Study of polyaniline films degradation by thin-layer bidimensional spectroelectrochemistry

Electrochimica Acta 52 (2006) 234–239

Study of polyaniline films degradation by thin-layer bidimensional spectroelectrochemistry ´ Jes´us L´opez-Palacios ∗ , Emma Mu˜noz, M. Ar´anzazu Heras, Alvaro Colina, Virginia Ruiz Departamento de Qu´ımica, Universidad de Burgos, Pza. Misael Ba˜nuelos s/n, E-09001 Burgos, Spain Received 10 March 2006; received in revised form 28 April 2006; accepted 28 April 2006 Available online 5 June 2006

Abstract Degradation of polyaniline films electrodeposited on optically transparent gold electrodes has been followed by thin-layer bidimensional spectroelectrochemistry. The influence of the anodic potential reached during multiple potential scans has been assessed. Correlation of changes in the spectral signals in normal and parallel arrangement has enabled identifying species of different nature, both soluble and insoluble, electroactive and inert, during the degradation of the polymer film. The three bidimensional spectroelectrochemistry signals have demonstrated the occurrence of different types of degradation processes depending on the applied potential, ranging from the simple release of oligomers and hydrolysis products retained in the film during electropolymerization to more dramatic modifications of the film. © 2006 Elsevier Ltd. All rights reserved. Keywords: Spectroelectrochemistry; Electrochemistry; Conducting polymers; Polyaniline; Degradation

1. Introduction Electrochemistry of conducting polymers (CPs) has been under deep and ceaseless study in the last years, mainly due to their characteristic and important electronic properties and the wide variety of application fields in which they can be used. Among conducting polymers, polyaniline (PANI) continues to be among the most extensively investigated ones because of its high conductivity, its electrochromic properties, the atmospheric stability of its conducting form, etc. Even though PANI can be easily synthesized both chemical and electrochemically in aqueous medium, the electrosynthesis and degradation mechanisms are still subject of controversy [1–5]. Degradation of PANI films during doping/de-doping processes is one of their main disadvantages when they are used as base material in rechargeable batteries [6,7], corrosion inhibitors [8,9] or sensors [10,11]. A vast assortment of techniques has been used to evaluate the stability of PANI films during their electro-oxidation in acidic aqueous solutions: electrochemical quartz crystal microbalance (EQCM) [4,12], electrochemical impedance spectroscopy (EIS) [13], spectroelectrochem-

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istry (SEC) [14,15], etc. The usual characterization techniques have made possible to formulate different hypothesis about the PANI film degradation process [4,14,16–18] in view of the detected degradation products (p-benzoquinone, hydroquinone, p-aminodiphenylamine, p-aminophenol, quinoneimine, etc.) [16,19,20]. Among these techniques, UV–vis spectroelectrochemistry has been of undeniable usefulness due to the absorption of UV–visible radiation by oligomers, polymer films and degradation products. Typical spectroelectrochemical experiments allow to follow the evolution of the degradation products easily, although they have two main limitations: the lack of specificity due to the joint absorption of adsorbed and soluble products, and the fact that, in some studies, spectroscopic and electrochemical measurements have not been carried out simultaneously. A recent technique, bidimensional spectroelectrochemistry (BSEC) [21], helps to circumvent these limitations. BSEC is based on coupling the two classical arrangements in spectroelectrochemistry: (a) normal configuration, in which the electromagnetic radiation goes through an optically transparent working electrode and (b) parallel configuration, in which the light passes along the electrode surface. This technique has proven to be very helpful in the follow-up of electrogeneration of different kinds of polymers [22–24].

