Comparison of the Stability of Surface-Modified SWNTs and DWNTs

Feb 24, 2010 - Conductivity measurements disclosed that gold ion and HNO3 treatment of the nanotube network films increased the electrical conductivit...
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Comparison of the Stability of Surface-Modified SWNTs and DWNTs Network Films Seung Bo Yang,† Byung-Seon Kong,‡ Dae-Woo Kim,† and Hee-Tae Jung*,† Department of Chemical and Biomolecular Engineering, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, and KCC Central Research Institute, 83 Mabook-dong, Giheung-gu, Yongin-si, Gyunggi-do 446-912, Korea ReceiVed: December 14, 2009; ReVised Manuscript ReceiVed: January 12, 2010

We studied the stability of single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) films as a function of surface modification. Conductivity measurements disclosed that gold ion and HNO3 treatment of the nanotube network films increased the electrical conductivity by more than a factor of 2 with negligible loss of transmittance. However, the long-term stability of the films varied depending on nanotube type and post-treatment method, with gold ion-treated nanotubes exhibiting higher stability than HNO3-treated nanotubes. Moreover, DWNTs showed better stability than SWNTs when treated with gold ions. Work function and optical absorption spectral measurements suggested that the dedoping process and the contact resistance of the nanotube networks may be important for stability under ambient conditions. These results are important for the development of surface-modified SWNTs and DWNTs for potential applications in solar cells, light-emitting diodes, sensors, and field-effect transistors. Introduction Transparent, conductive thin films of carbon nanotubes (CNTs) have received considerable interest due to their exceptional electrical, optical, and mechanical properties,1,2 making them candidates for use in sensors,3 solar cells,4-6 organic light emitting diodes (OLEDs),7,8 and field effect transistors (FETs).3,9,10 To extend the range of potential applications, the conductivity of nanotube films must be improved while maintaining a high optical transmittance. Several approaches have been developed to enhance the electrical conductivity of films without decreasing transmittance: immersion of nanotube films in acids, such as HNO3,11 H2SO4,12 and H2SO3;13 immersion in HNO3 followed by dipping in SOCl2;14 or doping with iodine,13 SOCl2,13,15 or Au.16,18 Although single-walled carbon nanotubes (SWNTs) have been primarily used for conducting films, surface modification of double-walled carbon nanotubes (DWNTs) has been shown to produce highly conductive π-conjugated pathways in the network films.17,18 Indeed, the band structure of DWNTs rarely depends on interlayer interactions, and the potential barriers are strongly affected by the chirality pairs.19 Furthermore, the inner and outer tubes of DWNTs may be independently doped, which may convey novel properties to the nanotube networks.19 Recently, the electrical property of DWNTs were found to be improved by treatment with H2SO4,20 Br2,21 potassium,22 nitrogen,23 HNO3,18 Au,18 or SOCl2.24 In the present study, we investigated the stability, in ambient air, of transparent SWNT and DWNT films treated with gold ions (Au3+) or HNO3. Stability is an essential requirement for transparent conducting film applications. The acid and gold ion treatments enhanced the conductivity of CNT network films without significantly decreasing the transmittance, as described previously.11,12,16 However, we found that the film stability with respect to air exposure depended strongly on the type of nanotube and method used to introduction surface modifications. * To whom correspondence should be addressed. Tel.: +82-42-350-3931. Fax: +82-42-350-3910. E-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ KCC Central Research Institute.

