- Email: [email protected]
polypyrrole composites via two-step inverse microemulsion method
European Polymer Journal 43 (2007) 300–306
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Synthesis of graphite nanosheets/AgCl/polypyrrole composites via two-step inverse microemulsion method Zunli Mo *, Dandan Zuo, Hong Chen, Yinxia Sun, Ping Zhang MACROMOLECULAR NANOTECHNOLOGY
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China Received 2 October 2006; accepted 21 November 2006
Abstract An effective process for the synthesis of graphite nanosheets/AgCl/polypyrrole (NanoGs/AgCl/PPy) composites was developed. NanoGs were prepared by treating the expanded graphite with sonication in aqueous alcohol solution. Then nanocomposites were fabricated via in situ polymerization of pyrrole in the presence of NanoGs and AgCl particles through two-step sequentially inverse microemulsion method. The nanoscale dispersion of NanoGs and AgCl particles was evidenced by the SEM and TEM examinations. From the thermogravimetric analysis, the introduction of inorganic nano-AgCl and NanoGs exhibited a beneficial effect on the thermal stability of pure PPy. According to the four-pointprobe test, the conductivity of the final NanoGs/AgCl/PPy composites was dramatically increased compared with pure PPy. 2006 Elsevier Ltd. All rights reserved. Keywords: Graphite nanosheets (NanoGs); Nano-AgCl; Polypyrrole; Nanocomposites
1. Introduction Recently, composite materials based on inorganic and organic components have received an increasing interest assumingly due to their new favorable properties connected to the presence and combination of materials of different character [1,2]. Research on the topic of composite materials involves challenges and opportunities. The foremost challenge is that how to synthesize composite materials which can keep or enhance the best properties of each of the components while eliminating or * Corresponding author. Tel.: +86 931 7971829; fax: +86 931 7971933. E-mail address: [email protected] (Z. Mo).
reducing their particular limitations. This challenge provides the opportunity to develop new materials with specific behavior. And this behavior can lead to improved performances or to the finding of new and useful properties [3]. So far electronically conducting polymers (ECPs) are one type of the most frequently combined materials [4,5]. The considerable research of ECPs for a wide array of applications has been conducted over the past 30 years. Of all the synthetic ECPs, especially polypyrrole (PPy), polyaniline (PANi), polystyrene and polythiophene as conductive base in the composites or nanocomposites are used diffusely [6–10]. In this paper we chose electronically conducting PPy as one of the components to produce nanocomposite materials. As we know, PPy is one
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.11.023
Z. Mo et al. / European Polymer Journal 43 (2007) 300–306
make graphite nanosheets. Then NanoGs/AgCl/ PPy nanocomposites were synthesized by chemical oxidative polymerization of pyrrole monomer in the presence of graphite nanosheets and ultrafine AgCl. The structural property, thermal property and electrical conductivity of composites were investigated by different experimental techniques, including transmission electron microscope (TEM), scanning electron microscope (SEM), Fourier-transform infrared spectra (FT-IR), thermogravimetric analysis (TG), and four-point-probe method. Other properties and potential applications of the material will be given in future report. 2. Experimental section 2.1. Materials The graphite used for preparing graphite nanosheets was expandable graphite, supplied by Shandong Qingdao Graphite Company (China). The pyrrole monomer (Py), supplied by Shanghai Chemical Company (China), was purified by water vapour distillation before use. The initiator of polymerization, FeCl3 Æ 6H2O (A.R.) was purchased from Jinshan Chemical Plant (China). Trichloromethane (CHCl3), butyl alcohol (C4H10O), cetyltrimethylammonium bromide (CTAB), acetone (C3H6O), ethyl alcohol (C2H6O), AgNO3 and NaCl at analytical reagent grade were obtained from commercial suppliers and used as received. 2.2. Preparation of NanoGs The expandable graphite was heat treated at 950 ± 10 C for 30 s to obtain expanded graphite particles. The expanded graphite was immersed in a 75% of aqueous alcohol solution in an ultrasonic bath. The mixture was sonicated for 10 h, and then was filtered and washed with enough distilled water and ethyl alcohol. The obtained graphite powers called graphite nanosheets were dried under vacuum at ambient temperature for 12 h. 2.3. In situ polymerization to prepare nanocomposites The synthesis of NanoGs/AgCl/PPy nanocomposites was carried out by means of an oxidative polymerization of pyrrole monomer, in the presence of NanoGs and AgCl through two-step sequentially inverse microemulsion method. Initially, certain amount of AgNO3, NaCl was dissolved in redistilled
