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2009, 113, 10346–10349 Published on Web 05/27/2009
Template-Free Growth of Aligned Bundles of Conducting Polymer Nanowires Yue Wang, Henry D. Tran, and Richard B. Kaner* Department of Chemistry and Biochemistry and California NanoSystems Institute, UniVersity of California, Los Angeles, California, 90095-1969 ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: May 15, 2009
Aligned bundles of conducting polymer nanowires are generated in situ during the polymerization of o-anisidine. The presence of an appropriate initiator, a prolonged reaction time, and careful tuning of reaction conditions are crucial parameters for producing aligned nanowires. Mechanistic studies suggest hydrogen bonding between the substituents on the monomer and the polymer backbone is important for promoting alignment. Introduction One-dimensional (1D) nanostructures of conducting polymers such as wires, fibers, rods, and tubes have been intensively investigated because they possess many unique properties and potential advantages over their conventional counterparts. In particular, polyaniline nanofibers, because of their stability and simple, reversible acid-base doping-dedoping chemistry, have been one of the most studied conducting polymers. Many methods exist in order to synthesize polyaniline nanofibers.1 However, these routes typically produce nanofibers that are entangled and randomly oriented; this hinders the development of devices that would benefit from ordered arrangements of nanomaterials over macroscopic areas. While many techniques for synthesizing ordered arrays of 1-D nanowires for inorganic semiconductors exist, examples of alignment of organic materials such as polyaniline are sparse. Aside from using hard templates,2 only a few template-free methods exist in order to generate ordered arrays of polyaniline nanofibers. These methods include stepwise electrochemical deposition,3 dilute polymerization,4 and polymerization followed by a recrystallization process.5 The oriented nanofibers produced using the first two methods are often short and possess a low aspect ratio, while the oriented arrays of polyaniline wires synthesized via the last method are on the micrometer-scale. Furthermore, several of these techniques require in situ deposition of polyaniline onto a solid substrate during polymerization. It is therefore highly desirable to create an alternative method that can produce aligned polyaniline nanofibers in bulk quantities in solution rather than on a solid substrate in order to increase the versatility of these materials. This work explores the polymerization of substituted anilines for possible in situ alignment of the polymer because the sidechains may facilitate interchain stacking, which has been shown to be beneficial for small molecule organic semiconductors.6,7 Side groups capable of creating extended hydrogen bonding networks that thereby promote interchain π-π stacking are explored. Interestingly, we have found that while techniques to synthesize substituted polyaniline nanofibers typically produce randomly oriented bundles,8,9 modifications in reaction param* To whom correspondence should be addressed. E-mail: Kaner@ chem.ucla.edu. Tel.: +1 310 825 5346. Fax: +1 310 206 4038.
10.1021/jp903583e CCC: $40.75
eters such as reaction time and polymerization additives can change the bulk morphology from randomly oriented nanofibers to aligned bundles of nanowires when certain monomers, such as o-anisidine, are polymerized. Experimental Methods In a typical reaction to produce aligned bundles of polyanisidine nanowires, 2-4 mg of N-phenyl-1,4-phenylenediamine (the optimal amount of p-aniline dimer in this study) is predissolved in a minimal amount of methanol and mixed with a 3.2 mmol solution of o-anisidine in 10 mL of 1 M HCl. This solution is then rapidly mixed with a 0.8 mmol solution of ammonium peroxydisulfate (APS), in 10 mL of 1 M HCl. The reaction mixture is left undisturbed for 3 days. The product is purified by dialysis or centrifugation against deionized water until the pH of the solution becomes neutral. Scanning electron microscopy (SEM) samples are prepared by drop-casting an ∼1 g/L conducting polymer dispersion onto a piece of silicon wafer. SEM images are taken with a JEOL JSM-6700-F field emission SEM microscope. UV-vis spectra are taken on a HP 8452 spectrometer. Cyclic voltammetery (CV) is collected with a Princeton Applied Research Potentiostat/Galvanostat Model 263A in 1 M HClO4 with a fluorinated tin oxide (FTO) glass slide as the working electrode at a scan rate of 50 mV/s. Solid state FT-IR samples are prepared with FT-IR grade KBr and the spectra are taken with a JASCO FT/IR-420 