2566
Langmuir 1999, 15, 2566-2574
Improvement of the Electrosynthesis and Physicochemical Properties of Poly(3,4-ethylenedioxythiophene) Using a Sodium Dodecyl Sulfate Micellar Aqueous Medium Nacer Sakmeche, Salah Aeiyach, Jean-Jacques Aaron,* Mohamed Jouini, Jean Christophe Lacroix, and Pierre-Camille Lacaze Institut de Topologie et de Dynamique des Syste` mes de l'Universite´ Paris 7-Denis Diderot, associe´ au CNRS, UPRES-A 7086, 1, rue Guy de la Brosse, 75005 Paris, France Received July 20, 1998. In Final Form: December 10, 1998 Electrosynthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) films was performed in a micellar aqueous solution containing sodium dodecyl sulfate (SDS) by cyclic voltammetry, chronoamperometry, and chronopotentiometry on a platinum electrode. The electrocatalytic effect of SDS was characterized by a significant decrease of the EDOT oxidation potential (Ep) in the micellar medium relative to 0.1 M LiClO4 acetonitrile as well as aqueous solutions. Linear variation of Ep with SDS concentration indicated the formation of a pseudocomplex (Keq ) 5.4 × 103 M-1). PEDOT films were characterized electrochemically and spectroscopically (UV-visible, X-ray photoelectron spectroscopy, IR, Raman spectra). Regular, wellordered, and adherent films were obtained in SDS medium. The PEDOT film morphologies investigated by atomic force microscopy suggested the possible presence of columnar structures when the electrosynthesis is performed in the micellar medium.
Introduction Numerous studies have shown that polypyrrole and polyaniline films can be electrosynthesized by electrochemical oxidation of the corresponding monomers in aqueous solutions.1-3 In contrast, electropolymerization of thiophene and its derivatives requires generally anhydrous organic media4-18; in a few cases, polythiophene (PT) conducting films also have been electrodeposited, using very acidic aqueous solutions.19-22 (1) Diaz, A. F.; Kanazawa, K. K.; Gardini, G. P. J. Chem. Soc. Chem. Commun. 1979, 635. (2) McDiarmid, A. G.; Chiang, J. C.; Halppern, M.; Huang, H. S.; Mu, S. L.; Somasiri, N. L. D.; Wu, W.; Yaniger, S. Y. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (3) Genie`s, E. M.; Tsintavis, C.; Syed, A. A. Mol. Cryst. Liq. Cryst. 1985, 121, 181. (4) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 111, 111. (5) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173. (6) Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983, 87, 1459. (7) Sato, M.; Tanaka, S.; Kaeriyama, K. J. Chem. Soc. Chem. Commun. 1985, 713. (8) Roncali, J.; Garnier, F.; Lemaire, M.; Garreau, R. Synth. Met. 1986, 15, 323. (9) Tanaka, S.; Sato, M.; Kaeriyama, K. Makromol. Chem. 1984, 185, 1295. (10) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J. Appl. Phys. 1982, 21, L567. (11) Kaneto, K.; Yoshino, K.; Inuishi, Y. Jpn. J. Appl. Phys. 1982, 22, L412. (12) Kaneto, K.; Yoshino, K.; Inuishi, Y. J. Chem. Soc. Chem. Commun. 1983, 382. (13) Ingana¨s, O.; Liedberg, B.; Chang-Ru, W.; Wynberg, H. Synth. Met. 1985, 11, 239. (14) Roncali, J.; Yasser, A.; Garnier, F. J. Chem. Soc. Chem. Commun. 1988, 581. (15) Yasser, A.; Roncali, J.; Garnier, F. Macromolecules 1989, 22, 804. (16) Barsch, U.; Beck, F.; Hambitzer, G.; Holtze, R.; Lippe, J.; Stassen, I. J. Electroanal. Chem. 1994, 369, 97. (17) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994, 369, 87. (18) Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986, 206, 139; 147. (19) Dong, S.; Zhang, W. Synth. Met. 1989, 30, 359. (20) Bazzaoui, E. A.; Aeiyach, S.; Lacaze, P. C. J. Electroanal. Chem. 1994, 364, 63.
