J. Phys. Chem. C 2008, 112, 7001-7004
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Electric Field Frequency and Strength Effects on Au-Electrode Damage for an Electroactive Paper Actuator Coated with Polypyrrole K. Y. Cho, H. G. Lim, S. R. Yun, Jaehwan Kim, and K. S. Kang* CreatiVe Research Center for ElectroactiVe Paper (EAPap) Actuator, Mechanical Engineering Department, Inha UniVersity 253 Yonghyun-Dong Nam-Ku 402-751, Incheon, South Korea ReceiVed: October 26, 2006; In Final Form: June 7, 2007
The causes of Au-electrode damage to an electroactive paper (EAPap) actuator coated with Au and polypyrrole (PPy) were investigated with various electric field frequencies and strengths. The resonance frequency of 3.5 Hz was obtained for 3 and 4 V. Electric field frequencies below (2 Hz) the resonance frequency yielded a faster bending displacement reduction than those of the higher resonance frequency. High electric field strength (4 V) shows a faster reduction of bending displacement than lower field strength (3 V). The degree of Auelectrode damage after a certain period of actuation is shown in field emission scanning electron microscope (FESEM) images. The electric field strength and frequency and bending displacement reduction were found to be closely related to the degree of Au-electrode damage.
1. Introduction Inherently conductive polymers (ICPs), such as poly(phenylene vinylene) (PPV), poly(ethylene-dioxythiophene) (PEDOT), polypyrrole (PPy), polythiophene (PTh), and polyaniline (PAn) have garnered considerable attention over the past 2 decades. The remarkable light emission, electronic properties, and ion exchange redox properties of these conducting polymers have led to their use in application fields as diverse as flat panel displays, lightings, sensors, polymer batteries, artificial muscles, mechanical actuators, and membrane separations.1-6 Among these ICPs, PPy and PAn have been actively investigated for actuator applications via use of ion exchange redox processes. PPy undergoes a volume change when the oxidation (or doping) level is changed. This expansion or contraction is related to the insertion or expulsion of ions. Since this change in volume can be controlled electrochemically, it can be utilized for actuation. Generally, when ions enter, the polymer expands, and when they exit, it contracts.7 Electroactive polymers (EAPs) are attractive due to their large displacement, low power requirement, mechanical flexibility, ease of processing and control, and nontoxicity to humans, offering advantages over traditional electroactive ceramic materials.8,9 EAPs have been used in many industrial fields, such as actuators, sensors, noise control systems, artificial muscles, valves, medical imaging systems, acoustic transducers, electrooptic modulators, and so forth.10-12 Recently, Kim at el.13-16 reported on the application of cellulose as a smart material, revealing its electroactive paper (EAPap) properties, and actuating behavior. They also discovered that EAPap has a large displacement upon low actuation voltage as well as low electrical power consumption. There have been a few reports on the degradation of metal electrodes after operation of EAP actuators. In general, the overall resistance and capacitance changes of EAPs under an electric field lead to variation in the bending displacement of the actuator. Surface reactions of Pt and Au electrodes under * To whom correspondence should be addressed. E-mail: kkang@ inha.ac.kr.
aqueous and nonaqueous environments in an ionic polymer actuator with metal electrodes were presented through an electrochemomechanical analysis including voltammetry and electrochemical impedance spectroscopy on the electrode surface. The authors observed electrochemically irreversible metal oxide layer formation.17 Metal electrode damage was also reported after actuation in the case of polymer actuators. As the actuation time increases, increased contact between the gold electrode and electrolyte and eventually complete delamination of the PPy film from the device were reported for the PPy actuator.10 The Pt electrode damage of a poly(vinylidene fluoride) actuator after operation was also reported.11 In this paper, we present the causes of displacement reduction for the EAPap actuator after actuation. The effects of AC-electric field frequency and strength on gold electrode damage were investigated. FESEM images of the Au-electrode surface after a certain period of actuation are presented. 2. Experimental Section Au electrodes (thickness of about 100 nm) of 10 mm × 40 mm size were deposited on cellophane (20 µm thickness) before conductive polymer coating. Acetonitrile was utilized as a solvent for the PPy preparation medium instead of an aqueous medium due to the potential for cellophane deformation in the latter medium. Electrochemical polymerization was performed in an acetonitrile solution containing 0.2 M pyrrole and 0.1 M LiClO4 using a Solartron electrochemical interface unit (model SI 1287) at a current density of 0.5 mA/cm2 for various time intervals. Figure 1 shows a schematic view of the actuator. The following evaluations were performed for the EAPap actuator with fixed temperature (23 °C) and a relative humidity (RH) of 85%. The prepared sample was placed in an environmental chamber that can control temperature and RH. The displacement measurement was performed using a high precision laser doppler vibrometer (LDV) (Bru¨el&Kjær, 8336), the environmental chamber (KMS, CTH3-2S), a current probe (Tektronix, TCP 300), LabView on a personal computer, and a function generator (Agilent, 33220A). The gold electrode was
10.1021/jp067012c CCC: $40.75 © 2008 American Chemical Society Published on Web 04/04/2008
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Cho et al.