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of the most popular ECPs for various electronic applications such as the secondary battery [11], EMI shielding [12], light emitting diode [13], chemical and biochemical sensors [4] and conducting composite materials [3], because of its high conductivity, environmental stability to water and oxygen and ease of synthesis [14]. By an electrochemical or by a chemical oxidative polymerization, the PPy and its derivatives have been successfully synthesized. Generally, PPy is brittle, insoluble and infusible, and hence inprocessible. Therefore, in order to develop PPy-based conductive materials, several approaches including graft copolymerization containing pyrrole as grafted groups, electrochemical polymerization of pyrrole in the presence of latex particles with anionic surface [15], and oxidant-impregnated polymerization using pyrrole vapor [16], have been applied. Furthermore, Ruckenstein and Hong [17] introduced emulsion pathway to synthesize PPy-based conductive composites. Graphite has been widely used as electronically conducting filler for preparing conducting polymer/graphite composites in the last decade [18–25]. However, because of the small inter spaces (0.335 nm) of the graphite layers and the lack of the affinity for either hydrophilic or hydrophobic polymers, it is very difficult to prepare the polymer/graphite nanocomposites via direct intercalation method [23]. Thus, with certain chemical or physical modification to the graphite, it can achieve nanoscale dispersion of graphite in polymer matrix [20,21]. Expanded graphite is composed of a large number of delaminated graphite sheets. There are many pores and galleries in its structure [24,25]. But they might not be able to reach closed cavities inside the expanded graphite. The graphite sheets around the cavities tend to overlap and accumulate during processing. The other disadvantage is that the loading level of expanded graphite within a conductive polymeric nanocomposite is high [26,27]. For solving these problems, we can deal with expanded graphite and prepare graphite nanosheets (NanoGs). Nowadays, NanoGs are the best filler for making conducting polymer/graphite nanocomposite [1,20–22]. Great success has been achieved in polymer/graphite nanocomposites, but reports on the nanocomposites of graphite nanosheets, inorganic nanoparticles with polymer were rare. In this study, we reported on the preparation of NanoGs/AgCl/PPy nanocomposites through twostep sequentially inverse microemulsion method. Firstly, the expanded graphite was powdered to
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water to form 0.1 mol/l AgNO3, 0.1 mol/l NaCl aqueous solution. Secondly, 2.0 ml AgNO3 aqueous solution obtained above, 1.0 g CTAB, 2.0 ml C4H9OH and 0.9 ml pyrrole were charged into 15.0 ml CHCl3, and the mixture was sonicated for about 30 min. Once the inverse emulsion formed, it was mixed with 0.11 g NanoGs, which was kept in an ultrasonic bath for another 15 min. Then, added 2.0 ml 0.1 mol/l NaCl aqueous solution into the mixture with stirring for 1 h. Finally, 5.0 g FeCl3 Æ 6H2O was then dropped into the reactor and a rapid oxidation occurred. The resulting suspension was stirred at 10 C for 24 h. Excess acetone was then introduced into the reaction mixture to terminate the reaction. Upon being filtrated, the precipitate was washed with redistilled water and CHCl3 repeatedly. The solid was dried under vacuum at ambient temperature for 24 h. 2.4. Characterization and measurements Scanning electron microscope (SEM; Hitachi, Japan, JEOL, JSM-6330F) was used to observe the morphologies of NanoGs and nanocomposite. Prior to the examination, the specimens were coated with a very thin layer of gold. Transmission electron microscope (TEM; JEOL100CX-II model) was run at 100 kV accelerated voltage to observe the morphologies. The observations were carried out after retrieving the slices onto Cu grids. Fourier-transform infrared spectra (FT-IR) of the samples in KBr pellets were recorded on an EQUINOX55 FTIR spectrometer (Bruker). The weight loss temperatures of the composites were determined with a Perkin–Elmer thermogravimetric analyzer (TGDTA; model SSC-5200) from 20 to 900 C under environment atmosphere (10 C/min). The electrical conductivity measurements were made by the conventional four-point-probe method on pressed pellets of composite particles prepared at ambient temperature (25 C).
Fig. 1. SEM micrograph of NanoGs.
diameter ranging 0.5–20 lm, named NanoGs. This can also been seen in the TEM micrograph shown in the following part. The result was similar to the reports of other researchers [28,29]. The spherical AgCl particles were prepared through reverse microemulsion as template, in which CTAB was used as surfactant, C4H9OH was designated as assistant surfactant and CHCl3 was applied as the oily phase. These AgCl particles with particle diameters from 5 to 30 nm were clearly confirmed by the TEM image (Fig. 2). It indicated that the formation of nano-AgCl took place in the ‘‘water pools’’. The small-sized, fairly distributed ‘‘water pools’’ limited the formation space and shape of AgCl particles. An inverted emulsion of water containing the oxidant (FeCl3) in CHCl3, the CTAB surfactant was used as dispersant. Then pyrrole was introduced dropwise in the inverted emulsion. As a result, colloidal-sized PPy particles generated. The FT-IR spectra of the prepared PPy (Fig. 3) clearly exhibited characteristic absorption peaks with respect to PPy. The band at 3425 cm 1 corresponded to the N–H
3. Results and discussion 3.1. Structure of NanoGs, ultrafine AgCl and PPy We adopted ultrasonic irradiation technique to break down the expanded graphite and then obtained NanoGs. Fig. 1 showed the SEM micrograph of the as-prepared NanoGs. It can be seen clearly that the expanded graphite was torn into sheets with a thickness ranging 30–80 nm and a
Fig. 2. TEM micrograph of AgCl nanoparticles.
