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

Electrochimica Acta 176 (2015) 926–940

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

A R T I C L E I N F O

A B S T R A C T

Article history: Received 30 April 2015 Received in revised form 8 July 2015 Accepted 8 July 2015 Available online 18 July 2015

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. ã 2015 Elsevier Ltd. All rights reserved.

Keywords: Poly(o-aminophenol) redox switching EQCM XPS analysis

1. Introduction The electrosynthesis of PoAP from acidic solution of its monomer brings to the formation of a conducting film with interesting electrochemical and electrochromic properties [1–3]. The conductive behavior of organic polymers with conjugated p bonds, comes from a doping process consisting in their redox oxidation and progressive formation of polaron (specie with a positive charge and a spin), and then bipolaron (specie with a double positive charge) type defects. Polaron is the intermediate

* Corresponding author. Present address: Dipartimento di Scienze - Università degli Studi della Basilicata, Viale dell’Ateneo Lucano 10 - 85100 POTENZA (ITALY). Tel.: +39 0971 205944; fax: +39 0971 205678. E-mail address: [email protected] (R. Ciriello). http://dx.doi.org/10.1016/j.electacta.2015.07.047 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.

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. Transport parameters relevant to electron and ion movements depend on the type and concentration of the electrolyte solution contacting the polymer but also on polymer thickness. In swollen films, for example, electron hopping is slow in comparison with ion transport determining an electron transport control [4]. Charge conduction processes underlying PoAP redox switching have been studied and reviewed [5]. The existence of intermediate charged species, i.e. radical cations, was associated to an oxidation process that occurs through two consecutive reactions from the totally reduced phenoxazine form to the completely oxidized one [6]. Controversies, indeed, arise about the two redox or single

M.E. Carbone et al. / Electrochimica Acta 176 (2015) 926–940

redox nature of polymer oxidation. The possibility to discern a two redox oxidation was sometimes dependent on the experimental conditions adopted. As an example, for the potentiodinamically prepared PoAP it was shown that its cyclic voltammetric profile with one redox couple in 1 M HClO4 was splitted into two pairs when the HClO4 concentration was increased to 5 M [7]. Holze and coworkers [6] showed that the cyclic voltammograms of PoAP films, synthesized potentiostatically at 0.7 and 0.8 V vs SCE, display two redox processes rather than a single redox one as reported in literature for the potentiodynamically synthesized. If controversies are present on the sequences of the redox oxidation, it is instead undoubted that a chemical reaction occurs coupled to the electron-transfer process. Particularly, being the response highly dependent on the solution pH, the chemical step was simply associated to protonation/deprotonation of nitrogen functionalities necessary to assure polymer conductivities. We have already demonstrated that conducting PoAP becomes non conducting if cycled at neutral pH [8]. In addition, we outlined the extreme importance of the electrolyte solution pH also on the electrosynthesis of the polymer which showed insulating behavior if grown in neutral media [9]. Till now several techniques have been employed to understand and describe the PoAP redox behavior and to highlight the existence of the intermediate polaron specie such as ESR [10], in situ Raman Spectroscopy [6,7], in situ UV-vis Spectroscopy [6]. Few studies were based on the employment of EQCM, despite the capability of this technique to provide useful information on polymer redox switching from mass transfer processes occurring at the polymer/solution interface. Levin et al. [11] limited the EQCM investigation to demonstrate that the effects of supporting electrolyte anion are negligible. The apparent exchanged molar mass, derived by relating the mass variation to the charge consumed during oxidation, was explained in terms of an insignificant insertion of the anions into the films. Zhang et al. [12] performing EQCM experiments by varying the anion present in solution (perchlorate, nitrate, solphate) deduced that ion doping and dedoping occurred in the oxidation and reduction processes of the polymer film. Ortega [13] employed quartz crystal impedance measurements to determine the number of molecules of water per polymer equivalent in its oxidized form. Other EQCM studies concern rather co-polymer based on PoAP [14,15]. In the present work, to gain a further insight into the mechanism of PoAP redox cycling, a systematic and in deep EQCM investigation has been carried out by varying experimental variables like scan rate, composition, concentration and pH of the supporting electrolyte. The results of this analysis have been combined with the important aspects coming from the XPS investigation of PoAP in different redox states. The film in the reduced state has already been characterized by the authors [16]. Relying on those results, here we focused our spectroscopic characterization on the partially oxidized and oxidized PoAP. The combined EQCM and XPS study allowed to derive a reaction scheme of the redox processes, confirming some assumptions while evidencing novel interesting aspects, like a chemical deprotonation step before and after the oxidation, as it will be shown later on. To our knowledge this is the first study in which an XPS investigation on different redox states has been attempted and combined with EQCM results. 2. Materials and methods 2.1. Chemicals o-aminophenol (oAP) was obtained from Sigma-Aldrich (Germany) and purified by re-crystallization in ethyl acetate [10]. All other chemicals were of analytical grade and were used

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without further purification. Monomer solutions were prepared in supporting electrolyte just before their use. Pure water supplied by Milli-Q RG unit from Millipore (Bedford, MA, USA) was used throughout. 2.2. Electrochemical and EQCM apparatus Polymer preparation and characterization was performed by an EG&G (Princeton Applied Research, Princeton, NJ) Model 263A potentiostat/galvanostat. Data acquisition and potentiostat control were accomplished with a desktop computer running the M270 electrochemical research software (EG&G) version 4.23. All experiments were carried out at room temperature in a standard three-electrode cell employing an Ag/AgCl/KCl (sat) reference electrode and a platinum counter electrode. For XPS analysis, the working electrode was a platinum foil (10
15
0.127 mm) 99.99% pure (Aldrich). No action was taken to remove oxygen from solutions. The simultaneous cyclic voltammetric and microbalance experiments were carried out with a Seiko EG&G QCA 917 electrochemical quartz crystal microbalance connected to the EG&G 263A potentiostat. The experimental setup consisted of a RG100 glass cell bottom mounted on the top of a Well-type Teflon quartz crystal holder (EG&G model QA-CL4), a 9 MHz AT-cut platinum plated quartz crystal (EG&G), a Pt rod auxiliary electrode and an Ag/AgCl/KCl (sat) reference electrode. The geometrical area of the electrode surface was 0.2 cm2. Microgravimetric data reported in terms of mass change were calculated using the Sauerbray equation. The proportionality constant, K, dependent on the characteristic of the piezoelectric crystal, was experimentally evaluated [17] and was equal to (10.7  0.3)108 Hz g1 (mean  SD, n=3). All the cyclic voltammograms and microgravimetric profiles where realized by scanning the potential from -0.2 V (starting and stopping potential) to +0.5 V (inversion potenzial) vs Ag/AgCl, saturated KCl. In all EQCM figures positive potentials were set to the right and anodic currents upward according to the common use. At the purposes of simplification polarization direction was specified by placing arrows on both the current and the gravimetric profile of the fist EQCM figure. 2.3. PoAP films deposition Before modification, platinum substrates were cleaned following the optimized procedure already indicated by the authors [8]. PoAP films were electrochemically grown by cyclic voltammetry, 125 scan cycles, using a 5 mM oAP solution in perchlorate electrolyte (HClO4 0.1 M/KClO4 0.1 M, pH 1.1). The potential ranged from 0.2 to +0.9 V (vs. Ag/AgCl, saturated KCl) at a scan rate of 50 mV/s. Electrochemically synthesized PoAP films were then washed with double-distilled water, unless otherwise stated, and dried at room temperature in nitrogen atmosphere before their ex situ XPS analysis. 2.4. XPS measurements and curve-fitting procedure XPS analysis of the electrochemically synthesized PoAP films was performed by a Phoibos 100-MCD5 spectrometer, operating in Medium Area lens mode (spot of Ø = 2 mm, entrance slit of 7
20 mm). Spectra were acquired with achromatic Al Ka radiation (1486.6 eV) operating at 10 kV and 10 mA. The pressure in the analysis chamber was typically about 109 mbar during acquisition. A pass energy of 9 eV was used to collect the wide and the detailed spectra, using fixed analyzer transmission (FAT) operation mode with channel widths of 1.0 and 0.1 eV, respectively. XP spectra were acquired at a 0 take off angle, unless otherwise stated. The energy scale of the spectrometer was calibrated with

