Facile Synthesis of Polyaniline Nanofibers Using Chloroaurate Acid

It is interesting to study the reaction between HAuCl4 and aniline and the structure or properties of the resultant products. In this present work, we...
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Articles Facile Synthesis of Polyaniline Nanofibers Using Chloroaurate Acid as the Oxidant Yong Wang, Zhimin Liu,* Buxing Han,* Zhenyu Sun, Ying Huang, and Guanying Yang Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100080, China Received October 18, 2004 This work demonstrated a facile route to the synthesis of polyaniline (PANI) nanofibers by polymerization of aniline using chloroaurate acid (HAuCl4) as the oxidant. The reduction of AuCl4- is accompanied by oxidative polymerization of aniline, leading to uniform PANI nanofibers with a diameter of 35 ( 5 nm and aggregated gold nanoparticles which can precipitate from the liquid phase during the reaction. The resultant PANI nanofibers and gold particles were characterized by means of different techniques, such as UV-vis, FTIR spectroscopy, and scanning and transmission electron microscopy methods. It is found that the gold aggregates are capped with polyaniline, and the conductivity of the fibers is around 0.16 S/cm.

* To whom correspondence may be addressed. Fax: 86-1062562821. E-mails: [email protected], [email protected].

liquids.9 Zhang et al. has developed a “nanofiber seeding” technique, in which very small amounts of biological, inorganic, or organic nanofibers are used as seeds in the conventional chemical oxidative polymerization of aniline, resulting in bulk quantities of PANI nanofibers.10 In the chemical synthesis of polyaniline, various oxidants, including ammonium peroxydisulfate (APS),1,8-10 potassium bichromate,11 hydrogen peroxide,12 etc., have been employed for the oxidative polymerization of aniline, often in the presence of acid, though APS is the most often used oxidant. Recently, it is found that chloroaurate ions (AuCl4-) can also be applied as an oxidant in the oxidative polymerization of pyrrole, leading to gold nanostructures capped with polypyrrole.13 Sastry’s group demonstrated that AuCl4- could be reduced by amine-containing molecules including hexadecylaniline, diamine-containing oxyethylene linkage, resulting in metallic gold particles accompanied with the formation of corresponding polymers.14-16 In the above-mentioned work, much more emphasis was focused on the structure and properties of gold nanoparticles, and few efforts were made to investigate the morphology and properties of polymers simultaneously generated with the gold nanoparticles. Very recently, there is a report on the synthesis of poly(ophenylenediamine) nanobelts from the o-phenylenedi-

(1) Wei, Z. X.; Zhang, Z. M.; Wan, M. X. Langmuir 2002, 18, 917. (2) (a) Skotheim, T. A.; Elsenbaumer, R. L.; Reynolds, J. R. Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1997. (b) Chiang, J. C.; MacDiarmid, A. G. Synth. Met. 1986, 13, 193. (c) Xia, Y. N.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995, 7, 443. (3) (a) Vincent, B.; Waterson, J. J. Chem. Soc. Chem. Commun. 1990, 683. (b) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstedt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814. (c) Yang, C. Y.; Smith, P.; Heeger, A. J.; Cao, Y.; Osterholm, J. E. Polymer 1994, 36, 1142. (4) Wu, C. G.; Bein, T. Science 1994, 264, 1757. (5) (a) Martin, C. R. Chem. Mater. 1996, 8, 1739. (b) Parthasarathy, R. V.; Martin, C. R. Chem. Mater. 1994, 6, 1627. (6) Wang, C. W.; Wang, Z.; Li, M. K.; Li, H. L. Chem. Phys. Lett. 2001,341, 431. (7) Michaelson, J. C.; McEvoy, A. J. Chem. Commun. 1994, 79. (8) Qiu, H. J.; Wan, M. X. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 3485.