Indeed, many difficulties appear for electrosynthesizing such materials in aqueous media: (i) the thiophene structures are very weakly soluble in water; (ii) the oxidation potential of thiophene (1.8 V) is higher than that of water (1.23 V)23; and (iii) the electropolymerization process is inhibited by water, because of the formation of thienyl cation radicals, which react rapidly in this nucleophilic medium.18 To solve these problems, it recently has been proposed to add significant amounts of anionic surfactants such as sodium dodecyl sulfate (SDS)23-25 to aqueous solutions of thiophenes. As a result, the solubility of thiophene derivatives is increased and their oxidation potential is lowered. Indeed, the use of surfactant-containing media for the polymerization of heteroaromatic compounds produces several important effects. The presence of micelles provides an interesting solvent system for solubilization of these water-insoluble compounds. Also, micellar media affect the electrochemical reactions by irreversible adsorption, leading to a change in the solution-electrode interface properties and producing some template effects at the electrode.26,27 Moreover, surfactantcontaining media can also stabilize charged species such as anions or cation radicals.28-30 (21) Bidan, G.; Genie`s, E. M.; Lapokowski, M. Synth. Met. 1989, 31, 327. (22) Lapokowski, M.; Bidan, G.; Fournier, M. Synth. Met. 1991, 41, 407. (23) Bazzaoui, E. A.; Aeiyach, S.; Lacaze, P. C. Synth. Met. 1996, 83, 159. (24) Sakmeche, N.; Aaron, J. J.; Fall, M.; Aeiyach, S.; Jouini, M., Lacroix, J. C.; Lacaze, P. C. J. Chem. Soc. Chem. Commun. 1996, 2723. (25) Sakmeche, N.; Bazzaoui, E. A.; Fall, M.; Aeiyach, S.; Jouini, M.; Lacroix, J. C.; Aaron, J. J.; Lacaze, P. C. Synth. Met. 1997, 84, 191. (26) Damaskin, B. B.; Petrii, O. A.; Batrakov, V. V. Adsorption of Organic Compounds on Electrodes; Plenum: New York, 1971. (27) Damaskin, B. B.; Kazarinov, V. E. Comprehensive Treaty of Electrochemistry; Bockris, J. O’M., Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Vol. 1, p 353. (28) Rusling, J. F.; Kaman, G. K. J. Electroanal. Chem. 1985, 187, 355. (29) McIntire, G. L. CRC Crit. Rev. Anal. Chem. 1990, 21, 257. (30) Pelizzetti, E.; Pramauro, E. Anal. Chim. Acta 1985, 169, 1.
10.1021/la980909j CCC: $18.00 © 1999 American Chemical Society Published on Web 03/05/1999
Electrosynthesis of PEDOT in SDS Micellar Aqueous Medium
Despite these advantages, relatively few works have been devoted to the electrosynthesis of conducting organic polymer films in micellar media.23-26,31-39 The effect of SDS on the electropolymerization of pyrrole in aqueous solutions has been investigated.32-36 Polypyrrole (PPy) films formed in the presence of dodecyl sulfate have better mechanical properties than those obtained with other anions such as NO3- or ClO4-.31-35 PPy films with perpendicularly oriented columnar structures have been synthesized from the electrochemical polymerization of pyrrole in anionic micellar solutions, including SDS or sodium dodecylbenzene sulfonate (SDBS) as surfactants.36 Also, it was demonstrated that electropolymerization of water-insoluble 3-alkylpyrroles in SDS aqueous solutions leads to partially ordered and conductive films with improved mechanical