Observation of Surface-Enhanced Raman Scattering for Transition-Metal Hexaammine Cations at the Outer Helmholtz Plane: Implications for Enhancement Mechanisms at Electrochemical Interfaces

摘要

The recent observation of surface-enhanced Raman scattering (SERS) for a variety of adsorbates at silver in electrochemical and gas-phase environments has generated a plethora of theoretical models to account for the remarkable (ca.1,000,000 fold) observed enhancement of the Raman scattering intensities. These models can be divided into those demanding only the presence of the Raman scatterer at or close to the metal surface ('physical models'), and those requiring specific adsorbate-surface interactions ('chemical models'). Prompted in part by the especially intense SERS seen for specifically adsorbed species at silver electrodes, a widely held viewpoint is that the enhancement is due in part to chemical interactions between the Raman scatterer and the metal surface, possibly involving as adsorbate-adatom 'complex'. The report examines SERS of inorganic adsorbates at silver-aqueous interfaces. One objective is to examine systematically how the SER spectra are influenced by the nature of the adsorbate-surface interactions. A valuable class of adsorbates for this purpose is provided by substitutionally-inert Cr(III), Co(III), Ru(III) and Os(III) ammine complexes containing ligands such as thiocyanate, bromide, or pyrazine that bind strongly to silver electrodes.