19836
J. Phys. Chem. C 2008, 112, 19836–19840
Facile Synthesis of Hierarchical Polyaniline Nanostructures with Dendritic Nanofibers as Scaffolds Xu-Sheng Du,*,† Cui-Feng Zhou,‡ and Yiu-Wing Mai† Centre for AdVanced Materials Technology (CAMT), School of Aerospace Mechanical and Mechatronic Engineering J07, UniVersity of Sydney, Sydney, NSW 2006, Australia, and Australian Key Centre for Microanalysis and Microscopy F09, UniVersity of Sydney, Sydney, NSW 2006, Australia ReceiVed: August 04, 2008; ReVised Manuscript ReceiVed: September 11, 2008
Hierarchical polyaniline nanostructures with branched nanofibers as scaffolds are prepared by a solution method using ferric chloride oxidant without using any organic structure directing reagent or hard template. The synthesis parameters, such as reaction temperature and time, on the morphologies and crystal structures of polyaniline nanomaterials have been investigated. After 24 h of polymerization at 25 °C, the surfaces of polyaniline dendritic nanofibers are decorated with polyaniline nanorods with diameters of 5-20 nm. Increasing the reaction temperature to 60 °C, the polyaniline nanofibers become smoother, longer, and thinner. XRD, TGA, FTIR, and UV-vis analyses confirm that the products are emeraldine salts with good crystallinity. Also, the product polymerized at 60 °C yields higher thermal stability but lower doping level than that fabricated at 25 °C, which may be caused by the higher content of the phenazine-like segment in the product. The growth process has been discussed. 1. Introduction Among the large family of conductive polymers, polyaniline is one of the most remarkable and has been studied and used in many fields,1 such as electrode materials in batteries, anticorrosion coatings, chemical sensors, superhydrophobic materials, and light-emitting and electronic devices. Recently, many efforts have been made on the synthesis of polyaniline nanomaterials.2 The conductive polymers can be synthesized by a variety of methods, such as electrochemical oxidation polymerization,1,3 solid-state polymerization,4 chemical vapor deposition,5 and the often-used chemical oxidation polymerization in solutions.2 Although various 1D or 2D polyaniline nanomaterials such as nanofibers, nanoplates, nanotubes, and nanorods have been prepared and studied,2 there are few reports on the more complicated hierarchical structure of this functional polymer. In a recent study, we prepared dendritic polyaniline nanofibers with a solid-state mechanochemical polymerization method in the absence of any structure-control organic molecules or porous templates.4a However, it was found that when the same reaction system was transferred to a solution polyaniline hierarchical structures with nanorods growing on dendritic nanofibers could be produced, which is very different from previously reported polyaniline nanostructures.2 Moreover, both molecular structure and morphology of the product are easily tuned by adjusting the reaction time and temperature in the case of the solution method. In this paper, details of the synthesis of such assembly structures are reported and the corresponding growth mechanisms are discussed. 2. Experimental Details In a typical synthesis, aniline hydrochloride (Aldrich, 98%) (10.0 mmol) was directly dissolved in deionized water (30 mL) * Corresponding author. Fax: +61-2-9351-3760. E-mail:
[email protected] (XS Du). † School of Aerospace Mechanical and Mechatronic Engineering J07. ‡ Australian Key Centre for Microanalysis and Microscopy F09.
in a vial, and FeCl3 · 6H2O (10.0 mmol) was also dissolved in deionized water (20 mL) in a separate vial. The two solutions were vigorously mixed at room temperature for 60 s and then remained without disturbance at the same temperature. After 24 h, the precipitate was collected and purified by using a Buchner funnel with a water aspirator to wash the product with 1 mol/L HCl solution, deionized water, and ethanol. A small portion of the wet product was then dispersed in 10 mL of ethanol. The suspension was transferred to copper grids for microscopy studies. The other part of the product was dried under vacuum at room temperature for further characterization. Wide-angle X-ray diffraction patterns (XRD) were obtained using an X-ray diffractometer (Siemens D5000). Transmission electron microscopy (TEM) observations were performed on a Philips CM12 at 120 kV accelerated voltage. Scanning electron microscopy (SEM) observations were performed on a Philips XL30 at 20 kV accelerated voltage. Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra and the UV-vis spectra of the products were recorded on a Bruker IFS66V FTIR spectrometer and Cary 5-UV-vis spectrometer, respectively. Weight loss temperatures of the products were measured with a TA thermogravimetric analyzer (TGA 2950) at a heating rate of 20 °C/min from 50 to 700 °C in a N2 atmosphere. 3. Results and Discussion Figure 1 shows TEM images of polyaniline nanostructures synthesized at 25 °C. A large quantity of polyaniline nanofibers with many small rods on their surfaces can be seen in Figure 1a. The diameters of most fibers are less than 150 nm, and their lengths are up to 1 µm. The magnified TEM image (Figure 1b) shows that many polyaniline short nanorods with a diameter of 5-20 nm and average length of ∼70 nm grow from the nanofibers to form a tree coralloid hierarchical structure. Occasionally, a few nanorods on the nanofibers are several hundred nanometers long and connect different nanofibers (indicated by arrows in Figure 1a). The hierarchical nanostructure of the product is also confirmed by SEM observations
10.1021/jp8069404 CCC: $40.75 2008 American Chemical Society Published on Web 11/18/2008
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Figure 1. TEM images (a, b) and SEM image (c) of the products obtained after reaction for 24 h at 25 °C.