Figure 2. Resonance frequency for the EAPap/PPy actuators. A resonance frequency of 3.5 Hz was obtained for both 3 and 4 V.
Figure 1. Graphical representation of EAPap/PPy actuator.
completely removed from the cellophane. The gold electrode (cellophane side) surface was investigated using a S4300 FESEM (Hitachi). 3. Results and Discussion The polymer backbone is changed during the redox reaction. The change of the polymer backbone is balanced by counterions of opposite charge in the polymer matrix. The following reactions occur during the conducting polymer oxidation (eq 1) and reduction (eq 2) process.
(P0)n + nyA- T (Py+Ay-)n + nye-
(1)
(P0Ay-My+)n T (Py+Ay-)n + nyM+ + nye-
(2)
where n is the degree of polymerization for the conducting polymer backbone (P) and y is the degree of doping. A- and M+ are the anion and cation, respectively.18 Two theories have been proposed for the volume change of ICP. The first involves a thermodynamic description including the solvent activities, and the other entails osmotic expansion and depends on the concentration of the surrounding electrolyte. Christensen and Hammett19 reported a 30% decrease in the thickness of PPy during oxidation. The electrochemical deposition process of PPy was performed using LiClO4 as an electrolyte. The prepared actuator, consisting of PPy/Au/cellophane/Au/Ppy, was bent toward the negative electrode direction during DC-electric filed application. This implies that the ClO4- ions and water molecules moved toward the positive electrode and expanded the positive electrode side. All other tests were performed using an AC-electric field. Figure 2 shows the relationship between frequency and displacement. The maximum displacement occurred at 3.5 Hz for both 3 and 4 V. The free ions (ClO4-) and water molecules move toward the gold electrode during the positive field application. These ions and water molecules remained for a longer time near or on the Au electrode when a lower frequency electric field was applied. Therefore, the gold electrode has more time to execute the electrochemical reaction at lower frequency than at higher frequency. The resonance frequency was investigated in terms of application of frequency lower or higher than the resonance frequency and to verify the assumption that gold
Figure 3. Normalized displacement reduction of the actuator for (a) 3 V, 2 and 5 Hz and (b) 4 V, 2 and 5 Hz.
electrode will be damage more rapidly at lower frequency than at higher frequency. MaQuarrie proposed that the electromechanical properties of materials degrade due to electrical fatigue.20 Wang et al.21 also reported ferroelectric fatigue of piezoelectric ceramics in actuator applications. Degradation and delamination of a PPy film coated with an Au electrode were observed due to purely mechanical origins, that is, a volume change of the PPy(DBS) film and charge-induced surface stress from the interaction of anions and Au.22 Figure 3a shows the relative displacement reduction for 3 V, 2 Hz and 3 V, 5 Hz. Surprisingly, although the number of actuations was 10 200 and 36 000 for 2 and 5 Hz, respectively,
Electroactive Paper Actuator Coated with PPy
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Figure 4. SEM images for (a) 4 V, 2 Hz; (b) 4 V, 5 Hz; (c) 3 V, 2 Hz; and (d) 3 V, 5 Hz.
