Electrografting Polyaniline on Carbon through the Electroreduction of

Sep 19, 2008 - Luís Santos , Jalal Ghilane , Pascal Martin , Pierre-Camille Lacaze , Hyacinthe Randriamahazaka and Jean-Christophe Lacroix. Journal o...
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J. Phys. Chem. C 2008, 112, 16103–16109

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Electrografting Polyaniline on Carbon through the Electroreduction of Diazonium Salts and the Electrochemical Polymerization of Aniline Luı´s Miguel Santos,†,‡ Jalal Ghilane,† Claire Fave,† Pierre-Camille Lacaze,† Hyacinthe Randriamahazaka,† Luisa Maria Abrantes,‡ and Jean-Christophe Lacroix*,† Interfaces, Traitements, Organisation et Dynamique des Syste`mes, UniVersite´ Paris 7-Denis Diderot, UMR 7086, Baˆtiment LaVoisier, 15 rue Jean de Baı¨f, 75205 Paris Cedex 13, France, and Centro de Quı´mica e Bioquı´mica, Departamento de Quı´mica e Bioquı´mica, Faculdade de Cieˆncias, UniVersidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal ReceiVed: May 14, 2008; ReVised Manuscript ReceiVed: July 31, 2008

Electroreduction of diazonium salts and electropolymerization of conductive polymers are two widely used techniques for producing modified electrodes. In this work, these two techniques are combined in order to functionalize glassy carbon electrodes. In a first step, a thin layer of diphenyl amine (DPA) is grafted through the electrochemical reduction of the 4-aminodiphenylamine (ADPA) diazonium salt. The DPA grafted layer has been studied using XPS and SECM. It appears to act as a switch giving diode-like behavior for probes with a redox potential below 0.4 V. As a consequence, aniline oxidation remains possible on this substrate, and polyaniline (PANI) can easily be deposited on the DPA-modified electrodes. The PANI films generated on such modified electrodes show electrochemical behavior similar to that of PANI electrodeposited on bare carbon electrodes but exhibit better chemical stability and high resistance to aggressive environments, that is, high temperatures and ultrasonic etching. Such results, associated with the scanning electrochemical microscopy response of the modified electrodes, strongly suggest that PANI is grafted covalently on the DPA layer. From a fundamental point of view, the results show that surface graft polymerization can be extended to surface graft electropolymerization. From a practical point of view, the method described in this work is a simple and efficient process for improving the stability of an electrodeposited conducting polymer, that is, polyaniline, on a carbon substrate and could be of considerable importance in the synthesis of PANI/carbon nanotube composites. 1. Introduction Conducting polymers (CP), owing to their unique properties, are a focus of intense materials science research.1,2 Their deposition as a thin layer on various materials, such as plastics, glass, and metals, as well as on micro- and nanoporous materials, has attracted considerable attention in the past two decades. One of the major problems to be tackled is the usually weak and short-term adhesion between the CP layers and the substrates of a completely different nature. The most desirable situation is then by far the covalent bonding of the CP chains. Such control of the substrate/CP interface is not reached when CPs are chemically or electrochemically deposited on surfaces through oxidation of the monomer. Surface graft polymerization can overcome this problem. This surface chemical modification method is achieved by grafting the appropriate macromolecular chains on the surface of materials through covalent bonding. It allows the surface of the materials to be modified or adapted in order to acquire very distinctive properties through the choice of different grafting monomers, maintaining the substrate properties and ensuring an easy and controllable introduction of graft chains with a high density and exact location onto the surface. Compared with physically coated polymer chains, the covalent attachment of the grafted chains onto a surface avoids their desorption and confers long-term chemical stability of the introduced chains. Cathodic electropolymerization initiated from * Corresponding author. † Universite ´ Paris 7-Denis Diderot. ‡ Universidade de Lisboa.