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Cu2p3/2 (932.7 eV), Ag3d5/2 (368.3 eV) and Au4f7/2 (84.0 eV) signals using pure metals (Johnson Matthey) for spectroscopic analysis. The XPS spectra were analyzed using a curve-fitting program New Googly which gives to each individual peak its own intrinsic Shirley-like background [18] and extrinsic tail, as fully described in previous works [19,20]. Peak areas were converted to atomic composition using established procedures and the appropriate sensitivity factors, SF [21]. The criteria adopted for data elaboration were based on preliminary analyses of reference compounds and literature data to assure the correct sample stoichiometry, in the limit of XPS accuracy [22,23]. The wide spectra are reported, as acquired, in kinetic energy. The XPS figures of the detailed regions and peak assignments reported in the relevant tables are converted to binding energies (BEs) and corrected for surface charging by referring to C1s aromatic carbon, used as an internal standard, at 284.8 eV. 3. Results and discussion 3.1. EQCM behaviour of conducting PoAP The voltammetric and electrogravimetric behaviour of a typical PoAP film in acidic supporting electrolyte (HClO4 0.1 M/KClO4 0.1 M, pH 1.1) is shown in Fig. 1. As it is possible to see from the voltammogram, current peaks associated with the reduction and oxidation of the film are not symmetrical showing a peak potential separation of about 121 mV. In addition, current intensities underlying oxidation and reduction processes are quite different and the oxidation peak appears considerably broader with respect to the reduction one. Such a behavior has already been reported in literature and has been mostly interpreted in terms of reducing and oxidizing activity modulated by the swelling of the film. The presence of a significant swelling of the polymer in one of its redox state, in the present case the oxidized one, will decrease the internal number of active sites per volume unit so that a small and broad peak could be expected [10,12]. Broad cyclic peak shape with nonzero DEp has been ascribed in the case of cross-linked donor bound polymer films [24] to the considerable degree of bound donor site flexibility and to the heterogeneous distribution of intersite distances. In this context Tucceri [25] showed that the increase of resistance during POAP oxidation can be explained in terms of an interfacial distribution of imine sites in the oxidized state with a spacing among them larger than that corresponding to amine sites in the reduced state. The shape and width of current-potential peaks for immobilized molecular charge-transfer states have been explained

1 5

4

5.0e-5

0 -5

3 -5.0e-5

-10

-1.0e-4

m/ng

Current/A

0.0

-15

2

-20

-1.5e-4

-25 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Potential/V vs Ag/AgCl Fig. 1. Voltammetric and microgravimetric profiles of a Pt quartz crystal electrode modified by PoAP acquired in the supporting electrolyte (HClO4/KClO4 0.1 mol dm3, pH 1.1) at a scan rate of 50 mV/s.

also in terms of interactions with their microenvironment: repulsive interactions among oxidized and reduced states cause broadened current potential waves [26]. In order to better understand the mechanisms that characterize the oxidation and reduction of PoAP and then to justify the voltammetric peaks shape, in the present work cyclic voltammetry has been combined with in situ QCM. During oxidation, polymer undergoes a process of doping consisting in the formation of positive charges, polaron and then bipolaron type, which requires the mass transport of negatively charged ions from the external solution for the electro-neutrality maintenance within the film. From the overlap of the voltammetric and gravimetric profiles (see Fig. 1), a rather unusual behavior however comes out: the expected mass increase, due to the entrance of ClO4 anions as the film is positively charged, is observed only in the first portion of the oxidation scan (stage 1 in the gravimetric profile). From a potential value approximately equal to the anodic peak potential, a mass decrease is observed until the end of the oxidation scan which slightly continues after reversing the potential scan (stage 2). A net mass increase is observed in conjunction with the incoming of the reduction peak (stage 3). Reduction scan ends with a small mass decrease (stage 4). The interpretation of the gravimetric profile is not so immediate. The mass number calculated by EQCM does not match the molecular weight of perchlorate as already found by Levin et al. [11]. In that case an insignificant insertion of the anions into the films was postulated. We think, instead, that this is rather the consequence of the interplay of compensating events, i.e. anion and proton entrance or expulsion at the polymer/electrolytic solution interface along with solvent molecules. In order to deepen this topic the instantaneous mass/charge ratio (F*dm/dQ) was evaluated during the anodic scan [27]. The values of such a function calculated punctually at any potential were not indicative of perchlorate ions molecular mass. Notable was instead the change of the function sign that was positive in the potential interval corresponding to stage 1 and became negative passing to stage 2. This evidence attests that during stage 1 mass is inserted into the polymer (during oxidation dQ>0 and then the function is positive for dm>0); during stage 2 mass is released from the polymer (during oxidation dQ>0 and then the function is negative for dm<0). The change in the mass profile during the anodic scan is therefore effective in indicating mass transfer towards or from the polymer. A more detailed treatment of the differential mass data will be given later on when discussing the influence of anion (see section 3.1.4 Influence of anion and cation). An interpretation of the gravimetric profile of PoAP was given by Zhang et al. [12]. The initial decrease in frequency was attributed to the doping into the film of negative charged ions consequent to the formation of a positive charge upon losing an electron at a specified potential. Proceeding in the positive scan, the film was further oxidized and another electron and two protons were lost. The polymer film gets back to its neutral state and then the negatively charged ions were dedoped from the film justifying the increase in frequency. During the reduction the neutral oxidized film was reduced and then, upon protonation, converted into positively charged PoAP; negatively charged ions were doped into the film with a frequency decrease. Such an interpretation was only qualitative and does not justify the imbalance between mass inserted on stage 1 and mass expelled on stage 2. Being the electrogravimetric profile shown in Fig. 1 not exhaustive in explaining all the processes involved in the redox cycling of PoAP, a more systematic EQCM study has been carried out by varying experimental variables like scan rate, pH, composition and ionic strength of the supporting electrolyte. A qualitative and quantitative study was performed comparing the

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modifications induced on the Dm values associated to each stage depicted in Fig. 1. It is important to outline that the EQCM study was made possible since films were shown to be rigid under the experimental conditions adopted in this work, even if the polymer was sufficiently hydrated (see XPS experiments). The quality factor was derived from the admittance spectra as the ratio between the frequency at maximum and the full width at half maximum. Passing from bare Pt electrode to Pt covered by PoAP, both immersed in the electrolyte solution, a negligible variation was estimated (1873.7 and 1873.4 respectively, data not shown). Thus, the frequency variations can be described in terms of mass movements rather than viscoelastic changes. 3.1.1. Influence of pH The electrochemical response of PoAP is highly dependent on the solution pH. Barbero et al. [28] showed that at pH higher than 7 no response was observed. Exactly, at pH close to 5 cathodic response disappears whereas this occurs for the anodic peak at pH 7. A process of addition/elimination of protons coupled with a reversible electron transfer was proposed. In order to investigate the role played by H+ ions in the redox switching of PoAP, the electrogravimetric profile has been acquired at pH 0, 1 and 2 while fixing perchlorate concentration as it is specified in the caption of Fig. 2. The pH decrease causes a shift of both peaks to more positive potential values highlighting that the reduction process is favored, while the oxidation one is hindered at higher proton concentrations. Such evidence confirms that a process of deprotonation

