In situ Raman spectroelectrochemical study of electrocatalytic oxidation of ascorbate at polyaniline- and sulfonated polyaniline-modified electrodes

Electrochimica Acta 51 (2006) 5761–5766

In situ Raman spectroelectrochemical study of electrocatalytic oxidation of ascorbate at polyaniline- and sulfonated polyaniline-modified electrodes R. Maˇzeikien˙e, G. Niaura, A. Malinauskas ∗ Institute of Chemistry, Goˇstauto Str. 9, LT-01108 Vilnius, Lithuania Received 30 December 2005; received in revised form 8 March 2006; accepted 9 March 2006 Available online 18 April 2006

Abstract Electrocatalytic oxidation of ascorbate at electrodes, covered with polyaniline and sulfonated polyaniline, has been studied by in situ Raman spectroelectrochemistry with the green laser excitation (532 nm). Characteristic Raman features have been identified and their changes during electrooxidation of ascorbate have been analysed. It has been shown that an increase of ascorbate concentration causes an increase of the relative content of the reduced polyaniline form within the film. From this, it has been concluded that electrocatalytic oxidation of ascorbate proceeds within a polyaniline film rather than at an outer polyaniline/solution boundary, i.e. electrooxidation follows a redox-catalysis mechanism. © 2006 Elsevier Ltd. All rights reserved. Keywords: Polyaniline; Raman spectroscopy; Spectroelectrochemistry; Electrocatalysis; Ascorbate

1. Introduction Electrocatalysis at modified electrodes is an interesting phenomenon that finds diverse applications in electrochemical synthesis and electroanalysis. Among many electrode modifiers, conducting polymers like polyaniline, polypyrrole and polythiohene are attracting a great deal of attention due to simple synthesis, relative environmental stability, relatively broad range of electrocatalysed reactions, and other useful properties. In any electrocatalytic process, the material flux of solution species towards the reaction zone should be balanced by the flux of charge carriers (electrons or holes) through a modifier layer placed at electrode surface. For simple metal electrodes with indefinitely high mobility of charge carriers, the rate of electrocatalytic process should be limited either by the flux of solution species to reaction zone, or by the rate of electrocatalytic conversion. However, the conductivity of the most conducting polymers, despite of a few known exceptions, is by one or more orders of magnitude lower than that of metals. Therefore, an overall rate of electrocatalytic process could be determined

∗

Corresponding author. E-mail address: [email protected] (A. Malinauskas).

0013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2006.03.011

by the slow transportation of charge carriers within the layer of conducting polymer. Thus, two principal possibilities for electrocatalysis at conducting polymer modified electrodes are possible: 1. ‘Metal-like’ electrocatalysis, that occurs at a high conductivity of electrode material. In this case, an overall rate is determined by the flux of solution species, or by the rate of catalytic conversion, whereas electrocatalytic reaction occurs at the conducting polymer/solution interface. 2. Redox catalysis, that occurs at a limited conductivity of a modifier layer. Here, the reaction occurs within the conducting polymer layer. There are a limited number of papers that address this problem. Cooper and Hall studied electrocatalytic conversion of benzoquinone/hydroquinone (BQ/HQ) redox couple at polyanilinemodified electrode, proceeding probably through the chargecompensating complex of polyaniline and a transient anionradical, and claimed the shift of a reaction site from the polymer/solution boundary towards the bulk of polymer film at decreasing solution acidity due to decreasing conductivity of the film [1]. Mandic and Duic discussed the location of electrocatalytic reaction for BQ/HQ and Fe2+/3+ at polyaniline elec-