conductive surfaces has been used for this purpose and gives strongly adherent films but remains restricted to (meth)acrylic derivatives.3-6 Conducting surfaces can also be easily functionalized by organic molecules through diazonium salt electroreduction.7-12 The generated layers usually have thicknesses of just a few nanometers and are covalently attached onto the carbon. These layers have been used as a primary layer for the surface polymerization of various nonconducting polymers.13-15 However, this method has, to the best of our knowledge, never been used for the electrochemical deposition of a second material. The main reason for this is that the grafted layers resulting from the reduction of diazonium salts are poorly conducting, and once deposited, any electrochemical processes are hardly triggered on these modified surfaces. In this work, we show that the electroreduction of the diazonium salt of the aniline dimer (4-aminodiphenylamine or ADPA) gives a modified electrode with diode like behavior. This grafted layer has been characterized using X-ray photoelectron spectroscopy (XPS), scanning electrochemical microscopy (SECM), and basic electrochemical techniques. As a consequence of the diode like behavior, aniline oxidation remains possible on this surface, and a strongly adherent and stable polyaniline (PANI) film can be deposited on the modified electrode from aqueous sulfuric acid (Figure 1). We show that this procedure considerably improves the stability of the PANI layer deposited on carbon electrodes. Moreover, a SECM study is used to compare the electrochemical properties of this material to PANI directly deposited on bare glassy carbon (GC). We anticipate that these results will be useful to control the interface

10.1021/jp8042818 CCC: $40.75  2008 American Chemical Society Published on Web 09/19/2008

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Figure 1. Scheme of ADPA grafting and subsequent ANI polymerization on a carbon conducting surface.

between PANI and carbon nanotubes for nanotube solubilization16-25 and in technological applications such as plastic electronics and optoelectronic devices,26-29 sensors,30 actuators,31-33 supercondensators,34,35 batteries,36 and fuel cells.37 2. Materials and Methods 2.1. Chemical and Reagents. All reagents were used as received. Sulfuric acid (H2SO4, 18 M) was supplied by Prolabo; aniline was supplied by Alfa Aesar; ferrocenemethanol (FcMeOH) and perchloric acid (HClO4, 17 M) were from Acros Organics; ADPA (4-aminodiphenylamine) was from Fluka, and lithium perchlorate (LiClO4) was from Aldrich. The diazonium salt was obtained in situ and was not isolated prior to grafting. All of the DPA-modified electrodes were prepared in aqueous solution of 3 mM ADPA in 6 mM NaNO2, 0.1 M HClO4 using 5 electroreduction cycles at 50 mV/s between 0.2 and -1 V/SCE except the DPA-modified electrode shown in Figure 4, which was prepared in aqueous solution of 3 mM ADPA in 6 mM NaNO2, 0.1 M HClO4 using 10 electroreduction cycles at 50 mV/s between 0.2 and -1 V/SCE. 2.2. Apparatus and Procedures. Electrochemistry. Basic electrochemical experiments were carried out in a singlecompartment three-electrode cell using an EG&G PAR 362 potentiostat in the potentiodynamic mode. The auxiliary electrode was a platinum grid; a saturated calomel electrode, SCE (3 M KCl) was used as reference electrode. Working electrodes were homemade GC disks (area ) 0.07 cm2) fabricated from carbon rods (Tokai, Japan) embedded in Teflon. Before use, electrodes were polished with 3 µm, then 1 µm, diamond paste (Struers, Denmark) and ultrasonicated in analytical grade acetone for 3 min and rinsed with water. Water was used for ultrasonication of the DPA-modified and PANI electrodes. Scanning Electrochemical Microscopy (SECM). SECM measurements were performed using a commercial SECM instrument with close-loop piezoelectric motors, CHI 900B (CH Instruments, Austin, TX). A three-electrode setup was employed with Ag/AgCl serving as reference electrode; a platinum wire was used as counter-electrode and a carbon electrode (3 mm diameter) was used as substrate. The ultramicroelectrodes (UME) were prepared in the laboratory following a published procedure.38 A disk-shaped UME 10 µm in diameter was made by sealing platinum wires (Goodfellow) in a soft glass tube that was subsequently ground at one end. The glass edge was conically shaped with an outer diameter of 100 µm giving an RG value between 5 and 10. Prior to use, the UME was polished

