Formation Mechanism of Conducting Polypyrrole Nanotubes in

The reverse cylindrical micelle phase was characterized, and the key factors affecting the formation of PPy ... Effect of Hydrophobicity of Monomers o...
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Formation Mechanism of Conducting Polypyrrole Nanotubes in Reverse Micelle Systems Jyongsik Jang* and Hyeonseok Yoon Hyperstructured Organic Materials Research Center, School of Chemical and Biological Engineering, College of Engineering, Seoul National University, Shinlimdong 56-1, Seoul 151-742, Korea Received June 2, 2005. In Final Form: September 11, 2005 Polypyrrole (PPy) nanotubes were readily fabricated through chemical oxidation polymerization in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse (water-in-oil) emulsions. The reverse cylindrical micelle phase was characterized, and the key factors affecting the formation of PPy nanotubes were systematically inspected. AOT reverse cylindrical micelles were prepared via a cooperative interaction between an aqueous FeCl3 solution and AOT in an apolar solvent. In the H2O/FeCl3/AOT/apolar solvent system, the aqueous FeCl3 solution played a role in increasing the ionic strength and decreasing the second critical micelle concentration of AOT. As a result, AOT reverse cylindrical micelles could be spontaneously formed in an apolar solvent. In addition, iron cations were adsorbed to the anionic AOT headgroups that were capable of extracting metal cations from the aqueous core. Under these conditions, the addition of pyrrole monomer resulted in the chemical oxidation polymerization of the corresponding monomer at the surface of AOT reverse cylindrical micelles, followed by the formation of tubular PPy nanostructures. In a typical composition (74.0 wt % hexane, 22.4 wt % AOT, and 3.6 wt % aqueous FeCl3 solution at 15 °C), the average diameter of PPy nanotubes was approximately 94 nm and their length was more than 2 µm. The PPy nanotube dimensions were affected by synthetic variables such as the weight ratio of aqueous FeCl3 solution/AOT, type of apolar solvent, and reaction temperature. Moreover, the relationship between the diameter and the conductivity of the nanotubes was investigated.

Introduction A variety of nanotubes that are one-dimensional (1-D) nanostructures with hollow interiors have attracted a great deal of interest from the viewpoint of scientific research and practical applications. They possess novel physical and chemical properties derived from structural versatility and provide opportunities for advanced applications in the fields of electronics, optics, catalysis, energy storage, and biological systems.1-5 Compared with other nanostructures (e.g., spheres, rods, and fibers), it is quite difficult to fabricate tubular nanostructures due to their exquisite morphology. Nevertheless, various nanotubes have been made of carbon, ceramics, metals, conducting polymers, and so forth. Most approaches for fabricating these nanotubes were focused on the use of hard templates such as porous polymer films, anodic aluminum oxide (AAO) membranes, and mesoporous silica.6-9 Martin et al. pioneered the template synthesis using AAO and track-etched a polycarbonate membrane.10 They fabricated various nanotubes and nanofibers through the infiltration of cylindrical pores with an appropriate * To whom correspondence should be addressed. Phone: 82-2880-7069. Fax: 82-2-888-7295. E-mail: [email protected]. (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (2) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (3) Zhang, X.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 12714. (4) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (5) Hughes, M.; Chen, G. Z.; Shaffer, M. S. P.; Fray, D. J.; Windle, A. H. Chem. Mater. 2002, 14, 1610. (6) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wehrspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Go¨sele, U. Science 2002, 296, 1997. (7) Demoustier-Champagne, S.; Stavaux, P.-Y. Chem. Mater. 1999, 11, 829. (8) Jang, J.; Oh, J. H. Chem. Commun. 2004, 882. (9) Jang, J.; Lim, B.; Lee, J.; Hyeon, T. Chem. Commun. 2001, 83. (10) Martin, C. R. Acc. Chem. Res. 1995, 28, 61.

