Ind. Eng. Chem. Res. 2009, 48, 6303–6307
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Wettability Controlled Fabrication of Highly Transparent and Conductive Carbon Nanotube/Silane Sol Hybrid Thin Films Joong Tark Han, Sun Young Kim, Hee Jin Jeong, and Geon-Woong Lee* Nano Carbon Materials Research Group, Korea Electrotechnology Research Institute, Changwon 641-120, Korea
The stable few-walled carbon nanotube (FWCNT)/methyltrimethoxysilane sol mixture solution was successfully prepared by direct mixing of H2O2-treated FWCNT solution and silane sol. The stabilization of the solution was achieved by the intermolecular interaction between the hydroxyl groups on the FWCNTs and silanol groups in silane sol. FWCNT/silane sol hybrid thin films were then deposited on the model surfaces treated with piranha solution and silane molecules containing different end functionalities of NH2, CH3, and CF3 by spraying. The water contact angles of the model surfaces could be tuned from 112° to below 5°. It was found that a hydrophilic surface is more effective than a hydrophobic one in improving the conductivity of thin films by forming the uniform film structure, while the dewetting of the FWCNT/silane sol mixture solution on the most hydrophobic surface (CF3) slightly improved the transparency of the films. 1. Introduction Transparent conducting films based on carbon nanotubes (CNTs) have received considerable attention in various fields including display technologies, solar cells, flexible electronic devices, automobiles, and optical devices.1-9 CNT-based conductive-coating technologies have many potential applications in areas such as electrostatic dissipation (ESD), electromagneticinterference (EMI) shielding, and corrosion protection, as well as in the development of transparent film heaters and new electrode materials. To improve the mechanical and interfacial properties of the CNT-based conductive films on the substrate, binder materials such as polymer, silane compound, titanium compound, etc. should be added to the CNT solution. During development of high-quality films with controlled transparency and conductivity, it is also important to consider the long-term stability of the CNT/binder coating solution. The conductivity of the transparent CNT-based conducting film is then controlled by material properties such as purity, diameter, defects, metallicity, and degree of dispersion of the CNTs,10 as well as a tunneling effect through the insulating layer around the nanotubes.11 At the present time, for the transparent conductive coating based on the CNTs, membrane filtration and spraying methods were usually used. Spray application over a large and irregular area is advantageous for fast throughput fabrication. Here the wettability of the CNT/binder coating solutions on the substrates should be controlled to fabricate the highly transparent and conductive thin films because the film thickness should be below several hundred nanometers. In this respect, the surface free energy of the substrate is a characteristic factor that affects the surface properties and interfacial interactions, such as adsorption, wetting, and adhesion.12-14 Moreover, regarding the control of the wettability and the optical properties, we believe that the mixture solution of CNTs and a silane sol represents a promising candidate for producing multifunctional coatings because the use of sol-gel chemistry to modify the properties of the gel with functionalized silane precursors has significant advantages. The sol-gel technique is a well-known method for fabricating ceramic materials, which has been used to modify the ceramic materials, e.g., silica, TiO2, etc., with the CNTs. By incorporating selected functional precursor molecules care* To whom correspondence should be addressed. E-mail: gwleephd@ keri.re.kr. Fax: +82-55-280-1590. Tel.: +82-55-280-1677.
fully, they have found numerous applications in various fields such as superhydrophobic surfaces, optical materials, electrodes, and sensors. However, there have been a few reports about the transparent and conductive CNT/silane sol hybrid thin films fabricated with the dilute solution.15-27 In this study, we have therefore focused on the opto-electrical properties of the transparent CNT/silane sol hybrid films prepared on the surface of energy-controlled substrates. To achieve this goal, we modified the glass substrate with three kinds of functionalized (-NH2, -CH3, -CF3) silane layers, which have different chemical properties to that of the pirahnatreated glass substrate, which has -OH functional groups. In addition, a stable CNT/silane binder coating solution was prepared by controlling the chemical affinity between the CNTs and the silane compound. 2. Experimental Methods 2.1. Materials. The few-walled carbon nanotubes (FWCNTs), with an average diameter of 3-5 nm and a length ranging from hundreds of nanometers to micrometers (obtained from Iljin nanotechnology, Inc.), were used in this study. Methyltrimethoxysilane (MTMS) was purchased from Toshiba and used as received. Heptadecafluoro-(1,1,2,2-tetrahydrodecyl)trichlorosilane (containing CF3 groups), octadecyltrichlorosilane (containing CH3 groups), aminopropyltriethoxysilane (containing NH2 groups), and toluene were obtained from Aldrich and used as received. All the alkylsilanes, as well as anhydrous toluene, were stored in a desiccator prior to use.
Figure 1. Schematic representation of the spray coating of FWCNT/silane solutions on surface-modified model substrates.