using diamond pastes with decreasing grain sizes. All aqueous solutions were prepared with Milli-Q water and deoxygenated by N2 bubbling before use. Between measurements in different solutions, the electrodes were rinsed with distilled water. SECM experiments were performed in aqueous solution. Lithium perchlorate was used at 0.1 M as supporting electrolyte, and FcMeOH was used at 1 mM as redox mediator. X-ray Photoelectron Spectroscopy (XPS). XPS signals were acquired using a VG Scientific ESCALAB 250 system equipped with a microfocused, monochromatic Al KR X-ray source (1486.6 eV) and a magnetic lens which increases the electron acceptance angle and hence the sensitivity. A 650 µm X-ray beam was used at 20 mA × 15 kV. The spectra were acquired in the constant analyzer energy mode with pass energies of 150 and 40 eV for the survey and the narrow regions, respectively. Charge compensation was achieved with an electron flood gun combined with an argon ion gun. The argon partial pressure in the analysis chamber was 2 × 10-8 mbar. Avantage software, version 2.20 (Thermo Electron), was used for digital acquisition and data processing. Spectral calibration was determined by setting the main C1s component at 285 eV. Surface compositions (in atom %) were determined by considering the integrated peak areas of C1s, N1s, and O1s and the corresponding sensitivity factors corrected for the analyzer transmission. 3. Results and Discussion 3.1. Stability of PANI Deposited onto Carbon Electrode. In a preliminary experiment, a PANI film was prepared in an aqueous solution containing 0.5 M aniline (ANI) in 1 M acid,39,40 by sweeping the voltage applied to the carbon electrode between -100 and 900 mV/SCE at 50 mV/s. The electroactivity of the deposited film in H2SO4 is shown in Figure 2 and clearly points to the presence of a PANI film at the surface of the carbon electrode, as can be seen by the occurrence of a pair of oxidation-reduction peaks at 170 mV/ 30 mV and 450 mV/ 370 mV, respectively. This second oxidation peak (450 mV/370 mV) is often seen when the inversion potential used during PANI electropolymerization is not reduced upon successive cycles. It can be assigned to the hydrolysis product of the polymer or to byproduct like benzoquinone or hydroquinone which remains adsorbed in the polymer matrix during the synthesis.41 The modified carbon electrode was then ultrasonicated for 10 min and heated for 2 h at 100 °C in water. The electroactivity of the deposited PANI in the H2SO4 solution was followed at different moments of

Electrografting Polyaniline on Carbon

Figure 2. Electroactivity in 1 M H2SO4 of a PANI film generated on a carbon electrode in a solution of 1 M H2SO4 containing 0.5 M aniline after ultrasonication and heating (100 °C). Sweep rate: 100 mV/s. Figure in the inset: electroactivity of the same PANI film before and after sonication in a solution of 1 M H2SO4. Sweep rate: 100 mV/s.

Figure 3. Carbon disk electrode in aqueous solution of 3 mM ADPA in 6 mM NaNO2 and 0.1 M HClO4 (a) first scan (b) second to fifth scan. Sweep rate: 50 mV/s.

the procedure and is presented in Figure 2. Similar electroactivity was recorded before and after ultrasound treatment. However, the electrochemical response of the polymer film subjected to high temperature (100 °C) shows a gradual decrease in current intensity of both pairs of peaks initially well-defined on the voltammogram, indicating (i) desorption of the byproduct of PANI electrodeposition and (ii) a continued decrease in PANI electroactivity at the carbon electrode surface due to a poor physisorption of the polymer on the carbon surface and/or to a loss of conjugation due to thermal degradation. 3.2. Grafting of ADPA onto Carbon by Diazonium Salt Reduction. Another carbon electrode surface was modified by electrochemical reduction of an in situ generated diazonium cation, that is, the aniline dimer 4-aminodiphenylamine (ADPA). The diazonium cation was synthesized in the electrochemical cell by reaction of the ADPA with NaNO2 in aqueous HClO4. This deposition method, which involves simple reagents and does not require the isolation and purification of the diazonium salt, enables the grafting of covalently bonded layers which exhibit properties very similar to those of layers obtained by the classical derivatization method involving isolated diazonium salt dissolved in acetonitrile or aqueous acid solution.42

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Figure 4. Cyclic voltammetry in 1 M H2SO4 water solution of a DPAmodified carbon electrode (film generated using 10 electroreduction cycles between 0.2 and -1 V/SCE). Sweep rate: (-) 50 mV/s and (s) 100 mV/s.