precursor, followed by the conversion of this precursor to the desired materials. Hard template synthesis can provide nanotubes with a controlled diameter and length. However, this approach requires complicated synthetic steps including the dissolution of the template in corrosive media. Therefore, the use of hard templates has the potential drawback that scaleup for industrial applications is highly difficult. Recently, soft template synthesis has emerged as an alternative strategy against hard template synthesis. Grady and co-workers synthesized conducting polymer nanostructures with controlled morphologies on flat surfaces using adsorbed surfactant molecules as the template.11 In addition, Stupp et al. prepared various semiconductor superlattices by direct templating in a lyotropic liquid crystal.12 In the case of tubular nanostructures, soft templates such as surfactants, organic crystals, and organogels have been limitedly employed. Mann and co-workers prepared hollow silica fibers using organic crystal templating,13 and Shinkai et al. have employed organic gelators (e.g., cholesterol-based gelators and sugar-appended azonaphthol gelators) to prepare tubular silica structures.14 In terms of surfactant templating, diverse reverse micelles have been utilized as the nanoreactor for synthesizing nanomaterials.15-21 A reverse micelle is defined (11) Carswell, A. D. W.; O′Rear, E. A.; Grady, B. P. J. Am. Chem. Soc. 2003, 125, 14793. (12) Braun, P. V.; Osenar, P.; Tohver, V.; Kennedy, S. B.; Stupp, S. I. J. Am. Chem. Soc. 1999, 121, 7302. (13) Miyaji, F.; Davis, S. A.; Charmant, J. P. H.; Mann, S. Chem. Mater. 1999, 11, 3021. (14) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (15) Cao, M.; Wang, Y.; Guo, C.; Qi, Y.; Hu, C. Langmuir 2004, 20, 4784. (16) Yoo, S. I.; Sohn, B.-H.; Zin, W.-C.; Jung, J. C. Langmuir 2004, 20, 10734. (17) Shi, H.; Qi, L.; Ma, J.; Cheng, H. J. Am. Chem. Soc. 2003, 125, 3450.

10.1021/la051447u CCC: $30.25 © 2005 American Chemical Society Published on Web 10/19/2005

Formation Mechanism of Polypyrrole Nanotubes

as an aggregate of surfactant molecules containing a nanometer-sized water pool in the oil phase. The most widely used surfactant to form reverse micelles is sodium bis(2-ethylhexyl) sulfosuccinate (AOT), which is an anionic surfactant with two hydrophobic tailgroups. AOT molecules favorably form reverse micelles in the oil phase because of their bulky hydrophobic tailgroups compared with the hydrophilic headgroups. A variety of methods have been exploited to prepare metal, ceramic, and polymeric nanomaterials in AOT reverse emulsions.22 BaCrO4 nanofilaments, BaSO4 nanofibers, and BaWO4 nanorods have been synthesized by using Ba(AOT)2 reverse micelles.23 Moreover, silver nanowires were formed via electrodeposition in the AOT reverse hexagonal liquidcrystalline phase.24 These materials could be synthesized in the form of wires or fibers inside the reverse cylindrical micelles. Conducting polymer nanotubes are fascinating materials with versatile functions that originate from their 1-D hollow structure, high surface area, and enhanced transport property. However, to date, there has been only limited information with respect to soft template synthesis for the production of conducting polymer nanotubes. PPy is one practically developed conducting polymer owing to its inherent characteristics of high electrical conductivity, redox property, and easy synthesis. Several research groups have reported the chemical polymerization of PPy in the presence of AOT. Omastova´ et al. found that the addition of AOT during the polymerization reaction of pyrrole brought about an enhancement in conductivity,25 and Ruckenstein et al. proposed the formation of PPy particles using interfacial polymerization.26 Recently, we reported the facile fabrication of PPy nanotubes using AOT reverse cylindrical micelle templating for the first time.27 This methodology does not require an atypical temperature and pressure, and the simple process offered a great possibility to produce PPy nanotubes in large quantities. Here, we describe the formation mechanism of PPy nanotubes in AOT reverse micelle systems. The AOT reverse cylindrical micelle phase was characterized, and the factors affecting the formation of PPy nanotubes were systematically investigated. Experimental Section Materials. AOT (98%) and ferric chloride (FeCl3; 97%) were purchased from Aldrich Chemical Co. AOT was analyzed by 1H nuclear magnetic resonance (NMR) spectroscopy to investigate the possible hydrolysis of AOT into sulfosuccinic acid and 2-ethylhexanol.28 The alcoholic signal could not be observed in the 1H NMR spectrum of AOT, and this fact indicated that there was no hydrolysis in the sample. Apolar solvents such as hexane (99%), heptane (99+%), and octane (99+%) were also obtained from Aldrich Chemical Co. and used without further purification. Rhodamine B base (Aldrich, ∼97%) was chosen to clarify the formation of AOT reverse cylindrical micelles. Pyrrole (Aldrich, 98%) monomer was used as received, and distilled water was used. (18) Raez, J.; Tomba, J. P.; Manners, I.; Winnik, M. A. J. Am. Chem. Soc. 2003, 125, 9546. (19) Xu, S.; Zhou, H.; Xu, J.; Li, Y. Langmuir 2002, 18, 10503. (20) Goren, M.; Lennox, R. B. Nano Lett. 2001, 1, 735. (21) Jang, J.; Chang, M.; Yoon, H. Adv. Mater. 2005, 17, 1616. (22) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (23) Li, M.; Mann, S. Langmuir 2000, 16, 7088. (24) Huang, L.; Wang, H.; Wang, Z.; Mitra, A.; Bozhilov, K. N.; Yan, Y. Adv. Mater. 2002, 14, 61. (25) Omastova´, M.; Trchova´, M.; Pionteck, J.; Proke×f0, J.; Stejskal, J. Synth. Met. 2004, 143, 153. (26) Ruckenstein, E.; Hong, L. Synth. Met. 1994, 66, 249. (27) Jang, J.; Yoon, H. Chem. Commun. 2003, 720. (28) Eastoe, J.; Robinson, B. H.; Heenan, R. K. Langmuir 1993, 9, 2820.