properties.37,38 Recently, we investigated the effect of anionic surfactants such as SDS on the electropolymerization of waterinsoluble thiophene derivatives in aqueous solutions.23-25 We found that SDS increased the solubility and lowered the oxidation potential of these monomers; moreover, films of PT derivatives obtained in micellar media had welldefined structures. Among PT derivatives, poly(3,4ethylenedioxythiophene) (PEDOT), which can be electrosynthesized either in organic media40,41 or in aqueous solutions in the presence of sodium polystyrene sulfonate,42 has the advantage of yielding very stable and highly conductive films42; moreover, it presents remarkable electrochromic properties.41,43 PEDOT also has been applied to the development of electroluminescent diodes,42 preparation of antistatic, transparent films,44,45 and metallization of insulating devices.46 In a preliminary communication, we showed the usefulness of SDS-containing aqueous media for the electrosynthesis of conductive and electroactive PEDOT films at Pt electrodes.24 Subsequently, similar results on PEDOT have been obtained by other authors in SDBS aqueous solutions.47 Here, we describe in detail the advantages of a novel method of electrochemical preparation of PEDOT, based on the ability of anionic surfactants to form micelles in aqueous media. We demonstrate that the electropolymerization process carried out in the presence of SDS (31) Wernet, W.; Monkenbusch, M.; Wegner, G. Mol. Cryst. Liq. Cryst. 1985, 118, 193. (32) Perres, R. C. D.; Pernault, J. M.; De Paoli, M. A. Synth. Met. 1989, 28, 59. (33) De Paoli, M. A.; Panero, S.; Prosperi, P.; Scrosati, B. Electrochim. Acta 1990, 35, 1145. (34) De Paoli, M. A.; Peres, R. C. D.; Panero, S.; Scrosati, B. Electrochim. Acta 1992, 37, 1173. (35) Peres, R. C. D.; De Paoli, M. A.; Torresi, P. M. Synth. Met. 1992, 48, 259. (36) Naoi, K.; Oura, Y.; Maeda, M.; Nakamura, S. J. Electrochem. Soc. 1995, 142, 417. (37) Barr, G. E.; Sayre, C. N.; Connor, D. M.; Collard, D. M. Langmuir 1996, 12, 1395. (38) Moussa, I.; Hedayatullah, Mir; Aaron, J. J. J. Chim. Phys. 1998, 95, 1551. (39) Fall, M.; Aaron, J. J.; Sakmeche, N.; Dieng, M. M.; Jouini, M.; Aeiyach, S.; Lacroix, J. C.; Lacaze, P. C. Synth. Met. 1998, 93, 175. (40) Dietrich, M.; Heinze, J.; Heywang, G.; Jonas, F. J. Electroanal. Chem. 1994, 369, 87. (41) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Ingana¨s, O. Polymer 1994, 35, 1347. (42) Granstro¨m, M.; Berggren, M.; Ingana¨s, O. Science 1995, 267, 1479. (43) Gustafsson, J. C.; Liedberg, B.; Ingana¨s, O. Solid State Ionics 1994, 69, 145. (44) Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116; Jonas, F., Morrison, J. T. Synth. Met. 1997, 85, 1397. (45) Bayer, A. G. U.S. Patent 5,035,926, 1991. (46) De Leeuw, D. M.; Kraakman, R. A.; Bongaerts, P. F. G.; Mutsaers, C. M. J.; Klaasen, D. B. M. Synth. Met. 1994, 66, 263. (47) Lima, A.; Schottland, P.; Sadki, S.; Chevrot, C. Synth. Met. 1998, 93, 33.
Langmuir, Vol. 15, No. 7, 1999 2567
Figure 1. Effect of SDS concentration on the EDOT (10-4 M) UV absorption spectra in 0.1 M LiClO4 aqueous solution: (a) No SDS; (b) 0.01 M SDS; (c) 0.05 M SDS; (d) 0.1 M SDS. Insert A, variation of the EDOT molar absorption coefficient () as a function of SDS concentration.
