Dispersion of Single-Walled Carbon Nanotubes by Nafion in Water

Publication Date (Web): September 25, 2008 ... Fax: +86-21-52413122. ... The best performance of SWCNT-Nafion-water/ethanol (50:50) films was attribut...
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16370

J. Phys. Chem. C 2008, 112, 16370–16376

Dispersion of Single-Walled Carbon Nanotubes by Nafion in Water/Ethanol for Preparing Transparent Conducting Films Jing Zhang,† Lian Gao,*,† Jing Sun,† Yangqiao Liu,† Yan Wang,† Jiaping Wang,† Hisashi Kajiura,‡ Yongming Li,‡ and Kazuhiro Noda‡ State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, People’s Republic of China, and Materials Laboratories, Sony Corporation, Atsugi Tec. No. 2, 4-16-1 Okata Atsugi, Kanagawa 243-0021, Japan ReceiVed: June 18, 2008; ReVised Manuscript ReceiVed: August 11, 2008

Single-wall carbon nanotubes (SWCNTs) dispersed by Nafion in water/ethanol with different ratios (100:0, 75:25, 50:50, 25:75, 0:100) were used to prepare transparent conducting SWCNT films both on glass and on polyethylene terephthalate (PET) substrates by filtration method. SWCNT-SDS films were also made to compare with different SWCNT-Nafion films, and the factors influencing the electrical conductivity of SWCNT films were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. It was found that SWCNT-Nafion-water/ethanol (50:50) films exhibited the best performance with the sheet resistance of 500-600 Ω/sq at transmittance of 85% and the best thermal stability at 60 °C with change of sheet resistance less than 2% in 200 h. The best performance of SWCNT-Nafion-water/ethanol (50:50) films was attributed to the following: (1) the bundle size of SWCNTs dispersed by Nafion was small, (2) the residual substance Nafion in the network of SWCNTs was more conductive than insulating SDS, (3) SWCNTs were p-doped by Nafion, and (4) the thermal stability of Nafion was good. Introduction Recently, SWCNTs have been widely used to make flexible transparent conducting thin films for potential application in electronic devices such as touch screens, liquid crystal displays (LCDs), organic light-emitting diodes (OLEDs), and photovoltaics.1-6 As compared to the conventional indium tin oxide (ITO) films, which have brittle nature and are easy to crack after repeated use, SWCNT films offer good durability and flexibility, ease of processing, low reflectance, and natural color. These advantages allow SWCNT films to replace ITO films for a variety of touch and display applications in the future. There are several methods for fabricating transparent nanotube films, including drop casting from solvents,7 spin coating,5 air brushing,1 dip casting,8 and Langmuir-Blodgett deposition.9 However, those methods have some limitations, such as lack of film homogeneity, low efficiency of film production methods, lack of control of film thickness, and flocculation due to van der Waals interactions between nanotubes.10 The vacuum filtration method, recently developed by Wu,1 is a simple and efficient one for producing homogeneous films with controllable thickness. Before transparent films are made by filtration method, SWCNTs should be debundled and dispersed well in liquid. Different strategies have been developed to achieve stable dispersions of debundled SWCNTs. Among the various methods reported, surfactants like sodium dodecyl sulfate (SDS) are widely used4,11 to disperse SWCNTs because noncovalent functionalization produced almost no damage to SWCNT structures. Although it is expected that surfactants are removed by washing with water in filtration, there are still some residual * Corresponding author. Tel.: +86-21-52412718. Fax: +86-21-52413122. E-mail: [email protected]. † Chinese Academy of Sciences. ‡ Sony Corp.

