NANO LETTERS
A Multi-Wall Carbon Nanotube Tower Electrochemical Actuator
2006 Vol. 6, No. 4 689-693
YeoHeung Yun,† Vesselin Shanov,‡ Yi Tu,§ Mark J. Schulz,*,† Sergey Yarmolenko,| Sudhir Neralla,| Jag Sankar,| and Srinivas Subramaniam‡ Department of Mechanical Engineering, Smart Materials Nanotechnology Laboratory, and Department of Chemical and Materials Engineering, UniVersity of Cincinnati, 45211, First Nano, a DiVision of CVD Equipment Corporation, 1860 Smithtown AVenue, Ronkonkoma, New York 11779, and Department of Chemical and Mechanical Engineering, North Carolina A&T State UniVersity, Greensboro, North Carolina 27411 Received December 8, 2005; Revised Manuscript Received February 5, 2006
ABSTRACT Patterned multiwall carbon nanotube arrays up to four millimeters long were synthesized using chemical vapor deposition. Electrochemical actuation of a nanotube array tower was demonstrated in a 2 M NaCl solution at frequencies up to 10 Hz with 0.15% strain using a 2 V square wave excitation. The synthesis and electrochemical modeling approach outlined in the paper provide a foundation for the design of nanotube smart materials that actuate and are load bearing.
Introduction Smart materials are called solid-state transducers because they have sensing and actuating properties that are intrinsic to the material. The transduction properties are based on piezoelectric, pyroelectric, electrostrictive, magnetostrictive, piezoresistive, electroactive, and other effects. Piezoelectric ceramic materials produce a charge when strained and conversely expand when a voltage is applied and are the most important smart material today. However, high modulus piezoceramic materials are heavy and brittle, and need high voltage to operate, and low modulus ferroelectric ceramics have reduced force. The strains of ferroelectric ceramics are 0.15% for high modulus materials and several percent for low modulus materials. Shape memory alloy materials have up to 8% strain, but they require constant power to heat and a cooling part of the cycle is needed for operation. Although shape memory alloys are generally slower in cooling than heating, the rate response can be above 10 Hz by using thin sections and active cooling of the material. Electroactive polymers have large strain, but hysteresis makes precision motion difficult. Demonstrated actuation stresses of conducting polymers are as high as tens of megapascals. Other smart materials such as electrostrictive and magnetostrictive ac* Corresponding author. E-mail:
[email protected]. † Department of Mechanical Engineering, Smart Materials Nanotechnology Laboratory, University of Cincinnati. ‡ Department of Chemical and Materials Engineering, University of Cincinnati. § First Nano, a Division of CVD Equipment Corporation. | North Carolina A&T State University. 10.1021/nl052435w CCC: $33.50 Published on Web 03/14/2006
© 2006 American Chemical Society
tuators, thermal bimorphs, and other actuators require magnetic fields, large voltage, or have large sizes or weights. Therefore, no existing smart materials can meet the needs for many current advanced and future applications. Recently, smart materials based on nanotechnology have shown the potential to improve the way we generate and measure motion in devices from the nano- to the macroscale in size. The mechanical and electrochemical properties are coupled in carbon nanotubes (CNT), which is a characteristic of smart materials. In our experimentation, for example, operating with no electrolyte, no significant piezoelectric effect was observed for multiwall carbon nanotubes (MWCNT). MWCNTs are probably too electrically conductive for piezoelectric actuation to be observable. The piezoelectric effect in boron nitride nanotube bundles is predicted to be about 20 times smaller than that for piezoelectric ceramic materials, and this material is not readily available for making a new smart material at this time. The first CNT actuator developed was a macroscale sheet of nanotubes termed “buckypaper”.1-3 This actuator produced strain because of the change in dimension of the nanotube in the covalently bonded direction caused by an applied electric potential. Our group4-6 developed CNF (carbon nanofiber) nanocomposite actuators for both liquid electrolyte and solid polymer electrolyte applications. Despite these advances, there are still many challenges to developing tailored practical devices mainly because of the uncontrollable properties of nanotube buckypaper and polymer nanocomposite actuators. The short length and difficulty in processing bundles of nonoriented nanotubes has restricted their applications. These actuators
Figure 1. ESEM images of aligned nanotube patterned array at high magnification: (a) top view of the nanotube tower at 10 000x, 200 nm scale bar; (b) one nanotube tower at 125x, 200 µm scale bar; (c) side view with high magnification; and (d) final fabricated device on the glass substrate. Growth conditions: 200 SCCM of H2 flow, 200 SCCM of C2H4 flow, 100 SCCM carrier gas flow through a bubbler with water, 750 °C growth temperature.