at an oxidation potential lower than in an organic medium yields better organized PEDOT films. The improved physicochemical and structural properties of PEDOT obtained in these conditions can be related to the electrocatalytic effect of SDS. Experimental Section Chemicals. 3,4-Ethylenedioxythiophene (EDOT, Bayer) and SDS (Fluka, purity >99%) were used as received. Water used as a solvent was purified with a Millipore system. Electrochemistry. All voltammetric and galvanostatic experiments were performed using a one-compartment, threeelectrode electrochemical cell driven by an EG&G PAR system (model 262) potentiostat/galvanostat, connected to a Sefram TGM 164 XY recorder. Working electrodes were platinum disks (2mm diameter) or Pt-coated glasses; the counter electrode was a stainless steel grid and the reference, a KCl-saturated calomel electrode (Tacussel). Polymer Film Analysis. Photoelectron (XPS) spectra were carried out with a vacuum generator Escalab MKI spectrometer (50-eV pass energy) equipped with Mg KR (1253 eV) and Al KR (1486.6 eV) sources, operating at 200 W. Pressure in the analysis chamber ranged from 10-8 and 10-9 mbar, and low-intensity X-rays were focused on an area of about 1 to 4 cm2. The calibration of XPS spectra was performed by taking the C (1s) electron peak (Eb ) 285 eV). Infrared spectra were obtained on a Nicolet FT-IR 60-SX spectrometer. Scanning electron microscopy (SEM) studies were carried out on a Cambridge Stereoscan 250 instrument.
Results and Discussion EDOT Solubility and UV Absorption Spectra in Aqueous SDS Solutions. The solubility of EDOT in SDScontaining aqueous solutions was determined by UVvisible absorption spectroscopy. The EDOT absorption spectrum, recorded in water, is characterized by a broad band with a maximum at 255 nm and a shoulder at 244 nm (Figure 1a). Taking into account the max value of about 8400 L mol-1 cm-1, the 255-nm band can be attributed to π-π* electronic transitions. Addition of SDS (g5 × 10-2 M) to the aqueous solution above does not modify these general spectral features but produces a slight red-shift of the EDOT main band (∆λ ) 3 nm) and a significant decrease of the max value (Figure
2568 Langmuir, Vol. 15, No. 7, 1999
Sakmeche et al. Table 1. Effect of the Electrolytic Medium on the EDOT Oxidation Peak Potential electrolytic medium LiClO4b/CH3CN LiClO4b/H2O Aqueous CTAClc Aqueous Triton X-405d Aqueous SDSe
Ep(a) (mV)a 1380 1340 1340 1340 1250
a Versus SCE; 1 mM monomer in the electrolytic medium, first cycle. b 0.1 M LiClO4. c 0.01 M CTACl. d 0.01 M Triton X-405. e 0.01 M SDS.
Figure 2. Cyclic voltammograms of EDOT (10-3 M) obtained in: (a) 0.1 M LiClO4 acetonitrile, (b) 0.1 M LiClO4 water, (c) aqueous 0.01 M SDS + 0.1 M LiClO4; first cycle shown; Pt electrode area, 3.14 mm2; potential measured vs SCE; scan rate 20 mV/s; intensity scale amplified 1.7 times for (c).
1b).48 This weak red-shift indicates that EDOT incorporation within either the micellar core or the sufactant polar headgroup region perturbs only moderately the chemical environment of the monomer.48-50 In contrast, the EDOT absorption molar coefficient considerably decreases when SDS concentration increases (Figure 1b). This spectral behavior, which has already been observed in other organic compounds can be attributed to the change of the medium dielectric constant provoked by the increase of surfactant amount in the aqueous solution.48-50 EDOT absorbance measurements in the presence of various SDS concentrations allowed us to evaluate its limiting solubility. A value of 7.3 × 10-2 M was found for EDOT at 20 °C in the presence of 0.1 M SDS, against 1.1 × 10-2 (( 0.3 × 10-2) M in pure water. Electrochemical Behavior of EDOT in Surfactant Solutions. The redox properties of EDOT have been investigated in aqueous anionic, cationic, and neutral micelles. Indeed, recent studies on the electrochemistry of organic compounds in micellar media have shown that important effects, depending on the type of surfactant, could be induced either in the catalysis redox reaction or in the orientation of the electrochemical reaction.26,51,52 The cyclic voltammogram of EDOT (10-3 M) in an anionic micellar solution containing 0.01 M