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Enhanced Electromechanical Performance of Graphite Oxide-Nafion Nanocomposite Actuator Yuanfeng Lian,† Yuexian Liu,‡ Tao Jiang,‡ Jing Shu,§ Huiqin Lian,*,‡ and Minhua Cao*,§ Department of Computer Science and Technology, China UniVersity of Petroleum, Beijing, 102249, China, Department of Chemical Engineering, Yanbian UniVersity, Jilin, 133002, China, and Department of Chemistry, Beijing Institute of Technology, Beijing, 100081, China ReceiVed: February 11, 2010; ReVised Manuscript ReceiVed: April 15, 2010
We report for the first time the fabrication of the graphene oxide (GO)-Nafion actuators with carbon nanotubes (CNTs) as electrodes. The exfoliated GO was homogeneously dispersed in the Nafion matrix with doping level of 0.5-10 wt %. CNT electrodes on both sides of the membrane were made by dip-coating method; that is, the composites membranes were first emerged into the CNT-Nafion dimethylformamide (DMF) solution and then pulled out to dry. The as-prepared GO-Nafion actuators were tested in terms of conductivity, bulk and surface morphology, tip displacement, and blocked force. The results demonstrated that the electrochemical behavior of the polymer nanocomposite was significantly improved because of the efficient distribution of the high aspect ratio GO sheets, and the blocking force of the nanocomposite actuator with doping level 10 wt % is 4 times that of the virgin nafion, and the displacement is nearly 2 times that of nafion. 1. Introduction Electroactive polymer, ionic polymer-metal composite (IPMC), has attracted much attention as a good candidate for robotic actuators, artificial muscle, and dynamic sensors because of its outstanding advantages including flexibility, lighter weight, biocompatibility, and a large displacement under low potential (1-5 V) stimuli.1-4 In general, a typical IPMC consists of a thin ion-exchange polymer membrane covered with a thin metal layer from both sides, and the metal layers serve as surface electrodes. Its electromechanical mechanism is that hydrated metal cations inside the actuator migrate toward cathode under electrical field, which causes the large deformation, and thus strong blocking force and displacement are produced. However, the existing IPMC actuators have two significant drawbacks, low generative blocking force and short nonwater working time,5 both of which are closely related to the solvent losing in the polymer because of the evaporation, electrolysis, and the low mechanical property of polymer. Therefore, increasing solvent capacity and enhancing the mechanical property are critical to improving the performance of IPMC actuators. It is well known that composites formed from two or more distinct materials have desirable combinations of properties, which could not be found in the individual components.6 In inorganic nanoscale fillers-polymer composites, the inorganic component, such as layered silicate or carbon nanotubes (CNTs), can provide dramatic improvement in mechanical properties, electromechanical coupling, thermal stability, dimensional stability, heat-distortion temperature, and barrier properties.7-11 Recently, graphite oxide (GO) has attracted increasing interest as a filler for polymer nanocomposites because of its high dispersive capacity, long coherence length, and the barrier property. GO with a typical pseudo-2D structure generally contains hydroxyl, carboxyl, and ether groups, which make GO * Corresponding authors. E-mail:
[email protected] (M.C.). † China University of Petroleum. ‡ Yanbian University. § Beijing Institute of Technology.
absorb polar molecules and polar polymers easily, and thus GO/ polymer composites could be formed. Such structural nanocomposite can provide reinforcement to the base polymer matrix. Also, it may tailor to the needs of other desirable properties, such as mass diffusion coefficients, coefficients of thermal expansion, dielectric constants, thermal/chemical stability, solvent resistance, selectivity, conductivity, and resistivity to membrane fouling and poisoning. Moreover, in IPMC actuator, the electrodes on the both sides of the composite membrane play an important role in its actuation performance. Usually, the electrodes of IPMC were made by chemical plating method; that is, the membrane first was soaked in the metal cationic solution, followed by a reduction process, and the soaked metallic cations were reduced.12 In this process, it is important to control the soaking time, avoiding the electric short circuit of the membrane. To achieve a good electrode, this soak-reduce procedure is often repeated several times. The method is time-consuming and less repeatable; therefore, electrode plating becomes a bottleneck in fabricating IPMC. It is well known that CNTs have the merit of lightweight, super conductivity, and high strength. As the electrode of IPMC, nanotubes could hold solvent to hinder its losing through leaking and evaporation. In this article, we report for the first time a simple method to fabricate IPMC actuator based on GO-Nafion nanocomposite with CNTs as electrode plated by the dip-coating method. The GO nanosheets were homogeneously dispersed in the Nafion matrix with doping level of 0.5-10 wt %. Compared with the pure Nafion, the resulting nanocomposite actuators exhibit dramatic enhancement of actuation properties. 2. Experimental Section 2.1. Materials. The starting materials include Nafion dispersion (5 wt %), graphite (40 nm), and CNTs. Nafion dispersion and graphite were purchased from DuPont, Aladdin, respectively. CNTs were kindly provided by Tsing Hua University.
