Full Electrochemical Synthesis of Conducting Polymer Films

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Full Electrochemical Synthesis of Conducting Polymer Films Chemically Grafted to Conducting Surfaces D. E. Labaye,† C. Je´roˆme,† V. M. Geskin,‡ P. Louette,§ R. Lazzaroni,‡ L. Martinot,|,⊥ and R. Je´roˆme*,† Laboratory of Macromolecular Chemistry and Organic Materials, Centre de Recherche en Science des Mate´ riaux Polyme` res (CRESMAP), Universite´ de Lie` ge, B6 Sart-Tilman, B-4000 Liege, Belgium, Service de Chimie des Mate´ riaux Nouveaux, Centre de Recherche en Science des Mate´ riaux Polyme` res (CRESMAP), Universite´ de Mons-Hainaut, Place du Parc 20, B-7000 Mons, Belgium, Laboratoire Interde´ partemental de Spectroscopie Electronique (LISE), Faculte´ s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium, and Coordination Chemistry and Radiochemistry, University of Lie` ge, B16 Sart-Tilman, B4000 Lie` ge, Belgium Received September 17, 2001. In Final Form: January 24, 2002 This paper reports on the full electrochemical synthesis of electrically conducting polymers chemically grafted to conducting surfaces (e.g., glassy carbon, stainless steel, nickel, gold). It is based on new functional acrylate monomers, i.e., 3-(2-acryloyloxyethyl)thiophene and N-(2-acryloyloxyethyl)pyrrole, whose the synthesis is reported in this work. The polymerization process consists of two electrochemical steps. The first one is the cathodic electrografting of polyacrylate chains bearing a precursor of the conducting polymer in the ester group, either thiophene or pyrrole. In the second step, this precursor is polymerized under anodic polarization, in the presence or not of additional unsubstituted monomer in the electrochemical bath. Cyclic voltammetry was used to confirm that the two-component film is conducting and electrochemically active (reversible doping and dedoping). The chemical composition and the microscopic morphology of these composites were characterized by X-ray photoelectron spectroscopy and atomic force microscopy, respectively.

Introduction Nowadays, the field of electrically conducting polymers, such as polypyrrole (PPy) and polythiophene (PTh), is a topic of very active research.1 The 2000 Nobel Prize in chemistry was awarded to pioneers of the field. These polymers have potential in many applications, such as light emitting diodes,2 batteries,3 electrochromic devices,4 sensors,5 electromagnetic shielding,6 and corrosion inhibition.7 Electropolymerization is a commonly used technique for the synthesis of conducting polymers. Among other advantages, a film is directly deposited on the conducting surface and the film thickness can be easily controlled, which is of prime importance whenever surface coating is concerned.8 The poor interfacial adhesion of the conducting polymer to the electrode is, however, a major † Laboratory of Macromolecular Chemistry and Organic Materials, CRESMAP, Universite´ de Lie`ge. ‡ Service de Chimie des Mate ´ riaux Nouveaux, CRESMAP, Universite´ de Mons-Hainaut. § LISE, Faculte ´ s Universitaires Notre-Dame de la Paix. | Coordination Chemistry and Radiochemistry, University of Lie`ge. ⊥ Research Associate of the Inter-University Institute for Nuclear Sciences (Brussels).

(1) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2d ed.; Marcel Dekker: New York, 1998. (2) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int. Ed. 1998, 37, 402-428. (3) Killian, J. G.; Coffey, B. M.; Gao, F.; Pochler, T. O.; Searson, P. C. J. Electrochem. Soc. 1996, 143, 936. (4) Leventis, N. Polym. News 1995, 20, 5-18. (5) Barisci, J. N.; Conn, C.; Wallace, G. G. Trends Polym. Sci. 1996, 4, 307-311. (6) Olmedo, L.; Hourquebie, P.; Jousse, F. Adv. Mater. 1993, 3, 373377. (7) Beck, F.; Michaelis, R. J. Coat. Technol. 1992, 64, 59-67. (8) Mekhalif, Z.; Lazarescu, A.; Hevesi, L.; Pireaux, J. J.; Delhalle, J. J. Mater. Chem. 1998, 8, 545-551.