Transmittance/%
Pure PPy
26.5
25
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
Fig. 3. FT-IR absorption spectra of pure PPy.
stretching vibration and that at 1512 cm 1 reflected C@C stretching vibration. The broad band from 1400 to 1250 cm 1 was attributed to C–H or C–N in-plane deformation modes and had a maximum at 1291 cm 1 [30]. The absorptions at 779 and 671 cm 1 were related to the C–H outerbending vibrations [31]. The above results indicated the formation of PPy. 3.2. Structure of NanoGs/AgCl/PPy nanocomposites The reverse microemulsion containing different components was prepared firstly. Then the droplets of reverse microemulsion collided to exchange molecules. In this step, the AgCl precipitate formed after NanoGs were mixed. Lastly pyrrole began to polymerize by addition of FeCl3 Æ 6H2O on the base of inorganic substances and the so-formed nuclei grew. Because NanoGs were in nanoscale, they tended to accumulate each other and were difficult to be dispersed in polypyrrole by traditional ways. Ultrasonic irradiation has been widely applied in chemical reaction and engineering. When an ultrasonic wave passes through a liquid medium, the effect of ultrasonic cavitation will take place and generate a very strong stirring environment. Therefore, ultrasound has been extensively used in dispersion, emulsifying, crushing, and activation of particles. In this case, sonication was an effective way for the optimal dispersion of the NanoGs in reverse microemulsion system and in the polypyrrole matrix. When the instant in situ polymerization of the pyrrole occurred, the NanoGs and inorganic nano-AgCl were dispersed and fixed among the polypyrrole molecules. Using this chemical reaction, we synthe-
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sized NanoGs/inorganic nanoparticles AgCl/PPy composites successfully. The typical SEM pictures of NanoGs/AgCl/PPy composites were presented in Fig. 4. Fig. 4a and b were the samples magnified 1.2 · 104 times and 2.0 · 104 times, respectively. We can observe clearly that NanoGs dispersed in PPy matrix and the PPy also embedded the NanoGs. Furthermore, NanoGs were distributed quite uniformly within the PPy matrix and no aggregates can be seen in the mixture, and then the inorganic nanoparticles AgCl were covered in the PPy matrix and NanoGs separately. Due to this especial structure, the interfacial affinity of NanoGs and inorganic AgCl with the PPy increased and led to an ensured thermal stability and electrical contact. Fig. 5 showed TEM images of NanoGs/AgCl/ PPy composites dispersed in ethyl alcohol. The gray lines and flakes represented the graphite layers. It can be seen that the nanoscale patterns of the graphite layers well dispersed in PPy matrix. This kind of NanoGs as observed had thickness of 40–90 nm, which was a little bigger than that of the result in Fig. 1 due to the presence of modified layer of PPy [32]. And the size of dark spots in the cross section of NanoGs/AgCl/PPy composites was in the range of 50–80 nm. Compared with the size of
Fig. 4. SEM of NanoGs/AgCl/PPy composites: (a) lower magnification and (b) higher magnification.
Fig. 5. TEM of NanoGs/AgCl/PPy composites: (a) lower magnification and (b) higher magnification.
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Z. Mo et al. / European Polymer Journal 43 (2007) 300–306
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Transmittance/%
1.0
0.8
0.6 4000
3500
3000
2500
2000
1500
1000
500
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Wavenumber/cm-1
Fig. 6. FT-IR absorption spectra of NanoGs/AgCl/PPy composites.
above-mentioned pure AgCl (5–30 nm), it was apparent that, the dark spots were the core–shell nanoparticles with AgCl as the core and PPy as the conducting shell. Moreover, this structure of composites effectively confirmed that there was strong interaction among NanoGs, nano-AgCl and PPy. The structural changes caused by deposition of PPy on NanoGs and nano-AgCl were followed through FT-IR spectra (Fig. 6). From Fig. 6, remarkable differences between NanoGs/AgCl/PPy composites and pure PPy could be observed. The N–H stretching region near 3425 cm 1 was obvious in the PPy yet not clear in the composites. Meanwhile, comparing with pure PPy, the characteristic peaks of the PPy molecules in the composites shifted to higher wavenumbers. This may be reasonably concluded that there was strong interaction between PPy and nanoparticles. The addition of nanoparticles probably resulted in the formation of hydrogen-bonding, which weakened the N–H as well as its stretching intensity. These evidences indicated that PPy was obtained by in situ polymerization of pyrrole monomer and the nanocomposites were successfully synthesized through two-step sequentially inverse microemulsion method. 3.3. Thermogravimetric analysis Fig. 7 showed the TG curves of pure PPy and NanoGs/AgCl/PPy composites, respectively. It can be seen that the weight loss curve of nanocomposites appeared above of pure PPy, indicating the enhanced thermal stability for the nanocomposites. The detailed analyze was shown as follows: there were two important weight losses that were easily
Fig. 7. TG curves of pure PPy (1) and NanoGs/AgCl/PPy composites (2).