SDS and 0.1 M LiClO4 presents a shape similar to that observed in a 0.1 M LiClO4 aqueous medium, but is markedly different in an acetonitrile solution (Figure 2). Indeed, only one EDOT oxidation peak appears at a potential value Ep(a) ) 1.38 V vs Ag/AgCl in acetonitrile, whereas two oxidation peaks (P1 and P2) occur in micellar and nonmicellar aqueous solutions. The prepeak P1, which is poorly resolved and is located at about 1.05-1.10 V vs Ag/AgCl, in both media can be caused by the adsorption of oxidized EDOT species at the Pt electrode. This interpretation is confirmed by the linear increase of P1 (48) McIntire, L. G.; Blount, H. N. Solution Behavior of Surfactants. Theoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2., p 1101. (49) Sepulveda, L. J. Colloı¨d Interface Sci. 1974, 46, 372. (50) Le Rosen, A. L.; Reid, C. E. J. Chem. Phys. 1952, 20, 233. (51) McIntire, G. L.; Blount, H. N. J. Am. Chem. Soc. 1979, 101, 7720; McIntire, G. L.; Blount, H. N. Solution Behavior of Surfactants, Theoretical and Applied Aspects; Mittal, K. L.; Fendler, E. J.; Eds.; Plenum Press: New York, 1982; Vol. 2., p 1101. (52) McIntire, L. G.; Chiappardi, D. M.; Casselberry, R. L.; Blount, H. N. J. Phys. Chem. 1982, 86, 2632.
intensity with scan rate (v). On the other hand, the second oxidation peak, P2, is well resolved and occurs at potentials (Ep(a)) of 1.34 and 1.25 V vs Ag/AgCl, respectively, in nonmicellar and SDS micellar solutions; it is attributed to the oxidation of EDOT species diffusing close to the electrode, as suggested by the linear variation of P2 intensity with a scan rate value intermediate between v1/2 and v. We can conclude from these electrochemical results that the oxidation mechanism of EDOT in aqueous micellar and nonmicellar media takes place in two steps: in the first one, an adsorbed layer of oxidized EDOT appears at the surface of the electrode and, in the second step, EDOT species diffusing close to the electrode and/or dimers or oligomers formed during the first step are oxidized. As can be seen in Table 1, the EDOT oxidation potential is lowered by approximately 90 and 130 mV in 0.01 M SDS aqueous medium relative to water and acetonitrile, whereas no significant charge of Ep(a) occurs in cationic (0.01 M cetyltrimethylammonium chloride, CTACl) as well as nonionic (0.01 M Triton X-405) aqueous media (Table 1). This invariance of EDOT oxidation potential indicates that no or very weak interaction takes place between cationic or nonionic surfactant and the monomer or its oxidation product (EDOT+•). In contrast, the variation of the EDOT Ep(a) value observed in aqueous SDS solutions demonstrates the existence of specific interactions between the SDS micelles and one form of the redox couple (EDOT/ EDOT+•).48,51 The effect of varying SDS concentrations (5 × 10-4 8 × 10-3 M) on the electrochemical oxidation potential of 10-4 M EDOT has been studied in a 0.1 M LiClO4 aqueous solution. Upon increasing the SDS concentration, the EDOT oxidation potential decreases, reaching a plateau value for SDS concentrations larger than the CMC (Figure 3, point A). A similar behavior has already been observed in the literature.48,52,53 It can be explained as follows: below critical micelle concentration (CMC), the extent of surfactant aggregation is small, the solution including essentially monomeric species; in these conditions, it occurs as a strong interaction between the EDOT+• cation radical and dodecyl sulfate (DS-) anion; as a consequence a pseudo complex is formed, which decreases the EDOT redox potential. Beyond point A (Figure 3), which corresponds to the electrochemical system effective CMC value, the free SDS monomer concentration remains practically constant and all possibly SDS species are complexed. As a result, essentially all EDOT molecules become incorporated into the micellar phase, which implies the invariance of the EDOT oxidation potential.51,53 Therefore, the shift of the EDOT oxidation potential toward less anodic values upon increasing SDS concentration below CMC provides definitive evidence for the formation of a pseudocomplex between EDOT+• and DS- species. More (53) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975.