cannot provide the high modulus and fast response needed for practical applications. One solution to improve CNT actuation performance is to synthesize vertically aligned arrays of CNTs that have well-defined properties with uniform length and diameter. Recently, efficient chemical vapor deposition (CVD) synthesis of SWCNT arrays has been demonstrated where the activity of the catalyst is enhanced by water during the synthesis.7 This approach produced patterned growth of SWCNT arrays up to 2.5 mm using water vapor to oxidize synthesis byproducts such as amorphous carbon or benzene without damaging the nanotubes. The present paper reports a technique for growing patterned CNT array towers for electrochemical actuators. MWCNT towers 1 mm square by 4 mm high were grown using water-assisted CVD. Because water provides longer catalyst life, continuous and long MWCNT arrays were grown with a growth time up to 3 h. The MWCNT tower was peeled off the Si substrate, and one end of the CNT tower was connected to a wire with conducting epoxy on a glass substrate. Electrochemical actuation was tested in a 2 M NaCl electrolyte solution and displacement of the CNT towers was measured using a laser displacement sensor. Electrochemical impedance spectroscopy (EIS) was carried 690
out to characterize the electrochemical properties of the nanotube tower actuator. Because EIS can provide the frequency-dependent complex impedance of an electrochemical cell, this result is directly related to the dopant concentration of the liquid-based actuator. The actuation properties of nanotube tower actuators were studied in a preliminary way based on the relationship between applied voltage and tower displacement. Actuator Fabrication. The procedure for synthesis of the nanotube tower is described in detail elswhere.4-6 Figure 1a-c shows the ESEM results of water-assisted CVD for the following growth conditions: 200 SCCM of H2 flow, 200 SCCM of C2H4 flow, and a 750 °C growth temperature. The Fe/Al2O3/SiO2/Si substrate used for the growth is cut into 5 × 5 mm2 wafers, and Fe is patterned into 1 × 1 mm2 blocks with 100 µm spacing between blocks. The as-grown CNT array has a high density of nanotubes and few impurities except on the top of the array. The adhesion of the tower to the substrate is weak, and the CNT array is peeled off easily. Figure 1b shows one nanotube array tower taken from the CNT array in Figure 1a by tweezers without damage. Individual nanotube towers 1 × 1 mm2 in size can be harvested easily from the substrate and used for electrochemical actuation. Figure 1c shows the MWCNT high Nano Lett., Vol. 6, No. 4, 2006
Figure 2. Electrochemical analysis setup for testing the nanotube tower actuator, consisting of one nanotube tower that is fixed on a glass substrate. A Ag/AgCl reference electrode and a Pt counter electrode are used.
density array at high magnification. Figure 1d shows the final fabricated device. Conductive epoxy is used to connect a wire to the nanotube tower on a glass plate. The wire is then electrically insulated using nonconductive epoxy. Each tower (1 × 1 mm2) of the patterned array contains on the order of 25 million nanotubes. The nanotube average diameter is 20 nm, and the nanotube length to diameter aspect ratio is 200 000:1. The surface area of each tower (for an average MWCNT 20 nm in diameter and 4 mm length) is about 6000 mm2. To measure the resistivity of the nanotube tower, epoxy was cast into the tower, both ends of the tower were polished, and wires were attached using conductive silver epoxy. The volume resistivity of the nanocomposite was about 0.11 Ω‚cm, and this includes the contact resistance of the wire to nanotube connection using conductive epoxy. Supporting Information Figure S1 shows the fabrication steps to make the nanotube tower electrode. First, the CNT tower is peeled off the Si substrate by tweezers. One drop of preheated conducive epoxy is deposited on the glass substrate, and the nanotube tower is placed on the glass. A copper wire is connected to the conducting epoxy and cured in an oven at 80 °C for 1 h. Exposed conducting epoxy is sealed by Epon Resin 862 and EPICURE curing agent W at 120 °C for 4 h. Resin 862 is manufactured from epichlorohydrin and Bisphenol-F and is a practical material for insulation because it has a low viscosity; provides good stiffness, toughness, and heat-resistant properties, is chemically resistant, and is an electrical insulator. The EPICURE curing agent W is used because it reacts with the resin without the formation of any byproducts. Glass bead tape is pasted on the top of nanotube tower to reflect the laser displacement sensor optical signal. Actuation Testing. The displacement of the nanotube tower actuators was measured using a laser displacement sensor (Keyence, LC-2400 Series) and a specially built test cell for characterizing the electrochemical properties of nanotubes as shown in Figure 2. Square wave potentials were applied between the working and counter electrodes using a National Instruments (NI) PCI board. To supply enough power from the NI board, a voltage follower using a Nano Lett., Vol. 6, No. 4, 2006