observed in the derivative curve (2). The initial weight loss took place between 325 and 580 C, corresponding to the combustion of organic phase PPy. Subsequently, a slow and somewhat gradual weight loss profile after 620 C was due to the oxidation of NanoGs. In addition, the last remaining weight percentage related to the inorganic part of the composites. The TG curve (1) of pure PPy showed only a single step weight loss that started around 280 C was attributed to the loss of polymer. These results indicated that the inorganic nanoAgCl and NanoGs dispersed into the PPy matrix. In general, for polymer/inorganic substance nanocomposite, the unparalleled ability was found to boost the thermal stability of polymer matrix. The thermal stability of our composites also increased with the increasing of the decomposition temperature of matrix. From TG data, it might be concluded that the thermal weight loss temperature of pure PPy was 288 C, but that of NanoGs/AgCl/ PPy composites was 325 C. Within the structure of nanocomposite, there was strong interaction between large numbers of surface atoms of AgCl nanoparticles, NanoGs, and PPy molecule chains. This interaction formed the barrier character for the degradation of PPy matrix [33]. Consequently the needed energy of thermal decomposition increased, namely the thermal stability of composites increased [26]. 3.4. Conductivity characteristics Nanocomposites with excellent conducting property were achieved by this way. Table 1 summarized the results of the dc conductivity (conventional
Z. Mo et al. / European Polymer Journal 43 (2007) 300–306
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Table 1 Conductivity values of pure PPy, NanoGs and NanoGs/AgCl/PPy composites Samples
Pyrrole (wt.%)
Nano-AgCl (wt.%)
NanoGs (wt.%)
Conductivity value (S/cm)
A B C D
100 0 90 90
0 0 3.0 3.0
0 100 7.0 7.0
3.3 · 10 2 1.0 · 102 0.37 0.34
4. Conclusions Facile process for the synthesis of graphite nanosheets was developed via sonication of the expanded graphite. The conducting NanoGs/AgCl/PPy composites can be fabricated by in situ polymerization of pyrrole in the presence of NanoGs and nanoAgCl. The nanoscale dispersion of inorganic substances within the PPy matrix was evidenced by SEM and TEM examinations. From FT-IR spectra analysis, it reasonably concluded that there was strong interaction between PPy and nanoparticles. The thermal stability of PPy exhibited a beneficial effect due to the introduction of nano-AgCl and NanoGs. Electrical conductivity measurements indicated that the conductivity of final NanoGs/ AgCl/PPy composites increased compared with pure PPy owing to the introduction of NanoGs. Acknowledgements We thank the National Natural Science Foundation of China (29875018), the Natural Science Foun-
dation of Gansu Province (3ZS051-A25-50), Gansu Key Laboratory of Polymer Materials (DZ04-14), and the Doctorate Foundation of Northwestern Polytechnical University (CX200309) for financial support of this work. References [1] Chen GH, Weng WG, Wu DJ, Wu CL. Eur Polym J 2003;39:2329. [2] Liu YC, Yang SJ, Chuang TC, Wang CC. J Electroanal Chem 2004;570:1. [3] Kim JW, Liu F, Choi HJ, Hong SH, Joo J. Polymer 2003;44:289. [4] Gerard M, Chaubey A, Malhotra BD. Biosens Bioelectron 2002;17:345. [5] Chatterjee N, Basumallick IN. J Power Sources 1996;63:271. [6] Singh RN, Lal B, Malviya M. Electrochim Acta 2004; 49:4605. [7] Geng LN, Wang SR, Zhao YQ, Li P, Zhang SM, Huang WP, et al. Mater Chem Phys 2006;99:15. [8] Bissessur R, MacDonald J. Solid State Sci 2006;8:531. [9] Chattopadhyay S, Datta A. Synth Met 2005;155:365. [10] Zhang ZP, Wang F, Chen FE, Shi GQ. Mater Lett 2006; 60:1039. [11] Otero TF, Cantero I, Grande H. Electrochim Acta 1999; 44:2053. [12] Lee CY, Lee DE, Joo J, Kim MS, Lee JY, Jeong SH, et al. Synth Met 2001;19:429. [13] Kilmartin PA, Wright GA. Electrochim Acta 2001;46:2787. [14] Calvo PA, Rodrı´guez J, Grande H, Mecerreyes D, Pomposo JA. Synth Met 2002;26:111. [15] Franc¸ois G, Siyu Y, Guylaine L, Daniel B. J Electroanal Chem 1992;334:35. [16] Makhlouki M, Berne`de JC, Morsli M, Bonnet A, Conan A, Lefrant S. Synth Met 1994;62:101. [17] Ruckenstein E, Hong L. Synth Met 1994;66:249. [18] Bourdo SE, Viswanathan T. Carbon 2005;43:2983. [19] Xiao P, Xiao M, Liu PG, Gong KC. Carbon 2000;38:626. [20] Du XS, Xiao M, Meng YZ. Eur Polym J 2004;40:1489. [21] Chen GH, Wu CL, Weng WG, Wu DJ, Yan WL. Polymer 2003;44:1781. [22] Xiao P, Xiao M, Gong KC. Polymer 2001;42:4813. [23] Xiao M, Sun LY, Liu JJ, Li Y, Gong KC. Polymer 2002; 43:2245. [24] Wang WP, Pan CY. Eur Polym J 2004;40:543. [25] Zheng GH, Wu JS, Wang WP, Pan CY. Carbon 2004; 42:2839. [26] Mo ZL, Sun YX, Chen H, Zhang P, Zuo DD, Liu YZ, et al. Polymer 2005;46:12670. [27] Du XS, Xiao M, Meng YZ, Hay AS. Polymer 2004;45:6713.