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Formation of an AOT Reverse Cylindrical Micelle Phase. An AOT reverse cylindrical micelle phase could be determined by tuning the weight ratios of each ingredient in the ternary mixture of apolar solvent (hexane, heptane, octane), AOT, and aqueous FeCl3 solution (16.2 M) at 15-30 °C. Typically, the compositions that are amenable to form the cylindrical micelle phase in hexane at 15 °C were inspected and mapped on a ternary phase diagram. The reverse cylindrical micelle phase was prepared as follows: In the first stage, AOT was dissolved in hexane. The solution was transparent without turbidity. Then, aqueous FeCl3 solution was progressively added. The system evolved from a limpid state to a viscous yellow state in appearance, indicating that reverse cylindrical micelles were formed. The reverse cylindrical micelle phase was identified by polarizing optical microscopy (POM). The micrograph was taken on a Leica MPS30 microscope. The small-angle X-ray scattering (SAXS) pattern was examined in the scattering vector (q) range of 0.01.0 nm-1 from a Bruker AXS Nanostar spectrometer. A fluorescence image of dye-impregnated AOT reverse cylindrical micelles was taken with a Carl Zeiss LSM510 confocal laser scanning microscope. Rhodamine B base as a laser dye was dissolved in water to produce a 6.9 mM solution, and 6 wt % was used relative to the amount of aqueous FeCl3 solution. Fabrication of PPy Nanotubes. Pyrrole (pyrrole/aqueous FeCl3 solution ) 20/ 80 by weight percent) was introduced into the reverse cylindrical micelle phase; the weight ratio of pyrrole to FeCl3 was approximately 0.3. Pyrrole monomer was added to an aged AOT reverse cylindrical micelle phase. The reactant became black within a few seconds, and the chemical oxidation polymerization of pyrrole monomers proceeded for 2 h. The resulting product was moved to a separation funnel and washed several times with ethanol to remove residual reagents. The colored supernatant was isolated from the desirable products by centrifugation and carefully decanted. Finally, the products could be retrieved and allowed to dry in a vacuum oven at room temperature. Characterization. A Bomem MB 100 Fourier transform infrared (FT-IR) spectrometer was used to characterize the PPy polymerization. A representative IR spectrum was recorded over 32 scans in absorption mode. The IR spectrum showed the featureless rise in the absorbance at energies above 1600 cm-1 owing to the plasma reflection phenomenon. The slanted baseline was corrected with a multiple-point level. An ultraviolet (UV)visible (vis) spectrum was taken with a Perkin-Elmer Lambda 20 spectrometer. CHNS elemental analysis was conducted with an EA 1110 apparatus (CE instruments). The transmission electron microscopy (TEM) images, electron diffraction pattern, and energy-dispersive X-ray (EDX) spectrum were taken with a Phillips CM-20 microscope and a JEOL EM-2000 EX II microscope. For TEM observation, the samples were diluted in ethanol and then the diluted solution was deposited on a perforated carbon foil supported on a copper grid. For EDX analysis, a copper grid coated with a silicon monoxide film was used. A field emission scanning electron microscopy (FE-SEM) image was obtained with a JEOL JSM-6700 F microscope. A specimen was coated with a thin layer of gold to eliminate charging effects. The measurement of electrical conductivity was performed at ambient temperature by a four-probe method using a Keithley 2400 SourceMeter.

