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Doping of polyaniline by some redox active organic anions
European Polymer Journal 36 (2000) 1347±1353
Doping of polyaniline by some redox active organic anions R. MazÏeikiene, A. Malinauskas* Institute of Chemistry, GosÏtauto Street 9, LT-2600 Vilnius, Lithuania Received 19 April 1999; received in revised form 10 June 1999; accepted 16 July 1999
Abstract Electropolymerized ®lms of polyaniline (PANI) were doped by electrochemically driven anion exchange with some redox active species bearing anionic sulfogroups: indigotetrasulfonate (ITS), hydroquinone-2,5-disulfonate (HQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). It was shown that ITS and HQDS can be bound by doping into PANI by simple potential cycling, provided in an acid solution containing the corresponding dopants. Both ITS and HQDS retain their own redox activity when bound to PANI and can be released from PANI by potential cycling in the supporting electrolyte. NQS was shown to react with PANI during potential cycling and to increase the redox capacity of PANI itself. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polyaniline (PANI) is often considered as one of the most promising conducting polymers [1]. Many unique electrical, electrochemical, optical and other properties, characteristic for PANI and its derivatives, make them very attractive in many branches of engineering. One of the most striking properties of PANI is its potential-driven ion exchange ability. It is well known that the reversible electrochemical oxidation of the leucoemeraldine form of PANI, provided in acidic solution, is followed by reversible binding of anions, present in solution, i.e. anion doping of PANI. Adversely, electrochemical reduction of PANI to its leucoemeraldine form results in expelling of anions from the polymer ®lm. This reversible potential-driven anion exchange can be in principle involved in many applications of PANI, based on the modi®cation of native PANI by inserting
* Corresponding author. Tel.: +37-2-729-350; fax: +37-2617-018. E-mail address: [email protected] (A. Malinauskas).
into its structure some anions of special interest, or reversible controllable binding and expelling of certain anions from/into diverse solutions [2]. One numerous class of such anions are redox active substances which bear anionogenic groups. When bound to PANI through anion doping, these substances can in principle show their own electroactivity, complementing that of a polymer. The resulting composite materials should have an increased redox capacity, as compared to PANI. Also, they can show the redox activity in dierent regions of the electrode potential, dierent from that characteristic for PANI itself. Yano doped PANI with red quinone dye, that contains an anionic sulfogroup (1-amino-4-bromoanthraquinone-2-sulfonate) and showed the electrochromic properties of the resulting composite polymer layer [3]. Zagal et al. studied the stability and electrical properties of PANI ®lms, doped with EDTA and FeEDTA [4]. Some works are devoted to PANI, doped with complex heteropolyions. Fabre and Bidan described the electrosynthesis of various conducting polymers doped with iron-substituted heteropolytungstate and showed the suitability of a resulting material for the electrocatalytic reduction of nitrite ions [5]. Heteropo-
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R. MazÏeikiene, A. Malinauskas / European Polymer Journal 36 (2000) 1347±1353
lyacid-doped polymers were shown to possess catalytic properties also in other processes [6,7]. Catalytic conversion of isopropanol was shown to proceed at PANI, doped with H3PW11MoO40 [8]. Ping et al. investigated the spectroelectrochemical properties of PANI, doped with perrhenate [9] and Nakayama et al. reported electrodes, modi®ed by a composite of Prussian blue and PANI, modi®ed with dierent anionic Fe(II) complexes, which are active as mediators for the electroreduction of carbon dioxide [10]. Some important macromolecular biologically active substances like e.g. enzymes were also included into PANI by anion doping [11,12]. Closely related potential-driven ion exchange properties are characteristic to PANI, but also to some other related polymers. A ®lm of poly(5-amino-1-naphthol) was doped with heteropolyanions and showed catalytic properties [13]. Polypyrrole was doped with polytungstate anions [14], heteropolytungstate anions [15], copper phthalocyanine tetrasulfonate [16] and ferricyanide [17]. Electrocatalytic properties were shown for polypyrrole, doped with heteropolyanions [18]. Polypyrroles, doped with anionic complexing ligands, were synthesized and shown to be useful in electroanalysis, e.g. alizarin S doped polypyrrole was used in electroanalysis of copper containing species [19,20]. In the present paper, we report electrochemical doping of polyaniline ®lm, deposited onto inert electrode, with some sulfonate containing redox active compounds, known as electron transfer mediators. These mediators were selected from dierent classes of compounds: para-quinones ( p-hydroquinone-2,5-disulfonate), ortho-quinones (1,2-naphthoquinone-4-sulfonate) and heterocyclic compounds (indigotetrasulfonate).
containing 50 mM of aniline, within potential scan limits of ÿ0.1 to 0.9 or 1.0 V. Modi®cation of the PANI electrodes was provided in a solution containing 10 mM of ITS, HQDS, or NQS.
3. Results and discussion As possible redox active anionogenic dopants for polyaniline (PANI), three dierent well known compounds were used in the present work, selected from dierent groups of redox mediators. These are: indigo5,5 ',7,7'tetrasulfonate (ITS), p-hydroquinone-2,5-disulfonate (HQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). All these compounds have one or more anionogenic sulfonate groups, thus they likely might be
2. Experimental Indigo-5,5 ',7,7'-tetrasulfonic acid dipotassium salt (ITS), p-hydroquinone-2,5-disulfonic acid dipotassium salt (HQDS), both from Reachim and 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQS) from Merck were used as received. Aniline was distilled before use. All solutions used contained 0.5 M sulfuric acid. A PI-50-1 model potentiostat was used in the experiments. Electrochemical experiments were performed in a three electrode cell, containing a Pt wire working electrode, 5 mm in length and 0.5 mm in diameter, a Pt wire counter electrode and a saturated Ag/AgCl reference electrode. All potentials reported are referred to this reference electrode. In almost all potential cycling experiments, a sweep rate of 100 mV/s was applied. Polyaniline modi®ed electrodes were prepared by the potential cycling procedure, performed in a solution
Fig. 1. (a) Cyclic voltammogram of the platinum electrode in a solution containing 10 mM ITS. (b) Cyclic voltammogram of the PANI electrode in a solution of 10 mM ITS, obtained for the 1st, 30th and 90th cycle (as indicated). The PANI electrode was prepared by 60-fold potential cycling within the limits of ÿ0.1 to 0.9 V in a solution of 50 mM aniline. (c) Cyclic voltammogram of the ITS-doped PANI electrode in supporting electrolyte, obtained for the 1st, 3rd, 5th and 30th cycle (as indicated). The electrode was prepared as in (b), followed by cycling within the limits of ÿ0.1 to 0.6 V in a solution of 10 mM ITS for 30 min.