The gold ion-treated nanotube network films exhibited better stability than did the acid-treated CNT networks. Notably, the DWNT-Au nanoparticle hybrid films showed the highest stability among the surface-treated nanotubes examined, making them a suitable candidate for applications in transparent conducting films. Experimental Section Materials. Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs), synthesized by the high pressure CO disproportionation (HiPco) process, were obtained from Unidym (purified grade). Additional purification steps, using dry oxidation and acid treatment, were not carried out to avoid creation of defects on the CNT sidewalls. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2O, 99.9% g pure) and Nafion perfluorinated ion-exchange resin, 5 wt % in a mixture of lower aliphatic alcohols and H2O (45% water), were purchased from Aldrich. NaOH (97%) and ethanol (99.9% pure) were purchased from Merck. HNO3 (60% pure) was purchased from Junsei Chemicals. A porous alumina membrane filter (200 nm pore size, 47 nm diameter) was purchased from Whatman International Ltd., England. The untreated glass slide substrates (75 × 25 × 1.1 mm) were obtained from Paul Marienfeld GmbH and Co. KG. Deionized water (18 MΩ cm) was purified using an ultrapure water system (Milli-Q, Millipore.). Fabrication of CNT Films and their Post-Treatment. A CNT dispersion solution was prepared as reported previously:18 (1) DWNTs and SWNTs (20 mg) were dispersed in a solution containing 160 mL of H2O, 40 mL of 2-propanol, and 4 g Nafion. The solution was ultrasonicated using a probe-type sonicator (Ultrasonic processors VCX 750, 750W) at 90% amplitude for 10 min. (2) The suspension was centrifuged at 15000 rpm for 1 h to remove CNT bundles. CNT network films were prepared on glass substrates using the supernatant CNT suspensions (the upper 50% volume) by vacuum filtration methods, as reported previously.16,26,27 This procedure allowed the nanotube network density to be easily controlled by varying the volume of suspension filtered. Ti-Au contact electrodes

10.1021/jp9118265  2010 American Chemical Society Published on Web 02/24/2010

Stability of DWNT-Gold Nanohybrid Films

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Figure 1. SEM images of CNT network films: (a) pristine-SWNT films, (b) SWNT-HNO3 films, (c) SWNT-Au films, (d) pristine-DWNT films, (e) DWNT-HNO3 films, and (f) DWNT-Au films with 82% transmittance at 550 nm. The scale bar represents 1000 nm.

Figure 2. (a) Variation of electrical resistance of (a) SWNT-Au, (b) SWNT-HNO3, (c) DWNT-Au, and (d) DWNT-HNO3 vs time in air.

(10 and 90 nm thick, respectively) were formed on the CNT films (15 mm spacing) by using electron-beam evaporation techniques. The CNT films were immersed in a 1 mM gold salt solution for 10 min, followed by concentrated HNO3 for 1 h. The CNT films after post-treatment were rinsed thoroughly with deionized water and dried under nitrogen gas. All procedures were carried out under ambient conditions with 28 ( 2% humidity and a temperature of 25 ( 1 °C. Characterization. SEM images of SWNT and DWNT films were obtained using a field emission scanning electron microscope (FESEM, Philips, XL30SFEG). The optical transmittance of each CNT film was measured at 550 nm on a UV-vis-NIR spectrophotometer (V-570, JASCO). The electrical conductance was measured in the linear sweep voltammetry mode (threeelectrode mode: working electrode, reference, and counterelectrode system) using a CHI 600C potentiostat (CH Instruments Inc.) at ambient temperature. The electrical resistance of each CNT film was obtained from the reciprocal of the slope of the current-voltage curve. The potential was swept at a scan rate of 0.1 V s-1 from -3 to +3 V. The Kelvin probe system

(KP-6500, McAllister Technical Services) was used to measure the work function of CNT films. Results and Discussion Figure 1 shows scanning electron microscope (SEM) images of the nanotube network films at 82% transmittance, fabricated by the vacuum filtration method, both before (Figure 1a) and after (Figure 1b) HNO3 treatment and electroless reduction of gold ions (Figure 1c) onto the nanotube films. The SWNT films thoroughly and uniformly covered the substrate surface (Figure 1a). Similar images were observed in the HNO3-treated nanotube network films (Figure 1b), perhaps because the HNO3 did not significantly affect the electron contrast. Several gold nanoparticles were observed on the SWNT networks after electroless reduction of gold ions, due to their high electron densities (Figure 1c). SEM images of the DWNTs were similar to images of the surfaces of treated SWNT network films (Figure 1d-f). Figure 2 shows the variation of resistance (%) of four nanotube network films, with 82% transmittance, as a function of exposure time in air. The electrical resistance of each CNT