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trode of different thickness, and showed a metal-like behaviour within the potential window where the most conducting emeraldine form of polyaniline exists, and a redox-polymer behaviour at potentials of a fully oxidised pernigraniline form [2]. For electrocatalytic oxidation of coenzyme NADH at polyanilinepolyvinylsulfonate composite electrode, Bartlett et al. showed the reaction to proceed within the polymer film rather than at polymer/solution boundary (i.e. redox rather than metal-like catalysis) [3]. The problem on the location of electrocatalytic process has been studied with the use of UV–vis spectroelectrochemical techniques at transparent ITO electrodes. It has been shown that, under controlled potential conditions, electrocatalytic oxidation of HQ, as well as reduction of BQ, Fe3+ ions, and 1,2-naphthoquinone-4-sulfonate proceeds within the modifying layer of polyaniline or poly(N-benzylaniline), whereas the depth of the reaction zone, located within the polymer layer, depends on its thickness and the concentration of solution species [4–6]. Adversely, the cathodic reduction of dichromate ion has been found to proceed at an outer polymer/solution interface because of a slow electrocatalytic reaction of this species [7]. Recently, with the use of in situ resonance Raman spectroscopy (RRS), we studied electrocatalytic oxidation of BQ and ascorbic acid at electrodes, covered with polyaniline and sulfonated polyaniline [8]. In both cases, the redox (versus metal-like) mechanism of electrocatalytic conversion has been deduced from Raman spectra. The use of dynamic in situ Raman spectroscopy during electrolysis meets known difficulties concerned mainly with the short time available for recording and averaging of spectra, as opposite to static experiments where no electrolysis proceeds, or even to ex situ techniques. The excitation of spectra in our recent study [8] has been done with a He–Ne gas laser at 632.8 nm. The reduced form of polyaniline shows an optical absorbance maximum at 310–320 nm, whereas its oxidised form shows a strong absorbance with a maximum located between 700 and 800 nm [4]. Taking into account a strong light absorbance in the red spectral region for oxidised form of polyaniline [4–6], it could be concluded that the characteristic Raman features of oxidised polymer form should be greatly enhanced by resonance with the red laser excitation [9]. Thus, the characteristic features for the reduced form of polyaniline should be less expressed. Therefore, it would be of interest to apply any different excitation wavelength, being not in a strong resonance with the oxidised form of polymer. The present work has been aimed to in situ Raman spectroelectrochemical study of ascorbic acid oxidation at polyaniline modified electrode with the use of a green laser excitation (532 nm). As far as we know, the present work, along with our previous paper [8], is the only study on dynamic in situ application of Raman spectroscopy to electrocatalytic processes taking place at polyaniline-modified electrodes.

into a Teflon rod, as a working electrode, platinum wire as a counter electrode, and a KCl saturated Ag/AgCl reference electrode. All potential values reported below refer to this reference electrode. The working electrode was placed at approximately 5 mm distance from the cell window. The 532 nm beam of the diode pumped solid-state laser (Viasho Technology Co. Ltd.) with the power of ca. 50 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- and thermoeffects, and a 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/s with the help of a custom built equipment [10]. The experiments were performed in 90◦ scattering geometry. The Raman scattered light was analysed by a 400 mm focal length, f/2.5 custom built spectrograph equipped with 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. The cut-off filter 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 have been mounted onto a massive optical bench. PI-50-1 model potentiostat, arranged with PR-8 model programmer, was used in experiments. Before each experimental set, the working electrode has been cleaned for 1 h in a Piranha solution (a mixture of 30% hydrogen peroxide and concentrated sulfuric acid, 3:1 (v/v)). The deposition of polyaniline layer onto the pretreated gold electrode has been performed in a separate electrochemical cell by applying of a controlled potential of 0.8 V for 7 or 14 min (for obtaining of ‘thin’ and ‘thick’ polyaniline films, respectively) in a solution of 0.5 M of sulfuric acid, containing 0.05 M of aniline. The polymer films thus obtained are characterized by the specific electrochemical redox charge of ca. 11 and 35 mC/cm2 for ‘thin’ and ‘thick’ films, respectively. Similarly, ‘thin’ and ‘thick’ films of sulfonated polyaniline have been electrodeposited at a controlled potential of 0.8 V for 30 or 90 min, respectively, 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). ‘Thin’ and ‘thick’ films of sulfonated polyaniline prepared have a specific redox charge of ca. 9 and 14 mC/cm2 , respectively. After the deposition, the electrode was rinsed in a supporting electrolyte, and mounted into a spectroelectrochemical cell. As supporting electrolytes for Raman spectroelectrochemical experiments, either a solution of 0.5 M of sulfuric acid, or 0.1 M phosphate buffer solution pH 7.1, were used. Definite amounts of separately prepared stock solution of ascorbic acid have been added stepwise into Raman spectroelectrochemical cell to get a desired concentration of this species. After each addition, the working electrode has been poised at an appropriate potential for at least 3 min to establish an equilibrium before recording of the spectrum.