pending problem.9-14 Some strategies were proposed to tackle this problem, e.g., formation of the conducting polymer from polymerizable precursors (pyrrole, thiophene) preadsorbed on a metallic substrate (gold, nickel, or platinum) by thiol functions. For example, ω-(N-pyrrolyl)alkanethiols and alkanethiols bearing a 3-substituted pyrrole unit were adsorbed onto gold electrodes.9-11 Alkanethiols substituted by an aromatic ring and 6-[2′,5′di(2′′-thienyl)pyrrol-1′-yl]hexanethiol were also chemisorbed onto platinum and nickel, respectively.8,12 In another approach, conducting polymers were grafted onto metal oxide surfaces, such as indium tin oxide (ITO). For instance, poly(p-phenylene) oligomers end-capped by a carboxylic acid were chemically grafted onto ITO.13 The electropolymerization of pyrrole was also carried out on electrodes pretreated by N-(3-(trimethoxysilyl)-propyl)pyrrole, which is reactive toward hydroxyl groups available on the surface.14 In all these two-step processes, the key step is the chemisorption of either the preformed conducting polymer or its precursor as result of a chemical reaction. This paper aims at reporting a novel strategy, which is a completely electrochemical two-step procedure. As shown in Scheme 1, the first step consists of the cathodic electrografting15 of a (meth)acrylate monomer, which contains the precursor of the conducting polymer (e.g., thiophene and pyrrole) in the ester group. It is known, (9) McCarley, R. L.; Willicut, R. J. J. Am. Chem. Soc. 1998, 120, 9296-9304. (10) Smela, E. Langmuir 1998, 14, 2996-3002. (11) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302-306. (12) Lang, P.; Mekhalif, Z.; Garnier, F. J. Chim. Phys. 1992, 89, 1063-1070. (13) Nuësch, F.; Si-Ahmed, L.; Franç ois, B.; Zuppiroli, L. Adv. Mater. 1997, 9, 222-225. (14) Simon, R.; Ricco, A.; Wrighton, M. J. Am. Chem. Soc. 1982, 104, 2031-2034.

10.1021/la011439n CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002

Synthesis of Electrically Conducting Polymers Scheme 1. Two-Step Formation of a Conducting Polymer Strongly Adhering to the Electrode, Where the Cathodic and Anodic Polarizations of the Electrode Are Indicated by the Signs - and +a

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or not of additional monomer (pyrrole or thiophene) in solution. The redox properties, the chemical composition, and the microscopic morphology of the final two-component films have been characterized by electrochemical techniques, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM), respectively. Experimental Section

a The structures of the APy and ATh monomers with numeration of the C and O atoms are included.

indeed, that a series of (meth)acrylates can be electropolymerized under conditions of cathodic potential15 and solvents16,17 such that the final polymer is chemisorbed to the electrode. Because this cathodic electrografting technique is effective in the case of carbon-carbon double bonds substituted by an electron-withdrawing group,17 two acrylic monomers have been synthesized, 3-(2acryloyloxyethyl)thiophene (ATh) and N-(2-acryloyloxyethyl)pyrrole (APy), and electropolymerized with the purpose to have a prepolymer containing precursors of a conducting polymer grafted to the electrode. The structure of these monomers is shown in Scheme 1. In the second step (Scheme 1), the precursor of the conducting polymer is polymerized under anodic polarization in the presence

