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Langmuir 2007, 23, 11430-11433
Electrochemical Synthesis and Surface Characterization of Poly(3,4-ethylenedioxythiophene) Films Grown in an Ionic Liquid Shahzada Ahmad,* M. Deepa, and S. Singh Electronic Material DiVision, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012 ReceiVed August 8, 2007. In Final Form: September 17, 2007 We report a facile method to synthesize poly(3,4-ethylenedioxythiophene) (PEDOT) films at room temperature in a waterproof ionic liquid, 1-ethyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide (EMIPFSI), by electropolymerization. The ionic liquid leads to the formation of randomly oriented nanofibers and particles confined to submicrometer-sized domains in the film microstructure. X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) studies provide information about the intercalation of the cation apart from the reported anion in the polymer film, and on how the imidazolium ion controls the growth of PEDOT nanostructures.
Introduction Since the discovery of conductivity in conjugated polymers, researchers in the field of electronically conductive polymers have focused on conjugated systems, which include polyacetylene, polypyrrole, polyaniline, polythiophene, and similar derivatives.1-3 It would not be an exaggeration to claim that thiophenes occupied the top position among different types of conjugated materials for organic electronics. Superior chemical, physical, and electronic properties (high mobility), synthetic availability, and structural flexibility gave rise to an extraordinary growth of interest in these compounds that stimulate interest of both academic and industrial research on electroconductive thiophenes. 3,4-Ethylenedioxythiophene (EDOT) is a commercially available monomer, and since 1990, after its synthesis, PEDOT occupies a prominent position among conducting polymers owing, among other things, to the multiple well-established technological applications of its various conducting forms.4,5 PEDOT displays many interesting properties including low half-wave potential and band gap, high environmental stability, high conductivity, and excellent transparency in its doped state. In addition, this polymer exhibits outstanding electrochemical stability upon cycling, as well as superior air, thermal stability, and electrical properties compared to other polythiophenes.6,7 One of the most salient features pertaining to the recent chemistry of EDOT concerns its progressive emergence as a unique building block for the synthesis of different classes of molecular functional π-conjugated systems. In addition to a strong electron donor effect, EDOT gives rise to self-rigidification of linearly π-conjugated structures by means of intramolecular * To whom correspondence should be addressed. Telephone: 91-1125742610 Ext. 2283. Fax: 91-11-25726938. E-mail:
[email protected]. ernet.in. (1) Park, S. M. Electrochemistry of π-conjugated polymers. Handbook of Organic ConductiVe Molecules and Polymers ConductiVe Polymers: Spectroscopy and Physical Properties; Nalwa, H. S., Ed.; J. Wiley & Sons: New York, 1997; Vol. 3, p 429. (2) Higgins, S. J.; Lovell, K. V.; Rajapakse, R. M. G.; Walsby, N. M. J. Mater. Chem. 2003, 13, 2485-2489. (3) Han, M. G.; Foulger, S. H. Small 2006, 10, 1164-1169. (4) (a) Kirchmeyer, S.; Reuter, K. J. Mater. Chem. 2005, 115, 2077-2088. (b) Cho, S.; Choi, D. H.; Kim, S. H.; Lee, S. B. Chem. Mater. 2005, 17, 45644566. (5) Zhang, X.; Lee, J. S.; Lee, G. S.; Cha, D. K.; Kim, M. J.; Yang, D. J.; Manohar, S. K. Macromolecules 2006, 39, 470-472. (6) Feng, W.; Li, Yu.; Wu, J.; Noda, H.; Fujii, A.; Ozaki, M.; Yoshino, K. J. Phys.: Condens. Matter. 2007, 19, 186220-186228. (7) (a) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. AdV. Mater. 2000, 12, 481-494. (b) Hohnholz, D.; MacDiarmid, A. G.; Sarno, D. M.; Jones, W. E., Jr. Chem. Commun. 2001, 2444-2445.