spectrometer. Results and Discussion We recently reported a scalable method to produce polyanisidine nanofibers.8 However, as the reaction time is prolonged to 3 days, we discovered that small amounts of aligned nanowires exists underneath the nanofibers upon closer inspection of a SEM cross-section from a drop-cast sample of polyo-anisidine (Figure 1a). Because the aligned portions of the product occur as bundles, they are denser than the highly porous nanofibrous portion and thus deposit onto the solid substrate first when drop-cast. The top nonoriented nanofibrous layer can be peeled off by a piece of cellophane tape, which leaves a clearer view of the aligned nanowire area (Figure 1b). The nanowires are quite straight and highly aligned with lengths 2009 American Chemical Society
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Figure 1. Scanning electron microscope (SEM) images of poly-o-anisidine nanowires. (a) A drop-cast sample is composed of 2 layers, entangled, nonoriented nanofibers on top of a bottom layer of highly aligned nanowires. (b) A view of the bottom layer after peeling off the top layer with a piece of cellophane tape. (c) A magnified view of an aligned array of nanowires. (d) Poly-o-anisidine synthesized without any initiators.
Figure 2. (a) UV-vis spectra of doped poly-o-anisidine after purification with water and dedoped poly-o-anisidine after exposure to 0.1 M NH4OH. (b) CV of poly-o-anisidine collected in 1 M HClO4 with a FTO glass slide as the working electrode at a scan rate of 50 mV/s.
extending up to several micrometers and diameters between 30-100 nm (Figure 1c). The extra reaction time may facilitate more ordered molecular stacking and growth along the fibers’ long axis. Further extended reaction time beyond 3 days does not change the proportion between the aligned nanowires and the randomly oriented nanofibers. Characterization of the product by UV-vis, CV, and FT-IR reveals that the polymer is indeed poly-o-anisidine.9-12 The UV-vis spectrum indicates the final product is in the doped emeraldine oxidation state with peaks at 338, 431, and 718 nm (Figure 2a).9 The addition of NH4OH yields a typical dedoped spectrum. The same conclusion can be drawn from the CV (Figure 2b),8,10 which produces oxidation peaks at 0.19 and 0.68 V with corresponding reduction peaks at 0.07 and 0.55 V. The FT-IR data confirm the molecular structure of poly-o-anisidine11,12 (Supporting Information 1) with peak assignments presented in Supporting Information, Table S1. The aligned nanowires likely form for poly-o-anisidine but not for the parent polymer, polyaniline, because of the differences in molecular structure of the two monomers and polymers. The methoxy moiety in poly-o-anisidine can provide additional intermolecular noncovalent interactions such as hydrogen bonding, which has been demonstrated to be important in modulating supramolecular morphology.13 Hydrogen bonding is evident in the FT-IR spectra by a broad absorption between 1800-3600
cm-1 and a peak centered around 3260 cm-1, which represents the overlapping of N-H · · · N and N-H · · · O hydrogen bonds.12-14 In order to investigate this effect, we polymerized two structurally similar monomers, N-methyl-1,2-diamine and 2-methylthioaniline, in which the oxygen in o-anisidine is replaced by nitrogen and sulfur, respectively. Under identical reaction conditions, long and nonentangled nanowires form for poly(Nmethyl-1,2-diamine) samples, which exhibit some degree of alignment (Figure 3a). However, only short, entangled, and randomly oriented nanofibers, along with an occasional large sheet, form for poly(2-methylthioaniline) (Figure 3b). The nitrogen atom in poly(N-methyl-1,2-diamine) can also form strong N-H · · · N hydrogen bonds with the protonated backbone nitrogens between individual polymer chains, but these are not as strong as the N-H · · · O hydrogen bonds in poly-o-anisidine. This may explain the higher degree of order that nanowires of poly(N-methyl-1,2-diamine) display versus polyaniline and other derivatives,6 yet not as oriented as those observed for poly-oanisidine. Conversely, due to the inability of the sulfur atom in 2-methylthioaniline to form strong hydrogen bonds, the resulting polymer does not exhibit any degree of alignment. A number of other structurally similar aniline derivatives have also been studied in order to further elucidate the importance of substituents in determining polymer alignment. For example, in addition to the importance of the hydrogen bonding moieties,
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Figure 3. SEM images of (a) poly(N-methyl-1,2-diamine) and (b) poly(2-methylthioaniline) synthesized using N-phenyl-1,4-phenylenediamine as an additive.
