EQCM and XPS investigations on the redox switching of conducting poly(o-aminophenol) films electrosynthesized onto Pt substrates

The redox behaviour of conducting poly(o-aminophenol) films (PoAP), potentiodynamically electrosynthesized onto Pt substrates, was studied by means of in situ Electrochemical Quartz Crystal Microbalance (EQCM), varying the composition, concentration and pH of the acidic supporting electrolyte. PoAP films at different oxidation stages were also characterized by ex situ X-ray Photoelectron Spectroscopy (XPS), stopping the anodic scan at +0.1 V (semi-oxidized PoAP) and +0.5 V vs Ag/AgCl (oxidized PoAP). The results were interpreted by comparison with previous investigations carried out on the reduced PoAP. Polymer oxidation proceeds through the deprotonation of aminic site susceptible then to oxidation. The incorporation of perchlorate ions occurs mostly at the beginning of the anodic scan till the peak potential is reached. At this stage of the oxidation positively charged nitrogens, polaron type, are present which then recombine each other to give bipolaron and, upon deprotonation, neutral immines. The overall PoAP redox oxidation is a reversible two electrons process complicated by chemical deprotonation steps before and after the oxidation itself. On the reverse scan immines require protonation in order to be reduced. A diffusional type limitation on the cathodic process was demonstrated and attributed to counter ions diffusion through the polymer accompanying its protonation. The XPS investigation allowed to unambiguously prove the presence of water inside the film, already suggested by the authors for the reduced PoAP by heating experiments in ultra-high vacuum conditions. Rinsing the polymer with acetonitrile before the XPS analysis, the relevant detailed C1s, N1s and O1s regions evidenced the presence of ammonium acetate coming from nitrile hydrolysis. A higher amount of water was evidenced in the oxidized states with respect to the reduced form. The exchanged molar mass calculated by EQCM revealed, indeed, solvent entrance in the last part of the oxidation. Accordingly, the Binding Energies characteristic of neutral nitrogen functionalities suggested that polymer chains are more distant in the oxidized state, preventing the incoming of hydrogen bonds.