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Graphene Oxide: Surface Activity and Two-Dimensional Assembly By Franklin Kim, Laura J. Cote, and Jiaxing Huang*
membranes, colloids and amphiphile. GO sheets are characterized by two drastically different length scales, with their thickness determined by a single atomic layer and the lateral size extending up to tens of micrometers. This gives GO sheets very high aspect ratios and a large surface area, since a single layer is essentially completely surface. In many applications including transparent conductors,[17,22,28,29] electrical energy storage,[2,3,5,30], polymer composites,[14] and catalytic support,[31,32] GO is used in bulk quantities. Due to the highly anisotropic morphology of GO, the properties of the final product are determined, not only by the quality of the individual sheets but also by how they are assembled. Therefore, controlled assembly of these 2D building blocks is important for both fundamental scientific curiosity and technical applications. For example, since GO forms stable single-layer dispersion in water, solution processing is often used to create thin films. However, common thin-film preparation methods such as drop-casting, spraying, or spin-coating generally
Graphene oxide (GO) is a promising precursor for preparing graphene-based composites and electronics applications. Like graphene, GO is essentially one-atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two-dimensional assemblies over large areas is discussed.
1. Introduction More than a century after its first reported synthesis,[1] newborn interest in graphite oxide has been rapidly growing, largely towards graphene-related applications such as transistors, sensors, and energy storage devices.[2–8] A typical synthesis of graphite oxide involves the reaction of graphite powder with strong oxidizing agents such as potassium permanganate in concentrated sulfuric acid (Fig. 1a).[9] After oxidation, the carbon sheets are derivatized by carboxylic acid at the edges or by phenol, hydroxyl, and epoxide groups, mainly at the basal plane (Fig. 1b) and can readily exfoliate to form a stable, light-brown-colored single-layer suspension in water.[10,11] The apparent thickness of a graphite oxide single layer, now often termed graphene oxide (GO), is around 1 nm, as measured by atomic force microscopy (AFM; Fig. 1c).[12,13] The oxygenated defects break the p–p conjugation of the sp2 carbon network, thus making the sheets insulating. However, their electrical conductivity can be partially restored through chemical,[8,14,15] thermal,[16,17] photothermal,[18] or electrochemical reduction,[19,20] producing chemically modified graphene sheets (also known as reduced GO).[14,15,17,21] The ease of synthesizing GO and its solution processability has made GO a very attractive material for polymer composite and graphene-related electronics applications.[7,8,22–25] We view GO sheet as an unconventional type of soft material[13,26,27] in that it possesses characteristics of polymer, [*] Prof. J. Huang, Dr. F. Kim, L. J. Cote Department of Materials Science and Engineering Northwestern University Evanston, IL 60208 (USA) E-mail: [email protected]
DOI: 10.1002/adma.200903932
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Figure 1. a) Typical preparation of graphene oxide (GO). Pristine graphite powder is reacted with a strong oxidant. Upon oxidation and purification, a stable light-brown suspension of GO is obtained. b) Structural model of GO, which is composed of a partially broken sp2-carbon network with phenol hydroxyl and epoxide groups on the basal plane and carboxylic acid groups at the edges. c) AFM image of a GO sheet. The apparent thickness of a single sheet is around 1 nm. The bar shows 5 mm.
