pubs.acs.org/Langmuir © 2009 American Chemical Society
Shape and Size Control of Oriented Polyaniline Microstructure by a Self-Assembly Method Qunwei Tang, Jihuai Wu,* Xiaoming Sun, Qinghua Li, and Jianming Lin The Key Laboratory for Functional Materials of Fujian Higher Education, Institute of Material Physical Chemistry, Huaqiao University, Quanzhou 362021, China Received November 20, 2008. Revised Manuscript Received February 21, 2009 Oriented polyaniline (PANI) microstructures (e.g., bulks, spheres, flakes, and fibers) were prepared without a template by polymerization at 80 °C and crystal growth at 0 °C and using a self-assembly method in the presence of hydrochloric acid (HCl) as the dopant. It was found that the shape and size of the resulting PANI microstructures depended on the HCl dosage and aniline concentration. Polyanilinium salt precipitates, as a templete, drive the formation of highly oriented PANI microstructures. The infrared and UV-vis absorption spectra and X-ray diffraction were used to characterize the molecular structures of the PANI microstructures. The results showed that their main structure was identical to those of the emeraldine salt form of PANI.
Introduction Among conducting polymers such as polypyrrole, polythiophene, polyphenylenevinylene, and polyacetylene, polyaniline (PANI) has been most extensively studied because it exhibits good environmental stability and its electrical properties can be modified by both the oxidation state of the main chain and protonation. These allow it to be applied in various electrochemical devices including batteries, capacitors, electrochromic windows and displays, actuators, photovoltaic cells, and lightemitting electrochemical cells.1-4 One of the key strategies for synthesizing conducting polymers with dimensions on the microscale or nanoscale is templatedirected synthesis. Templates used via this route can be classified into two major categories, namely, “hard” and “soft” templates. Hard templates include many kinds of conventional hard porous materials, such as porous zeolites,5 opals,6 and controlled poresize membranes.7 However, soft templates, such as block polymers8 and surfactants,9 which provide well-defined rooms or channels for conducting polymer chains to grow into micro-/ nanometer-sized products, have been introduced in the synthesis of conducting polymer on small scales. Template synthesis is a common and effective method for synthesizing these micro-/ nanostructured conducting polymers. However, to get pure conducting polymers, the templates have to be removed after the polymerization. This is difficult in most cases because the microstructure of resulting materials is drastically altered or even destroy during recovery from the templates.10 Recently, several oxidation polymerization methods used to fabricate polyaniline *Corresponding author. Tel: +86 595 22693899. Fax: +86 595 22693999. E-mail:
[email protected] (1) Shimomura, T.; Akai, T.; Abe, T.; Ito, K. J. Chem. Phys. 2002, 116, 1753. (2) Li, W.; Zhu, M. F.; Zhang, Q. H.; Chen, D. J. Appl. Phys. Lett. 2006, 89, 103110. (3) Kaul, P. B.; Day, K. A.; Abramson, A. R. J. Appl. Phys. 2007, 101, 83507. (4) Li, D.; Huang, J.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135–145. (5) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (6) Misoska, V.; Price, W.; Ralph, S.; Wallace, G. Synth. Met. 2001, 121, 1501– 1502. (7) Cepak, V. M.; Martin, C. R. Chem. Mater. 1999, 11, 1363–1367. (8) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1997; pp 423. (9) Zhang, X. Y.; Manohar, S. K. Chem. Commun. 2004, 20, 2360–2361. (10) Lasic, S. S. Liposomes: From Physics to Applications; Plenum Press: New York, 1993.
