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ARTICLES The Cause of Nanohole and Nanoparticle Formation on Au-Electrode after Actuation of Electro-Active Paper Actuator K. Y. Choi, H. G. Lim, S. R. Yun, Jaehwan Kim, and K. S. Kang* CreatiVe Research Center for Electro-ActiVe Paper (EAPap) Actuator, Mechanical Engineering Department, Inha UniVersity 253 Yonghyun-Dong Nam-Ku 402-751, Incheon, South Korea ReceiVed: July 31, 2007; ReVised Manuscript ReceiVed: July 11, 2008
Cellulose has been discovered as a smart material, which is termed as electro-active paper (EAPap). EAPap actuator revealed large displacement upon low actuation voltage and low electrical power consumption. However, the performance of the actuator was reduced as the actuation time increased. To investigate the performance degradation of the actuator, field emission scanning electron microscope (FESEM) images were taken on the surfaces of gold electrode of the actuator, and energy dispersive spectroscopy (EDS) was performed on them. Nanoparticles and nanoholes were observed on the surfaces of gold electrode after actuation, which might be strongly associated with the degradation of the actuator performance. The compositions of nanoparticles were gold and sodium. The degradation rate of the actuator performance and the number of nanoparticles at a low actuation frequency (2 Hz) were larger than those of a higher frequency (5 Hz). As the actuation voltage increased, the actuator performance degraded rapidly. Introduction Biocomposites including bioplastics from crops or biofibers from plants are novel materials for the next generation due to their importance not only as a solution of growing environmental intimidations but also as a solution of assuaging the uncertainty of the fossil fuel supply. The widespread reliance on petroleum demands a growing urgency to develop biobased composites to reduce the petroleum usage.1 Cellulose is one of the most naturally abundant biopolymers and can be found in many products, such as textiles,2-5 papers,6-9 and pharmaceuticals.10 Cellulose derivatives have demonstrated their versatility in numerous applications, for instance, as substrates for enzyme immobilization,11 semipermeable membranes,12 substrate for the immobilization of electroactive species,13 periodontal treatments,14 canine gastrointestinal tracts,15 purification of waters,16 and antimicrobial agents.17 Although Pb(Zr, Ti)O3 (PZT) is an important piezoelectric ceramic due to its high dielectric constant and charge coefficient, its high density, low hydrostatic piezoelectric charge, high voltage coefficient, and high stiffness are the major drawbacks for certain applications, such as hydrophones and artificial muscles. These disadvantages could be resolved by incorporating PZT into polymer. Recently, electro-active polymers (EAPs) have been attractive due to their large displacement, mechanical flexibility, and harmlessness to humans, which offer many advantages over traditional piezoelectric ceramic materials.18,19 EAPs can be used as actuators, sensors, artificial muscles, valves, acoustic transducers, electro-optic modulators, and noise control systems.20-22 Recently, Kim et al.23-25 have discovered cellulose as a smart material, which is termed electro-active paper. EAPap actuator exhibits a large bending displacement upon low * Corresponding author. E-mail:
[email protected].
actuation voltage and low electrical power consumption. Because EAPap is made with cellulose, it is biodegradable, biocompatible, cheap, and dry. There have been a few reports on the electrochemical behaviors of the electrode surface for EAPs. Electrode surfaces of EAP actuators significantly affect the performance of the actuators. Thus, the electrode behavior needs to be investigated to improve the performance and durability of EAP actuators. In general, the overall change of electrical resistance and capacitance of EAP actuators leads to the change of actuation behaviors of the actuators. Surface reactions of Pt and Au electrodes under aqueous and nonaqueous environments for ionic polymer metal composites were investigated via electrochemo-mechanical analysis using voltammetry and electrochemical impedance spectroscopy.26 They observed an electrochemically irreversible metal oxide layer formation. Metal electrode damage has been also reported for conductive polymer actuators. As the contact between gold electrode and electrolyte increased, the polypyrrole conductive polymer was completely delaminated from the electrode, as the actuation time increased.21 The Pt electrode damage of a poly(vinylidene fluoride) actuator has been also reported.21 In this Article, we investigated the performance degradation of EAPap actuator. Activating frequencies near the resonance frequency were applied to obtain the performance degradation mechanism. Au-electrode surface (air-Au interface) and its inside (Au-cellulose interface) were investigated after a certain time of actuation, using FESEM and EDS. Experimental Section The configuration of EAPap actuator comprised of cellulose film and gold electrodes is shown in Figure 1a. Gold electrodes (approximately 100 nm thick) of 10 mm × 40 mm were
10.1021/jp802635x CCC: $40.75 2008 American Chemical Society Published on Web 09/25/2008
Nanohole and Nanoparticle Formation on Au-Electrode
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Figure 1. (a) Configuration of EAPap actuator comprised of cellulose film and gold electrodes on both sides of cellulose paper and (b) schematic diagram of the displacement measurement system.