R. MazÏeikiene, A. Malinauskas / European Polymer Journal 36 (2000) 1347±1353
bound to PANI by an anion doping process. However, the compounds selected dier in their electrochemical and chemical activity, showing their electrode redox processes in dierent potential regions. ITS shows at the platinum electrode a pair of anodic and cathodic peaks at 00.1 V, which correspond to the quasireversible reduction and oxidation of this structure, involving two electrons and two protons [21] (Fig. 1a). At high anode potentials, exceeding 00.7 V, also an irreversible oxidation process is observed. The PANI-covered electrode in the ITS containing solution shows both peaks characteristic for the intrinsic redox transitions of the PANI ®lm and of the ITS redox process (Fig. 1b). An anodic peak at 00.2 V, together with its cathodic counterpart at 00.05 V, correspond to the redox transition from reduced leucoemeraldine to the half-oxidized emeraldine form of PANI, whereas the second pair of peaks, located at 0.8 V, indicate a reversible emeraldine to pernigraniline transition of PANI to proceed. The position of these peaks, characteristic for PANI, is not in¯uenced by the presence of ITS. Anodic oxidation of ITS appears at 0.1 V as a partially overlapped prewave of the ®rst anodic peak of PANI, whereas the cathodic peak of ITS fully overlaps with that of PANI. These observations indicate that ITS undergoes redox processes at the PANI-covered electrode in the potential region where PANI shows its own redox transformations. A reduced form of PANI is known to be insulating and no redox processes of solute species like e.g. viologens could be detected in the potential range lower than a ®rst redox process of PANI [22]. In contrast, a conducting emeraldine form of PANI, existing in the potential region exceeding the potential of the ®rst redox process of PANI, allows electrode redox processes of many solution redox couples, like e.g. hydroquinone/benzoquinone and Fe2+/3+, to proceed very eciently [23]. Redox processes of ITS proceed in a potential region where the redox transformation of PANI from its insulating to conducting form takes place. By prolonged potential cycling, a degradation of the PANI ®lm occurs, resulting in a decrease of all characteristic anodic and cathodic peaks, whereas the observed prewave at 0.1 V remains unchanged in its height, indicating clearly its origin from ITS redox processes (Fig. 1b). When held for a de®nite time in ITS solution under open circuit conditions and then transferred into the supporting electrolyte not containing ITS, the PANIcovered electrode shows on cyclic voltammogram only peaks, characteristic for PANI itself, whereas no prewave, characteristic for ITS, is observed. This indicates that, under open circuit conditions, no ITS is bound as dopant anion to PANI. Contrary, when the PANI electrode is redox cycled in ITS solution and then transferred to the supporting electrolyte, a clear pre-
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wave or, depending on conditions, an anodic peak is seen on the cyclic voltammogram, indicating that some amount of ITS is bound to the PANI layer during its redox cycling in ITS solution (Fig. 1c). We observed an increase of this prewave by extending of the cycling duration in the range of 10±30 min, provided in ITS solution. In these experiments, potential cycling within the range of ÿ0.1±0.4 V, or ÿ0.1±0.6 V, was applied, however, no essential dierence in the CVs was observed. The height of both anodic peaks of ITS and PANI depends near linearly on the potential sweep rate within the limits of 20 to 100 mV/s, indicating their `diusionless' nature. By extending the upper potential scan limit up to 0.9 V, no ITS included into PANI ®lms was found, most probably because of irreversible anodic degradation of ITS at the electrode potential, exceeding 0.7 V, as it is seen in Fig. 1a. The results obtained show that ITS can be included into the electropolymerized PANI ®lm by the potential cycling procedure, provided in ITS solution. During the cathodic potential sweep, reduction of the emeral-
Fig. 2. (a) Cyclic voltammograms of a thin PANI electrode in a solution of 10 mM HQDS, recorded at each of 30 cycles from the 1st to the 120th cycle. The electrode was prepared by 3-fold potential cycling from ÿ0.1 to 1.0 V in aniline solution. (b) Multicycle voltammogram of a thick PANI electrode in a solution of 10 mM HQDS. The electrode was prepared by 60-fold cycling from ÿ0.1 to 0.9 V in aniline solution.