noninverting amplifier was designed using an operational amplifier with a gain of 10:1. Various square wave amplitudes were applied with frequencies ranging from 0.2 to 20 Hz. The EIS was performed on three-electrode cells, with the nanotube tower actuator as the working electrode, a Ag/ AgCl electrode used as the reference electrode, and a platinum plate as the counter electrode. EIS measurements were performed using a Gamry Potentiostat (Model PCI4/ 750) coupled with the EIS (Gamry, EIS300) software. The cell was equilibrated for several hours after each step. A 2 M NaCl electrolyte solution was used for the experiments. Figures 3 and 4 show the relationship between the strain and the voltage applied to the nanotube tower actuator. As shown in Figure 3, there is a nonuniform strain response with the square wave input. This problem would be solved by depositing an electrode layer on the top of the nanotube tower to bind the CNT electrodes to provide a higher voltage and uniform displacement. Strain of the nanotube tower closely follows the applied square wave potential of (2 V. With the increase of frequency, strain of the nanotube array decreases as shown in Figure 4. The nanotube tower has a high Young’s modulus resulting in a wide bandwidth actuator. The Young’s modulus is calculated using nanoindentation of vertically aligned MWCNT arrays.8 The exact actuation behavior of the MWCNT needs further study to determine if the strain is uniform along the length and if only the outer surface of the nanotubes are expanding and actuating. If all of the shells of the MWCNT could be made to actuate, then the force would be the greatest possible. The results herein verified the actuation effect and that increasing the magnitude of the applied potential increases the strain. The higher potential probably increases the charge accumulation at the nanotube tower/electrolyte interface and causes the faster response. However, too high of voltage would cause electrolysis of water and generate the bubbles on the surface of nanotube tower. This would decrease the lifetime of the actuator. Therefore, there is a limitation to increasing voltage to achieve high strain. A maximum strain of 2% has been predicted1 based on basal plane theory. Actual strains of 0.1-1% have been reported based on filmtype single-wall carbon nanotube actuators. The MWCNT tower actuator shows strain up to 0.15%. The straight aligned nanotube tower may produce higher strain compared to entangled nanotube-film actuators. Excellent mechanical properties and high strain generation of the nanotube tower actuator might provide a solution for a new intelligent material. Electrochemical Characterization. The electrochemical impedance spectroscopy (EIS) and potential step chronocoulogram analyses are performed to provide an understanding of the complex phenomenon of nanotube electrochemical actuation. The kinetics of ion exchange between the nanotube tower and electrolyte solution can control the actuator performance. Modeling the kinetics can provide the parameters and values for design and optimization of electrochemical actuators, including increasing the bandwidth. EIS and potential step chronocoulogram analyses were carried out using the same environmental condition in which the nanotube tower actuator works. 691
Figure 3. Strain of the nanotube tower at excitation frequencies of (a) 0.2 Hz, (b) 0.5 Hz, (c) 1 Hz, and (d) 5 Hz.
Figure 4. Strain as a function of frequency and applied square wave voltage for the nanotube tower actuator.
Figure 5b shows the EIS spectra for all frequencies between 0.5 Hz and 10 kHz. Depressed semicircles were observed at higher frequencies in Figure 5b, and the lines drawn through the rising part of the second loops at 45° angles represent the Warburg impedance that appeared at lower frequencies. This Warburg impedance indicates the finite diffusion of ions in the electrolyte solution. We also found that with a high open circuit potential at very low 692
Figure 5. (a) Randles’s circuit with Warburg impedances and (b) electrochemical impedance spectra for the tower actuator at DC cell potentials of 0.5 and 1 V; frequencies between 0.5 Hz and 10 kHz.