Results and Discussion Formation of a Reverse Cylindrical Micelle Phase. A reverse cylindrical micelle phase was formed via a cooperative interaction between aqueous FeCl3 solution and AOT in an apolar solvent. AOT molecules are assembled into spherical aggregates with a diameter of

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Figure 1. Partial ternary phase diagram for the hexane/AOT/ aqueous FeCl3 solution system determined at 15 °C. The molar concentration of the aqueous FeCl3 solution was 16.2 M. The marked area corresponds to the AOT reverse cylindrical micelle phase region. The compositions are given as weight ratios. Typically, composition A (74.0 wt % hexane, 22.4 wt % AOT, and 3.6 wt % aqueous FeCl3 solution) was chosen to characterize the AOT reverse cylindrical micelle phase.

a few nanometers in the absence of water,29 whereas they form relatively large aggregates in the presence of water. When water is continuously added into the AOT/apolar solvent system, the aggregation number of the reverse micelles and the hydrodynamic radius of the aqueous micelle core (water pool) concurrently increase. Consequently, spherical reverse micelles are finally rearranged to ellipsoidal or cylindrical micelles that provide a relatively larger interfacial area for the energetic interaction between water and AOT headgroups.30,31 Reverse micelle growth is facilitated in the presence of metal salt. The incorporation of metal salts into AOT emulsions strongly affects the micelle aggregation number and second critical micelle concentration (CMC II); the micelle aggregation number describes the number of surfactant molecules required to form a micelle, and CMC II means the critical concentration at which spherical micelles undergo structural transition into cylindrical micelles. The aggregation number of AOT reverse micelles increases and the CMC II value of AOT decreases with increasing salt concentration because the electric double layers of reverse micelles are compressed and thus the electrostatic repulsion among the adjacent AOT molecules is reduced.32-35 Figure 1 represents a partial ternary phase diagram of hexane/AOT/aqueous FeCl3 solution. The experimental procedure for constructing the phase diagram was as follows: At 15 °C, AOT was dissolved in hexane at a given concentration, and aqueous FeCl3 solution was added progressively to the AOT/hexane mixture. When the emulsion composition favored the formation of the reverse cylindrical micelles, there was a slow transition from a transparent phase to a viscous yellow phase. “Composition A” (74.0 wt % hexane, 22.4 wt % AOT, and 3.6 wt % aqueous FeCl3 solution) was representatively chosen to characterize the AOT reverse cylindrical micelle phase. In Figure 2, a POM photograph of the AOT reverse cylindrical micelle phase (composition A) reveals clearly (29) Kotlarchyk, M.; Huang, J. S.; Chen, S.-H. J. Phys. Chem. 1985, 89, 4382. (30) Yoshimura, Y.; Abe, I.; Ueda, M.; Kajiwara, K.; Hori, T.; Schelly, Z. A. Langmuir 2000, 16, 3633. (31) Petit, C.; Lixon, P.; Pileni, M. P. Langmuir 1991, 7, 2620. (32) Zheng, L.; Li, F.; Hao, J.; Li, G. Colloids Surf., A 1995, 98, 11. (33) Jang, J.; Bae, J. Angew. Chem., Int. Ed. 2004, 43, 3803. (34) Jang, J.; Yoon, H. Adv. Mater. 2004, 16, 799. (35) Jang, J.; Yoon, H. Adv. Mater. 2003, 15, 2088.

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Figure 2. POM micrograph of the AOT reverse cylindrical micelle phase (composition A) showing birefringent striations arising from the undulations of the reverse cylindrical micelles. The scale bar is 5 µm.