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In this work, we propose the use of BSEC to study the influence of the electrode potential in PANI film degradation and the fate of the products generated during the degradation. Degradation products can be soluble or insoluble, electroactive or inert, and only a powerful technique as BSEC allows to determine the moment when this mixture of compounds is generated. 2. Experimental 2.1. Chemicals All chemicals were reagent grade, used without further purification. Aqueous solutions have been prepared using high-quality water (MilliQ gradient A10 system, Millipore, Bedford, MA). 2.2. Instrumentation A conventional three-electrode system controlled by a PGSTAT 20 potentiostat (Eco Chemie B.V., The Netherlands) and an SD2000 Fibre Optic Spectrometer from Ocean Optics (USA) made up of two 2048-element diode arrays were used in all the experiments. A light beam supplied by a halogen–deuterium light source (Avalight DH-S Avantes, The Netherlands) was taken to the sample cell through a 200-␮mfibre optic system, fitted with suitable lenses at its ends. Optically transparent working electrodes (OTE) were made of a thin-layer of gold sputtered (Emitech K550, Emitech, UK) on a piece of glass. A micro reference electrode Ag/AgCl/KCl 3 M, constructed in our laboratory, and a platinum counter electrode were used. Bidimensional spectroelectrochemical measurements were carried out using a home-made thin-layer cell, which has been described in detail previously [21,22]. Spacer thickness, w = 190 ␮m, was measured with a Mitutoyo Digimatic Indicator (0.001 mm resolution). The optical path length in parallel arrangement was
= 3.5 mm. Impedance spectra of PANI films before and after electrochemical degradation were performed using an Autolab PGSTAT 20 potentiostat (Ecochemie, Utrecht, The Netherlands), equipped with a frequency response analyzer (FRA) module. The amplitude of the ac voltage was 10 mV and measurements were carried out in automatic sweep mode from high to low frequency with 100 points in two frequency ranges (a) between 30 and 3 kHz before polymer film degradation and (b) between 30 kHz and 30 Hz after PANI film degradation. 2.3. Electrosynthesis of polyaniline films PANI films were grown potentiostatically by applying a potential of +0.90 V for 100 s in 0.1 M aniline and 0.25 M H2 SO4 solution (pH 1.18). Subsequently, the polymer was discharged at −0.10 V for 100 s. After the electrosynthesis step, the working electrode with the PANI film was removed from the cell, and small particles of green material were apparent in solution. The same phenomenon has been previously reported, and the particles were attributed to PANI not-adhered to the electrode [20].

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After having been removed from the BSEC cell, films were thoroughly rinsed with deionized water and then immersed in 0.25 M H2 SO4 solution for characterization. 3. Results and discussion The spectroelectrochemical characterization of PANI films was performed by BSEC during several voltabsorptometric experiments, in which information about the evolution of the original sulphuric acid solution and the polymer film was collected. The films were scanned anodically reaching different vertex potentials. Absorbance changes in normal configuration (AN ) were measured taking the substrate with polyaniline discharged at −0.10 V as blank. The reference for absorbance in parallel configuration (AP ) was the 0.25 M H2 SO4 solution. The normal configuration signal was corrected to remove the contribution of soluble species, using the expression [22]: AC N = AN − AP

w


(1)

where w and
are the optical path lengths in normal and parallel configuration, respectively. This correction allows to handle separately the spectral changes taking place only in the solution adjacent to the electrode and those occurring in the polymer film adsorbed on the electrode surface. 3.1. BSEC experiments between −0.10 and +0.90 V The cyclic voltammogram and the evolution of the absorption spectra in the two arrangements, normal and parallel, during a potential scan experiment between −0.10 and +0.90 V are shown in Fig. 1. The three responses provide complementary information on the progressive modification of the PANI film during five potential cycles. The peaks in the voltammogram (Fig. 1a) have been previously assigned to: (a) oxidation of leucoemeraldine PANI form (reduced state) to the polaron or emeraldine form [15], peaks A/A ; (b) different processes, such as ortho-coupling of polymer chains generating phenazine rings [25], breaking of polymer chains and crosslinking of neighboring chains [26], degradation products, such as p-benzoquinone, hydroquinone, p-aminophenol or quinoeimine [17,19,27], etc., peaks B/B ; (c) oxidation of emeraldine form to the pernigraniline PANI state (bipolaron) [15], peaks C/C . Upon multiple potential cycling, peaks A/A and C/C decrease progressively with concomitant increase of peaks B/B , indicating the partial degradation of the polymer film. No information about the nature and solubility of degradation products can be obtained only from the electrical signal. The degradation process is corroborated by the absorbance in parallel configuration (Fig. 1b). Spectra measured in this configuration exhibit a relative maximum at 300 nm, a band with maximum located at 500 nm, and a very broad band centered in 750 nm. Absorbance at these wavelengths increases cycle by cycle. Therefore, soluble species with spectra clearly different to that of the polymer (Fig. 1c) go into solution. Changes of AP with potential in each cycle indicate that some of these soluble species are electroactive (vide infra). Spectra of the polymer (Fig. 1c) show three main bands. The

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Fig. 1. Cyclic voltammogram (a) and three-dimensional plots of evolution of spectra during potential scan in parallel (b) and normal (c) arrangement. PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +0.90 V, v = 0.05 V s−1 , 5 cycles.

