SURFACE-ENHANCED RAMAN SPECTROSCOPY OF TRANSITION-METAL COMPLEXES AT SILVER AND GOLD ELECTRODES
Abstract
Surface-Enhanced Raman (SER) spectra have been obtained at the silver-aqueous interface for transition-metal ammine complexes containing bridging ligands. SERS bands due to metal-ammine, internal ammine, and ligand-surface vibrations were observed. The intensities and frequencies of most of these bands were mildly potential dependent. However, the peak frequency and bandshape of the thiocyanate C-N stretch mode were potential sensitive, this can be understood in terms of electron withdrawal by silver surfaces. SERS have been observed for transition-metal ammine cations which cannot bind to the silver surface and are separated from it by a monolayer of halide anions. Their SERS intensities were comparable to those seen for closely related surface bound complexes. The formal potential of the Ru(NH(,3))(,6)('3+/2+ )couple obtained from SERS and electrochemical data was consistent with the Raman scatterers being located at the outer Helmholtz plane. The intensities and frequencies of SERS bands for these cations were not potential dependent, provided the halide coverage remained a monolayer. SER spectra were also obtained for unadsorbed anions. The observation of SERS for unbound ions is understood in terms of their electrostatic attraction with counter ions on the surface.(') Comparisons are made between the kinetic behaviors extracted from the SERS data and from faradaic measurements. For electrostatically attracted cations, the SERS and electrochemical rate behaviors are similar, while for directly bound cations, the reactivity at the SERS-active sites are higher than for those of prevalent adsorbates. Spectra have been obtained for cyanide and transition-metal complexes, at gold electrodes. The frequency and bandshape of the cyanide C-N stretching mode were sensitive to its surface concentration. The SERS frequencies of transition-metal complexes were indicative of their redox states. Observation of SERS for electrostatically bound reactants suggests their strong diffuse-layer adsorption, this enabled unimolecular rate constants, k(,et) (sec ('-1)) and transfer coefficients, (alpha)(,et), to be determined for Co(III) ammine cations on a halide coated silver electrode, using fast linear sweep voltammetry. Comparison with the corresponding values for the solution reactants at the mercury-aqueous interface enabled the influence of precursor stability upon the rate parameters to be assessed.
Degree
Ph.D.
Subject Area
Analytical chemistry
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