A Simple Approach for Preparing Transparent Conductive Graphene

Aug 9, 2010 - A Simple Approach for Preparing Transparent Conductive Graphene Films Using the Controlled Chemical Reduction of Exfoliated Graphene Oxi...
0 downloads 13 Views 429KB Size
Download PDF
J. Phys. Chem. C 2010, 114, 14433–14440

14433

A Simple Approach for Preparing Transparent Conductive Graphene Films Using the Controlled Chemical Reduction of Exfoliated Graphene Oxide in an Aqueous Suspension Jianxin Geng, Leijing Liu, Seung Bo Yang, Sang-Cheon Youn, Dae Woo Kim, Ji-Sun Lee, Jong-Kil Choi, and Hee-Tae Jung* National Research Laboratory for Organic Opto-Electronic Materials, Department of Chemical and Biomolecular Engineering (BK-21), Korea AdVanced Institute of Science and Technology, 373-1 Guseong-dong, Yuseong-gu, Daejeon 305-701, Korea ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: July 23, 2010

We report a simple method for preparing transparent conductive graphene films using a chemically converted graphene (CCG) suspension that was obtained via controlled chemical reduction of exfoliated graphene oxide (GO) in the absence of dispersants. Upon thermal annealing of the CCG films, the films displayed a sheet resistance on the order of 103 Ω · 0-1 at 80% transparency (550 nm), with a bulk conductivity on the order of 102 S · cm-1. FT-IR, UV-visible, and X-ray photoelectron spectroscopy results showed that the combination of the controlled reduction of GO in suspension and thermal annealing of the CCG films efficiently restored the sp2 carbon networks of the graphene sheets, facilitating charge carrier transport in the individual CCG sheets. Furthermore, grazing-incidence X-ray diffraction results showed that the thermal annealing of the CCG films reduced the interlayer distance between the CCG sheets to a distance comparable to that in bulk graphite, facilitating charge carrier transport across the CCG sheets. Polymer solar cell devices composed of the CCG films as transparent electrodes showed power conversion efficiencies, η, of 1.01 ( 0.05%, which corresponded to half the value (2.04 ( 0.1%) of the reference devices, in which indium tin oxide-covered glass was used for the transparent electrode. Introduction The preparation of transparent conductive graphene films over large areas is a key issue in graphene research due to the potential applications of such films in transparent electrodes,1 field effect transistors,2 field emission displays,3 and bioelectronic devices.4 To date, chemical methods that involve the chemical oxidation of graphite powders and the reduction of exfoliated graphene oxide (GO) sheets have been primarily used to prepare graphene films because the products of such methods display several advantages, such as the single-layer character of the chemically converted graphene (CCG) sheets, high throughput preparation, low cost, and the simplicity of the fabrication technique over other methods.5 Ultrahigh vacuum annealing of single crystal SiC(0001)6 and chemical vapor deposition (CVD)7 methods have also been used. These methods generate graphene films with perfect sp2 carbon networks and high conductivity. However, they suffer from the high temperature requirements for graphene film growth, a lack of efficient control over the film structures, high-cost processing conditions, and the difficulty in transferring the prepared films onto desired substrates. Currently, the chemical methods for the preparation of graphene films rely on two reduction routes: (i) hydrazine vapor reduction of GO films, followed by thermal annealing of the reduced GO films,5a,b,8 or (ii) chemical reduction of GO in suspension in the presence of dispersing agents, followed by thermal annealing of the CCG films prepared from the CCG suspension.1b,9 In the case of hydrazine vapor reduction of GO films, the sheet resistance of the CCG films reaches saturation as the film thickness increases because vapor reduction is only * Author for correspondence. E-mail: [email protected]. Telephone: +82-42-350-3931. Fax: +82-42-350-8890.

