Relationship between SERS Intensity and Both Surface Coverage and Morphology for Pyrazine Adsorbed on a Polycrystalline Gold Electrode

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Abstract

The dependence of the surface-enhanced Raman scattering (SERS) of pyrazine (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 force microscopy (AFM), and the surface coverage data were obtained from previous electrochemical measurements. The plots between the SERS intensity of the pz ring-breathing mode (ca. 1016 cm-1) and the surface morphology parameters (roughness factor) were obtained. The SERS intensity reached a maximum between 20 and 30 ORCs. Independent AFM measurements indicated that roughness features with an average size of ca. 100 nm are present in the surface which yielded the strongest SERS signal. The dependence of the SERS intensity on the surface morphology agrees well with the electromagnetic calculations of the enhancement factor for gold spheres. The SERS intensities from pz adsorbed on an “unroughened” 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 interactions between the induced molecular dipoles present on a heavily packed surface. The SERS intensities from a smooth surface (zero ORC) were obtained by extrapolation from the spectrum of pz adsorbed on gold electrodes with different degrees of roughness. The calculated SERS intensities were compared to the surface coverage, and a linear relationship was obtained for a wide range of potentials.

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