(9) (a) Huang, J. X.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (b) Huang, J. X.; Kaner, R. B. J. Am. Chem. Soc. 2004, 126, 851. (10) Zhang, X. Y.; Goux, W. J.; Manohar, S. K. J. Am. Chem. Soc. 2004, 126, 4502. (11) Genie´s, E. M.; Tsintavis C.; Syed, A. A. Mol. Cryst. Liq. Cryst. 1985, 121, 181. (12) Sun, Z.; Geng, Y.; Li, J.; Jing, X.; Wang, F. Synth. Met. 1997, 84, 99. (13) (a) Selvan, S. T.; Spatz, J. P.; Klok, H. A.; Moller, M. Adv. Mater. 1998, 10, 132. (b) Bhattacharjee, R. R.; Chakraborty, M.; Mandal, T. K. J. Nanosci. Nanotechnol. 2003, 3, 487. (14) Selvakannan, P. R.; Mandal, S.; Pasricha, R.; Adyanthaya, S. D.; Sastry, M. Chem. Commun. 2002, 13, 1334. (15) Selvakannan, P. R.; Kumar, P. S.; More, A. S.; Shingte, R. D.; Wadgaonkar, P. P.; Sastry, M. Langmuir 2004, 20, 295. (16) Selvakannan, P. R.; Kumar, P. S.; More, A. S.; Shingte, R. D.; Wadgaonkar, P. P.; Sastry, M. Adv. Mater. 2004, 16, 966.

Introduction Polyaniline (PANI), as the most extensively studied conducting polymer,1 has been widely investigated for electronic and optical applications due to its good environmental stability and tunable electrical and optical properties.2 Different morphologies of PANI have been obtained through different synthesis or processing routes.3 In recent years, one-dimensional (1-D) PANI nanostructures, including nanofibers and nanotubes, have attracted much research interest since they are a kind of desirable molecular wire due to their 1-D structure and metal-like and controllable conductivity. In general, 1-D PANI nanostructures are synthesized chemically or electrochemically through a “template synthesis” route. Different templates such as zeolite channels,4 track-etched polymer membranes,5 and anodized alumina membrane,5a,6 or soft templates including surfactants,7 and micelles,1,8 have been employed to direct the growth of the 1-D nanostructures of PANI. Recently, some new methods have been reported to synthesize 1-D PANI. For example, an interfacial polymerization method has been applied to prepare PANI fibers at the interface of two immiscible

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amine-HAuCl4 aqueous solution.17 Although so many aniline derivates can react with HAuCl4 and give interesting gold nanoparticles or polymer structures, no research work is carried out on the reaction of AuCl4with aniline, the most widely used monomer in the synthesis of conducting polymer. The synthesis of Au nanoparticle-polyaniline composite using H2O2 as both oxidizing and reducing agent has been reported.18 It is interesting to study the reaction between HAuCl4 and aniline and the structure or properties of the resultant products. In this present work, we studied the oxidative polymerization of aniline using HAuCl4 as the oxidant via simply mixing aniline acidic aqueous solution with HAuCl4 solution at room temperature, resulting in PANI nanofibers and aggregated gold particles simultaneously. We paid more attention to the produced PANI, which was characterized with different techniques including UVvis, FTIR spectroscopy, and scanning and transmission electron microscopy (SEM and TEM). The resultant gold particles were also investigated.

Wang et al.

Figure 1. FTIR spectrum of the dark green product (PANI).

Experimental Section All chemicals were purchased from Beijing Chemicals Plant with reagent purity. Double distilled aniline (465 mg, 5 mmol) was dissolved in 20 mL of 1 M HCl aqueous solution, and HAuCl4‚ 3H2O (394 mg, 1 mmol) was dissolved in 20 mL of water, respectively. These two solutions were then mixed at room temperature under vigorous agitation. Within 10 min, the color of the mixture changed from the original buff to blue and then dark green. After 24 h, the stirrer was stopped, and the golden particles precipitated slowly. The precipitate was collected via leaching out the upper liquid phase, and the liquid phase was centrifuged at 4000 rpm, resulting in a dark green precipitate. The golden and dark green precipitates were washed with distilled water several times, respectively, and dried under vacuum at 60 °C. The morphology of these two kinds of products was examined with a JEOL 2010 transmission electron spectroscope (TEM) operated at 200 keV, and a Hitachi S-4300 field emission scanning electron spectroscope (FE-SEM) operated at 15 keV, equipped with a Phoenix energy dispersive analysis X-ray (EDAX) attachment. The samples were observed directly under SEM without the sputter-coating of a conductive thin layer due to the good electrical conductivity of these two products. The dark green precipitate was dispersed into water to form a relatively stable suspension, several drops of which were cast onto a glass slide, and then dried in air at room temperature, finally resulting in a dark green film. The film was used for the cross-sectional SEM observation. The X-ray diffraction (XRD) was performed on a Bruker D8 Advance X-ray diffractometer with high-intensity Cu KR radiation. The FTIR spectrum of the dark green product was obtained from a Burker Tensor 27 spectrometer in KBr pellet form, and the UV-vis spectrum was recorded on a TU-1201 UVvis spectrometer. The conductivity of the resulting dark green product was determined using the standard four-probe method.