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four-probe) values of pure PPy and NanoGs/AgCl/ PPy composites. Incorporation of conducting NanoGs in PPy matrix could lead to an increase in the conductivity value of pure PPy. The average electrical conductivity value of the composites was 0.36 S/cm, one order of magnitude higher than that of pure PPy (3.3 · 10 2 S/cm). The electrical conductivity of NanoGs were not deteriorated to a great degree compared with the original graphite flake. In this case, the nanocomposites prepared using NanoGs as conducting fillers exhibited highly electrical conductivity. The results also showed that the electrical conductivity of NanoGs/AgCl/PPy nanocomposites increased comparing with pure PPy. The augmentation of the electrical conductivity can be ascribed to the nanoscale dispersion of graphite nanosheets and the formation of conducting networks in PPy matrix [18,20]. Based on this factor, we can conclude that NanoGs within PPy matrix served as electrically conductive bridge.
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[28] Wang JJ, Zhu MY, Outlaw-Ron A, Zhao X, Manos-Dennis M, Holloway-Brian C. Carbon 2004;42:2867. [29] Chen GH, Weng WG, Wu DJ, Wu CL, Lu JR, Wang PP, et al. Carbon 2004;2:753. [30] Stejskal J, Trchova M, Ananieva IA, Janca J, Prokes J, Fedorova S, et al. Synth Met 2004;146:29.
[31] Park HJ, Ko MJ, Park OO, et al. J Power Sources 2002; 105:20. [32] Boukerma K, Piquemal J, Chehimi MM, Mravcˇa´kova´ M, Omastova´ M, Beaunier P. Polymer 2006;47:569. [33] Yang XM, Dai TY, Lu Y. Polymer 2006;47:441.
EUROPEAN POLYMER JOURNAL www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Synthesis of graphite nanosheets/AgCl/polypyrrole composites via two-step inverse microemulsion method Zunli Mo *, Dandan Zuo, Hong Chen, Yinxia Sun, Ping Zhang MACROMOLECULAR NANOTECHNOLOGY
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, PR China Received 2 October 2006; accepted 21 November 2006
Abstract An effective process for the synthesis of graphite nanosheets/AgCl/polypyrrole (NanoGs/AgCl/PPy) composites was developed. NanoGs were prepared by treating the expanded graphite with sonication in aqueous alcohol solution. Then nanocomposites were fabricated via in situ polymerization of pyrrole in the presence of NanoGs and AgCl particles through two-step sequentially inverse microemulsion method. The nanoscale dispersion of NanoGs and AgCl particles was evidenced by the SEM and TEM examinations. From the thermogravimetric analysis, the introduction of inorganic nano-AgCl and NanoGs exhibited a beneficial effect on the thermal stability of pure PPy. According to the four-pointprobe test, the conductivity of the final NanoGs/AgCl/PPy composites was dramatically increased compared with pure PPy. 2006 Elsevier Ltd. All rights reserved. Keywords: Graphite nanosheets (NanoGs); Nano-AgCl; Polypyrrole; Nanocomposites
1. Introduction Recently, composite materials based on inorganic and organic components have received an increasing interest assumingly due to their new favorable properties connected to the presence and combination of materials of different character [1,2]. Research on the topic of composite materials involves challenges and opportunities. The foremost challenge is that how to synthesize composite materials which can keep or enhance the best properties of each of the components while eliminating or * Corresponding author. Tel.: +86 931 7971829; fax: +86 931 7971933. E-mail address: [email protected] (Z. Mo).
