pubs.acs.org/Langmuir © 2010 American Chemical Society
Crystal-like Growth of a Metal Oxide/CNT Composite Fiber with Electroplated “Seed” from a CNT-Dispersed Nonaqueous Electrolyte Wal Jun Kim,† Eui Yun Jang,† Dong Kyun Seo,† Tae June Kang,† Kyoung Cheol Jin,‡ Dae Hong Jeong,‡ and Yong Hyup Kim*,† †
School of Mechanical and Aerospace Engineering and the Institute of Advanced Aerospace Technology and ‡ Department of Chemistry Education, Seoul National University, San 56-1, Sillim-dong, Kwanak-gu, Seoul 151-742, Korea Received April 6, 2010. Revised Manuscript Received September 7, 2010
A fabrication technique is developed for the preparation of metal oxide/CNT composites. An essential feature of the technique lies in the use of nonaqueous electrolyte in place of the usual aqueous electrolyte, which ensures well-dispersed CNTs without surfactants. After a “seed” is formed by electroplating on the anode, the seed is simply pulled up at a certain speed to grow a 1D CNT composite structure. The technique leads to a uniform distribution of metal oxide and a high weight fraction of CNT in the composite structure. Moreover, the conductivity of the composite is much higher than that of the CNT fibers fabricated with polymer.
Introduction Carbon nanotubes (CNTs) have extraordinary mechanical, chemical, and electrical properties. One way of exploiting these excellent properties is to produce composite materials of metal, metal oxide, and ceramic with CNT. It has already been shown that CNT is an excellent reinforcement material for polymer/ ceramic-based CNTs1-5 and metal/CNT composites.6-9 Recent interest in metal and metal oxide/CNT composites stems from their utilization as an emitter in field emission displays (FEDs), a thermal interface material, a supercapacitor, and interconnects in ultra-large-scale integrated circuits.10-19 Metal and metal oxide/ CNT composites have been prepared mostly via an electroplating technique,11-18 and in particular, an aqueous electrolyte has been *Corresponding author. E-mail:
[email protected]. (1) Mamedov, A. A.; Kotov, N. A.; Prato, M. D.; Guldi, M.; Wichsted, J. P.; Hirsch, A. Nat. Mater. 2002, 1, 190–194. (2) Thostenson, E. T.; Ren, Z.; Chou, T. W. Compos. Sci. Technol. 2001, 61, 1899–1912. (3) Coleman, J. N.; Khan, U.; Gun’ko, Y. K. Adv. Mater. 2006, 18, 689–706. (4) Ahir, S. V.; Terentjev, E. M. Nat. Mater. 2005, 4, 491–495. (5) Padture, N. P. Adv. Mater. 2009, 21, 1767–1770. (6) He, C.; Zhao, N.; Shi, C.; Du, X.; Li, J.; Li, H.; Cui, Q. Adv. Mater. 2007, 19, 1128–1132. (7) Cha, S. I; Kim, K. T.; Arshad, S. N.; Mo, C. B.; Hong, S. H. Adv. Mater. 2005, 17, 1377–1381. (8) Jeong, Y. J.; Cha, S. I.; Kim, K. T.; Lee, K. H.; Mo, C. B.; Hong, S. H. Small 2007, 3, 840–844. (9) Kim, K. T.; Eckert, J.; Menzel, S. B.; Gemming, T.; Hong, S. H. Appl. Phys. Lett. 2008, 92, 121901. (10) Cho., Y.; Choi, G.; Kim, D. Electrochem. Solid-State Lett. 2006, 9, G107– G110. (11) Lyth, S. M.; Oyeleye, f.; Curry, R. J. J. Vac. Sci. Technol., B 2006, 24, 1362– 1364. (12) Kang, H.; Lee, S.; Lee, H. J. Vac. Sci. Technol., B 2005, 23, 563–565. (13) Arai, S.; Endo, M.; Sato, T.; Koide, A. Electrochem. Solid-State Lett. 2006, 9, C131–C133. (14) Chen, X. H.; Cheng, F. Q.; Li, S. L.; Zhou, L. P.; Li, D. Y. Surf. Coat. Technol. 2002, 155, 274–278. (15) Arai, S.; Endo, M.; Kaneko, N. Carbon 2004, 42, 641–644. (16) Kim, I. H.; Kim, J. H.; Cho, B. W.; Lee, Y. H.; Kim, K. B. J. Electrochem. Soc. 2006, 153, A989–A996. (17) Xe, K. X.; Wu, Q. F.; Zhang, X. G.; Wang, X. L. J. Electrochem. Soc. 2006, 153, A1568–A1574. (18) Kim, I. H.; Kim, J. H.; Cho, B. W.; Lee, Y. H.; Kim, K. B. J. Electrochem. Soc. 2006, 152, A2170–A2178. (19) Liu, P.; Xu, D.; Li, Z.; Zhao, B.; Kong, E. S.; Zhang, Y. Microelectron. Eng. 2008, 85, 1984–1987.