Figure 2. TEM images (a, b) and SEM image (c) of the products obtained after reaction for 24 h at 60 °C.
(Figure 1c). Here, the polyaniline nanostructure is comparable to that synthesized by vanadic acid as oxidant.6a However, when compared to the expensive and toxic vanadic acid, ferric chloride is an economically available and environmentally friendly chemical reagent. The influences of experimental conditions on the morphology of the products have been investigated. Increasing the reaction temperature is found to significantly affect the polymer nanostructure. Figure 2a shows the polyaniline products synthesized at 60 °C are branched nanofibers with diameters between 30-50 nm and lengths up to several microns. A highly magnified image (Figure 2b) reveals that some unique cone-shaped protuberances appear on the surfaces of the branched polyaniline nanofibers, along with a few tiny nanorods. Comparing Figures 1a,b and 2a,b, it is seen that the polyaniline nanofibers prepared at high temperature are obviously much smoother, thinner, and longer. This is also confirmed by SEM observations, whereby the abundant entangled clean nanofibers in Figure 2c can be contrasted with those heavily decorated nanofibers prepared at 25 °C (Figure 1c). Stejskal’s group has also reported that the
morphology of polyaniline particles can be varied from submicron spheres to coral-like objects by increasing the polymerization temperature from 0 to 40 °C, when the dispersion polymerization is in the presence of hydroxypropylcellulose stabilizer and ammonium peroxodisulfate (APS) oxidant.2n It is suggested that the high polymerization rate at high temperature and the autoacceleration effect in the polymerization of polyaniline play an important role in the formation of the shape of the products. Note that the polymerization yield at 60 °C (∼67%) is much higher than that at 25 °C (∼25%). Compared to the ∼8% yield for solid-state polymerization with the same oxidant,4a the polymerization in solution is obviously beneficial for mass production of dendritic polyaniline nanostructures. The effect of stirring on the product morphology is also studied. It is found that the products prepared with stirring have a hierarchical structure similar to that synthesized under static conditions (Figure S1, see Supporting Information). Figure 3 shows the XRD patterns of the polyaniline nanomaterials prepared at 25 and 60 °C, respectively. Three broad peaks centered at 2θ ) 9° (001), 14.6° (011), and 20.3° (100)
19838 J. Phys. Chem. C, Vol. 112, No. 50, 2008
Figure 3. XRD pattern of the polyaniline products obtained after reaction for 24 h at 25 and 60 °C.
Figure 4. FTIR-ATR spectrum of the polyaniline products obtained after reaction for 24 h at 25 and 60 °C.
and an intense peak at 2θ ) 25.3° (110) are observed. These sharp peaks indicate the resultant polymers are in the highly doped emeraldine salt form.4a,2m,7 The similarity of XRD patterns of polyaniline prepared at both temperatures indicates the same emeraldine salt form of the product. All these results confirm that the products with good crystallinity can be synthesized in a range of temperatures despite their different morphologies. The FTIR spectra of the synthesized polyaniline nanostructures shown in Figure 4 are similar to those reported products prepared by a solid state polymerization method,4a again confirming the emeraldine salt form of both samples. The characteristic peaks at 1477 and 1558 cm-1 belong to the CdC stretching vibration mode of benzenoid and quinonoid rings, respectively. The absorption bands at 1282, 1100, and 1230 cm-1 correspond to the C-N stretching modes and the C-H in-plane bending and the protonated C-N group, respectively. Compared to the spectrum of polyaniline prepared at 25 °C, a new peak appears at 1440 cm-1 and a weak one at 1640 cm-1 in the product synthesized at 60 °C, and both peaks can be attributed to the phenazine-like segment.8 As the phenazinelike structure is formed through both ortho- and para-linked aniline units, it can be assumed that the oxidation polymerization rate at higher temperature is much faster than that at lower temperature from the viewpoint of chemical reaction dynamics theory, resulting in a higher content of phenazine-like segments in the product. In the spectrum of both of the fabricated polyaniline materials (Figure 5), the absorption peak at ∼350 nm can be ascribed to
Du et al.
