Application of surface-enhanced Raman spectroscopy to mechanistic electrochemistry

The electrooxidation of dilute (1 mM) iodide at the gold-aqueous interface has been examined by rotating disk voltammetry combined with surface-enhanced Raman spectroscopy (SERS) in order to identify the surface species formed and hence to shed light on the electrooxidation mechanism. Marked changes in the SER spectra occur upon shifting the electrode potential through the region where faradaic current flows, the characteristic 123 and 158 cm−1 bands associated with adsorbed iodide being supplemented and eventually supplanted by bands at 110, 145 and 160–175 cm−1, the latter two being especially intense. The new bands are assigned to higher polyiodides and molecular iodine. The latter species appears to be the major interfacial product associated with faradaic current flow. Iodide forms an irreversibly adsorbed and electroinactive layer at gold in the absence of solution iodide, as evidenced by the survival of the 123 and 158 cm−1 SERS bands even at far positive potentials under these conditions. The results obtained for dilute iodide solutions are compared and contrasted with those obtained at higher iodide concentrations. For the latter conditions, the observed “surface” Raman spectra arise from resonance enhancement of the thick insoluble iodine films and solution triiodide formed in the convective diffusion layer rather than from SERS of species present in the double layer. Criteria for distinguishing between these two possibilities for systems involving such electrogenerated species are described.