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dine to the leucoemeraldine form of PANI proceeds, followed by expelling dopant anions, usually SO2ÿ or 4 , present in excess in the supporting electrolyte HSOÿ 4 used. In the course of reoxidation of the PANI ®lm, attained in an anodic potential scan, the anions are bound back to the emeraldine form of PANI. ITS, when present in solution, can obviously compete with sulfate or hydrosulfate anions for the doping sites in PANI because of the presence of sulfonate groups in its structure. Thus, a consecutive potential cycling in ITS containing solution, leads to the exchange of at least part of the sulfate anions in the polymer ®lm to ITS anions. Adversely, when cycled in supporting electrolyte, the ITS-doped PANI ®lm losses gradually ITS dopant, as it is evidenced in Fig. 1c.Thus, a reversible doping and dedoping of PANI by ITS anions can be achieved by simple potential cycling in the corresponding electrolyte. A similar doping/dedoping of PANI layer is known for some other organic anions, like e.g. long chain alkyl sulfonates, however, ITS diers from those in that it presents a redox couple, that can be reversible bound to PANI by a doping mechanism. HQDS presents a well characterized p-quinone twoelectron redox system and possesses two sulfonate functionalities in its structure. At a Pt electrode, HQDS shows a pair of anodic and cathodic peaks, characterized by a midpoint potential of Em=0.59 V (Em=(Epa+Epc)/2) and DEp=0.55 V. Such a great value of DEp indicates a high degree of irreversibility of electrode redox interconversion of HQDS and its oxidized form, p-benzoquinone-2,5-disulfonate. At the PANI-covered electrode, HQDS shows a pair of anodic and cathodic peaks with nearly same Em, but a much lower peak separation. Fig. 2a (trace 1) shows the CV of HQDS, obtained at a very thin layer of PANI, deposited at substrate electrode. Despite of the very low thickness of the PANI layer (no characteristic current peaks of PANI are observed for this modi®ed electrode) a substantially lower value of DEp=0.17 V is observed. The thickening of the PANI layer, provided usually by increasing the potential cycle number, performed in aniline solution, causes the lowering of DEp for the HQDS redox couple. Fig. 2b shows the CV for HQDS, obtained at thick PANI coating on the Pt electrode, where characteristic anodic peaks at 00.2 and 0.8 V as well as their cathodic counterparts are observed. Well de®ned sharp anodic and cathodic peaks corresponding to redox interconversion of HQDS are observed, separated by DEp=0.03 V, i.e. coinciding with the theoretical value for two-electron redox processes. Thus, redox processes of HQDS should be considered as fully reversible in the PANI layer, which acts as an ecient electrocatalyst, transferring electrons between the electrode and the solute HQDS redox couple. In this respect, the redox system studied is closely re-
lated to the redox couple benzoquinone/hydroquinone [24] and diering in some higher redox potential, which is located on the potential scale within the limits of the conducting emeraldine form of PANI. By repeating the potential cycling at the PANI electrode in HQDS solution, both anodic and cathodic peaks diminish in height and shift to higher and lower potentials, respectively (Fig. 2a). This indicates a gradual decomposition of the PANI layer which proceeds by potential cycling. Fig. 3 shows an increase of DEp for HQDS on prolonged potential cycling for the PANI-coated electrode. The highest values of DEp are obtained at electrodes modi®ed by the thinnest PANI ®lms. Fig. 3, top, shows DEp pro®les for PANI electrodes, obtained by a dierent number of cycles in the electropolymerization electrolyte, i.e. for PANI ®lms of dierent thickness. An increase of ®lm thickness leads to a drop of DEp. It is seen from Fig. 3 that the increase of DEp proceeds on prolonged potential cycling much more faster for thin PANI ®lms. For thick polymer ®lms, only a small increase of DEp can be detected on prolonged potential cycling. After holding in HQDS solution, the PANI elec-
Fig. 3. Dependence of (EpaÿEpc) for the HQDS redox couple on the cycle number, obtained at PANI-®lmed electrodes within scan limits of ÿ0.1 to 0.9 V. PANI-®lmed electrodes were prepared by manyfold potential cycling (as indicated on the corresponding curves) in aniline solution within the limits of ÿ0.1 to 0.9 V (top), or ÿ0.1 to 1.0 V (bottom).
R. MazÏeikiene, A. Malinauskas / European Polymer Journal 36 (2000) 1347±1353
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trode shows in a few ®rst potential scans in a supporting electrolyte anodic and cathodic peaks, characteristic for the HQDS redox couple. In a few ®rst potential scans, a relative high anodic peak of PANI at E = 0.26 V is observed, which diminishes in height and shifts to lower potentials in a few next potential scans (Fig. 4), due to the well known `®rst cycle eect' or `slow relaxation', taking place in the PANI ®lm [25,26]. Also, a pair of anodic and cathodic peaks, located at around 0.6 V, characteristic for reversible oxidation and reduction of HQDS, is well seen (Fig. 4). By repeating the potential cycling, both these peaks diminish in height, whereas the main peak of PANI at 00.2 V remains unchanged. This indicates that the HQDS dopant is released slowly from the polymer ®lm, most probably it is replaced by sulfate anions, present in excess in the bulk solution. In a control experiment using hydroquinone instead of HQDS, no binding of redox active substances was observed under closely similar experimental conditions. Thus, it may be assumed that HQDS is bound as dopant due to the electrostatic interaction of its sulfonate groups with positively charged nitrogens in a polymer chain. A closely similar picture was observed by simply holding the PANI electrode in HQDS solution at an open circuit for a few tens of minutes, or by potential cycling in the same solution for the same time within the limits of ÿ0.1±0.4, or ÿ0.1±0.6 V. Thus, HQDS competes
for doping sites with sulfate anion independent of the redox state of PANI, contrary to ITS studied. Fig. 5 shows the simultaneous decrease of the peak current for the leucoemeraldine to emeraldine transition of PANI (trace 1) and HQDS. It shows a relative fast release of HQDS from the polymer ®lm (with a half value attained at about 5±7 potential cycles made), as compared to the relative slow degradation of the PANI ®lm (with a half value at 0200 cycles). The extent of redox active anions, incorporated into the PANI ®lm, can be veri®ed by comparing the redox charge needed for redox transformation of the species included and PANI itself. As with HQDS (Fig. 4), an electric charge of 11.4 mC/cm2 passes during the anodic oxidation of the PANI ®lm, obtained in the second potential scan (not in¯uenced by the `®rst cycle eect'). Accordingly, an anodic charge of 6.6 mC/cm2 is consumed in electrooxidation of HQDS included into the PANI ®lm. It follows that HQDS does not replace all sulfate anions, involved in PANI doping. Assuming that one molecule of HQDS is needed to compensate one electron involved in the redox transformation of PANI and taking into account a twoelectron oxidation of HQDS, it can be deduced from the data obtained that roughly 29% of the electric charge of PANI are compensated by HQDS anions, whereas the remaining part is compensated obviously by sulfate/hydrosulfate anions. Unexpected results were obtained by cycling the
Fig. 4. Cyclic voltammograms of the HQDS-doped PANI electrode in supporting electrolyte, obtained for the 1st to the 5th cycle (as indicated). The electrode was prepared by 55fold potential cycling from ÿ0.1 to 1.0 V in aniline solution, followed by holding in a solution of 10 mM HQDS for 10 min.