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2. Surface Activity of Graphene Oxide Sheets 2.1. Enrichment of Graphene Oxide at Liquid/Liquid Interfaces One way to verify the amphiphilic nature of GO is to observe how it interacts with an oil/water interface. Figure 2a and 2b illustrates a simple, yet effective experiment for this purpose. When toluene is added onto an aqueous GO suspension, a clear interface is observed. However, when the vial is shaken a few times by hand, toluene droplets immediately form, which stay stable for at least several months. In contrast, when deionized water is mixed with toluene the interface does not change. This is similar to the Pickering emulsions, in which the oil/water interface is stabilized by solid colloidal particles, forming stable droplets.[33] The droplets appear yellowish due to the enriched GO at the oil/water interface. The experiment clearly shows that GO is capable of stabilizing an oil/water interface and thus is an amphiphile. Preliminary observations show that the droplets form more
readily with aromatic organic solvents, such as benzene and toluene, than with non-aromatic solvents, such as chloroform and hexane, likely due to the p–p interactions between the residual p-conjugated domains in GO and the aromatic solvents. As shown in Figure 2b, GO can be enriched at the oil/water interface. Therefore, a volatile organic solvent could be used to ‘‘pick up’’ GO from its aqueous dispersion, as illustrated in Figure 2c. First, a few drops of chloroform were spread onto the dispersion, forming a thin oil layer, which should attract GO sheets to the vicinity of the oil/water interface. To verify whether there is any GO left on the water surface after the chloroform was evaporated, we utilized a surface imaging technique— Brewster-angle microscopy (BAM).[34,35] In a BAM setup, the reflection of a p-polarized incident beam irradiating at a surface is collected by a camera. At the Brewster angle (538 for the air/water interface), reflection is prohibited, and therefore the surface appears black. Materials such as molecular monolayers or particles floating on the water surface change the local refractive index, thus allowing reflection of the incident beam again and resulting in a bright appearance of the area (Fig. 2d). Therefore, BAM can detect floating GO on water without the addition of markers or label molecules. In our BAM experiments, the surface of the GO suspension was initially observed to be free of any scattering points, indicating that no GO was present at the interface (Fig. 2e). After chloroform is applied and evaporated, a large number of bright spots appeared in the BAM image, confirming the enrichment of surface active particles (Fig. 2f). The floating particles on the water surface can be transferred to a silicon wafer using vertical dip-coating for further characterization by scanning electron microscopy (SEM), which shows that the shinning spots observed in the BAM images are indeed GO sheets (Fig. 2g). The surface GO sheets appeared to be stable, as BAM showed that they did not re-disperse into the water subphase, even after being left on water overnight.
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results in multilayer aggregates and crumpled sheets due to the uncontrolled capillary flow and de-wetting during solvent evaporation, which force the soft sheets to fold and wrinkle.[25] Although GO has been commonly described as a hydrophilic, water dispersible material, we believe GO should be amphiphilic, since it is composed of a largely hydrophobic basal plane with hydrophilic edges. This inspires us to explore whether GO will assemble at interfaces, mimicking molecular surfactants. In this Research News article, we discuss the surface activity of GO and show how it can be utilized to achieve well-controlled 2D assemblies.
2.2. Enrichment of Graphene Oxide at Gas/Liquid Interfaces
Figure 2. Enrichment of GO at the liquid/liquid interface. a,b) A clear interface is initially formed when toluene is added to the aqueous GO suspension. After brief shaking, GO stabilized toluene droplets immediately form, which are stable for several months. c) Enrichment of GO at the oil/water interface. When a small amount of oil, such as chloroform, is applied to aqueous GO suspension, the sheet in the vicinity of the surface gets attracted and trapped at the surface. d) Schematic representation of surface imaging using BAM. e,f) BAM images from the GO-dispersion surface before and after chloroform spreading, respectively. Significant increase of surface active materials on the air/water interface can be observed after applying chloroform. The material is confirmed to be GO sheets by SEM (g). Scale bars in (e,f) and (g) are 0.5 mm and 20 mm, respectively.
Adv. Mater. 2010, 22, 1954–1958
The experiments shown in Figure 2 suggest that GO should also be active at gas/liquid interfaces. When a dilute GO suspension was left in air for a prolonged period of time, we observe a thin film formation at the water surface. This phenomenon was also reported in a recent paper.[36] This, in fact, poses a problem when preparing GO films by drop-casting, since the surface GO sheets tend to block the water evaporation path, leading to extended evaporation time. To speed up the enrichment of GO sheets at the air/water interface, we used a flotation process where gas bubbles were blown through the dispersion to catch GO sheets and lift them up to water surface (Fig. 3a). BAM studies showed that the gas bubbles indeed served as very effective ‘‘carriers’’ of GO as shown in Figure 3b and 3c. After injecting only a few bubbles, floating sheets were immediately observed by BAM. More quantitative results were obtained using a Langmuir–Blodgett (LB) trough (Fig. 3d). First, a dilute GO dispersion was used as the subphase. Then N2 bubbles were blown through the dispersion using a glass tube with a fritted end. Surface pressure was monitored using a tensiometer attached to a Wilhelmy plate. Moving barriers were used to
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Figure 3. Enrichment of GO by flotation with gas bubbles. a) Schematic representation of collecting GO to the air/water interface by blowing air bubbles into the dispersion. GO is caught onto the bubbles as they rise to the surface. b,c) BAM images from the GO solution surface before and after bubbles are applied. Instantaneous enrichment of GO at the air/water interface can be observed. d) Experimental setup for the bubbling of GO in a LB trough. The trough is filled with diluted GO dispersion. A glass tube with a fritted end is placed on the center of the trough, through which N2 gas is bubbled. e) Surface-pressure/area plot before and after blowing of N2. After bubbling, the surface pressure increases rapidly, indicating enrichment of GO at the air/water interface. The scale bars in BAM images (b,c) denote 0.5 mm.