Pyrrole and thiophene were distilled at 60 °C under vacuum before use. Acryloyl chloride was dried over calcium hydride for 24 h and distilled under reduced pressure. 2-(3-Thienyl)ethanol was used as received. 3-((2-Acryloyloxy)ethyl)thiophene (thiophene acrylate, ATh) was synthesized by reaction of 2-(3-thienyl)ethanol with acryloyl chloride in dry tetrahydrofuran (THF) as reported elsewhere.18 The yield was 90%, and the purity of the monomer was higher than 95% (gas chromatography (GC)). N-((2-Acryloyloxy)ethyl)pyrrole (pyrrole acrylate, APy) was similarly prepared by reaction of N-(hydroxyethyl)pyrrole with acryloyl chloride (75% yield, 90% purity). N-(Hydroxyethyl)pyrrole19 was prepared by reaction of the potassium pyrrolyl salt19 with 2-bromoethanol (75% yield, 95% purity). 2-Bromoethanol was purified by the repeated washing (three times) of a solution in methylene chloride (50 vol %) with a saturated aqueous solution of potassium carbonate, followed by a saturated aqueous solution of sodium chloride and finally by deionized water. Methylene chloride was distilled off, and 2-bromoethanol was dried over magnesium sulfate and then over calcium hydride for 2 days, before being distilled under reduced pressure. ATh and APy were dried over calcium hydride and distilled at 80 °C under reduced pressure prior to use. Acetonitrile (ACN) was dried over calcium hydride and distilled. N,N′-Dimethylformamide (DMF) was dried over phosphorus pentoxide and distilled at 70 °C under reduced pressure. Tetraethylammonium perchlorate (TEAP) was heated in vacuo at 80 °C for 12 h. In cathodic experiments, cyclic voltammetry (CV) was carried out with ATh or APy (0.1-2.5 M) dissolved in DMF, in the presence of TEAP (5 × 10-2 M) as a conducting salt. The water content of DMF was measured by the Karl Fischer method (Tacussel aquaprocessor) and found to be lower than 5 ppm. Then, no parasitic electrochemical signal for water could be detected in CV experiments. The anodic experiments, i.e., CV and chronoamperometry (CA), were carried out in ACN containing TEAP (5 × 10-2 M) as a conducting salt. All these experiments were carried out in a glovebox under an inert and dry atmosphere at room temperature. The working electrode was placed between the reference electrode and the counter-electrode. Qox and Qred were calculated from the area delimited by the (I vs E) curve, the horizontal at I ) 0 and the vertical line at each limit potential noted in the text (end of the second section of the “Results and Discussion” section). All the potentials were measured in this work with respect to a Pt wire used as a quasi-reference electrode. For this reason, the potentials may not be directly compared. The potentiostat, and the general procedures for the electrode preparation and the electropolymerization, were detailed elsewhere.16 X-ray photoelectron spectroscopy (XPS) was performed with a Surface Science Instrument SSX-100 spectrometer, equipped with a monochromatized Al KR X-ray source. AFM was carried out with a Nanoscope III microscope (Digital Instruments, USA) operating in the tapping mode, with commercial silicon tips, in air. The as-acquired images contained 256 × 256 points, even though only part of them may be shown for the sake of comparison. These images were usually not filtered or merely treated by a plane-fitting technique.

Results and Discussion (15) (a) Lećayon, G.; Boizeau, C. La Recherche 1988, 19, 890. (b) Je´roˆme, R.; Mertens, M.; Martinot, L. Adv. Mater. 1995, 7, 807-809. (16) (a) Mertens, M.; Calberg, C.; Martinot, L.; Je´roˆme, R. Macromolecules 1996, 29, 4910-4918. (b) Baute, N.; Martinot, L.; Je´roˆme, R. J. Electroanal. Chem. 1999, 472, 83-90. (17) (a) Baute, N.; Teyssie´, P.; Martinot, L.; Mertens, M.; Dubois, P.; Je´roˆme, R. Eur. J. Inorg. Chem. 1998, 1711-1720. (b) Crispin, X.; Lazzaroni, R.; Geskin, V.; Baute, N.; Dubois, P.; Je´roˆme, R.; Bre´das, J. L. J. Am. Chem. Soc. 1999, 121, 176-187.

The cathodic electrografting of the new thiophene (ATh) and pyrrole (APy) containing monomers (Scheme 1) has been first studied on glassy carbon, which is a stable electrode in both cathodic and anodic regimes. In a second (18) Kock, T. J. J. M.; de Ruiter, B. Synth. Met. 1996, 79, 215-218. (19) Bidan, G. Tetrahedron Lett. 1985, 26, 735-736.

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Labaye et al. Table 1. Concentration Dependence of the Intensity of the First Voltammetric Peak for 3-(2-Acryloyloxyethyl)thiophene (ATh) on Glassy Carbon in a DMF Solution of TEAP (0.05 M) concn of ATh (M)

first peak intensity (mA)

1 1.5 2 2.5

3.22 1.93 1.54 1.07

Figure 1. Voltammogram for the reduction of APy (1 M) in DMF containing TEAP 0.05 M. Glassy carbon electrode, 20 mV/s. The first scan (1), the second scan (2), and the “grafting peak” (I) are shown. The inset shows the complete cathodic polarization recorded on glassy carbon electrode (a) for the APy (0.5 M) solution in DMF (b) DMF (+ conducting salt). Peak II corresponds to the diffusion peak.