5.0e-5

Current/A

0.0

-5.0e-5

pH 0 pH 1 pH 2

-1.0e-4

-1.5e-4

929

and protonation of the polymer is associated respectively to its redox oxidation and reduction. The pH dependence of the voltammograms was quantitatively evaluated by plotting the formal potential, E0 , estimated as (Epa + Epc)/2, against pH. The plot, here not showed, was linear with a slope of about -73  22 mV/pH (Student’s t evaluated for 1 degree of freedom and a 95% confidence level, r2= 0.9994), indicating that protons and electrons take part in the electrode reaction in a 1/1 ratio, as it was already outlined [29]. Also Komura et al. [4] found that one proton is released for each electron transferred during oxidation. The whole oxidation process therein described is electrochemical – chemical type (EC): oxidation ends with immine deprotonation whereas reduction starts with immine protonation. Peak potentials shifts, experimented in our work by varying pH, are in accordance with this interpretation. A careful investigation of the influence of pH on the gravimetric profiles showed, however, a marked difference at the beginning of the oxidation scan (stage 1 in Fig. 1) when, presumably, anion are inserted into the polymer. The relative mass changes are listed in Table 1 where roughly a halving of Dm by increasing pH of one unit was observed at this stage. The increase of Dm in the stage 1 could be justified, as a first approximation, in terms of the more extended potential interval relevant to this stage, consequently to the shift of anodic peak potentials as pH was decreased. This possibility was ruled out by fitting the linear part of the gravimetric profile relevant to this region. The slopes of the corresponding regression lines evidenced an increase with H+ concentration, indicating therefore an absolute increase of Dm for a fixed potential interval. Slopes of 34.6  0.5, 26.6  0.5 and 17.4  0.7 ng/V were found respectively at pH 0, 1, and 2. Another important consideration comes out from the beginning of the gravimetric profile where mass increase is observed after a short rest period. This leads us to exclude that the mass increase could be due to a higher degree of film protonation, and then anion concentration within the film, since this process would be immediate and not initiated by the potential scan towards oxidation values. Certainly, the strong dependence on pH would suggest that stage 1 is not based on the solely entrance of counterions upon oxidation, as often described in literature. Indeed at acidic pH values, the amino groups are protonated in the reduced form [30]. The loss of a proton is therefore necessary in order to remove one electron. A flux of protons paired to perchlorate anions is set inside the film towards the external solution. A counter flux of perchlorate anions goes in the opposite direction to compensate Table 1 Mass variations associated to the four stages of Fig. 1.

5

Dm1 (ng) Dm2 (ng) Dm3 (ng) Dm4 (ng)

Experimental variable pHa

pH 0 pH 1 pH 2

7.77 4.04 2.75

17.10 14.64 14.62

14.29 13.16 13.14

2.88 1.16 0.60

Scan rateb (mV/s)

10 20 50 100 150 200

8.88 9.61 9.29 10.34 9.40 9.41

26.73 25.76 23.20 21.10 18.07 15.57

20.19 19.42 18.15 16.17 13.81 11.20

4.49 4.73 4.24 5.29 5.01 4.82

[NaClO4]a (mol/dm3)

0.0 0.1 0.5 1.0 2.0

2.82 3.76 4.30 7.29 7.76

23.83 22.30 20.37 17.40 7.10

20.92 20.39 16.45 12.41 4.02

0.09 0.28 0.37 2.26 4.86

m/ng

0

-5

-10

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Potential/V vs Ag/AgCl Fig. 2. Voltammetric (upper part) and microgravimetric (lower part) profiles of a Pt quartz crystal electrode modified by PoAP acquired by varying pH of the supporting electrolyte while fixing perchlorate concentration: HClO4 1 mol dm3 (pH 0), HClO4 0.1 mol dm3 /NaClO4 0.9 mol dm3 (pH 1), HClO4 0.01 mol dm3/NaClO4 1 mol dm3 (pH 2). Scan rate: 50 mV/s.

a b

scan rate: 50 mV/s supporting electrolyte: HClO4 1 mol dm3, pH 0

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the positive charges generated upon oxidation. At lower pH, the H+ gradient concentration between polymer and the external electrolyte solution prevents the transport of H+ and ClO4 coming from amine deprotonation towards the outside and then an apparent higher mass increase is observed. The excess of positive charge generated upon oxidation acts in the stage 2 as a driving force that promotes anyway the flux of the ionic pair H+ ClO4 coming this time from immine deprotonation towards the outside of the film. This explains the scarce influence of pH on this stage. In order to elucidate further the experimental evidences illustrated so far, and to check if pH influences only the protonation/deprotonation processes or a hindered diffusion of H+ ions within PoAP could be hypothesized, the effect of scan rate was studied. The value of 0 was chosen for the pH since a greater symmetry between oxidation and reduction peaks was observed while assuring the maximum film protonation. 3.1.2. Influence of scan rate As it was already stressed, organic polymers become conducting upon a doping process based on their oxidation. Electron transport is ensured by inter-chains electron hopping and requires ion transport in order to assure electro-neutrality. Depending on the experimental conditions, one of the two carriers, electron and ions, moves much faster than the other. In swollen films, for example, electron hopping is slow in comparison with ion transport [4]. Barbero et al. [28] considered the protonation of the oxidized form of PoAP as the chemical reaction coupled to the redox one, which may be a rate controlling step due to the restriction of H+ diffusion through the film matrix. In this respect

Current/A

2.0e-4

0.0

-2.0e-4 10 mV/s 20 mV/s 50 mV/s 100 mV/s 150 mV/s 200 mV/s

-4.0e-4

-6.0e-4

10

m/ng

0

-10

-20 -0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Potential/V vs Ag/AgCl Fig. 3. Voltammetric (upper part) and microgravimetric (lower part) profiles of a Pt quartz crystal electrode modified by PoAP acquired by varying the scan rate in the range 10–200 mV/s. Supporting electrolyte: HClO4 1 mol dm3. Arrows indicate the trend experimented by increasing scan rate.

scan rate is an experimental variable that can provide useful information. Fig. 3 shows the cyclic voltammograms acquired in HClO4 1 M by varying scan rate in the range 10–200 mV/s. A greater influence on the reduction peak was immediately observed: increasing scan rate the reduction potential is shifted towards more cathodic values while for the oxidation peak there is not a substantial variation. A similar trend was found by Ortega et al. [10] highlighting an inhibited PoAP reduction as scan rate is increased. The dependence of the cathodic (Ipc) and anodic (Ipa) peak currents from the scan rate was evaluated. For the anodic process a better linear dependence was observed with the scan rate, r2=0.9788, whereas the cathodic peak exhibited a linear dependence with the square root of scan rate, r2=0.9974. In light of these findings it is possible to assume that there is a diffusional type limitation on the cathodic process, which could be the proton hindered transport through the film matrix, as already found by Barbero et al. [28]. This is in accordance with the influence of pH on the polymer reduction previously described, i.e. decreasing the pH protonation is favoured and then reduction as well. In literature there are controversies about the dependence of peak currents on the scan rate. Zhang et al. [12] evidenced a cathodic peak current linearly correlated with the scanning rate. Both Ipa and Ipc were found to scale linearly with v(1/2) in the range of v from 30 to 200 mV/s by Kunimura et al. [29], indicating that the charge transport process within the film (the thickness was 0.4 mm) is diffusion controlled. Finally also for Ohsaka et al. [31] Ipa and Ipc were proportional to v(1/2), indicating that both processes are diffusion controlled. In order to strengthen our voltammetric findings and to dissipate controversies on the topic, we have analyzed the gravimetric profiles acquired by varying scan rate as it is shown in Fig. 3. Also in this case mass variations were calculated in the four stages of a typical redox cycle and the corresponding values are given in Table 1. The mass variation of the second and third stage of the gravimetric profile, which would correspond respectively to the expulsion and entrance of H+ and perchlorate ions, are comparable, indicating that the mass transfer from and into the polymer is counterbalanced. These stages are also the most influenced showing an evident attenuation with increasing scan rate. Presumably, at higher scan rates mass transfer has not enough time to take place, thus confirming the diffusional limitation found from voltammetric analysis. In light of such evidence, it can be stated that the process of polymer deprotonation (protonation) and the corresponding movement of counterions, could be the rate determining step. In order to separate the contribution arising from protons from that of counter ions, perchlorate concentrations was varied, while keeping the pH solution constant. 3.1.3. Influence of ionic strength Fig. 4 shows the voltammetric profiles acquired by gradually increasing perchlorate concentration in the supporting electrolyte, while maintaining pH at about 1. As it can be seen from the figure, an increase of perchlorate concentration, as previously seen for proton concentration, causes a shift of both peak potentials towards more positive values. Again, the cathodic peak suffers a more pronounced shift than the anodic one showing that a higher ionic concentration favors the reduction process which just contemplates proton and anion transfer from the solution into the film. Opposite consideration must be done for the oxidation process. As concern peak currents, for both processes there is an increase with increasing perchlorate concentration. Barbero et al. [28] reported as well that a high concentration of perchlorate makes PoAP more easily reducible. Large quantity of