Fig. 5. Dependence of ipa for PANI (1) and HQDS (2) peaks on the potential scan number for a HQDS-doped PANI ®lm, within scan limits of ÿ0.1 to 0.8 V. The PANI ®lm was prepared by potential cycling for 20 min between ÿ0.1 and 1.0 V in aniline solution, followed by holding for 10 min in HQDS solution.
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PANI electrode in a solution of 1,2-naphthoquinone-4sulfonate (NQS). At a bare platinum electrode, HQS shows a pair of cathodic and anodic peaks with Em=0.43 V and DEp of 00.4±0.5 V under the conditions used. At thin PANI-coated electrodes, DEp decreases gradually by thickening of the PANI layer, indicating an increasing reversibility of the redox processes for NQS. In this respect, the system behaves very similar to the HQDS redox system and also closely related peculiarities were observed for both redox systems. However, thick electrodeposited ®lms of PANI showed an unexpected behavior. By prolonged potential cycling, both redox peaks belonging to NQS redox processes, show no or little changes, retaining a constant value of Em and DEp of 00.3±0.4 V, whereas the anodic peak of PANI grows in height and shifts to higher potentials (Fig. 6). Also, some growth of the cathodic peak is seen. Depending on the conditions used, 02-fold increase of electric charge, passed in the ®rst anodic peak of PANI, is attained after 150±200 potential cycles. Thus, an increase of redox capacity is obtained by cycling of the PANI ®lm in NQS solution. In the absence of NQS, only 00.5 of initial redox capacity of the ®lm is retained by potential cycling, provided for 150±200 cycles (see Fig. 5), due to degradation of the PANI ®lm. Fig. 7a, shows CVs for the PANI electrode held in NQS solution. Characteristic anodic and cathodic peaks indicate some amount of NQS bound into the polymer ®lm and then released by repeating potential cycling in the supporting electrolyte. When cycled in a NQS containing solution, the PANI electrode shows in CVs obtained in the supporting electrolyte both
characteristic redox peaks, however, NQS is obviously stronger bound to the PANI matrix, since the redox peaks diminish in height slower than in the case when no cycling in NQS solution was done (Fig. 7b). It is seen from Fig. 7b, that some dierences in the shape of both anodic and cathodic peaks for PANI appear on cycling in NQS, indicating a more complex interaction of the polymer with NQS than electrostatic binding only. The dierences in interaction of PANI with two similar (quinoid) compounds, HQDS and NQS, can be understood taking into account a relative high chemical reactivity of NQS. It is known that NQS reacts with many kinds of aliphatic amines via the substitution of its sulfonate group, yielding colored 4-substituted 1,2-naphthoquinones [27]. Thus, a chemical reaction between NQS and PANI is very likely. As a result, 1,2-naphthoquinone moieties bound to PANI chains yield p-conjugated structures, extending the number of possible redox sites in the polymer. This should result in an increase of the redox capacity, that is observed experimentally (Fig. 6).
Fig. 6. Cyclic voltammograms of the PANI electrode in a solution of 10 mM NQS, obtained for the 15th, 60th, 90th, 120th and 180th potential scan (as indicated). The electrode was prepared by 30-fold potential cycling between ÿ0.1 and 1.0 V.
Fig. 7. Cyclic voltammograms, obtained in supporting electrolyte, not containing NQS, (a) for the PANI electrode, held initially in 10 mM NQS solution for 30 min; (b) for the PANI electrode, cycled in NQS solution for 30 min between ÿ0.1 and 0.6 V.
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4. Conclusions It was found that some redox active species containing anionic sulfogroups, like ITS and HQDS, can be reversibly bound by anion doping into the PANI layer by the potential cycling procedure. The redox active dopants, however, replace only part of the sulfate anions, present in a virgin PANI ®lm. When bound to the PANI layer, both dopants used retain their own redox activity. Both dopants can be reversibly expelled from the PANI ®lm by potential cycling in sulfuric acid solution. Indirect evidence was obtained from cyclic voltammetry: NQS reacts with PANI during potential cycling, leading to chemical modi®cation of PANI followed by an increase of its redox capacity.
References [1] Evans GP. In: Gerischer H, Tobias CW, editors. Advances in electrochemical science and engineering, Vol. 1. Weinheim: VCH, 1990. p. 1. [2] Walton DJ, Hall CE, Chyla A. Synth Metals 1991;45:363±71. [3] Yano J. J Electrochem Soc 1997;144:477±81. [4] Zagal JH, Del Rio RR, Retamal BA, Biaggio SR. J Appl Electrochem 1996;26:95±101. [5] Fabre B, Bidan G. Electrochim Acta 1997;42:2587±90. [6] Stochmal-Pomarzanska E, Quillard S, Hasik M, Turek W, Pron A, Lapkowski M, Lefrant S. Synth Metals 1997;84:427±8. [7] Hasik M, Pron A, Kulszewicz-Bajer I, Pozniczek J, Bielanski A, Piwowarska Z, Dziembaj R. Synth Metals 1993;55±57:972±6. [8] Qu LY, Lu RQ, Peng J, Chen YG, Dai ZM. Synth Metals 1997;84:135±6.