Figure 5. UV-vis spectrum of the polyaniline products obtained after reaction for 24 h at 25 and 60 °C.
a π-π* transition of the benzenoid rings,2e,4c,9 while the peaks at∼430 and ∼830 nm can be attributed to polaron-π* transition and π-polaron transition, respectively. Interestingly, for the product synthesized at 60 °C, the intensity of the peak at ∼830 nm is less than the one at ∼350 nm, while the result is the opposite for the product prepared at 25 °C. As the doping level could roughly be estimated from the UV-vis absorption spectra of polyaniline, e.g., the ratio of absorbance at ∼830 and ∼350 nm indicates the doping level of polyaniline salts,2e,4c,9 the results imply that the doping level of polyaniline prepared at 25 °C is higher than the one synthesized at 60 °C. This difference could also result from the higher content of the phenazine-like segments in the product fabricated at 60 °C. Figure 6a shows typical TG curves of polyaniline products prepared at 25 and 60 °C. Its derivative TG curves (Figure 6b) show clearly three stages of weight loss below 700 °C. The results are similar to those previously reported and confirm the emeraldine phase of the products.4a,2l,m It is evident from Figure 6b that the maximum weight loss temperature of the third stage (attributed to the decomposition of the polyaniline main chains) increases from 490 to 510 °C when the reaction temperature increases from 25 to 60 °C. Moreover, gradual weight loss over the wide temperature range in Figure 6a indicates enhanced thermal stability of polyaniline prepared at 60 °C, which may be due to the higher content of phenazine-like segments in the product, as indicated in the above FTIR analysis (Figure 4). To investigate the growth process of the nanostructured polymer, the morphology of the products synthesized at different reaction time was monitored by TEM. Figure 7 shows these TEM images which demonstrate the shape evolution from fine branched nanofibers to hierarchical structure with increasing reaction time. After 0.5 h, the products consist of mainly smooth dendritic nanofibers but not isolated fibers or bundles of single fibers (Figure 7a,b). The diameters of the branched fibers are in the range of 25-40 nm, and their lengths are up to 1 µm. Although similar polyaniline interconnected nanofibers prepared with the assistance of special surfactants2g or in organo-gel6b have been reported recently, our method is simpler as no such additional organic molecules are involved. Some asperities can be found on the surfaces of polyaniline nanofibers in Figure 7b, which may be heterogeneous nucleations for the secondary growth of polyaniline.4a After 3 h, the branched nanostructure remains, but some rods with diameter 5-20 nm and length 10-30 nm (indicated by arrows) appear on the surfaces of the branched nanofibers as shown in Figure 7c and its magnified image in Figure 7d. In comparison with Figure 7a,b, the diameter of the polyaniline nanofibers increases slightly to 30-50 nm.
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J. Phys. Chem. C, Vol. 112, No. 50, 2008 19839
Figure 6. (a) TGA and (b) DTG curves of the branched polyaniline products prepared at 25 and 60 °C.
Figure 7. TEM images of the products obtained after reaction at 25 °C for 0.5 h (a, b) and 3 h (c, d).
When the reaction time is further increased to 24 h, the branched nanofibers are densely decorated with more nanorods and accompanied by an obvious increase of the length of the nanorods (Figure 1). As suggested previously,10 the formation of 1D polyaniline nanofibers is related to the homogeneous nucleation, while heterogeneous nucleation leads to granular particulates. As a competition between a 1D growth process and grain growth occurred during the growth of the polymer,2a many nucleation centers rapidly form on the preformed nanofibers, being accompanied by the growth of 1D fibers. During the polymerization of polyaniline with FeCl3, HCl is expected to produce continuously in the reaction system. The constant formation of HCl around the fiber may benefit the autocatalytic polymerization on the active spots. Obviously, the preformed fibers become the scaffolds for the secondary growth of the nanorods. The small protuberances (Figure 7c,d) on the nanofibers may act as active sites for further growth of nanorods.4a At the later stage of the polymerization, such polyaniline nanorods cannot grow as thick as those preformed fibers due to the greatly
decreased concentration of the reactive species (such as aniline, oligomers, and Fe3+ ions). In the event of high reaction temperature, the competition is in favor of 1D growth due to the higher oxidation reaction rate and diffusion rate of the reactive species in the solution than those at low temperature.10 Thus, the growth rate of the nanofibers is so fast that fewer active species take part in the secondary growth of polyaniline, resulting in relatively smoother 1D nanofibers. This agrees with the recent report on polyaniline nanofibers prepared with APS oxidant,11 where it is suggested that the anisotropic growth of polymer could be attributed to a depletion region along the length of the growing fibers, and the fresh polymer growth occurred predominantly at the ends of the fibers. The experimental results that the length of nanofibers fabricated at 60 °C is always longer than that prepared at 25 °C while their diameter is smaller can also be explained by the same consideration. Moreover, the presence of the peak from the phenazine-like units in the FTIR spectrum of the product obtained at 60 °C (Figure 3) is also consistent with the above proposed growth mechanism.