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verify this, the nature of anion in the electrolyte solution was varied. 5.0e-5

Current/A

0.0

-5.0e-5 NaClO4 0.0 mol dm-3 NaClO4 0.1 mol dm-3

-1.0e-4

NaClO4 0.5 mol dm-3 NaClO4 1.0 mol dm-3 NaClO4 2.0 mol dm-3

-1.5e-4

10 5

e

m/ng

0 -5

d -10

c b a

-15 -20

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Potential/V vs Ag/AgCl Fig. 4. Voltammetric (upper part) and microgravimetric (lower part) profiles of a Pt quartz crystal electrode modified by PoAP acquired in HClO4 0.1 mol dm3 by varying sodium perchlorate concentration: a 0.0 mol dm3, b 0.1 mol dm3, c 0.5 mol dm3, d 1.0 mol dm3, e 2.0 mol dm3. Scan rate: 50 mV/s. Arrows indicate the trend experimented by increasing perchlorate concentration.

electrolyte, indeed, can shield the redox centres from interaction between them. The existence of a more compact distribution of these centres increases the rate of the charge-transport process by intrinsic electron hopping. Fig. 4 shows the trends of the gravimetric profiles that seem to greatly suffer the increase of perchlorate concentration. By increasing this parameter, a constriction of the profile is observed across the range of the explored potential, with the exception of the initial portion of the oxidation scan and the last portion of the reduction scan. The Dm values calculated in the four stages of the scan cycle are reported in Table 1. As it is possible to see, varying perchlorate concentration from 0 M to 2 M, Dm1 and Dm4 increase while Dm2 and Dm3 notable decrease. The stage 1 reflects a higher amount of ions entering into the polymer thanks to the higher electrolyte concentration. When ion concentration in the film is sufficiently high, a loss of permselectivity occurs which allows the indiscriminate permeation of all ions, practically the concentration gradient is flattened. This typically happens when the electrolyte concentration outside is of the order of magnitude of the concentration of the fixed charged sites within the polymer [32]. In conclusion, the electrogravimetric experimental results illustrated so far evidenced that the influence of counter ions concentration on Dm1 and Dm4 is the same exerted by H+ ions. While pH showed no effect on the stages 2 and 3, perchlorate concentrations showed a marked variation also on these stages. Then, reasonably, the diffusional limitation found by varying scan rate precisely during stages 2 and 3 could be more dependent on anion transport across the polymer rather than on proton ones. To

3.1.4. Influence of anion and cation A crucial effect of the electrolyte composition has been reported on the charge conduction process of the polymer [33]. The influence of the type of the electrolyte on the charge transport and charge transfer rate at thick PoAP was explained in terms of the incorporation of the electrolyte into their open structure which facilitates the electron hopping process by reducing the repulsive interactions between redox sites [33]. In this regard it is possible that ClO4 anions were more effective than Cl to shield the redox centres from interaction between them. Strong interaction of ClO4 are presumed whose size corresponds to the distance between two aminophenol units. The large conjugated polymer chains with delocalized positive charges can be considered as soft acid (Pearson theory). The best fitted dopant should be a soft base in which a charge delocalization occurs. The radius of ClO4 is larger than Cl carrying a highly delocalized charge [34]. Elsewhere it is reported that the shape of the Cl anions is spherical so they tend to have stronger interaction with the polymer chains and also cause the planarity of the chains increasing conjugation [35]. In order to confirm the dynamics of the mass transport upon the change of the charge state of the polymer during its redox cycling, a study on the influence of relative size of the anions present in solution has been carried out by choosing just the two anions mentioned. It is important to remark that it is mainly the electropolymerization process to be influenced by the presence in the deposition solution of different anions, both in terms of speed of growth and polymer morphology and conductivity. As an example, the films electrodeposited in the presence of HCl are more compact and present less surface defects than those deposited in presence of nitric and sulphuric acids. On the other hand the PoAP thin films deposited in the presence of H2SO4 are very thick and present a rough surface compared to the others [36]. In the present work, however, the composition of the deposition solution has never been varied and the influence of the anion has been studied only on the formed film. The electrogravimetric profile of PoAP in two different electrolyte solutions, HClO4/KClO4 0.1 M and HCl/KCl 0.1 M both at pH=1, has been acquired, as it is shown in Fig. 5 A and B. The presence of a smaller anion, like chloride one, determines, as expected, an attenuation on mass variation. Apart from this aspect, the gravimetric behaviour is similar thus indicating that the interplay between entrance and exit processes through the polymer does not undergo appreciable changes. The voltammetric profiles look also similar, evidencing just a reduction peak slightly more pronounced and shifted towards higher potential values in the case of chloride. A systematic study of the influence of the anion on the exchanged apparent molar mass has been carried out. The interpretation of this study was made possible thanks to the results so far exposed. Fig. 5C and D shows the mass vs charge plots acquired respectively for the oxidation and the reduction processes in the presence of perchlorate and chloride ions. For both processes a notable difference was noticed by varying the anion thus confirming its involvement during the entire redox cycling of PoAP. The apparent exchanged molar mass Mapp, defined as Mapp = zF(Dm/DQ)

(1)

where z is the charge of the ion and F is the Faraday constant, can be evaluated as the slope of the mass vs charge plot using linear regression. This evaluation was complicated by the pronounced

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Fig. 5. Voltammetric (A) and microgravimetric (B) profiles of a Pt quartz crystal electrode modified by PoAP acquired by varying the anion in the supporting electrolyte: HClO4/KClO4 0.1 mol dm3 (solid line) and HCl/KCl 0.1 mol dm3 (dot line), pH 1. Scan rate: 50 mV/s. Comparison of the relevant mass vs charge balance for the anodic (C) and the cathodic (D) scan.

curvature of the plot, especially at the beginning of the oxidation scan. As previously discussed, when anodic scan starts different processes happen like aminic nitrogen deprotonation necessary for oxidation to take place, anion ingress upon oxidation along with solvent movement. Due to the simultaneously exchanged species, Mapp is not constant showing a potential dependence. In the second part of the oxidation, however, linearity extends over a wider potential interval. Indeed we hypothesized that at this stage of the oxidation the deprotonation of the oxidized moieties occurs. It is important to outline that the change in the sign of the slope from positive (net mass entrance) to negative (net mass ejection)

exactly mirrors the two regions depicted in the anodic scan of the gravimetric profile (stages 1 and 2). As concern reduction, an extended linear interval was envisaged in which mass entrance occurs, corresponding to the stage 3 depicted in Fig. 1. Only at the end of the reduction, plot curves, defining a poorly evidenced region in which an apparent mass decrease occurs (stage 4 of Fig. 1). After the evident mass entrance that we attributed to imine protonation, the incoming of reduction causes anion expulsion and subsequent protonation of the reduced functionalities. Again the establishment of various processes justifies the plot curvature, complicating the estimation of Mapp.