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[9] Ping Z, Neugebauer H, Neckel. Electrochim Acta 1996;41:767±72. [10] Nakayama M, Iino M, Ogura K. J Electroanal Chem 1997;440:251±7. [11] Skinner NG, Hall EAH. J Electroanal Chem 1997;420:179±88. [12] Shaolin M, Jinqing K. Electrochim Acta 1995;40:241±6. [13] Pham MC, Bouallala S, Le LA, Dang VM, Lacaze PC. Electrochim Acta 1997;42:439±47. [14] Ohtsuka T, Wakabayashi TO, Einaga H. Synth Metals 1996;79:235±9. [15] Sung H, So H, Paik WK. Electrochim Acta 1994;39:645± 50. [16] Reynolds JR, Pyo M, Qin YJ. J Electrochem Soc 1994;141:35±40. [17] Tolgyesi M, Szucs A, Visy C, Novak M. Electrochim Acta 1995;40:1127±33. [18] Wang P, Li Y. J Electroanal Chem 1996;408:77±81. [19] Shiu KK, Chan OY. J Electroanal Chem 1995;388:45± 51. [20] Shiu KK, Chan OY, Pang SK. Anal Chem 1995;67:2828±34. [21] Ottaway JM. In: Bishop E, editor. Indicators. Oxford: Pergamon Press, 1972. p. 469±530. [22] Malinauskas A, Holze R. J Electroanal Chem 1999;461:184±93. [23] Malinauskas A, Holze R. Electrochim Acta 1998;43:2563±75. [24] Malinauskas A, Holze R. Ber Bunsenges Phys Chem 1996;100:1740±5. [25] Odin C, Nechtschein M. Synth Metals 1991;41±43:2943± 6. [26] Odin C, Nechtschein M. Synth Metals 1993;55±57:1281± 6. [27] Asahi Y, Tanaka M, Shinozaki K. Chem Pharm Bull 1984;32:3039±49.
Doping of polyaniline by some redox active organic anions R. MazÏeikiene, A. Malinauskas* Institute of Chemistry, GosÏtauto Street 9, LT-2600 Vilnius, Lithuania Received 19 April 1999; received in revised form 10 June 1999; accepted 16 July 1999
Abstract Electropolymerized ®lms of polyaniline (PANI) were doped by electrochemically driven anion exchange with some redox active species bearing anionic sulfogroups: indigotetrasulfonate (ITS), hydroquinone-2,5-disulfonate (HQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). It was shown that ITS and HQDS can be bound by doping into PANI by simple potential cycling, provided in an acid solution containing the corresponding dopants. Both ITS and HQDS retain their own redox activity when bound to PANI and can be released from PANI by potential cycling in the supporting electrolyte. NQS was shown to react with PANI during potential cycling and to increase the redox capacity of PANI itself. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction Polyaniline (PANI) is often considered as one of the most promising conducting polymers [1]. Many unique electrical, electrochemical, optical and other properties, characteristic for PANI and its derivatives, make them very attractive in many branches of engineering. One of the most striking properties of PANI is its potential-driven ion exchange ability. It is well known that the reversible electrochemical oxidation of the leucoemeraldine form of PANI, provided in acidic solution, is followed by reversible binding of anions, present in solution, i.e. anion doping of PANI. Adversely, electrochemical reduction of PANI to its leucoemeraldine form results in expelling of anions from the polymer ®lm. This reversible potential-driven anion exchange can be in principle involved in many applications of PANI, based on the modi®cation of native PANI by inserting
* Corresponding author. Tel.: +37-2-729-350; fax: +37-2617-018. E-mail address: [email protected] (A. Malinauskas).
into its structure some anions of special interest, or reversible controllable binding and expelling of certain anions from/into diverse solutions [2]. One numerous class of such anions are redox active substances which bear anionogenic groups. When bound to PANI through anion doping, these substances can in principle show their own electroactivity, complementing that of a polymer. The resulting composite materials should have an increased redox capacity, as compared to PANI. Also, they can show the redox activity in dierent regions of the electrode potential, dierent from that characteristic for PANI itself. Yano doped PANI with red quinone dye, that contains an anionic sulfogroup (1-amino-4-bromoanthraquinone-2-sulfonate) and showed the electrochromic properties of the resulting composite polymer layer [3]. Zagal et al. studied the stability and electrical properties of PANI ®lms, doped with EDTA and FeEDTA [4]. Some works are devoted to PANI, doped with complex heteropolyions. Fabre and Bidan described the electrosynthesis of various conducting polymers doped with iron-substituted heteropolytungstate and showed the suitability of a resulting material for the electrocatalytic reduction of nitrite ions [5]. Heteropo-
0014-3057/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 1 9 1 - 3
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R. MazÏeikiene, A. Malinauskas / European Polymer Journal 36 (2000) 1347±1353
lyacid-doped polymers were shown to possess catalytic properties also in other processes [6,7]. Catalytic conversion of isopropanol was shown to proceed at PANI, doped with H3PW11MoO40 [8]. Ping et al. investigated the spectroelectrochemical properties of PANI, doped with perrhenate [9] and Nakayama et al. reported electrodes, modi®ed by a composite of Prussian blue and PANI, modi®ed with dierent anionic Fe(II) complexes, which are active as mediators for the electroreduction of carbon dioxide [10]. Some important macromolecular biologically active substances like e.g. enzymes were also included into PANI by anion doping [11,12]. Closely related potential-driven ion exchange properties are characteristic to PANI, but also to some other related polymers. A ®lm of poly(5-amino-1-naphthol) was doped with heteropolyanions and showed catalytic properties [13]. Polypyrrole was doped with polytungstate anions [14], heteropolytungstate anions [15], copper phthalocyanine tetrasulfonate [16] and ferricyanide [17]. Electrocatalytic properties were shown for polypyrrole, doped with heteropolyanions [18]. Polypyrroles, doped with anionic complexing ligands, were synthesized and shown to be useful in electroanalysis, e.g. alizarin S doped polypyrrole was used in electroanalysis of copper containing species [19,20]. In the present paper, we report electrochemical doping of polyaniline ®lm, deposited onto inert electrode, with some sulfonate containing redox active compounds, known as electron transfer mediators. These mediators were selected from dierent classes of compounds: para-quinones ( p-hydroquinone-2,5-disulfonate), ortho-quinones (1,2-naphthoquinone-4-sulfonate) and heterocyclic compounds (indigotetrasulfonate).
containing 50 mM of aniline, within potential scan limits of ÿ0.1 to 0.9 or 1.0 V. Modi®cation of the PANI electrodes was provided in a solution containing 10 mM of ITS, HQDS, or NQS.
