Enhanced Electrical Properties of Transparent Carbon Nanotube

Jun 25, 2010 - ... by the silane sol, whereas the D-to-G ratio and the G band position of the unbundled FWNTs-COOH is noticeably influenced by the sil...
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Enhanced Electrical Properties of Transparent Carbon Nanotube/Binder Hybrid Thin Films: Effects of the Silane Sol and the Bundle Size of the Carbon Nanotubes Joong Tark Han, Sun Young Kim, Jun Suk Kim, Hee Jin Jeong, Seung Yol Jeong, and Geon-Woong Lee* Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute, Changwon 641-120, South Korea

We report the effects of intermolecular interactions at the few-walled carbon nanotubes (FWNTs)/binder interface and the bundle size of the acid-treated FWNTs on the electrical properties of FWNTs/silane sol hybrid films. The bundle size of the FWNTs was successfully controlled by using acid treatment; assembled bundle structures due to hydrogen bonding were observed only for FWNTs functionalized with hydroxyl groups (FWNTs-OH). It was also confirmed by measuring the UV absorbance that the FWNTs-OH even form bundled structures in the solution state. Tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTMS) sols were used to manipulate the intermolecular interactions between the nanotubes and binder. We found that the hybridization of relatively large bundled FWNTs with TEOS sol unfavorable to hydrophobic nanotube surfaces enhanced the electrical properties of thin films prepared by spray coating. Moreover, the Raman spectrum of the bundled FWNTs-OH is not affected by the silane sol, whereas the D-to-G ratio and the G band position of the unbundled FWNTs-COOH is noticeably influenced by the silane sol. 1. Introduction Carbon nanotube (CNT)-based transparent conductive coating technology has potential applications in electrostatic dissipation, electromagnetic interference shielding, and transparent film heating as well as in the development of alternative electrode materials.1-10 The manipulation of the optoelectrical properties of CNT-based conductive films is performed by varying the material properties of the nanotubes, including their purity, diameter, chirality, defects, and metallicity.11-17 Moreover, organic materials such as surfactants, conjugated polymers, block copolymers, polyelectrolytes, pyrene, and DNA have been used to control the dispersion and stabilization of CNTs in various solvent media and polymer matrices.18-20 In particular, to fabricate highly transparent and durable CNT-based conductive films, binder materials such as cross-linkable polymers, silane compounds, and titanium compounds can be added to the CNT solution. In addition to the electronic properties of the nanotubes, the intermolecular interactions between the nanotubes and the binder material are the most important influence on the degree of dispersion of the CNTs and on the tunneling through the insulating layer around the nanotubes.21 The bundle size of the nanotubes is also important in terms of its effects on the resistance in the junction network structure since the higher the porosity of the CNT film, the smaller will be the conductivity.22,23 Previously, it has been reported that the bundle size of few-walled CNTs (FWNTs) can be controlled by surface modification of nanotubes through acid treatment.24 Therefore, FWNTs are suitable for studying the relation between the bundle size and intermolecular interaction between nanotubes and binder materials. In this study, we present the effect of the bundle size of FWNTs and the intermolecular interactions at the interface between the nanotubes and the functional groups of the silane on the optoelectrical properties of the FWNT/binder thin films. * To whom correspondence should be addressed. E-mail: gwleephd@ keri.re.kr. Fax: +82-55-280-1590. Tel.: +82-55-280-1677.

The bundle size of the FWNTs was successfully controlled by acid treatment with nitric acid and hydrogen peroxide, and its effects on the thermogravimetric and solution properties were characterized. We used silane sols as binder materials since the use of sol-gel chemistry to modify the properties of the gel with functionalized silane precursors has significant advantages.25 It has been found that there is a clear correlation between the optoelectrical properties of the FWNTs/binder thin films and both the intermolecular interactions at the nanotubes/binder interface and the bundle size of the nanotubes. 2. Experimental Details The FWNTs produced by chemical vapor deposition (CVD) method was purchased from Hanwha Nanotech Inc. The FWNTs with an average diameter of 3-5 nm and a length ranging from hundreds of nanometers to micrometer were used in this study. First, the FWNTs were immersed and refluxed in hydrogen peroxide for 48 h and 50% nitric acid for 4 h to attach hydroxyl and carboxylic groups onto the side wall of the FWNTs, respectively. The samples were then extracted several times by vacuum filtration using an alumina filter, until the solution reached a value of pH 7. After, functionalized FWNTs were dried by freeze-dryer to remove moisture for 24 h. Tetraethylorthosilicate (TEOS) and methyltrimethoxysilane (MTMS) were purchased from Aldrich and used as received. First, a silane sol solution was prepared by mixing 5 g of tetraethoxysilane, 1.73 g of water (1.98 g of water in the case of methyltrimethoxysilane), 3.33 g of ethanol, and 8 mg of 30.9 M HCl and by a continuous sol-gel reaction of it at 60 °C and 250 rpm. After that, the functionalized FWNTs (-OH or -COOH) were dispersed in ethanol for 2 h in an ultrasonic bath at a concentration of 100 mg/L, following the treatment with highpressure homogenizer. The FWNT solution was stable at a concentration of 100 mg/L, which was confirmed by UV-vis absorption spectroscopy (Varian, Cary 5000). The absorbance values of the CNT solutions were not changed even after 1 month. The concentration of the stable CNT solution depends