Table 2 Apparent molar mass for the oxidation and reduction of PoAP by varying the anion of the electrolyte solution. Scan rate 50 mV/s. Anion

Potential interval (V vs Ag/AgCl)

Mapp  Std Errc g/mol1

ad

Average Mapp  Std Deve g/mol1

Oxidationa

ClO4 Cl ClO4 Cl

-0.120/-0.030 -0.124/-0.040 +0.090/+0.480 +0.030/+0.490

+10.3  0.2 +2.12  0.04 -8.8  0.1 -4.78  0.03

(-5) (-2) +5 +2

+11  6 (n=6) +2.3  0.9 (n=5) -8.7  0.3 (n=20) -4.9  1.1 (n=23)

Reductionb

ClO4 Cl ClO4 Cl

+0.070/-0.070 +0.070/-0.070 -0.146/-0.180 -0.154/-0.180

-6.58  0.02 -4.49  0.03 +2.94  0.02 +1.87  0.04

+5 +2 (-5) (-2)

-6.0  0.9 (n=8) -4.0  0.8 (n=8) +2.6  0.3 (n=3) +1.2  0.9 (n=3)

a

oxidation: positive mass are inserted, negative one are rejected reduction: negative mass are inserted, positive one are rejected from F*(slope of m/Q plot  standard error) d the number of the exchanged solvent molecules, a, is to be considered accurate only in the potential interval where mass vs charge plot was almost linear and the derivative function constant (see text for more details) e mean value  standard deviation of the n data point comprised in the specified potential interval of the dm/dQ plot b c

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Table 2 lists the values of Mapp calculated from the slope of the mass vs charge plot in the specified potential intervals. Particularly for each of the four stages of Fig. 1 the potential interval in which the plot was almost linear was individuated. Alternatively we used the derivative F*dm/dQ to obtain the instantaneous potential dependent value of Mapp. The apparent molar masses obtained from the slope of the mass vs charge plot are to be considered as the average values of those calculated from the derivative [37]. To verify this, the average of the potential dependent Mapp values, evaluated in the same potential interval, has been added in Table 2. Considering the associate standard errors, a good agreement was noticed. The behavior of the Mapp, derived from the derivative dm/ dQ, as a function of the applied potential is reported in the supplementary Figs. S1 (perchlorate) and S2 (chloride). Again a marked variation of the function at the beginning of the oxidation was noticed, which reaches a more constant value in the second part of the oxidation from about 0.1 V. In Table 2 the number of solvent molecules transferred per ion, a, was evaluated by means of the following equation: Mapp = Mion + aMsolv

(2)

where Msolv is the molar mass of water. As concern Mion, in the first part of the anodic scan one perchlorate replaces five solvent molecules in the film whereas in the second part one perchlorate and one proton coming from imine deprotonation, are replaced as well by five water molecules. The first region of the reduction mirrors the second one of the oxidation: one ClO4 H+ replaces five solvent molecules. In the last part of the reduction perchlorate ejection is probably attenuate by amine protonation and then the water content estimated may not be true. Indeed we think that the number of exchanged water molecules should be estimated only in

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the potential interval where a single process happens and then derivative function remains constant, i.e. the second region of the oxidation and the first one of the reduction. These are the two zones where mass fluxes are more balanced (mass ejected is similar to mass inserted). Then it is reasonably to suppose that oxidation causes a more hydration of the polymer. The apparent molar mass and the solvent exchange behavior depicted in Table 2 is in agreement with the different dimensions of the investigated anions. The smaller chloride ion replaces ca. two water molecules. It is noteworthy that for chloride a mass balance is present between the first region of the oxidation and the second one of the reduction, and between the second region of the oxidation and the first one of the reduction. Considering a possible diffusion limitation of the larger perchlorate, the same treatment has been done for this anion at lower scan rate (data not shown). Indeed at 10 mV/s and pH 0, higher apparent molar mass values were obtained. Water molecules exchanged during the second region of the oxidation were about 3, instead of 5. This means that, fixing the number of exchanged water molecules, the amount of transferred anion is higher at lower scan rate, as it would be expected. Finally, the apparent molar mass of the first part of the reduction was now clearly lower than that relevant to the second region of the oxidation, confirming the diffusion limitation already evidenced on imine protonation. A similar study was conducted by varying the cation present in the supporting electrolyte (see Fig. 6). The PoAP modified electrode was then cycled from -0.2 to 0.5 V in a solution of HNO3/KNO3 0.1 M and HNO3/CsNO3 0.1 M both at pH=1. In this case voltammograms (Fig. 6 A) and the gravimetric profiles (Fig. 6 B) are practically superimposable. Also in the mass vs charge plots (Fig. 6C and 6 D) no significant changes were detected by varying the cation. It can

Fig. 6. Voltammetric (A) and microgravimetric (B) profiles of a Pt quartz crystal electrode modified by PoAP acquired by varying the cation in the supporting electrolyte: HNO3/KNO3 0.1 mol dm3 (dot line) and HNO3/CsNO3 0.1 mol dm3 (solid line), pH 1. Scan rate: 50 mV/s. Comparison of the relevant mass vs charge balance for the anodic (C) and the cathodic (D) scan.

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therefore be concluded that cation present in solution takes no part in both the redox reactions and the following mass transfer processes within the polymer. 3.2. XPS investigation of PoAP in different redox states The chemical structure variations on PoAP determined upon its redox cycling were highlighted by employing ex-situ XPS, a technique with notable analytical capability. PoAP films were analyzed in different redox states by stopping the anodic scan at about the anodic peak potential (+0.1 V, semi-oxidized film), and then at the end of the anodic scan (+0.5 V, oxidized film). In order to avoid any influence from the exposure to X-ray irradiation and from the ultra-high vacuum conditions present in the analysis chamber, the characterization of the two redox states was carried out on different polymers grown onto Pt substrate by employing exactly the same experimental conditions. This was possible thanks to the reproducibility of the electrosynthesis procedure already testified by the XPS characterization of PoAP in the reduced state [16]. The corresponding modified electrodes were rinsed with water, dried under nitrogen and then analyzed ex situ by XPS. Ex situ analysis could probably result in the loss of details relevant to the different redox states investigated due to post synthesis processes favored by the external environment, but, as it will be shown later on, still helpful to grasp important information. 3.2.1. Analysis of the oxidized polymer Fig. 7 and Table 3 show the wide spectra (7A) and detailed regions (7B) acquired for the oxidized PoAP together with fitting results. The analog XPS characterization of the reduced PoAP (potential scan stopped at the reducing potential of -0.2 V) has already been reported by the authors [16].

Oxidized PoAP shows the same spectral features and functionalities already described for the reduced one with different relative ratios as it will be shown later on. The main carbon component (peak C1) due to aromatic carbon and set at 284.8 eV provided also in this case the reference peak for spectral features assignment which was made on the basis of results from reference spectra of purified monomer reported in previous work [8] and literature references therein cited, also retrievable from online database [38]. Passing from the reduced to the oxidized polymer, the highest binding energies shifts were evident for nitrogen functionalities (DBE = -0.4 eV for N1 peak, DBE = -0.6 eV for N2 peak). Also for shake up (Peak n. 5 in C1s region) a DBE of -0.4 eV was evaluated which however could not be significant due to the difficulty in identifying the exact location of this feature, considering the contribution of the extrinsic tail of the main peaks. The BE of nitrogens are typical of neutral functionalities whereas in the case of reduced film intermediate positions between fully protonated and neutral functionalities were observed and justified in terms of hydrogen bonding with water (iminic nitrogen) and inter chains (aminic nitrogen) [16]. To understand these differences a quantitative analysis has been carried out. As for previous work, being nitrogen the specific component of the polymer, its area has been considered as a reference in order to compute the overall and partial C:O:N atomic ratios. Stoichiometric mass balance among C1s, O1s and N1s detailed regions, were verified, in the limit of our accuracy [22,23], from the corrected area reported in Table 3. The overall C:O:N ratio was for the oxidized PoAP 8.0:2.8:1 thus revealing immediately an excess of oxygen contribution with respect to the reduced film in which case the overall ratio was 8.2:2.3:1 [16]. In order to assess the distribution of this excess in the O1s region, the mass balance among C1s and the corresponding

Fig. 7. Wide scan (A) and curve-fitted detailed C1s, O1s and N1s regions (B) acquired for oxidized PoAP. Arresting potential: +0.5 V vs Ag/AgCl.

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Table 3 Peak positions, FWHM and area for conducting PoAP at the arresting potential of + 0.1 V (partially oxidized) and + 0.5 V (oxidized) vs Ag/AgCl. Element

Peak number

Arresting potential (V vs Ag/AgCl)

BE corr. (ev)

FWHM (eV)

Corrected area (arbitrary units)

Assignment

C1s

1

+0.1 +0.5 +0.1 +0.5 +0.1 +0.5 +0.1 +0.5 +0.1 +0.5

284.8 284.8 285.5 285.1 286.3 286.2 287.4 287.0 – 292.1

1.58 1.55 1.58 1.55 1.58 1.55 1.58 1.55 – 1.95

7151.08 6920.36 2884.62 2492.82 4326.92 5216.32 1670.04 1451.04 – 282.83

Carom.