surfactants covering the SWCNTs, which increase the contact resistance of SWCNT films because surfactants are insulators. To remove the surfactants in SWCNT films and thus increase the electronic properties of films, various post treatment methods are used, such as acid treatment,12 and so on. However, post treatment is limited by substrates and may destroy SWCNT films. On the basis of those problems, new reagents are expected to replace surfactants for dispersing SWCNTs. These new reagents are expected to have two characteristics: (1) disperse SWCNTs well, and (2) not act as an insulator. Conductive polymers such as poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT/PSS) are candidates for dispersing SWCNTs, and then the conductive polymers between SWCNT networks may decrease the contact resistance of SWCNT films. However, PEDOT/PSS demonstrates undesirable optical transmittance over the visible region, which may influence the transmittance of SWCNT films. Therefore, new reagents are still needed for dispersing SWCNTs so that high conductive SWCNT films with high transmittance are obtained. In this study, Nafion, a perfluorosulfonated cation-exchange polymer, was used to disperse SWCNTs, and then transparent conducting films were made by the vacuum filtration method. Similar to other polymers used to wrap and solubilize SWCNTs, Nafion bears a polar side chain, through which the hydrophobic part can interact with SWCNTs. Wang et al.13 first reported that Nafion was a useful solubilizing agent for SWCNTs in preparing carbon nanotube based biosensors. Lee et al.14 also reported dispersion stability of SWCNTs using Nafion in a bisolvent and found that SWCNTs could be dispersed well by Nafion in the mixture of water and 1-propanol in a noncovalent way. On the other hand, unlike insulating surfactants, Nafion films (made by coating a specific amount of 5 wt % Nafion solution on glass or PET and dried at 150 °C) were tested to have sheet resistances in an order of 105 Ω/sq, with transmittance at 102% (greater

10.1021/jp8053839 CCC: $40.75  2008 American Chemical Society Published on Web 09/25/2008

Dispersion of SWNTs by Nafion in Water/Ethanol than 100% because of the exceptional optical property of Nafion15,16). It means that when SWCNTs are dispersed by Nafion and made into transparent films, the residual Nafion might have less negative effective in decreasing the conductivity and the transmittance of films as compared to surfactants. Besides, Nafion is viscous and has good thermal stability,17-19 which is helpful for increasing the adhesion of SWCNTs with substrates, and the thermal stability of SWCNT films. Therefore, in this Article, SWCNTs dispersed by Nafion in water, ethanol, and the mixture of water/ethanol with different ratios (which were marked as Nafion-water, Nafion-ethanol, and Nafion-water/ethanol solutions, respectively) were used to prepare SWCNT films both on glass and on polyethylene terephthalate (PET) substrates. SWCNT-SDS films were also made in the same experimental condition to compare with different SWCNT-Nafion films. Transmission electron microscopy (TEM) was used to characterize SWCNTs dispersed in different solutions. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to characterize different films. We investigated the conducting mechanism and the thermal stability of different films and found that SWCNT-Nafion-water/ethanol (50:50), SWCNT-Nafion-water/ethanol (75:25), and SWCNT-Nafionethanol films exhibited better performance than SWCNT-SDS films including the higher conductivity and the better thermal stability. Our results also showed that SWCNT-Nafion-water/ ethanol (50:50) films exhibited the best performance, with the sheet resistance of 500-600 Ω/sq at transmittance of 85%, suggesting that they could be promising candidates for replacing indium tin oxide (ITO) in electronic devices, such as touch screens12 and organic photovoltaics.11,20-22 Finally, we concluded that preparing transparent conducting SWCNT films using Nafion as dispersant was an effective, easy, and convenient method. Experimental Methods The SWCNTs synthesized by CVD were obtained from the Chengdu Organic Institute. The length of SWCNTs was about 50 µm, and the purity of SWCNTs was more than 90 wt %. To remove metal catalysts, 1.7 g of SWCNTs was refluxed in 2.6 M HNO3 at 140 °C for 48 h. Nafion purchased from DuPont had a concentration of 5 wt %, and it was diluted to 0.5 wt % with the mixture of water/ethanol in different ratios (100:0, 72: 25, 50:50, 25:75, and 0:100) to get different Nafion solutions. Vacuum filtration method was used to make transparent conducting films of SWCNTs. First, 10 mg of SWCNTs was dispersed in 200 mL of different Nafion solutions by bath ultrasonication for 2 h. SWCNTs were also dispersed in 1 wt % SDS aqueous solution to make films for comparison. Second, the sonicated solution was centrifuged at 13 000 rpm for 30 min. The supernatant was carefully collected and subjected to another round of 30 min centrifugation at 13 000 rpm. The supernatant was diluted with water or water/ethanol (75:25, 50: 50, 25:75) or ethanol, respectively, for 10-fold according to different SWCNT solutions, and then 10-150 mL solutions were used to filtrate and prepare films. 220 nm Millipore ester membranes were used to allow the deposition of SWCNT films of different thickness and densities. No water or ethanol was used to wash SWCNT-Nafion films. Yet for SWCNT-SDS films, water was used to wash SDS away. After filtration, the filter membranes were then transferred onto glass or PET substrates, dried in air at 90 °C for 1 h, and dipped in acetone for 30 min after ortho-dichlorobenzene was dropped onto the membrane filter, leaving behind SWCNTs thin films on the substrates. The obtained films were finally dried at 150 °C for 1 h.