Figure 3 shows the reduction of ADPA diazonium generated using 3 mM ADPA and 6 mM NaNO2 in 0.1 M HClO4 water solution on the carbon electrode. It shows a broad irreversible wave near -500 mV (curve a), indicating that an irreversible reaction (i.e., the cleavage of the C-N bond and elimination of dinitrogen) is associated with the electron transfer. After the second scan (curve b), the wave disappears almost completely and indicates the blocking of the surface by the organic groups which become attached to the surface. It is now widely accepted that such a reaction leads to the covalent grafting of diphenylamine (DPA) monomers or oligomers (see Figure 1).43 This electrode will be denoted as the DPA-modified carbon electrode in the following. 3.3. Characterization of the DPA-Modified Electrode. DPA-modified electrodes were rinsed in acetonitrile and their electrochemical responses were studied in aqueous 1 M H2SO4 and 0.1 M NaOH solutions. Figure 4 shows the electroactivity of the electrode in 1 M H2SO4 (film generated using 10 electroreduction cycles between 0.2 and -1 V/SCE). It shows an electroactive redox couple at 440 mV and 390 mV versus SCE. The peak-to-peak separation is around 50 mV (at 50mV/s sweep rate), and the ratio of anodic to cathodic peak currents is close to unity. In addition, a linear dependence of the current versus the scan rate is also observed, which is indicative of a surface process. From the integrated charge of the electrochemical response, a surface coverage of 10-9 mol · cm-2 can be estimated (depending on the number of cycles used during electroreduction; note that the film shown in Figure 4 is thicker than that used in Figures 5 and 8). The reported surface coverage on GC substrates7-12,43 varies in the range of 4 to 12 × 10-10 mol · cm-2 with the theoretical maximum surface concentration of a closed-packed monolayer on GC surfaces being 1.35 × 10-9 mol · cm-2.43 The surface coverage of 10-9 mol · cm-2 clearly indicates that the GC electrode is modified with a thin film of DPA or oligo(DPA) molecules rather than a thick film. Furthermore, this redox couple disappears when the electrode is studied in 0.1 M NaOH and is recovered when the electrode is studied in 1 M H2SO4. This film electroactivity dependence upon pH is characteristic of oligoaniline derivatives and strongly suggests that the film is composed of oligo(DPA) and DPA moieties grafted on the carbon surface. A fresh DPA-modified carbon electrode was then plunged into a ferrocenemethanol (FcMeOH) solution to probe its

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Figure 5. Cyclic voltammetry in 1 mM FcMeOH + 10 mM LiClO4 solution of a bare carbon electrode and DPA-modified carbon electrode (film generated using 5 electroreduction cycles between 0.2 and -1 V/SCE). Sweep rate: 100 mV/s.