effective at the uppermost surface layer due to the poor vapor permeability of the graphene materials.5b On the other hand, the chemical reduction of GO in suspension by hydrazine requires more complicated procedures that involve covalent10 or noncovalent9,11 modification of the GO or CCG sheets to keep the resultant CCG sheets efficiently dispersed in suspension. The dispersants lead to contamination of the resulting CCG films, causing irreversible negative effects on the opto-electronic properties of the CCG films. Here, we report a simple and efficient chemical method that mildly reduces GO in an aqueous suspension without the use of dispersants for the preparation of transparent conductive CCG films. After vacuum filtration of the reduction-controlled CCG suspension and subsequent thermal annealing of the CCG films, we obtained high transparency (80% transparency at 500 nm) and high conductivity graphene films with a low sheet resistance on the order of 103 Ω · 0-1. We show that the sp2 carbon networks of the mildly reduced CCG sheets were further restored during thermal annealing. In addition, the interlayer distance between the CCG sheets was found to approach the theoretical value after thermal annealing of the CCG films. These structural changes were the major factors that determined the conductivity of the resultant CCG films. Polymer solar cells prepared with the CCG films as transparent electrodes exhibited power conversion efficiencies, η, of 1.01 ( 0.05%, which corresponded to half the efficiency of solar cell devices in which the electrode was made using indium tin oxide (ITO). Experimental Methods Materials. GO was synthesized from graphite powder (Graphit Korpfmu¨hl AG) via a modified Hummers’ method.9,12 Paper-like graphite oxide, which could be easily dispersed in

10.1021/jp105029m  2010 American Chemical Society Published on Web 08/09/2010

14434

J. Phys. Chem. C, Vol. 114, No. 34, 2010

water or select organic solvents by sonication, was obtained for the following experiments. CCG suspensions were obtained by reducing the GO in an aqueous suspension using hydrazine hydrate (50-60% aqueous solution) at a concentration ratio of GO:hydrazine )1 mg:1 mmol. The reduction was performed at room temperature (25 °C) for 36 h to easily control the reactions. The resultant suspension was stable for one week at room temperature. This stable period allowed sufficient time for sample preparation and characterization steps. Preparation of the CCG films. CCG films were prepared by the vacuum filtration method.13 Two types of anodic aluminum oxide (AAO) membrane, one 25 mm in diameter with 0.1 µm pores and the other 47 mm in diameter with 0.1 µm pores, were used. Note that the effective diameter of the CCG films prepared using the 25 mm diameter membranes was 18.8 mm, while the effective diameter was 38.4 mm for the 47 mm diameter membranes. Thus, CCG films of a given thickness could be prepared by adjusting the volume of the filtered CCG suspension if different types of AAO membrane were used. The 47 mm diameter membranes were used to make films for solar cell preparation, whereas the 25 mm diameter membranes were used for other purposes. The volumes mentioned in this paper denote the volumes used with the 25 mm diameter membranes. The concentration of the CCG suspension used for film preparation was 10 mg · L-1. In brief, a specific volume of the CCG suspension was diluted with DI water (40 mL) and sonicated for 5 min. The diluted suspension was subjected to vacuum filtration, leading to the deposition of a CCG film on the surface of the AAO membrane. After the CCG film was dried in a 60 °C oven, the AAO membrane was floated on the surface of a 3 M NaOH solution to dissolve the AAO membrane and deposit the CCG film on the solution surface. The NaOH solution was exchanged with copious amounts of DI water, and the CCG film was transferred to the surface of a glass or quartz slide and dried in a 60 °C oven. Thermal Annealing of the CCG Films. The CCG films were thermally annealed at temperatures of 200, 400, and 800 °C, which were selected on the basis of the thermogravimetric (TG) and differential thermogravimetric (DTG) curves (see Figure S1 in Supporting Information). Thermal annealing was performed in a tube furnace equipped with a vacuum system. Because the CCG films were sensitive to thermal treatment, careful control of the vacuum was important. In our experiments, the thermal treatment was not applied until the vacuum reached CCG > CCG-A200 > CCG-A400 > CCG-A800 (see Figure S3 in Supporting Information). Figure 3b shows the UV-visible spectra of GO, CCG films, and thermally annealed CCG films. The films used for the UV-visible measurements were prepared using a constant volume of the suspensions (560 µL of a 10 mg · L-1 GO or CCG suspension). The absorption across the entire UV-visible range increased significantly, and the characteristic peak shifted from 236 to 276 nm upon chemical reduction of GO and thermal annealing of the CCG films. These results supported the effective restoration of the sp2 carbon networks in the resultant CCG sheets.20 Figure 4 shows the C 1s XPS of GO, CCG, thermally annealed CCG, and graphite. GO films generated spectra with two dominant peaks at 284.5 and 286.4 eV corresponding to C-C and C-O species and two weak peaks at 287.5 and 288.2 eV corresponding to CdO and O-CdO species (Figure 4a).21 The peaks at 286.4, 287.5, and 288.2 eV indicated the presence of hydroxyl, epoxy, carbonyl, and carboxylic acid groups on the surfaces and edges of the GO sheets. Upon chemical reduction, the peaks at 288.2, 287.5, and, in particular, 286.4 eV decreased (Figure 4b), indicating that the mild reduction of GO was efficient at removing the oxygen-containing functional groups, especially hydroxyl and epoxy groups. Meanwhile, a new peak ascribed to a C-N species, resulting from bond formation during hydrazine reduction, appeared at 285.7 eV in the CCG spectra. The peak at 284.5 eV in the thermally annealed CCG films (CCG-A200, CCG-A400, and CCG-A800, respectively in Figure 4c, d, and e) became more dominant and the peak corresponding to the C-O species became weaker and shifted slightly to lower binding energies. It should be noted that the graphite sample did not generate a symmetric peak corresponding to C-C species (Figure 4f). The spectrum of CCG-A800 was very similar to that of the graphite, indicating that the CCG-A800 film was similar to graphite in chemical composition. The FT-IR, UV-visible, and XPS results together led to the conclusion that an efficiently dispersed CCG aqueous suspension was obtained by the chemically controlled reduction of GO in the absence of dispersants due to a small number of residual oxygen-containing groups (Figures 3a and 4b) on the CCG surfaces; the overall effective reduction of the CCG sheets was fulfilled during the subsequent thermal annealing of the CCG films (Figures 3 and 4e). The structural changes induced by the chemical reduction of GO and thermal annealing of CCG films were further investigated by GIXRD experiments (Figure 5). All samples exhibited GIXRD patterns with diffraction spots only in the out-of-plane direction because the films were composed of GO or CCG sheets that flatly overlapped on the substrate. The distances between the first diffraction spot and the incident beam varied, revealing a change in the interlayer distance between the graphene sheets upon chemical reduction and thermal annealing. Figure 5a shows the GIXRD pattern from a GO film. The interlayer distance was calculated to be 0.675 nm (Figure 5f), which was greater than that of the graphite film (0.34 nm) due to the presence of