J. Phys. Chem. C, Vol. 112, No. 42, 2008 16371 The morphology of SWCNTs dispersed using Nafion or SDS was observed by TEM (JEM-2100F, JEOL, Tokyo, Japan). SEM images of SWCNT films were taken on a field emission scanning electron microscope (FESEM, JEOL, JSM-6700F). The transmittance at 550 nm of films was measured via a UV-vis spectrometer (Lambda 950, Perkin-Elmer, Shelton, USA). Measurements of the sheet resistances were carried out with a four-point probe resistivity meter (Loresta EP MCP-T360, Mitsubishi Chemical, Japan). X-ray photoelectron spectra (XPS) analysis was conducted using the Al KR (1486.6 eV) monochromatic X-ray source (Axis Ultra DLD, Kratos). Raman spectra of the films were recorded using a Renishaw MicroRaman spectrometer with an excitation length of 633 nm. Results and Discussion The TEM images of the supernatant of SWCNTs dispersed by Nafion in different ratios of water/ethanol and SWCNTs dispersed in SDS aqueous solution were shown in Figure 1. Long nanotubes were observed for all of the SWCNT-Nafion solutions, indicating that SWCNTs were dispersed well using Nafion in a noncovalent way as reported by Wang et al.13 and Lee et al.14 Bundle diameter distributions measured from those images in Figure 1 were presented in Figure 2. For SWCNTs dispersed in (a) Nafion-water and (b) Nafion-water/ethanol (75: 25) solutions, the bundle sizes were from 2 to 33 nm. About 5% of SWCNTs had diameters larger than 20 nm. For SWCNTs dispersed in (c) Nafion-water/ethanol (50:50), (d) Nafion-water/ ethanol (25:75), and (e) Nafion-ethanol solutions, small sizes of bundles occupied more proportion (no bundle sizes larger than 20 nm were observed). However, the stability of SWCNTs dispersed in (d) Nafion-water/ethanol (25:75) and (e) Nafionethanol solutions seemed not so good, because precipitation was observed by naked eyes after being left for several days. SWCNTs dispersed in (c) Nafion-water/ethanol (50:50) solution had the best stability and the narrowest diameter distribution, with bundle sizes from 3 to 9 nm. This phenomenon was similar to Lee’s result14 that SWCNTs were dispersed best using Nafion in water/1-propanol with ratio of 80:20. Our result could be explained according to the mechanism proposed by Lee et al.14 For SWCNTs dispersed in (a) Nafion-water, the hydrophobic backbone in Nafion interacted with SWCNTs, and the hydrophilic polar group dissolved well in water. Yet Nafion was easy to aggregate in water, which caused large bundles of SWCNTs, like SWCNTs dispersed in (a) Nafion-water and (b) Nafionwater/ethanol (75:25) solutions. As ethanol was further added to water, the solubility of Nafion in the liquid medium was improved, resulting in the conformational change of Nafion23 adsorbed on the walls of SWCNTs from an aggregated to a stretched-out structure,14 and finally SWCNTs with small bundles and good stability were obtained, like SWCNTs dispersed in (c) Nafion-water/ethanol (50:50) solution. However, as the proportion of ethanol in the mixed solvent increased, contacts between hydrophobic Nafion and the solvent were preferred,14 which led to a decrease in the amount of adsorbed Nafion on the SWCNTs’ surface. That is why the small bundles of tubes for SWCNTs dispersed in (d) Nafion-water/ethanol (25: 75) and (e) Nafion-ethanol were easy to precipitate after being left for several days. From the TEM image and bundle diameter distributions, we also found that the bundle sizes of nanotubes dispersed in SDS aqueous solution were as small as those in Nafion-water/ethanol (50:50), with diameter from 3 to 10 nm. SEM images of different SWCNT films were displayed in Figure 3. Consistent with the results of TEM, the bundle sizes of nanotubes in (a) SWCNT-Nafion-water film and (b) SWCNT-

