Organic Single-Crystal Surface-Induced Polymerization of Conducting

pubs.acs.org/Langmuir © 2009 American Chemical Society

Organic Single-Crystal Surface-Induced Polymerization of Conducting Polypyrroles Sang Soo Jeon,† Jun Kyu Park,† Chong Seung Yoon,‡ and Seung Soon Im*,† †

Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-dong, Seoul, 133-791 Korea, and ‡Division of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seoul, 133-791 Korea Received May 1, 2009. Revised Manuscript Received July 7, 2009 Polypyrrole hexagonal microplates (PHMs) (50-100 μm long, 10-20 μm wide, and 0.8-1.2 μm thick) with a quasicrystalline structure and high electrical conductivity (up to 400 S/cm) are simply fabricated using single crystals of 4-sulfobenzoic acid monopotassium salt (KSBA) in aqueous medium. Moreover, the fabrication process described here differs strikingly from traditional methods, such as template-free, soft template, and hard template methods. Synthetic time-resolved polypyrrole (PPy) morphology dynamics reveals that the fabrication process of PHMs composed of PPy nanostructures combines a shape-copying process for forming a PPy preform that imitates the shape of a KSBA single crystal and the self-assembly process of PPys on the preform. The PHMs exhibit the improved π-stacking and bipolaron structure. The strong π-stacks among PPy rings of bipolaron structures lead to a high quasicrystalline structural order and the metallic conduction. Other single organic crystals that can act as dopants could also be grown using this approach, which will also enable the fabrication of complex micro/nanostructures on organic single crystals.

Introduction Conducting polymer nanostructures and microstructures composed of nanosized structures have attracted much attention owing to their unique properties originating from their size, surface, and quantum effect and promising potential applications in various electronic devices.1-6 However, most studies on conducting polymer micro/nanostructures have been limited to their fibrous, tubular,2,3,7-11 or particulate12-16 structures fabricated using a template-free method3,7 or hard/soft template synthesis.8-17 The template-free method includes interfacial and dispersion polymerization of aniline monomers. Hard template methods use the pores of anodic aluminum oxide, tracketched polycarbonate, and zeolite channels. On the other hand, *Corresponding author. Phone: þ82-2-2220-0495. Fax: þ82-2-2297-5859. E-mail: [email protected]. (1) MacDiarmid, A. G. Synth. Met. 1997, 84, 27–34. (2) Jang, J.; Yoon, H. Adv. Mater. 2003, 15, 2088–2091. (3) Huang, J. X.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314–315. (4) Akkerman, H. B.; Blom, P. W.; de Leeuw, D. M.; de Boer, B. Nature 2006, 441, 69–72. (5) M€oller, S.; Perlov, C.; Jackson, W.; Taussig, C.; Forrest, S. R. A. Nature 2003, 426, 166–169. (6) Jurewicz, K.; Delpeux, S.; Bertagna, V.; Beguin, F.; Frackowiak, E. Chem. Phys. Lett. 2001, 347, 36–40. (7) Jang, J.; Bae, J.; Lee, K. Polymer 2005, 46, 3677–3684. (8) Wu, C. G.; Bein, T. Science 1994, 264, 1757–1759. (9) Martin, C. R. Chem. Mater. 1996, 8, 1739–1746. (10) Han, M. G.; Foulger, S. H. Small 2006, 2, 1164–1169. (11) Jang, J.; Yoon, H. Chem. Commun. 2003, 720–721. (12) Jang, J.; Oh, J. H. Adv. Funct. Mater. 2005, 15, 494–502. (13) Marinakos, S. M.; Shultz, D. A.; Feldheim, D. L. Adv. Mater. 1999, 11, 34–37. (14) Kim, B. J.; Oh, S. G.; Han, M. G.; Im, S. S. Langmuir 2000, 16, 5841–5845. (15) Choi, J. W.; Han, M. G.; Kim, S. Y.; Oh, S. G.; Im, S. S. Synth. Met. 2004, 141, 293–299. (16) Jang, J.; Oh, J. H.; Stucky, G. D. Angew. Chem. 2002, 114, 4188–4191. Jang, J.; Oh, J. H.; Stucky, G. D. Angew. Chem., Int. Ed. 2002, 41, 4016–4019. (17) Yanai, N.; Uemura, T.; Ohba, M.; Kadowaki, Y.; Maesato, M.; Takenaka, M.; Nishitsuji, S.; Hasegawa, H.; Kitagawa, S. Angew. Chem. 2008, 120, 10031– 10034. Yanai, N.; Uemura, T.; Ohba, M.; Kadowaki, Y.; Maesato, M.; Takenaka, M.; Nishitsuji, S.; Hasegawa, H.; Kitagawa, S. Angew. Chem., Int. Ed. 2008, 47, 9883– 9886.