control the density of GO sheets trapped at the air/water interface. If there is material floating on water, compression will increase its surface concentration and thus its surface pressure. Before bubbling, the plot was a flat line, indicating that the surface was free of GO. After bubbling for 10 minutes, a surface-pressure increase was observed as the area is compressed (Fig. 3e), suggesting the presence of GO sheets. The final surface pressure on the plot increased with extended bubbling, indicating that more GO was transferred to the water surface. Bubbling experiments with CO2 show similar results. There should be no limitation on the type of carrier gases that can be used to collect GO from solution.
3. 2D Langmuir–Blodgett Assembly of Graphene Oxide Sheets With the knowledge of surface activity of GO, one can now envision its assembly at interfaces that may mimic molecular amphiphiles. A fluidic, 2D interface, such as a water surface, should be an ideal platform to assemble the 2D sheets, since it allows free movement of GO. In addition, the water surface tension can help to keep the floating sheets flat and free from wrinkling and crumpling. One may even find similarities with water lilies lying on the surface of a pond. This kind of assembly will allow one to investigate the 2D, edge-to-edge interaction between neighboring sheets, something that is difficult to achieve using any other experimental method. LB should be an ideal technique for such studies as it allows control over the density of the materials on water surface as well as real time monitoring of
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surface pressure. In this section, we will discuss the controlled 2D assembly of GO sheets using LB. In a typical LB experiment, deionized water was used as the supporting subphase. GO dispersion in a 1:5 water/methanol mixture was carefully deposited drop-wise onto the water surface using a glass syringe. Commonly used hydrophobic spreading solvents, such as chloroform or toluene, were not suitable since they do not disperse GO well. Methanol, the simplest polar protic alcohol, can yield a stable, surfactant-free GO dispersion. Since it is miscible with water, we found that the most effective way to spread methanol on water was to touch the small pendant droplets one by one at the water surface.[13] BAM observation confirmed that a large amount of GO can be easily deposited onto the water surface using this method. The density of the sheets can be tuned by moving the barriers while the surface pressure is monitored by a tensiometer. A gradual increase in surface pressure was recorded as the barrier was closed, as shown in the isothermal surface-pressure–area plot (Fig. 4d). The GO layer was collected at different stages of compression by vertical dip-coating and was imaged with SEM. In each stage, large-scale assemblies (on the order of several square centimeters) of micrometer-sized GO sheets could be obtained (Fig. 4a–c). During the isothermal compression, an initial gas phase existed where the surface pressure essentially remained constant during compression. Films collected at this stage were found to consist of dilute, well-isolated, individual GO sheets. A gradual pressure increase began as the compression continues (region a), and the films obtained in this region consisted of closely packed GO sheets that were about to touch each other, essentially tiling over the entire 2D surface (Fig. 4a). The smallest gaps between two neighboring GO sheets were often too small to be resolved with SEM or even AFM. This suggests that LB assembly may be used for creating nanogaps between GO or graphene sheets. The GO sheets in the monolayers were well dispersed and free of multilayer aggregates often seen in other solution-processed GO films. This can be attributed to the strong edge-to-edge electrostatic repulsion between neighboring GO sheets originating from the ionized carboxylic groups decorating their edges. It is also worth noting that the GO sheets collected from LB assembly were free of wrinkles and folds despite of their large, micrometer-scale sizes. In comparison, alternative methods such as drop-casting, spin-coating, and spraying usually produces wrinkled sheets, even with sub-micrometer-sized GO.[17,22,37,38] When the monolayer was compressed beyond the closepacking region, a further increase in surface pressure was observed. This was in contrast to monolayers of small molecules or hard colloids, which would collapse into multilayers, leading to constant or reduced surface pressure.[39] The strong edge-to-edge repulsion resisted stacking or overlapping between layers, even when the closely packed monolayer was further compressed. Instead, the soft and flexible GO sheets started to fold at the touching points along their edges (Fig. 4b) to accommodate the increased surface pressure, thus leaving their interior largely flat and free of wrinkling. At even higher compression, partial edge-overlapping was observed, leading to a nearly complete, interlocked GO monolayer (Fig. 4c). Upon further compression, the interlocked monolayer buckled like a whole piece of thin film, generating macroscopic wrinkles at the millimeter scale, which can be seen with the naked eye. This eventually led to the collapse
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Figure 4. LB assemblies of GO sheets. a–c) SEM images of a large-area assembly of GO monolayers collected on a silicon wafer at different stages of isothermal compression. The packing density was continuously tuned: a) monolayer of closely packed GO, b) over-packed monolayer with sheets folded at interconnecting edges, and c) over-packed monolayer with folded and partially overlapped sheets interlocking with each other. The assembly is uniform over macroscopic areas. d) Isothermal surface pressure–area plot showing the corresponding regions (a–c) at which the monolayers were collected. e) Transparent conducting thin film obtained by chemical reduction of an over-packed, interlocking GO layer, such as those collected from region (c). The film was reduced at 500 8C in argon environment. The film transmits 95% of light. Four-probe resistivity measurements give a sheet resistance of 4 MV & 1. The scale bar indicates 1 cm.