reducing their particular limitations. This challenge provides the opportunity to develop new materials with specific behavior. And this behavior can lead to improved performances or to the finding of new and useful properties [3]. So far electronically conducting polymers (ECPs) are one type of the most frequently combined materials [4,5]. The considerable research of ECPs for a wide array of applications has been conducted over the past 30 years. Of all the synthetic ECPs, especially polypyrrole (PPy), polyaniline (PANi), polystyrene and polythiophene as conductive base in the composites or nanocomposites are used diffusely [6–10]. In this paper we chose electronically conducting PPy as one of the components to produce nanocomposite materials. As we know, PPy is one
0014-3057/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2006.11.023
Z. Mo et al. / European Polymer Journal 43 (2007) 300–306
make graphite nanosheets. Then NanoGs/AgCl/ PPy nanocomposites were synthesized by chemical oxidative polymerization of pyrrole monomer in the presence of graphite nanosheets and ultrafine AgCl. The structural property, thermal property and electrical conductivity of composites were investigated by different experimental techniques, including transmission electron microscope (TEM), scanning electron microscope (SEM), Fourier-transform infrared spectra (FT-IR), thermogravimetric analysis (TG), and four-point-probe method. Other properties and potential applications of the material will be given in future report. 2. Experimental section 2.1. Materials The graphite used for preparing graphite nanosheets was expandable graphite, supplied by Shandong Qingdao Graphite Company (China). The pyrrole monomer (Py), supplied by Shanghai Chemical Company (China), was purified by water vapour distillation before use. The initiator of polymerization, FeCl3 Æ 6H2O (A.R.) was purchased from Jinshan Chemical Plant (China). Trichloromethane (CHCl3), butyl alcohol (C4H10O), cetyltrimethylammonium bromide (CTAB), acetone (C3H6O), ethyl alcohol (C2H6O), AgNO3 and NaCl at analytical reagent grade were obtained from commercial suppliers and used as received. 2.2. Preparation of NanoGs The expandable graphite was heat treated at 950 ± 10 C for 30 s to obtain expanded graphite particles. The expanded graphite was immersed in a 75% of aqueous alcohol solution in an ultrasonic bath. The mixture was sonicated for 10 h, and then was filtered and washed with enough distilled water and ethyl alcohol. The obtained graphite powers called graphite nanosheets were dried under vacuum at ambient temperature for 12 h. 2.3. In situ polymerization to prepare nanocomposites The synthesis of NanoGs/AgCl/PPy nanocomposites was carried out by means of an oxidative polymerization of pyrrole monomer, in the presence of NanoGs and AgCl through two-step sequentially inverse microemulsion method. Initially, certain amount of AgNO3, NaCl was dissolved in redistilled
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of the most popular ECPs for various electronic applications such as the secondary battery [11], EMI shielding [12], light emitting diode [13], chemical and biochemical sensors [4] and conducting composite materials [3], because of its high conductivity, environmental stability to water and oxygen and ease of synthesis [14]. By an electrochemical or by a chemical oxidative polymerization, the PPy and its derivatives have been successfully synthesized. Generally, PPy is brittle, insoluble and infusible, and hence inprocessible. Therefore, in order to develop PPy-based conductive materials, several approaches including graft copolymerization containing pyrrole as grafted groups, electrochemical polymerization of pyrrole in the presence of latex particles with anionic surface [15], and oxidant-impregnated polymerization using pyrrole vapor [16], have been applied. Furthermore, Ruckenstein and Hong [17] introduced emulsion pathway to synthesize PPy-based conductive composites. Graphite has been widely used as electronically conducting filler for preparing conducting polymer/graphite composites in the last decade [18–25]. However, because of the small inter spaces (0.335 nm) of the graphite layers and the lack of the affinity for either hydrophilic or hydrophobic polymers, it is very difficult to prepare the polymer/graphite nanocomposites via direct intercalation method [23]. Thus, with certain chemical or physical modification to the graphite, it can achieve nanoscale dispersion of graphite in polymer matrix [20,21]. Expanded graphite is composed of a large number of delaminated graphite sheets. There are many pores and galleries in its structure [24,25]. But they might not be able to reach closed cavities inside the expanded graphite. The graphite sheets around the cavities tend to overlap and accumulate during processing. The other disadvantage is that the loading level of expanded graphite within a conductive polymeric nanocomposite is high [26,27]. For solving these problems, we can deal with expanded graphite and prepare graphite nanosheets (NanoGs). Nowadays, NanoGs are the best filler for making conducting polymer/graphite nanocomposite [1,20–22]. Great success has been achieved in polymer/graphite nanocomposites, but reports on the nanocomposites of graphite nanosheets, inorganic nanoparticles with polymer were rare. In this study, we reported on the preparation of NanoGs/AgCl/PPy nanocomposites through twostep sequentially inverse microemulsion method. Firstly, the expanded graphite was powdered to
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water to form 0.1 mol/l AgNO3, 0.1 mol/l NaCl aqueous solution. Secondly, 2.0 ml AgNO3 aqueous solution obtained above, 1.0 g CTAB, 2.0 ml C4H9OH and 0.9 ml pyrrole were charged into 15.0 ml CHCl3, and the mixture was sonicated for about 30 min. Once the inverse emulsion formed, it was mixed with 0.11 g NanoGs, which was kept in an ultrasonic bath for another 15 min. Then, added 2.0 ml 0.1 mol/l NaCl aqueous solution into the mixture with stirring for 1 h. Finally, 5.0 g FeCl3 Æ 6H2O was then dropped into the reactor and a rapid oxidation occurred. The resulting suspension was stirred at 10 C for 24 h. Excess acetone was then introduced into the reaction mixture to terminate the reaction. Upon being filtrated, the precipitate was washed with redistilled water and CHCl3 repeatedly. The solid was dried under vacuum at ambient temperature for 24 h. 2.4. Characterization and measurements Scanning electron microscope (SEM; Hitachi, Japan, JEOL, JSM-6330F) was used to observe the morphologies of NanoGs and nanocomposite. Prior to the examination, the specimens were coated with a very thin layer of gold. Transmission electron microscope (TEM; JEOL100CX-II model) was run at 100 kV accelerated voltage to observe the morphologies. The observations were carried out after retrieving the slices onto Cu grids. Fourier-transform infrared spectra (FT-IR) of the samples in KBr pellets were recorded on an EQUINOX55 FTIR spectrometer (Bruker). The weight loss temperatures of the composites were determined with a Perkin–Elmer thermogravimetric analyzer (TGDTA; model SSC-5200) from 20 to 900 C under environment atmosphere (10 C/min). The electrical conductivity measurements were made by the conventional four-point-probe method on pressed pellets of composite particles prepared at ambient temperature (25 C).