the position of the group on the monomer as well as the size of the actual substituent plays a role in whether or not agglomerates, nanofibers, or aligned nanowires are observed (Supporting Information). The presence of oligomeric initiators during polymerization is another important reaction parameter in order to generate aligned nanowires. Oligomer molecules such as p-aniline dimer can promote nanofiber formation under conditions that normally only produce polymer agglomerates.8,15 As a control reaction, o-anisidine was polymerized under identical conditions but without the addition of any initiators. In this instance, a very small amount of aligned nanowires can also be found (Figure 1d). However, the majority of the sample forms only agglomerates. p-Aniline dimer has been shown to promote nanofiber formation by promoting homogeneous nucleation, which thereby results in linear semirigid rod oligomeric segments that become the core for nanofiber growth.15 The linear and relatively planar semirigid rod oligomer chains have a higher tendency to form well-ordered supramolecular structures by π-π stacking and are further supported by the extended hydrogen-bonding network formed by the ortho-substituted methoxy group in poly-oanisidine. Therefore, formation of aligned nanowires competes with the formation of randomly oriented nanofiberssthis results in a mixture of the two morphologies in the final product. In addition, the presence of the aligned areas, despite the small amounts, in the initiator-free control reaction further suggests that o-anisidine has the necessary molecular structure for alignment since oriented nanowires form even under conditions that do not promote nanostructure (i.e., nanofiber) formation. We used other small molecule additives that also promote nanostructure formation to further elucidate the role of the initiator molecules. We observed that, along with p-aniline dimer, 4,4′-diaminodiphenylamine can also serve as an effective initiator for nanofiber formation for aniline derivatives including o-anisidine. When minute amounts of this initiator are incorporated into the polymerization of o-anisidine, the same results are obtained: entangled and randomly oriented nanofibers as the top layer and highly aligned nanowires as the bottom layer (Supporting Information 3c). Careful tuning of reaction conditions is also important for producing aligned bundles of nanowires. The choice of dopant acid is crucial as HCl is the only dopant acid found that promotes alignment. This effect may be attributed to the size of the dopant acid molecules; larger dopants such as the perchloric ions from perchloric acid or camphorsulfonic ions from CSA could hinder intermolecular stacking and thus obstruct the formation of aligned nanowires (Supporting Information 5). APS is also the best oxidant for obtaining aligned bundles of
polyanisidine nanowires. For instance, when iron(III) chloride is used as the oxidant, a mixture of aligned nanowires and micrometer-sized spheres are obtained instead of the nanofiber/ aligned nanowire bilayered morphology observed when APS is used. No alignment is observed when APS is replaced with iron(II) chloride or hydrogen peroxide (Supporting Information 6). In addition, a 4:1 monomer-to-oxidant ratio appears to be optimal for alignment as the aligned nanowires appear to coagulate and eventually become featureless when the ratio is