Figure 2. Displacement of the EAPap actuator with frequency. The resonance frequency of 3.5 Hz was obtained for both 3 and 4 V.
TABLE 1: The Composition of Cellulose Film (ICP-MASS Was Utilized To Analyze These Compositions (Unit: ppm)) element
amount
element
amount
Na Al Cr Co As
1693.08 391.95 97.81 1.281 3.34
S Cu Pb Zn Si
2867.59 89.47 117.08 179.13 306.54
deposited onto both sides of regenerated cellulose film (thickness of 20 µm). The actuator performance was evaluated by measuring the bending displacement of the actuator at a fixed temperature (23 °C) and relative humidity (85% RH) as shown in previous research.25 The schematic view of the measurement process is shown in Figure 1b. The prepared sample was placed in an environmental chamber (KMS, CTH3-2S) that can control the temperature and humidity. Bending displacement measurements were performed using a high precision laser doppler vibrometer (LDV, Bru¨el&Kjær, 8336), a current probe (Tektronix, TCP 300), LabView software on a personal computer,
Figure 3. Displacement decay with time for the EAPap actuator: 3 V-2 Hz, 3 V-5 Hz, 4 V-2 Hz, and 4 V-5 Hz cases. The higher electric field and lower frequency showed faster displacement degradation than the lower electric field and higher frequency.
and a function generator (Agilent, 33220A). The bending displacement was measured during 2 h by changing its actuation voltage (3 and 4 V) and frequency (2 and 5 Hz). After actuating 2 h, the surfaces of the outside (interface of air and Au) and the inside (interface of Au and cellophane) of the Au electrode were investigated using FESEM (S4300, Hitach). The composition of nanoparticles on the outside Au electrode was analyzed using EDS. To investigate the ion contents in the cellulose film, an ICPMASS (ELAN6100, PerkinElmer) was used. Approximately 0.1 g of cellulose film was dissolved in 10 mL of HCl:HNO3 solution (3:1 ratio) by stirring for the ICP-MASS sample preparation. Deionized water (10 mL) was added to the solution, followed by filtering through a polycarbonate filter (0.2 µm). The composition of the solution was analyzed using the ICPMASS.
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Choi et al.
Figure 4. FESEM images for the gold electrodes for (a) 3 V-5 Hz, (b) 3 V-2 Hz, (c) 4 V-5 Hz, and (d) 4 V-2 Hz. Nanoparticles appeared after 2 h of actuation on the outside of the gold electrodes.
TABLE 2: The Composition of Particle Area and Nonparticle Areaa particle area
nonparticle area
element
wt %
atom %
wt %
atom %
Na(K) Au(M) total
0.32 99.68 100.00
2.68 97.32 100.00
0.00 100.00 100.00
0.00 100.00 100.00
a The acceleration voltage was 15 kV, and the analyzed spot size was about 1 µm2.
Results and Discussion Before we investigated the electrode degradation of EAPap actuator, the bending displacement of the EAPap actuator was measured. Figure 2 shows the displacement with various excitation frequencies and voltages. When the excitation frequency was near the resonance frequency (3.5 Hz), the displacement became a maximum. The actuation frequencies of below (2 Hz) and above (5 Hz) the resonance frequency were chosen to investigate the possibly different degradation mechanisms of electrodes. When we started the investigation, we assumed that ions in the cellulose film would move to Au-electrode and stay near the electrode a longer time at lower frequency than higher frequency, which would result in a higher probability of electrochemical reaction at lower frequency than
at higher frequency. Figure 3 shows the displacement amplitude reduction of the actuators with various frequencies and electric field strengths. The initial amplitudes of the displacement were normalized to one, and the reductions of the displacements were recorded with time. The amplitude reductions for 3 V-2 Hz, 3 V-5 Hz, 4 V-2 Hz, and 4 V-5 Hz were about 22%, 15%, 60%, and 38% after 7200 s actuation, respectively. The decay rates for a low frequency (2 Hz) were larger than those of a high frequency (5 Hz) for both 3 and 4 V cases. For the different electric field strengths, high field strength (4 V) showed higher degradation rates with respect to low field strength (3 V) cases. The degradation rates for the low frequency and high electric field strength were faster than those for the high frequency and low electric field strength. The electromechanical degradation of the materials due to the electrical fatigue was proposed by MaQuarrie.27 Wang et al.28 also reported ferroelectric fatigue of the piezoelectric ceramics for actuator application. Degradation and delamination processes of the polypyrrole film coated with gold were observed due to the purely mechanical source caused by the volume change of the polypyrrole film and charge-induced surface stress from the interaction of anions and gold.29 It is well-known that alkali metals can form metallic or semiconducting alloys depending on the actual nature of the alkali metal and its concentration. Usually, the work function drastically decreases and then reaches saturation as the alkali metal coverage
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Figure 5. FESEM images for the gold electrodes for (a) 3 V-5 Hz, (b) 3 V-2 Hz, (c) 4 V-5 Hz, and (d) 4 V-2 Hz. Nanoholes appeared after actuation on the inside of the gold electrodes.