J. Phys. Chem. C, Vol. 114, No. 34, 2010 14437

Figure 4. C 1s XPS of (a) GO, (b) CCG, (c) CCG-A200, (d) CCGA400, (e) CCG-A800, and (f) graphite.

carboxylic acid, carbonyl, hydroxyl, and epoxy groups on the GO sheet surfaces.22 Figure 5b shows the GIXRD pattern of a CCG film containing the first- and second-order diffraction spots of the CCG sheets, from which the interlayer distance was calculated to be 0.838 nm (Figure 5f). This value was greater than that of the GO film due to the formation of aminoaziridine moieties, which are larger than the epoxide groups on the GO surfaces, produced by the ring-opening reaction of the epoxides.14,22a The presence of nitrogen-containing groups on the CCG surfaces was verified by FT-IR and XPS (Figures 3a and 4b). Upon thermal annealing at 200 °C, the interlayer distance in the CCG-A200 films was abruptly reduced to 0.375 nm (Figure 5c and f), due to the thermal removal of the functional groups (see Figure S1 in Supporting Information).14 Elevation of the annealing temperature reduced the interlayer distance in the CCG-A800 films to 0.354 nm, which approached the value in bulk graphite, 0.34 nm (Figure 5e and f). The structural changes in the CCG films inevitably influenced the electrical properties of the films. The conductivity of the CCG films composed of individual CCG sheets was determined

14438

J. Phys. Chem. C, Vol. 114, No. 34, 2010

Geng et al.

Figure 5. GIXRD patterns of (a) a GO film, (b) a CCG film, (c) a CCG-A200 film, (d) a CCG-A400 film, and (e) a CCG-A800 film. (f) Onedimensional XRD patterns in the out-of-plane direction of the GO, CCG, CCG-A200, CCG-400, and CCG-A800 films.