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Figure 3. UV-vis-NIR optical absorption spectra of (a) pristine-SWNT and SWNT-Au, (c) pristine-SWNT and SWNT-HNO3, (e) pristineDWNT and DWNT-Au, (f) pristine-DWNT and DWNT-HNO3 with 82% transmittance at 550 nm and UV-vis-NIR optical absorption spectra of (b) SWNT-Au, (d) SWNT-HNO3, (f) DWNT-Au, and (h) DWNT-HNO3 as a function of air exposure.

network film was obtained in the linear sweep voltammetry mode using a CHI 600C potentiostat. All surface-treated nanotube network films were exposed to ambient air with 28 ( 2% humidity and 25 ( 1 °C temperature, during >200 h of measurement. It is noteworthy that the resistance of the gold ion-treated SWNT and DWNT network films did not vary significantly after 216 h of exposure to air. As expected, the electrical resistance of pristine DWNTs and SWNTs decreased significantly after gold ion and HNO3 treatment, as a result of electron depletion caused by the doping process.16,18 The sheet resistance of total six types of films with 82% transmittance are presented as follows: pristine-SWNT (1960 Ω), pristineDWNT (1131 Ω), SWNT-Au (808 Ω), SWNT-HNO3 (823 Ω), DWNT-Au (553 Ω), and DWNT-HNO3 (640 Ω). Interestingly, the stability of the nanotube network films was strongly influenced by the type of nanotube and the post-treatment method: The resistance of films composed of SWNT-Au (black squares), SWNT-HNO3 (red circles), DWNT-Au (blue triangles), and DWNT-HNO3 (purple down-facing triangle) after post-treatment increased with increasing air exposure time. The

stability of the nanotube films treated with HNO3 was fairly insensitive to the type of nanotube used in the film. In contrast, the stability of nanotube films treated with gold ions was strongly dependent on the type of nanotube film. SWNT-Au films showed a 20% increase in electrical resistance after 48 h of air exposure, 21% after 100 h of air exposure, and 35% after 196 h of air exposure. In contrast, the DWNT-Au films showed a 6% increase in the conductivity after 48 h of air exposure, 12% after 100 h of air exposure, and 20% after 196 h of air exposure. This difference in stability may be largely due to π-conjugated pathways that are more numerous, and therefore more conductive, in DWNTs than in SWNTs.17 Furthermore, we found that the nanotube films treated with gold ions were more stable than the nanotube films treated with HNO3: The nanotube films treated with HNO3 showed significant increases in the electrical resistance by 50% after 48 h, 60% after 100 h, and 80% after 196 h of air exposure. The percent increase in electrical resistance of the post-treated nanotube network films showed the following order: DWNTAu (20%), SWNT-Au (35%), SWNT-HNO3 (85%), DWNT-

Stability of DWNT-Gold Nanohybrid Films

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Figure 4. Work function difference of (a) SWNT-Au, (b) SWNT-HNO3, (c) DWNT-Au, and (d) DWNT-HNO3 as a function of air exposure.