3. Results and discussion As possible redox active anionogenic dopants for polyaniline (PANI), three dierent well known compounds were used in the present work, selected from dierent groups of redox mediators. These are: indigo5,5 ',7,7'tetrasulfonate (ITS), p-hydroquinone-2,5-disulfonate (HQDS) and 1,2-naphthoquinone-4-sulfonate (NQS). All these compounds have one or more anionogenic sulfonate groups, thus they likely might be
2. Experimental Indigo-5,5 ',7,7'-tetrasulfonic acid dipotassium salt (ITS), p-hydroquinone-2,5-disulfonic acid dipotassium salt (HQDS), both from Reachim and 1,2-naphthoquinone-4-sulfonic acid sodium salt (NQS) from Merck were used as received. Aniline was distilled before use. All solutions used contained 0.5 M sulfuric acid. A PI-50-1 model potentiostat was used in the experiments. Electrochemical experiments were performed in a three electrode cell, containing a Pt wire working electrode, 5 mm in length and 0.5 mm in diameter, a Pt wire counter electrode and a saturated Ag/AgCl reference electrode. All potentials reported are referred to this reference electrode. In almost all potential cycling experiments, a sweep rate of 100 mV/s was applied. Polyaniline modi®ed electrodes were prepared by the potential cycling procedure, performed in a solution
Fig. 1. (a) Cyclic voltammogram of the platinum electrode in a solution containing 10 mM ITS. (b) Cyclic voltammogram of the PANI electrode in a solution of 10 mM ITS, obtained for the 1st, 30th and 90th cycle (as indicated). The PANI electrode was prepared by 60-fold potential cycling within the limits of ÿ0.1 to 0.9 V in a solution of 50 mM aniline. (c) Cyclic voltammogram of the ITS-doped PANI electrode in supporting electrolyte, obtained for the 1st, 3rd, 5th and 30th cycle (as indicated). The electrode was prepared as in (b), followed by cycling within the limits of ÿ0.1 to 0.6 V in a solution of 10 mM ITS for 30 min.
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bound to PANI by an anion doping process. However, the compounds selected dier in their electrochemical and chemical activity, showing their electrode redox processes in dierent potential regions. ITS shows at the platinum electrode a pair of anodic and cathodic peaks at 00.1 V, which correspond to the quasireversible reduction and oxidation of this structure, involving two electrons and two protons [21] (Fig. 1a). At high anode potentials, exceeding 00.7 V, also an irreversible oxidation process is observed. The PANI-covered electrode in the ITS containing solution shows both peaks characteristic for the intrinsic redox transitions of the PANI ®lm and of the ITS redox process (Fig. 1b). An anodic peak at 00.2 V, together with its cathodic counterpart at 00.05 V, correspond to the redox transition from reduced leucoemeraldine to the half-oxidized emeraldine form of PANI, whereas the second pair of peaks, located at 0.8 V, indicate a reversible emeraldine to pernigraniline transition of PANI to proceed. The position of these peaks, characteristic for PANI, is not in¯uenced by the presence of ITS. Anodic oxidation of ITS appears at 0.1 V as a partially overlapped prewave of the ®rst anodic peak of PANI, whereas the cathodic peak of ITS fully overlaps with that of PANI. These observations indicate that ITS undergoes redox processes at the PANI-covered electrode in the potential region where PANI shows its own redox transformations. A reduced form of PANI is known to be insulating and no redox processes of solute species like e.g. viologens could be detected in the potential range lower than a ®rst redox process of PANI [22]. In contrast, a conducting emeraldine form of PANI, existing in the potential region exceeding the potential of the ®rst redox process of PANI, allows electrode redox processes of many solution redox couples, like e.g. hydroquinone/benzoquinone and Fe2+/3+, to proceed very eciently [23]. Redox processes of ITS proceed in a potential region where the redox transformation of PANI from its insulating to conducting form takes place. By prolonged potential cycling, a degradation of the PANI ®lm occurs, resulting in a decrease of all characteristic anodic and cathodic peaks, whereas the observed prewave at 0.1 V remains unchanged in its height, indicating clearly its origin from ITS redox processes (Fig. 1b). When held for a de®nite time in ITS solution under open circuit conditions and then transferred into the supporting electrolyte not containing ITS, the PANIcovered electrode shows on cyclic voltammogram only peaks, characteristic for PANI itself, whereas no prewave, characteristic for ITS, is observed. This indicates that, under open circuit conditions, no ITS is bound as dopant anion to PANI. Contrary, when the PANI electrode is redox cycled in ITS solution and then transferred to the supporting electrolyte, a clear pre-
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wave or, depending on conditions, an anodic peak is seen on the cyclic voltammogram, indicating that some amount of ITS is bound to the PANI layer during its redox cycling in ITS solution (Fig. 1c). We observed an increase of this prewave by extending of the cycling duration in the range of 10±30 min, provided in ITS solution. In these experiments, potential cycling within the range of ÿ0.1±0.4 V, or ÿ0.1±0.6 V, was applied, however, no essential dierence in the CVs was observed. The height of both anodic peaks of ITS and PANI depends near linearly on the potential sweep rate within the limits of 20 to 100 mV/s, indicating their `diusionless' nature. By extending the upper potential scan limit up to 0.9 V, no ITS included into PANI ®lms was found, most probably because of irreversible anodic degradation of ITS at the electrode potential, exceeding 0.7 V, as it is seen in Fig. 1a. The results obtained show that ITS can be included into the electropolymerized PANI ®lm by the potential cycling procedure, provided in ITS solution. During the cathodic potential sweep, reduction of the emeral-
Fig. 2. (a) Cyclic voltammograms of a thin PANI electrode in a solution of 10 mM HQDS, recorded at each of 30 cycles from the 1st to the 120th cycle. The electrode was prepared by 3-fold potential cycling from ÿ0.1 to 1.0 V in aniline solution. (b) Multicycle voltammogram of a thick PANI electrode in a solution of 10 mM HQDS. The electrode was prepared by 60-fold cycling from ÿ0.1 to 0.9 V in aniline solution.
