Raman spectroelectrochemical study on the kinetics of electrochemical degradation of polyaniline

Polymer Degradation and Stability 93 (2008) 1742–1746

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Raman spectroelectrochemical study on the kinetics of electrochemical degradation of polyaniline _ G. Niaura, A. Malinauskas* R. Ma zeikiene, Department of Organic Chemistry, Institute of Chemistry, Gosˇtauto Strasse 9, LT-01108 Vilnius, Lithuania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2008 Received in revised form 21 July 2008 Accepted 28 July 2008 Available online 6 August 2008

The kinetics of electrochemical degradation of polyaniline and a copolymer of aniline and metanilic acid have been studied by in situ Raman spectroscopy at a gold electrode. It has been concluded that probably no drastic changes in polymer structure occur on prolonged electrochemical treatment of polymer films at a high electrode potential (0.8 V vs. Ag/AgCl). Instead, most prominent changes relate to a gradual decrease of an overall intensity of spectra, viz. to gradual degradation of a polymer layer. The degradation proceeds faster at pH 1.0, compared to pH 7.0. The kinetic results obtained have been analyzed following simple 2- or 3-parameter exponential decay equations, and compared with the known degradation rate constants. Ó 2008 Elsevier Ltd. All rights reserved.

Keywords: Polyaniline Degradation Stability Kinetics Raman spectroscopy Spectroelectrochemistry

1. Introduction The stability of polyaniline (PANI) and its derivatives is of primary importance for many applications of these conducting polymers. In electrochemistry-related systems, PANI is known to undergo decomposition processes, especially at high electrode potentials [1]. At least two kinds of electrochemical PANI degradation are known: (i) breaking of polymer chains accomplished by formation of soluble low molecular weight products like quinones or quinoneimines, and (ii) transformation of polymer structure, leading to chemical and morphological changes, and loss of its electric conductivity [2]. In the former case, the loss of polymer mass, accomplished by changes in concomitant characteristics like optical absorbance and electrochemical redox capacity, are observed. In the latter case, however, no or little changes in polymer mass are expected. Thus, different techniques should be applied to study both the mechanism and kinetics of electrochemical degradation of PANI and related polymers. Earlier, we studied the kinetics of electrochemical degradation of PANI with cyclic voltammetry [3,4]. The degradation process was found to proceed as a simple first-order reaction. Within the range of relatively low electrode potentials (0.3–0.6 V vs. Ag/AgCl), the decomposition proceeds at a relatively low rate constant of ca.

* Corresponding author. E-mail address: [email protected] (A. Malinauskas). 0141-3910/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2008.07.028

105 s1, whereas an increase of electrode potential causes drastic increase of degradation rate constant up to ca. 3.6  103 s1 at 0.9 V [3]. Mechanistically, the data have been interpreted by the different degradation rate for two different forms of PANI, viz. emeraldine (slow decomposition), and pernigraniline (fast decomposition) [4]. The data obtained by cyclic voltammetry, however, relate to electrochemically (redox) active part of PANI, whereas inactive degradation products cannot be detected. The same degradation processes have been studied by in situ UV–vis spectroelectrochemical technique [5,6]. Again, nearly exponential decay of optical absorbance has been disclosed, and the corresponding first-order decomposition constants have been calculated. This technique relates to time-resolved detection of coloured species that absorb in a visible region of spectrum. Therefore, some possible degradation products like dimers of aromatic amines can interfere to changes in optical absorbance caused by PANI itself. The degradation kinetics of PANI and related polymers has been also studied by different authors with the use of cyclic voltammetry [7,8], thin-layer spectroelectrochemistry [2], and electrochemical quartz crystal microgravimetry [9,10]. In search of other techniques that could be applied to study the kinetics of degradation of conducting polymers, we recently used in situ Raman spectroelectrochemistry, and obtained kinetic data for electrochemical decomposition of poly(N-methylaniline) [11], and self-doped sulfonated polyaniline [12]. The present work has been aimed at kinetic Raman spectroelectrochemical study of electrochemical degradation of polyaniline.