HNO3 (95%). The percent increase was defined as r ) ((R)/ (R0) - 1) × 100(*), where r, R0, and R denote the increase ratio, the resistance of post-treated nanotube network films, and the resistance of nanotube network films after air exposure, respectively. DWNT-Au exhibited the highest conductance among the CNT network films examined in this study, although the long-term stability of the pristine-DWNTs was slightly better than the DWNT-Au films. To examine the unexpected stability trends observed for the post-treated SWNT and DWNT nanotube networks, the electronic transitions, both before and after application of surface treatments to the nanotube films with 82% transmittance, were measured by UV-vis-NIR absorbance spectra (Figure 3): pristine SWNT and SWNT-Au (Figure 3a); pristine SWNT and SWNT- HNO3 (Figure 3c); pristine DWNT and DWNT-Au (Figure 3e); and pristine DWNT and DWNT- HNO3 (Figure 3g). Characteristic absorption peaks corresponding to M11, S22, and S11 transitions were observed in the pristine SWNTs, which result from electronic transitions related to van Hove singularities. Treatment of the pristine SWNT network films with gold ions or HNO3 produced considerable bleaching of the first and second interband transitions of the semiconducting tubes,28 demonstrating significant changes in the electronic structure of the pristine nanotubes. In addition, p-type doping with acid or gold ions reduced the S22 and S11 peak intensities by effectively pinning the Fermi level inside the valence band,28,29 indicating electronic structural properties of semiconducting tubes close to metallic tubes. The pristine DWNTs did not exhibit characteristic peaks, in contrast with the pristine SWNT films. The absence of peaks may be a property of the unsorted as-prepared DWNTs.17,24 Similar peaks were observed for DWNT-Au and DWNT-HNO3 films (Figure 3e,g). Figure 3b and d show the restoration of the S11 peak as a function of air exposure time for the SWNT-Au and SWNT-HNO3 films, respectively. Within 48 h of doping, the S11 peak intensities of post-treated SWNT films reached levels similar to that of the pristine SWNTs (insets of Figure 3b and d), confirming the dedoping of SWNT films. However, DWNT-Au and DWNT-HNO3 films did not show significant differences in peak intensity, suggesting similar dedoping characteristics (Figure 3f and h).

To further investigate electronic structural changes before and after the post-treated nanotubes, we measured the work functions using a Kelvin probe system (KP-6500, McAllister Technical Services). The work functions were obtained relative to the work function of pristine CNT films. Initial Kelvin probe measurements were conducted at ambient conditions, within a 24 h period after doping. Figure 4 shows the work function of the post-treated SWNT and DWNT network films as a function of air exposure time under ambient conditions. The work functions of the nanotube films with 82% transmittance increased considerably after HNO3 and gold ion treatments, indicating significant changes in the surface potential of the pristine CNTs.30,31 The observed change in work function was attributed to electron depletion, that is, an increase in the hole concentration, from specific bands in the semiconducting nanotubes due to doping with electron acceptors such as Au3+ and oxygen.32 However, the sensitivity of the work function to air exposure was not directly related to the electrical resistance of the four types of CNT network films (Figure 2). The work function was reduced to approximately 100 meV after 216 h. The following two factors may account for the decrease in the work function: (1) a change in the surface condition of the CNT films due to, for example, adsorption of moisture, oxygen, or other impurities from the air;16 and (2) chemically induced defects on the CNTs, which increase the density of states at the Fermi level thereby producing a band bending effect at the surface that leads to a reduction in the work function.25 The initial work functions of post-treated CNT network films were obtained, for pristine CNT network films: SWNT-Au: 270 meV (Figure 4a); SWNT-HNO3: 250 meV (Figure 4b); DWNT-Au: 350 meV (Figure 4c); and DWNT-HNO3: 390 meV (Figure 4d). Within 24 h of posttreatment, the work functions decreased slightly. The work functions shifted by 90 meV (SWNT-Au), 40 meV (SWNTHNO3), 90 meV (DWNT-Au), and 90 meV (DWNT-HNO3) after 72 h of air exposure. After a 216 h exposure to air, the work functions had returned to levels only slightly above those of the pristine CNT network films prior to post-treatment. It is likely that the higher stability conferred by gold ion treatment arose from the reduced contact resistance between metallic and semiconducting nanotubes. Thus, the higher relative stability,