Langmuir 2010, 26(20), 15701–15705
used for the electroplating. It is difficult to disperse carbon nanotubes uniformly in aqueous electrolyte without surfactants because of strong van der Waals interactions among them as a result of their hydrophobic nature. As a result, agglomerated CNTs form in the composite matrix and the composite properties become much worse than expected. In this work, we utilize a nonaqueous electrolyte in place of an aqueous electrolyte in preparing a metal oxide/CNT composite. The nonaqueous electrolyte, which is sodium tungstate dehydrate (Na2WO4 3 2H2O, STD) dissolved in an organic solvent of N,Ndimethylformamide (DMF), readily dissolves acid-treated singlewalled carbon nanotubes (SWNTs), resulting in a very stable CNT colloidal solution (Figure S1 in Supporting Information). The good dispersion of CNTs that is made possible with the use of a nonaqueous electrolyte ensures well-dispersed CNTs in the prepared composite. We expect that the CNT-dispersed nonaqueous electrolyte could contribute to the development of a fabrication technique for metal oxide/CNT composites with various dimensional structures. Metal oxide/CNT composites have been fabricated in the form of a thin film.16,18 However, a 1D metal oxide/CNT composite has not yet been reported because a homogenously dispersed CNT colloidal solution could not be obtained with an aqueous electrolyte. A homogeneous dispersion is an essential prerequisite for forming a continuous 1D structure. We successfully obtained a macroscopic 1D tungsten oxide/CNT composite fiber by electroplating and dipcoating with a nonaqueous electrolyte. In this fabrication of the tungsten oxide/CNT composite, tungsten oxide intermixed with CNTs is electroplated on the tungsten tip, which acts as a “seed” for the further growth of the composite (Figure S2 in Supporting Information). Once the seed is formed, it is simply pulled upward at a certain speed with the bias still on, creating a neck at the end of the seed, and then a small but enlarged cylindrical ingot forms at the neck in a manner similar to Czochralski crystal growth20 that is typically used for the growth of silicon and III-V compound semiconductors. Although the structure of the grown ingot is not that of a crystal, the CNTs in the grown ingot are well aligned in the direction of growth, as (20) Zulehner, W. J. J. Cryst. Growth 1983, 65, 189–213.
Published on Web 09/15/2010
DOI: 10.1021/la101358q
15701
Letter
Kim et al.
Figure 1. Schematic of the procedure for fabricating the 1D WO3/CNT composite structure by crystal-like growth: (a) immersing the tungsten (W) tip into a stable CNT suspension and electrolyte in DMF; (b) beginning of electroplating with a bias applied between the W tip and the counter electrode; (c) pulling up the WO3/CNT composite from the solution; and (d) ending the growth by pulling up at a faster rate. The bottom-row frames show digital snapshots of each step in the procedure.
shown later. It is for this reason that the process is referred to as crystal-like growth of a CNT composite.