electrochemical behavior toward reversible outer-sphere redox species. Figure 5 compares the FcMeOH electrochemical response on bare and DPA-modified carbon electrodes. No current is observed on the DPA-modified electrode in the potential range where FcMeOH redox reactions usually occur on bare electrodes. The organic layer totally blocks the electrode in such a potential window. Above 400 mV, the current dramatically increases and an “irreversible” wave is observed with an oxidation peak at 680 mV as compared with 320 mV on the bare electrode. The modified electrode was then ultrasonicated for 10 min and plunged into the same FcMeOH solution, and its behavior is recorded in the same potential range. As can also be seen in Figure 4, there are no significant differences between the voltammograms recorded before and those recorded after the electrode had been subjected to ultrasound. This is a clear indication that the DPA layer is strongly attached to the carbon surface and suggests covalent grafting on carbon. It is important to note here that, contrary to surface modifications generated by the reduction of other diazonium salts such as those resulting from the diazotization of aniline, nitroaniline, aminobiphenyl, or para-aminobenzoic acid,7-12,42,43 the layer obtained through the electroreduction of ADPA diazonium salt behaves like an electrochemical switch. Above some threshold potential, oxidation of soluble external probes is possible whereas the reduction of the oxidized form of this external probe remains impossible. This behavior is similar to that recently reported for layers grafted on carbon surfaces through the electroreduction of the diazonium salt of 1-(2-bisthienyl)-4aminobenzene.44 Such behavior is linked to the electroactivity of the grafted layer which acts as a dopable organic semiconductive material and switches from an insulating state to a conductive state above a potential threshold which is here close to 0.4 V/CSE (as mentioned above, this DPA layer, generated using five electroreduction cycles between 0.2 and -1 V, as depicted in Figure 3, is about one monolayer thick).44 The modified carbon electrode was characterized by X-ray photoelectron spectroscopy (XPS). C1s and O1s peaks are observed at 285 and 532.7 eV, respectively, together with the expected N1s peak at 400 eV. These peaks confirm the presence of the grafted DPA molecules on the surface. A more detailed analysis of the surface composition of the modified electrode

element peak peak BE (eV) fwhm (eV) atom % C1s N1s C1s N1s

285.00 400.50 285.00 400.20

1.38 1.95 1.79 2.01

86.95 0.47 76.66 6.43

can be obtained from the core level spectra. The C1s peak at 285 eV, mainly corresponds to aromatic carbons of the phenyl groups. Analysis of the N1s core level gives two peaks centered at 400 eV and at 403 eV (minor peak). According to the literature, the first one can be attributed to amino groups and the minor peak at 403 eV can be attributed to the ammonium groups, obtained by protonation of amino groups in acidic medium.45 The atomic surface concentrations obtained by the integration of the core level peaks are presented in Table 1 and are compared to those of bare carbon. The C1s/N1s ratio changes from 185 to 11.9 upon DPA grafting and reaches a value close to the expected value of 12 for DPA or oligo(DPA) grafted layers. Therefore, XPS indicates that a DPA layer has been successfully grafted on the carbon electrode surface. 3.4. SECM Experiments on PANI Carbon Electrode and DPA-Modified Electrode. The modified electrode has also been characterized using scanning electrochemical microscopy (SECM). The SECM approach curve was obtained above polyaniline, deposited onto carbon electrode by electrodeposition. Figure 6 shows the experimental approach curve obtained using ferrocenemethanol as redox mediator. For a normalized distance, L > 3, I(L) is close to 1; this value reflects the absence of interaction between the ferrocinium methanol electrogenerated at the UME and the substrate for this distance. However, for L < 3, the current rapidly increases with the microelectrode/ substrate distance, and positive feedback is observed. This enhancement demonstrates the regeneration of the mediator at the polyaniline substrate. Furthermore, the experimental data match with the theoretical curve for a conducting substrate under diffusion control.46 This result demonstrates that the regeneration of the mediator above polyaniline is under diffusion control and that the polyaniline behaves as a metallic substrate. Following that, SECM approach curve was obtained above DPA attached to the carbon (Figure 7). As expected, when the UME is located far from the substrate, L > 3, the value of the normalized current I(L) is close to unity. When the separation distance, L, decreases, the normalized current diminishes, demonstrating that the mediator is not regenerated at the DPAcarbon substrate. This behavior corresponds to negative feedback. Moreover, the experimental approach curve is in good agreement with the theoretical variation expected for an insulating substrate, demonstrating the insulating properties of the surface under these conditions. The approach curve was recorded in different areas of the surface, and the same behavior was observed. (It is worth to noting that the approach curve recorded above the carbon substrate exhibits total positive feedback.) These results demonstrate that the DPA layer is homogeneous on the carbon substrate and suggest the absence of pinholes in such a surface. It could be proposed as a good candidate to replace SAMs attached to gold electrode which often show pinhole defects in similar SECM experiments and lack stability.47,48 By considering the CV obtained with the DPAmodified carbon electrode in the presence of FcMeOH as redox species, (see Figure 6), the insulating behavior deduced from

