Our research interest in the last years has been focussed on the electrosynthesis and characterization of poly(o-aminophenol) films (PoAP)1-3. This polymer shows the peculiarity of having tuneable properties depending on pH, evidencing a conducting or passivating behaviour if synthesized in acidic and neutral/basic media, respectively. We have recently investigated the polymerization route underlying the electrosynthesis of conducting poly(o-aminophenol) onto Pt substrates4. Relying on these results, in the present work a systematic and in deep investigation has been carried out to gain a further insight into the mechanism of PoAP redox cycling, by combining in situ (Electrochemical Quartz Crystal Microbalance) and ex situ (X Ray Photoelectron Spectroscopy) techniques.
While poly(o-aminophenol) oxidation has been always described as an electrochemical process complicated by the final imine deprotonation, our investigation revealed a marked influence of pH on the initial stage of the oxidation. This has led us to derive a reaction scheme in which a preliminary deprotonation step allows amine functionalities to undergo oxidation and then again deprotonation so that the whole PoAP oxidation can be regarded as a Chemical- Electrochemical-Chemical process. A marked influence of the nature and concentration of anions in the electrolyte solution was evidenced. The incorporation of perchlorate counter ions upon film oxidation occurs mostly at the beginning of the anodic scan. Conversely, mass flow at the polymer/solution interface does not involve cations.
The presence of uncoupled polarons type defects in correspondence of the anodic peak potential was postulated by combining EQCM and XPS experimental evidence. Polaron is the intermediate oxidized form responsible of the charge conduction by inter chain electron hopping assisted by the corresponding motion of counter ions coming from the external solution to maintain electroneutrality. From the anodic peak potential polarons begins to recombine each other to give bipolaronic structure (=NH+) and then neutral imine. Imines deprotonation establishes a mass transfer towards the external solution along with solvent inclusion into the polymer. The uptake of five water molecules for each H+×ClO4– ejected was detected.
Due to the prolonged mass ejection and solvent inclusion, changes in the polymer structure occur. Particularly our study has highlighted extremely important features related to the redox switching of poly(o-aminophenol) such as a hydration degree depending on the redox state. In the reduced state two water molecules every three nitrogens were evaluated. Polymer chains are held together by the establishment of hydrogen bonds mediated by water and by the same functional groups of the polymer. In the fully oxidized film water amount equilibrates nitrogen (about three water molecules every three nitrogens). The absence of hydrogen bonds proved by the Binding Energies of the nitrogen peaks, confirmed that the film is more swollen in the oxidized state than in the reduced one with chains more distant from each other, according to the broad shape of the anodic peak current.
Our experimental evidences are certainly an important achievement in view of possible applications of this polymer. Indeed the use of poly(o-aminophenol) for practical purpose is closely related not only to the knowledge of the functional groups present along the polymer skeleton but also to the variations in the hydration and compactness of the polymer chains upon charge conduction processes underlying its redox switching. In this context the proper evaluation of the specimen involved in the mass transfer processes occurring at the polymer/solution interface is of paramount relevance.
“Poly(o-aminophenol) electrosynthesized onto platinum at acidic and neutral pH: comparative investigation on the polymers characteristics and on their inner and outer interfaces”, M.E. Carbone, R. Ciriello, A. Guerrieri, A.M. Salvi, International Journal of Electrochemical Science 9 (2014) 2047
“XPS investigation on the chemical structure of a very thin, insulating, film synthesized on platinum by electropolymerization of o-aminophenol (oAP) in aqueous solution at neutral pH” M.E. Carbone, R. Ciriello, A. Guerrieri, A.M. Salvi, Surface and Interface Analisys 46 (2014) 1081
“XPS, AFM and electrochemical investigation on the inner composition of insulating poly(o-aminophenol), PoAP, deposited on platinum by CV, as a function of the number of cycles” M.E. Carbone, R. Ciriello, A. Guerrieri, A.M. Salvi, Surface and Interface Analisys DOI: 10.1002/sia.5910, 2015 in press
“Electrosynthesis of conducting poly(o-aminophenol) films on Pt substrates: a combined electrochemical and XPS investigation” M.E. Carbone, R. Ciriello, S. Granafei, A. Guerrieri, A. M. Salvi, Electrochimica Acta 144 (2014) 174
Figure Legend: Electrogravimetric profile (upper part) relevant to the oxidation of Poly(o-aminiphenol) and its detailed N1s XPS spectra (lower part) in the reduced (1), semioxidized (2) and fully oxidized (3) state
Maria E. Carbone, Rosanna Ciriello, , Sara Granafei, Antonio Guerrieri,Anna M. Salvi.
Dipartimento di Scienze, Università degli Studi della Basilicata, Via dell’Ateneo Lucano 10, 85100 Potenza, Italy.
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 poly(o-aminophenol) 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 poly(o-aminophenol) 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.Go To Electrochimica Acta