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2. Experimental Aniline has been distilled before use. Other chemicals of analytical or reagent grade were used as received. A PI-50-1 model potentiostat, arranged with PR-8 model programmer and a custom built A/D converter for data acquisition, was used in experiments. Electrochemical experiments have been done in a conventional one-compartment cell arranged with an Ag/ AgCl (sat. KCl) reference electrode. All potential values reported below are referred to this reference electrode. A flat circular gold electrode of ca. 5 mm in diameter, press-fitted into a Teflon rod, was used as a working electrode. Before experiments, the working electrode has been cleaned for 30 min in a Piranha solution (a mixture of 30% hydrogen peroxide and concentrated sulfuric acid, 3:1 by vol.), polished with 0.3 mm alumina powder (Kemet, UK), and ultrasonicated for 2 min in ethanol and water mixture. Then, the surface of Au electrode was roughened by cycling for 50 cycles in 0.1 M KCl solution within the potential limits of 0.3 and þ1.4 V at a potential scan rate of 200 mV s1. Electropolymerisation of aniline has been performed in a solution of 0.5 M of sulfuric acid containing 0.05 M of aniline by applying a controlled potential of 0.8 V for 3.5 min. Copolymerization of aniline with metanilic acid has been performed in a solution of 0.1 M of sulfuric acid containing 0.01 M of aniline and 0.1 M of metanilic acid (m-aminobenzenesulfonic acid). After the electropolymerisation, the electrode was rinsed in a supporting electrolyte, and mounted onto an electrochemical or spectroelectrochemical cell. Raman spectroelectrochemical experiments were done in a cylindrical three electrode moving cell, arranged with a gold working electrode as described above, platinum wire as a counter electrode, and a saturated Ag/AgCl reference electrode. The cell was been filled either with 0.1 M solution of sulfuric acid (pH 1.0), or with 0.01 M phosphate buffer solution (pH 7.0) containing 0.1 M of sodium sulfate. The flat surface of a working electrode was placed at approx. 2 mm distance from the cell window. The 676.4 nm beam of the Kr-ion laser (Coherent, model Innova 90-K.) with a power of ca. 20 mW was incident onto the surface at ca. 60 and focused to a spot of ca. 1 mm2 in area. In order to reduce photo-effects and thermal effects, and possible degradation of a polymer film by the incident light as well, the cell holder was moved periodically with respect to the laser beam at ca. 20 mm s1 with the help of a custom built equipment [13]. The experiments were performed in 90 scattering geometry. The Raman scattered light was analyzed by a 500 mm focal length, f/6.4 aperture ratio spectrograph (Acton Research Co., Model: SpectraPro-2500i) equipped with a 600 lines/ mm grating, and recorded by the thermoelectrically cooled at 70  C CCD camera (Princeton Instruments, Model: Spec-10:256E). The integration time was 1 s. Each spectrum was recorded by accumulation of 100 scans. The cut-off filter (Semrock Inc.) was put in front of the spectrograph to eliminate Rayleigh scattering from the electrode. The Raman frequencies were calibrated using the toluene spectrum. The spectrometer, electrochemical cell, detector and laser were mounted onto a massive optical bench. 3. Results and discussion Fig. 1 shows Raman spectra of polyaniline (PANI) layer at a gold electrode in pH 1.0 and pH 7.0 solutions. The bands observed could be assigned based on the known data [14–19]. Generally, most characteristic vibrations could be divided into three spectral regions. Within a high-frequency region, C–C and C]C stretching vibrations of benzene and quinone type rings are dominating. At a relatively high electrode potential of 0.8 V applied, PANI presents in its half-oxidised emeraldine form containing both benzene and quinone type rings, thus, both C–C and C]C stretching vibrations are observed. From these, C–C stretching vibrations appear at 1630