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surfactants, liquid crystals, and block copolymers can be used as soluble soft templates. Although efficient fabrication routes for conducting polymer micro/nanostructures have been devised and can be used to produce various micro/nanostructures with unique properties, adapting these methods to synthesize various conducting polymer micro/nanostructures with unique properties has become a fundamental issue in the field. Therefore, controlling the morphology and size of conducting polymer micro/nanostructures using a simple, effective method remains a challenging goal. In particular, the development of methods for producing anisotropic arrays or ordered growth from nano- to microsized structures is an important technology to fabricate micro/nanodevices for template and various electronic applications. Recently, Suarez-Herrera and Feliu reported that electropolymerzation of polypyrrole and polythiophene in aqueous and nonaqueous media, respectively, is a structure-sensitive process on platinum single-crystal electrodes with basal orientations.18,19 The platinum surface structure induced enhanced anisotroic properties of the synthesized conducting polymer films. Herein, we report on the organic single-crystal surface-induced polymerization of conducting polypyrrole hexagonal microplates (PHMs) (50-100 μm long, 10-20 μm wide, and 0.8-1.2 μm thick) with an improved structural order and high electrical conductivity (up to 400 S/cm), using organic single crystals of 4-sulfobenzoic acid monopotassium salt (KSBA) as the soluble template. To the best of our knowledge, the hexagonal platelike morphology of the micro/nanostructured conducting polymers has been obtained for the first time, in which the ordered πstacking structure allows for high electrical conductivity. Significantly, the fabrication process described here differs from traditional methods, such as soft template and hard template methods. (18) Suarez-Herrera, M. F.; Feliu, J. M. Phys. Chem. Chem. Phys. 2008, 10, 7022–7030. (19) Suarez-Herrera, M. F.; Feliu, J. M. J. Phys. Chem. B 2009, 113, 1899–1905.

Published on Web 08/14/2009

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This peculiar morphology and high electrical conductivity motivated us to study the mechanism of formation of the PHMs and characterize their structure. The formation mechanism of conducting PHMs was studied by synthetic time-resolved morphology dynamics using scanning electron microscopy (SEM) and Xray diffraction (XRD).

Experimental Section Materials. Pyrrole monomer (Sigma-Aldrich, 99%) was vacuum-distilled and stored at 2 °C. The oxidant, iron(III) chloride (Riedel, 98%), was dissolved in water and filtered using a syringe filter with 0.1 μm pores. 4-Sulfobenzoic acid monopotassium salt (KSBA) (Sigma-Aldrich, 95%) was purified using the recrystallization method in water and acetone. Synthesis of PHMs. The PHMs were synthesized by dissolving 0.015 mol of KSBA and 0.015 mol of Py monomer in 100 mL of purified water and stirring at room temperature for 2 h. The solution was kept at the desired temperature (0, 5, 10, or 15 °C) with vigorous stirring for 1 h. An oxidant solution, 0.02 mol of iron(III) chloride dissolved in 20 mL of distilled water, was added to this solution at a rate of 15 mL/h. The polymerization solution was stirred magnetically, and polymerization was allowed to proceed for 5 h. The precipitated polypyrroles (PPys) were filtered and washed with distilled water and acetone several times and then dried in a vacuum oven at 60 °C for 24 h. To compare the properties of the PHMs to those of the bulk PPys, pure PPys were synthesized using the same procedure in the absence of KSBA. Characterization. The electrical conductivity (298 K) of a compressed pellet of the dried precipitated PPys was measured using the four-point microprobe method using a Jandel contact probe connected to a Keithley 238 high-current source measure unit. The electrical conductivity data in the inset to Figure 2d were measured from three independent samples of a single synthesis. The morphological changes in the KSBA crystals and synthesized PPys were observed using SEM (JEOL, JSM6340) and TEM (JEOL 2010). To check the crystal structures of the PPy samples, wide-angle X-ray diffraction measurements were carried out on a Rigaku Denki X-ray generator (Rigaku, D/MAX-2500), using Cu KR (λ=1.5418 A˚) radiation operated at 40 kV and 100 mA. The scan angle covered 5° < 2θ < 50° (2θ is the scattering angle, θ is the Bragg angle) at a speed of 5°/min. Fourier transform infrared (FT-IR) spectra in attenuated total reflection (ATR) mode were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer.