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Figure 1. Schematic representation for the fabrication of GO-Nafion nanocomposite membrane with parallel GO sheets.
2.2. Preparation of Graphite Oxide-Nafion Nanocomposite Membrane. The methods to oxidize the graphite and CNT have been reported elsewhere.13,14 The GO dispersion in water and Nafion in alcohol were mixed, with a weight percent of GO in Nafion of 0.5-10 wt %. Meanwhile, the same amount of DMF was added, and the mixture was ultrasonicated for 0.5 h, followed by mechanical stirring at 65 °C for 24 h. Water and alcohol were removed by slowly heating at 65 °C. After that, the membrane was made by casting in a vacuum oven at 80 °C for 24 h. 2.3. Preparation of CNT Electrode. The procedure to make CNT electrode is as follow: first, CNT oxide-Nafion DMF solution was prepared; second, the strip of nanocomposite membrane was dipped into the electrode solution; finally, the memebrane was pulled out and dried at 150 °C for 5 min. This process was repeated three times to ensure a dense and uniform CNT layer on the membrane. After that, Li+-IPMC was obtained by ion-exchanging the H+-IPMC in 0.1 N LiOH solution at room temperature for 24 h. 2.4. Characterization. The as-made membrane was characterized by X-ray powder diffraction (XRD, Scintag PAD X diffractometer, Cu KR source, operated at 45 kV and 40 mA) and scanning electron microscopy (SEM, Tecnai T12,100 kV). Keithley SourceMeter was used to measure electric resistance. Characterization of the actuation performance was carried out using a Labview-based system device. The blocking force of IPMC is measured using a balance under the stimulating of programmable power. The programmable power is composed of a MCU of Mega8, digitally controlled potentiometer, adjustable voltage regulator AS1117, and relay, and it provides electrical signal with periods of 1 s to 5 min, voltage of (1.25 to (5 V, and square wave. The balance with the sensitivity of 0.1 mg collects data through RS232 using Labview. Displacement of the IPMCs under different electrical stimuli was captured by a digital camera. All IPMC samples were cut into 25 × 5 mm2 besides 5 mm of the electrode contact area. One end of the IPMC strip was fixed between two Cu electrodes. All determinations were carried out in air at room temperature. 3. Results and Discussion The preparation process of the GO-Nafion nanocomposite membrane is shown in Figure 1. The GO dispersion in water is added to Nafion alcohol solution to form a colloidal suspension; then, DMF is added to the colloidal suspension with evaporating water and alcohol, resulting in the formation of GO-Nafion gel.
Figure 2. XRD patterns of the as-prepared GO-Nafion nanocomposite membrane, graphite, and graphite oxide.