of the monolayer. The 2D assembly of GO sheets, shown in Figure 4, was reversible even after many cycles of compression and expansion. SEM study confirmed that the folds, wrinkles, and partial overlapping observed in Figure 4c completely disappeared when the film was opened. As shown above, large areas of GO monolayers can be collected at the desired surface pressure, yielding uniform coverage of different densities. The GO sheets can be reduced by known methods (hydrazine, hydrogen, or thermal annealing) to generate chemically modified graphene.[14,17] The closely packed monolayers (Fig. 4a) would readily produce single-layer graphene sheets in high yield for large-scale device fabrication. The interlocked monolayers (Fig. 4b and 4c) already constitute continuous electrical pathways that can be potentially useful for transparent-conductor applications.[17,22,28,40] As a proofof-concept experiment, we collected a monolayer at the overpacked region of the pressure–area plot (region c) on a glass slide. The film was then thermally reduced by annealing at 500 8C in an argon atmosphere. Four gold electrodes were patterned onto the film for electrical measurement (Fig. 4e). Transmission measurement showed that the film has an average of 95% transmittance in the visible region of the spectrum. The current–voltage plot obtained by four-probe measurements shows sheet resistance of 4 MV & 1, which is comparable to previous reports on reduced GO films.[17]
Adv. Mater. 2010, 22, 1954–1958
The surface activity of GO is demonstrated and utilized to achieve controlled 2D assembly. GO is found to be amphiphilic and stable at air/water and oil/water interfaces. GO sheets can be transported to the water surface from its dispersion by flotation using gas bubbles and by spreading a volatile organic solvent. They can also stabilize oil droplets, forming Pickering emulsions. Surfactant-free LB monolayers of GO can be prepared at an air/water interface, creating large-scale 2D assemblies with controllable density. The strong edge-to-edge electrostatic repulsion effectively prevents stacking or aggregation that is often seen in other solution-processed films. With the advances in the reduction method of GO, the electrical properties of reduced GO have been continuously improved. Therefore, parallel progresses in controlled assembly of these 2D materials should lead to many exciting functional graphene-based materials.
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4. Conclusions
Acknowledgements
This work was supported by the National Science Foundation through a CAREER award (DMR 0955612) and the Northwestern University Materials Research Science & Engineering Center (NU-MRSEC, NSF DMR-0520513) through a capital equipment fund. L.J.C. gratefully acknowledges the National Science Foundation for a graduate research fellowship. We thank the NUANCE Center at Northwestern, which is supported by NU-NSEC, NU-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University, for use of their facilities. This article is part of a Special Issue on USTC Materials Science.
[1] B. C. Brodie, Philos. Trans. R. Soc. London 1859, 149, 249. [2] S. R. C. Vivekchand, C. S. Rout, K. S. Subrahmanyam, A. Govindaraj, C. N. R. Rao, J. Chem. Sci. 2008, 120, 9. [3] M. D. Stoller, S. Park, Y. Zhu, J. An, R. S. Ruoff, Nano Lett. 2008, 8, 3498. [4] J. T. Robinson, F. K. Perkins, E. S. Snow, Z. Wei, P. E. Sheehan, Nano Lett. 2008, 8, 3137. [5] E. Yoo, J. Kim, E. Hosono, H.-S. Zhou, T. Kudo, I. Honma, Nano Lett. 2008, 8, 2277. [6] J. D. Fowler, M. J. Allen, V. C. Tung, Y. Yang, R. B. Kaner, B. H. Weiller, ACS Nano 2009, 3, 301. [7] D. Li, R. B. Kaner, Science 2008, 320, 1170. [8] S. Park, R. S. Ruoff, Nat. Nanotechnol. 2009, 4, 217. [9] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339. [10] T. Nakajima, A. Mabuchi, R. Hagiwara, Carbon 1988, 26, 357. [11] H. He, M. Forster, J. Klinowski, J. Phys. Chem. B 1998, 102, 4477. [12] S. Stankovich, D. A. Dikin, R. D. Piner, K. A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu, S. T. Nguyen, R. S. Ruoff, Carbon 2007, 45, 1558. [13] L. J. Cote, F. Kim, J. Huang, J. Am. Chem. Soc. 2009, 131, 1043.