Results and Discussion Most organic acid salts that can act as dopants, including KSBA,20,21 exhibit crystallographic polymorphism, and the crystal size can range from tens of nanometers to several micrometers depending on temperature, concentration, and solvent composition. We have found that 0.15 M KSBA is precipitated as single microsized crystals, shaped like hexagonal plates in an aqueous medium at temperatures below 7 °C. In addition, when a Py monomer was added to the KSBA solution together with iron(III) chloride as an oxidant, the Py was polymerized with a hexagonal platelike morphology in imitation of the KSBA single-crystal shape. SEM images show that the morphology and size of the PPys are similar to those of the KSBA single crystal (Figure 1). In addition, we discovered that typical PHMs are formed only when polymerized in the presence of hexagonal KSBA single crystals in an aqueous medium (Figure 2a). The formation of KSBA crystals at a solution temperature lower than the crystallization temperature of 0.15 M KSBA (7 °C) results in sudden turbidity observable to the naked eye (Figure 2a, inset). Although KSBA crystals were formed at 0 °C, well-defined PHMs were not (20) Gunderman, B. J.; Squattrito, P. J. Inorg. Chem. 1994, 33, 2924–2931. (21) Kariuki, B. M.; Jones, W. Acta Crystallogr. 1995, C51, 867–871.

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Figure 1. Typical SEM images of (a) a KSBA single crystal before polymerization and (b) a PHM after polymerization at 0 °C.

Figure 2. (a-c) SEM images (scale bar = 10 μm) of PPys synthesized under different conditions: (a) mechanical stirring at 0 °C, (b) unperturbed state at 0 °C, and (c) mechanical stirring at 15 °C. The insets in (a-c) show the turbidity of the polymerization solution before adding the oxidant. (d) XRD patterns of samples polymerized at various temperatures. The inset in (d) shows that conductivity decreases as the polymerization temperature increases.

produced without vigorous mechanical stirring (Figure 2b and inset). Polymerization temperatures above 7 °C, at which temperature KSBA crystals do not form, produce agglomerates of PPy nanoparticles instead of the microplates (Figure 2c and inset). Additional SEM images of the PPys synthesized under different temperatures are included in the Supporting Information (Figure S1). The XRD patterns of PPys synthesized at various temperatures are shown in Figure 2d. The reported typical XRD patterns of chemically synthesized pristine PPys have a broad scattering amorphous structure composed of two or three indistinct peaks ranging from 2θ=10° to 2θ=35°,22,23 whereas the XRD patterns (22) He, C.; Yang, C.; Li, Y. Synth. Met. 2003, 139, 539–545. (23) Song, M. K.; Kim, Y. T.; Kim, B. S.; Kim, J.; Char, K.; Rhee, H. W. Synth. Met. 2004, 141, 315–319.