Results and Discussion In this work, the dark green product could be readily dispersed in water to form a clear, green suspension, which was stable for several days, while golden product was easily precipitated from water. This difference gives the convenience for the separation of these simultaneous formed products by step precipitation. We first examined the spectroscopic behavior of the dark green product, and its FTIR and UV-vis spectra are shown in Figure 1 and Figure 2, respectively. Obviously, they are the typical spectra of HCl-doped PANI,19 which means that the dark green product is PANI produced from the (17) Sun, X.; Dong, S.; Wang, E. Chem. Commun. 2004, 1182. (18) Sarma, T. K.; Chowdhury, D.; Paul, A.; Chattopadhyay, A. Chem. Commun. 2002, 10, 1048.

Figure 2. UV-vis spectrum of the dark green product (PANI) in water

reaction between aniline and HAuCl4. In Figure 1, the large descending baseline in the spectral region of 40002000 cm-1 has been attributed to free-electron conduction in the doped polymer,20 and the main peaks at 1574 and 1493 cm-1 correspond to stretching deformations of quinonimine and benzene rings, respectively.18,19c,21 As shown in Figure 2, the UV-vis spectrum of the dark green product dispersed in water shows two adsorption peaks at ca. 400 and 800 nm, indicating it is PANI in conducting state.22 Four-probe pressed-pellet conductivity of the HCl-doped PANI nanofibers is about 0.16 S/cm, which is in the range of common HCl-doped PANI.9b The morphology of the resultant PANI was observed under an electron microscope. As shown in Figure 3, both the TEM and SEM images consistently indicate that the resultant PANI consists of uniform nanofibers with diameter around 35 nm, and the length of the fibers ranges from hundreds of nanometers to several micrometers. In addition, it is observed that many fibers join with others and form branched structures or interconnected networks, as shown in Figure 3b. It can be known from the cross section of the PANI nanofiber film shown in Figure 3d that these fibers are curved and entangled, demonstrating that the PANI nanofibers are very flexible. (19) (a) Zeng, X. R.; Ko. T. M. J. Polym. Sci., Part B: Polym. Phys. 1997, 35, 1993. (b) Quillard, S.; Louarn, G.; Buisson, J. P.; Boyer, M.; Lapkowski, M.; Pron, A.; Lefrant, S. Synth Met 1997, 84, 805. (c) Gao, H. X.; Jiang, T.; Han, B. X.; Wang, Y.; Du, J. M.; Liu, Z. M.; Zhang, J. L. Polymer 2004, 45, 3017. (20) Cao, Y.; Li, S.; Xue Z.; Guo, D. Synth. Met. 1986, 16, 305. (21) Sun, Y.; MacDiarmid, A. G.; Epstein, A. J. J. Chem. Soc., Chem. Commun. 1990, 529. (22) Athawale, A. A.; Kulkarni, M. V.; Chabukswar, V. V. Mater. Chem. Phys. 2002, 73, 1.

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Figure 3. (a) TEM image of PANI nanofibers. (b) Enlarged TEM image of a branched network of PANI nanofibers. (c) SEM image of PANI nanofibers. (d) Cross-sectional view of a PANI nanofibers film.

Figure 4. XRD pattern of the prepared golden deposit. Inset is its EDAX spectrum.

The above studies reveal that the PANI synthesized in the present work using HAuCl4 as the oxidant has a similar chemical structure with PANI prepared from the conventional route in which peroxydisulfate is used as the oxidant. And they also have close electrical conductivity. The main difference between the polymers from the two methods is that the morphology of the PANI obtained using peroxydisulfate or many other kinds of substances as the oxidant is nonfibrous particulates with hardly any order,10 while the PANI prepared in the present work through the oxidative polymerization of aniline in the presence of HCl has a uniform, highly ordered fiberlike structure with adiameter in the nanometer range. There are four main peaks in the XRD pattern of the golden product, as shown in Figure 4, which correspond to (111), (200), (220), and (311) Bragg reflections of Au,18 indicating that the golden product is gold. This is also confirmed by elementary analysis during the TEM observation, which shows that the product is mainly composed of Au. The mean size of the Au nanocrystals, calculated from the width of the (111) Bragg reflec-