step, the formation of the conducting polymer has been investigated by electrochemical oxidation of the grafted polyacrylate, in the presence or not of additional monomer (pyrrole or thiophene) in the electrolytic bath. 1. Cathodic Electrografting of ATh and APy. Figure 1 (curve 1) shows a typical voltammogram recorded for an APy solution (1 M) in DMF/TEAP on a glassy carbon electrode. The curve consists of a shoulder at -1.9 V, designated as the “grafting peak” or peak I. This observation is common to the electroreduction of (meth)acrylic monomers on conducting substrates, such as Ni, Cu, stainless steel, gold, etc.16,17 A prepolymer film is deposited at this potential. It is strongly adhering to the cathode as confirmed by resistance to prolonged washing by a good solvent for the polymer (DMF in this case) and by peeling tests.20 Whenever the potential scanning does not go beyond -2 V, this grafting peak is no longer observed when the scanning is repeated, which is the signature of an apparent passivation (Figure 1, curve 2). Actually, the grafted (chemisorbed) chains rapidly prevent additional chains from being “grafted” from the cathode surface (surface saturation).15,16 For the polymer grafting to be successful, the monomer must be adsorbed on the cathode. Indeed, whenever the solvent used is able to compete with the monomer for adsorption, no grafting occurs.17 The electroreduction of the adsorbed monomer (at our best knowledge a (meth)acrylic monomer) results in a species which remains attached to the surface and initiates the polyaddition reaction which does not need current for being sustained. The polyaddition is stopped by a noncontrolled termination process which limits the film thickness. Only the increase of the monomer concentration can involve an increase in the thickness of the grafted film (up to ∼100 nm) as reported by Baute et al.16b This mechanism is thus basically different from that one responsible for the anodic electrodeposition of a conducting polymer (e.g., polypyrrole), which is an oxidative coupling (polycondensation type), whose the extent is controlled by the amount of current and in parallel the polymer thickness. In the electrografting process, because of the rapid apparent passivation of the cathode, the intensity of peak I is very low. At higher cathodic potentials (inset in Figure 1, curve b), a second electrochemical reaction occurs (peak II), (20) Je´roˆme, C.; Geskin, V.; Lazzaroni, R.; Bre´das, J. L.; Thibaut, A.; Calberg, C.; Bodart, I.; Mertens, M.; Martinot, L.; Rodrigue, D.; Riga, J.; Je´roˆme, R. Chem. Mater. 2001, 13, 1656-1664.

Figure 2. Reflection-absorption FTIR spectrum of PAPy grafted onto a gilt nickel surface.

which is controlled by the monomer diffusion to the cathode.15 The pregrafted chains are desorbed, and the monomer reduction forms a species which does not remain adsorbed on the electrode and initiates the (meth)acrylate polymerization in solution. For argument’s sake, no electrochemical reaction is observed in the absence of monomer (curve a, inset in Figure 1), which shows that the solvent and the conducting salt do not react in the same cathodic potential range as the monomer. As was previously reported for acrylonitrile (AN),15,16 the intensity of the grafting peak decreases when the monomer (ATh, APy) concentration is increased (Table 1). This observation is related to the chain growth which is as fast as the monomer concentration is high, which results in earlier apparent passivation of the cathode and in a lower electrografting peak intensity.16a The chemical structure proposed for the electrografted polymer film (Scheme 1, first step) has been confirmed by reflection-absorption infrared spectroscopy. Figure 2 shows the main absorptions characteristic of the grafted PAPy film. The absorptions at 3100 and 735 cm-1 are typical of the stretching and wagging vibrations of the dC-H bonds. The peaks at 1500, 1090, and 1070 cm-1 confirm that pyrrole rings are available on the carbon surface. Moreover, whenever a carbon electrode grafted by a PATh film at -1.7 V is dipped in a monomer-free solution in DMF (which is a solvent for the polymer) added with the conducting salt, a cathodic peak is observed at -2.35 V, which is characteristic of the polymer degrafting (vide supra). The surface analysis by reflection-absorption FTIR confirms the complete desorption of the pregrafted PATh film from the substrate. Moreover, the film or pieces of it have never been observed in suspension in the electrolytic bath. All these experimental observations support that the resistance of the originally deposited PATh film against dissolution in DMF does not result from cross-linking but rather from chemisorption of the growing polymer chains. In this work, PATh and PAPy films have been grafted onto 1 cm2 carbon electrodes by scanning the cathodic potential up to the maximum of the shoulder ([M] ) 1.5 M, scanning rate ) 20 mV/s, Ec ) ca. -1.7 V/Pt) of the

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Figure 3. XPS spectra for PAPy (A) and PATh (B) grafted on glassy carbon.