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nanofibers without surfactant or template have been developed, such as interfacial polymerization,11 rapidly mixed reaction,12 and dilute polymerization.13 However, these methods are fairly complicated, and the yield and orientation of products are not good. In this article, we report the synthesis of oriented PANI microstructures (e.g., bulks, spheres, flakes, and fibers) by polymerization at 80 °C and crystal growth at 0 °C and using a selfassembly method without any templates or surfactants. The influence of HCl dosage and aniline concentration on the shape and size of PANI products can be used for the controllable synthesis. Polyanilinium salt precipitates, as a template, drive the formation of highly oriented PANI microstructures. The chemical and electronic structures of the PANI products are also studied by Fourier transform and UV-vis absorption spectroscopy and X-ray diffraction, respectively. Experimental Section Materials. Aniline monomer was distilled under pressure prior to use. Potassium peroxydisulfate (K2S2O8, KPS) as a radical oxidant for PANI was purified by recrystallization from a 66 wt % ethanol/water solution. An inorganic acid (HCl) was used as a dopant for PANI. All aqueous solutions were prepared in 18 MΩ water obtained by the purification of deionized water with a Millipore Milli-Q system. All reagents were purchased from Shanghai Chemical Reagents Co., China. Preparation of PANI Microstructures. PANI was synthesized by an aqueous solution polymerization method. A typical synthesis procedure is the following: 2 mL of aniline monomer (ANI) is mixed with 250 mL of deionized water and the desired amount of HCl (1 M) solution. Under an air atmosphere, the reaction mixture was stirred and heated to 80 °C in a water bath, and then 2.97 g of KPS (molar ratio of KPS to aniline was 1/2) was quickly added to the above mixture. After all of the KPS was dissolved and green PANI was formed, the reactant was cooled to a temperature of 0 ( 3 °C and was kept at this temperature for 10 h without stirring (caution: any stirring or shaking is absolutely not allowed), and a green product was grown gradually. The product was filtered and washed with deionized water five
(11) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314–315. (12) Huang, J.; Kaner, R. B. Angew. Chem., Int. Ed. 2004, 43, 5817–5821. (13) Chiou, N. R.; Epstein, A. J. Adv. Mater. 2005, 17, 1679–1683.
Published on Web 3/20/2009
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Figure 1. SEM images of the PANI microstructures synthesized at different molar ratios of [HCl]/[ANI]: (a) 0, (b) 1/1, (c) 3/2, (d) 2/1, (e) 4/1, and (f) 8/1 at a molar ratio of [KPS]/[ANI] of 1/2 and 8.8 10-2 M [ANI]. times and finally vacuum-dried for 24 h to obtain the dark-green PANI product. Characterization. The sample was mounted on a metal stub and coated with gold, and the morphology of the sample was observed and photographed with a scanning electron microscopy (SEM) and identified by infrared spectroscopy on a Nicolet Impact 410 FTIR spectrophotometer using KBr pellets. The PANI microstructures were monitored by UV-vis absorption spectra (Shimadzu UV-3100 UV-vis-IR spectrophotometer). The powder X-ray diffraction (XRD) pattern of the sample was recorded with a D8 Advance X-ray diffractometer (Bruker Co., Germany) at a Cu KR wavelength of 0.154 nm, operating at 40 kV and 40 mA and scanning from 2 to 40° at 5 deg min-1.
Results and Discussion Morphology Control. The typical influence of the HCl dosage on the morphology of PANI is shown in Figure 1. The resulting PANI is composed of a powder, spheres, and fibers (Figure 1a) in the absence of HCl. The average diameter of the 5254
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spheres is about 500 nm, and the length of the fibers (60 nm in diameter) is about 1 μm. When the molar ratio of [HCl]/[ANI] is 1/1, the resulting bulk product is composed of spheres (600 nm in diameter), and fibers (50 nm in diameter) with an average length of 1.2 μm (Figure 1b). In the molar ratio of [HCl]/[ANI] at 3/2, it can be seen that the products are composed of microsized flakes without any other morphologies (Figure 1c). Therefore, we can control the shapes of PANI by varying the [HCl]/[ANI] molar ratio. When the molar ratio of [HCl]/[ANI] reaches 2/1, PANI flakes and fibers with a length of 250-300 μm and a width of 50 μm are observed (Figure 1d). Therefore, we call them microsized products. Most of the segments are flakes, and there are few fibers with an average diameter of 600 nm. Upon increasing the molar ratio of [HCl]/[ANI] to 4/1, microsized flakes and fibers are also found, but at a molar ratio of 2/1, fibers dominate (Figure 1e). Furthermore, when the molar ratio of [HCl]/[ANI] reaches 8/1, a large number of microsized fibers with an average diameter of 1.2 μm and high orientation are produced (Figure 1f), and the longest fibers are nearly 5 cm, Langmuir 2009, 25(9), 5253–5257
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Figure 2. SEM images of the PANI microstructures synthesized in different aniline concentrations: (a) [ANI] = 7.8 10-2 M and (b) [ANI]
= 4.9 10-2 M at a molar ratio of [KPS]/[ANI] of 1/2 and [HCl]/[ANI] at 1/1.