+0.1 +0.5 +0.1 +0.5

531.7 531.5 533.5 533.6

2.20 2.03 2.20 2.13

1574.47 1572.04 4009.92 4264.41

C=O

+0.1 +0.5 +0.1 +0.5

399.3 398.9 400.5 399.9

2.00 1.98 2.00 1.98

730.36 1571.51 1016.20 480.07

C=N

2 3 4 5

O1s

1 2

N1s

1 2

O1s functionalities was verified. The cross-checked analysis revealed for the oxidized PoAP that, being the ratio C=O/C=O close to unity, the excess of oxygen is only under peak O2. Particularly, considering that in accordance with the structure already proposed for reduced PoAP [16] ether like oxygen is equal to total nitrogen regardless of its oxidation state, if such a content is subtracted from O2 peak, an excess of 1.08:1 is found with respect to total nitrogen, corresponding to about 3.24 oxygen atoms for every three nitrogen ones. We attributed this excess to water molecules whose presence was already supposed for the reduced PoAP but to a lesser extent (2 water molecules for every three nitrogens) [16]. It would therefore seem that PoAP is more hydrated in the oxidized redox state. As derived by experiments under heating [16], in the reduced state water interacted with p electrons and imine functionalities while amine groups gave almost inter chains hydrogen bonds. In the oxidized state, from the positions of the nitrogen peaks, it would appear that there are no hydrogen bonds. However, since the polymer is more hydrated in the oxidized state, we may think that the film is more swollen with chains more distant from each other thereby hindering the possibility of formation of hydrogen bonds. This is in accordance with the broad shape of the oxidation peak and also with the higher repulsion observed by Tucceri [25] between oxidized sites than between reduced ones at PoAP, causing a more extended configuration of oxidized sites. Oxidized terminal functionalities carbonyl type, both on the same ring, already demonstrated for the reduced film, are present also in the oxidized state with a similar ratio with respect to total nitrogen (C=O/Ntot = 0.71). In the N1s region the ratio between iminic and aminic nitrogen is 3.27:1. Indeed for the reduced PoAP it was shown that some iminic functionalities were present along the polymer chains (about 28 % of the total nitrogen), i.e. oxidized nitrogen, even if the polymerization step ended at reduction potentials. In order to

C-N C-O + C=N C=O Shake up

C-O-C + H2O

C-N

evaluate the nitrogen fraction that undergoes to oxidation during the anodic scan, 28% of the total nitrogen area, i.e. nitrogen already oxidized, was subtracted from the area of iminic peak. Then a ratio of 2.08:1 was found between iminic and aminic nitrogen in the oxidized polymer. On the basis of the above experimental findings the structure reported in Scheme 1 was derived for the oxidized PoAP. The sampled portion of oxidized PoAP contains two iminic and one aminic nitrogen, being this last collocated towards the outer part. Indeed if the emission angle, u, is varied from 0 to 50 , the iminic percentage on total nitrogen decreases (from 77% to 43%). The proposed structure best meets, in the limit of curve fitting procedure accuracy, the total and partial experimental C:O:N atomic ratios and the “cross-checking” of chemical groups. As it was already verified for the reduced film, also in the oxidized state the aminic carbon (C2 peak area) is counterbalanced by two times N2 peak area (for each aminic nitrogen there are two corresponding carbon bonded to it) plus N1 area (every iminic nitrogen is bonded to one carbon by a double bond and to another one, on the other side, by a single bond C=N-C). Moreover if we consider that for each nitrogen there is an oxygen atom in the monomer structure, total nitrogen area, doubled (ether contribution from oxygen) and added of N1 (iminic contribution from nitrogen), is approximately equal to C3 area (ether and iminic contribution from carbon). Finally, within the C1s region, the experimental ratios of aromatic carbon (C1 plus its shake up), ether-like plus carbon bonded to nitrogen (C2 + C3) and carbonyl one (C4) with respect to total nitrogen give Caromatic: (C2+C3): C=O: N = 3.5: 3.8: 0.7: 1 and then, considering an accuracy of typically 10%, again very close to the one derived by the structure of Scheme 1 (3.3: 4.0: 0.7: 1).

Scheme 1. Schematic representation of oxidized PoAP, according to XPS curve-fitting results.

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3.2.2. Analysis of semioxidized polymer A semi-oxidized state was also analyzed by stopping the anodic scan at the potential of +0.1 V, i.e. approximately in correspondence of the peak potential. This potential was chosen to combine XPS analysis with the experimental findings coming from EQCM. In correspondence of such a potential, in fact, an inversion of the frequency trend was noticed in the gravimetric profile. Fig. 8 (B) shows the detailed regions acquired. As it is possible to see from Fig. 8 and Table 3, the semi-oxidized PoAP shows the usual spectral features and then functionalities already described for the oxidized PoAP. A difference in peak position is however observed for C2 peak shifted towards slightly higher BE (DBE = +0.4 eV) as sensing a partial positive charge. A similar or even higher shift is observed for N1 (+0.4 eV) and N2 (+0.6 eV) peaks. Nitrogen positions are then more similar to the situation observed for the reduced film, showing BE values intermediates between neutral and positively charge functionalities. Apparently, no shake up peaks are evident in the C1s region but at this aim we want to mark again the difficulty in fitting such a contribution. The quantitative analysis showed a total C: O: N ratio of 9.2: 3.2: 1 and then an evident excess of both carbon and oxygen with respect to the other redox states. The ratio C=O/Ntot, close to unity, allowed to hypothesize that probably polymer chain sampled is shortened of about one monomer unit. In this last

case the total ratio would be 9: 2: 1. Oxygen is however in excess and this could be rationalized as usual in terms of water content. With regard to this, being the ratio C=O/C=O close to unity, water contribution must be only under peak O2, as in the case of reduced and oxidized PoAP. Following the same procedure adopted for the oxidized state, an excess of water of 1.30: 1 was found with respect to total nitrogen (i.e. about 4 water molecules every three nitrogens). The higher water amount could justify the attenuation of the sampled polymer fraction. The hypothesis of sampling a monomer unit less was confirmed also by the accordance with the total and partial experimental C: O: N atomic ratios and the “cross-checking” of chemical groups. In accordance was also, within the C1s region, the experimental ratio of aromatic carbon (C1), ether-like plus carbon bonded to nitrogen (C2 + C3) and carbonyl one (C4) with respect to total nitrogen (experimental ratio 4.1: 4.1: 0.96: 1, expected ratio 4: 4: 1: 1). Interestingly, the relative area ratio N1(immine)/N2(ammine) increases passing from the reduced to the semi-oxidized and then to the fully oxidized states, thus indicating a progressive film oxidation as potential is scanned towards more positive values (see Table 4). Particularly, the major increase in the oxidized component is observed from the semi-oxidized to the oxidized state (imminic percentage increment of +83%) rather than from the reduced to the semi-oxidized one (+49%). Considering that the

Fig. 8. Wide scans (A) and curve-fitted detailed C1s, O1s and N1s regions acquired for semi-oxidized PoAP after rinsing with water (B) and with acetonitrile (C). Arresting potential: +0.1 V vs Ag/AgCl.