Experimental Section Preparation of a CNT Colloidal Solution Based on a Nonaqueous Electrolyte and a Sharpened Tungsten Tip. To prepare purified SWNT powder, raw SWNTs (ASP-100F, Hanwha Nanotech Corp.) were sonicated in nitric acid for 30 min. Then the SWNTs were neutralized with deionized (DI) water and filtered on a membrane filter (Millipore, 0.2 μm pore size, 47 mm diameter) with a vacuum filtration method. The SWNTs were dried in a vacuum oven chamber at 80 °C for 48 h. Then, sodium tungstate and the purified SWNT powder, both of which were prepared to have a concentration of 0.1 mg/mL, were mixed. The solution was continuously sonicated for 10 h. The CNT colloidal solution-based nonaqueous electrolyte was placed in a Teflon vessel that is 3 mm in diameter and 5 mm in depth, and a counter electrode was placed on the bottom. Meanwhile, a 0.3-mm-diamter tungsten wire (The Nilaco Corporation, Japan) was electrochemically etched in KOH solution (1.5 M) with an applied voltage of 30 V to produce a sharpened tungsten tip with a radius of curvature of 250 nm (SEM image in Figure S1 of the Supporting Information). Electroplating System. The tungsten tip was immersed in the CNT colloidal solution in a container. The counter electrode was placed in the bottom of the container. We carried out electroplating with a Keithley 6220 precision current source, which provided the tungsten tip and the counter electrode with a constant current ranging from 2 nA to 100 mA. Elemental Analysis. An electron probe microanalyzer (EPMA) was employed for the quantitative analysis of the CNT/tungsten oxide composite. The electron beam for generating X-rays was accelerated up to 15 kV and focused on a small spot of less than 1 μm diameter. Tensile Test. A tensile test of the composite structure was carried out using a microbalance (a modified K100 Kr€ uss tensiometer). We have performed two different tests. First, a tungsten tip with the 1D composite was loaded onto the microbalance, and then the free end of the composite was tightly fixed to a UV-curable polymer. This tensile specimen was slowly pulled using a precise translator with a speed of 0.1 mm/min, and the loading force was simultaneously measured with a microbalance. At a certain loading, the specimen fractures at the interface between the composite and the tungsten tip. After the first tensile test, the second tensile test was carried out to obtain a tensile 15702 DOI: 10.1021/la101358q
stress-strain curve of the composite itself. The end of the composite, which was taken off the tungsten tip, was again fixed on a UV-curable polymer. We prepared a tensile specimen for the composite itself, both ends of which were fixed on the polymers, and then carried out the second tensile test. Measurement of the Conductivity of the Composite. For the measurement, the grown tungsten oxide/CNT composite was manually separated from the tungsten tip. The composite was then placed across two pads that are electrically isolated. The electrodes on both ends of the composite were coated with silver paste. The measurements were then made with the help of a probe station.
Results and Discussion Shown in Figure 1 is a schematic of the procedure involved in the fabrication of the tungsten oxide/CNT composite with a nonaqueous electrolyte. The bottom frames show actual optical pictures of the fabrication steps. As shown in Figure 1a, a sharpened tungsten tip with a radius of curvature of 250 nm is immersed in the electrolyte solution, which is composed of CNT and STD dissolved in DMF solvent for the WO3/CNT composite being fabricated (oxidation state of tungsten oxide in Figure S3 in the Supporting Information). In the schematic, the red and blue dots are WO4-2 ions and WO3, respectively, and the dark, straight lines represent CNTs. Upon applying bias to the tungsten anode with the container bottom grounded, negatively charged CNTs that are acid-treated gather around the anode along with the WO4-2 ions. The exact mechanism by which WO4-2 is transformed to WO3 is not clear. However, it could be postulated that the ion reacts with the carboxyl group or the hydroxyl group that is typically present on acid-treated CNTs to form the oxide or the ions recombine to yield the oxide. The darker black color in the bottom frame of Figure 1b clearly shows CNTs densely gathered around the electrode. After electroplating is carried out for 5 min, the electrode is pulled up as shown in Figure 1c at a speed of 0.3 mm/min with the solution of CNTs and WO4-2 attached to the tungsten tip. The bias is still on during the pulling. The high density of CNT in the tail attached to the electrode is also apparent from the color in the bottom frame. After the desired length of the macroscopic 1D CNT is reached, the electrode is quickly pulled up to complete the electroplating, as shown in Figure 1d. During and after electroplating, the solvent in the exposed column of solution evaporates, eventually leading to the Langmuir 2010, 26(20), 15701–15705
Kim et al.
