New strategies for surface-enhanced Raman scattering at transition-metal interfaces: Thickness-dependent characteristics of electrodeposited Pt-group films on gold and carbon

The surface-enhanced Raman scattering (SERS) properties of rhodium films electrodeposited onto a SERS-active gold substrate are examined as a function of overlayer thickness, using carbon monoxide and thiocyanate as chemisorbate probes in aqueous solution. Although the adsorbate Raman signals obtained for ultrathin (less than or equal to 10 monolayers, ML) Rh layers emanate from the Sold template, a significant or even dominant SERS component for thicker films is identified as arising instead from the transition-metal overlayer itself. The latter contribution was diagnosed and characterized from similar experiments undertaken using SERS-inactive gold and carbon substrates, and from the observation of comparable Raman enhancements obtained with green (514.5 nm) as well as red (632.8 nm) excitation. While the surface enhancement factors for these layers, of the order of 10(3), are markedly less than those obtained for ultrathin films on SERS-active gold, the signals even for very weak Raman scatterers (such as adsorbed hydrogen) on palladium and platinum, as well as rhodium, electrochemical interfaces are detectable with a confocal Raman spectrometer. The broad-based potential utility of such films for the detailed vibrational characterization of transition-metal ambient interfaces is also pointed out.