Fig. 1. Raman spectra from a gold electrode covered with polyaniline, as obtained at excitation wavelength of 647.1 nm in pH 1.0 or 7.0 solutions (as indicated), by holding electrode at a controlled potential of 0.8 V for a definite time (as indicated).

and 1626 cm1 for pH 1.0 and pH 7.0 solutions, respectively, whereas C]C stretches are observed at 1595 cm1 in both solutions. Within the mid-frequency region, ranging from 1520 to 1210 cm1, C–N and C]N stretching vibrations are most characteristic. From these, C]N stretches corresponding to imine sites in emeraldine form of PANI appear dominating at 1488 and 1483 cm1 for pH 1.0 and 7.0 solutions, respectively. Also, C–N stretches corresponding to amino sites are well developed at 1228 cm1. Within a spectral region below 1210 cm1, C–H bending vibrations of aromatic rings appear most prominent at 1172 and 1168 cm1 for pH 1.0 and pH 7.0 solutions, respectively. In the Raman spectra there are several minor bands which cannot be unambiguous interpreted. Among these, weak bands located at 1411 and 1417 cm1 for pH 1.0 and pH 7.0 solutions, respectively, are of interest. It has been pointed out earlier that some minor structures, next to PANI, can be formed during electrochemical or even chemical oxidative polymerization of aniline. Genies et al. claimed a heterocyclic phenazine-like structure to be formed at a higher electrode potential during electropolymerization, and by oxidation of the polymer as well [20]. Similar conclusions have been drawn, based on FTIR spectroscopy, for PANI prepared by chemical polymerization with the use of ammonium peroxydisulfate as oxidising agent [21]. Phenazine-like structures like poly(o-phenylenediamine) show a characteristic Raman band

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Fig. 2. The same as in Fig. 1, obtained for electrode, covered with SPAN, in pH 1.0 solution.

around 1420–1410 cm1 [22]. Thus, some minor polymer structures, next to PANI, can present in the bulk of the electropolymerised layer. A self-doped copolymer of aniline and metanilic acid (SPAN) shows close similar Raman spectrum, as depicted in Fig. 2. Assignments of Raman bands are summarized in Table 1. By holding PANI modified electrode at a high electrode potential of 0.8 V up to 1 h, no significant qualitative changes in the band number or positions occur, except for some changes in a relative intensity for selected bands. An increase of a relative intensity of the band at 1595 cm1 as compared to that at 1630 or 1626 cm1 (Fig. 1) shows probably an increase in the number of quinone type rings relative to benzene type ones, occurring at a prolonged oxidation of the film at a high electrode potential. For SPAN films, however, a decrease of intensity ratio for these two bands is observed (Fig. 2). Also, some changes in the shape of overlapped bands within the range of 1400–1300 cm1 should be noted for SPAN (Fig. 2). The bands within this region present stretching Table 1 Assignments of Raman bands, obtained at an excitation wavelength of 647.1 nm and at electrode potential of 0.8 V vs. Ag/AgCl for PANI and SPAN films at a gold electrode Wavenumbers (cm1) PANI

PANI

Assignment SPAN

pH 1.0

pH 7.0

pH 1.0

1630s 1595s 1488s 1411w

1626s 1595s 1483s 1417w

1625s 1590s 1480s

C–C stretching in B (8a) C]C stretching in Q (8a) C]N stretching in emeraldine (imine site)

1338m 1265w 1226m 1172s 809m 751w 633w 585w 518m

CwNþ stretching in polaronic form (polarons)

1280w 1228w 1172s

1228w 1168s

C–N stretching in emeraldine (amine site) C–H bending (9a) Amine deformation (C–N–C bending) Imine deformation (C–N]C bending) Ring deformation (6b) Ring deformation (6b) Amine in-plane deformation

Abbreviations used: B: benzene type ring, Q: quinone type ring, w: bond intermediate between the single and double bonds. The numbers given in parentheses (6b), (8a), and (9a) refer to Wilson’s notation of aromatic species vibration modes.