noncovalent interactions between oxygen and sulfur, and this undoubtedly represents an important result for the synthetic chemistry of functional π-conjugated systems.8-10 This selfstructuring effect has successfully been used for the optimization of the optoelectronic properties of various classes of molecular functional π-conjugated systems. PEDOT based electrochromes are apparently faster and have a higher contrast ratio than any other existing polymer.11 Similarly, room-temperature ionic liquids (RTILs) have experienced a cometlike boost in the past few years in a plethora of chemical processes. RTILs are a new class of solvents where molecules are composed of ions and the charge on the cation and the counterion is distributed over a large volume of the molecule through resonance. As RTILs can exercise a strong influence on chemical or physical properties, they can be used as media for electropolymerization.12-17 The chemistry and the impact on film properties formed thereof are often controlled by the choice of anion. Environmentally friendly and electrochemically stable imidazolium cations with stable anions were used herein to synthesize ionic liquids. It is known that a hydrophobic medium facilitates deposition of uniform well-adherent thick films unlike highly polar media, which interact weakly with EDOT and are known to cause the polymers’ flaking off the substrate following growth. There is a small number of reports of similar phenomena, but all are limited to different ionic liquids.13,16,17 Contrary to doping of PEDOT in traditional organic electrolytes, wherein only the anions are incorporated in the polymer structure, here, when electropolymerized in ionic liquids, both the cation (imidazolium) and the anion (PFSI-) of the RTIL (8) Smith, R. R.; Smith, A. P.; Stricker, J. T.; Taylor, B. E.; Durstock, M. F. Macromolecules 2006, 39, 6071-6074. (9) Groenendaal, L. B.; Zotti, G.; Aubert, P. H.; Waybright, S. M.; Reynolds, J. R. AdV. Mater. 2003, 15, 855-878. (10) Roncali, J.; Blanchard, P.; Frere, P. J. Mater. Chem. 2005, 15, 15891610. (11) Cutler, C. A.; Bouguettaya, M.; Kang, T. S.; Reynolds, J. R. Macromolecules 2005, 38, 3068-3074. (12) Pang, Y.; Xu, H.; Li, X.; Ding, H.; Cheng, Y.; Shi, G.; Jin, L. Electrochem. Commun. 2006, 8, 1757-1763. (13) Lu, W.; Fadeev, A. G.; Qi, B.; Smela, E.; Mattes, B. R.; Ding, J.; Spinks, G. M.; Mazurkiewicz, J.; Zhou, D.; Wallace, G. G.; MacFarlane, D. R.; Forsyth, S. A.; Forsyth, M. Science 2002, 297, 983-987. (14) Fukushima, T.; Kosaka, A.; Ishimura, Y.; Yamamoto, T.; Takigawa, T.; Ishii, N.; Aida, T. Science 2003, 300, 2072-2074. (15) Dobbelin, M.; Marcilla, R.; Salsamendi, M.; Gonzalo, C. P.; Carrasco, P. M.; Pomposo, J. A.; Mecerreyes, D. Chem. Mater. 2007, 19, 2147-2149. (16) Wagner, K.; Pringle, J. M.; Hall, S. B.; Forsyth, M.; Macfarlane, D. R.; Officer, D. L. Synth. Met. 2005, 153, 257-260. (17) Randriamahazaka, H.; Plesse, C.; Teyssie, D.; Chevrot, C. Electrochem. Commun. 2004, 6, 299-305.
10.1021/la702442c CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007
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Langmuir, Vol. 23, No. 23, 2007 11431
Figure 1. XRD patterns of (a) SnO2/F coated glass substrate and (b) PEDOT grown on SnO2/F coated glass substrate.