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[14] S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen, R. S. Ruoff, Nature 2006, 442, 282. [15] W. Gao, L. B. Alemany, L. Ci, P. M. Ajayan, Nat. Chem. 2009, 1, 403. [16] H. C. Schniepp, J. L. Li, M. J. McAllister, H. Sai, M. Herrera-Alonso, D. H. Adamson, R. K. Prud’homme, R. Car, D. A. Saville, I. A. Aksay, J. Phys. Chem. B 2006, 110, 8535. [17] H. A. Becerril, J. Mao, Z. Liu, R. M. Stoltenberg, Z. Bao, Y. Chen, ACS Nano 2008, 2, 463. [18] L. J. Cote, R. Cruz-Silva, J. Huang, J. Am. Chem. Soc. 2009, 131, 11027. [19] G. K. Ramesha, S. Sampath, J. Phys. Chem. C 2009, 113, 7985. [20] Z. J. Wang, X. Z. Zhou, J. Zhang, F. Boey, H. Zhang, J. Phys. Chem. C 2009, 113, 14071. [21] S. Stankovich, R. D. Piner, X. Chen, N. Wu, S. T. Nguyen, R. S. Ruoff, J. Mater. Chem. 2006, 16, 155. [22] G. Eda, G. Fanchini, M. Chhowalla, Nat. Nanotechnol. 2008, 3, 270. [23] D. Li, M. B. Mueller, S. Gilje, R. B. Kaner, G. G. Wallace, Nat. Nanotechnol. 2008, 3, 101. [24] X. Li, X. Wang, L. Zhang, S. Lee, H. Dai, Science 2008, 319, 1229. [25] M. J. Allen, V. C. Tung, R. B. Kaner, Chem. Rev. 2010, 110, 132. [26] R. A. L. Jones, Soft Condensed Matter, 1st ed., Oxford Univeristy Press, Oxford, UK 2002. [27] I. W. Hamley, Introduction to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid Crystals, 2nd ed., Wiley, Chichester, UK 2007.
[28] X. Wang, L. Zhi, N. Tsao, Z. Tomovic, J. Li, K. Mullen, Angew. Chem. Int. Ed. 2008, 47, 2990. [29] S. Watcharotone, D. A. Dikin, S. Stankovich, R. Piner, I. Jung, G. H. B. Dommett, G. Evmenenko, S.-E. Wu, S.-F. Chen, C.-P. Liu, S. T. Nguyen, R. S. Ruoff, Nano Lett. 2007, 7, 1888. [30] D. Pan, S. Wang, B. Zhao, M. Wu, H. Zhang, Y. Wang, Z. Jiao, Chem. Mater. 2009, 21, 3136. [31] X. Zhou, X. Huang, X. Qi, S. Wu, C. Xue, F. Y. C. Boey, Q. Yan, P. Chen, H. Zhang, J. Phys. Chem. C 2009, 113, 10842. [32] C. Xu, X. Wang, J. Zhu, J. Phys. Chem. C 2008, 112, 19841. [33] S. U. Pickering, J. Chem. Soc, Trans. 1907, 91, 2001. [34] S. M. Danauskas, M. K. Ratajczak, Y. Ishitsuka, J. Gebhardt, D. Schultz, M. Meron, B. Lin, K. Y. C. Lee, Rev. Sci. Instrum. 2007, 78, 103705. [35] M. M. Lipp, K. Y. C. Lee, J. A. Zasadzinski, A. J. Waring, Rev. Sci. Instrum. 1997, 68, 2574. [36] C. Chen, Q.-H. Yang, Y. Yang, W. Lv, Y. Wen, P.-X. Hou, M. Wang, H.-M. Cheng, Adv. Mater. 2009, 21, 3007. [37] D. A. Dikin, S. Stankovich, E. J. Zimney, R. D. Piner, G. H. B. Dommett, G. Evmenenko, S. T. Nguyen, R. S. Ruoff, Nature 2007, 448, 457. [38] S. Gilje, S. Han, M. Wang, K. L. Wang, R. B. Kaner, Nano Lett. 2007, 7, 3394. [39] C. Ybert, W. Lu, G. Moeller, C. M. Knobler, J. Phys. Chem. B 2002, 106, 2004. [40] X. Wang, L. Zhi, K. Muellen, Nano Lett. 2008, 8, 323.
