Molecular conformation changes in alkylthiol ligands as a function of size in gold nanoparticles: X-ray absorption studies

Abstract

The bonding of hexanethiols to gold nanoparticles of 1.5, 2.0 and 3 nm was studied using X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS). The XANES spectra revealed that a substantial fraction of hexanethiol molecules were weakly bound, in addition to the molecules forming covalent bonds with Au surface atoms. The weakly bound molecules could be easily removed by washing in dichloromethane. After removal of the weakly bound molecules the S K-edge XANES spectrum reveals peaks due to S-Au and S-C bonds with intensities that change as a function of particle size. We explain this as the result of distortions occurring in the molecules adsorbed on the smaller particles. The distortions arise from the poor packing due to the high curvature of the particles. In addition, EXAFS results show that the smaller particles bind more molecules per Au atom than the larger ones, which is again interpreted as a result of the curved nature of the surface.