formation of the macroscopic 1D CNT structure. The resulting structure is an interwoven network of aligned CNTs with WO3 acting as binder, and as a result, the structure is mechanically strong. Note in this regard that the atomic ratio of carbon in the composite is close to 92%, as revealed later. Therefore, the WO3/ CNT composite contains only a small amount of WO3. It is also noted here that the pulling of the liquid column as shown in Figure 1 was not possible in the absence of sodium tungstate dihydrates. The length of the macroscopic 1D CNT structure is entirely determined by the pulling time of the liquid column when the pulling speed is fixed, with the length being longer for longer pulling times (Figure S4 in Supporting Information). The diameter of the 1D structure can be varied by controlling the electroplating current. Shown in Figure 2 is the structure diameter plotted with respect to the current. Various currents of 2 nA, 10 μA, 100 μA, 1 mA, and 3 mA are applied to samples 1-5,
Figure 2. Diameter of the WO3/CNT composite vs the electroplating current in a controlled chamber at 30 °C and 27% relative humidity.
Letter
respectively. The plot shows that the diameter increases linearly with the logarithmic current. Varying the current implies adjusting the voltage; therefore, a higher current indicates a higher applied bias. A higher bias draws more CNTs into the vicinity of the tungsten tip, which results in a larger diameter. An example of the WO3/CNT composite fabricated according to the procedure in Figure 1 is shown in Figure 3a. The macroscopic 1D CNT structure is 2.5 mm in length and 25 μm in diameter and was fabricated at a current of 1 mA with a pulling time of 10 min. The magnified scanning electron microscopy (SEM) image of the structure in Figure 3b,c reveals the outside texture that appears to be interwoven cylindrical cords bound together in the direction of growth. The alignment is a result of the surface tension of the solution exerted on the WO3/CNT composite network that was generated by electroplating and the upward movement of the electrode.24,25 It is noted in this regard that the contact angle of the meniscus with the tungsten tip is acute as verified in our experiment with digital snapshots. The surface tension and the mass of the composite network below the solution surface act to generate vertical stress, which leads to the alignment of CNTs. The cross section of the composite was examined by tearing the composite off the tungsten tip, which is shown in Figure 3d where the tungsten tip is visible through the hole. The thickness of the composite on the tungsten tip was estimated to be about 5 μm. An examination of the inner surface of the delaminated composite that is shown in Figure 3e reveals that the CNT bundles are tightly embedded in tungsten oxide. To investigate the mechanical properties of the grown composite, tensile and shear stress-strain curves were obtained and are shown in Figure 3f. From these tests, we obtained ultimate stress values (Young’s modulus values) of 32.8 (3.63), 34.7 (4.84), 52.9 (8.79), and 72.7 MPa (50.8 GPa), respectively, for samples 2-5. The mechanical strength of sample 1 was not strong enough to be measured within the range of the microbalance because of its very small diameter. It was found that the ultimate stress and Young’s modulus increase with increasing electroplating current level. However, the strain corresponding to ultimate stress decreases
Figure 3. (a) SEM image of the whole WO3/CNT composite structure. (b) Cylindrical shape (25 μm diameter) of the composite. (c) Magnified SEM micrograph of well-aligned SWNT bundles along the axis of the composite. (d) Cross section of the fractured composite. (e) Morphology of SWNT bundles mixed in tungsten oxide as revealed by the inside of the delaminated composite. (f ) Stress-strain curves of the composite. The numbers indicate the tensile strength with the Young’s modulus in parentheses. Langmuir 2010, 26(20), 15701–15705
DOI: 10.1021/la101358q
15703
Letter
Kim et al.
Figure 4. (a) Quantitative analysis of all elements of the WO3/CNT composite as determined by electron probe microanalysis (EPMA) given as a function of the electroplating current. (b) Tungsten atomic ratio at different positions on the composite for five samples. (c) EPMA mapped image showing the distribution of tungsten (red dots) and carbon (green) over the whole composite.
from 0.0221 to 0.0027. Therefore, the composite gets more brittle with increasing electroplating current. These mechanical properties are comparable to those of the macroscopic 1D CNT structures fabricated by the usual methods.21-26 It should be noted, however, that the CNTs in those studies were multiwalled long tubes whereas CNTs in this study are single-walled short tubes (