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Zhang et al.

Figure 1. TEM images of the supernatant of 10 mg of SWCNTs dispersed by Nafion in (a) water, (b) water/ethanol (75:25), (c) water/ethanol (50:50), (d) water/ethanol (25:75), (e) ethanol, and (f) SWCNTs dispersed in SDS aqueous solution.

Figure 2. Bundle diameter distributions measured from corresponding images in Figure 1, SWCNTs dispersed by Nafion in (a) water, (b) water/ ethanol (75:25), (c) water/ethanol (50:50), (d) water/ethanol (25:75), (e) ethanol, and (f) SWCNTs dispersed in SDS aqueous solution.

Nafion-water/ethanol (75:25) film were larger than those in (c) SWCNT-Nafion-water/ethanol (50:50) film, (d) SWCNT-Nafionwater/ethanol (25:75) film, and (e) SWCNT-Nafion-ethanol film. Those large bundles would decrease the conductivity of SWCNT films, because the intertube resistance of SWCNTs increased. Besides, some residuals wrapped around the tubes or in the network of SWCNTs were also observed in SEM images, and those residuals were attributed to Nafion and SDS according to the XPS results. Figure 4 showed the high-resolution XPS of SWCNT-Nafion-water/ethanol (100:0, 50:50, 0:100) films and SWCNT-SDS film on PET substrate. The main C1s peak of all of the SWCNT films at 284.6 eV was assigned as CdC from carbon nanotubes,24 with a tail near 285 eV for C-C and C-H25 (Figure 4A). The peaks at higher binding energy, C-O from 286.5 to 286.7 eV and COO from 288.4 to 288.9 eV, were attributed to the oxygen-related groups on SWCNTs24 and

contamination introduced by the ambient atmosphere.25 For SWCNT-Nafion films, a new peak at 291.6 eV appeared, while no peak was observed for SWCNT-SDS films. According to Susac et al.,25 the peak at 291.6 eV was interpreted as CF2, CF-O, CF2-O groups from Nafion (F1s peak was also observed at 688.8 eV (not shown)), indicating that Nafion existed in SWCNT-Nafion films. In addition, the S2p peaks (Figure 4B) of SWCNT-Nafion films were observed at 168.5 eV, which were attributed to sulfur in the -SO3H groups from Nafion,26 while for SWCNT-SDS films, the S2p peak was observed at higher binging energy 169.0 eV, which corresponded to the sulfur in the SO4- from SDS.27 Those SDS residuals would affect the sheet resistance of SWCNT films, which will be discussed next. The transmittance and sheet resistance of different SWCNT films on different substrates were shown in Figure 5. The

Dispersion of SWNTs by Nafion in Water/Ethanol

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Figure 3. SEM images of (a) SWCNT-Nafion-water film, (b) SWCNT-Nafion- water/ethanol (75:25) film, (c) SWCNT-Nafion-water/ethanol (50:50) film, (d) SWCNT-Nafion-water/ethanol (25:75) film, (e) SWCNT-Nafion-ethanol film, and (f) SWCNT-SDS film on PET substrate.