band located around 400 nm is related to the first oxidation step of PANI film (peaks A/A in the voltammogram), while the one around 480 nm is mainly related to the peaks C/C in Fig. 1a. Finally, the band around 650 nm changes in the whole potential range. PANI film evolution can be followed better by selecting a characteristic wavelength of the polymer spectrum (650 nm) from Fig. 1c. The voltabsorptogram at this wavelength (Fig. 2a) shows the doping and de-doping of the polymer film between 0 and +0.75 V approximately and a progressive diminution of AC N with the number of cycles. This diminution could be due to gradual loss of film mass. Although, AC N decreases cycle by cycle, at +0.90 V and AC differences between AC N N at −0.10 V at the beginning of each cycle remain almost constant, as Table 1a shows. In order to understand this phenomenon, the parallel arrangement signal has to be considered. Absorbance around 300 nm has been previously assigned [26] to oligomers with short chains and hydrolysis products arising from the film degradation. These compounds can be either retained in the PANI film during electropolymerization or generated by breaking of the polymer chains. Fig. 2b displays the voltabsorptogram in parallel arrangement at 300 nm. Absorbance increment at potentials as low as +0.15 V evidences

Table 1 Absorbance values in normal (a) and parallel (b) arrangement at the initial (−0.10 V) and vertex (+0.90 V) potential, and differences between them Cycle

AC N at −0.10 V

(a) AC N at 650 nm 0 0.000 1 0.004 2 −0.003 3 −0.008 4 −0.011 5 −0.014 Cycle

AP at −0.10 V

(b) AP at 300 nm 0 0.001 1 0.021 2 0.028 3 0.036 4 0.042 5 0.045

AC N at +0.90 V

C,+0.90 V C,−0.10 V AN − AN

– 0.234 0.236 0.231 0.228 0.226

– 0.234 0.232 0.234 0.236 0.237

AP at +0.90 V

V V A+0.90 − A−0.10 P P

– 0.054 0.067 0.076 0.083 0.091

– 0.053 0.046 0.048 0.047 0.050

Spectroelectrochemical characterization of PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +0.90 V, v = 0.05 V s−1 , 5 cycles.

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Fig. 3. Cyclic voltammogram of PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +1.00 V, v = 0.05 V s−1 , 10 cycles.

Fig. 2. Voltabsorptograms in normal configuration at 650 nm (a) and in parallel arrangement at 300 nm (b). PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +0.90 V, v = 0.05 V s−1 , 5 cycles.

the diffusion into solution of soluble species, retained during electropolymerization, as a consequence of polymer swelling. The voltabsorptogram shape reveals the electroactivity of some of these compounds but also the presence of inert species that are accumulated in solution cycle by cycle. Table 1b summarizes AP values at the initial and vertex potentials of each potential scan. Even though AP increases in each cycle, differences between AP at +0.90 V and AP at −0.10 V at the beginning of each scan remain almost constant. This fact proves that the soluble electroactive species are mainly released during the first cycle, while the non-electroactive ones diffuse to solution during the whole experiment. The release of these compounds implies structural changes in the PANI film, provoking the shift of its absorption band from 650 to 665 nm. This bathochromic shift explains the AC N changes observed in Table 1a and Fig. 2a. 3.2. BSEC experiments between −0.10 and +1.00 V PANI film degradation was more pronounced when the vertex potential value was increased to +1.00 V. As it has been previously reported [1,20,26,28,29], noticeable changes in the voltammogram upon cycling were observed, both in position and height of the three pairs of peaks. The most evident modifications

corresponded to the C/C peaks, while the progressive degradation of the film did not imply considerable changes in B/B and A/A peaks (Fig. 3). The two spectroscopic signals obtained simultaneously with BSEC measurements during PANI degradation allow us to give a better explanation of this phenomenon. Fig. 4 shows the spectra at +0.90 V in the backward scan of ten cycles carried out selecting +1.00 V as vertex potential. Spectra at +0.90 V are shown for a better comparison with the experiment displayed in Figs. 1 and 2. During the first two cycles, AP spectra increase considerably while AC N spectra hardly change in the wavelengths around the maximum of absorbance (665 nm). This has to be mainly due to the release of compounds retained during electropolymerization. However, in the last eight cycles the linear decrease of AC N spectra around 665 nm after each scan is strongly correlated (R2 = 0.99) with the linear increase of AP spectra. This experimental observation indicates that soluble electroactive species are generated along with the PANI film degradation in acid solution; these compounds are most likely oligomers and hydrolysis products originating from the polymer chain breaking. From the decrease of AC N spectra, it is possible to estimate that around 25% of the polymer mass was lost. Concomitantly with these changes, a bathochromic shift in AC N maximum from 665 to 680 nm and a hypsochromic shift in AP maximum from 750 to 700 nm were detected. This shift in AC N maximum could be related to a change in the average conjugation length of PANI chains [14]. The same hypothesis could be applied to oligomers present in solution. 3.3. BSEC experiments between −0.10 and +1.20 V Reaching a more anodic potential (+1.20 V) in the voltammetric scan, more significant changes take place, leading to a polymer with completely different electrical, electrochromic and morphological properties. Pristine PANI films have a porous morphology, but after the degradation process at +1.20 V the polymer film morphology changes drastically to a more compact structure (JEOL JSM-6460LV Microscope). Fig. 5 shows the evolution of AC N at 600 nm and AP at 700 nm with time during

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Fig. 4. Spectra evolution with the number of cycles at +0.90 V in the backward cycle in normal (a) and parallel (b) arrangement of PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +1.00 V, v = 0.05 V s−1 , 10 cycles.