Electrografting Polyaniline on Carbon

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Figure 6. (O) Experimental approach curve of Pt UME above polyaniline deposit on carbon substrate in aqueous solution containing 1 mM FcMeOH and 0.1 M LiClO4. The substrate potential is 0.1 V/Ag/ AgCl. (solid line) theoretical approach curve for a conducting substrate. Figure 9. Electroactivity in 1 M H2SO4 of a PANI film generated on a DPA-modified carbon electrode (a) after ultrasonication and (b) after 2 h heating (100 °C). Sweep rate: 100 mV/s. Figure in the inset: electroactivity of the same PANI film before and after sonication in a solution of 1 M H2SO4. Sweep rate: 100 mV/s.

Figure 7. (]) Experimental approach curve of Pt UME above DPA layer grafted onto carbon substrate in aqueous solution containing 1 mM FcMeOH and 0.1 M LiClO4. (Film generated using 5 electroreduction cycles between 0.2 and -1 V/SCE.) The substrate potential is 0.1 V/Ag/AgCl, solid line represents the theoretical approach curve for an insulating substrate.

Figure 8. Cyclic voltammograms recorded for a DPA-modified carbon electrode (film generated using 5 electroreduction cycles between 0.2 and -1 V/SCE). immersed in a 1 M sulfuric acid containing 0.5 M aniline. Sweep rate: 50 mV/s.

SECM is related to the absence of the reduction of ferrocinum methanol at the DPA-carbon surface. SECM experiments confirm the result obtained using CV and demonstrate that the oxidation of FcMeOH on DPA-carbon substrate is irreversible. At this stage, the DPA layer has a blocking effect of the

Figure 10. Experimental (×) approach curve of Pt UME above polyaniline electrodeposit onto ADPA-carbon substrate in aqueous solution containing 1 mM FcMeOH and 0.1 M LiClO4. The substrate potential is 0.1 V/Ag/AgCl. The solid line (-) is the theoretical approach curve for conducting substrate.

reduction process and could be considered as insulating layer toward this electrochemical reaction. Similar experiments were also performed using ADPA as mediator. The results obtained are identical to that observed with FcMeOH mediator. This behavior confirms the insulating character of the DPA layer and clearly demonstrates that ADPA cannot diffuse through the layer despite a molecular structure close to that of the layer. Using this probe, pinhole defects are not observed. 3.5. Electrochemical Grafting of PANI on the DPAModified Electrode. Since the DPA-modified carbon electrode shows a switch behavior and makes it possible to oxidize FcMeOH, we have studied the electrooxidation of aniline on this electrode in order to check if surface graft electropolymerization can be achieved. The DPA-modified electrode was plunged into an aqueous solution containing 0.5 M aniline and 1 M sulfuric acid, and a PANI film was prepared using the same conditions as on bare GC. The voltammogram is shown in Figure 8. It is important to note that aniline oxidation does not seem to be strongly hindered by the DPA layer, thanks to the switch of such a modified electrode. Nevertheless, in order to obtain a PANI film with the same thickness as that generated on bare carbon, two supplementary cycles were necessary. This

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Figure 11. Schematic of SECM in feedback mode (A) above polyaniline layer, (B) above DPA layer, and (C) above polyaniline attached to DPA layer.