voltammogram and by holding this potential until the current has fallen down. A second cathodic scan has been carried out in order to saturate the surface by grafted chains. The films have been thoroughly washed with pure DMF, to remove any unreacted monomer and soluble contaminant, and then with acetonitrile, i.e., a solvent more volatile than DMF. Finally, the samples have been dried in a vacuum. The chemical composition of the films grafted onto glassy carbon has been characterized by XPS. The general XPS spectra of the films are shown in Figure 3. The constitutive elements of the grafted PAPy (C, O, and N) and PATh (C, O, and S, Figure 4) are clearly detected. No chlorine is visible, which indicates that the tetraethylammonium perchlorate conducting salt has been completely removed by washing. The observation of nitrogen in the PAPy films and sulfur in the PATh films confirms that the heteroaromatic rings, which are the precursors of the conducting polymers, are attached to the surface. Furthermore, the XPS spectra have been analyzed more quantitatively. As an example, the experimental data C/O ) 5.7, C/S ) 8.8, and O/S ) 1.5 for the grafted PATh are in agreement with the theoretical values, i.e., C/O ) 4.5, C/S ) 9, and

O/S ) 2, except for a too low oxygen content which remains unexplained. Moreover, the C1s, S2p, and O1s spectral regions have been further analyzed by peak reconstruction. The C1s peak can be decomposed into three contributions at 289.0, 286.6, and 285.0 eV assigned to the C3, C4, and C1-2,6-9,5 (not close to the O2) atoms of PATh, respectively (Figure 4A). The O1s peak shows two components at 532.2 (O1) and 533.6 eV (O2) (Figure 4B). As expected, the S2p peak is a spin-orbit coupling induced doublet with a splitting of 1.2 eV (Figure 4C). Its binding energy (164.4 eV) is typical of thiophene. These results are consistent with the chemical structure of the grafted PATh and confirm that thiophene rings are available at the surface. The morphology of PATh and PAPy grafted onto the carbon substrate has been observed by atomic force microscopy (AFM). A flat film of a rather homogeneous structure is deposited (Figure 5A,B), as was the case when poly(ethyl acrylate) (PEA) was grafted onto carbon.20 Similarly to the surface of the neat glassy carbon electrode (Figure 5C), the organic films consist of grains, whose size is smaller for PAPy compared to PATh. These grains are, however, larger than those of the glassy carbon surface. The organic films must be very thin, because the

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Figure 4. XPS peak fittings for PATh (C1s (A), O1s (B), and S2p (C)] grafted on glassy carbon.

long straight grooves originally seen on the carbon surface remain visible after deposition (Figure 5A,B). In one of the samples, a crack happened to be formed in the film which allowed the thickness of the PATh film to be estimated at ca. 20 nm by AFM (step height of the dislocation). 2. Anodic Polymerization of the Grafted Pyrrole (Thiophene) Units. The glassy carbon electrodes pregrafted by a PAPy or a PATh film have been anodically polarized in order to trigger the polymerization of the pendant aromatic rings and thus to form a conducting polymer layer (PAPyOx and PAThOx). These electrodes were dipped in a solution of TEAP (5 × 10-2 M) in acetonitrile, which is a good solvent for the electropolymerization of thiophene and pyrrole.21 (21) Evans, G. P. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH: Weinheim, New York, 1990; Vol. 1, pp 1-74.

Figure 5. AFM micrographs for PATh (A) and PAPy (B) grafted on glassy carbon. A neat glassy carbon electrode (C) is also shown.

A well-defined oxidation peak starting at 1.2 V/Pt for the PAPy (Figure 6A, curve 1) and at 1.8 V/Pt for the PATh was observed by cyclic voltammetry. Because films of grafted poly(alkylacrylate), e.g., PEA, do not react upon anodic polarization, this peak can only be attributed to the oxidation of the aromatic thiophene or pyrrole rings. The reduction of the accordingly formed PAPyOx and PAThOx two-component films is observed at 0.88 and 1.3 V/Pt, respectively, during the reverse scan. All these observations confirm that the thienyl and pyrrolyl rings,


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Figure 7. Chronoamperometry of a PATh film grafted onto glassy carbon at 2 V/Pt for 40 s in absence (a) and in the presence of Th (0.1 M) (c). The same for Th under the same experimental conditions (b).