tion using the Debye-Scherrer equation,23 is about 26 nm. Figures 5a and 5b display the SEM images of the obtained metal gold, which presents as particles with diameters in the range of several hundred of nanometers to several micrometers. A closer look at the particles reveals that they have a rough surface and are actually aggregates of nanoparticles with diameters around 3040 nm, which is slightly larger than the size determined by XRD analysis. TEM observations (Figures 5c and 5d) also confirm the size distribution and the rough surface of these gold microparticles. More importantly, at high magnification, the gold particles seem to be coated by a thin polymeric sheath. Energy dispersive analysis of X-rays (EDAX) on these particles shows that they are composed of mainly Au and considerably large amounts of C and N. (See inset in Figure 4. Cu peaks originated from the copper stub used to support samples in SEM observations.) Therefore, we can conclude that these gold particles are coated by a PANI layer. In other words, the gold particles are actually a core-shell composite material consisting of a gold core and PANI shell. And this coreshell structure was also obtained in the reactions of HAuCl4 with aniline derivates and pyrrole.13,15 As discussed above, the sizes of the nanoparticles observed through SEM observation are slightly larger than that determined from XRD analylsis. The main reason is that these nanoparticles are coated by a layer of PANI, As mentioned above, the present method of synthesis PANI is similar to other routes with the exception of using different oxidants, and the products possess similar chemical structure and conductivity but different morphology. Therefore, the formation of the fibrous PANI in the present method must arise from the unique oxidant. It is known that aniline is protonated in acidic solution. Therefore, in the experiment, when aniline acidic solution was mixed with chloroaurate acid, the protonated aniline is first electrostatically complexed with AuCl4-, and then reduction of HAuCl4 and oxidation of aniline occur (23) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley: Reading, MA, 1978.

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Figure 5. SEM images of gold particles at low magnification (a), showing the size distribution of the particles, and at high magnification (b), revealing that an individual gold microparticle is the aggregation of many nanoparticles. TEM images of gold particles at low magnification (c), showing their size distribution, and at high magnification (d), revealing that they are coated by a thin layer of polymer.

simultaneously, leading to the formation of gold nanoparticles and aniline oligomers, respectively. It was reported that gold nanoparticles can serve as catalysts to promote the oriented growth of one-dimensional nanostructures, such as carbon and silicon nanotubes25 and siloxane nanowires, filaments, and tubes.26 Sun et al. suggested a gold nanoparticle catalyzed oriented growth mechanism of the polymer nanobelts.17 In our present work, gold nanoparticles may also act as catalysts to direct the oriented growth of PANI nanofibers. However, the exact growth process of the PANI fibers is not very clear. Considering the fact that the size of the gold nanoparticles is comparable to the diameter of the PANI nanofibers and gold nanoparticles are capped with PANI, we think the polymer growth may start from the attachment of the aniline monomer and oligomer to some specific crystalline facets on the catalyst nanoparticles and more monomer molecules react with the propagating polymer chain, leading to the oriented growth of the PANI. (24) Grisel, R.; Weststrate, K. J.; Gluhoi, A.; Nieuwenhuys, B. E. Gold Bull. (London) 2002, 35, 39. (25) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (26) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Zaikovski, V.; Klabunde, K. J. J. Am. Chem. Soc. 2003, 125, 10488.

Conclusion In this work, we demonstrate a facile synthesis route to prepare PANI nanofibers using HAuCl4 as oxidant in the polymerization of aniline at room temperature. The generated PANI fibers possess a uniform diameter around 35 nm, some of which join together to form branched structures or interconnected networks. These fiberlike PANI products have similar chemical structure and conductivity to the polymer generated from the conventional method using APS to oxidize aniline. Simultaneously with the formation of PANI nanofibers, HAuCl4 was reduced to metallic gold nanoparticles. These nanoparticles aggregated to form microscale gold clusters and were capped with a thin layer of PANI. The PANI nanofibers may be formed through a gold nanoparticle catalyzed oriented growth mechanism. Acknowledgment. We gratefully acknowledge the helpful discussion with Professor Meixiang Wan of the Institute of Chemistry, the Chinese Academy of Sciences. This work is financially supported by National Natural Science Foundation of China (No. 20374057). LA047442Z