A Surfaced-Enhanced Raman Spectroscopy and Density Functional Theory Study of [Au(CN)2]-/[Au(CN)4]- Adsorbed on Gold Nanoparticles

Surface-enhanced Raman spectroscopy (SERS) can be used to study chemical reactions on metal surfaces. Its detection limit makes it possible to observe long-lived intermediates and conformational changes to reactants and products. However, the interaction between the surface and the adsorbate makes interpretation of SERS spectra difficult. A case in point is provided by our studies of gold nanoparticles exposed to aqueous cyanide solution. These systems are characterized by time-dependent SERS spectra, suggesting either conformational or chemical changes to gold adsorbed cyanide species. Though there has been conjecture as to the nature of the process responsible for the evolving spectra, there is no definitive answer. Herein, we report the results of first-principles studies that, when combined with the SERS data, suggest that the observed shift of the SERS signals is attributable to the transformation of dicyanoaurate ions ([Au(CN)(2)](-)) to tetracyanoaurate ions ([Au(CN)(4)](-)) adsorbed to the metal. In the former case, we argue that the C-infinity axis of the ion is oriented parallel to the surface, and in the latter, the 4-fold axis is normal to the gold (111) surface. Further, the [Au(CN)(4)](-) complex is slightly distorted from its D-4h symmetry to enhance surface bonding.