X-ray photoelectron and in situ and ex situ resonance Raman spectroscopic investigations of polythiophene overoxidation

Polythiophene films have been electrochemically synthesized on platinum electrodes by anodic oxidation of thiophene in CH3CN-LiClO4 organic solution, then galvanostatically polarized in the same but monomer-free electrolytic medium to obtain the reduced, oxidized, and overoxidized states of the polymer. X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate the changes within the chemical composition accompanying the overoxidation of the polythiophene (PT) film and connected to degradation mechanisms reported in the literature. Ex situ resonance Raman spectroscopy (RRS) studies of the three oxidation states, with red and near-infrared excitation laser lines, lead to complementary insights about the degradation process of this polymer. Particularly, marked modifications of the positions, widths, and relative intensities of the Raman bands are attributed to structural transformations and the appearance of defects in the polymer chains. Moreover, in situ RRS experiments, during the gradual transition between the reduced and overoxidized states, allow to determine the electrochemical stability threshold of the PT. In the reversibility domain, the quinoid to aromatic intensity ratio of the ring C=C symmetric stretching obeys a Nernst-like equation when the applied potential to the PT/Pt working electrode is varied.