Letter pubs.acs.org/Langmuir
Electropolymerization of Polypyrrole by Bipolar Electrochemistry in an Ionic Liquid Shuwei Kong,†,‡ Olivier Fontaine,§ Jérôme Roche,†,‡ Laurent Bouffier,†,‡ Alexander Kuhn,†,‡ and Dodzi Zigah*,†,‡ †
Université Bordeaux, ISM, UMR 5255, F-33400 Talence, France CNRS, ISM, UMR 5255, F-33400 Talence, France § Université Montpellier 2 UMR 5253, CC1701 Place Eugène Bataillon, 34095 Montpellier, France ‡
S Supporting Information *
ABSTRACT: Bipolar electrochemistry has been recently explored for the modification of conducting micro- and nanoobjects with various surface layers. So far, it has been assumed that such processes should be carried out in low-conductivity electrolytes in order to be efficient. We report here the first bipolar electrochemistry experiment carried out in an ionic liquid, which by definition shows a relatively high conductivity. Pyrrole has been electropolymerized on a bipolar electrode, either in ionic liquid or in acetonitrile. The resulting polymer films were characterized by scanning electron microscopy and by contact profilometry. We demonstrate that the films obtained in an ionic liquid are thinner and smoother than the films synthesized in acetonitrile. Furthermore, a well-defined band of polypyrrole can be obtained in ionic liquid, in contrast to acetonitrile for which the polypyrrole film is present on the whole anodic part of the bipolar electrode.
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INTRODUCTION Surface modification intends to give specific properties for welldefined applications. Depending on the chemical nature of this surface modification, physical, biological, or chemical characteristics can be modified, allowing various applications ranging from molecular electronics and energy storage to biochemical or chemical sensors.1−3 Bipolar electrochemistry (BE) is a technique that allows one to perform wireless electrochemistry on a conducting object immersed in solution under the influence of an electric field.4 This technique intrinsically leads to the modification of only one part of a conducting object; therefore, one of its main applications is the bulk production of Janus particles (JPs) with asymmetric features.5 BE is a versatile approach that can be used in different fields, such as analytical chemistry, electrochemical motors, and materials science.6,7 The principle of BE is simple, and the setup consists of placing a conductive object in a solution between two feeder electrodes that are connected to a power supply. When a potential is applied between these two electrodes, it induces a potential drop in the solution and the conductive object becomes a bipolar electrode (Figure 1). If the applied electric field is high enough, then oxidation reactions are induced at one extremity of the object, simultaneously with reduction reactions at the other extremity.8 This break in symmetry leads in most cases to the formation of JPs,5 which can be used in different fields, from electronics to targeted drug delivery or energy conversion.9,10 Because an ohmic drop in the solution is necessary to trigger the bipolar reactions, solutions with low conductivities (∼0.1 mS/cm) are © 2014 American Chemical Society
Figure 1. Scheme of a bipolar gold electrode exposed to an electric field between two gold feeder electrodes. The reduction of benzoquinone and the oxidative polymerization of pyrrole occur, respectively, at the cathodic and anodic parts of the bipolar electrode.
usually employed. Ionic liquids with their significantly higher conductivity (∼3 mS/cm) seem to be at first sight unsuitable for this kind of experiment. However, they have several interesting properties, such as a high viscosity, that might be useful in the framework of BE. We therefore decided to test a room-temperature ionic liquid (RTIL) as an ionically conductive phase to produce original Janus surfaces and JPs. RTIL do not evaporate because of their low volatility, and they Received: December 24, 2013 Revised: March 3, 2014 Published: March 6, 2014 2973
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have a large electrochemical window (>2 V).11,12 In the proofof-principle experiments in this work, we use BE to produce JPs modified with a conducting polymer generated in the environment of an ionic liquid. Conducting polymers have applications in various electrochemical devices, such as capacitors, batteries, and photovoltaic cells.13 Their electrochemical synthesis in RTIL has already been explored, for example, in the case of poly(paraphenylene), polypyrrole (PPy), and polythiophene.14−18 The conductive polymer produced in this work is PPy, which is one of the most popular conducting polymers because of its good conductivity, the solubility of its monomer in different solvents, its commercial availability, and its low price. It is also well known that PPy produced in ionic liquids has a higher conductivity and also better mechanical stability.15,16 BE has already been used to generate conducting polymers19,20 but so far not with RTIL as a solvent because increasing the conductivity of the electrolyte should lead to a smaller potential drop in the BE cell and thus should make the experiments much more difficult to carry out because high electric fields are mandatory. To test the general feasibility of a deposition by BE in such an unusual environment, the first experiments were carried out with relatively long objects having a characteristic length on the millimeter scale. Indeed, the maximum potential difference arising between the two extremities of a bipolar electrode is directly proportional to the electric field intensity and to the characteristic dimension of the object, according to eq 1
TBAPF6 as supporting electrolytes. The latter was added in order to raise the conductivity to 4 mS cm−1 at 25 °C, which corresponds to the conductivity of the RTIL12,21,22 measured under the same conditions, in order to allow a direct comparison between different experiments. Benzoquinone was added to the solution in order to lower the potential difference (ΔV) required at the extremities of the bipolar electrode by decreasing the potential needed for the reduction reaction (Figure S2). The gold substrate was placed between two gold feeder electrodes that were connected to the power supply delivering a constant potential (Figure 1). All of the experiments were performed for a constant period of time (30 s) but with different electric field values. In Figure 3, one can observe a dark PPy deposit on the gold surfaces. The modification obtained with electric fields of
(1)
ΔV = εd −1
with ε being the electric field (V m ) and d being the size of the bipolar object (m). PPy in RTIL has been generated first on one side of a flat gold surface (Figure 1). In a second step, the experiments were scaled down and performed in an analogous way with glassy carbon microbeads as bipolar electrodes in order to obtain JPs.