Figure 4. The high-resolution XPS of (A) C1s and (B) S2p peaks for (a) SWCNT-Nafion-water film, (b) SWCNT-Nafion-water/ethanol (50:50) film, (c) SWCNT-Nafion-ethanol film, and (d) SWCNT-SDS film on PET substrate.

transmittance shown in all of the figures was the transparency of SWCNT films after arithmetically subtracting the transparency of substrates. It was found that there were not many differences in the sheet resistances of films on glass or PET, which meant that sheet resistances of SWCNT films were independent of the substrate. For (a) SWCNT-Nafion-water and (b) SWCNT-Nafion-water/ethanol (75:25) films, the sheet resistances were larger than the others, while for (c) SWCNTNafion-water/ethanol (50:50), (d) SWCNT-Nafion-water/ethanol (25:75), and (e) SWCNT-Nafion-ethanol films, the resistances almost decreased 3-10-folds at the same transmittance. SWCNTNafion-water/ethanol (50:50) films showed the best performance with sheet resistance of 500-600 Ω/sq at transmittance of 85%. Gruner2 reported, for SWCNT film at the same transparency, which has the same nanotube density, well-dispersed SWCNTs would lead to higher conductivity. In our case, SWCNTs dispersed in Nafion-water/ethanol (100:0, 75:25) had some large size bundles, which increased the intertube resistance of SWCNTs, and finally induced the higher sheet resistance of films. The better performance of SWCNT-Nafion-water/ethanol (50:50, 25:75, 0:100) films was mainly attributed to the better dispersion of SWCNTs with small bundle sizes. SWCNT-SDS films (as shown in Figure 4f) were also made to compare to different SWCNT-Nafion films. The sheet

resistance of (f) SWCNT-SDS films was lower than that of (a) SWCNT-Nafion-water film and (b) SWCNT-Nafion- water/ ethanol (75:25) film, but higher than that of (c) SWCNT-Nafionwater/ethanol (50:50) film, (d) SWCNT-Nafion-water/ethanol (25:75) film, and (e) SWCNT-Nafion-ethanol film. Although the bundle sizes of SWCNTs dispersed in SDS aqueous solution were comparable to or even smaller than SWCNTs dispersed in Nafion-water/ethanol (50:50, 75:25, 0:100), the sheet resistance of SWCNT-SDS films was about 2 times greater than that of SWCNT-Nafion-water/ethanol (50:50, 75:25, 0:100) films. Using SDS as dispersant, Gruner et al.10 prepared SWCNT (hipco) films by filtration method and got the sheet resistance of 1000 Ω/sq at transmittance of 85%. Using SDS as dispersant, Chhowalla et al.11 prepared SWCNT (hipco) films with sheet resistance of 2500 Ω/sq at transmittance of 85% by filtration method and decreased the sheet resistance to 500 Ω/sq after SOCl2 post-treatment. Comparing the two reference results, the sheet resistance of SWCNT films was much smaller by using Nafion as dispersant in water/ethanol (50:50) media in the present work. Besides, no post-treatment was used and the resistance was comparable to Chhowalla’s work after SOCl2 post-treatment, indicating that Nafion was more effective and convenient than SDS as a dispersant for making SWCNT films. One possible reason for the better properties of SWCNT-Nafion

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Figure 5. The transmittance and sheet resistance of (a) SWCNTNafion-water films, (b) SWCNT-Nafion-water/ethanol (75:25) films, (c) SWCNT-Nafion-water/ethanol (50:50) films, (d) SWCNT-Nafionwater/ethanol (25:75) films, (e) SWCNT-Nafion-ethanol films, and (f) SWCNT-SDS films on different substrates: (A) glass and (B) PET.