by two factors: the charge carrier transport in the individual CCG sheets and the ease of charge carrier transport across the CCG sheets. In this study, both factors showed improved efficiency. The restoration of the sp2 carbon network was key for increasing the charge carrier transport in individual CCG sheets. The two-step reduction (chemical reduction of GO in suspension and thermal annealing of the CCG films) ensured the effective restoration of the sp2 carbon networks. Furthermore, thermal annealing of the CCG films reduced the interlayer distance in the annealed CCG films to a level that was very close to the value in bulk graphite. As a result, charge carrier transport across the CCG sheets improved. Therefore, a clear conclusion could be drawn that the high conductivity of the CCG-A800 films resulted from the improved sp2 carbon networks in the CCG sheets and the reduced interlayer distance between the CCG sheets in the films. The superior performance of the CCG films prepared in this study was demonstrated by fabricating CCG line patterns (Figure 6) and polymer solar cell devices containing CCG transparent electrodes formed from the CCG films (Figure 7). Figure 6a illustrates the scheme used to manufacture CCG line patterns by conventional photolithography and reactive ion etching. This process included (i) the preparation of photoresist (PR) patterns on the surfaces of the CCG film (steps 1-4), (ii) preparation of CCG patterns via reactive ion etching (step 5), and (iii) removal of the PR patterns (step 6). Parts b and c of Figure 6 show the prepared CCG line patterns with line widths of 10 and 3 µm, respectively. The line patterns were obtained by the protection of the PR patterns, and the unprotected areas were removed by O2 plasma etching. Both the 10 and 3 µm CCG patterns showed high contrast and clear line edges, indicating that the lines could be used as conductive microwires. The CCG films prepared in this study offer promise for applications such as transparent electrodes and integrated circuits. Figure 7 shows the I-V curves of a polymer solar cell in which the CCG-A800 films (78% transparency at 550 nm and

Figure 6. (a) Illustration of the preparation of CCG patterns by photolithography and reactive ion etching, (b and c) AFM images of CCG line patterns with line widths of 10 and 3 µm, respectively.

a sheet resistance of 6 kΩ · 0-1) were used as the transparent electrode. The device configuration is displayed in the inset. The performances of this device and the reference device containing an ITO transparent electrode are listed in Table 1. The CCG-A800 electrode device showed an η of 1.01 ( 0.05% (a 5% device variation), comparable to the highest values reported previously for devices containing conductive carbon films as the transparent electrodes.1b,c,5b,8b,23 Several features of these graphene films employed as transparent electrodes in solar cell devices are notable. First, the dark current could only be detected if a forward bias voltage was applied, indicating that the heterojunction in the active layer of this device was stable and well-established. Second, the CCG-A800 electrode devices

Preparing Transparent Conductive Graphene Films

J. Phys. Chem. C, Vol. 114, No. 34, 2010 14439 Acknowledgment. This work was supported by the National Research Laboratory Program of the Korea Science and Engineering Foundation (KOSEF), the World Class University Program (R32-2008-000-10142-0), and the Center for Nanoscale Mechatronics & Manufacturing (08K140100414, CNMM). Supporting Information Available: (1) TG and DTG curves of GO, (2) sheet resistance and transparency of CCG films as a function of annealing time for annealing at 200, 400, and 800 °C, and (3) XPS survey spectra of GO, CCG, CCG-A200, CCGA400, and CCG-A800. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. I-V curves for a P3HT:PCBM solar cell device made using the CCG-A800 film as a transparent electrode. The inset shows the configuration of the solar cell device.

TABLE 1: Comparison of the Performance of the Polymer Solar Cells Made Using CCG-A800 and ITO As the Transparent Electrodes ITO CCG-A800

ISCa (mA · cm-2)

VOCb (V)

FFc

ηd (%)

5.39 4.18

0.69 0.67

0.55 0.36

2.04 ( 0.1 1.01 ( 0.05

a ISC: short circuit current. b VOC: open circuit voltage. factor. d η: power conversion efficiency.