Figure 3. CLSM image of the AOT reverse cylindrical micelle phase. An aqueous solution of rhodamine B base (6.9 mM) as a laser dye was added to the reverse cylindrical micelle phase prepared with composition A. The amount of aqueous dye solution used was 6 wt % relative to the amount of aqueous FeCl3 solution. The scale bar is 10 µm.

the birefringent striations arising from the undulations of the reverse cylindrical micelles. The anisotropic refractive index of the AOT reverse cylindrical micelle phase alters the polarization of transmitted polarized light, leading to red and blue texture. The characteristic striations were similar to those of the hexagonal liquidcrystalline phase.36 However, the AOT reverse cylindrical micelle phase showed more random orientation due to some disordered arrangement of the cylinders compared with the hexagonal phase. Confocal laser scanning microscopy (CLSM) observation was performed to further verify the formation of reverse cylindrical micelles in the H2O/FeCl3/AOT/apolar solvent system (Figure 3). An aqueous solution of rhodamine B base was used as a laser dye and added dropwise to the AOT reverse cylindrical micelle phase prepared with composition A. The aqueous dye solution is spontaneously introduced into the aqueous core of the reverse cylindrical micelles owing to its hydrophilicity. A CLSM image could visualize the 1-D water channels compartmentalized by AOT molecules. Figure 3 exhibits red emissions resulting from the dye molecules embedded inside the reverse cylindrical micelles. In an apolar solvent, the aggregates of the dyeimpregnated reverse cylindrical micelles gave rise to the emergence of needle-like red emission domains with a width of 0.2-3 µm and a length of 5-30 µm. AOT reverse cylindrical micelles can assemble into parallel bundles owing to their unique anisotropic mor(36) Hulvat, J. F.; Stupp, S. I. Angew. Chem., Int. Ed. 2003, 42, 778.

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Figure 4. SAXS pattern of the AOT reverse cylindrical micelle phase (composition A).

phology. Moreover, the aggregation of the AOT cylinders is promoted by the evaporation of solvent (e.g., hexane, boiling point ∼69 °C) during the observation process. When the sample was mounted onto the microscope, heating was caused by incident light. Thus, the solvent slowly evaporated with the aid of heat at the focusing spot, and the solution became more concentrated. At a higher solution concentration, AOT reverse cylindrical micelles prefer to form reverse cylindrical micelle bundles. Therefore, it can be considered that a needle-like red emission results from an AOT reverse cylindrical micelle bundle, not a single AOT reverse cylindrical micelle. This fact confirmed the formation of AOT reverse cylindrical micelles containing 1-D water channels in the core. The size of individual AOT reverse cylindrical micelles could be obtained from SAXS measurement (Figure 4). SAXS analysis was performed on the reverse cylindrical micelle phase prepared with composition A. The SAXS pattern revealed a distinct scattering peak at q ) 0.092 nm-1, indicating that AOT reverse cylindrical micelles prepared at a given condition involved 1-D water channels with a diameter of ca. 65 nm. Fabrication of PPy Nanotubes. In general, spherical PPy particles have been obtained by chemical polymerization using surfactant or stabilizer in an aqueous medium,37-39 and PPy films have been synthesized by electrochemical polymerization on the electrode.40 To fabricate tubular PPy nanostructures, AOT reverse cylindrical micelles were employed as the soft template. At composition A, AOT molecules were allowed to form reverse cylindrical micelles. Pyrrole monomer was introduced into the reverse cylindrical micelle phase and then rapidly polymerized by iron cations (oxidizing agent) along the surface of the reverse cylindrical micelles. The color of the mixture was changed from yellow to black during the polymerization. To remove AOT and other residual reagents, the resulting product was thoroughly washed with an excess of ethanol. For the successful fabrication of PPy nanotubes, it is noteworthy that aqueous FeCl3 solution is stably homogenized with the AOT/apolar solvent mixture. When the AOT reverse cylindrical micelle phase was disturbed by external factors, phase separation occurred in the system and PPy nanotubes were hardly produced. An FT-IR spectrum of PPy nanotubes indicated a pyrrole ring stretching band at 1548 cm-1, a conjugated C-N stretching band at 1473 cm-1, a dCsH in-plane vibration (37) Jang, J.; Nam, Y.; Yoon, H. Adv. Mater. 2005, 17, 1382. (38) Jang, J.; Oh, J. H. Adv. Mater. 2003, 15, 977. (39) Jang, J.; Oh, J. H., G. D. Stucky, Angew. Chem., Int. Ed. 2002, 41, 4016. (40) Cho, G.; Glatzhofer, D. T.; Fung, B. M.; Yuan, W.-L.; O’Rear, E. A. Langmuir 2000, 16, 4424.