is mainly due to the presence of the organic DPA layer grafted at the carbon electrode surface which, despite its nonblocking effect on aniline oxidation, offers a supplementary resistance to electron transfer from the carbon to the aniline monomers present in solution. One has also to note that PANI electrochemical peaks are seen during the entire electrodeposition process, despite the fact that its potential (250 mV) is below the threshold potential of the DPA layer (400 mV). This result is a strong indication that PANI is covalently attached to the DPA layer and constitutes a continuous polymer chain anchored to the carbon surface. Indeed, if a bilayer was obtained (i.e., PANI adsorbed on the DPA layer), it is likely that the underlying DPA layer would have blocked PANI reduction and/or shifted the PANI oxidation peak toward higher potential. The modified electrode with approximately the same PANI thickness as that synthesized on a bare electrode was then rinsed with Milli-Q water and transferred to a new electrochemical cell containing a 1 M sulfuric acid. The recorded voltammetric response clearly confirmed the presence of a PANI film at the surface of the carbon electrode (Figure 9, after polymerization). This modified electrode was then subjected to the same conditions experienced by the electrode modified with a PANI film without any previous grafted DPA layer, that is, ultrasonicated for 10 min and heated for 2 h at 100 °C (Figure 9). The electroactivity of the PANI film present at the electrode surface was followed at different times. In contrast with the electrochemical response of the PANI film polymerized on a bare carbon electrode after it has been subjected to a high temperature treatment (Figure 2), this modified electrode surface resulting from electropolymerization of ANI on a DPA-modified carbon electrode does not show any evidence for matter or electroactivity loss. In fact, the current intensity of both pairs of peaks present on the voltammogram recorded just after the polymerization (Figure 9) remains almost unchanged upon 2 h exposure to 100 °C, for a set of five PANI/ DPA/GC electrodes, in contrast with the gradual decrease observed for both peak intensities in the electrode modified only with a PANI coating (Figure 2). Considering these results, we can state that the PANI deposited on a DPA-modified carbon electrode is most likely also grafted to this organic layer (DPA). 3.6. SECM Experiments on Electrografted PANI on DPAModified Electrode. The strategy developed in this paper leads to the strong attachment of polyaniline to a carbon electrode. This surface was also investigated by CV in FcMeOH solution to probe its electrochemical behavior toward reversible outersphere redox species. It is found that the peculiar diode-like behavior observed with GC/DPA electrodes disappears when PANI has been deposited. This effect has also been studied using SECM, and the data obtained are illustrated in the approach

curve (Figure 10) which shows a positive feedback, indicating the regeneration of the mediator at the substrate. The experimental curve matches the theoretical behavior expected for a conducting substrate under diffusion control. This behavior is similar to that observed for polyaniline deposited on a carbon electrode. This result, associated to the CV results of this electrode in FcMeOH, suggests that the DPA layer has lost its blocking effect regarding to reduction processes and that the whole layer behaves as a metallic substrate. Figure 11 summarizes the result obtained using SECM in feedback mode. In the case of direct electrodeposition of polyaniline at carbon electrode (Figure 11A), positive feedback is observed because of the regeneration of the mediator, and the polymer layer behaves as a metallic substrate. In the case of DPA grafted on the carbon electrode, SECM experiments demonstrate the insulating properties of this layer. This behavior is attributed to the irreversibility of FcMeOH which induces negative feedback. The DPA layer blocks the regeneration of the mediator (Figure 11B). Finally, after polyaniline electrodeposition on the DPA layer, the SECM investigation shows a positive feedback, indicating regeneration of the mediator, and the layer recovers its properties as a conducting substrate (Figure 11C). This result proves that the DPA has lost its blocking effect and that a continuous polyaniline layer attached to the carbon surface is obtained. All of these results strongly suggest the absence of a bilayer structure (DPA/PANI) and the covalent attachment of the polyaniline onto DPA leading to a continuous conductive chain bonded to the carbon electrode with enhanced stability. 4. Conclusion In this work, we have combined two widely used techniques to produce modified electrodes, that is, the electroreduction of diazonium salts and the electropolymerization of conductive polymers. We have shown that a strongly adherent conducting PANI film can be electrodeposited on carbon electrode surfaces modified by electrochemical reduction of the diazonium salt of the 4-aminodiphenylamine (ADPA). The grafted DPA layer is electroactive, shows a diode-like behavior for ferrocenemethanol (it hinders its electrochemical reduction whereas its oxidation is shifted to a more positive potential), and acts as a conductive switch. As a consequence, aniline oxidation remains possible on this substrate, and PANI can easily be deposited on the modified GC electrode. These polyaniline films show an electrochemical behavior identical with that of electrodeposited polyaniline on bare carbon electrodes but exhibit a better chemical stability and high resistance to hostile environments, that is, high temperatures and ultrasonication. Such results,