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with respect to HNO3-treated CNT films, might be due to the contact resistance induced by Au nanoparticles within the nanotube network, although the nanotubes dedoped to some extend upon exposure to air. The relative stability of DWNTAu films was likely due to the higher structural stability, the independence of the band structure in DWNTs with respect to interlayer interactions, and the independent doping of the inner and outer tubes of DWNTs.19 However, it is possible that the relatively poor stability of HNO3-treated CNT films may result from the deterioration of adhesion properties produced by acid treatment, although a precise explanation for this mechanism has not yet been investigated. Fluoropolymers, such as Nafion, may protect the CNT films against the environmental stresses of temperature and humidity.33 The protecting Nafion layers may be partially removed by HNO3 treatment.34 Conclusions We characterized the long-term stability SWNTs and DWNTs network films in air as a function of surface treatments. We found that the nanotube network films, after gold ion and HNO3 doping, exhibited varying stabilities upon exposure to air. Gold ion-treated nanotube network films were more stable than HNO3-treated nanotube network films. Notably, DWNT-Au showed the highest stability among the surface-treated network films, resulting from higher structural stability, an independence of DWNT band structure with respect to the interlayer interactions, more stable Au doping. These results suggest a route for optimizing the stability of CNT films for use as transparent electrodes in a variety of optoelectronic devices, such as solar cells, light-emitting diodes, sensors, and field-effect transistors. Acknowledgment. This work was supported by the National Research Laboratory Program (R0A-2007-000-20037-0, KOSEF) and the Center for Nanoscale Mechatronics and Manufacturing (08K140100414, CNMM). Supporting Information Available: SEM images of CNT films (82% transmittance) as a function of air exposure time (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; Heer, W. A. Science 2002, 297, 787. (2) Gruner, G. J. Mater. Chem. 2006, 16, 3533. (3) Cao, Q.; Rogers, J. A. AdV. Mater. 2009, 21, 29. (4) Pasquier, A. D.; Unalan, H. E.; Kanwal, A.; Miller, S.; Chhowalla, M. Appl. Phys. Lett. 2005, 87, 203511. (5) van de Lagemaat, J.; Barnes, T. E.; Rumbles, G.; Shaheen, S. E.; Coutts, T. J. Appl. Phys. Lett. 2006, 88, 233503.