10.1021/ie900301v CCC: $40.75 2009 American Chemical Society Published on Web 05/27/2009
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Figure 3. Transmittance versus sheet resistance of FWCNT/MTMS hybrid films on surface-energy-controlled substrates.
were then heated under vacuum for 3 h (at 150 °C) in order to remove the remaining chemicals and cure the silane binder. 2.4. Characterization. The Raman spectra were measured to characterize the disordering of FWCNTs after acid treatment at room temperature using a high-resolution Raman spectrometer (LabRAM HR800 UV) under excitation at wavelengths λ of 633 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed to confirm the functionalization of FWCNTs on an ESCALAB 250 (VG Scientific) spectrometer with monochromatized Al KR X-ray radiation as the X-ray source for excitation. The corresponding images of the resulting films were obtained by scanning electron microscopy (SEM, Hitachi S4800). The water contact angle measurement was performed to estimate the wettability of the surface-modified substrates. The sheet resistance was measured by a four-probe tester (Mitsubishi, MCP-T610). 3. Results and Discussion
Figure 2. (a) XPS and (b) Raman spectra of FWCNTs oxidized with hydrogen peroxide for 65 h at 60 °C.
2.2. Surface Modification of Substrates. Vacuum-dried reaction flasks were charged with anhydrous toluene and the pirahna-cleaned glass substrates under argon. The alkylsilanes were then added to the flasks (10 mM) and left to self-assemble on the wafers for 3 h under argon. The glass substrates were removed from the solution, rinsed several times with toluene and ethanol, and then baked in an oven at 120 °C for 1 h. After baking, the substrates were cleaned by ultrasonication in toluene, rinsed thoroughly with toluene and ethanol, and then vacuumdried prior to use. 2.3. Preparation of CNT/Silane Sol Hybrid Films. The FWCNTs were immersed and refluxed in hydrogen peroxide to attach hydroxy groups onto the side wall of FWCNTs for 65 h at 60 °C.15 The samples were then extracted several times by vacuum filtration using an alumina filter, until the solution reached a pH value of 7. Finally, 20 mg of the filtered FWCNTs were dispersed in 200 mL of ethanol for 2 h in an ultrasonic bath. The silane sol solution was prepared as follows. A silane sol solution was prepared by mixing 5 g of MTMS, 2 g of water, and 50 mL ethanol, and a sol-gel reaction was performed at 60 °C. The silane sol solution was then mixed with the CNT solution at the concentration of 70 wt %. The fabrication of CNT/silane sol hybrid film was achieved by the automated spray coater (Fujimori Co., NVD200) with a nozzle of 1.2 mm diameter at 70 °C. The prepared FWCNT/silane sol hybrid films
In this work, to study the surface energy effect, we chose four different glass substrates with water contact angles (CAs) from 112° to below 5°. Highly hydrophilic glass substrates (CA < 5°) were prepared by treatment with piranha solution (H2SO4/ H2O2 ) 7:3). The surface energies of the glass slides were systematically modified with silane layers containing various end functionalities. The CAs of silane-modified glasses were 67° for an NH2-functionalized surface, 96.5° for a CH3functionalized surface, and 112° for a CF3-functionalized surface (Figure 1). The FWCNTs were functionalized with hydroxyl groups by means of refluxing in hydrogen peroxide for 65 h at 60 °C. The functional groups on the nanotube surface were confirmed by XPS (Figure 2a). The main binding-energy peak (at 284.6 eV) was attributed to the C-C 1s, while the other three peaks were assigned to -C-OH (285.2 eV), -CdO (286.5 eV), and -COOH (289.5 eV).27 From XPS spectra, it was confirmed that hydroxyl groups were dominantly attached by refluxing in hydrogen peroxide. In addition, Figure 2b shows typical G and D bands of the Raman spectra at an excitation wavelength of 633 nm for the as-received and H2O2-treated FWCNTs. It was found that the ratio of the G-to-D peak intensity in the Raman spectra increased from 0.124 to 0.246 after the H2O2 treatment. The H2O2-treated FWCNTs were then dispersed in ethanol for 2 h in an ultrasonic bath (at a concentration of 100 mg/L). A typical silane sol solution was prepared by mixing MTMS, water, ethanol, and HCl with the sol-gel reaction being carried out at 60 °C. The prepared FWCNT solution was then mixed with the silane sol solution, which resulted in the formation of
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Figure 4. Scanning electron microscopy images of FWCNT/MTMS thin films on various substrates; the surface functionalities are as follows: (a) OH, (b) NH2, (c) CH3, and (d) CF3.
