl6 adsorbed into nanoporous alumina membrane was found to retain its integrity inside the membrane but with slightly longer Au−Au bonds due to some aggregation. The same cluster thermally transformed into colloidal gold within the alumina membrane was found to be almost identical structurally and electronically to the bulk. Gold nanowires electrochemically grown within the nanoporous alumina were found to be composed on average of 120 Å diameter crystallites. These have the same structure as the bulk, but with smaller Debye−Waller factors, indicating either a better crystallinity or that the gold atoms are more tightly held than in the bulk. The difference of area method L3 − kL2 was used to quantify the d orbital occupancy. The two ligand-stabilized Au55 clusters both had a smaller value (2.7) than the bulk material (4.1). The nanomaterials inside the membrane also showed smaller L3 − kL2 values. The geometrical and electronic structures of these gold materials show a very clear pattern of buildup as the number of gold atoms increases from Au55 clusters through Au colloids and nanowires to the bulk metal.