Finally, the GO-Nafion membrane is obtained by casting the GO-Nafion gel. The XRD patterns of the as-prepared membranes, graphite, and graphite oxide and are shown in Figure 2. The sharp diffraction peak around 26.5° for pristine graphite in Figure 2a shows that the basal spacing is 0.34 nm. Because of the strong van der Waals force and static electric force between the sheets of graphite, it is difficult to disperse and easily flocculates in the solution, which directly influences the actuation and other physical properties of as-prepared hybrid membrane. Therefore, a relatively strong oxidative acid is used to oxidize the graphite to improve its hydrophilic property in the solution. As shown in Figure 2b, the diffraction peak centering at 9.8° with the basal spacing of 0.90 nm is observed and that at 26.5° disappears. Therefore, it can be deduced that the graphite has been oxidized into GO completely. The XRD patterns of GO-Nafion membranes with different doping levels are presented in Figure 2c-f. It can be seen that the diffraction peak ascribed to GO does not appear in all of the XRD patterns, indicating the complete exfoliation of the GO in Nafion. SEM images of the cross-section surfaces of the as-made 10 wt % GO-Nafion nanocomposite and Nafion membranes are show in Figure 3. From the low magnification SEM images of both membranes (Figures 3a,c), it can be observed that the GONafion nanocomposite membrane has a rough cross section, which may result from the doping of the GO sheets in Nafion. GO sheets could be clearly seen in the high magnification SEM image (Figure 3b, marked by the arrows). It can be seen that GO sheets not only were uniformly dispersed in the Nafion matrix but also tended to align parallel to the film surface. The preferential orientation of GO sheets may originate from gravitational forces experienced by the nanosheets while remaining well dispersed in the solution, whereas the pure Nafion membrane shows a relatively smooth cross section (Figures 3c,d). It can be deduced that the polar groups in the surface of GO sheets, such as hydroxyl, carboxyl, or ether groups, benefit the dispersion of GO in polar Nafion solution and at the same time enhance the interaction between GO sheets and Nafion. Therefore, it inhibits the aggregation of GO sheets in Nafion during the casting process. The CNT electrodes on the surface of hybrid membrane were accomplished using dip-coating method. The interaction between the CNT with the hybrid membrane surface was well characterized by SEM. The SEM image (Figure 4a) of the horizontal
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Figure 3. SEM images of cross-section of 10 wt % GO-Nafion composite membrane (a) low magnification and (b) high magnification; SEM images of cross-section of Nafion membrane (c) low magnification and (d) high magnification.
Figure 4. SEM images of Nafion film with CNT as electrode: (a) horizontal surface and (b) cross-section.
surface of the CNT electrodes indicated that the CNT electrode surface is relatively uniform. From the cross-section view of the membrane (Figure 4b), the thickness of the CNT layer is determined to be ∼0.8 µm. In addition, Figure 4b also disclosed that the CNT electrode layer connected to the membrane very well and aligned parallel to surface of film. Considering the fact that the electrical conductivity of the membrane is one of the important parameters affecting the actuator performance, conductivity measurements were performed, as shown in Figure 5. As clearly shown in Figure 5, the conductivity of pure nafion is only 2 × 10-9 S/cm, whereas those of the GO-Nafion membranes are evidently increased with the increase in the doping levels of GO from 2 × 10-9 to 2 × 10-5 S/cm. Ye et al. reported that the electrical conductivity of GO was ∼5 × 10-5 S/cm.15 It is clear that the conductivity of
Figure 5. Relationship between the conductivity of GO-Nafion membrane and GO content.
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Figure 6. Displacement photo of an IPMC based on the pure Nafion membrane (a) before test and (b) driven under a 3 V electrical filed. The displacement photo of an IPMC based on the Nafion hybrid membrane (c) before test and (d) driven under a 3 V electrical filed. (e) Dependence of the generated strain (ε) on the GO content of the membranes. (f) Dependence of the blocking force on the GO content of the membranes.