Fig. 1. SEM micrograph of NanoGs.
diameter ranging 0.5–20 lm, named NanoGs. This can also been seen in the TEM micrograph shown in the following part. The result was similar to the reports of other researchers [28,29]. The spherical AgCl particles were prepared through reverse microemulsion as template, in which CTAB was used as surfactant, C4H9OH was designated as assistant surfactant and CHCl3 was applied as the oily phase. These AgCl particles with particle diameters from 5 to 30 nm were clearly confirmed by the TEM image (Fig. 2). It indicated that the formation of nano-AgCl took place in the ‘‘water pools’’. The small-sized, fairly distributed ‘‘water pools’’ limited the formation space and shape of AgCl particles. An inverted emulsion of water containing the oxidant (FeCl3) in CHCl3, the CTAB surfactant was used as dispersant. Then pyrrole was introduced dropwise in the inverted emulsion. As a result, colloidal-sized PPy particles generated. The FT-IR spectra of the prepared PPy (Fig. 3) clearly exhibited characteristic absorption peaks with respect to PPy. The band at 3425 cm 1 corresponded to the N–H
3. Results and discussion 3.1. Structure of NanoGs, ultrafine AgCl and PPy We adopted ultrasonic irradiation technique to break down the expanded graphite and then obtained NanoGs. Fig. 1 showed the SEM micrograph of the as-prepared NanoGs. It can be seen clearly that the expanded graphite was torn into sheets with a thickness ranging 30–80 nm and a
Fig. 2. TEM micrograph of AgCl nanoparticles.
Transmittance/%
Pure PPy
26.5
25
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber/cm-1
Fig. 3. FT-IR absorption spectra of pure PPy.
stretching vibration and that at 1512 cm 1 reflected C@C stretching vibration. The broad band from 1400 to 1250 cm 1 was attributed to C–H or C–N in-plane deformation modes and had a maximum at 1291 cm 1 [30]. The absorptions at 779 and 671 cm 1 were related to the C–H outerbending vibrations [31]. The above results indicated the formation of PPy. 3.2. Structure of NanoGs/AgCl/PPy nanocomposites The reverse microemulsion containing different components was prepared firstly. Then the droplets of reverse microemulsion collided to exchange molecules. In this step, the AgCl precipitate formed after NanoGs were mixed. Lastly pyrrole began to polymerize by addition of FeCl3 Æ 6H2O on the base of inorganic substances and the so-formed nuclei grew. Because NanoGs were in nanoscale, they tended to accumulate each other and were difficult to be dispersed in polypyrrole by traditional ways. Ultrasonic irradiation has been widely applied in chemical reaction and engineering. When an ultrasonic wave passes through a liquid medium, the effect of ultrasonic cavitation will take place and generate a very strong stirring environment. Therefore, ultrasound has been extensively used in dispersion, emulsifying, crushing, and activation of particles. In this case, sonication was an effective way for the optimal dispersion of the NanoGs in reverse microemulsion system and in the polypyrrole matrix. When the instant in situ polymerization of the pyrrole occurred, the NanoGs and inorganic nano-AgCl were dispersed and fixed among the polypyrrole molecules. Using this chemical reaction, we synthe-
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sized NanoGs/inorganic nanoparticles AgCl/PPy composites successfully. The typical SEM pictures of NanoGs/AgCl/PPy composites were presented in Fig. 4. Fig. 4a and b were the samples magnified 1.2 · 104 times and 2.0 · 104 times, respectively. We can observe clearly that NanoGs dispersed in PPy matrix and the PPy also embedded the NanoGs. Furthermore, NanoGs were distributed quite uniformly within the PPy matrix and no aggregates can be seen in the mixture, and then the inorganic nanoparticles AgCl were covered in the PPy matrix and NanoGs separately. Due to this especial structure, the interfacial affinity of NanoGs and inorganic AgCl with the PPy increased and led to an ensured thermal stability and electrical contact. Fig. 5 showed TEM images of NanoGs/AgCl/ PPy composites dispersed in ethyl alcohol. The gray lines and flakes represented the graphite layers. It can be seen that the nanoscale patterns of the graphite layers well dispersed in PPy matrix. This kind of NanoGs as observed had thickness of 40–90 nm, which was a little bigger than that of the result in Fig. 1 due to the presence of modified layer of PPy [32]. And the size of dark spots in the cross section of NanoGs/AgCl/PPy composites was in the range of 50–80 nm. Compared with the size of
Fig. 4. SEM of NanoGs/AgCl/PPy composites: (a) lower magnification and (b) higher magnification.
Fig. 5. TEM of NanoGs/AgCl/PPy composites: (a) lower magnification and (b) higher magnification.