the displacement reduction at 5 Hz was far less than that at 2 Hz. For investigation of the applied electric field strength effect, an electric field strength of 4 V was applied with 2 and 5 Hz. The results are shown in Figure 3b. The displacement reductions were roughly 90% for 4 V, 2 Hz after 2000 s actuation and 80% for 4 V, 5 Hz after 5000 s actuation. The bending displacement reduction at 4 V was faster than that at 3 V. The frequency effect showed a similar tendency. Faster displacement reduction occurred for 2 Hz at both 3 and 4 V than in the case of 5 Hz. This indicates that the electric field frequency and strength are the major factors underlying displacement reduction. After operation of the EAPap actuator, the Au-PPy layer was easily peeled off from the cellophane. FESEM images of the Au electrode (cellophane side) were obtained. Large area damage is seen in Figure 4a, corresponding to 4 V, 2 Hz after 2000 s actuation. A relatively large number of small holes is exhibited in Figure 4b, corresponding to 4 V, 5 Hz after 5000 s actuation. A large number of holes is concentrated on some areas of the sample of 3 V, 2 Hz after 5100 s actuation (Figure 4c). However, a small number of holes is distributed through the electrode surface for the case of 3 V, 5 Hz after 7200 s actuation. This trend closely corresponds with the displacement reduction trend. The holes may be produced by an electrochemical reaction with ions and Au. More electrochemical reactions may occur when a lower frequency of electric field is applied than at a higher frequency. The main causes of bending displacement reduction were the frequency and magnitude of the electric field.
4. Conclusions Application of a low-frequency electric field leads to faster displacement reduction than the use of a high-frequency electric field. The displacement reduction may be attributed to the electrochemical reactions with ions and Au. Application of high strength electric field also leads to faster displacement reduction than low electric field strength. FESEM images indicate that the displacement reduction is closely correlated with Au-electrode damage. The present results support that electrochemical reaction of the Au electrode with ions may be the main cause of Au-electrode damage and displacement reduction. Acknowledgment. This work was performed under the support of Creative Research Initiatives (EAPap Actuator) of KOSEF/MOST. References and Notes (1) Calleja, M.; Tamayo, J.; Nordstrom, M.; Boisen, A. Appl. Phys. Lett. 2006, 88, 113901. (2) Killian, J. G.; Coffey, B. M.; Cao, F.; Poehler, T. O.; Searson, P. C. J. J. Electrochem. Soc. 1996, 143, 936. (3) Baughman, R. H. Synth. Met. 1996, 78, 339. (4) Kim, J.; Song, C. S.; Yun, S. R. Smart Mater. Struct. 2006, 15, 719. (5) Kim, J.; Kang, Y. K.; Yun, S. R. Key Eng. Mater. 2005, 297, 671. (6) Ding, J.; Price, W. E.; Ralf, S. F.; Wallace, G. G. Synth. Met. 2000, 110, 123. (7) Smela, E.; Gadegaard, N. J. Phys. Chem. B 2001, 105, 9395. (8) Okamoto, S.; Kuwabara, K.; Otsuja, K. Sens. Actuators, A 2006, 125, 376.
7004 J. Phys. Chem. C, Vol. 112, No. 17, 2008 (9) Otake, M.; Kagami, Y.; Kuniyoshi, Y.; Inaba, M.; Inoue, H. Proc. IEEE, Int. Conf. Rob. Autom. 2002, 3224. (10) Pelrine, R. H.; Kornbluh, R. D.; Joseph, J. P. Sens. Actuators, A 1998, 64, 77. (11) Lee, C. S.; Joo, J.; Han, S.; Lee, J. H.; Koh, S. K. Synth. Met. 2005, 152, 49. (12) Ann, S. W.; Steier, W. H.; Kuo, Y. H.; Oh, M. C.; Lee, H. J.; Zhang, C.; Fetterman, H. R. Opt. Lett. 2002, 27 (23), 2109. (13) Kim, J.; Yun, S.; Ounaies, Z. Macromolecules 2006, 39, 4202. (14) Kim, J.; Lee, J. H. Smart Mater. Struct. 2005, 14, 934. (15) Kim, J.; Desphande, S. D.; Yun, S. R.; Li, Q. Polym. J. 2006, 38 (7), 659.
Cho et al. (16) Kim, J.; Song, C. S.; Yun, S. R. Smart Mater. Struct. 2006, 15, 719. (17) Jager, E. W.; Smela, E.; Inganas, O. Science 2000, 290, 1540. (18) Bay, L.; Jacobsen, T.; Skaarup, S.; West, K. J. Phys. Chem. B 2001, 105, 8492. (19) Christensen, P. A.; Hammett, A. Electrochim, Acta 1991, 36, 1263. (20) Maquarrie, M. J. Appl. Phys. 1953, 24, 1334. (21) Wang, D.; Fotinich, Y.; Carman, G. P. J. Appl. Phys. 1998, 83, 5342. (22) Tabard-Cossa, V.; Godin, M.; Grutter, P.; Burgess, I.; Lennox, R. B. J. Phys. Chem. B 2005, 109, 17531.
