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

Significance Statement

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

 

 

About the author

Maria Elvira Egidia Carbone received the degree in Chemistry at the University of Basilicata (Italy) in 2011, where she is currently a PhD student (XXVIII cycle) in the Department of Science. Her research interests include the electrosynthesis and characterization of conducting and insulating polymers. In 2015 she was a doctoral visiting student at the University of Surrey (UK) for six months in order to explore by in situ Electro Chemical – Atomic Force Microscopy the polymer growth and to deepen the knowledge about advanced surface methods. The actual research field concerns mainly the investigation of poly(o-aminophenol), poly(o-aminophenol), films by combined electroanalytical and surface techniques particularly aimed to the development of a biosensor for amyloids based on insulating poly(o-aminophenol).

About the author

Rosanna Ciriello received her B.Sc Degree in Chemistry in 1995 from the University of Basilicata, Potenza Italy. During her studying she spent the period September – December 1992 at the University of St. Andrews (Scotland) as Erasmus Student. She received the Ph.D degree in Chemistry on February 2000 from University of Basilicata (Thesis: “Novel methods of electrodes modification for biomolecules immobilization and the realization of advanced electrochemical sensors”), carrying out part of the research activity at the Electrochemistry Laboratory of the University of Southampton (England) under the supervision of  Prof. P.N. Bartlett. Other academic position: postdoctoral fellow at the Chemistry Department of University of Basilicata (2000 – 2003) within the research project “Realization and studying of novel electrochemical biosensors for the development of advanced analytical devices of clinical, food and environmental interest”. She joined the Department of Science of University of Basilicata in 2005 as a Researcher in Analytical Chemistry. The research activity concerns the development of analytical methods in food and biological fields based on amperometric detection. Her current research interests include biosensors, electrosynthesis and characterization of insulating and conducting polymers for electrochemical and gravimetric devices. She is author of over 50 contributions including publications on international indexed journals and conference proceedings. [email protected]  

About the author

Sara Granafei graduated in Chemistry at the University of Basilicata in 2012. Her first academic research project was focused on electrosynthesis and EQCM characterization of conducting poly(o-aminophenol) films. She is currently PhD student at University of Bari in Department of Chemistry. In 2015 she received the “Research Grants – Short-Term Grants” in the Scholarship programme number 57130097 from German Academic Exchange Service (DAAD) allowed her to perform a project at the University Hospital of Regensburg about a focused lipidomic analysis of a food waste, European sea bass brain, to explore its nutraceutical properties. Her actual activity is centered on lipidomics and metabolomics based on liquid chromatography coupled to mass spectrometry for applications in food analysis, mainly seafood, and microbiology field, particularly aimed to bacteria for bioremediation.

About the author

Antonio Guerrieri graduated in Chemistry at the University of Bari (Italy). He did his postgraduate studies at the Department of Chemistry, University of Bari, receiving his Ph.D. in Chemical Sciences (curriculum Analytical Methodologies and Instrumentation). Researcher at the Faculty of Science, University of Bari, from November 1992 till now Antonio Guerrieri is Associate Professor in Analytical Chemistry at the Faculty of Science, now Department of Sciences, at University of Basilicata (Potenza, Italy). He teaches analytical chemistry in the chemistry and biotechnologies courses of his University. Antonio Guerrieri was Head of Chemistry Courses, Head of Biotechnology Courses as well as Member of Board of Governors and Vice Rector for Triennial Planning at University of Basilicata. His research interests mainly deal with bioelectrochemistry and the application of biological electron transfer processes in bioelectronic devices, the interaction between electropolymerised films and biological systems, the development of amperometric biosensors based on electropolymerised thin polymer films and the study of protein/ biomaterial interactions. Antonio Guerrieri, author of more than 100 scientific publications on international journals and congress meetings, acts from a long time as referee for several international journals,  manly in his research area.

About the author

Anna Maria Salvi: Associate Professor – CHIM/01 – Analytical Chemistry. University of Basilicata, Science Department

Teaching activity: Analytical Chemistry I – Analytical Chemistry of Surfaces and Interfaces (Triennial degree) Superior Analytical Chemistry (Masterly degree). The scientific activity is centred on the study of surfaces and interfaces by XPS (X-ray Photoelectron Spectroscopy) and specialises in the analysis of spectra by curve fitting using a program for data elaboration developed in the course of a long-standing collaboration with the University of Surrey, UK (PhD thesis). Work based on the analytical use of XPS in combination with AFM, SEM/EDS, SIMS and other surface/bulk analytical techniques has covered several projects of National and European significance aimed to the characterization of technological materials such as carbon fibres, meso and micro-porous catalysts, modified electrode surfaces as analytical sensors in complex matrices, intermetallic compounds and electrochromic devices based on lithium ion intercalation/de-intercalation into vanadium-based oxides (Intercalnet-RTN 2002-2006). Current researches, based on the combined use of XPS and other microscopic, spectroscopic and electro-analytical techniques are related to the molecular and supra-molecular characterization of biological samples for biomedical and pharmaceutical applications and of conducting and insulating polymers electrodeposited on surfaces for biosensor devices. The XPS laboratory for which I have full responsibility is steadily operative for demonstrative courses and applicative researches on environmental issues and on the preservation and restoration of our artistic and monumental heritage for regional and national needs. Over 65 published papers (ISI journals and peer reviewed long-abstracts) and as many Congress contributions were produced. 

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

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

Journal Reference

Electrochimica Acta, Volume 176, 2015, Pages 926–940.

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.

Abstract

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.

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