c

FF: fill

prepared in this study showed higher fill factors than the carbon film electrode devices reported previously.5b,8b,23b,c This was due to the well-established heterojunction and the low serial resistance conveyed by the device configuration. Third, η for the CCG-A800 electrode devices was ∼50% that of the ITO electrode devices. Further studies are in progress to improve the performance of the graphene film electrode solar cell devices by optimizing the device fabrication via methods such as improving the conductivities of the graphene films and controlling the interfacial structures between the graphene film electrodes and the PEDOT:PSS layers. Conclusions Transparent conductive graphene films were prepared by a simple two-step reduction method that consisted of the controlled chemical reduction of GO in an aqueous suspension and the thermal annealing of the resultant CCG films. The chemical reduction of GO proceeded such that the CCG sheets remained well-dispersed in the aqueous suspension. This two-step reduction approach not only effectively restored the sp2 carbon networks of the CCG sheets but also reduced the interlayer distance in the CCG films. These features led to enhanced charge carrier transport in the individual CCG sheets and increased charge carrier transport across the CCG sheets. As a result, the CCG-A800 films showed 80% transparency at 550 nm and a sheet resistance on the order of 103 Ω · 0-1. The CCG films prepared in this study could be uniformly patterned by conventional photolithography and reactive ion etching methods. Furthermore, the polymer solar cell devices made using the CCG-A800 films as transparent electrodes yielded values for ISC, VOC, FF, and η of 4.18 mA · cm-2, 0.67 V, 0.36, and 1.01 ( 0.05%, respectively. These transparent conductive graphene films may be employed as transparent electrodes in a variety of optoelectronic devices such as solar cells, light-emitting diodes, sensors, and field effect transistors.

(1) (a) Wang, X.; Zhi, L. J.; Tsao, N.; Tomovic, Z.; Li, J. L.; Mullen, K. Angew. Chem., Int. Ed. 2008, 47, 2990–2992. (b) Su, Q.; Pang, S. P.; Alijani, V.; Li, C.; Feng, X. L.; Mullen, K. AdV. Mater. 2009, 21, 3191– 3195. (c) Wang, Y.; Chen, X. H.; Zhong, Y. L.; Zhu, F. R.; Loh, K. P. Appl. Phys. Lett. 2009, 95, 063302-1-063302-3. (d) Yin, Z. Y.; Wu, S. X.; Zhou, X. Z.; Huang, X.; Zhang, Q. C.; Boey, F.; Zhang, H. Small 2010, 6, 307–312. (2) (a) Pang, S. P.; Tsao, H. N.; Feng, X. L.; Mullen, K. AdV. Mater. 2009, 21, 3488–3491. (b) Eda, G.; Chhowalla, M. Nano Lett. 2009, 9, 814– 818. (c) He, Q. Y.; Sudibya, H. G.; Yin, Z. Y.; Wu, S. X.; Li, H.; Boey, F.; Huang, W.; Chen, P.; Zhang, H. ACS Nano 2010, 4, 3201–3208. (3) Eda, G.; Unalan, H. E.; Rupesinghe, N.; Amaratunga, G. A. J.; Chhowalla, M. Appl. Phys. Lett. 2008, 93, 233502-1–233502-3. (4) Lu, C. H.; Yang, H. H.; Zhu, C. L.; Chen, X.; Chen, G. N. Angew. Chem., Int. Ed. 2009, 48, 4785–4787. (5) (a) Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. ACS Nano 2008, 2, 463–470. (b) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270–274. (c) Zhou, X. Z.; Huang, X.; Qi, X. Y.; Wu, S. X.; Xue, C.; Boey, F. Y. C.; Yan, Q. Y.; Chen, P.; Zhang, H. J. Phys. Chem. C 2009, 113, 10842–10846. (6) (a) Berger, C.; Song, Z. M.; Li, T. B.; Li, X. B.; Ogbazghi, A. Y.; Feng, R.; Dai, Z. T.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. J. Phys. Chem. B 2004, 108, 19912–19916. (b) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; Conrad, E. H.; First, P. N.; de Heer, W. A. Science 2006, 312, 1191–1196. (7) (a) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Ahn, J. H.; Kim, P.; Choi, J. Y.; Hong, B. H. Nature 2009, 457, 706–710. (b) Reina, A.; Jia, X. T.; Ho, J.; Nezich, D.; Son, H. B.; Bulovic, V.; Dresselhaus, M. S.; Kong, J. Nano Lett. 2009, 9, 30–35. (8) (a) Watcharotone, S.; Dikin, D. A.; Stankovich, S.; Piner, R.; Jung, I.; Dommett, G. H. B.; Evmenenko, G.; Wu, S. E.; Chen, S. F.; Liu, C. P.; Nguyen, S. T.; Ruoff, R. S. Nano Lett. 2007, 7, 1888–1892. (b) Wu, J. B.; Becerril, H. A.; Bao, Z. N.; Liu, Z. F.; Chen, Y. S.; Peumans, P. Appl. Phys. Lett. 2008, 92, 263302-1263302-3. (9) Geng, J.; Jung, H. J. Phys. Chem. C 2010, 114, 8227–8234. (10) (a) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Nature 2006, 442, 282–286. (b) Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Carbon 2006, 44, 3342–3347. (c) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217–224. (11) (a) Stankovich, S.; Piner, R. D.; Chen, X. Q.; Wu, N. Q.; Nguyen, S. T.; Ruoff, R. S. J. Mater. Chem. 2006, 16, 155–158. (b) Bai, H.; Xu, Y. X.; Zhao, L.; Li, C.; Shi, G. Q. Chem. Commun. 2009, 1667–1669. (c) Hao, R.; Qian, W.; Zhang, L. H.; Hou, Y. L. Chem. Commun. 2008, 6576– 6578. (d) Xu, Y. X.; Bai, H.; Lu, G. W.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2008, 130, 5856–6857. (e) Qi, X. Y.; Pu, K. Y.; Zhou, X. Z.; Li, H.; Liu, B.; Boey, F.; Huang, W.; Zhang, H. Small 2010, 6, 663–669. (f) Geng, J.; Kong, B. S.; Yang, S. B.; Jung, H. T. Chem. Commun. 2010, 46, 5091– 5093. (12) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339–1339. (13) Kong, B. S.; Geng, J. X.; Jung, H. T. Chem. Commun. 2009, 2174– 2176. (14) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon 2007, 45, 1558–1565. (15) Li, X. L.; Zhang, G. Y.; Bai, X. D.; Sun, X. M.; Wang, X. R.; Wang, E.; Dai, H. J. Nat. Nanotechnol. 2008, 3, 538–542. (16) (a) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. (b) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; Boland, J. J.; Niraj, P.; Duesberg, G.; Krishnamurthy, S.; Goodhue, R.;