form an integral part of the polymer matrix. RTILs based on an imidazolium cation and with different inorganic anions were used by us, and the best results were obtained with 1-ethyl-3methylimidazolium bis(perfluoroethylsulfonyl)imide (EMIPFSI) due to its lower symmetry. To this end, heretofore unreported, we used EMPFSI as the polymerizing medium and as a dopant for electropolymerization of EDOT. Our preliminary results confirm the need to devote more research efforts on the elucidation of the structure-property relationship in PEDOT grown in RTILs. Experimental Section The ionic liquid, 1-ethyl-3-methylimidazolium bis(perfluoroethylsulfonyl)imide (EMIPFSI), was prepared by using a previously reported method and was obtained as a transparent liquid.18 The working electrode, a SnO2/F coated glass plate obtained from Pilkington commonly known as “K-glass” (∼16 Ω/cm2) having a surface roughness of ∼4 nm, was placed vertically into the cell, completely through the ionic liquid containing 0.05 M EDOT (Aldrich), and the counter electrode used was a platinum rod (parallel to the working electrode). The film was grown under potentiostatic conditions, and a DC potential of +0.8 V was applied for 5 min versus a Ag/AgCl reference electrode at 25 °C. The film was removed from the growth solution, washed repeatedly with deionized water (DIW) and alcohol (1:1) solution, and then dried at room temperature for 5-6 h. The charge capacity of 0.025 C/cm2 allows the formation of uniform and outstanding quality films. Transmission and scanning electron microscopy images were recorded on JEOL, JEM 200 and LEO 440 microscopes, respectively. For TEM, the film was detached from the working electrode, diluted with N-methyl pyrrolidine, and plastered onto a carbon coated copper grid. X-ray diffraction (XRD) patterns were recorded on a Philips PW3710 instrument from 2θ ) 3 to 70° with a scan step of 0.02°. XPS was carried out on Perkin-Elmer 1257 model, operating at a base pressure of 6.66 × 10-8 Pa at 27 °C, with a non-monochromatized Mg KR line at 1253.6 eV, an analyzer pass energy of 60 eV, and a hemispherical sector analyzer of 25 meV resolution.
Results and Discussion The X-ray diffraction pattern recorded at a very low scan rate at a step of 0.02°/s of electropolymerized PEDOT is illustrated in Figure 1. Figure 1a illustrates the diffractogram of the SnO2/F coated glass plate alone, while Figure 1b represents the diffractogram of a PEDOT film grown on a SnO2/F coated glass plate. The peaks at higher 2θ values are due to tetragonal tin oxide originating from the SnO2/F substrate, while the distinguishing features of PEDOT nanofibers are the multiple diffraction (18) (a) Ahmad, S.; Deepa, M. Electrochem. Commun. 2007, 9, 1635-1638. (b) Susan, M. A. B. H.; Kaneko, T.; Noda, A.; Watanabe, M. J. Am. Chem. Soc. 2005, 127, 4976-4983.
Figure 2. (a) Morphological profile of the electrodeposited PEDOT fibers and (b) EDX elemental analysis of PEDOT films grown on conducting substrates, with PEDOT nanofibers shown in the inset.
peaks at low scattering angles of 2θ ) 5.74° (15.384 Å), 12° (7.369 Å), and 18° (4.924 Å), which are in good agreement with the diffraction data reported by other researchers.3,19 At lower 2θ values, due to a low signal-to-noise ratio, the peak at 5.74° is not easily perceptible. However, on the other hand, the polymer sample grown on SnO2/F coated glass plates is showing diffraction activity, and this can be easily differentiated by observing the pattern. Though the diffraction peak at 26.48° corresponding to a d-spacing of 3.363 Å along the (020) reflection plane (of polycrystalline PEDOT) involves overlapping contributions from the substrate as well, the polymer’s contribution can be easily realized on the basis of its higher intensity and lower full width at half-maximum (FWHM) (Figure 1b). This was attributed to the interchain planar ring stacking distance of the polymer.3,6,19 Polymerization of EDOT in an ionic liquid medium even by a simple method can induce remarkable changes in the morphology of PEDOT thus produced. The SEM image of PEDOT in (19) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Inganas, O.; Johansson, T.; Feidenhans, R. Synth. Met. 1999, 101, 561-564.
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Figure 3. (a) Electron diffraction pattern of PEDOT nanofibers and (b-d) TEM images of PEDOT nanorods and particles under different magnifications.
