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.

In Situ Observation of Nucleation and Growth Process of Gold Nanoparticles by Quick XAFS Spectroscopy

“…The possible reason for the difference is the variation in the 5d state of AuNPs with the size and surrounding environment of AuNPs. [4,25,26] The size and protecting agents of AuNPs can cause variations in the d-d interaction of Au atoms and the d charge transfer to protecting agents. In fact, the difference can be observed, particularly in the energy region around the edge, where the 5d state of Au atoms mainly contributes to the spectral feature.…”

“…The purified AuNPs, covered with DT having 3.3 AE 0.5 nm diameter, and Au 11 (PPh 3 ) 8 Cl 3 were prepared as reported elsewhere. [11,25] The detailed synthesis methods and characterizations are described in the Supplementary Information.…”

Electronic structure, magnetic properties, and microstructural analysis of thiol-functionalized Au nanoparticles: role of chemical and structural parameters in the ferromagnetic behaviour

“…charge depletion in the NPs just above the Fermi level (López-Cartes et al, 2005), as it happens in the studied systems. Regarding the meaning of this change in the d-hole population, it has been previously stated by Benfield et al (2001) that a change in the local geometry of a given system could lead to a variation on the hybridation of the valence orbitals which would be reflected as an intensity change of the white line. However, the systems studied in the current research work present XANES oscillations at the very same points as bulk gold do.…”

Structure of metal nanowires in nanoporous alumina membranes studied by EXAFS and X-ray diffraction

“…Gold nanowires were found to have the same fcc structure and Au-Au distance as the bulk metal, but with smaller EXAFS Debye-Waller factors indicating a higher degree of crystallinity [10]. Figure 1 shows the WAXS patterns of nanowires in the 20V and the 60V membranes.…”

“…X-ray absorption near-edge spectroscopy (XANES) has shown the electronic structure of these nanowires to be intermediate between that of Au clusters and bulk Au [10].…”