Fig. 5. Absorbance vs. time in normal and parallel configuration at 700 and 600 nm, respectively, during 30 potential cycles. Inset: From bottom to top spectra, evolution of AC N at vertex potential (+1.20 V) with the number of cycles. PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +1.20 V, v = 0.05 V s−1 , 30 cycles.

thirty potential cycles. Contrary to expected, AC N increases and AP decreases simultaneously with the number of cycles, reaching a constant value from the 20th cycle onwards. In contrast with the behavior observed in experiments reaching +0.90 and +1.00 V, the increment of absorbance in normal configuration points out the transformation of the PANI film without generation of soluble species. Moreover, concomitant decrease of AP indicates that some soluble species are being depleted from solution during this transformation. This phenomenon is favored by working in a thin-layer cell because soluble compounds cannot diffuse far from the electrode/solution interface. Transformation of PANI film is corroborated by the spectral changes observed with cycling, as shown in the inset of Fig. 5. The major increment of absorbance occurs during the first 20 cycles. Between the 20th and 30th cycles, the spectra at wavelengths shorter than 600 nm remains constant indicating that the main transformation of the PANI film has finished. Besides, a slight decrease of absorbance at wavelengths longer than 600 nm is observed. The AC N maximum shifts hypsochromically during the first 20 cycles from 680 to 600 nm, indicating that the transformed film has a lower conjugation length and, therefore, a poorer conductivity. Electrochemical impedance spectroscopy confirmed the different conducting properties of a PANI film before and after degradation. The ionic charge-transfer resistance (Rct ) was measured [30,31] at +0.25 V for the PANI film: (a) freshly prepared and (b) after the voltammetric treatment at +1.20 V. The remarkably higher Rct value of the degradated polymer film (Rbct = 1.11 k) confirms that this film is less conductive than the freshly prepared one (Ract = 0.31 ). According to previous results [25,26], this new film could stem from the linkage of neighboring polymer chains where the appearance of phenazine units cause the conductivity to decrease, from the reincorporation of some degradation products resulting in an assortment of polymers, and/or from shortening of the chain length.

Fig. 6. Cyclic voltammograms of PANI film in 0.25 mol L−1 H2 SO4 . Ei = −0.10 V, EV = +1.20 V, v = 0.05 V s−1 , 30 cycles. (—) 1st cycle, () 10th cycle, (
) 20th cycle, (×) 30th cycle.

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These structural changes have dramatic effects on the cyclic voltammogram (Fig. 6). During the first ten cycles the C/C peaks, corresponding to the fully oxidized polymer, disappear, and the B/B peaks intensity decreases, indicating a strong modification of the starting PANI film. The total disappearance of A/A peaks is also observed between the 10th and 20th cycles. In the last 10 cycles only a slight decrease of the prevailing pair of peaks takes place. All these phenomena are in complete agreement with the spectral changes explained above (Fig. 5). Complementary BSEC signals provide an unambiguous interpretation of the process, that otherwise could be really difficult to attain. 4. Conclusions Depending on the applied potential, PANI films can suffer three different kinds of degradation: (a) Release of soluble compounds retained during the electropolymerization that is accompanied by a slight bathochromic shift of the PANI film absorption band. (b) Breaking of polymer chains and concomitant generation of soluble species leading to a significant loss of polymer mass. (c) A deep transformation of the PANI film resulting in unexpected electrochromic behaviour, important morphological changes and a dramatic decrease of the conducting properties of the polymer. All these processes cannot be easily inferred from the voltammetric signal alone, and incorrect conclusions could be drawn without the complementary aid of other analytical techniques. From the results presented in this work, it is evident that bidimensional spectroelectrochemistry provides essential information to understand PANI film degradation. Acknowledgments Support of Junta de Castilla y Le´on (BU011A05) and Ministerio de Educaci´on y Ciencia (MAT2003-07440) are gratefully acknowledged. V.R. also thanks the Ministerio de Educaci´on y Ciencia for a Juan de la Cierva contract.

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