Potential-Dependent Surface-Enhanced Raman Scattering from Adsorbed Thiocyanate for Characterizing Silver Surfaces with Improved Reproducibility

Abstract

Surface-enhanced Raman scattering (SERS) spectroelectrochemistry is used to characterize electrochemically roughened and highly polished polycrystalline silver SERS-active substrates. Changes in the nitrile stretching vibrational mode of adsorbed thiocyanate are used as an in situ spectroscopic probe: the potential dependence of band position (Stark tuning), shape, and scattering intensity of this mode are measured in order to investigate differences between SERS-active sites found on smooth and roughened electrode surfaces. Results obtained from thiocyanate adsorbed onto two different types of highly polished Ag surfaces (alumina and diamond polishing) show discrete populations of SERS-active adsorption sites that remain stable over a wide potential range. This behavior stands in contrast to that observed on electrochemically roughened surfaces, where very strong Stark tuning, large vibrational bandwidths, and irreversible loss of SERS enhancement upon negative potential excursions can be attributed to a diverse population of labile SERS-active sites that exhibit strong charge-transfer interactions with the adsorbate and large chemical SERS enhancement.