10.1021/ie100305g  2010 American Chemical Society Published on Web 06/25/2010

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on the functionality of the CNTs and the chemical affinity or the solubility parameter matching between CNTs and solvents. However, the addition of foreign materials in the stable CNT dispersion solution can cause severe aggregation of the CNTs if that is unfavorable to the CNTs and solvent. Thus, to prepare the coating solution having a long-term stability after mixing with silane sol, the FWNT solution was diluted to 50 mg/L concentration. Various amounts of sol (0, 25, 50, and 75 wt %) were added to FWNTs and mixed thoroughly to ensure uniform solution. The fabrication of FWNT/silane sol hybrid thin film was then achieved by automatic spray coater (Fujimori Co., NVD200) with a nozzle of 1.2 mm diameter at room temperature (RT). The glass substrates were cleaned with piranha solution (H2SO4:H2O2 (7:3)) at 130 °C for 20 min before use. The thicknesses (or transmittances) of the films were controlled by varying the coating times. The prepared FWNT/silane sol hybrid films were then heated under air for 3 h (at 100 °C) in order to remove the remaining chemicals and cure the silane binder. Thermogravimetric analysis (TGA) was performed to measure the impurity content and decomposition temperature of the pristine and acid-treated FWNTs. The samples were preheated to 200 °C in N2 atmosphere. The TG measurements by using TA TGA 2050 were done in air with a heating rate of 2 °C/min from room temperature up to 1000 °C. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB 250 (VG Scientific) spectrometer with monochromatized Al KR X-ray radiation as the X-ray source for excitation. The sheet resistance of the films was measured by a four-probe tester (Loresta, MCP-T610). The critical binder contents graph of the film was obtained by transmittance at 82%. The taping test of the films was performed by detaching the tape (3M Scotch tape) after rolling with a rubber roller). The corresponding images of the resulting films were obtained by field-emission scanning electron microscopy (FE-SEM, HITACHI S4800). The Raman spectra were measured to characterize the disordering of FWNTs at room temperature using a highresolution Raman spectrometer (LabRAM HR800 UV) under excitation at wavelengths λ of 633 nm. The transmittances of the prepared films were measured by UV-vis spectroscopy (Varian, Cary 5000). 3. Results and Discussion It is clear that many more carbon atoms bonded with hydroxyl (FWNTs-OH) and carboxyl groups (FWNTs-COOH) after H2O2 and HNO3 treatment, as previously reported.24 TGA provides a straightforward means of characterizing the thermal stability of an oxidized nanotube sample. The differential thermogravimetric (DTG) and TGA curves of pristine FWNTs and FWNTs oxidized with H2O2 and HNO3 are shown in Figures 1 and S1 (Supporting Information), respectively. The thermal stability of the oxidized FWNTs is lower than that of the pristine FWNTs. Above 300 °C, the weight of the acidtreated FWNTs starts decreasing, whereas the pristine FWNTs were not oxidized until 400 °C. This result indicates that functionalization and defect formation result from the acid treatment. The pristine FWNTs were completely burned near 600 °C, whereas the temperature of complete decomposition of the acid-treated FWNTs is 650 °C. We also note that the peak positions of the acid-treated FWNTs arise at higher temperatures than those of the pristine FWNTs. The presence of transition metals in the raw samples in fact assists the burning of carbonaceous particles and nanotubes.26 The remaining materials are transition metals, below approximately 3 wt %. The composition ratio can be determined from the DTG curve.

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Figure 1. TGA thermal graphs for (black) pristine FWNTs, (blue) FWNTsOH (treated with H2O2 at 60 °C for 48 h), and (red) FWNTs-COOH (treated with HNO3 at 100 °C for 4 h).