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1.0
0.8
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Fig. 6. FT-IR absorption spectra of NanoGs/AgCl/PPy composites.
above-mentioned pure AgCl (5–30 nm), it was apparent that, the dark spots were the core–shell nanoparticles with AgCl as the core and PPy as the conducting shell. Moreover, this structure of composites effectively confirmed that there was strong interaction among NanoGs, nano-AgCl and PPy. The structural changes caused by deposition of PPy on NanoGs and nano-AgCl were followed through FT-IR spectra (Fig. 6). From Fig. 6, remarkable differences between NanoGs/AgCl/PPy composites and pure PPy could be observed. The N–H stretching region near 3425 cm 1 was obvious in the PPy yet not clear in the composites. Meanwhile, comparing with pure PPy, the characteristic peaks of the PPy molecules in the composites shifted to higher wavenumbers. This may be reasonably concluded that there was strong interaction between PPy and nanoparticles. The addition of nanoparticles probably resulted in the formation of hydrogen-bonding, which weakened the N–H as well as its stretching intensity. These evidences indicated that PPy was obtained by in situ polymerization of pyrrole monomer and the nanocomposites were successfully synthesized through two-step sequentially inverse microemulsion method. 3.3. Thermogravimetric analysis Fig. 7 showed the TG curves of pure PPy and NanoGs/AgCl/PPy composites, respectively. It can be seen that the weight loss curve of nanocomposites appeared above of pure PPy, indicating the enhanced thermal stability for the nanocomposites. The detailed analyze was shown as follows: there were two important weight losses that were easily
Fig. 7. TG curves of pure PPy (1) and NanoGs/AgCl/PPy composites (2).
observed in the derivative curve (2). The initial weight loss took place between 325 and 580 C, corresponding to the combustion of organic phase PPy. Subsequently, a slow and somewhat gradual weight loss profile after 620 C was due to the oxidation of NanoGs. In addition, the last remaining weight percentage related to the inorganic part of the composites. The TG curve (1) of pure PPy showed only a single step weight loss that started around 280 C was attributed to the loss of polymer. These results indicated that the inorganic nanoAgCl and NanoGs dispersed into the PPy matrix. In general, for polymer/inorganic substance nanocomposite, the unparalleled ability was found to boost the thermal stability of polymer matrix. The thermal stability of our composites also increased with the increasing of the decomposition temperature of matrix. From TG data, it might be concluded that the thermal weight loss temperature of pure PPy was 288 C, but that of NanoGs/AgCl/ PPy composites was 325 C. Within the structure of nanocomposite, there was strong interaction between large numbers of surface atoms of AgCl nanoparticles, NanoGs, and PPy molecule chains. This interaction formed the barrier character for the degradation of PPy matrix [33]. Consequently the needed energy of thermal decomposition increased, namely the thermal stability of composites increased [26]. 3.4. Conductivity characteristics Nanocomposites with excellent conducting property were achieved by this way. Table 1 summarized the results of the dc conductivity (conventional
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Table 1 Conductivity values of pure PPy, NanoGs and NanoGs/AgCl/PPy composites Samples
Pyrrole (wt.%)
Nano-AgCl (wt.%)
NanoGs (wt.%)
Conductivity value (S/cm)
A B C D
100 0 90 90
0 0 3.0 3.0
0 100 7.0 7.0
3.3 · 10 2 1.0 · 102 0.37 0.34
4. Conclusions Facile process for the synthesis of graphite nanosheets was developed via sonication of the expanded graphite. The conducting NanoGs/AgCl/PPy composites can be fabricated by in situ polymerization of pyrrole in the presence of NanoGs and nanoAgCl. The nanoscale dispersion of inorganic substances within the PPy matrix was evidenced by SEM and TEM examinations. From FT-IR spectra analysis, it reasonably concluded that there was strong interaction between PPy and nanoparticles. The thermal stability of PPy exhibited a beneficial effect due to the introduction of nano-AgCl and NanoGs. Electrical conductivity measurements indicated that the conductivity of final NanoGs/ AgCl/PPy composites increased compared with pure PPy owing to the introduction of NanoGs. Acknowledgements We thank the National Natural Science Foundation of China (29875018), the Natural Science Foun-
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four-probe) values of pure PPy and NanoGs/AgCl/ PPy composites. Incorporation of conducting NanoGs in PPy matrix could lead to an increase in the conductivity value of pure PPy. The average electrical conductivity value of the composites was 0.36 S/cm, one order of magnitude higher than that of pure PPy (3.3 · 10 2 S/cm). The electrical conductivity of NanoGs were not deteriorated to a great degree compared with the original graphite flake. In this case, the nanocomposites prepared using NanoGs as conducting fillers exhibited highly electrical conductivity. The results also showed that the electrical conductivity of NanoGs/AgCl/PPy nanocomposites increased comparing with pure PPy. The augmentation of the electrical conductivity can be ascribed to the nanoscale dispersion of graphite nanosheets and the formation of conducting networks in PPy matrix [18,20]. Based on this factor, we can conclude that NanoGs within PPy matrix served as electrically conductive bridge.
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