Figure 6. Raman spectrum taken at 633 nm: (a) SWCNT-Nafionwater film, (b) SWCNT-Nafion-water/ethanol (75:25) film, (c) SWCNTNafion-water/ethanol (50:50) film, (d) SWCNT-Nafion-water/ethanol (25:75) film, (e) SWCNT-Nafion-ethanol film, (f) SWCNT-SDS film, and (o) SWCNTs powder on PET substrate.

films was that Nafion (with sheet resistance of 105 Ω/sq for the Nafion film) was more conductive than SDS insulator, and therefore the remnant Nafion for SWCNT-Nafion-water/ethanol (50:50) films less increased the contact resistances of SWCNTs as compared to the residual SDS affected on SWCNT-SDS films. Another possible reason was that SWCNTs were doped for SWCNT-Nafion films, which were confirmed by Raman spectra as shown in Figure 6. Raman spectroscopy was used to analyze the structural and electronic properties of different SWCNT films, as shown in Figure 6. There are two main domains for typical Raman spectra of SWCNTs: D-band around 1300 cm-1 resulting from disordered graphite and the degree of conjugation disruption,28 and

Zhang et al. G-band around 1600 cm-1 resulting from the tangential C-C stretching vibrations.28 Thus, the intensity ratio of the D-band to the G-band (ID/IG) is widely used as a measure of sidewall covalent derivation or defect introduction.29,30 In our spectrum, the intensity of different films was normalized to the same intensity for the G-band. ID/IG values were 0.37, 0.39, 0.34, 0.30, 0.30, 0.31, and 0.20 for (a) SWCNT-Nafion-water film, (b) SWCNT-Nafion-water/ethanol (75:25) film, (c) SWCNT-Nafionwater/ethanol (50:50) film, (d) SWCNT-Nafion-water/ethanol (25:75) film, (e) SWCNT-Nafion-ethanol film, (f) SWCNT-SDS film, and (o) SWCNTs powder, respectively. In comparison with that of SWCNTs powder, the ID/IG values of all of the SWCNTs films became a little increased, which might be induced from the damage of the wall of nanotubes by ultrasonication when SWCNTs were dispersed. Moreover, the ID/IG of the SWCNTNafion films was almost the same as that of SWCNT-SDS films, indicating that SWCNTs were dispersed by Nafion in a noncovalent way, similar to the case of SDS-dispersed SWCNTs. The noncovalent nature of SDS-dispersed SWCNTs has been reported in many literature works.31-34 Charge transfer was also confirmed by Raman spectroscopy after SWCNTs were dispersed by Nafion in water/ethanol and made into films. As shown in Figure 6, the G-band of (o) SWCNTs powder was at 1586.9 cm-1, and it blue-shifted by 23.6, 27.0, 21.9, 25.3, and 18.6 cm-1 for the (a) SWCNT-Nafion-water film at 1610.5 cm-1, (b) SWCNT-Nafion-water/ethanol (75:25) film at 1613.9 cm-1, (c) SWCNT-Nafion-water/ethanol (50:50) film at 1608.8 cm-1, (d) SWCNT-Nafion-water/ethanol (25:75) film at 1612.2 cm-1, and (e) SWCNT-Nafion-ethanol film at 1605.5 cm-1, respectively. According to previous reports, the G-band of Raman spectra was shifted to the higher frequency (blue shift) by p-doping (like bromine35 as electron-acceptor) or oxidizing (like by HNO336 or H2SO437). In our case, the blue shift of SWCNT-Nafion films indicated that SWCNTs became p-doped by Nafion, because the CF2 groups on Nafion backbone were electron-acceptor due to their electronegativity, and the sulfonic acid group on the end of Nafion was capable of protonating SWCNTs in similarity to HNO336 and H2SO4,37 which made the electrons on SWCNTs transfer to Nafion. Although Geng et al.38 reported that p-doped SWCNTs by Nafion would decrease the conductivity of metallic SWCNTs, Chhowalla et al. demonstrated that acyl chlorides11 and acyl bromide39 functional groups, which acted as electron acceptors and made SWCNTs p-doped, increased the conductivity of SWCNT films, because those groups tended to move the Fermi level toward the valence band. In our case, the p-doped nanotubes by Nafion increased the conductivity of SWCNT films as compared to SWCNT-SDS films. The Raman spectrum of SWCNT-SDS films was shown in Figure 6f, and almost no blue-shift of the G-band was found, which meant that no charge transfer occurred and no doping of SWCNTs by SDS existed. Although the bundle sizes of SWCNTs dispersed in SDS aqueous solutions were almost the same as or even smaller than those in Nafion-water/ethanol (50: 50, 75:25, 0:100), the sheet resistances were higher for the former than for the latter. One possible reason was that the p-doped nanotubes by Nafion increased the hole density of SWCNTs and finally increased the conductivity of SWCNT films. Another noticeable point in our Raman spectrum was that a broadened, asymmetric peak with a Breit-Wigner-Fano (BWF) line shape at 1586.9, 1590.3, and 1589.5 cm-1 appeared at the lower energy side of the G-band for (c) SWCNT-Nafion-water/ ethanol (50:50) film, (d) SWCNT-Nafion-water/ethanol (25:75) film, and (e) SWCNT-Nafion-ethanol film, respectively. Because of the blue-shift of the G-band for (c), (d), and (e), the frequency of those BWF lines was higher than the region of 1530-1560