Figure 5. (a) FT-IR spectrum, (b) EDX spectrum (the silicon peak originates from a copper grid coated with a silicon monoxide film), and (c) UV-vis spectrum of PPy nanotubes. The PPy nanotubes were prepared through the chemical polymerization of the corresponding monomer (pyrrole/aqueous FeCl3 solution ) 20/80 by weight percent) at composition A.

band at 1301 and 1183 cm-1, and a dCsH out-of-plane vibration band at 916 cm-1 (Figure 5a). The EDX spectrum of PPy nanotubes showed the presence of C (54.8%), N (15.7%), Fe (9.2%), and Cl (16.7%) (Figure 5b). In addition, CHNS elemental analysis provided the composition of C (50.4%), H (3.2%), N (14.6%), and S (0.0%). The C/N ratio of pristine PPy is 3.43 since a repeating unit consists of a nitrogen atom and four carbon atoms. The C/N ratio of PPy nanotubes was approximately 3.5 for both the EDX data and elemental analysis, and the C/N/H ratio (15.9/ 4.7/1.0) was very similar to that of pristine PPy.41 These results supported that the product was composed of PPy doped with iron complex. The UV-vis spectrum showed that PPy nanotubes were in a highly oxidized state (Figure 5c). The absorption at 425 nm is due to the transition from the valence band to the antibonding polaron state, and the broad absorption at more than 1000 nm is ascribed to the transition from the valence band to the bonding polaron state. Figure 6 displays the FE-SEM and the TEM images of PPy nanotubes prepared with composition A. The diameter of the PPy nanotubes was 94 ( 13 nm, and their length was more than 2 µm. The nanotube wall (12 ( 4 nm thickness) revealed a smooth inside and rugged outside, demonstrating that the surface of the AOT reverse cylindrical micelles served as the template in fabricating the nanotubes. The electron diffraction pattern (Figure 6b inset), which displayed a diffusive ring, indicated that the PPy nanotubes were amorphous. This was consistent with the results obtained from conventional PPy materials.37,38 The frame of the AOT reverse cylindrical micelles directly affects the morphology of the PPy nanotubes. To (41) Jang, J.; Oh, J. H. Chem. Commun. 2002, 2200.

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Figure 7. Diameter change of PPy nanotubes as a function of the weight ratio of aqueous FeCl3 solution to AOT. The weight ratio of aqueous FeCl3 solution relative to AOT varied from 0.16 to 0.22 in composition A (pyrrole/aqueous FeCl3 solution ) 20/80 by weight percent).

Figure 8. Conductivity variations of PPy nanotubes with different diameters.

Figure 6. (a) FE-SEM and (b, c) TEM images of PPy nanotubes (inset: electron diffraction pattern of PPy nanotubes). The PPy nanotubes were prepared through the chemical polymerization of the corresponding monomer (pyrrole/aqueous FeCl3 solution ) 20/80 by weight percent) with composition A.

investigate the influence of aqueous FeCl3 solution and AOT on the fabrication of PPy nanotubes, the weight ratio of aqueous FeCl3 solution relative to AOT was varied from 0.16 to 0.22. In general, AOT reverse micelle systems are subjected to a simple linear relationship between the size of the micelle core and the weight ratio of H2O to AOT. Namely, when the weight ratio of H2O to AOT increases, the hydrodynamic radius of the aqueous micelle core increases monotonically. Under our experimental conditions, water was introduced into the reverse micelle core in proportion to the input amount of aqueous FeCl3 solution, which induced swelling of the water pool inside the reverse cylindrical micelles. As a result, the diameter of the PPy nanotubes increased with increasing weight ratio of aqueous FeCl3 solution to AOT (Figure 7). The conductivity of conducting polymers is commonly dependent on the concentration of oxidizing agent, type of solvent, reaction time, and temperature. In the case of conducting polymer nanostructures, additionally, the conductivity is susceptible to size variation. Martin et al. synthesized PPy fibers inside the cylindrical pores of polycarbonate membranes and investigated the dependence of the PPy fiber conductivity on the diameter.42 They reported that, as the fiber diameter decreased, the conductivity gradually increased with conjugation length;