Figure 3. Formation process of the PANI fiber and flake.
which is the highest value in the current reports. This result indicates that the shape and size of the PANI microstructure can be controlled by adjusting the HCl dosages in the selfassembly process. The morphology and size of PANI also depend on the aniline concentration [ANI]. Typically, for the synthesis of PANI at a molar ratio of [HCl]/[ANI] of 1/1 and [ANI] at 7.8 10-2 M, PANI shows a uniform flake array with an individual thickness of 0.5-3 μm and a width of 6-15 μm (Figure 2a). When [ANI] is 4.9 10-2 M, a large number of PANI fiber arrays with an average diameter of 1.2 μm are produced (Figure 2b). Formation Mechanism. Different from other methods for preparing PANI microstrcture, the specificity of our method is two-step (Figure 3): one is the oxidation polymerization of ANI at 80 °C to form PANI molecules, and the other is the crystal growth of PANI molecules in the temperature range of 0 ( 3 °C. If the preparation is carried out at 80 °C only, although more PANI molecules are produced, the PANI crystal cannot grow effectively because of the higher solubility of PANI at this higher temperature. However, if the preparation takes place in the temperature range of 0 ( 3 °C, then as reported by most authors, fewer PANI molecules are produced, the PANI crystal also cannot grow effectively, and only a small quantity of PANI crystals are formed. When polymerization at higher temperature is combined with crystal growth at lower temperature, the highly oriented PANI microstructure are obtained in high yield. The oxidation polymerization of aniline occurs under acidic conditions, and weakly basic aniline molecules with Langmuir 2009, 25(9), 5253–5257
Kb = 3.8 10-10 can react with HCl to form anilinium cations.14 Whereas two oxidation processes are possibly initiated from aniline molecules and anilinium cations, the neutral aniline molecule is easily oxidized to produce a dimer and is added to the aniline molecule to produce a higher oligomer.15 Owing to the stereohindrance in the ortho position, para coupling mainly occurs. The anilinium cation cannot be oxidized directly by KPS because the electron pair on the nitrogen atom, which is delocalized in the neutral aniline molecule, becomes localized in the anilinium cation.16 Although anilinium cations are difficult to oxidize, they are easily added to the propagation of pernigraniline to form a PANI chain. After reduction by the residual aniline, the desired product, a green emeraldine form of PANI, is thus obtained. The growth process of PANI microstructures can be described as follow (Figure 3b): when the temperature decreases to 0 °C, the solubility of the polyanilinium salt decrease, which results in the supersaturation and separation of the polyanilinium salt as a crystal seed or crystal nucleus. As a templete, polyanilinium salt possesses a linear 1D backbone, which induces the self-assembly of PANI with a higher length/ radii ratio. From Figures 1 and 2, it can be seen that the lower ANI concentration produces more highly ordered PANI long fibers. (14) Ding, H. J.; Wan, M. X.; Wei, Y. Adv. Mater. 2007, 19, 465–469. (15) Zimmermann, A.; Kunzelmann, U.; Dunsch, L. Synth. Met. 1998, 93, 17–25. (16) Kang, E. T.; Neoh, K. G.; Tan, K. L. Prog. Polym. Sci. 1998, 23, 277–324.