Fig. 3. Time dependence of a relative intensity for selected Raman bands (as indicated), as obtained by holding PANI (top) or SPAN-coated (bottom) gold electrode at a controlled potential of 0.8 V in pH 1.0 or 7.0 solutions (as indicated).

vibrations of an intermediate bond CwNþ, characteristic for polaronic form of a polymer. It could be concluded from these observations that probably no drastic changes in polymer structure occur at a prolonged electrochemical treatment of polymer films at a high electrode potential. Most prominent changes relate to a gradual decrease of an overall intensity of spectra. Evidently, this decrease proceeds for PANI in both solutions studied (Fig. 1). In a pH 1.0 solution, this decrease appears to be some faster than in pH 7.0 solution. Since the decrease in intensity of Raman bands is concerned with the decomposition of a polymer layer, it is of interest to obtain quantitative characteristics of this process. Two most prominent Raman bands have been chosen to characterize the kinetics of decomposition, viz. those centred around 1630 and 1170 cm1. Fig. 3 shows time dependence for the decrease of these two bands. The kinetic results obtained could be analyzed within the frames of two distinct models. Table 2 Fitting of kinetic data according to a single 2-parameter exponential decay equation I ¼ a expðbtÞ Polymer

pH

Band, cm1

r

a

b

Average b, s1

PANI

1.0

1630 1172 1626 1168 1625 1172

0.8324 0.8560 0.9566 0.9907 0.9243 0.8890

0.875 0.886 0.945 0.980 0.926 0.902

0.0282 0.0315 0.0139 0.0121 0.0218 0.0238

5.00  104

7.0 SPAN

1.0

2.17  104 3.80  104

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Table 3 Fitting of kinetic data according to a single 3-parameter exponential decay equation I ¼ I0 þ a expðbtÞ Polymer

pH

Band, cm1

r

I0

a

b

Average b, s1

PANI

1.0

1630 1172 1626 1168 1625 1172

0.9973 0.9966 0.9985 0.9990 0.9985 0.9989

0.295 0.267 0.414 0.331 0.321 0.317

0.702 0.730 0.582 0.670 0.681 0.680

0.105 0.102 0.0408 0.0231 0.0603 0.0758

1.73  103

7.0 SPAN

1.0

5.33  104 1.13  103

Table 4 Electrochemical decomposition rate constants (k) for PANI and its derivatives, as obtained at a controlled electrode potential of 0.8 V vs. Ag/AgCl Polymer

Technique and conditions

k (s1)

References

PANI Polystyrenesulfonate-doped PANI PANI Poly(N-benzylaniline) Poly(N-methylaniline)

Cyclic voltammetry, 0.5 M H2SO4 Cyclic voltammetry, 0.5 M H2SO4 UV–vis spectrometry, 0.5 M H2SO4 (1.0 V vs. RHE) UV–vis spectrometry, 0.5 M H2SO4 (1.0 V vs. RHE) Cyclic voltammetry, 0.5 M H2SO4 Raman spectrometry: 0.5 M H2SO4 0.1 M H2SO4 Raman spectrometry, pH 1.0

8.50  104 11.0  104 1.82  104 5.50  104 8.72  104

[3] [4] [5] [6] [11]

9.60  104 2.80  104 6.6  104

[12]

Sulfonated PANI

First, the decomposition process could be described as a simple first-order chemical hydrolysis reaction. At a high electrode potential used (0.8 V), PANI presents mainly in its fully oxidised pernigraniline form, which is subjected to hydrolysis in an acidic solution, producing quinones or quinoimines as hydrolysis products. Following this model, the decomposition kinetics should follow a single 2-parameter exponential decay equation:

I ¼ a expð  btÞ

(1)

where I denotes the band intensity at any moment, the coefficient a denotes an initial value of I, t is reaction time, and coefficient b represents a first-order decomposition rate constant. Earlier, we used a similar equation to characterize the decomposition of PANI and some of its derivatives with cyclic voltammetry [3,4], or with UV–vis spectroelectrochemical technique [5,6], and obtained good correlation of the data. Treatment of the present data according to (1), however, results in a poor correlation both for PANI and SPAN, especially in pH 1.0 solutions, as presented in Table 2. Despite this, the decomposition rate constant obtained for PANI (5.0  104 s1) does not differ drastically from that obtained by cyclic voltammetry in 0.5 M sulfuric acid solution (8.5  104 s1) [3]. In order to get a better fit for kinetic data, the results have been analyzed following a more complex 3-parameter equation.

I ¼ I0 þ a expð  btÞ

(2)

Treatment of kinetic data according to (2) yielded good mathematical correlation, however, the decomposition rate constants obtained for PANI or SPAN appear to be significantly higher than those obtained following Eq. (1) (Table 3). The physical meaning for I0 in (2) is a mean residual intensity of spectral bands, attained at indefinitely high t. It follows from our previous study on decomposition of PANI and related polymers performed with cyclic voltammetry [3,4], and UV–vis spectrophotometry as well [5,6], that the polymers decompose and dissolve nearly completely from electrode surface at a sufficient high electrolysis time. Thus, no residual signal should be obtained. In the present case, however, the residual intensity of spectra could be caused by a very thin (probably even monomolecular) layer of polymer placed directly at a gold surface. Because of surface enhancement of Raman spectra from a roughened gold electrode surface, the spectra from a monolayer could be enormously high in intensity, causing themselves an apparent residual intensity. Earlier, we

demonstrated a high increment to a total intensities of Raman spectra, caused by surface enhancement [23]. Thus, a high increment to a total intensity arising from a thin PANI layer at a roughened gold surface, as compared to the bulk of electrode modifier, seems to be very probable. According to the data presented in Table 3, the increment from a constant residual intensity for PANI ranges within the limits of 0.27–0.30, and 0.33–0.41 in pH 1.0 and pH 7.0 solutions, respectively. It is of interest to compare the data obtained with the results presented in our earlier studies. A summary of decomposition rate constants obtained by electrochemical or UV–vis spectroelectrochemical techniques is presented in Table 4. Most of the results of Table 4 have been obtained in 0.5 M solution of sulfuric acid, where degradation proceeds faster than in its 0.1 M solution [11]. In a chemical sense, the decomposition of PANI represents acidic hydrolysis of fully oxidised polymer, thus, it proceeds faster in more acidic solutions. Despite the different techniques used, all decomposition rate constants obtained vary within one order of magnitude. The best fit of the present results, obtained with Raman spectroscopy, to our previous results, obtained by cyclic voltammetry or UV–vis spectroscopy, has been obtained by using the simplest 2-parameter exponential decay following Eq. (1), despite a poor mathematical correlation. Treatment of the data within the model of 3-parameter decay (Eq. (2)) yield higher values for decomposition constants, exceeding those of Eq. (1) by the factor ranging from ca. 2.5 (for PANI in pH 7.0 solution) to ca. 3.5 (for pH 1.0 solution). In summary, although more complicated as compared to cyclic voltammetry and UV–vis spectrometry, Raman spectroscopy presents a useful tool to study the decomposition of PANI, yielding rate constants closely related to those, obtained by other techniques. Acknowledgements Financial support of this work by the Lithuanian Science and Study Foundation (Project No. N-07009) is gratefully acknowledged. References [1] Pud AA. Synth Met 1994;66:1–18. [2] Lopez-Palacios J, Munoz E, Aranzazu Heras M, Colina A, Ruiz V. Electrochim Acta 2006;52:234–9. [3] Mazeikiene R, Malinauskas A. Synth Met 2001;123:349–54.

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