Figure 6. The image of EDS area (two circled areas) of nanoparticles. The composition was analyzed using EDS. The actuator was operated for 3 h at 4 V-2 Hz. The small arrows indicate the nanohole position.
increases. The alloy formation can be formed by thermal evaporation to the cold film30 or mixing two vapors.31 The free sodium ions with water molecules can move toward gold electrode when negative field is applied to the gold electrode, and more sodium ions can stay a longer time near the gold electrode when the actuation frequency is low. Therefore, the
gold electrode has a better chance of some electrochemical reaction with sodium ions at lower actuation frequency. The actuation principle of cellulose EAPap actuator has been explained as a combination of ion migration and piezoelectric effects associated with constituents of the cellulose paper.23 For the EAPap actuator system, -S- ions are attached to the
16208 J. Phys. Chem. C, Vol. 112, No. 42, 2008 cellulose backbone and can be considered as fixed ions, but Na+ ions are free when the cellulose film is wet. The free ions (Na+) move to the negative electrode and create osmotic pressure in the cellulose matrix with the application of electric field. The amount of ions in the EAPap is shown in Table 1. Although the amount of Na+ ions is small, a large bending displacement of 10 mm was achieved. This may be due to the additional piezoelectric effect of the cellulose.23 To investigate the electrode degradation of EAPap actuator, FESEM images were taken at the inside and outside surfaces of gold electrode after 7200 s actuation. Figure 4 shows the FESEM images of the outside gold surfaces contacting with air after actuation for (a) 3 V-5 Hz, (b) 3 V-2 Hz, (c) 4 V-5 Hz, and (d) 4 V-2 Hz. Various sizes of many nanoparticles were spread all over the gold surfaces for the four samples. The number of nanoparticles is closely related to the rate of actuator performance degradation. As the number of nanoparticles increased, the degradation rate of the actuator displacement increased. For a better understanding of the origin of the nanoparticles, the surfaces of the inside (between Au and cellophane) Auelectrodes were investigated using FESEM. The inside Auelectrode surfaces exhibited many nanoholes with different sizes and shapes, depending on the actuation frequencies and field strengths as shown in Figure 5a-d. The number of nanoholes shows a trend similar to that of nanoparticles. This indicates that the nanoparticles are closely related to the nanoholes. The numbers and sizes of the holes increased, as the degradation rate of actuator increased. The nanoparticles might be formed between Au and cellulose film with the reaction of Na+, Au, and electron, which result in pushing out Na+ toward the outside of the Au-surface through the nanoholes by osmotic pressure during the actuation process. From this result, the following electrochemical reaction is proposed:
2Au + Na+ + e- f Au2Na The gold and sodium alloy can be easily formed by electrochemical reaction. Qiao et al.32 obtained Au2Na-alloy electrode by cathodic electrolysis on Au electrode at 0.3 V for 2 h. However, only Au was obtained by anodic electrolysis on Au2Na electrode at 0.6 V for 1 h. The composition of the nanoparticles was analyzed using an EDS (Table 2). The acceleration voltage was 15 kV, and the analyzed spot area was about 1 µm2 for the EDS analysis (Figure 6). Although the electron beam usually can pass through several micrometers in depth for soft samples, it seems that the beam did not pass through the gold layer in our case. In case of the electron pass through the Auelectrode, the composition should be carbon and oxygen with gold. However, the composition of the nonparticle area was pure gold (100%), which proves that the electron beam did not pass through the gold layer. The EAPap actuator was operated for 3 h at 4 V-2 Hz to analyze the composition of the nanoparticles. As mentioned earlier, the nonparticle area composition was pure gold. However, the particle area has a relatively large amount of sodium, approximately 2.68 atom % (0.32 wt %). In summary, the performance degradation of the EAPap actuator was related to the electrode damage, causing the electrochemical reaction between gold and sodium with assistance of electron.