The curve shows that the main peak position is shifted by 39° by the acid treatment. Further, the DTG peak of the FWNTsOH arises at a lower temperature and is broader than that of the FWNTs-COOH, even though the ID/IG ratio (0.22) of the FWNTs-OH in Raman spectra is smaller than that (0.27) of the FWNTs-COOH, as shown in Figure 5. This phenomenon might therefore be due to the highly defective bundled structure of the FWNTs-OH. If the thermal oxidation of a defective nanotube in a bundled formation is initiated, the surrounded nanotubes could be oxidized even at low temperatures due to heat conduction. Solubility tests were performed on the FWNTs treated with H2O2 for 48 h and on those treated with HNO3 for 4 h. The solubility of the selected FWNTs in ethanol was determined with UV-vis spectroscopy (Figure S2 of the Supporting Information). The prepared FWNT solutions have concentrations less than 100 mg/L. In Figure 2, the relationship between the absorbance and the concentrations of the CNT solutions is linear and in good agreement with the Beer-Lambert law.27 However, note that the UV absorbance value of the FWNTs-OH solution is higher than that of the FWNTs-COOH solution at the same concentration (Figure 2B). The extinction coefficient of the FWNTs-OH solution (40.1 mL · mg · cm-1) is larger than that of the FWNTs-COOH solution (22.7 mL · mg · cm-1). In the case of the highly functionalized CNTs, the UV absorbance values of the CNT solution can be influenced by the quantity of the functional groups after acid treatment. However, in this study, the nanotube surface was not damaged by acid treatment,24 which means that the functional groups were mainly attached in nanotube ends. We therefore believe that this phenomenon is due to the bundled structure of the FWNTs-OH; the extinction coefficient is modulated by the size of the particles in solution according to the Beer-Lambert law. The size of the FWNTsCOOH is below 20 nm, while the bundle size of the FWNTsOH is 20-50 nm after spray coating, as shown of Figure 2C,D. Thus, to investigate the effect of the silane sol and bundle size of the FWNTs on the optoelectrical properties of the FWNT/silane sol hybrid film, coating solutions were prepared with FWNTs treated with H2O2 for 48 h and with those treated with HNO3 for 4 h. The FWNTs/binder coating solutions were prepared by direct mixing of the FWNT solutions with a silane sol. The transmittance of the FWNTs/silane sol thin films could be controlled easily by varying the coating times. Moreover, the transmittance and sheet resistance of the spray-coated CNTs/ binder films are expected to depend on the amount of deposited

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Figure 2. UV absorbance intensity at various wavelengths and concentrations of (A) a FWNTs-COOH/ethanol solution and (B) a FWNTs-OH/ethanol solution. C and D show the SEM images of the FWNT-COOH and FWNT-OH film surfaces, respectively; the scale bars are 200 nm in length.

CNTs and the binder material, and on the ratio of CNTs and the binder. From the sheet resistance vs binder content relation, it has been shown that there is a critical binder content (Xc) above which the sheet resistance increases dramatically.18-20 Although both carboxyl and hydroxyl groups can interact favorably with silanol groups in sols, the factor to mainly affect the film morphology is interaction between the hydrophobic nanotube surface and functional groups of sols because the carboxyl or hydroxyl groups are mostly attached in nanotube ends. The unpaired electrons of the hydroxyl groups of the TEOS sol can polarize the negative charges on the nanotubes’ surfaces, so favorable interactions between the TEOS sol and FWNTs do not result.28 Hydrophobic interactions can arise between the methyl groups in a MTMS sol and the nanotubes’ surfaces.29-31 Regarding the surface tension of FWNTs, it is well-known that the pristine CNTs are hydrophobic and have a surface tension around 150 mN/m, which is an upper limit for the surface tension of the liquid for favorable wetting.32 Therefore, the wettability of hydrophilic TEOS sol on nanotube surfaces is poor, while MTMS sol containing hydrophobic methyl groups is favorable to wet on nanotube surfaces. This interaction is expected to affect the intertube or interbundle distance in FWNTs/binder thin films, which can be directly correlated with the electrical properties of the FWNTs/binder film because the sheet resistance (Rs) of the film is determined by the intrinsic resistance of the FWNTs and the junction resistance between the nanotubes or bundles. It is expected that when binder material is intercalated between the FWNTs-COOH or FWNTs-OH bundles, the junction resistance will increase. On the basis of the intermolecular interactions between the CNTs and silane, it is expected that the films containing a more favorable silane binder to the CNTs such as MTMS will have

Figure 3. Sheet resistance change by increasing the amount of silane sol in acid-treated FWNTs/silane sol hybrid thin films. The red squares and blue circles indicate the FWNTs-OH and FWNTs-COOH, respectively. The red and blue lines indicate the addition of TEOS sol and MTMS sol, respectively.