2. Experimental 3. Results and discussion Raman spectroelectrochemical experiments have been done in a cylinder-shaped three electrode moving cell, arranged with a flat circular gold electrode of ca. 5 mm in diameter, press-fitted

Fig. 1 shows Raman spectra for thick polyaniline films, as obtained at different electrode potentials in two different sup-

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Fig. 1. Raman spectra, obtained at excitation wavelength of 532 nm from gold electrode, covered with thin polyaniline films in a solution containing 0.5 M of sulfuric acid or 0.1 M phosphate buffer pH 7.0 at different electrode potentials (as indicated).

porting electrolytes. Nearly same pictures have been obtained for thin polyaniline films as well, except that the intensities of Raman bands are lower. Also, the intensity ratio for some bands, and their positions as well, vary depending on the film thickness, although to a less extent. The interpretation of the bands observed could be well done based on the known assignments [11–17]. In an acidic solution, the oxidised form of polyaniline shows at 0.8 V a strong band at 1152 cm−1 that corresponds

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to C–H in-plane bending mode of quinone-like rings. During a stepwise reduction of a polymer film, this band diminishes in intensity, and a shoulder at higher wavelength appears at 0.4 V, which develops into a separate band at 1172 cm−1 for a fullliney reduced form, that corresponds to C–H bending mode of benzene-like rings. The band at 1206 cm−1 , corresponding to C–N stretching, is well expressed for the oxidised form, but disappears upon electrochemical reduction. Within the region of 1300–1350 cm−1 , a few overlapping bands are present. Some earlier work [18], dealing with a detailed study on H-D isotope exchange, ascribed the two overlapping bands to different kinds of intermediate C–N+ bond stretch vibrations, corresponding to different kinds of polarons present in an oxidised conducting form of polyaniline. Upon electrochemical reduction, the polaron band(s) almost disappear by turning the polymer into its nonconducting leucoemeraldine form. Also, the band located at ca. 1481 cm−1 , which corresponds to stretch vibrations of a double C N bond, coupled to quinone-like rings, disappears as it could be expected upon electrochemical reduction of a polymer film. The two well expressed bands around 1600 cm−1 are well known characteristics for different oxidation states of polyaniline. From these, the band at 1569 cm−1 corresponds to C C stretches of quinone-like rings, and disappears upon reduction, whereas the band at 1610 cm−1 corresponds to C–C stretches of benzene-like rings, and is characteristic for the reduced form of a polymer [9–14]. In a pH-neutral solution, essential changes in Raman spectra for polyaniline are observed (Fig. 1). At pH exceeding 4, polyaniline exists in its deprotonated (proton-dedoped) non-conducting form. Accordingly, the polaron bands at 1300–1350 cm−1 appears diminished even for oxidised form of a polymer. The most intense band at pH 7.0, located at 1480 cm−1 , corresponding to stretch vibrations of deprotonated C N bond, greatly diminishes in intensity upon electrochemical reduction of a polymer. Also, changes in the bands located around 1600 cm−1 are well seen upon changing of oxidation state. The changes observed in a pH-neutral solution show that, despite of the low electric conductivity of deprotonated polyaniline form, it is still possible to reduce or oxidise reversibly the deposited film by changing the electrode potential. Consider-

Table 1 Assignment of vibrational bands (cm−1 ) of Raman spectra (excitation wavelength 532 nm) of polyaniline (PANI) and sulfonated polyaniline (SPAN) PANI

SPAN

0.5 M H2 SO4

pH 7.0

Assignment

pH 7.0

0.8 V

0.0 V

0.8 V

0.0 V

0.2 V

1152 (s) – 1206 (w) 1311–1328

– 1172 (m) – 1318 (w)

1152 (s) – 1206 (m) 1312–1340 1410 (w) 1480 (vs) 1549 (w) 1574 (m) 1608 (m)

1152 (m) – 1206 (w) ∼1340 (m) – 1476 (m) 1542 (w) – 1602 (m)

1151 (m) – 1202 (w) 1320–1350

δ(CH)Q (9a) δ(CH)B (9a) ν(C–N) ν(C–N) + Q (19a)

1495 (s)

ν(C N) + Q (19a)

1578 (m) 1618 (m)

ν(C C)Q (8a) ν(C–C)B (8a)

1481 (s)

–

1569 (m) 1610 (m)

– 1610 (m)

vs: very strong, s: strong, m: middle, w: weak, B: benzenoid ring, Q: quinoid ring or benzene ring with quinone character, ∼: intermediate bond (between a single and a double bond), ν: stretching vibration mode, δ: in-plane bending vibration mode, Wilson’s notations of aromatic species modes are given in parentheses.