M.E. Carbone et al. / Electrochimica Acta 176 (2015) 926–940 Table 4 Amminic and imminic composition of nitrogen peak for the reduced, partially oxidized and oxidized PoAP. Redox state

C=N %a

C-N %a

C=N/C-N

Reduced Partially oxidized Oxidized

28.0% 41.8% 76.6%

72.0% 58.2% 23.4%

0.40 0.72 3.27

a

percentage with respect to the area of total nitrogen

semi-oxidized state is to be referred to the anodic peak potential, a major oxidation fraction would be expected at this stage. Really, the electrogravimetric profile showed that counter ions coming from film oxidation are incorporated mostly at the beginning of the anodic scan till the peak potential. A plausible explanation could be that under amminic peak, positively charged nitrogens, in the form of unpaired polarons, fall. Accordingly, as it was previously stressed, the position of N2 peak is intermediate between charged and neutral ammine [39]. BE for polaron has been reported at 400.8 eV [39,40]. It could be supposed that the shift in BE is due to a coalescence of polaron, and amminic nitrogens under the same peak set at 400.5 eV. Moreover it is important to stress that XPS analysis was carried out ex situ and then, even if it represents a trusty picture of the in situ situation, some details may be lost. Effectively, as it was already demonstrated [16], water final rinsing before XPS analysis removes positive charges (protons) and perchlorate counter ions from nitrogen. However, nitrogen which were positively charged tend to form hydrogen bonds with water thus suffering a partial positive charge. Therefore there is not a net positive charge on nitrogen and, coherently, in the wide scan of Fig. 8A perchlorate counter ions are absent. The memory of original polaron type nitrogen would justify also the unusual BE of amminic carbon which is shifted towards more positive values (285.5 eV) as

937

suffering a positive charge. The net increase in imminic component going from the semi-oxidized to the oxidized state is due mostly to the recombination of unpaired polarons to give bipolarons (protonated immine) and then upon deprotonation neutral immines (N1 peak), rather than to a further oxidation. To check whether water commonly found into PoAP could come from the final sample rinsing, a different solvent was used. At this aim acetonitrile was employed, being an organic solvent sufficiently volatile (vapor pressure equal to 9700 Pa at 293 K) and oxygen free. The anodic scan was stopped again at the oxidation potential of +0.1 V, being the semi oxidized state the most hydrated ones. The film was rinsed with acetonitrile, dried under nitrogen and then transferred into the XPS instrumentation. From the wide spectrum showed in Fig. 8, a variation of intensities in the signals typically acquired for PoAP was noticed. The curve fitting of the detailed C1s, N1s and O1s regions showed in Fig. 8C evidenced the presence of additional components (red lines) to those commonly observed for PoAP, attributable, therefore, to the action of acetonitrile. Both in C1s and O1s regions, new peaks were fitted at 288.5 and at 535.5 eV respectively. These signals were attributed to carboxylic functionality by comparison with literature data [38]. In the N1s region the new and most intense peak at 402.3 eV was attributed to NH4+. An explanation comes from the capability of nitrile functionalities to generate carboxylic acids upon hydrolysis, as shown in the following reaction [41]: CH3CN + H2O + H3O+ ! CH3COOH + NH4+

(3)

The reaction is usually carried out under heating, but probably the conditions of ultra-high vacuum present in the analysis chamber facilitated the process. It is worth noting that in the reaction the presence of water molecules is fundamental to provide the oxygen necessary to

Scheme 2. Schematic representation of the mechanism underlying PoAP redox switching at acidic pH. A in the Scheme represents perchlorate ion.

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generate the carboxylic acid. Then this could be an unquestionable proof of the presence of water entrapped into the polymer matrix and not deriving from the final rinsing. A detailed quantitative analysis of this last sample was prevented from the alteration of polymer signals caused by the products of acetonitrile hydrolysis. Moreover, although it is reasonable to assume that acetonitrile in excess is completely vaporized in UHV, an eventual surplus would cause a superimposition of CH3CN signal with the C3 peak (286.3 eV), CH3CN with C4 peak (287.2 eV) and CH3CN with the N1 peak (399.4 eV) [38]. It is however encouraging that the area ratios of the new peaks are in accordance with the stoichiometry of the reaction: Area (NH4+) (980.20)  Area (COOH) (933.70)  1/2 Area (COOH) (1733.51). 3.3. Mechanism of PoAP redox cycling The experimental evidences so far illustrated allowed to derive the reaction path of PoAP redox cycling as summarized in Scheme 2. The repeating unit in the scheme was derived by comparing the charge underlying the reduction process during the last scan of the polymerization with the mass deposited on the electrode (Q = 616 mC, m = 1.625 mg). The moles of electron and of monomer, normalized for the electrode area (0.2 cm2), were computed resulting, respectively, 0.0319 mmol/cm2 and 0.0744 mmol/cm2. For each mole of electrons there will be 2.3 moles of monomer. Being the redox process two-electrons, the total monomer units will be 4.6. This means that two in every four or five amine sites are converted to the corresponding imine sites. The existence of inactive gaps within the distribution of oxidized sites of PoAP has generally been reported also at higher extent (one imine every four or five amine sites) [25]. The functional groups distributed along the polymer chains were derived from XPS results. The possibility of open structures was excluded since it would be in contradiction with the stoichiometric ratios evaluated. Four monomer units were postulated by XPS (Scheme 1). It is important however to specify that the portion of PoAP sampled by XPS is the terminating one which is affected by the influence of the external environment resulting, as it was already stressed, in the conversion of the external functionalities [16]. Combining the EQCM and XPS results the reaction path postulated in Scheme 2 can be rationalized in the following way. At the beginning of the anodic scan, PoAP is in a reduced and protonated state. A pKa of 2.5 indeed was reported for benzenoid diamine in the case of polyaniline [42]. Moreover the XPS investigation, already carried out on the polymer in its reduced state [16], demonstrated that part of nitrogen functionalities were effectively protonated and balanced by perchlorate counter ions when sample was rinsed with perchloric acid before the analysis. The oxidation process in order to occur requires a preliminary deprotonation of the involved ammine with subsequent removal of the corresponding anion. Neutral amminic functionality undergoes now to oxidation. The formation of the corresponding radical cation determines the incoming into the polymer of a perchlorate anion. The deprotonation and subsequent oxidation occurs twice. The mass variation is determined by the resultant of fluxes occurring in opposite directions: ion pairs H+ ClO4 coming from amines deprotonation move towards the external solution, ClO4 coming from the solution moves towards the film to compensate positive charges generated upon oxidation. A complete balancing of the two fluxes is to be excluded because it would not explain the mass increase of stage 1. Moreover this possibility is not compatible with the experimented dependence of stage 1 on the nature (perchlorate vs chloride) and concentration of the anion, and on pH. Therefore we hypothesized that ions expulsed

from protonated amine partially remain inside the film, not contributing to the mass variation. Totally, a net anion migration towards the film, and then a mass increase, is experimented. Particularly, the entity of the imbalance is dictated by the external concentration of H+: a higher H+ concentration in the electrolyte solution attenuates the migration of the film H+, and obviously of the paired ClO4, towards the outside. Such explanation justifies the dependence of stage 1 on pH. In conclusion, mass variation of stage 1 is maximized when ion concentration gradients are such to favor mass fluxes towards the film (higher external A concentration) or otherwise to attenuate mass flux towards the outside (higher external H+ concentration). The convolution of more processes under this first stage of the oxidation was confirmed by the curvature of the mass charge plot and by the rapid variation of the derivative function, that complicated the estimation of solvent transfer by EQCM. XPS analysis evidenced however an increase of water content at the anodic peak potential with respect to the reduced form. In correspondence of the anodic peak potential, polymer oxidation took place almost completely. The polymer structure contains mostly uncoupled polarons which from this potential value begins to recombine each other to give bipolaronic structure (=NH+) and then neutral immine. Obviously some imine functionalities have already been formed at this stage as it is attested by the increase of imine peak in the XPS spectra passing from the reduced to the semi-oxidized film. The major part of immine forms however from the peak potential by the expulsion of two protons and two anions. In the gravimetric profile a mass decrease is then recorded (stage 2). The high positive charge density generated upon oxidation force at this stage H+ ClO4 to exit, justifying the different mass variations of stage 1 and 2 (in stage 1 the mass of the anions entering upon oxidation is attenuated by the partial exit of H+ ClO4, in the stage 2 all anions coordinated to oxidized nitrogens are expelled along with protons). Mass expulsion continues to happen as long as polarons combine each other to give bipolarons, namely protonated immine. Mass vs charge plot showed a better linearity in this region with the derivative curve being almost constant. This allowed to estimate the inclusion of about five solvent molecules for the ejection of one ClO4 H+ moiety. Due to the prolonged mass ejection and solvent inclusion, polymer structure become less compact with chains more distant thus preventing the incoming of hydrogen bonds as evidenced by the nitrogen binding energies found by XPS. XPS revealed also a partial water reduction going from the semioxidized to the oxidized state. This may seem contradictory with the solvent movement evidenced by EQCM data. Indeed it is important to stress that XPS analyzes only the outer part of the polymer and that it certainly suffers from the sample exposure to the external environment and the UHV condition of the analysis chamber. The amount of water found is that firmly entrapped inside the polymer. If the polymer structure is less compact, as we postulated for the oxidized state, a more effective evaporation could take place in the analysis chamber, thus resulting in an apparent water reduction with respect to the semioxidized state. EQCM, on the other hand, measures the solvent that is effectively exchanged during the polymer redox switching. Coherently, both techniques evidenced a more swollen state of the oxidized film with respect to the reduced one. It is important to remark that the whole oxidation process is CEC, whereas in literature emphasis is given only to the deprotonation of the oxidized forms. On the reverse scan, when the potential becomes sufficiently cathodic polymer reduction occurs. In order to be reduced, imminic groups must be protonated. Hence reduction starts with the entrance of two protons and two anions. A mass increase is observed in the electrogravimetric profile (stage 3). The mass vs charge plot shows a good linearity in this stage and allowed to