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Graphene Oxide: Surface Activity and Two-Dimensional Assembly By Franklin Kim, Laura J. Cote, and Jiaxing Huang*
membranes, colloids and amphiphile. GO sheets are characterized by two drastically different length scales, with their thickness determined by a single atomic layer and the lateral size extending up to tens of micrometers. This gives GO sheets very high aspect ratios and a large surface area, since a single layer is essentially completely surface. In many applications including transparent conductors,[17,22,28,29] electrical energy storage,[2,3,5,30], polymer composites,[14] and catalytic support,[31,32] GO is used in bulk quantities. Due to the highly anisotropic morphology of GO, the properties of the final product are determined, not only by the quality of the individual sheets but also by how they are assembled. Therefore, controlled assembly of these 2D building blocks is important for both fundamental scientific curiosity and technical applications. For example, since GO forms stable single-layer dispersion in water, solution processing is often used to create thin films. However, common thin-film preparation methods such as drop-casting, spraying, or spin-coating generally
Graphene oxide (GO) is a promising precursor for preparing graphene-based composites and electronics applications. Like graphene, GO is essentially one-atom thick but can be as wide as tens of micrometers, resulting in a unique type of material building block, characterized by two very different length scales. Due to this highly anisotropic structure, the collective material properties are highly dependent on how these sheets are assembled. Therefore, understanding and controlling the assembly behavior of GO has become an important subject of research. In this Research News article the surface activity of GO and how it can be employed to create two-dimensional assemblies over large areas is discussed.
1. Introduction More than a century after its first reported synthesis,[1] newborn interest in graphite oxide has been rapidly growing, largely towards graphene-related applications such as transistors, sensors, and energy storage devices.[2–8] A typical synthesis of graphite oxide involves the reaction of graphite powder with strong oxidizing agents such as potassium permanganate in concentrated sulfuric acid (Fig. 1a).[9] After oxidation, the carbon sheets are derivatized by carboxylic acid at the edges or by phenol, hydroxyl, and epoxide groups, mainly at the basal plane (Fig. 1b) and can readily exfoliate to form a stable, light-brown-colored single-layer suspension in water.[10,11] The apparent thickness of a graphite oxide single layer, now often termed graphene oxide (GO), is around 1 nm, as measured by atomic force microscopy (AFM; Fig. 1c).[12,13] The oxygenated defects break the p–p conjugation of the sp2 carbon network, thus making the sheets insulating. However, their electrical conductivity can be partially restored through chemical,[8,14,15] thermal,[16,17] photothermal,[18] or electrochemical reduction,[19,20] producing chemically modified graphene sheets (also known as reduced GO).[14,15,17,21] The ease of synthesizing GO and its solution processability has made GO a very attractive material for polymer composite and graphene-related electronics applications.[7,8,22–25] We view GO sheet as an unconventional type of soft material[13,26,27] in that it possesses characteristics of polymer, [*] Prof. J. Huang, Dr. F. Kim, L. J. Cote Department of Materials Science and Engineering Northwestern University Evanston, IL 60208 (USA) E-mail: [email protected]
DOI: 10.1002/adma.200903932
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Figure 1. a) Typical preparation of graphene oxide (GO). Pristine graphite powder is reacted with a strong oxidant. Upon oxidation and purification, a stable light-brown suspension of GO is obtained. b) Structural model of GO, which is composed of a partially broken sp2-carbon network with phenol hydroxyl and epoxide groups on the basal plane and carboxylic acid groups at the edges. c) AFM image of a GO sheet. The apparent thickness of a single sheet is around 1 nm. The bar shows 5 mm.