Figure 6. Cyclic voltamperogram for the oxidation of PAPy grafted on glassy carbon in ACN added (A) with TEAP (0.05 M), 20 mV/s, and (B) in the presence of Py in solution (first and second scan). First (1) and second (2) scans.

attached to the conducting substrate as result of the cathodic grafting of the parent acrylates (ATh and APy), remain available to anodic polymerization and form polymers with a more extended π-electron conjugation. Whenever the voltammetric scan is repeated (Figure 6A curve 2; PAPyOx film), the polymerization peak is no longer observed, which indicates that most of the pyrrole rings have reacted during the first scan. The oxidation peak, which appears at a lower anodic potential (E ) 1.1 V/Pt), corresponds to the doping of the conducting polymer formed during the first anodic scan. The dedoping peak (E ) 0.9V/Pt) remains visible. These redox potentials are higher than the values reported for the pyrrole reduction under the same conditions (Ec ) -0.2 V/Pt and Ea ) 0.1 V/Pt). However, they are comparable to data reported by Balci et al.23 for copolymers of methyl methacrylate and pyrrole-containing methacrylate (Ec ) 0.7 V/Ag/Ag+, Ea ) 1 V/Ag/Ag+). A lower mobility of pyrrole attached to the grafted polyacrylate chains might explain the formation of chains with a shorter conjugation length. The oxidation wave charge (Qox) between 0.7 and 1.35 V is 8.09 mC/cm2, and the reduction wave charge between 0.5 and 1.2 V is 6.9 mC/cm2 (Qred). The relative rate of the charge exchange during oxidation and reduction is expressed by the Qox/Qred ratio (1.17).22 Being close to 1, this ratio indicates that the electroactivity of the conducting polymer is highly reversible.22 Moreover, additional scans show that the electroactivity of the polymer film (22) Li, G.; Kossmehl, G.; Kautek, W.; Plieth, W.; Melsheimer, J.; Dolbhofer, K.; Hunnius, W. D.; Zhu, H. Macromol. Chem. Phys. 1999, 200, 450-459. (23) Balci, N.; Toppare, L.; Akbulut, U.; Stanke, D.; Hallensleben, M. L. J. Macromol. Sci., Pure Appl. Chem. 1998, A35, 1727-1739.

(Ec ) 0.9 V/Pt, Ea ) 1.1 V/Pt) is stable, which is a major improvement compared to the copolymers studied by Balci et al.23 When the PATh film is concerned, the reduction peak that occurs during the first reverse scan is observed at 1.3 V/Pt. During the second scan, the oxidation peak is noted at 1.5 V/Pt and the reduction peak at 1.3 V/Pt. The ratio of Qox (between 1.2 and 1.7 V) and Qred (between 1 and 1.65 V) is also close to 1 (1.17). All these results are clear evidence that a network of conducting polypyrrole or polythiophene chains has grown from the pregrafted films of PAPy and PATh, respectively. 3. Anodic Copolymerization of the Grafted Pyrrole (Thiophene) Units and Additional Pyrrole (Thiophene) in Solution. The potentiostatic oxidation of the grafted PAPy (PATh) film in the presence of extra pyrrole (thiophene) in the electrolytic bath was studied. The synthesis of the mixed insulating-conducting film was compared to polypyrrole or polythiophene directly formed on a neat carbon electrode. Figure 6B shows the voltammogram recorded for the oxidation of a grafted PAPy film in the presence of pyrrole. The polymerization peak starts at ca. 1.3 V/Pt, thus the same potential as the oxidation of the PAPy film in the absence of Py. However, the redox peaks (Er ) 0.6 V and Eox ) 0.8 V) of the as-prepared conducting polymer are observed at less cathodic potentials compared to PAPyOx (but still higher than neat PPy), which is consistent with formation of chains with longer conjugated length when PAPy is polarized anodically in the presence of pyrrole. Because values of Epolym, Er, and Eox are quite different from those observed for pure PPy, the extra pyrrolesor at least a major part of itsis not polymerized independently of PAPyOx, e.g., in pinholes of the film. In parallel, the potentiostatic curve was recorded for a PATh film grafted onto glassy carbon in the absence of extra monomer in solution (Figure 7, curve a). A fast decrease in the current intensity is observed. When thiophene is added to the electrolytic bath (0.1 M), the current intensity is rapidly restored and maintained constant (Figure 7, curve c), as result of the oxidation of thiophene in solution. A nucleation phenomenon is observed as assessed by the slow rise of the current after a few seconds. In contrast, in the case of deposition of polythiophene (PTh) on a neat carbon electrode the current transient is not observed on the time scale of the measurement more likely because of a too fast nucleation (Figure 7, curve b). Under the same experimental conditions including the same surface area, the total charge is 20 mC for the PATh