Yang et al. (6) Rowell, M. W.; Topinka, M. A.; McGehee, M. D.; Prall, H.-J.; Dennler, G.; Sariciftci, N. S.; Hu, L.; Gruner, G. Appl. Phys. Lett. 2006, 88, 233506. (7) Wang, Y.; Di, C.-A.; Liu, Y.; Kajiura, H.; Ye, S.; Cao, L.; Wei, D.; Zhang, H.; Li, Y.; Noda, K. AdV. Mater. 2008, 20, 4442. (8) Li, J.; Hu, L.; Wang, L.; Zhou, Y.; Gruner, G.; Marks, T. J. Nano. Lett. 2006, 6, 2472. (9) Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Nature 2003, 424, 654. (10) Cao, Q.; Zhu, Z. T.; Lemaitre, M. G.; Xia, M. G.; Shim, M.; Rogers, J. A. AdV. Mater. 2006, 18, 304. (11) Geng, H.-Z.; Kim, K. K.; So, K. P.; Lee, Y. S.; Lee, Y. H. J. Am. Chem. Soc. 2007, 129, 7758. (12) Tantang, H.; Ong, J. Y.; Loh, C. L.; Dong, X.; Chen, P.; Chen, Y.; Hu, X.; Tan, L. P.; Li, L.-J. Carbon 2009, 47, 1867. (13) Ska´kalova´, V.; Kaiser, A. B.; Dettlaff-Weglikowska, U.; Hrnarikova´, K.; Roth, S. J. Phys. Chem. B 2005, 109, 7174. (14) Parekh, B. B.; Fanchini, G.; Eda, G.; Chhowalla, M. Appl. Phys. Lett. 2007, 90, 121913. (15) Dettlaff-Weglikowska, U.; Skakalova, V.; Graupner, R.; Jhang, S. H.; Kim, B. H.; Lee, H. J.; Ley, L.; Park, Y. W.; Berber, S.; Tomanek, D.; Roth, S. J. Am. Chem. Soc. 2005, 127, 5125. (16) Kong, B.-S.; Jung, D.-H.; Oh, S.-K.; Han, C.-S.; Jung, H.-T. J. Phys. Chem. C 2007, 111, 8377. (17) Li, Z.; Kandel, H. R.; Dervishi, E.; Saini, V.; Biris, A. S.; Bris, A. R.; Lupu, D. Appl. Phys. Lett. 2007, 91, 053115. (18) Yang, S. B.; Kong, B.-S.; Geng, J.; Jung, H.-T. J. Phys. Chem. C 2009, 113, 13658. (19) Carbon Nanotubes, Topics Appl. Physics 111; Jorio, A., Dresselhaus, G., Dresselhaus, M. S., Eds.; Springer-Verlag: Berlin, 2008; p 495. (20) Barros, E. B.; Son, H.; Samsonidze, Ge. G.; Souza Filho, A. G.; Saito, R.; Kim, Y. A.; Muramatsu, H.; Hayashi, T.; Endo, M.; Kong, J.; Dresselhaus, M. S. Phys. ReV. B. 2007, 76, 045425. (21) Souza Filho, A. G.; Endo, M.; Muramatsu, H.; Hayashi, T.; Kim, Y. A.; Barros, E. B.; Akuzawa, N.; Samsonidze, G. G.; Saito, R.; Dresselhaus, M. S. Phys. ReV. B. 2006, 73, 235413. (22) Chun, K.-Y.; Lee, C. J. J. Phys. Chem. C 2008, 112, 4492. (23) Chun, K.-Y.; Lee, H. S.; Lee, C. J. Carbon 2009, 47, 169. (24) Green, A. A.; Hersam, M. C. Nat. Nanotechnol. 2009, 4, 64. (25) Jackson, R.; Domercq, B.; Jain, R.; Kippelen, B.; Graham, S. AdV. Funct. Mater. 2008, 18, 1. (26) Wu, Z.; 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. Science 2004, 305, 1273. (27) Hu, L.; Hecht, D. S.; Gruner, G. Nano Lett. 2004, 4, 2513. (28) Hennrich, F.; Wellmann, R.; Malik, S.; Lebedkin, S.; Kappes, M. M. Phys. Chem. Chem. Phys. 2003, 5, 178. (29) Kim, K. K.; Bae, J. J.; Park, H. K.; Kim, S. M.; Geng, H.-Z.; Park, K. A.; Shin, H.-J.; Yoon, S.-M.; Benayad, A.; Choi, J.-Y.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 12757. (30) Introduction to Surface Physics; Prutton, M., Ed.; Oxford University Press: New York, 1994; Chapter 4. (31) Characterization of Organic Thin Films; Ulman, A., Ed.; Butterworth-Heinemann: Boston, 1995; Chapter 6. (32) Kaempgen, M.; Lebert, M.; Haluska, M.; Nicoloso, N.; Roth, S. AdV. Mater. 2008, 20, 616. (33) Zhang, J.; Gao, L.; Sun, J.; Liu, Y.; Wang, W.; Wang, J.; Kajiura, H.; Li, Y.; Noda, K. J. Phys. Chem. C 2008, 112, 16370. (34) Geng, H.-Z.; Kim, K. K.; Song, C.; Xuyen, N. T.; Kim, S. M.; Park, K. A.; Lee, D. S.; An, K. H.; Lee, Y. S.; Chang, Y.; Lee, Y. J.; Choi, J. Y.; Benayad, A.; Lee, Y. H. J. Mater. Chem. 2008, 18, 1261.

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