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of our samples synthesized in the presence of single KSBA crystals are quite different. In particular, these XRD patterns can be deconvoluted into two peaks (for example, see the XRD pattern of the sample synthesized at 15 °C in Figure 2d). As the polymerization temperature decreases from 15 to 0 °C, the amorphous shoulder peak at 2θ ≈ 21.5° decreases gradually, whereas the peak at 2θ ≈ 25.9° sharpens progressively and shifts toward the higher 2θ region (25.9° f 26.5°). In addition, the electrical conductivity increases to the maximum value of 412 S/ cm as the sharpness of the XRD peak at 26.5° increases (Figure 2d, inset). The peak at 26.5° (d-spacing ∼3.4 A˚) is associated with the average face-to-face distance of the inter-Py rings in the π-stacking of aromatic rings.24-27 This π-stacking is a strong noncovalent interplanar interaction that shares π orbitals among aromatic ring molecules in π-conjugated systems.28 The essentially planar, but not necessarily linear, PPy molecules24-27 can also form strong π-stacks with the assistance of aromatic and planar dopants, such as KSBA crystallized by π-stacking.29 (The mole fraction of the doped KSBA except for Cl- measured by Xray photoelectron spectroscopy and elemental analysis was ∼10% in all PPy samples polymerized in the presence of the KSBA, but data were not shown.) The sharper, stronger peak at 2θ ≈ 26.5° together with the peak shift toward the higher 2θ region with decreasing polymerization temperature reveals a more quasicrystalline, enhanced π-stacking order between oxidized Py rings. A similar result in which a more effective π-π interaction among the aromatic rings improved the electrical conductivity was also reported for polyaniline.30 Therefore, the increased degree of π-stacks for PPy chains likely contributed to the observed enhancement in electrical conductivity. The FT-IR spectra (see the Supporting Information, Figure S2) of the synthesized polypyrroles exhibited the characteristic PPy peaks at around 1573 and 1480 cm-1, indicative of the antisymmetric ring stretching mode and symmetric mode in the Py ring, respectively. The two peaks around 1212 and 934 cm-1 were attributed to C-H vibration mode in Py rings. Peaks at 1050 and 1331 cm-1 were attributed to C-H and N-H deformation vibration modes. These spectral characteristics mean that the PPys were successfully synthesized in the presence of the KSBA. Although the samples synthesized at different temperatures and doped with iron(III) chloride in the absence of KSBA exhibited widely varying electrical conductivity, a similar doping level (∼0.25 mol %) estimated from the N 1s core level spectra analysis (Nþ/N) using X-ray photoelectron spectroscopy was observed in all samples (see the Supporting Information, Figure S3). Surprisingly, electron spin resonance (ESR) spectroscopy studies (Figure 3) show that no ESR signal is observed in PPys synthesized in the presence of KSBA, whereas it is observed in the sample without KSBA. This indicates that the oxidation states of the samples synthesized in the presence of KSBA are spinless bipolaron structures. These results are in good agreement with the fact that little or no ESR signal is detected in metallic PPys.27,31 Therefore, the charge carriers responsible for conduction between (24) Geiss, R. H.; Street, G. B.; Volksen, W.; Economy, J. IBM J. Res. Dev. 1983, 27, 321–329. (25) Nogami, Y.; Pouget, J. P.; Ishiguro, T. Synth. Met. 1994, 62, 257–263. (26) Warren, M. R.; Madden, J. D. Synth. Met. 2006, 156, 724–730. (27) Saunders, B. R.; Fleming, R. J.; Murray, K. S. Chem. Mater. 1995, 7, 1082– 1094. (28) Sygula, A.; Fronczek, F. R.; Sygula, R.; Rabideau, P. R.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 3842–3843. (29) Wynne, K. J.; Street, G. B. Macromolecules 1985, 18, 2361–2368. (30) Lee, K.; Cho, S.; Park, S. H.; Heeger, A. J.; Lee, C. W.; Lee, S. H. Nature 2006, 441, 65–67. (31) Scott, J. C.; Pfluger, P.; Krounbi, M. T.; Street, G. B. Phys. Rev. B 1983, 28, 2140–2145.

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Figure 3. Electron spin resonance spectra of the samples. (a, b) PPys synthesized at 0 and 15 °C in the presence of KSBA, respectively. (c) PPys synthesized at 0 °C without KSBA. There is no ESR peak reflecting the existence of radical species in the samples synthesized in the presence of KSBA.