Electrosynthesis of PEDOT in SDS Micellar Aqueous Medium
Figure 3. Plot of EDOT oxidation potential versus the logarithm of SDS concentration. CMC, value of the critical micellar concentration obtained by fluorescence; A, effective CMC value for the electrochemical system.
generally, it was demonstrated in the literature that if one form of the redox couple of any organic compound preferentially interacts with the medium, its formal potential can be expected to be altered.54 For instance, McIntire and Blount51 have observed that the formal potential for the oxidation of 10-methylphenothiazine (MPTH) into MPTH+• was shifted to less anodic values with increasing SDS concentrations below the CMC and that the differential pulse voltammetric (DPV) peak potential varied linearly with log [DS-]. The authors attributed this behavior to the occurrence of strong interactions between MPTH+• cation radical and DSanion and to the appearance of a dominant complex between these species.51 In our case, we can also consider that the interaction of the cation radical (EDOT+•) with monomeric DS- leads to formation of a (EDOT+•): (DS-)F pseudo complex, where F represents the characteristic stoichiometry.
EDOT+• + F DS- h (EDOT+•): (DS-)F
(1)
The equilibrium constant, Keq, of this pseudocomplex is equal to:
Keq )
[(EDOT•+):(DS-)F] [EDOT•+] [DS-]F
(2)
The EDOT oxidation peak potential in the presence of SDS, EDSP(a) , is shifted relative to that observed in the absence of SDS, Ep(a). This shift can be expressed by eq 3.51,54-56 -
EDS P(a) ) Ep(a) - 0.059 Log Keq - 0.059 (F) Log [(DS )] (3) The variation of EDSP(a) as a function of log [DS ] below the CMC is linear with a slope of about 57 (( 3) mV, which corresponds to a 1:1 stoichiometry (F ≈ 1); it indicates that one EDOT+• species interacts with one DS- anion.48,51 From the intercept (1120 mV) and Ep(a) (1340 mV) values, one can calculate that the equilibrium constant (Keq) of the (EDOT+•):(DS-)F complex has a value of 5.4 × 103 M-1. This relatively high value suggests that complexation
(54) Galus, Z. Fundamentals of Electrochemical Analysis; Elis Horwood, Ltd: Chichester, 1976; Chapter 14. (55) Peover, M. E.; Davies, J. D. Electroanal. Chem. 1963, 6, 46. (56) Deford, D. D.; Hume, D. N. J. Am. Chem. Soc. 1952, 73, 5321.
Langmuir, Vol. 15, No. 7, 1999 2569
Figure 4. Cyclic voltammograms for 0.05 M EDOT obtained on a Pt electrode in: (a) aqueous 0.07 M SDS + 0.1 M LiClO4, (b) 0.1 M LiClO4 acetonitrile; scan rate: 100 mV/s.
of EDOT+• takes place with the DS- polar headgroup rather than with the micelle hydrocarbon core.51,57 EDOT Electropolymerization. By performing potential cyclic sweeps between 0.5 and 1.2 V vs saturated calomel electrode (SCE) in the micellar medium (0.07 M SDS- + 0.1 M LiClO4) containing 0.05 M EDOT, we observed on the first voltammetric curve a rapid growth of the anodic current density starting at 0.76 V (Eox1), which corresponds to the beginning of the EDOT oxidation wave (Figure 4). In contrast, in acetonitrile + 0.1 M LiClO4 an Eox1 value of 1.04 V was found. This important decrease (280 mV) of the oxidation potential in micellar medium relative to acetonitrile can be attributed to a specific effect of the anonic surfactant. During the successive potential cyclic sweeps, we observed in micellar medium and in acetonitrile a regular increase of the current anodic and cathodic densities, corresponding to the growth of an electroactive polymer film at the electrode, whose thickness increases regularly with the number of cycles (Figure 4). The obtained polymer film was homogeneous and adherent to the electrode. One can see also that the oxidation/reduction waves are better defined when PEDOT is synthesized in acetonitrile medium (PEDOT(org)) than when obtained in micellar solution (PEDOT(mic)) (Figure 4). This behavior suggests a weaker mobility of ionic species in PEDOT(mic) than in PEDOT(org) during the doping and undoping processes, probably because of a dense microstructure