a higher Rs than the FWNTs/TEOS films. On the other hand, in the case of the FWNTs/TEOS sol system, it is presumed that the Rs of the film will not be higher than that of pristine FWNT films because the van der Waals force of an intermolecular nanotube junction is greater than that of the interaction between the nanotubes and the oxygen or hydroxyl groups of the TEOS sol. Figure 3 shows the variation of the Rs of FWNTs/silane sol hybrid films with composition at a transmittance of 82%. As expected, the Rs of the FWNTs/MTMS films was found to increase gradually below a binder content 50 wt %; above 50 wt %, Rs dramatically increases. Note that below 50 wt % of MTMS sol, the Rs of the FWNTs-OH/MTMS films is virtually

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Figure 4. SEM images of (a, c, e) FWNTs-COOH, and (b, d, f) FWNTs-OH films (a, b) without silane sols, with 70 wt % of (c, d) MTMS sol, and (e, f) TEOS sol, respectively. Inset images indicate the high-magnification images.

constant, whereas that of the FWNTs-COOH/MTMS films gradually increases. It indicates that the films composed of large bundled nanotubes are less affected by the binder material than that composed of small bundled nanotubes. However, it is remarkable that the Rs of the FWNTs-COOH/TEOS sol films were constant and that of the FWNT-OH/TEOS sol films decreased with further additions of TEOS sol in spite of the insulating characteristics of TEOS sol. Figure 4 shows low- and high-magnification images of the FWNTs-COOH/silane sol and FWNTs-OH/silane sol films containing 75% silane sols, respectively. The dark regions in Figure 4 are a sol-rich area, in which the FWNTs are almost covered with sols, as shown in Figure S3 of the Supporting Information. The favorable interaction between the nanotubes and MTMS and in particular the wrapping of FWNTs-COOH and FWNTs-OH with the MTMS sol can be seen in Figure 4c,d, and the dark, sol-rich region was observed because the excess MTMS sol was added. Note that the small-sized FWNTs-COOH were effectively wrapped with MTMS sol. However, by adding TEOS sol, just the TEOSrich area was clearly observed in the low-magnification image and bare nanotube surfaces are exposed, as shown in Figure 4e,f. Above the critical binder content, the FWNTs are fully covered with MTMS sol (Figure 4c,d), which increases the contact resistance between the FWNTs and decreases the tunneling between the CNTs through the insulating silane layer. However, in the high-magnification images, bundled structures of the FWNTs-OH can be seen even after the addition of the silane sol. In particular, the bundled FWNT network structure of the FWNTs-OH/TEOS sol films is denser than that of the

Figure 5. Photographic image of FWNT/silane sol hybrid films before and after the taping tests, in which the dotted square indicates the detached area.

FWNTs-OH film without TEOS sol, which explains the decrease in the Rs of the FWNTs-OH/TEOS sol films. Moreover, in the bundled network structure, the addition of a small amount of MTMS sol is not expected to significantly affect the contact resistance at the junction because of the rich conduction channel in the bundled structure. Furthermore, we carried out taping tests to investigate the mechanical stabilities of the FWNTs/ silane sol hybrid films on the glass substrates. In Figure 5, the dotted areas show the photo images after the taping test, in which, with increasing binder content, the adhesive properties of the FWNTs/silane sol films on the substrate are gradually enhanced. However, even for 75 wt % TEOS, the FWNTs are slightly detached after the taping test. In particular, the FWNTs-

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Figure 6. Raman spectra (excitation at λ ) 633 nm) for (left) FWNTs-OH and FWNTs-OH/silane sol films and (right) FWNTs-COOH and FWNTsCOOH/silane sol films. The inset plot reveals the G band change by addition of silane sols.

OH/TEOS sol films are more detachable than other films, which can be also explained in terms of the larger bundle size. The conductivity, σDC, of disordered nanotube films depends on the number density of network junctions, NJ, which in turn scales with the network morphology though the film fill-factor, Vf, the mean diameter, 〈D〉, of the bundles, and the mean junction resistance, 〈RJ〉22,23 σDC )

