RELATIONSHIP BETWEEN SERS INTENSITY AND BOTH SURFACE COVERAGE AND MORPHOLOGY FOR PYRAZINE ADSORBED ON A POLYCRYSTALLINE GOLD ELECTRODE

The dependence of the surface-enhanced Raman scattering (SERS) of pyra zine (pz), adsorbed on polycrystalline gold electrodes (''smooth'' and rough), on both the surface coverage and morphology was investigated. The morphology of the rough gold electrode was observed by atomic for ce microscopy (AFM), and the surface coverage data were obtained from previous electrochemical measurements. The plots between the SERS inte nsity of the pz ring-breathing mode (ca. 1016 cm(-1)) and the surface morphology parameters (roughness factor) were obtained. The SERS inten sity reached a maximum between 20 and 30 ORCs. Independent AFM measure ments indicated that roughness features with an average size of ca. 10 0 nm are present in the surface which yielded the strongest SERS signa l. The dependence of the SERS intensity on the surface morphology agre es well with the electromagnetic calculations of the enhancement facto r for gold spheres. The SERS intensities from pz adsorbed on an ''unro ughened'' gold electrode track the surface concentration up to ca. two -thirds of a monolayer. As the amount of pz in the surface approaches the monolayer value, this dependence becomes inverse, due to the inter actions between the induced molecular dipoles present on a heavily pac ked surface. The SERS intensities from a smooth surface (zero ORC) wer e obtained by extrapolation from the spectrum of pz adsorbed on gold e lectrodes with different degrees of roughness. The calculated SERS int ensities were compared to the surface coverage, and a linear relations hip was obtained for a wide range of potentials.