in PEDOT(mic). Furthermore, this hypothesis is supported by SEM observations which indicate a structure apparently more compact for PEDOT(mic) than for PEDOT(org). Also, we performed the electropolymerization of EDOT in the potentiostatic mode at different potentials in both media. This approach allows monitoring of the film growth on the electrode and distinguishing the different steps of its electrosynthesis. General aspect of the PEDOT electrosynthesis chronoamperograms is comparable with that observed in the literature for other conductive polymers (Figure 5a).58-60 One can note three successive steps: (1) nucleation (part A of the curve), corresponding to creation of the first active centers; (2) polymer growth on the (57) Evans, C. A.; Bolton, J. R. J. Am. Chem. Soc. 1977, 99, 4502. (58) Li, F.; Albery, W. J. Electrochem. Acta 1992, 37, 393. (59) Asavapiriyanont, S.; Chandler, G. K.; Gunawardena, G. A.; Pletcher, D. J. Electroanal. Chem. 1984, 177, 299; 245. (60) Hillman, A. R.; Mallen, E. F. J. Electroanal. Chem. 1987, 220, 351.
2570 Langmuir, Vol. 15, No. 7, 1999
Sakmeche et al.
Figure 6. Chronopotentiometric curves of 0.05 M EDOT recorded on a Pt electrode (φ ) 5 mm) respectively in: (A) 0.07 M SDS + 0.1 M LiClO4 aqueous solution and (B) 0.1 M LiClO4 acetonitrile at the following current density (j) values: (a) 0.1 mA/cm2; (b) 0.5 mA/cm2; (c) 1 mA/cm2; (d) 5 mA/cm2; (e) 10 mA/cm2.
Figure 5. (a) General pattern of a chronoamperogram obtained for the potentiostatic electrosynthesis of a conducting polymer, showing three successive steps: (A) nucleation, (B) polymer growth, and (C) diffusion of the electroactive species (duration of steps A + B; from a few seconds to 1 min). (b) Chronamperograms of 0.05 M EDOT obtained in 0.07 M SDS + 0.1 M LiClO4 aqueous solution on a Pt electrode (φ ) 5 mm) at the following potential (E) values: (1) 0.76 V; (2) 0.78 V; (3) 0.80 V; (4) 0.82 V; (5) 0.84 V; (6) 0.86 V. (c) Chronamperograms of 0.05 M EDOT obtained in 0.1 M LiClO4 acetonitrile on a Pt electrode (φ ) 5 mm) at the following E values: (1′) 1.00 V; (2′) 1.02 V; (3′) 1.04 V; (4′) 1.06 V; (5′) 1.08 V; (6′) 1.10 V.
electrode (part B); and (3) diffusion of the electroactive species within the deposited film (part C). Therefore, an optimal potential value exists for which the current density remains constant during all the electrolysis time after the nucleation, which corresponds to the formation of a conductive polymer film.58-60 The initial stages of film growth imply the occurrence of an instantaneous nucleation process kinetically controlled by planar diffusion.60 For potentials greater than 0.76 V (potential threshold of the film formation), the chronoamperograms recorded in the presence of SDS after 50 s of polarization exhibit a constant electrosynthesis current which is attributed to deposition of a conductive polymer film on the surface (Figure 5b). In organic medium, the chronoamperometric curves (I-t) recorded between 1.0 and 1.1 V (Figure 5c) differ significantly from those obtained in micellar medium. Indeed, the minimum potential of 1.04 V at which starts the EDOT electropolymerization process is larger than that observed in micellar medium (0.78 V). An intensity plateau is only obtained at higher potentials and after a polarization time longer than in micellar solution (approximately 300 s). Obviously, this difference of behavior indicates that the EDOT electropolymerization process is more efficient in a micellar medium than in acetonitrile; moreover, the PEDOT nucleation process appears practically instantaneous in micellar solution. A similar film growth has been observed for the formation of polypyrrole