2 K Vf 〈RJ〉 〈D〉3

Here K is the proportionality factor that scales with the bundle length. Thus, the different electrical properties of FWNTs/TEOS and FWNTs/MTMS can be explained in terms of RJ, which can be influenced by the intermolecular interactions between the nanotubes and the silane binders. The hydroxyl groups of the TEOS sol do not result in favorable interactions with the nanotube surfaces and interact only with the carboxylic groups of the FWNTs, so the RJ does not increase upon the addition of TEOS. However, MTMS interacts favorably with the nanotube surfaces via hydrophobic interaction and so can be intercalated between nanotubes, which results in an increase in the RJ of the CNT networks. Moreover, the larger the bundle structure, the more porous the network structure of nanotube films. Vf ()1 - porosity) of the FWNTs-OOH is smaller than that of FWNTs-OH. Therefore, TEOS sol unfavorable to hydrophobic nanotube surface can much change the porosity of the FWNTsOH network films. Thus, hydridization of the FWNTs-OH with TEOS sol may reduce the Rs of the thin films. Previously, we have reported that the Rs of the single-walled carbon nanotube (SWNT) was increased slightly and not decreased by hybridization with TEOS sol,21 which may be due to the smaller bundle size (∼10 nm) of the SWNT than the FWNTs (>20 nm). To investigate the influence of the silane sol on the CNT structure, Raman spectroscopy was used at a laser excitation wavelength of 1.96 eV. Raman spectroscopy has been used widely as a tool for the characterization of various forms of carbon and in particular to examine the vibrational modes of carbon nanotubes and their composites. In a strongly aggregated state, e.g., CNT network films without binder materials, the van der Waals interactions arise essentially between bundles, whereas in CNTs/binder thin films, interactions between bundles and the functional groups of the binder materials can influence the Raman features. Figure 6 shows the Raman spectra of the FWNTs-COOH/silane sol and FWNTs-OH/silane sol films. The Raman spectra of the FWNTs-COOH/silane sol films are

significantly affected by the silane sols, whereas those of the FWNTs-OH/silane sol film do not vary upon the addition of the silane binder. The ID/IG ratios of the FWNTs-OH/silane sol films are constant or decrease slightly with the addition of the silane, whereas those of the FWNTs-COOH/silane sol films increase significantly. In addition, an upward shift of 3.5 cm-1 was observed in the G bands of the FWNTs-COOH/silane sol films, whereas the G bands of the FWNTs-OH/silane sol films are not affected. It might be that, in the bundled network structure of the FWNTs-OH networks, the van der Waals interactions between nanotubes are the dominant influence on the Raman spectrum. However, in the case of FWNTs-COOH, the intermolecular interactions between the FWNTs-COOH and silane binders are the main factor affecting the Raman spectra, because, as shown in the inset image in Figure 4e, the FWNTsCOOH are well-hybridized with TEOS sol due to the unbundled structure. 4. Conclusions We have systematically investigated the effects of the silane sol and the bundle size of the FWNTs on the optoelectrical properties of transparent FWNTs/silane sol hybrid thin films. We used two kinds of silane sols functionalized with only hydroxyl groups (TEOS) and methyl groups (MTMS), and the bundle size of the nanotubes was controlled by using acid treatment. The bundling of the FWNTs-OH even in ethanol solution was confirmed by measuring the UV absorbance. We found that there is a critical binder content in the FWNTs/ MTMS system of about 50 wt %, whereas the TEOS sol, which has unfavorable interactions with the CNT surface, does not affect the Rs. Remarkably, it was found that the Rs of the FWNTs-OH films decreases by increasing the amount of TEOS sol due to the densification of the network of the bundled FWNTs-OH. The Raman spectra confirmed that the bundled FWNT films are not affected by the silane sol, which could be due to the strong van der Waals interactions between the nanotubes. Acknowledgment This work was supported by a grant from the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, and from KERI (Grant No. 10-12-N0101-17), Republic of Korea. Supporting Information Available: Figures showing a thermogravimetric diagram and UV absorbance spectra of FWNTs. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Cao, Q.; Rogers, J. A. Ultrathin films of single-walled carbon nanotubes for electronics and sensors: A review of fundamental and applied aspects. AdV. Mater. 2009, 21, 29. (2) Wu, Z. C.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Transparent, conductive carbon nanotube films. Science 2004, 305, 1273. (3) Zhang, D.; Ryu, K.; Liu, X.; Plikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Transparent, conductive, and flexible carbon nanotube films and their application in organic light-emitting diodes. Nano Lett. 2006, 6, 1880. (4) Zhou, Y.; Hu, L.; Gru¨ner, G. A method of printing carbon nanotube thin films. Appl. Phys. Lett. 2006, 88, 123109–1. (5) Parekh, B. B.; Fanchini, G.; Eda, G.; Chhowalla, M. Improved conductivity of transparent single-wall carbon nanotube thin films via stable postdeposition functionalization. Appl. Phys. Lett. 2007, 90, 121913–1.

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ReceiVed for reView February 08, 2010 ReVised manuscript receiVed May 27, 2010 Accepted June 09, 2010 IE100305G