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ing the changes in band intensities, observed during reversible electrochemical reduction performed in either acidic or neutral solution used (Fig. 1), it could be concluded that the most prominent feature is a progressive decrease of intensities for the bands located at around 1150, 1205, 1480, and 1570 cm−1 . All these bands are characteristic for the oxidised form of polyaniline, as it follows from the assignments of the bands shown in Table 1. Upon addition of ascorbate to supporting electrolyte, characteristic changes in Raman spectra are observed. Fig. 2 shows a few examples of Raman spectral sets, obtained in pH-neutral solution at thick and thin polyaniline filmed electrodes at a consecutive addition of ascorbate into a supporting electrolyte. It is well seen that the intensity of all four characteristic Raman bands diminish in intensity at a progressive increase of ascorbate concentration. In both cases, the difference spectra reveal features, characteristic for more oxidised form of polyaniline (Fig. 3). The bands observed in difference spectra, located at about 1150, 1205, 1480, and 1570 cm−1 , correspond to δ(CH)Q , ν(C–N), ν(C N) + Q, and ν(C C)Q vibration modes, respectively (Table 2). Despite of negligible variations of band positions in the difference spectra, the four bands observed are characteristic for the more oxidised form of polyaniline (Fig. 3). In our previous study [8], we reported similar tendencies, obtained from difference spectra with the red laser (632.8 nm) excitation, however, the difference spectra reported here, as obtained with

Fig. 3. Difference spectra, obtained by subtracting spectra recorded in the presence of 25 mM of ascorbate from spectra recorded in absence of ascorbate. Original spectra were recorded at solution pH 7.0 for thin PANI films at operating potential of 0.2 or −0.1 V (as indicated).

a green laser excitation, are much better developed and of a low noise level. Obviously, there is a clear parallelism in spectral changes between variation of electrode potential and the concentration of ascorbate, as it could be concluded by comparing the spectra presented in Figs. 1 and 2. Particularly, both the shift of electrode potential to less positive values, and an increase of ascorbate concentration at a controlled electrode potential cause a clear decrease in intensity of Raman bands centered at around 1150, 1205, and 1480 cm−1 , characteristic for oxidised form of this polymer. This parallelism is illustrated in Fig. 4 for the three most characteristic Raman bands. From this, it could be concluded that an increase of ascorbate concentration in solution causes an increase of a relative content of a reduced form of polyaniline in the film. The process of electrocatalytic oxidation of ascorbate at polyaniline modified electrode could be supposed to consist of a few processes. Among them, the diffusion of ascorbate from solution to the reaction zone, and the diffusion of charge carriers through the polymer film seem to be essential in establishing of steady state. At a fast charge transfer, the reacTable 2 Assignment of vibrational bands (cm−1 ) of differential Raman spectra (excitation wavelength 532 nm), obtained by subtraction of spectra recorded with 25 mM of ascorbate from spectra recorded in absence of ascorbate at pH 7.0 for PANI and SPAN PANI

Fig. 2. Raman spectra, obtained at excitation wavelength of 532 nm from gold electrode, covered with thin polyaniline film, in 0.1 M phosphate buffer solution pH 7.0 at operating potential of 0.2 or −0.1 V in absence or presence of ascorbate at different concentrations (as indicated).

SPAN

Assignment

0.2 V

−0.1 V

Thin film 0.2 V

Thick film 0.2 V

1151 1203 1482 1571

1149 1204 1460 1576

1152 1206 1483 1576

1149 1203 1486 1574

δ(CH)Q (9a) ν(C–N) ν(C N) + Q (19a) ν(C C)Q (8a)

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Fig. 4. (Top) Dependence of relative intensities for Raman bands at 1152, 1206, and 1480 cm−1 (as indicated) on electrode potential for PANI films in pH 7.0 solution; (bottom) dependence of relative intensities for Raman bands at 1150, 1203, and 1481 cm−1 (as indicated) on ascorbate concentration for thick PANI film modified electrode, as obtained in pH 7.0 solution.