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dine to the leucoemeraldine form of PANI proceeds, followed by expelling dopant anions, usually SO2ÿ or 4 , present in excess in the supporting electrolyte HSOÿ 4 used. In the course of reoxidation of the PANI ®lm, attained in an anodic potential scan, the anions are bound back to the emeraldine form of PANI. ITS, when present in solution, can obviously compete with sulfate or hydrosulfate anions for the doping sites in PANI because of the presence of sulfonate groups in its structure. Thus, a consecutive potential cycling in ITS containing solution, leads to the exchange of at least part of the sulfate anions in the polymer ®lm to ITS anions. Adversely, when cycled in supporting electrolyte, the ITS-doped PANI ®lm losses gradually ITS dopant, as it is evidenced in Fig. 1c.Thus, a reversible doping and dedoping of PANI by ITS anions can be achieved by simple potential cycling in the corresponding electrolyte. A similar doping/dedoping of PANI layer is known for some other organic anions, like e.g. long chain alkyl sulfonates, however, ITS diers from those in that it presents a redox couple, that can be reversible bound to PANI by a doping mechanism. HQDS presents a well characterized p-quinone twoelectron redox system and possesses two sulfonate functionalities in its structure. At a Pt electrode, HQDS shows a pair of anodic and cathodic peaks, characterized by a midpoint potential of Em=0.59 V (Em=(Epa+Epc)/2) and DEp=0.55 V. Such a great value of DEp indicates a high degree of irreversibility of electrode redox interconversion of HQDS and its oxidized form, p-benzoquinone-2,5-disulfonate. At the PANI-covered electrode, HQDS shows a pair of anodic and cathodic peaks with nearly same Em, but a much lower peak separation. Fig. 2a (trace 1) shows the CV of HQDS, obtained at a very thin layer of PANI, deposited at substrate electrode. Despite of the very low thickness of the PANI layer (no characteristic current peaks of PANI are observed for this modi®ed electrode) a substantially lower value of DEp=0.17 V is observed. The thickening of the PANI layer, provided usually by increasing the potential cycle number, performed in aniline solution, causes the lowering of DEp for the HQDS redox couple. Fig. 2b shows the CV for HQDS, obtained at thick PANI coating on the Pt electrode, where characteristic anodic peaks at 00.2 and 0.8 V as well as their cathodic counterparts are observed. Well de®ned sharp anodic and cathodic peaks corresponding to redox interconversion of HQDS are observed, separated by DEp=0.03 V, i.e. coinciding with the theoretical value for two-electron redox processes. Thus, redox processes of HQDS should be considered as fully reversible in the PANI layer, which acts as an ecient electrocatalyst, transferring electrons between the electrode and the solute HQDS redox couple. In this respect, the redox system studied is closely re-
lated to the redox couple benzoquinone/hydroquinone [24] and diering in some higher redox potential, which is located on the potential scale within the limits of the conducting emeraldine form of PANI. By repeating the potential cycling at the PANI electrode in HQDS solution, both anodic and cathodic peaks diminish in height and shift to higher and lower potentials, respectively (Fig. 2a). This indicates a gradual decomposition of the PANI layer which proceeds by potential cycling. Fig. 3 shows an increase of DEp for HQDS on prolonged potential cycling for the PANI-coated electrode. The highest values of DEp are obtained at electrodes modi®ed by the thinnest PANI ®lms. Fig. 3, top, shows DEp pro®les for PANI electrodes, obtained by a dierent number of cycles in the electropolymerization electrolyte, i.e. for PANI ®lms of dierent thickness. An increase of ®lm thickness leads to a drop of DEp. It is seen from Fig. 3 that the increase of DEp proceeds on prolonged potential cycling much more faster for thin PANI ®lms. For thick polymer ®lms, only a small increase of DEp can be detected on prolonged potential cycling. After holding in HQDS solution, the PANI elec-
Fig. 3. Dependence of (EpaÿEpc) for the HQDS redox couple on the cycle number, obtained at PANI-®lmed electrodes within scan limits of ÿ0.1 to 0.9 V. PANI-®lmed electrodes were prepared by manyfold potential cycling (as indicated on the corresponding curves) in aniline solution within the limits of ÿ0.1 to 0.9 V (top), or ÿ0.1 to 1.0 V (bottom).