Dispersion of SWNTs by Nafion in Water/Ethanol cm-1,40 which was the common BWF feature. Many recent reports41,42 demonstrated that the appearance and disappearance of the BWF line was attributed to bundling/debundling effects. However, Blackburn et al.43 pointed out that the existence or lack of a BWF feature should not be used alone as a measure of SWCNTs aggregation, and, in fact, the BWF lines were also sensitive to interactions between nanotubes and surrounding molecules.44 Shim et al.45 have suggested that the local chemical environment (e.g., the adsorption of PEI) affected on isolated metallic SWCNTs could turn on or strongly enhance Fano coupling to a plasmon continuum by in-plane disorder. Our observation of the appearance of BWF lines for SWCNTNafion-water/ethanol (50:50, 25:75, 0:100) films should not be ascribed to the bundling of SWCNTs, because the bundles of SWCNTs were much smaller for the above films than for SWCNT-Nafion-water/ethanol (100:0, 75:25) films and SWCNTs powder, according to the results of TEM and SEM images. We suspected that the appearance of BWF lines was attributed to the more efficient interactions of SWCNTs with Nafion. Because Nafion-water/ethanol (50:50, 25:75, 0:100) dispersed SWCNTs better than Nafion-water/ethanol (100:0, 75:25), small bundles of SWCNTs might contain more individual metallic tubes and individual semiconductive tubes. Those tubes interacted with Nafion more efficiently than those tubes with large bundles dispersed in Nafion-water/ethanol (100:0, 75:25). In addition, many reports12,38,46,47 also claimed that the appearance or enchancement of BWF line was the evidence of the enhancement of metallicity for SWCNTs. For SWCNT-SDS films, no BWF line was observed, perhaps because the metallicity of SWCNTs was overwhelmed by SDS. Finally, we summarized the factors affecting the sheet resistances of SWCNT films. The first factor and also the dominant factor was the bundle sizes of SWCNTs. Large bundle size of tube definitely induced the large sheet resistance. That is why SWCNT-Nafion-water/ethanol (100:0, 75:25) films with larger bundle sizes had higher sheet resistance than that of SWCNT-Nafion-water/ethanol (50:50, 25:75, 0:100) films and SWCNT-SDS film. The second factor was the residual substances in the network of SWCNTs. The more conductive residual Nafion as compared to SDS could less increase the cross-junction resistances of SWCNT films. The third factor was whether SWCNTs were doped or not. The last two factors were able to explain why the doped SWCNTs films by Nafionwater/ethanol (50:50, 25:75, 0:100) had higher conductivity than undoped SWCNT films by SDS. At last, thermal stabilities of different films were tested. SWCNT films were kept at 60 °C for several days, and the sheet resistances were measured with intervals of 2 or 3 days as shown in Figure 7. It was found that (a) SWCNT-Nafionwater, (b) SWCNT-Nafion-water/ethanol (75:25), (c) SWCNTNafion-water/ethanol (50:50), and (d) SWCNT-Nafion-water/ ethanol (25:75) films showed better stability with change of sheet resistance less than 8% in 200 h. Eikos Inc. reported the use of fluoropolymers as binders for successfully protecting SWCNT films against mechanical damage and environmental stresses like temperature and humidity.48 In our case, the better thermal stabilities17-19 and viscosity of Nafion (might act like a binder) could explain the good thermal stability of SWCNT-Nafion films. In contrast, SWCNT-SDS films exhibited worse stability with 30% increase of sheet resistance in 213 h, because bare SWCNTs were sensitive to environmental conditions including moisture and heat.48 For SWCNT-Nafion-ethanol films, the stability was bad with 26% increase of sheet resistance in 177 h, because too much ethanol would destroy the adhesion of