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estimate the expulsion of about five solvent molecules for the inclusion of one ClO4 H+, almost balancing the mass ejected at the end of the oxidation. Then the loss of the positive charge upon the acquisition of one electron causes the removal of one anion. The reduced amine now undergoes protonation causing the entrance of one proton and one anion. Again the process occurs twice. Reduction and protonation probably are closer in time with respect to the oxidation scan. That is why stage 4 (slight mass decrease) is poorly defined in both the gravimetric profile and the mass vs charge plot. It is not to be excluded that amine protonation partially happens from ion pair already present in the film justifying an apparent net mass decrease during stage 4. It is not to be forgotten that passing from semioxidized to reduced film a more compact structure is obtained with solvent expulsion (XPS evidenced a halving of water content). Unfortunately derivative data were not effective in estimate solvent transfer in this stage. We are confident that the substantial mass transfer at the polymer/solution interface happens during stage 2 and 3 where dependence with scan rate was observed. These are the two regions in which the derivative function dm/dQ is more constant. Mass transfer, at least that related to deprotonation and protonation respectively in stage 1 and 4, partially involves ion pairs of the film. That the redox transition of PoAP occurs through two consecutive reactions in which a charged intermediate species takes part was sometimes stated [7]. During its oxidation the polymer was reported to incorporate anions in a first process and then expel protons in a second one. As we have just demonstrated, this is a simplified view of polymer oxidation that does not take into account of the interplay of various processes. Moreover it is important to remark that, while varying the experimental conditions, neither the anodic current peak nor the gravimetric profile showed any split so that a close occurrence of the two electron loss could be envisaged. About the reduction process the protonation of the oxidized film was described as a coupled chemical reaction which may be a rate controlling step due to the constraints of H+ diffusion through the film matrix [28]. We agree with such assumption highlighting that counter ions motion could be particularly hindered through the polymer matrix. 4. Conclusions The electrochemical study on the redox cycling of conducting PoAP, combined with a detailed characterization by QCM and XPS, has allowed to shed light on the processes involved during film oxidation and reduction and on the relevant chemical structure variation, highlighting interesting aspects. PoAP redox oxidation is a reversible two electron process complicated by chemical deprotonation steps. Particularly our investigation revealed that both reduced (amine) and oxidized (imine) functionalities undergo deprotonation, being the whole PoAP oxidation a CEC process. The two deprotonation steps, however, involve a different mass transfer with the external solution. While amines deprotonation produces ions which partially remain inside the film, imines deprotonation establishes a mass transfer towards the external solution along with solvent inclusion into the polymer, as evidenced by the gravimetric profiles and by the mass vs charge plot. A marked influence of the pH and of the nature and concentration of the anions in the electrolyte solution was evidenced. Conversely, mass flow at the polymer/ solution interface does not involve cations. XPS experiments demonstrated the presence of water inside the film, revealing a different degree of hydration with the redox state. Passing from the reduced to the semi-oxidized state, two additional water molecules every three nitrogens are incorporated

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into the polymer. In the fully oxidized film water amount equilibrates nitrogen (about three water molecules every three nitrogens). The absence of hydrogen bonds coming from the BEs 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. The solvent movement accompanying ion transfers during the redox switching was derived only by EQCM and effectively evidenced the uptake of five water molecules for each H+ClO4 ejected from about the anodic peak potential. Combining the electrogravimetric profile, showing the incorporation of counter ions upon film oxidation mostly at the beginning of the anodic scan, with the XPS distribution of the reduced and oxidized nitrogen functionalities, we postulated the presence of uncoupled polarons type defects in correspondence of the anodic peak potential. Polarons then recombine each other to give imminic neutral functionalities upon proton and counterions expulsion. Reduction process proceeds in a specular way with the preliminary protonation of immine. A diffusional type limitation was noticed on the cathodic process that was exactly attributed to the protonation of the imminic sites with consequent perchlorate anions transport through the film. ACKNOWLEDGEMENTS The authors thank Dr. Fausto Langerame for XPS assistance. Part of this work was already presented as poster communication at XXIV Congresso della Divisione di Chimica Analitica della Società Chimica Italiana, 15-19 Settembre 2013, SestriLevante (GE), Italy. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. electacta.2015.07.047. References [1] R. Tucceri, P.M. Arnal, A. Scian, Electrosynthesis and spectroscopic characterization of poly(o-aminophenol) film electrodes, ISRN Polymer Sci. (2012) , doi:http://dx.doi.org/10.5402/2012/942920. [2] R. Tucceri, P.M. Arnal, A.N. Scian, Spectroscopic characterization of poly(orthoaminophenol) film electrodes: a review article, J. Spectrosc. (2013) , doi:http:// dx.doi.org/10.1155/2013/951604. [3] R. Tucceri, P.M. Arnal, A.N. Scian, Poly(o-aminophenol) film electrodes: synthesis and characterization and formation mechanisms - A review article, Can. J. Chem. (2013) 91, doi:http://dx.doi.org/10.1139/cjc-2012-0031. [4] T. Komura, Y. Ito, T. Yamaguti, K. Takahasi, Charge-transport processes at polyo-aminiphenol film electrodes: electron hopping accompanied by proton exchange, Electrochim. Acta 43 (1997) 723, doi:http://dx.doi.org/10.1016/ S0013-4686(97)00202-8. [5] R. Tucceri, A review about the charge conduction process at poly(oaminophenol) film electrodes, The Open Physical Chemistry J. 4 (2010) 62, doi:http://dx.doi.org/10.2174/1874067701004010062. [6] A.U.H.A. Shah, R. Holze, Poly(o-aminophenol) with two redox processes: A spectroelectrochemical study, J. Electroanal. Chem. 597 (2006) 95, doi:http:// dx.doi.org/10.1016/j.elechem.2006.08.004. [7] H.J. Salavagione, J. Arias-Pardilla, J.M. Pérez, J.L. Vázquez, E. Morallón, M.C. Miras, C. Barbero, Study of redox mechanism of poly(o-aminophenol) using in situ techniques: evidence of two redox processes, J. Electroanal. Chem. 576 (2005) 139, doi:http://dx.doi.org/10.1016/j.jelechem.2004.10.013. [8] M.E. Carbone, R. Ciriello, A. Guerrieri, A.M. Salvi, Poly(o-aminophenol) electrosynthesized onto platinum at acidic and neutral pH: comparative investigation on the polymers characteristics and on their inner and outer interfaces, Int. J. Electrochem. Sci. 9 (2014) 2047. [9] M.E. Carbone, R. Ciriello, A. Guerrieri, A.M. Salvi, 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, Surf. Interface Anal. 46 (2014) 1081, doi:http://dx.doi.org/10.1002/ sia.5438. [10] J.M. Ortega, Conducting potential range for poly (o-amino-phenol), Thin Solid Films 371 (2000) 28, doi:http://dx.doi.org/10.1016/S0040-6090(00)00980-9.

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