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2. Surface Activity of Graphene Oxide Sheets 2.1. Enrichment of Graphene Oxide at Liquid/Liquid Interfaces One way to verify the amphiphilic nature of GO is to observe how it interacts with an oil/water interface. Figure 2a and 2b illustrates a simple, yet effective experiment for this purpose. When toluene is added onto an aqueous GO suspension, a clear interface is observed. However, when the vial is shaken a few times by hand, toluene droplets immediately form, which stay stable for at least several months. In contrast, when deionized water is mixed with toluene the interface does not change. This is similar to the Pickering emulsions, in which the oil/water interface is stabilized by solid colloidal particles, forming stable droplets.[33] The droplets appear yellowish due to the enriched GO at the oil/water interface. The experiment clearly shows that GO is capable of stabilizing an oil/water interface and thus is an amphiphile. Preliminary observations show that the droplets form more
readily with aromatic organic solvents, such as benzene and toluene, than with non-aromatic solvents, such as chloroform and hexane, likely due to the p–p interactions between the residual p-conjugated domains in GO and the aromatic solvents. As shown in Figure 2b, GO can be enriched at the oil/water interface. Therefore, a volatile organic solvent could be used to ‘‘pick up’’ GO from its aqueous dispersion, as illustrated in Figure 2c. First, a few drops of chloroform were spread onto the dispersion, forming a thin oil layer, which should attract GO sheets to the vicinity of the oil/water interface. To verify whether there is any GO left on the water surface after the chloroform was evaporated, we utilized a surface imaging technique— Brewster-angle microscopy (BAM).[34,35] In a BAM setup, the reflection of a p-polarized incident beam irradiating at a surface is collected by a camera. At the Brewster angle (538 for the air/water interface), reflection is prohibited, and therefore the surface appears black. Materials such as molecular monolayers or particles floating on the water surface change the local refractive index, thus allowing reflection of the incident beam again and resulting in a bright appearance of the area (Fig. 2d). Therefore, BAM can detect floating GO on water without the addition of markers or label molecules. In our BAM experiments, the surface of the GO suspension was initially observed to be free of any scattering points, indicating that no GO was present at the interface (Fig. 2e). After chloroform is applied and evaporated, a large number of bright spots appeared in the BAM image, confirming the enrichment of surface active particles (Fig. 2f). The floating particles on the water surface can be transferred to a silicon wafer using vertical dip-coating for further characterization by scanning electron microscopy (SEM), which shows that the shinning spots observed in the BAM images are indeed GO sheets (Fig. 2g). The surface GO sheets appeared to be stable, as BAM showed that they did not re-disperse into the water subphase, even after being left on water overnight.
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results in multilayer aggregates and crumpled sheets due to the uncontrolled capillary flow and de-wetting during solvent evaporation, which force the soft sheets to fold and wrinkle.[25] Although GO has been commonly described as a hydrophilic, water dispersible material, we believe GO should be amphiphilic, since it is composed of a largely hydrophobic basal plane with hydrophilic edges. This inspires us to explore whether GO will assemble at interfaces, mimicking molecular surfactants. In this Research News article, we discuss the surface activity of GO and show how it can be utilized to achieve well-controlled 2D assemblies.
2.2. Enrichment of Graphene Oxide at Gas/Liquid Interfaces
Figure 2. Enrichment of GO at the liquid/liquid interface. a,b) A clear interface is initially formed when toluene is added to the aqueous GO suspension. After brief shaking, GO stabilized toluene droplets immediately form, which are stable for several months. c) Enrichment of GO at the oil/water interface. When a small amount of oil, such as chloroform, is applied to aqueous GO suspension, the sheet in the vicinity of the surface gets attracted and trapped at the surface. d) Schematic representation of surface imaging using BAM. e,f) BAM images from the GO-dispersion surface before and after chloroform spreading, respectively. Significant increase of surface active materials on the air/water interface can be observed after applying chloroform. The material is confirmed to be GO sheets by SEM (g). Scale bars in (e,f) and (g) are 0.5 mm and 20 mm, respectively.
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The experiments shown in Figure 2 suggest that GO should also be active at gas/liquid interfaces. When a dilute GO suspension was left in air for a prolonged period of time, we observe a thin film formation at the water surface. This phenomenon was also reported in a recent paper.[36] This, in fact, poses a problem when preparing GO films by drop-casting, since the surface GO sheets tend to block the water evaporation path, leading to extended evaporation time. To speed up the enrichment of GO sheets at the air/water interface, we used a flotation process where gas bubbles were blown through the dispersion to catch GO sheets and lift them up to water surface (Fig. 3a). BAM studies showed that the gas bubbles indeed served as very effective ‘‘carriers’’ of GO as shown in Figure 3b and 3c. After injecting only a few bubbles, floating sheets were immediately observed by BAM. More quantitative results were obtained using a Langmuir–Blodgett (LB) trough (Fig. 3d). First, a dilute GO dispersion was used as the subphase. Then N2 bubbles were blown through the dispersion using a glass tube with a fritted end. Surface pressure was monitored using a tensiometer attached to a Wilhelmy plate. Moving barriers were used to
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Figure 3. Enrichment of GO by flotation with gas bubbles. a) Schematic representation of collecting GO to the air/water interface by blowing air bubbles into the dispersion. GO is caught onto the bubbles as they rise to the surface. b,c) BAM images from the GO solution surface before and after bubbles are applied. Instantaneous enrichment of GO at the air/water interface can be observed. d) Experimental setup for the bubbling of GO in a LB trough. The trough is filled with diluted GO dispersion. A glass tube with a fritted end is placed on the center of the trough, through which N2 gas is bubbled. e) Surface-pressure/area plot before and after blowing of N2. After bubbling, the surface pressure increases rapidly, indicating enrichment of GO at the air/water interface. The scale bars in BAM images (b,c) denote 0.5 mm.
