Characteristics of vibration modes of polypyrrole on surface-enhanced Raman scattering spectra

In this review, we report the advantage of evaluating the structural details of conducting polymers by using surface-enhanced Raman spectroscopy. Before the electropolymerization of PPy, Au electrodes were roughened by oxidation-reduction cycles (orc) in a separate cell for 25 scans. The characteristics of various vibrational modes of polypyrrole (PPy), including C-H in-plane deformation shown in the 1000-1150 cm^-1 range, ring stretching shown from 1300 to 1430 cm^-1 and C=C backbone stretching shown at ca. 1600 cm^-1, are examined by surface-enhanced Raman scattering (SERS) spectroscopy. It is found that the conductivity and the doping level of PPy are strongly related to their corresponding Raman peaks. Some results can be confirmed from systematic studies. The C=C backbone stretching of PPy films in various situations downshifts to lower wavenumbers when PPy films have higher conductivities. However, only for as-prepared, non-aged and non-over-oxidized PPy films, do the intensities of the characteristically oxidized peaks, showing at the higher frequencies of the double peaks of the C-H in-plane deformation and the ring stretching, increase when PPy films have higher conductivities. To examine the structural situation in more detail, we recently developed a sandwiched structure, Au monolayer/PPy/roughened Au substrate. This exhibits a higher intensity by one-order of magnitude and a better resolution, as compared with the normal SERS of PPy electrodeposited on a roughened Au substrate.