Figure 2a exhibits randomly distributed fine fiberlike structures with an average diameter of 20 nm. The fiber bundles composed of numerous single fibers (inset of Figure 2b), which, in all likelihood, nucleate from coarse nanoparticles, are heavily tangled with one another to form 3D networks. The primary particles act as an epicenter for the nucleation type growth of these fibers. When the external bias is applied, nucleation of the polymer is instantaneous and initially polymer particles are deposited onto the substrate. These act as a precursor phase or as heterogeneous nucleation sites, and the particles grow into elongated shapes (fibers), and this is assisted by the ionic liquid medium. The ionic liquid that steers the growth of the particulate fibrillar morphology is also apparent from the absence of any such structures when the film is grown in a conventional nonaqueous electrolyte (see the Supporting Information). A typical energy dispersion X-ray (EDX) graph of a PEDOT film grown on SnO2/F coated glass (Figure 2b) ratifies that the film embodies the pure polymer backbone along with the cation and anion of the ionic liquid, as strong signals from F, N, S, and O are seen which are due to the strong affinity of ionic liquids as a whole with the thiophenes. Elemental F and N signals are signature peaks for the ionic liquid. Figure 3a illustrates the electron diffraction pattern of the electrodeposited PEDOT, which displays well-resolved rings. One of the d-spacing values estimated from the diffraction pattern is 3.407 Å, which is very close to the value obtained from XRD, while the other calculated d-spacings are 2.916 and 2.23 Å and supplement the XRD data. The bright field images show faceted fiberlike shapes, which are misaligned with respect to each other and grow in all possible directions and show no specific orientation. The large polymer particles from which they emerge are the nucleating sites for these fibers with tapered ends. The diameter of these nanofibers is around 18 nm. It may be concluded that, under potentiostatic conditions, directed aggregation of polymer chains, which collapse onto the conducting substrate in a random manner, occurs with the fiberlike shapes confined to some domains in addition to the dense close packing of regular grains. The tail of the polymer deposit, chiefly comprising of the counterions (PFSI-) due to its large shape and extensive delocalization of negative charge, causes large separation between the polymer segments, and therefore, some portions of the film have an open ion accessible structure. The polymer structure also accommodates the bulky imidazolium cation, and the
presence of the latter reduces the effective charge density on the polymer and consequently steers the growth of the polymer film, which further leads to particles with varying shapes. The coexistence of particles with rectangular shapes and fibers indicates the RTIL directed faceted growth of PEDOT. The XPS general scan of the polymer film is shown in Figure 4. The core level spectrum of carbon, C1S, comprises of three main peaks, with the first one having peaks at 284.36, 285.36, 285.86, and 286.43 eV corresponding to C-C of imidazolium and C-C, C-S, and C-O of PEDOT, respectively. The second main peak at 290.1 eV arises from RTILs, and it has been deconvoluted into four peaks at 288.74, 289.85, 290.30, and 290.45 eV attributable to (i) the terminal methyl group of PFSI-, (ii) the C-N bond, where in the carbon atom it is that of the methylene/methyl group flanked with nitrogen in the imidazolium ring, and (iii) the N-C-N and (iv) N-C-C-N linkages of the imidazolium ring. Though these assignments clearly affirm the incorporation of the cation and the counterpart dopant anion in the polymer matrix, it is still unclear so as to how the bulky imidazolium ion attaches itself to the PEDOT backbone. The probability of its existence as free cations attracted electrostatically to the electron donating oxygen in the polymer chain far exceeds its ability to exist in the form of neutral undissociated molecules, as this is severely impeded by the extensive charge delocalization on the cation and the anion. The third prominent peak at 292.6 eV includes two components, with the lower binding energy component at 292.37 arising from the -CF2- moiety of PFSI- and its higher binding energy analogue at 292.90 eV being due to the -CF3- group of the anion. These assignments are fairly consistent with the reported binding energy values.20,21 Furthermore, the slight shifts in the peak positions and FWHMs, as compared to the literature values, originate from the different dopant moiety employed in the present study, as the use of this particular imide for doping PEDOT has not been reported to date. Contrary to literature reports, wherein the S2P core level spectrum comprises of two distinct peaks, here surprisingly a single, broad asymmetric peak is observed at 164.67 eV. In (20) Gottfried, J. M.; Maier, F.; Rossa, J.; Gerhard, D.; Schulz, P. S.; Wasserscheid, P.; Steinruck, H. P. Z. Phys. Chem. 2006, 220, 1439-1453. (21) Smith, E. F.; Gracia, I. J. V.; Briggs, D.; Licence, P. Chem. Commun. 2005, 5633-5635.