the membrane with 10 wt % GO (2 × 10-5 S/cm) has the same order with that of GO. It confirms such a fact that highly conductive nanofiller could improve the conductivity of the polymer membrane. The actuator performance of the as-prepared GO-Nafion and pure Nafion membranes was measured by a cantilevered actuation system. The deflection of the membrane was tested by fixing one end of the membrane in the apparatus and measuring the movement of the free end. Figure 6a,c show the photos of GO-Nafion-based and Nafion-based IPMC actuators with a piece of squared paper as background before test, respectively, both of which are upright. When a voltage of 3 V was used to drive the IPMC actuators, the 10 wt % GO-Nafion IPMCs exhibits a perceptibly larger displacement (Figure 6d) than that of the pure Nafion (Figure 6b). As defined in ref 16, the generated strain (ε) was calculated according to the tip displacement. The larger the strain, the higher the deflection of membrane and the better displacement behavior it exhibits. As shown in Figure 6e, the strains increase with the increase in the GO content of the membranes. Compared with the pure Nafion membrane, all hybrid membranes exhibit excellent displacement performance. Apparently, the membrane containing 10 wt % GO exhibits the largest displacement (Figure 6e), which may be due to its higher conductivity. Moreover, from the blocking force graph, it can be seen that GO-Nafion nanocomposite membranes exhibit higher blocked force measurements compared with pure Nafion membrane, as shown in
Figure 6f. Among them, the blocking force of membrane containing 10 wt % is also the highest, about four times that of pure nafion. This result is clearly better than that for the CNT/ Nafion actuator reported by Lee,4 in which the actuator with CNT doping lever of 1 wt % showed the largest blocking force of two times of that of pure Nafion. Because of the heterogeneousness of CNT, the blocking force evidently decreases with the increase in CNT content when the content of CNT is higher than 1 wt %, whereas in our case, the GO content of GO-Nafion actuator could reach as high as 10 wt % with the best performance. Therefore, GO with a layered structure is an attractive candidate filler for IPMC. In the following work, we will pay much attention to study the effect of size and dispersibility of GO on the actuation performance, and the actuation performance of GO-Nafion nanocomposite membranes has the possibility to be further improved. 4. Conclusions In summary, we for the first time reported the enhanced electromechanical performance of IPMCs based on GO-Nafion nanocomposite using CNT as electrodes. Here high aspect ratio GO sheets were uniformly dispersed in the Nafion polymer at doping levels between 0.5 and 10% w/w, and they were used as an efficient filler to enhance the actuator of Nafion membrane. The dispersion state and the compatibility of the GO sheets within the host polymer played an important role for the
GO-Nafion Nanocomposite Actuator electrical and mechanical properties of the hybrid membrane. Compared with pure Nafion membrane, the GO-Nafion membrane exhibited an enhanced actuator behavior. Acknowledgment. The CNT sample is provided by Tsinghua University, China. We are grateful to the assistance of Dr. Qian in conductivity measurement and SEM characterization. The work was supported by the Natural Science Foundation of China (NSFC, nos. 20771022 and 20973023) and Program for New Century Excellent Talents in University. References and Notes (1) Shahinpoor, M. Electrochim. Acta 2003, 2343. (2) Shahinpoor, M.; Kim, K. J. Smart Mater. Struct. 2004, 13, 1362. (3) Akle, B. J.; Bennett, M. D.; Leo, D. J. Sens. Actuators, A 2006, 126, 173.
J. Phys. Chem. C, Vol. 114, No. 21, 2010 9663 (4) Lee, D. Y.; Park, I. S.; Lee, M. H.; Kimb, K. J.; Heo, S. Sens. Actuators, A 2007, 133, 117. (5) Shahinpoor, M.; Kim, K. J. Smart Mater. Struct. 2005, 14, 197. (6) Pulickel, M. A.; James, M. T. Nature 2007, 447, 1066. (7) Tjong, S. C. Mater. Sci. Eng., R 2006, 53, 73. (8) Suresha, B.; Ravi Kumar, B. N.; Venkataramareddy, M.; Jayaraju, T. Mater. Des. 2010, 31, 1993. (9) Kim, H.; Christopher, W. Polymer 2009, 50, 3797. (10) Du, F. P.; Tang, C. Y.; Xie, X. L.; Zhou, X. P.; Tan, L. J. Phys. Chem. C 2009, 113, 7223. (11) Burgaz, E.; Lian, H.; Alonso, R. H.; Estevez, L.; Giannelis, E. P. Polymer 2009, 50, 2384. (12) Kim, K. J.; Shahinpoor, M. Smart Mater. Struct. 2003, 12, 65. (13) Paek, S. M.; Yoo, E. J.; Honma, I. Nano Lett. 2009, 91, 72. (14) Wang, J.; Musameh, M.; Lin, Y. J. Am. Chem. Soc. 2003, 125, 2408. (15) Ye, J. K.; Wang, G. C.; Yao, B.; Li, X. W. J. Inorg. Mater. 2008, 23, 945. (16) Sugino, T.; Kiyahara, K.; Takeuchi, I. Sens. Actuators, B 2009, 141, 179.
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