Raman spectroelectrochemical monitoring of conducting polymer electrosynthesis on reflective metallic electrode: Effects due to double excitation of the electrode/film/solution interfaces

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Abstract Application of back-scattering geometry in Raman spectroelectrochemistry of laser-absorbing polymer films on reflective metallic electrodes results in different degrees of doubling of excitation (by the incident and reflected beams) of the electrode/film/solution interfaces. Taking this into account, this method was first applied to study in situ galvanostatic polymerization of aniline on Pt-electrode in aqueous solutions of polymeric sulfonic acids distinguished by different rigidity of the main chain. Firstly, it was found that 532 nm laser radiation caused fluorescence in the solutions of phenyl-containing polyacids resulted in the Raman spectrometer overload. One can reduce fluorescence by increasing the incident angle of the laser beam on the electrode up to 20° (perpendicular direction is taken as 0°). Secondly, some of the polyacids were found to have intensive Raman bands in the range of characteristic Raman frequencies (1000–1800 cm−1) of polyaniline (PANI). In this case, subtraction of the solution background spectrum before starting the electropolymerization monitoring causes appearance of ever-growing negative Raman signals as the laser-absorbing PANI film grows on the reflective electrode. The above two issues are due to the double excitation of the thin solution layer before the electrode by the incident and reflected laser beams, the degree of this doubling decreasing as (1) the incident angle is increased and (2) the PANI film thickness grows. The distortive influence of the solution Raman spectrum on the real shape of PANI spectrum may be estimated and diminished by using the amplitude of ever-growing negative Raman signal in the area of water O H vibration (near 3440 cm−1) as a measure of the ever-decreasing reflectance of the working electrode. Using the developed approach, it was shown that evolutions of the relative portions of the imine nitrogen (near 1490 cm−1) and the radical-cation nitrogen (near 1340 cm−1) fragments in the Raman spectra of PANI recorded in the course of electropolymerization can be used as informative criteria for the explanation of the differences in the aniline polymerization rate in the presence of polyacids of different chemical structure.