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Figure 7. The change of sheet resistance along with the time for (a) SWCNT-Nafion-water film, (b) SWCNT-Nafion-water/ethanol (75:25) film, (c) SWCNT-Nafion-water/ethanol (50:50) film, (d) SWCNTNafion-water/ethanol (25:75) film, (e) SWCNT-Nafion-ethanol film, and (f) SWCNT-SDS film on PET substrate by keeping films at 60 °C.

SWCNTs on substrates because the surface tension of ethanol was smaller than that of water. Besides, too much ethanol distorted the Millipore ester membranes, which would affect the quality of SWCNT films during filtrating. SWCNT-Nafionwater/ethanol (50:50) films showed the best stability with change of sheet resistance less than 2% in 200 h, because the suitable proportion of water and ethanol (50:50) not only increased the dispersion of SWCNTs by Nafion, but also did not destroy the adhesion of SWCNTs and substrates. Conclusions Our findings indicated that the use of Nafion in water/ethanol as a solubilizing agent for SWCNTs provided a useful and effective avenue for preparing transparent conducting SWCNT films both on glass and on PET substrates. SWCNT-Nafionwater/ethanol (50:50, 25:75, 0:100) films displayed better conductivity than SWCNT-Nafion-water/ethanol (100:0, 25:75) films, because the bundle sizes of the former were much smaller than those of the latter, as supported by the TEM and SEM observation. As compared to SWCNT-SDS films, SWCNTNafion-water/ethanol (50:50, 25:75, 0:100) films showed the lower resistance at the same transmittance, because the more conductive Nafion as compared to the insulating SDS could less increase the resistances of SWCNT films, and, moreover, SWCNT films doped by Nafion were able to increase the hole density of SWCNTs and increased the conductivity of SWCNT films, as supported by the Raman spectrum. SWCNT-Nafionwater/ethanol (50:50) films exhibited the best performance including the lowest sheet resistance and the best thermal stability, due to the suitable proportion of water and ethanol. Sheet resistance of 500-600 Ω/sq with transmittance of 85% was obtained for SWCNT-Nafion-water/ethanol (50:50) films, which have met the requirement of touch screen and organic photovoltaics. Those transparent conducting SWCNT films would be applied in more flexible displays by improving their properties in the future. Acknowledgment. This work was financially supported by the National Basic Research Program (2005CB623605), the NSFC (Nos. 50372077 and 50572114), and the Hundred Talents Program of CAS. References and Notes (1) Wu, Z.; Chen, Z. H.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273.

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