R. MazÏeikiene, A. Malinauskas / European Polymer Journal 36 (2000) 1347±1353
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trode shows in a few ®rst potential scans in a supporting electrolyte anodic and cathodic peaks, characteristic for the HQDS redox couple. In a few ®rst potential scans, a relative high anodic peak of PANI at E = 0.26 V is observed, which diminishes in height and shifts to lower potentials in a few next potential scans (Fig. 4), due to the well known `®rst cycle eect' or `slow relaxation', taking place in the PANI ®lm [25,26]. Also, a pair of anodic and cathodic peaks, located at around 0.6 V, characteristic for reversible oxidation and reduction of HQDS, is well seen (Fig. 4). By repeating the potential cycling, both these peaks diminish in height, whereas the main peak of PANI at 00.2 V remains unchanged. This indicates that the HQDS dopant is released slowly from the polymer ®lm, most probably it is replaced by sulfate anions, present in excess in the bulk solution. In a control experiment using hydroquinone instead of HQDS, no binding of redox active substances was observed under closely similar experimental conditions. Thus, it may be assumed that HQDS is bound as dopant due to the electrostatic interaction of its sulfonate groups with positively charged nitrogens in a polymer chain. A closely similar picture was observed by simply holding the PANI electrode in HQDS solution at an open circuit for a few tens of minutes, or by potential cycling in the same solution for the same time within the limits of ÿ0.1±0.4, or ÿ0.1±0.6 V. Thus, HQDS competes
for doping sites with sulfate anion independent of the redox state of PANI, contrary to ITS studied. Fig. 5 shows the simultaneous decrease of the peak current for the leucoemeraldine to emeraldine transition of PANI (trace 1) and HQDS. It shows a relative fast release of HQDS from the polymer ®lm (with a half value attained at about 5±7 potential cycles made), as compared to the relative slow degradation of the PANI ®lm (with a half value at 0200 cycles). The extent of redox active anions, incorporated into the PANI ®lm, can be veri®ed by comparing the redox charge needed for redox transformation of the species included and PANI itself. As with HQDS (Fig. 4), an electric charge of 11.4 mC/cm2 passes during the anodic oxidation of the PANI ®lm, obtained in the second potential scan (not in¯uenced by the `®rst cycle eect'). Accordingly, an anodic charge of 6.6 mC/cm2 is consumed in electrooxidation of HQDS included into the PANI ®lm. It follows that HQDS does not replace all sulfate anions, involved in PANI doping. Assuming that one molecule of HQDS is needed to compensate one electron involved in the redox transformation of PANI and taking into account a twoelectron oxidation of HQDS, it can be deduced from the data obtained that roughly 29% of the electric charge of PANI are compensated by HQDS anions, whereas the remaining part is compensated obviously by sulfate/hydrosulfate anions. Unexpected results were obtained by cycling the
Fig. 4. Cyclic voltammograms of the HQDS-doped PANI electrode in supporting electrolyte, obtained for the 1st to the 5th cycle (as indicated). The electrode was prepared by 55fold potential cycling from ÿ0.1 to 1.0 V in aniline solution, followed by holding in a solution of 10 mM HQDS for 10 min.
Fig. 5. Dependence of ipa for PANI (1) and HQDS (2) peaks on the potential scan number for a HQDS-doped PANI ®lm, within scan limits of ÿ0.1 to 0.8 V. The PANI ®lm was prepared by potential cycling for 20 min between ÿ0.1 and 1.0 V in aniline solution, followed by holding for 10 min in HQDS solution.
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PANI electrode in a solution of 1,2-naphthoquinone-4sulfonate (NQS). At a bare platinum electrode, HQS shows a pair of cathodic and anodic peaks with Em=0.43 V and DEp of 00.4±0.5 V under the conditions used. At thin PANI-coated electrodes, DEp decreases gradually by thickening of the PANI layer, indicating an increasing reversibility of the redox processes for NQS. In this respect, the system behaves very similar to the HQDS redox system and also closely related peculiarities were observed for both redox systems. However, thick electrodeposited ®lms of PANI showed an unexpected behavior. By prolonged potential cycling, both redox peaks belonging to NQS redox processes, show no or little changes, retaining a constant value of Em and DEp of 00.3±0.4 V, whereas the anodic peak of PANI grows in height and shifts to higher potentials (Fig. 6). Also, some growth of the cathodic peak is seen. Depending on the conditions used, 02-fold increase of electric charge, passed in the ®rst anodic peak of PANI, is attained after 150±200 potential cycles. Thus, an increase of redox capacity is obtained by cycling of the PANI ®lm in NQS solution. In the absence of NQS, only 00.5 of initial redox capacity of the ®lm is retained by potential cycling, provided for 150±200 cycles (see Fig. 5), due to degradation of the PANI ®lm. Fig. 7a, shows CVs for the PANI electrode held in NQS solution. Characteristic anodic and cathodic peaks indicate some amount of NQS bound into the polymer ®lm and then released by repeating potential cycling in the supporting electrolyte. When cycled in a NQS containing solution, the PANI electrode shows in CVs obtained in the supporting electrolyte both
characteristic redox peaks, however, NQS is obviously stronger bound to the PANI matrix, since the redox peaks diminish in height slower than in the case when no cycling in NQS solution was done (Fig. 7b). It is seen from Fig. 7b, that some dierences in the shape of both anodic and cathodic peaks for PANI appear on cycling in NQS, indicating a more complex interaction of the polymer with NQS than electrostatic binding only. The dierences in interaction of PANI with two similar (quinoid) compounds, HQDS and NQS, can be understood taking into account a relative high chemical reactivity of NQS. It is known that NQS reacts with many kinds of aliphatic amines via the substitution of its sulfonate group, yielding colored 4-substituted 1,2-naphthoquinones [27]. Thus, a chemical reaction between NQS and PANI is very likely. As a result, 1,2-naphthoquinone moieties bound to PANI chains yield p-conjugated structures, extending the number of possible redox sites in the polymer. This should result in an increase of the redox capacity, that is observed experimentally (Fig. 6).
Fig. 6. Cyclic voltammograms of the PANI electrode in a solution of 10 mM NQS, obtained for the 15th, 60th, 90th, 120th and 180th potential scan (as indicated). The electrode was prepared by 30-fold potential cycling between ÿ0.1 and 1.0 V.
Fig. 7. Cyclic voltammograms, obtained in supporting electrolyte, not containing NQS, (a) for the PANI electrode, held initially in 10 mM NQS solution for 30 min; (b) for the PANI electrode, cycled in NQS solution for 30 min between ÿ0.1 and 0.6 V.
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4. Conclusions It was found that some redox active species containing anionic sulfogroups, like ITS and HQDS, can be reversibly bound by anion doping into the PANI layer by the potential cycling procedure. The redox active dopants, however, replace only part of the sulfate anions, present in a virgin PANI ®lm. When bound to the PANI layer, both dopants used retain their own redox activity. Both dopants can be reversibly expelled from the PANI ®lm by potential cycling in sulfuric acid solution. Indirect evidence was obtained from cyclic voltammetry: NQS reacts with PANI during potential cycling, leading to chemical modi®cation of PANI followed by an increase of its redox capacity.
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