the conjugation length means the length of the polyene segment along which the delocalization of the π-electron takes place. Figure 8 exhibits the conductivity of the PPy nanotubes measured as a function of their diameter. In the inner part of the PPy nanotubes, the polymer chains are highly oriented at the dictation of the AOT micelles, and thus, they can possess extended conjugation. The portion of ordered PPy chains decreases in PPy nanotubes with increasing wall thickness. Accordingly, a slight increment in conductivity was observed with decreasing diameter of the PPy nanotubes. This relationship between conductivity and nanotube diameter could be confirmed by the qualitative measurement of the conjugation length (the intensity ratio of the band at ca. 1550 cm-1 to the band at ca. 1470 cm-1) obtained from IR spectra.42 To explore the effects of solvent, PPy nanotubes were fabricated using apolar solvents with different hydrocarbon chain lengths (Figure 9). The diameters of PPy nanotubes fabricated in heptane and octane were 138 ( 16 and 176 ( 15 nm, respectively. Judging from these results, the diameter of the AOT reverse cylindrical micelles increased with increasing hydrocarbon chain length of the apolar solvents. Apolar solvent molecules of short chains can penetrate into the palisade layer consisting of hydrophobic AOT double tails. As the hydrocarbon chain length of the apolar solvents increases, it becomes difficult for the solvent molecules to penetrate into the palisade layer consisting of hydrophobic AOT double tails.43,44 Consequently, the packing parameter (p ) Vo/aLo, where Vo and Lo are the volume and length of the AOT tailgroup and a is the effective area of the AOT (42) Menon, V. P.; Lei, J.; Martin, C. R. Chem. Mater. 1996, 8, 2382. (43) Lang, J.; Jada, A.; Malliaris, A. J. Phys. Chem. 1988, 92, 1946. (44) Hirai, M.; Kawai-Hirai, R.; Sanada, M.; Iwase, H.; Mitsuya, S. J. Phys. Chem. B 1999, 103, 9658.

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Figure 9. TEM images of PPy nanotubes prepared in (a, b) heptane and (c, d) octane. The PPy nanotubes were fabricated using other apolar solvents in composition A.

Figure 10. Dependence of the PPy nanotube diameter on the reaction temperature. The reaction temperature was varied from 15 to 30 °C in composition A.

headgroup) is gradually reduced with decreasing volume of AOT tails, and therefore, the radius of curvature of the micelles increases. The polymerization temperature was also considered as an important factor in PPy nanotube fabrication. The aggregation number of micelles is sensitive to the variation of temperature.43 As the polymerization temperature increased in the AOT reverse micelle system, the solvent molecules were relatively unable to penetrate into the palisade layer as a result of the enhanced chain mobility. Therefore, the palisade layer is composed of more surfactant molecules, and the micelle aggregation number becomes higher than at lower temperature. Accordingly, as shown in Figure 10, the diameter of the PPy nanotubes increased substantially with increasing polymerization temperature in the range of 15-30 °C. Conclusions AOT reverse cylindrical micelle templating was demonstrated as an efficient tool for fabricating conducting

polymer nanotubes. The reverse cylindrical micelle phase was developed through a cooperative interaction between aqueous FeCl3 solution and AOT in an apolar solvent. Iron cations were adsorbed to the AOT headgroups and led the chemical oxidation polymerization of the corresponding monomer on the surface of the reverse cylindrical micelles. A partial ternary phase diagram was investigated for hexane/AOT/aqueous FeCl3 solution and used as a guide for reverse cylindrical micelle templating. POM observation exhibited the characteristic birefringence commensurate with the formation of AOT reverse cylindrical micelles. In addition, the water channel confined within AOT reverse cylindrical micelles could be visualized using a hydrophilic dye. The AOT reverse cylindrical micelle phase was controlled by changing the key experimental parameters such as the weight ratio of aqueous FeCl3 solution to AOT, type of solvent, and temperature. These parameters had significant effects on the nanotube fabrication. With increasing weight ratio of aqueous FeCl3 solution to AOT, the nanotube diameter increased. Likewise, the nanotube diameter increased when the hydrocarbon chain length of the apolar solvents increased and the polymerization temperature was raised. The conductivity of the PPy nanotubes was investigated as a function of the nanotube diameter. Importantly, our simple and novel approach may open up a facile route to fabricate new organic nanotubes and might be expanded to allow the fabrication of versatile nanotube composites (e.g., tubein-tube or rod-in-tube composites). Acknowledgment. This work was financially supported by the Brain Korea 21 program of the Korean Ministry of Education, and the Hyperstructured Organic Materials Research Center is supported by the Korea Science and Engineering Foundation. LA051447U