and polythiophene films.18,58-61 These results can be rationalized as follows: the adsorption of surfactant would take place at the electrode surface by the polar head, and the hydrophobic chains would be organized parallel to each other, creating a pseudoorganic, hydrophobic phase in which strong concentrations of monomer would remain. The electropolymerization process was also investigated using the galvanostatic mode. The chronopotentiometric curves (E-t) obtained on Pt electrode in micellar and acetonitrile solutions at different current density values show that the polymerization process in these media begins at very weak current densities, leading to homogeneous and adherent PEDOT films (Figure 6). In the presence of SDS and for j ) 0.1 mA‚cm-2, one observes a rapid variation of the electrode potential between the polarization initial value and a limiting value of approximately 0.8 V vs SCE, which yields a thick, deep-blue homogeneous film after 2 min (Figure 6A). In contrast, when experiments are performed in acetonitrile at the same current density, the potential varies much more slowly with time and does not stabilize at about 1 V before a 4-min electrolysis time, giving a film similar to that obtained in the micellar medium (Figure 6B). These results confirm our potentiostatic study indicating that EDOT electropolymerization reaction is strongly accelerated in the presence of surfactants. For current densities varying between 0.1 and 1 mA‚cm-2, whatever the electrosynthesis medium, the potential varies much more rapidly and it reaches a value corresponding to the monomer oxidation quasi-instantaneously (Figure 6). For current densities of about 5 mA‚cm-2, the chronopotentiograms drawn in micellar solution are characterized by a first potential plateau at 1.1 V, corresponding to the formation of the PEDOT film, followed by a rapid potential increase and a second plateau at 1.5 V. A gaseous release, probably caused by water decomposition, was also observed at the Pt electrode; the deposit obtained is extremely degraded. This phenomenon is even more pronounced for larger intensities. Thus, for j > 5 mA‚cm-2 the potential increases very rapidly until about 1.6 V after only 15 s of polarization. (61) Fleischman, M. F.; Thirsk, H. R. Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Wiley-Interscience: New York, 1963; Vol. 3, 127; Thirsk, H. R.; Harrison, J. A. A Guide of the Study of Electrode Kinetics; Academic Press: New York, 1972; p 115.
Electrosynthesis of PEDOT in SDS Micellar Aqueous Medium
Langmuir, Vol. 15, No. 7, 1999 2571
Figure 7. Electroactivity in 0.1 M N(Bu)4ClO4 acetonitrile of PEDOT films, electrosynthesized galvanostatically (j ) 0.5 mA/cm2; Q ) 0.12 C/cm2) in: (A) 0.07 M SDS + 0.1 M LiClO4 + 0.05 M EDOT aqueous solution; (B) 0.1 M LiClO4 + 0.05 M EDOT acetonitrile at various scan rates. Insert (C) plots of anodic current density versus scan rate in micellar aqueous (sbs) and acetonitrile (s2s) solutions.
Characterization of PEDOT Films. PEDOT films obtained in micellar and acetonitrile solutions have been characterized electrochemically and spectroscopically (UV-visible, IR, Raman, and XPS spectra). Electrochemical Properties. The electroactivity of PEDOT films, electrosynthesized in micellar or acetonitrile solutions (Q ) 0.12 C‚cm-2; j ) 0.5 mA/cm2; thickness ) 0.5 µm) was studied in CH3CN + 0.1 M TBAClO4. The voltammograms (Figure 7A and B) exhibit an oxidation peak at about 0.08 V vs SCE; its intensity increases linearly with scan rate in the range 20-200 mV‚s-1 (Figure 7, insert C). This behavior indicates that the redox processes are not controlled by diffusion. During reduction, two cathodic waves are observed at 0 and -0.6 V, respectively. The similarity of voltammograms of films prepared in both media and the absence of significant difference between the voltammograms obtained during the first cycle and the following ones seem to indicate that the cationic redox exchanges play an important role in the electroactivity of both PEDOT films. The polymer films prepared in micellar medium are more stable than those obtained in organic solution as demonstrated by the fact that, when submitted to a great number of redox cycles (n ≈ 50), there is no significant loss of their electroactivity (