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Figure 4. FTIR spectra of the PANI microstructures synthesized at different molar ratios of [HCl]/[ANI]: (a) 0, (b) 1/1, (c) 2/1, (d) 4/ 1, and (e) 8/1 under a molar ratio of [KPS]/[ANI] at 1/2 and 8.8 10-2 M [ANI].
This is due to the fact that PANI chains aggregate and grow slowly in low [ANI] as a template and the 1D arrangement of the polyanilinium salt backbone is retained and a highly oriented PANI fiber is formed. However, in high [ANI], the PANI chains aggregate and grow rapidly but are less highly oriented. Higher HCl concentration retards the growth of PANI, so higher [HCl] has a similar effect as low [ANI]. In particular, when [HCl] = 0, as a template, the polyanilinium salt cannot form, and PANI grows only in the sphere or granule particles. Structure Characterization. Figure 4 shows the FTIR spectra of PANI microstructures with different HCl dosages. The characteristic bands at 1561 and 1491 cm-1 are attributed to the CdC stretching deformation mode of the quinoid and benzenoid rings, and the 1300 cm-1 band is assigned to the C-N stretching of the secondary aromatic amine. The band at 1245 cm-1 could be interpreted as a C-N-C stretching vibration in the polaron structure. The aromatic C-H in-plane bending vibration mode, which is formed during protonation,15 moves to long wavenumbers from 1152 to 1184 cm-1 with increasing of HCl dosage (i.e., doped degree). Bands at 1048 and 692 cm-1 are the results of C-H out-of-plane bending of 1,2,4 ring and C-H out-of-plane bending of the 1,2 ring, respectively. Out-of-plane deformations of C-H on 1,4-disubstituted rings are located at 815 cm-1, and the absorption peak at 605 cm-1 is caused by the deformations of the benzene ring.17 The peaks observed at 800-900 cm-1 are characteristic of para substitution of the aromatic ring and reveal that the polymerization has proceeded via a head-to-tail mechanism. Upon acid protonation of the emeraldine base, the quinonoid units are believed to be converted to benzenoid units by a protoninduced spin-unpairing mechanism and have no absorption band at 1380 cm-1. In the case of protonated emeraldine, the long absorption tail above 2000 cm-1, which masks the N-H stretching vibration in the 3100-3500 cm-1 region, and the appearance of the intense broad band at about 1184 cm-1 have been associated with high electrical conductivity and a high degree of electron delocalization in PANI.18,19 These (17) Trchova, M.; Sedenkova, I.; Tobolkova, E.; Stejskal, J. Polym. Degrad. Stab. 2004, 86, 179–185. (18) Tang, J.; Jing, X.; Wang, B.; Wang, F. Synth. Met. 1988, 24, 231–238. (19) Lu, F. L.; Wudl, F.; Nowak, M.; Heeger, A. J. J. Am. Chem. Soc. 1986, 108, 8311–8313.
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Figure 5. UV-vis spectra of PANI microstructures dispersed in deionized water. The preparation conditions for samples a-c are the same as in Figure 1b,c,f.