14440

J. Phys. Chem. C, Vol. 114, No. 34, 2010

Hutchison, J.; Scardaci, V.; Ferrari, A. C.; Coleman, J. N. Nat. Nanotechnol. 2008, 3, 563–568. (17) McAllister, M. J.; Li, J. L.; Adamson, D. H.; Schniepp, H. C.; Abdala, A. A.; Liu, J.; Herrera-Alonso, M.; Milius, D. L.; Car, R.; Prud’homme, R. K.; Aksay, I. A. Chem. Mater. 2007, 19, 4396–4404. (18) (a) Hontorialucas, C.; Lopezpeinado, A. J.; Lopezgonzalez, J. D. D.; Rojascervantes, M. L.; Martinaranda, R. M. Carbon 1995, 33, 1585–1592. (b) Szabo, T.; Berkesi, O.; Dekany, I. Carbon 2005, 43, 3186–3189. (19) (a) Titelman, G. I.; Gelman, V.; Bron, S.; Khalfin, R. L.; Cohen, Y.; Bianco-Peled, H. Carbon 2005, 43, 641–649. (b) Ramesh, P.; Bhagyalakshmi, S.; Sampath, S. J. Colloid Interface Sci. 2004, 274, 95– 102. (20) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101–105.

Geng et al. (21) (a) Paredes, J. I.; Villar-Rodil, S.; Martinez-Alonso, A.; Tascon, J. M. D. Langmuir 2008, 24, 10560–10564. (b) Wu, Z. S.; Ren, W. C.; Gao, L. B.; Liu, B. L.; Jiang, C. B.; Cheng, H. M. Carbon 2009, 47, 493–499. (22) (a) Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szabo, T.; Szeri, A.; Dekany, I. Langmuir 2003, 19, 6050–6055. (b) Che, J. F.; Shen, L. Y.; Xiao, Y. H. J. Mater. Chem. 2010, 20, 1722–1727. (23) (a) Tung, V. C.; Chen, L. M.; Allen, M. J.; Wassei, J. K.; Nelson, K.; Kaner, R. B.; Yang, Y. Nano Lett. 2009, 9, 1949–1955. (b) Eda, G.; Lin, Y. Y.; Miller, S.; Chen, C. W.; Su, W. F.; Chhowalla, M. Appl. Phys. Lett. 2008, 92, 233305-1–233305-3. (c) Liu, Z. F.; Liu, Q.; Huang, Y.; Ma, Y. F.; Yin, S. G.; Zhang, X. Y.; Sun, W.; Chen, Y. S. AdV. Mater. 2008, 20, 3924–3930.

JP105029M