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Chart 1. Possible Explanations for the Increased Roughness of the Films When Py (or Th) Is Added: (A) Nucleation for PAPyOx (or PAThOx); (B and C) Two Possible Mechanisms for the Film Formation

grafted electrode. In contrast, this charge increases up to 290 mC when the solution contains thiophene, which is ca. 3.5 times higher than the charge measured when PTh is deposited on neat carbon (82 mC). This substantial increase in charge is in line with the high roughness of the surface which is observed by AFM (vida infra) when thiophene is coreacted with grafted PATh. The surface roughness could be explained by either the more extended growth of the nucleation spots as a result of Th incorporation (Chart 1b) or formation of additional nucleation spots by direct nucleation of Th on the conductive substrate (Chart 1c). The strong adherence of the grafted film to the substrate prevents it from being detached as free-standing film and conductivity from being measured by a traditional four points technique. An electrochemical technique reported by Zoppi et al.24 allows however the film resistance to be estimated roughly. The potential and current intensity of the oxidation wave peak (electroactivity peak) are measured at different scanning rates (Figure 8A). The film resistance is calculated from the slope of the current intensity versus potential plot (Figure 8B). The resistance of the PAPyOx film and PAPyOx+ (the symbol + in the acronym of the samples refers to the case of extra monomer) has been estimated at 100 ( 50 Ω. This value is not basically modified for films of PAPyOx+, which are prepared by oxidation of PAPy in the presence of pyrrole. The question may be addressed to know whether the morphology of the pregrafted PAPy and PATh films is changed upon oxidation of the pyrrole and thiophene units. The morphology of the oxidized grafted films has been investigated by AFM for samples prepared by cyclic voltammetry. The surfaces show a more granular morphology (Figure 9A,B for PAThOx and PAPyOx, respectively), compared to the grafted films before oxidation (Figure 5), which are above the glass transition temperature (as the parent polyethylacrylate is) and are flexible enough to form a smooth surface. Anodic oxidation leads to the formation of rigid polythiophene or polypyrrole segments which immobilizes the pregrafted polyacrylate chains and results in a more heterogeneous surface dominated by grains of various sizes. The addition of extra monomer to the electrolytic solution perturbs the morphology, as illustrated by the images in Figure 9C,D. The deposition is very heterogeneous, with large aggregates of grains in PAThOx+ (Figure 9C) and a large number of individual grains in PAPyOx+ (Figure 9D). So, the roughness observed for PAPyOx+ (24) Zoppi, R. A.; De Paoli, M. A. J. Electroanal. Chem. 1997, 437, 175.

Figure 8. (A) Electroactivity of PAPy grafted onto glassy carbon at different scanning rates (40, 60, 80, and 100 mV/s). (B) Plot of current intensity versus potential at the maximum of the oxidation wave.

might be consistent with a higher number of nucleation spots (Chart 1c) whereas the roughness for PAThOx+ might be explained by the more extended growth of the initially formed nuclei (Chart 1b). However, the AFM observations only provide qualitative pieces of information, being unable to report, e.g., on the nuclei composition. The growth mechanism thus remains hypothetical. Moreover, the films analyzed by AFM (Figure 9) have not been synthesized with the same current quantity, which may affect the surface morphology. The spatial distribution of these “nucleation” spots is irregular and so is the final distribution of the grains. However, the final morphology of the PAPyOx+ and PAThOx+ films shows that the “nucleation” spots are more uniformly spread on the PAPyOx surface than on the PAThOx one. Comparison of Figure 9C with deposition of pure polythiophene (Figure 9E) shows that the grafted precursor of the conducting polymer has an influence on the growth of the polythiophene film. Under the same experimental conditions, the polymer grows more rapidly on the grafted film than on the pristine electrode (see CA), because the roughness of the film is higher for PAThOx+ than for pure polythiophene (the difference in the vertical scale in images 9C and 9E must be noted). This difference is thought to result from a difference in the surface area of the pristine electrode and the PAThOx deposit. The larger surface area of PAThOx can account for the deposition of a larger amount of polymer and, in parallel, for a more intense anodic current as shown by chronoamperometry (Figure 7b,c). Figure 10 compares the C1s and S2p XPS spectra for PAThOx and PAThOx+. Chlorine from perchlorate ions is detected in these samples (50 and 16% doping, respectively), which indicates that polythiophene is in the oxidized doped state. The C1s spectrum for PAThOx (Figure 10A) is very similar to that one of PATh (Figure 4A), consistently with the polymerization mechanism of thiophene which merely