and within PPy chains cannot be polarons. Instead, the charge carriers are likely spinless bipolarons. On the basis of the facts that have been clarified so far, we conclude that stronger π-stacks among PPy rings of bipolaron structures give a higher quasicrystalline structural order and longer conjugation length with decreasing polymerization temperature. These properties coincide with the qualitative measurement of the effective conjugation length obtained from vibrational spectra based on the effective conjugation coordinate theory (see the Supporting Information, Figure S4).32,33 It is thought that the effect of strong π-stacking leads to the metallic conduction26,30 of the quasicrystalline PPys through effective interchain transport by electrons hopping between bipolarons as charge carriers. The improved ordering of PPy chains can also be attributed to a slow polymerization rate as a result of the low polymerization temperature and dropwise addition of oxidant. As the polymerization temperature decreases to 0 °C, KSBA molecules are precipitated as single crystals in an aqueous solution containing Py monomers as the saturation solubility is reached. On the dropwise addition of iron(III) chloride, the growing PPy chains remain in an oxidized state. The PPy chains then have sufficient time for physical bonding to the counteranions (-SO3-) exposed on the KSBA crystal surface as a doping group with vigorous stirring. Because strong π-π interactions generally play an important role in molecular self-assembly, KSBA crystals with strong π-π interactions induce π-stacks between pyrrole rings in the same manner as the KSBA molecules20,21 during PPy chain growth. Therefore, it follows that PPys are obtained in π-stacked, quasicrystalline form, resulting in a perfect PHM imitating the shape of a KSBA single crystal. Synthetic time-resolved SEM and XRD studies confirmed the ordered structure and hexagonal shape of PPys produced at 0 °C (32) Tian, B.; Zerbi, G. J. Chem. Phys. 1989, 92, 3886–3891. (33) Tian, B.; Zerbi, G. J. Chem. Phys. 1989, 92, 3892–3898.

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Figure 4. (a) Sequential SEM images (scale bar = 10 μm) showing the evolution of the PHMs at 0 °C. The polymerization time is indicated in each image. (b) XRD patterns of PPys for different polymerization times. The residual KSBA was removed completely by washing and filtering. The inset in (b) is the XRD pattern of the small angle (2θ) region.

Figure 5. SEM images of the KSBA crystal at given polymerization times t (min). (a) The magnified image of the circle in Figure 4a; t = 10 min. The white spots are PPy nanoparticles. (b) The surface of the KSBA crystal at t = 30 min. The cobweb-like PPys are fabricated by the aggregation of nanoparticles along the surface cracks.

(Figure 4a,b). Before adding iron(III) chloride, hexagonal platelike KSBA single crystals are visible in the polymerization solution (Figure 4a, t = 0 min). The crystal structure of KSBA obtained by recrystallization at 0 °C is monoclinic (β=94.96°), and the b axis corresponding to the stacking order of the aromatic rings is perpendicular to the KSBA crystal top surface.20,21 Langmuir 2009, 25(19), 11420–11424

Figure 6. TEM image of the pure PPys after washing at a polymerization time of t = 30 min. (a) The cobweb-like PPy nanofibers fabricated on the surface of a single KSBA crystal. (b) A PPy surface made of nanoparticles tens of nanometers in size fabricated on the side of a single KSBA crystal.

Consequently, exposed free active sites, such as sulfonate and carboxylic acid groups, are located chiefly on the sides of the KSBA crystal, rather than on the top surface. For this reason, oxidized PPys nucleate on the sides rather than on the top surface during the initial polymerization stage (Figure 4a, t=10 min, and DOI: 10.1021/la901563n

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separated from the KSBA crystal and then transforms into the preform structures by folding its sides (Figure 4a, t = 60 min). Because the π-stacking is a relatively strong interaction, residual Py monomers are mainly polymerized and self-assembled on the inner surface of the preform, which has more active π-stacking polymerization sites than the folded sides, while maintaining the ordered π-stacking structure (Figure 4a, t = 90 min). This is possible with the assistance of planar KSBA molecules in saturated solution when considering the highly anisotropic KSBA crystal structure. As a consequence of further polymerization, an XRD peak develops at 2θ=26.5° (Figure 4b, t=45-300 min), resulting from the perfect PHMs (Figure 4a, t = 300 min). The typical formation process for fabricating PHMs that imitates the shape of a KSBA single crystal is summarized schematically in Figure 7.