characteristic peaks are identical to those of PANI particles prepared via a common method.16 The results indicate that the backbone structures of PANI microstructures obtained by our method are identical to those of inorganic-acid-doped PANI particles synthesized using a traditional method. UV-vis absorption spectra of PANI microstructures dispersed in deionized water are shown in Figure 5. The peaks at around 246 and 289 are attributed to the π f π* transition, and the polaron band f π* transition of PANI microfibers, respectively.20 Two bands at 420 and 831 nm are assigned to the π f localized polaron band. These bands indicate that HCl-doped PANI products are in their conducting form (i.e., the emeraldine salt form of PANI), which is similar to that obtained by either chemical or electrochemical methods. Furthermore, the quinoid unit absorption peak at 610 nm appears. The red shift of the polaron transition band from 773 to 831 nm is due to the longer conjugation length in the oriented PANI microsize fibers in comparison with the spheres, short fibers, and flakes. From the red shift in the UV-vis absorption spectra, H+ attached to the N sites in the backbone can be expected to increase the torsional angle appended by the trans-cis-azobenzene photoisomerization between adjacent rings. This will cause the extent of orbital overlap between the phenyl π electrons and the nitrogen lone pairs. This further decreases the extent of π conjugation, resulting in increasing the energy lever of πq. XRD patterns for PANI samples synthesized by our method and the conventional method are given in Figure 6. The XRD patterns of PANI fiber by our mathod shows three typical diffraction peaks at 2θ = 6.0, 20.1, and 25.5° (d spacing = 14.71, 4.41, and 3.49 A˚, respectively). The peak centered at 2θ = 20.1° may be ascribed to periodicity parallel to the polymer chain, and the peak at 2θ = 25.5° may be caused by the periodicity perpendicular to the polymer chain.21 The three peaks at 20.1 and 25.5° are generally observed in PANI products, but the peak at around 2θ = 6.0° is observed only for highly ordered PANI structures in which the PANI chain distance increases by effective interdigitations of dopant molecules.22 As a comparison, PANI synthesized by a conventional (20) Anilkumar, P.; Jayakannan, M. Langmuir 2006, 22, 5952–5957. (21) Zhang, Z. M.; Wei, Z. X.; Wan, M. X. Macromolecules 2002, 35, 5937–5942. (22) Li, W.; Zhu, M.; Zhang, Q.; Chen, D. Appl. Phys. Lett. 2006, 89, 103110.
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anilinium cation, the PANI molecules cannot aggregate and grow in an orderly manner under vigorous stirring, and the anilinium cations aggregate randomly to form only amorphous powders.
Figure 6. XRD patterns of PANI synthesized under various conditions: (a) The synthesis condition is the same as in Figure 1f. (b) Conventional method (under vigorous stirring, [HCl]/[ANI] = 8, polymerization at 25 °C, and growth at 0 °C for 10 h).
method was also scanned for the XRD pattern. A broad diffraction band is observed, showing that the resulting PANI structure is amorphous. In the PANI fiber system, as mentioned above, the aniline monomer reacts easily with HCl to form polyanilinium salt As a templete, polyanilinium salt possesses a linear 1D backbone, which induces the self-assembly of PANI with a higher length/radii ratio and forms PANI flakes and fibers with good crystallinity. In the conventional method, although the aniline monomer can react with HCl to form an
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Conclusions Oriented polyaniline (PANI) microstructures (e.g., bulk, spheres, flakes, and fibers) were prepared without a template by polymerization at 80 °C and crystal growth at 0 °C and using a self-assembly method in the presence of hydrochloric acid (HCl) as the dopant. It is found that the lower aniline concentration [ANI] and higher [HCl] produce more highly ordered PANI long fibers. By adjusting the [HCl]/[ANI] molar ratio and [ANI], the shapes and sizes of PANI microstructures can be controlled. The formation mechanism of PANI microstructures is believed to be that as the polyanilinium salt is separated out, when the temperature decreases to 0 °C, as a template with a 1D backbone, polyanilinium salt induces the self-assembly of PANI with a higher length/radii ratio. FTIR and UV-vis absorption spectra and XRD measurements show that the main chain structures are identical to those of the emeraldine salt form of PANI. Moreover, the resulting structures are crystalline and different with conventional synthesized PANI powders. Acknowledgment. We are grateful for joint support by the National High Technology Research and Development Program of China (no. 2008AA03Z2470974), the National Natural Science Foundation of China (nos. 50572030 and 50842027), the Specialized Projiect of Fujian Province (no. 2007HZ0001-3), and the Specialized Research Fund for the Doctoral Program of Chinese Higher Education (no. 20060385001).
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