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Figure 4. XPS general scan of a PEDOT film grown in ionic liquid; insets show the core level N1S and C1S spectra.
concurrence with the reported data, this peak involves contributions from a spin-split doublet, S2P3/2 and S2P1/2, which possesses a 1.2 eV energy splitting and is characterized by a 2:1 intensity ratio. Accordingly, after fixing the aforementioned parameters in the overall curve fitting procedure, four components are revealed at 164.43 and 164.5 eV. These two are signature peaks of the sulfur atom in PEDOT and subpeaks at higher binding energies of 166.05 and 167.19 eV representing the sulfur atom in the dopant anion of PFSI-. The former doublet due to PEDOT appears at a lower binding energy, as sulfur is flanked by the two carbon atoms of the thiophene ring, whereas electronegative oxygen, nitrogen, and the CF2 group form the vicinal moieties of sulfur in PFSI-, which is responsible for the appearance of the S2P doublet at higher binding energies. The O1S signal is constituted by two components, with the first dominant one centered at 531.9 eV corresponding to the oxygen of the -SO2- group in PFSI- and the second one at 532.65 eV being ascribed to the oxygen of the -C-O-Cmonomer bonds in the polymer chain. The deconvolution of the F1S signal (at 688.7 eV) to identify the independent contributions from -CF2- and -CF3- groups of the dopant anion PFSI- is inhibited by the proximity of fluorine atoms, with nearly equivalent carbon atoms. Such a signature F1S peak has previously been observed in the high-resolution XPS spectrum of another fluorine containing RTIL.22 The two distinctive energetically well-separated (insets of Figure 4) imidazolium cation and the anion (PFSI-) emissions for N1S are observed at 402.18 and 399.42 eV, respectively, which reconfirms the presence of imidazolium moieties in the polymer film. This binding energy separation of 2.76 eV and the possible source of the origin for the appearance of N1S components of the (22) Hofft, O.; Bahr, S.; Himmerlich, M.; Krischok, S.; Schaefer, J. A.; Kempter, V. Langmuir 2006, 22, 7120-7123.
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imidazolium ion at a higher binding energy as compared to its analogue arising from the PFSI- anion are the effective shielding of the imidazolium ring by its alkyl groups. The calculated atomic ratios IC285/ICtotal = 0.47 and IO533/ ICtotal = 0.36 for PEDOT are close to the values of 0.50 and 0.24 observed for PEDOT doped by sodium dodecyl sulfate (SDS) in a micellar aqueous solution.23 The ratio of ICtotal/ISthiophene = 8.04, which is higher than the theoretical atomic ratios of 6.0 for PEDOT. The doping level of the synthesized PEDOT has been calculated in a similar fashion as that by Sakmeche et al.23 The doping level of PEDOT by PFSI- calculated from the atomic ratio of IN(PFSI-)/IS(thiophene) is 0.32, and this obtained value is quite close to the level of 0.24 determined from the IS(PFSI-)/ IS(thiophene) ratio. A doping level of 0.3 has been reported for PEDOT doped by tetrabutylammonium hexafluorophosphate in acetonitrile,24 and a value of 0.5 was observed by Sakemeche et al.23 for SDS doped PEDOT. Such a high doping level endows the film with superior redox activity, which will be demonstrated in future studies.
Conclusions In summary, we have demonstrated a facile and convenient method to synthesize PEDOT films encompassing nanofibers via electropolymerization of EDOT in an extremely hydrophobic ionic liquid medium without the use of any additional dopant. The incorporation of the imidazolium cation along with the PFSIanion in the polymer film matrix, affirmed by EDX and XPS studies, is responsible for the formation of films with a fibrillar structure. Further investigations on their bulk and electro-optical properties are underway to enable their use for different applications. We believe the present work will pave a way for the development of a new generation of nanostructured photonic and electronic devices. Acknowledgment. The authors gratefully acknowledge Mr. Abdullah for XRD and Dr. Govind for XPS measurements, and S.A. acknowledges CSIR for financial assistance and Dr. S. T. Lakshmi Kumar for necessary help in carrying out the experiments. Supporting Information Available: Cyclic voltammograms of the grown PEDOT films in EMIPFSI, SEM micrographs of a PEDOT film grown in conventional medium, and synthesis of the ionic liquid (EMIPFSI). LA702442C (23) Sakmeche, N.; Aeiyach, S.; Aaron, J. J.; Jouini, M.; Lacroix, J. C.; Lacaze, P. C. Langmuir 1999, 15, 2566-2574. (24) Randriamahazaka, H.; Noel, V.; Chevrot, C. J. Electroanal. Chem. 1999, 472, 103-111.