Choi et al. Conclusions Performance degradation of EAPap actuator occurred after a certain period of actuation. To investigate the degradation, the actuation frequencies near the resonance frequency and different electric field strengths were applied. Bending displacement degradation of cellulose EAPap actuator was strongly affected by the actuation voltage and frequency. Low actuation frequency led to faster displacement degradation. On the outside surface of the gold electrode, nanoparticles were observed after actuation, and many nanoholes were observed on the inside surface of the gold electrode. These nanoparticles contained sodium ions that might come from the inside of the cellulose film through the nanoholes. The number of nanoparticles and nanoholes is closely related to the displacement degradation: as the number of nanoparticles and nanoholes increased, the rate of displacement degradation increased. Acknowledgment. This work was performed under the support of Creative Research Initiatives (EAPap Actuator) of KOSEF/MEST. References and Notes (1) Park, H. M.; Liang, X.; Mohanty, A. K.; Misra, M.; Drzal, L. T. Macromolecules 2004, 37, 9076. (2) Lyons, W. J. J. Appl. Phys. 1958, 29, 1429. (3) Steinberger, R. L. J. Appl. Phys. 1934, 5, 53. (4) Quellet, C.; Schudel, M.; Ringgenberg, R. Chimia 2001, 55, 421. (5) Blow, C. M. J. Coc. Chem. Ind. 1938, 57, 116. (6) Horio, M.; Onogi, S. J. Appl. Phys. 1951, 22, 971. (7) Sennerfors, T.; Tiberg, F. J. Colloid Interface Sci. 2001, 238, 129. (8) Zauscher, S.; Klingenberg, D. J. J. Colloid Interface Sci. 2000, 229, 497. (9) Radtchenko, I. L.; Papastavrou, G.; Borkovec, M. Biomacromolecules 2005, 6, 3057. (10) Notley, S. M.; Wagberg, L. Biomacromolecules 2005, 6, 1586. (11) Silva, L. R. D.; Gushikem, Y.; Kubota, L. T. Colloids Surf., B 1996, 6, 309. (12) Rodrigues-Filgo, U. P.; Gushikem, Y.; Goncalves, M. D.; Cachichi, R. C.; Castro, S. C. Chem. Mater. 1996, 8, 1375. (13) Lazrin, A. M.; Borgo, C. A.; Gushikem, Y. J. Membr. Sci. 2003, 221, 175. (14) Khor, E.; Lim, L. Y. Biomaterials 2003, 24, 2339. (15) Okamoto, Y.; Nose, M.; Miyatake, K.; Sekine, J.; Oura, R.; Shigemasa, Y.; Minami, S. Carbohydr. Polym. 2000, 44, 211. (16) Lima, I. S.; Airoldi, C. Colloids Surf., A 2003, 229, 129. (17) Papineau, A. M.; Hoover, D. G.; Knorr, D.; Farkas, D. F. Food Biotechnol. 1991, 5, 45. (18) Okamoto, S.; Kuwabara, K.; Otsuka, K. Sens. Actuators, A 2006, 125, 376. (19) Otade, M.; Kagami, Y.; Kuniyoshi, Y.; Inaba, M.; Inoue, H. Proc. IEEE Intern. Conf. Robotics Automation 2002, 3224. (20) Pelrine, R. H.; Kornbluh, R. D.; Joseph, J. P. Sens. Actuators, A 1998, 64, 77. (21) Lee, C. S.; Joo, J.; Han, S.; Lee, J. H.; Koh, S. K. Synth. Met. 2005, 152, 49. (22) Ahn, S. W.; Steier, W. H.; Kuo, Y. H.; Oh, M. C.; Lee, H. J.; Zhang, C.; Fetterman, H. R. Opt. Lett. 2002, 27, 2109. (23) Kim, J.; Yun, S.; Ounaies, Z. Macromolecules 2006, 39, 4202. (24) Kim, J.; Deshpande, S. D.; Yun, S.; Li, Q. Polym. J. 2006, 38, 659. (25) Kim, J.; Song, C. S.; Yun, S. R. Smart Mater. Struct. 2006, 15, 719. (26) Jager, E. W.; Smela, E.; Inganas, O. Science 2000, 290, 1540. (27) McQuarrie, M. J. Appl. Phys. 1953, 24, 1334. (28) Wang, D.; Fotinich, Y.; Carman, G. P. J. Appl. Phys. 1998, 83, 5342. (29) Tabard-Cossa, V.; Godin, M.; Grutter, P.; Burgess, I.; Lennox, R. B. J. Phys. Chem. B 2005, 109, 17531. (30) Ageev, V. N.; Afanas’eva, E. Yu. Phys. Solid State 2006, 48, 2347. (31) Heiz, U.; Vayloyan, A.; Schumacher, E.; Yeretzian, C.; Stener, M.; Gisdakis, P.; Rosch, N. J. Chem. Phys. 1996, 105, 5574. (32) Qiao, H.; Nohira, T.; Ito, Y. Electrochim. Acta 2002, 47, 4543.
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