Fig. 5. Raman spectra, obtained at excitation wavelength of 532 nm for gold electrode, covered with thin or thick films (as indicated) of SPAN, in 0.1 M phosphate buffer solution pH 7.0 at operating potential of 0.2 V in absence or presence of ascorbate at different concentrations (as indicated).

tion zone appears to be located at an outer polyaniline/solution interface, whereas the redox state of an electrocatalyst should be determined by the electrode potential, and should not depend on the concentration of ascorbate. In the present case, however, the redox state of a polymer layer is influenced by ascorbate concentration. This means that both the diffusion of ascorbate and the diffusion of charge carriers within the polymer film proceeds at a comparable rate, whereas the reaction zone is located within a polymer film rather than at an outer polymer/solution boundary. The electrocatalytic oxidation of ascorbate proceeds following a redox mechanism. The relative intensities of both processes for any particular conditions (like electrode potential applied, thickness of polyaniline film et al.) could be verified from the dependence of Raman band intensities on ascorbate concentration. The dependence of band intensities on ascorbate concentration can be easily approximated following a 4-parameter sigmoidal equation, from which the inflection points can be calculated. For Raman bands located at 1150, 1203, and 1481 cm−1 , the inflection point of 17.85 ± 2.65, 16.17 ± 0.20, and 15.54 ± 0.68 mM has been obtained, respectively (with R2 not less than 0.998). For these close related values, a mean value of ca. 16.5 mM could be estimated. The physical meaning of this value is a definite concentration of ascorbate, for which the content of both oxidised and reduced forms of polyaniline appear to be

equal within the modifying layer. The mean “reaction zone” is thus located equidistant from underlying electrode surface, and a polymer/solution interface. At a lower concentration of ascorbate, the oxidised form of polyaniline becomes prevailing, and the reaction zone shifts towards polymer/solution interface. Adversely, at a higher ascorbate concentration, the reduced form of polyaniline prevails, whereas the reaction zone shifts towards an inner underlying electrode/polyaniline boundary. Next to polyaniline, we studied electrocatalytic oxidation of ascorbate at sulfonated polyaniline (SPAN), prepared by electrochemical copolymerisation of aniline with metanilic acid (m-aminobenzenesulfonic acid). As distinct from the parent polyaniline, this sulfonated derivative shows its electrochemical properties in pH-neutral solutions [19]. Fig. 5 shows Raman spectra for SPAN modified thin and thick electrodes, kept under a controlled potential in a solution containing ascorbate. Essentially same Raman features as for polyaniline are observed for SPAN. The interpretation of these bands is presented in Table 1. Although less expressed as for polyaniline, Raman bands of SPAN also undergo characteristic changes by the stepwise addition of ascorbate to solution, i.e. under electrolysis at a controlled potential. The difference spectra for SPAN, as depicted in Fig. 6, appear some noisier than for polyaniline under same experimental conditions, although better expressed than for polyaniline with the red laser excitation, as reported earlier [8]. The differ-

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respectively, by an increase or decrease of ascorbate concentration. Acknowledgements Financial support of this work from the Lithuanian State Science and Studies Foundation (Projects NanoBioPolymers C03047, and Biohemas C-03020) is gratefully acknowledged. References

Fig. 6. Difference spectra, obtained by subtracting spectra recorded in the presence of 25 mM of ascorbate from spectra recorded in absence of ascorbate. Original spectra were recorded at solution pH 7.0 for thin and thick SPAN films (as indicated) at operating potential of 0.2 V.

ence spectra, again, show the four most intense Raman bands with a dominating band centered at 1486 cm−1 , that correspond to stretch vibration of a double C N bond, coupled to quinone rings (Fig. 6). All these bands are characteristic for the oxidised form of SPAN (Table 2). The result obtained show that for SPAN films electrocatalytic oxidation of ascorbate proceeds within the polymer film via redox mechanism. In conclusion, the results presented here show that both for polyaniline and its sulfonated derivative, placed as a thin modifying film at an electrode surface, electrocatalytic oxidation of solution ascorbate occur within the polymer film, and presumably follows a redox catalysis mechanism (as opposed to a ‘metal-like’ catalysis). It has been also shown that the mean reaction zone, located within the polymer film, shifts to the underlying electrode/polymer, or to polymer/solution interface,

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