a stable FWCNT/MTMS sol (50/50 wt %) mixture solution. Here, the FWCNT solution can be stabilized by the intermolecular interaction occurring between the hydroxyl groups on the nanotubes and the silanol groups.28 The fabrication of the FWCNT/MTMS hybrid film was then achieved using an automatic spray coater at room temperature, in which the amount of coating solution was controlled to prepare thin films with high transmittances (above 90%). The transmittance and sheet resistance of the spray-coated CNT films depend on the amount of deposited FWCNTs and binder material (silane sol), as well as the ratio of CNT and binder. By the relationship of the binder content versus sheet resistance, a critical binder content (Xc) exists in the range of an amount less than a definite concentration. Above the Xc, the
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sheet resistance increases dramatically with increasing amount of binder content.28 Therefore, we prepared a FWCNT/silane sol mixture solution in the range of binder content (X), 0 < X < Xc, in our system, Xc ) 70 wt %. Figure 3 shows a plot of the transmittance (at 550 nm) versus the sheet resistance for FWCNT/MTMS films fabricated with the same amount of coating solution on substrates with increasing hydrophobicity. As shown in Figure 3, the sheet resistances were gradually decreased by increasing the surface energy of the substrate, which means that the electrical properties of the CNT/binder thin films can be controlled by changing the wettability of the coating solution on the substrates. Although the change of the transmittance of the films was very weak (T ≈ 92.3% to 91.2%, from CF3-functionalized to OH surfaces), the sheet resistance of the film on the OH surface is 1 order of magnitude lower compared to the counterparts prepared on the CF3-functionalized surface, which is a very low surface energy. This result is meaningful because, as reported in other literature, at the high-transmittance region, the sheet resistance can be changed dramatically.1-6 As shown in Figure 4, scanning electron microscopy (SEM) images of the FWCNT/MTMS sol hybrid films clearly show that they are more homogeneous on hydrophilic surfaces than on hydrophobic ones. A decrease in the surface energy leads to a more heterogeneous surface morphology. In particular, on the most hydrophobic surface (containing CF3 groups), a dewetted pattern is clearly formed after spray coating, which may explain the slightly high transmittance of the film. Nevertheless, the conductivity of this film is enough to use as a transparent ESD film because the CNT/MTMS sol is macroscopically connected. The dark region in the SEM images is the low-CNT-density area (mostly MTMS), as shown in Figure 5. Therefore, we analyzed the 2-D fractal dimension of the deposition pattern of the FWCNT/ MTMS sol on the CF3 surface based on the box counting of the digitized SEM image. The value of the fractal dimension
Figure 5. Magnified scanning electron microscopy images of FWCNT/MTMS thin films on various substrates; the surface functionalities are as follows: (a) CF3, (b) CH3, and (c) NH2. The insets show the shape-changing behavior of the silane sol droplets on the surface during spraying.
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of the film is obtained to be 1.77. This means that the FWCNT/ MTMS mixture forms the interconnected island-like structure after spraying. The lower sheet resistance of the film on CF3functionalized surface can be explained by the submicrometerscale disconnection of the FWCNTs as shown in Figure 5a. These results also indicate that the sheet resistance and the transmittance depend on the wettability of the binder materials on the substrate. In our study, the wettability of the silane sol (MTMS) could be visualized by observing high-magnification SEM images (Figure 5). During the spraying process, the droplet size of the coating solution (which contains CNTs, silane sol, and solvent) is about 10 µm. The silane sol also forms droplets with contact angles on the substrate after solvent evaporation. High-magnification SEM images taken on a CF3 surface (Figure 5a) indicate that the MTMS droplets have a high CA and are solidified without changing their diameter. However, upon increasing the surface energy, the MTMS sol forms a more filmlike structure. From the above SEM image analysis, the slight decrease of the transmittance of the film on the OH surface can be explained by the easy transmission of light through the dewetted area. Previously, there was also an attempt to improve the transparency of the CNT films by adjusting the CNT-network density using a two-dimensional colloidal-crystal template by Choi and co-workers.29 In another point of view, the transparency decrease can be speculated by the decreasing optical anisotropy of the films because of the low density of the CNT networks. It is indeed well-known that the individual CNTs possess intrinsic optical anisotropy due to different optical strengths of transitions allowed in tangential and parallel directions with respect to the tube axes.30-32 In films where CNTs are perfectly aligned to the substrate, the out-of-plane, extraordinary, optical absorption coefficient of the tubes (Ae) is much higher than that of the in-plane (Ao). An increase in mixing of Ae and Ao, due to reduced film anisotropy, will obviously lead to a lower transmittance at normal incidence. Our results have important implications for the fabrication of highly transparent and conductive films from CNT and binder mixed solutions. Although we use a polar solvent and a hydrophilic binder material in this study, our method can be applied to various coating solutions prepared with different solvents and binder materials on various substrates, such as poly(ethylene terephthalate), polyether sulfone, and polycarbonate. Moreover, we suggest that the transparency of CNT/ binder films can also be improved by manipulating the CNT density in the film, which can be achieved by adjusting the wettability of the coating solution or by forming the dewetted area of different surface energy surfaces, because the conductivity and transparency depend primarily on this CNT density. 4. Conclusion We have systematically investigated the effect of the surface energy of substrates on the optoelectrical properties of CNT/ silane sol hybrid films. To do this, we prepared a stable CNT/ silane sol mixture solution using FWCNTs, modified with hydroxyl groups, and an MTMS sol. We found that the wettability of the coating solution (which depends on its components and the nature of the substrate) affects the conductivity of the CNT-based thin films without the detrimental effect on the transparency. Therefore, we believe that this parameter should be considered in order to optimize the spraycoating process on various substrates, such as glass, polymer films, and others.
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ReceiVed for reView February 22, 2009 ReVised manuscript receiVed May 6, 2009 Accepted May 8, 2009 IE900301V