decreased (Supporting Information 7). A monomer concentration between 3.2 and 6.4 mmol at the onset of polymerization also appears optimal in obtaining a maximum amount of aligned bundles of polyanisidine nanowires (Supporting Information 8). Conclusions Aligned bundles of conducting polymer nanowires with relatively high aspect ratios have been synthesized by polymerizing o-methoxyaniline in the presence of appropriate additives. The resulting polymer has increased hydrogen bonding and intermolecular stacking when compared to polyaniline. A prolonged reaction time and careful tuning of reaction conditions are also important factors for promoting alignment. The high degree of alignment could improve applications that currently use nonoriented nanofibers such as chemical sensors,16 molecular memory devices,17 and capacitors.18 Acknowledgment. We thank Julio M. D’Arcy for assistance with CV. This work was supported by an NSF-NIRT award DMR-0507294 and the Microelectronics Advanced Research Corporation (MARCO) and its Focus Center Research Program in Functional Engineered NanoArchitectonics (FENA). Supporting Information Available: Characterizations, detailed reaction conditions, and mechanistic studies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Zhang, D. H.; Wang, Y. Y. Mater. Sci. Eng., B 2006, 134, 9– 19. (b) Li, D.; Huang, J. X.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135– 145. (c) Tran, H. D.; Li, D.; Kaner, R. B. AdV. Mater. 2009, 21, 1487– 1499. (2) Martin, C. R. Acc. Chem. Res. 1995, 28, 61–68. (3) Liu, J.; Lin, Y.; Liang, L.; Voigt, J. A.; Huber, D. L.; Tian, Z. R.; Coker, E.; Mckenzie, B.; Mcdermott, M. J. Chem.sEur. J. 2003, 9 (3), 604–611. (4) Chiou, N.-R.; Lu, C.; Guan, J.; Lee, J.; Epstein, A. J. Nat. Nanotechnol. 2007, 2, 354–357. (5) Tang, Q.; Wu, J.; Sun, X.; Li, Q; Lin, J. Langmuir 2009, 25, 5253– 5257. (6) Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.; Zang, L. J. Am. Chem. Soc. 2006, 128, 7390–7398.
Letters (7) Kastler, M.; Pisula, W.; Wasserfallen, D.; Pakula, T.; Mu¨llen, K. J. Am. Chem. Soc. 2005, 127, 4268–4296. (8) Tran, H. D.; Norris, I.; D’Arcy, J. M.; Tsang, H.; Wang, Y.; Mattes, B. R.; Kaner, R. B. Macromolecules 2008, 41, 7405–7410. (9) Tran, H. D.; Kaner, R. B. Chem. Commun. 2006, 3915–3917. (10) Koval’chuk, E. P.; Stratan, N. V.; Reshetnyak, O. V.; Bla · zejowski, J.; Whittingham, M. S. Solid State Ionics 2001, 141-142, 217–224. (11) Tan, Y.; Bai, F.; Wang, D.; Peng, Q.; Wang, X.; Li, Y. Chem. Mater. 2007, 19, 5773–5778. (12) Gruger, A.; Novak, A.; Re´gis, A.; Colomban, P. J. Mol. Struct. 1994, 328, 153–167. (13) Stejskal, J.; Sapurina, I.; Trchova´, M.; Konyushenko, E. N.; Holler, P. Polymer 2006, 47, 8253–8262.
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10349 (14) Coats, J. In Encyclopedia of Analytical Chemistry, Interpretation of Infrared Spectra, a Practical Approach; Meyers, R. A., Ed.; Wiley: Chichester, 2000; pp 10815-10837. (15) Tran, H. D.; Wang, Y.; D’Arcy, J. M.; Kaner, R. B. ACS Nano 2008, 2, 1841–1848. (16) (a) Virji, S.; Huang, J.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491–496. (b) Liu, H. Q.; Kameoka, J.; Czaplewski, D. A.; Craighead, H. G. Nano Lett. 2004, 4, 671–675. (17) Tseng, R. J.; Huang, J. X.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (18) Nadagouda, M. N.; Varma, R. S. Green Chem. 2007, 632–637.
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