Extending the metal interface generality of surface-enhanced Raman spectroscopy: Underpotential deposited layers of mercury, thallium, and lead on gold electrodes

Abstract

Surface-enhanced Raman (SER) spectra are reported for several adsorbates at underpotential deposited (upd) layers of mercury, thallium, and lead on an electrochemically roughened gold electrode. For upd mercury monolayers, SER bands were obtained for the surface-halide stretching mode, ν M-X, of adsorbed chloride and bromide that are of comparable intensities to those observed on the unmodified gold substrate. The ν M-X peak frequencies are downshifted by 15–24 cm −1 on the former relative to the latter surface, consistent with a smaller extent of halide bond covalency on mercury. These spectral changes induced by upd formation could be reversed by anodic stripping of the mercury monolayer. Comparable results were also obtained by prior formation of the upd monolayer in a separate solution followed by electrode transfer rather than by deposition in the solution of interest. Satisfactory SER spectra at upd mercury are reported additionally for thiocyanate and pyridine, as are similar experiments for upd thallium and lead layers on gold. Of the above adsorbates, only pyridine yielded easily measurable SER spectra for these layers, having 2–3 fold smaller signal intensities than on unmodified gold. The SERS intensity decreases upon thallium, and lead upd formation exhibited both irreversible and reversible components. Differential capacitance-potential plots for upd mercury indicate some similarities to liquid mercury interfaces. The results indicate that overlayers on gold provide a means of extending SERS to metals that in themselves do not exhibit suitable Raman scattering enhancements.