Conclusions Figure 7. Schematic diagram of the formation of PHMs. (a) KSBA single crystals are created by recrystallization at the saturation temperature in an aqueous medium. (b) On initiating polymerization, the top and bottom of the single KSBA crystals are covered with cobweb-like PPy nanowires and the sides are coated with fine PPy nanoparticles, forming smooth sides. (c) PPys are peeled off by dissolving the KSBA crystal and fragmenting it into small pieces with vigorous stirring; they are then transformed into preform structures when their sides fold. (d) PHMs are formed, covering the surfaces with the polymerization of residual Py monomers.

Figure 5a, magnified image of the circle in Figure 4a, t=10 min). PPys produced in this stage have a fully amorphous structure, as clearly seen in XRD patterns showing broad scattering at the peaks 2θ=10-35° (Figure 4b, t=10 min). As polymerization proceeds for up to 45 min, the sides become more densely covered with semiordered PPy nanoparticles tens of nanometers in size. The semiordered PPys may be responsible for the structural order of the sulfonate groups exposed on the sides of the single KSBA crystal. Evidence for the semiordered PPy nanoparticles on the sides of the single KSBA crystal is provided by the development of peaks under 2θ < 5° (Figure 4b, t=3045 min, and Figure 4b, inset). By contrast, the top surface is covered with cobweb-like PPy nanofibers that self-assemble from aggregated PPy nanoparticles under 100 nm in size (Figure 4a, t= 30 min, and Figure 5b). This phenomenon is readily explained in terms of the generation of the fine cracks on the KSBA crystal surface. Adding the oxidant solution to the polymerization mixture gradually generates these cracks through dissolution or etching of the KSBA crystal surface due to the increased solubility and the ionic strength of the FeIII ions. Because the crack edges are the same crystal orientation as that of the sides of the KSBA crystal, the cobweb-like PPy nanofibers preferentially nucleate at exposed cracks containing free sulfonate groups (Figure 5b). A difference in the morphology of the PPys fabricated on the side and top surfaces of the KSBA crystal at t=30 min is clearly seen in the transmission electron microscopy (TEM) image of the pure PPys (Figure 6) after washing and filtering it with acetone and water several times. After prolonged polymerization (t > 45 min), the continuous dissolution of the KSBA crystal surfaces weakens the interactions between the KSBA crystal surface and PPys. A KSBA crystal with many cracks fragments into small pieces on vigorous stirring (see the Supporting Information, Figure S5). Therefore, the PPy structure patterned after the pre-existing KSBA crystal is easily

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Compared with other methods used to fabricate conducting polymer nanostructures, the characteristics of organic crystal surface-induced polymerization of conducting PPys can be described as follows: (i) Our facile approach combines the shapecopying process as a quasi-template process with a self-assembly process. (ii) The produced PPys imitate the form of a single KSBA crystal. (iii) PHMs with a quasicrystalline structure and high electrical conductivity can be produced. Moreover, our method is not limited to the KSBA single-crystal system but is also applicable to other suitable dopant systems that can form various crystals. Although we have verified the fabrication and formation mechanism of PHMs based on a shape-copying effect, in principle, it is possible to obtain other PPy micro/nanostructures using the same approach if the crystal form is changed through polymorphism of the KSBA crystal under specific conditions. Preliminary experiments on PPys and other organic single crystals, such as 4-sulfobenzoic acid sodium salt, 3-sulfobenzoic acid sodium salt, and 2-sulfobenzoic acid ammonium salt, indicate that organic crystal surface-induced polymerization of conducting polymers is able to duplicate their shapes when the organic single crystals are present in polymerization medium. The unique conducting polymer microstructures composed of nanosized PPy structures with high electrical quality discussed here should have applications in a wide range of conducting polymers. These applications include sensors, templates, catalyst supports, and substitutes for carbon materials. In addition to hard template, soft template, and template-free methods, our proposed method provides another systematic approach to the fabrication of conducting polymer micro/nanostructures. Acknowledgment. This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (Grant No. R11-2008-08803003-0) and the research fund of Hanyang Universitiy (HYU2009-T). Supporting Information Available: FT-IR spectra of synthesized polypyrrole samples, N 1s core level spectra of X-ray photoelectron spectroscopy, qualitative measurement of the conjugation length obtained from IR spectra based on the effective conjugation coordinate theory, and SEM image of the KSBA crystals at polymerization time t=60 min. This material is available free of charge via the Internet at http:// pubs.acs.org.

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