Electrografting Polyaniline on Carbon associated with the SECM response of the electrode, strongly suggest that PANI is covalently grafted on the deposited DPA layer. From a practical point of view, the method described in this work is a simple and efficient process for improving the stability of an electrodeposited conducting polymer on a carbon substrate and could be of considerable importance for several applications and in the synthesis of PANI/carbon nanotube composites. From a more fundamental point of view, the surface electro-grafting method described here extends the possibilities of the already widely used surface grafting methods. Acknowledgment. This work was supported by the ANR Program (ANR BLAN 06-296) administered through the French Research Ministry and by a grant of the Fundac¸a˜o para a Cieˆncia e a Tecnologia (FCT) PhD fellowship (SFRH-BD-30209-2006). We are particulary grateful to Dr J. S. Lomas for revising our text and correcting the English. References and Notes (1) Tallman, D. E.; Bierwagen, G. P. Corrosion protection using conducting polymers. Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; 2007; 15/1-15/53. (2) Audebert, P.; Miomandre, F. Electrochemistry of conducting polymer. Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; 2007; 18/1-18/40. (3) Deniau, G.; Azoulay, L.; Je´jou, P.; Le Chevallier, G.; Palacin, S. Surf. Sci. 2006, 600, 675–684. (4) Palacin, S.; Bureau, C.; Charlier, J.; Deniau, G.; Mouanda, B.; Viel, P. ChemPhysChem 2004, 10, 1468–1481. (5) Baute, N; Teyssie´, P; Martinot, L; Mertens, M.; Dubois, P.; Je´roˆme, R. Eur. J. Inorg. Chem. 1998, 171, 1–1720. (6) Baute, N.; Calberg, C.; Dubois, P.; Je´roˆme, C.; Je´roˆme, R.; Martinot, L.; Mertens, M.; Teyssie´, P. Macromol. Symp. 1998, 134, 157–166. (7) Allongue, P.; Delamar, M.; Desbat, B.; Fagebaume, O.; Hitmi, R.; Pinson, J; Save´ant, J. M. J. Am. Chem. Soc. 1997, 119, 201–207. (8) Downard, A. J.; Prince, M. J. Langmuir 2001, 17, 5581–5586. (9) Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038–5045. (10) Anariba, F.; DuVall, S. H.; McCreery, R. L. Anal. Chem. 2003, 75, 3837–3844. (11) Guozhen, L.; Jingquan, L.; Bocking, T.; Eggers, P. K.; Gooding, J. J. Chem. Phys. 2005, 319, 136–146. (12) Solak, A. O.; Eichorst, L. R.; Clark, W. J.; McCreery, R. L. Anal. Chem. 2003, 75, 296–305. (13) Matrab, T.; Chancolon, J.; Mayne. L’Hermite, M.; Rouzaud, J.N.; Deniau, G.; Boudou, J.-P.; Chehimi, M. M.; Delamar, M. Colloids Surf. A: Physicochem. Eng. Aspects 2006, 287, 217–221. (14) Matrab, T.; Chehimi, M. M.; Perruchot, C.; Adenier, A.; Guillez, A.; Save, M.; Charleux, B.; Cabet.Deliry, E.; Pinson, J. Langmuir 2005, 21, 4686–4694. (15) Mevellec, V.; Roussel, S.; Tessier, L.; Chancolon, J.; Mayne. L’Hermite, M.; Deniau, G.; Viel, P.; Palacin, S. Chem. Mater. 2007, 19, 6323–6330. (16) Zhang, H.; Li, H. X.; Cheng, H. M. J. Phys. Chem. B 2006, 110, 9095–9099. (17) Sainz, R.; Benito, A. M.; Martinez, M. T.; Galindo, J. F.; Sotres, J.; Baro, A. M.; Corraze, B.; Chauvet, O.; Maser, W. K. AdV. Mater. 2005, 17, 278–281.

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