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EXPERIMENTAL SECTION
Pyrrole (reagent grade, 98%) was distilled before use. Tetrabutylammonium hexafluorophosphate (TBAPF6, ≥99.0%), acetonitrile (HPLC grade, ≥ 99.9%), and an RTIL (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][TFSI]) (Figure 2, ≥98%)
Figure 2. Cation [BMIM] and anion [TFSI] composing the roomtemperature ionic liquid.
were also used. RTIL was dried overnight prior to use. pBenzoquinone (≥98%) and the former products were purchased from Sigma-Aldrich. The gold electrodes were obtained from ACM, and glassy carbon beads (spherical powder 630−1000 μm) were purchased from Alfa Aesar.
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RESULTS AND DISCUSSION During the first screening experiments, PPy was electrogenerated on gold electrodes by BE in an acetonitrile solution containing 5 mM benzoquinone, 50 mM pyrrole, and 33 mM
Figure 3. Bipolar gold electrode: 5 × 5 mm2. (a−d) PPy prepared in acetonitrile. (e−h) PPy prepared in RTIL, deposited on the gold surface at different electric fields: (a, e) 10, (b, f) 15, (c, g) 20, and (d, h) 30 V/cm. 2974
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different amplitudes shows an increase in the modified area as a function of the value of the electric field, up to a limit of 30 V/ cm. Typically, one-third of the whole bipolar electrode surface was covered with the PPy deposit when this latter value was applied. The film morphology was studied in more detail with a contact profilometer. A tip in contact with the surface was moved laterally across the sample. Small surface variations in the vertical tip displacement as a function of position were measured. All of the films prepared in acetonitrile presented a rough surface with inhomogeneously large steps ranging between 1 and 5 μm (Figure 4A).
Figure 5. Glassy carbon beads obtained by BE in a solution containing 1 mM benzoquinone and 50 mM pyrrole (left) in acetonitrile with 33 mM TBAPF6 and (right) in [BMIM][TFSI]. The electric field in both cases was 60 V/cm.
As in the case of the gold plate, the PPy layer is thinner and smoother when it is synthesized in the presence of RTIL. In addition, in the latter case the mechanical stability is better, whereas the layers obtained in acetonitrile can be easily removed from the substrate.
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CONCLUSIONS We have successfully demonstrated that it is possible to carry out BE experiments in an ionic liquid. This has been illustrated by the site-selective deposition of PPy layers onto flat and spherical substrates with different characteristic sizes ranging from the millimeter to the micrometer scale. The PPy films generated in an ionic liquid have improved properties such as a smaller surface roughness, better-controlled thickness, and improved mechanical stability.15,16 These results are very encouraging for testing various ionic liquids, for example, those with increased viscosity, thus allowing the simultaneous immobilization of the beads mechanically in the cell during the application of the electric field. Furthermore, the bipolar electrochemical synthesis of other materials, obtained from precursors that are soluble in ionic liquids, will be explored in the near future.24,25
Figure 4. PPy film studied with a contact profilometer. (A) PPy in acetonitrile. (B) PPy in [BMIM][TFSI] obtained with an electric field of 30 V/cm.
In the next set of experiments, ionic liquid has been used as a deposition medium. The RTIL [BMIM][TFSI] has been chosen because it is known to allow the deposition of PPy.16 As in acetonitrile, the area where deposition can occur increased with the strength of the electric field. It is also remarkable that at 20 V/cm, no deposition of PPy was observed on the left part of the gold surface, where the polarization is most important. This was equally confirmed at 30 V/cm, as in this case the left part without visible deposition increased. For 20 and 30 V/cm, a band of PPy was generated, with the width being inversely proportional to the electric field. Several hypotheses can be made to explain this phenomenon. The formation of oxide layers on a gold surface can influence the rates of electrontransfer reactions, but because at high potential the oxide layers on gold are porous, they should not stop the polymerization. Another explanation is that at very high polarization PPy films cannot be formed because of the overoxidation of the polymer. After the dimers or tetramers are formed, the chain cannot grow anymore because it is overoxidized and starts to be insulating. We assumed that the anion of the RTIL induced an overoxidation state more easily than the anion of the supporting electrolyte, which is why this phenomenon was not observed in acetonitrile. Comparing the PPy structure obtained in RTIL with the one generated in acetonitrile reveals less roughness and thickness for the film produced in the ionic liquid (typically less than 0.5 μm thickness, Figure 4B). After this first study, we decided to switch from macroscopic anisotropic plates to submillimeter isotropic objects, in this case, carbon beads. The diameter of these beads is around 700 μm. These spherical particles do not exhibit any preferential orientation when subjected to an electric field and can possibly rotate freely. To avoid such movement of the beads during bipolar electrodeposition, we have already shown that immobilization either in a gel5 or in a capillary of comparable size is necessary.23 In the present case, we decided to immobilize the beads at the bottom of the cell with adhesive tape. The modified beads were observed by SEM (Figure 5).
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ASSOCIATED CONTENT
S Supporting Information *
Setup used to modify carbon beads. Cyclic voltammetry of benzoquinone and pyrrole in acetonitrile and in an ionic liquid. Description of instruments used to characterize polypyrrole. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially funded by the ANR Program Emergence (Project PROJANUS) under contract ANR-2011EMMA-007-01 and the maturation fund of the University of Bordeaux (Aquitaine Science Transfert).
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