Langmuir, Vol. 18, No. 13, 2002 5229

Figure 9. 3 × 3 µm2 AFM micrographs for oxidized PATh and PAPy grafted on glassy carbon by cyclic voltammetry. Oxidation in absence of Th (A) or Py (B) and in the presence of Th (C) or Py (D). Deposition of Th on a neat glassy carbon electrode (E). The vertical scale is 100 nm for A, B, and D, 300 nm for E, and 1000 nm for C.

results in loss of hydrogen atoms. However, the S2p spectrum (Figure 10C) shows a small contribution at ca. 168 eV, which suggests a slight overoxidation of the polythiophene chains. The XPS data recorded for PAThOx+ are markedly different from PATh. Indeed, the C/S ratio is 5.2:1,

compared to 9.0:1 for PAThOx, which is strong evidence that additional thiophene has been incorporated in the film. From the C/S ratio, the Th/ATh ratio can be approximated to 3:1. Moreover, the C1s spectrum (Figure 10B) shows a peak with a broad tail toward the high binding energy and the component at ca. 289 eV is hardly

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Figure 10. XPS peak fittings for PAThOx (A) C1s and (C) S2p and for PAThOx+ (B) C1s and (D) S2p.

visible. This asymmetry of the XPS core level peak is typical of doped conducting polymers, including polythiophene.25-27 It originates from nonequivalent sites along the doped polymer chains, in relation to the uneven distribution of both the positive charges and the polarization by the counterions. Similarly, the intensity ratio for the two components of the S2p doublet (Figure 10D) is no longer equal to 2:1; which is the signature of doped polythiophene.25-27 The XPS data for PAThOx+ strongly support that the film surface is dominated by polythiophene. The same conclusions hold for the pyrrole counterpart. Indeed, the anodic polymerization of the grafted PAPy leads to a doped conducting film, whose content of polypyrrole is higher whenever pyrrole is added before anodic polarization. Conclusions A new fast and all-electrochemical concept has been validated to anchor conjugated polymers to a conductive substrate. Substitution of acrylic double bonds by pyrrole or thiophene does not prevent them from being adsorbed onto a cathode and the parent polymer chains from being grown from the surface in a well-defined potential range. As a result, a pyrrole- or thiophene-containing polyacrylate (25) Jugnet, Y.; Tourillon, G.; Minh Duc, T. Phys. Rev. Lett. 1986, 56, 1862-1865. (26) Lazzaroni, R.; Sporken, R.; Riga, J.; Verbist, J.; Bre´das, J. L.; Zamboni, R.; Taliani, C. Springer Ser. Solid-State Sci. 1987, 76, 281284. (27) Lazzaroni, R.; Riga, J.; Verbist, J.; Bre´das, J. L.; Wudl, F. J. Chem. Phys. 1988, 88, 4257-4262.

film is chemisorbed (grafted) on the surface, which is an alternative to the chemical grafting of, e.g., thiols onto a gold surface. An advantage of this electrografting process is applicability to a variety of solid substrates, such as gold, stainless steel, glassy carbon, Ni, etc. Moreover, no pretreatment is required for making the surface reactive, it has just to be conductive and anodically stable. Electron transfer from the cathode to the adsorbed monomer is the key step for the chemisorption of the polymer, which grows independently of any further electron transfer. Pyrrole (or thiophene) is not modified in this preliminary step. Upon anodic polarization, these aromatic rings are oxidized and polymerized into electroactive conjugated chains in the presence or not of additional pyrrole (or thiophene) in solution. The higher potential of the redox peaks compared to, e.g., neat polypyrrole suggests that oligomers with short conjugation length are formed. This length can however be increased by the addition of monomer (pyrrole or thiophene) to the solution before the film is oxidized. Addition of pyrrole (or thiophene) derivatives and coating of carbon fibers and ITO-glass by this strategy are under current investigation. Acknowledgment. The authors are grateful to the “Services Fe´de´raux des Affaires Scientifiques, Techniques et Culturelles” in the frame of the “Poˆles d’Attraction Interuniversitaires: Chimie Supramolećulaire et Catalyze Supramolećulaire” (PAI 4/11). C.J. is “Charge´ de Recherche” and R.L. is “Directeur de Recherches” by the “Fonds National de la Recherche Scientifique” (FNRS). LA011439N

Full Electrochemical Synthesis of Conducting Polymer Films

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