19840 J. Phys. Chem. C, Vol. 112, No. 50, 2008 4. Conclusions Polyaniline hierarchical nanostructures with branched nanofibers as scaffolds have been synthesized using ferric chloride as an oxidant without hard templates or structure-control reagents. The polyaniline nanostructures with polyaniline nanorods standing on the surfaces of the nanofibers can be obtained at room temperature. The diameters and surface roughness of the polyaniline nanofibers can be tuned by varying the reaction time and temperature. The hierarchical highly branched structure of the prepared polyanilines will broaden their applications in many fields. Acknowledgment. X.S.D. and Y.-W.M. would like to thank the Australian Research Council for the financial support of this project. C.F.Z. also acknowledges the award of a PhD scholarship from the Australian Key Centre for Microscopy and Microanalysis at the University of Sydney. Supporting Information Available: TEM image of the polyaniline nanomaterials. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. I 1986, 82, 2385. (b) MacDiarmid, A. G.; Epstein, A. J. Faraday Discuss., Chem. Soc. 1989, 88, 317. (2) (a) Chiou, N.-R.; Epstein, A. J. AdV. Mater. 2005, 17, 1679. (b) Zhou, C.; Han, J.; Song, G.; Guo, R. Macromolecules 2007, 40, 7075. (c)
Du et al. Ding, H.; Wan, M.; Wei, Y. AdV. Mater. 2007, 19, 465. (d) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (e) Xia, H. S.; Wang, Q. J. Nanoparticle Res. 2001, 3, 401. (f) Zhong, W. B.; Yang, Y. X.; Yang, Y.; Sun, Y. F.; Deng, J. P.; Yang, W. T. J. Phys. Chem. B 2007, 111, 3918. (g) Zhong, W. B.; Deng, J. Y.; Yang, Y. S.; Yang, W. T. Macromol. Rapid Commun. 2005, 26, 395. (h) Zhou, C.; Han, J.; Guo, R. J. Phys. Chem. B 2008, 112, 5014. (j) Park, M.-C.; Sun, Q.; Deng, Y. Macromol. Rapid Commun. 2007, 28, 1237. (k) Liu, H.; Hu, X. B.; Wang, J. Y.; Boughton, R. I. Macromolecules 2002, 35, 9414. (l) Liu, D. F.; Du, X. S.; Meng, Y. Z. Mater. Lett. 2006, 60, 1847. (m) Du, X. S.; Xiao, M.; Meng, Y. Z. Eur. Polym. J. 2004, 40, 1489. (n) Stejskal, J.; Sˇpı´rkova´, M.; Riede, A.; Helmstedt, M.; Mokreva, P.; Prokesˇ, J. Polymer 1999, 40, 2487. (3) (a) Kim, B. H.; Park, D. H.; Joo, J.; Yu, S. G.; Lee, S. H. Synth. Met. 2005, 150, 279. (b) Du, X.; Wang, Z. Electrochim. Acta 2003, 48, 1713. (c) Huang, L.; Wang, Z.; Wang, H.; Cheng, X.; Mitra, A.; Yan, Y. J. Mater. Chem. 2002, 12, 388. (4) (a) Du, X. S.; Zhou, C. F.; Wang, G. T.; Mai, Y.-W. Chem. Mater. 2008, 20, 3806. (b) Huang, J.; Moore, J. A.; Acquaye, J. H.; Kaner, R. B. Macromolecules 2005, 38, 317. (c) Tursun, A.; Zhang, X. G.; Ruxangul, J. Mater. Chem. Phys. 2005, 90, 367. (5) Zaharias, G. A.; Shi, H. H.; Bent, S. F. Thin Solid Films 2006, 501, 341. (6) (a) Li, G.; Jiang, L.; Peng, H. Macromolecules 2007, 40, 7890. (b) Li, G.; Zhang, Z. Macromolecules 2004, 37, 2683. (7) Pouget, J. P.; Jozefowicz, M. E.; Epstein, A. J.; Tang, X.; MacDiarmid, A. G. Macromolecules 1991, 24, 779. (8) (a) Trchova´, M.; Sˇedeˇnkova´, I.; Konyushenko, E. N.; Stejskal, J.; Holler, P.; C´iric´-Marjanovic´, G. J. Phys. Chem. B 2006, 110, 9461. (b) Stejskal, J.; Sapurina, I.; Trchova´, M.; Konyushenko, E. N.; Holler, P. Polymer 2006, 47, 8253. (9) Shreepathi, S.; Holze, R. Chem. Mater. 2005, 17, 4078. (10) Li, D.; Kaner, R. B. J. Am. Chem. Soc. 2006, 128, 968. (11) Chiou, N.-R.; Epstein, A. J. Synth. Met. 2005, 153, 69.
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