frequencies the curving of the plots returns to the real axis, which indicated that the impedance is limited by convective steady-state diffusion. Nano Lett., Vol. 6, No. 4, 2006
The results show the typical Randles’s Circuit with Warburg impedance. Randles’s Circuit is an equivalent circuit representing each component at the interface and in the solution during an electrochemical reaction for comparison with physical components. Cdl is the double layer capacitance, Rp is the polarization resistance, and Rs is the solution resistance. The EIS spectra of Figure 5b are modeled using the simple equivalent circuit shown in Figure 5a. This circuit can be expressed by Z(ω) ) Rs + [jωCdl + (Rp + sω-1/2 - jσω-1/2)-1]-1 (1) The structural parameter, σ, is related to the electrochemical parameter σ ) Rp[kof (Do)-1/2 + kRf (DR)-1/2]
(2)
where kof and kRf are the forward and backward electrontransfer rate constants, respectively, Do and DR are the diffusion coefficients, and σ/x2ω is called the Warburg impedance.9 The values of nonlinear curve-fitting show that Cdl is 3.8 µF, Rs is 65 Ω, Rp is 2600 Ω , and the Warburg impedance is 24.11 × 10-6 S. In electrochemical actuators, charge is accumulated on the nanotube electrode surfaces resulting in a high double-layer capacitance. High capacitance might help increase the expansion of the C-C bond in nanotubes. The polarization resistance is a barrier for ion exchange that resists the charge separation. More detail of modeling nanotube electrochemical actuators is given in refs 4-6. Supporting Information Figure S2 shows the potential step chronocoulogram with a 0.1 V step input. We can assume that there is no faradic current because only a pure supporting electrolyte is used. The exponential decay of the current with time depends on the double-layer capacitance and the solution resistance, with the time delay τ ) RuCdl ) 0.25 ms. Compared to the time delay, the charging time is much longer up to 0.5 s. Ions probably could not penetrate between the high density of nanotubes fast enough, resulting in a longer charging time. However, the potential step chronocoulogram shows a small faradic current that might come from incomplete insulation of the silver-conducting epoxy. Complete insulation and lower ohmic contact between the nanotube tower and electrode might provide higher frequency operation of nanotube tower actuators. Future Work. For future study, the actuator stroke will be optimized by proper choice of potential.10 The commercial potentiostat (Gamry) used maintains a constant potential by controlling the current. However, it does not have the ability to measure the open-circuit potential. Although this setup is very useful for studying actuation properties, it is used mainly for the analysis of characteristics such as EIS and the chronocoulogram. A new procedure to apply and insulate the electrode is also being developed, which is expected to increase the actuation strain. Improvement in the work output of the CNT tower actuator may be possible by increasing the density of nanotubes in the towers, stabilizing the nanotubes against buckling by encapsulating the array in a Nano Lett., Vol. 6, No. 4, 2006
solid polymer electrolyte, and preloading the actuator to work in tension and compression, or in opposition as a bending actuator. The potential for increased strain is also indicated by reported results of sheet-based carbon single-walled nanotube actuators providing over a percent strain using an organic electrolyte. Stacking arrays of towers, forming planar actuators using the harvested towers, and growing longer nanotube towers should be investigated to provide larger actuators that will be useful in smart structure applications. Summary and Conclusions. The actuation of a highly aligned multiwall carbon nanotube tower was verified in this preliminary study based on the relationship between the applied voltage and deflection of the actuator. The nanotube array tower was synthesized on a patterned Si substrate with a multilayered structure. Water-assisted chemical vapor deposition was used to grow towers up to 4 mm long. The tower actuates up to 10 Hz with a small decrease in strain, and only 2 V are applied to achieve 0.15% strain. The long nanotube tower also provides unidirectional strain, which may increase the efficiency of the actuator as compared to sheet-based entangled nanotube actuators. Several areas that need to be improved to make the actuator suitable for practical applications are discussed in the paper. Overall, the excellent mechanical properties and potential for high strain generation indicate the nanotube tower actuator might become a new intelligent material for applications from the nanoscale to the macroscale. Acknowledgment. This work was supported in part by the University of Cincinnati Institute for Nanoscale Science and Technology; Dr. Thomas Mantei is the director. His support is gratefully acknowledged. Supporting Information Available: Figure S1 shows the fabrication steps for the nanotube tower actuator, Figure S2 shows the potential step chronocoulogram of the actuator, and suggestions for possible applications of the tower actuator. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; Rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (2) Roth, S.; Baughman, R. H. Curr. Appl. Phys. 2002, 2, 311 (3) Baughman, R. H. Synth. Met. 1996, 78, 339. (4) Yun, Y. H.; Shanov, V.; Schulz, M. J.; Narasimhadevara, S.; Subramaniam, S.; Hurd, D.; Boerio, F. J. Smart Mater. Struct. 2005, 14, 1526. (5) Yun, Y. H.; Miskin, A.; Kang, I.; Jain, S.; Narasimhadevara, S.; Hurd, D.; Schulz, M. J.; Shanov, V.; He, P.; Boerio, F. J.; Shi, D.; Srivinas, S. J. Intell. Mater. Syst. Struct. 2006, 17, 107. (6) Yun, Y. H.; Miskin, A.; Kang, I.; Jain, S.; Narasimhadevara, S.; Hurd, D.; Schulz, M. J.; Shanov, V.; He, P.; Boerio, F. J.; Shi, D.; Srivinas, S. J. Intell. Mater. Syst. Struct., in press. (7) Hata, K.; Futaba, N. D.; Mizuno, K.; Namai, T.; Yumura, M.; Ijima, S. Science 2004, 306, 1362. (8) Qia, H. J.; Teob, K. B. K.; Lauc, K. K. S.; Boycea, M. C.; Milneb, W. I.; Robertsonb, J.; Gleasonc, K. K. J. Mech. Phys. Solids 2003, 51, 2213. (9) Grahame, D. C. Chem. ReV. 1947, 41, 441. (10) Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Madden, J. D.; Baughman, R. H. Smart Materials and Structures; Institute of Physics Publishing: New York, 2003; Vol. 12, pp 549-555.
NL052435W 693