control the density of GO sheets trapped at the air/water interface. If there is material floating on water, compression will increase its surface concentration and thus its surface pressure. Before bubbling, the plot was a flat line, indicating that the surface was free of GO. After bubbling for 10 minutes, a surface-pressure increase was observed as the area is compressed (Fig. 3e), suggesting the presence of GO sheets. The final surface pressure on the plot increased with extended bubbling, indicating that more GO was transferred to the water surface. Bubbling experiments with CO2 show similar results. There should be no limitation on the type of carrier gases that can be used to collect GO from solution.
3. 2D Langmuir–Blodgett Assembly of Graphene Oxide Sheets With the knowledge of surface activity of GO, one can now envision its assembly at interfaces that may mimic molecular amphiphiles. A fluidic, 2D interface, such as a water surface, should be an ideal platform to assemble the 2D sheets, since it allows free movement of GO. In addition, the water surface tension can help to keep the floating sheets flat and free from wrinkling and crumpling. One may even find similarities with water lilies lying on the surface of a pond. This kind of assembly will allow one to investigate the 2D, edge-to-edge interaction between neighboring sheets, something that is difficult to achieve using any other experimental method. LB should be an ideal technique for such studies as it allows control over the density of the materials on water surface as well as real time monitoring of
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surface pressure. In this section, we will discuss the controlled 2D assembly of GO sheets using LB. In a typical LB experiment, deionized water was used as the supporting subphase. GO dispersion in a 1:5 water/methanol mixture was carefully deposited drop-wise onto the water surface using a glass syringe. Commonly used hydrophobic spreading solvents, such as chloroform or toluene, were not suitable since they do not disperse GO well. Methanol, the simplest polar protic alcohol, can yield a stable, surfactant-free GO dispersion. Since it is miscible with water, we found that the most effective way to spread methanol on water was to touch the small pendant droplets one by one at the water surface.[13] BAM observation confirmed that a large amount of GO can be easily deposited onto the water surface using this method. The density of the sheets can be tuned by moving the barriers while the surface pressure is monitored by a tensiometer. A gradual increase in surface pressure was recorded as the barrier was closed, as shown in the isothermal surface-pressure–area plot (Fig. 4d). The GO layer was collected at different stages of compression by vertical dip-coating and was imaged with SEM. In each stage, large-scale assemblies (on the order of several square centimeters) of micrometer-sized GO sheets could be obtained (Fig. 4a–c). During the isothermal compression, an initial gas phase existed where the surface pressure essentially remained constant during compression. Films collected at this stage were found to consist of dilute, well-isolated, individual GO sheets. A gradual pressure increase began as the compression continues (region a), and the films obtained in this region consisted of closely packed GO sheets that were about to touch each other, essentially tiling over the entire 2D surface (Fig. 4a). The smallest gaps between two neighboring GO sheets were often too small to be resolved with SEM or even AFM. This suggests that LB assembly may be used for creating nanogaps between GO or graphene sheets. The GO sheets in the monolayers were well dispersed and free of multilayer aggregates often seen in other solution-processed GO films. This can be attributed to the strong edge-to-edge electrostatic repulsion between neighboring GO sheets originating from the ionized carboxylic groups decorating their edges. It is also worth noting that the GO sheets collected from LB assembly were free of wrinkles and folds despite of their large, micrometer-scale sizes. In comparison, alternative methods such as drop-casting, spin-coating, and spraying usually produces wrinkled sheets, even with sub-micrometer-sized GO.[17,22,37,38] When the monolayer was compressed beyond the closepacking region, a further increase in surface pressure was observed. This was in contrast to monolayers of small molecules or hard colloids, which would collapse into multilayers, leading to constant or reduced surface pressure.[39] The strong edge-to-edge repulsion resisted stacking or overlapping between layers, even when the closely packed monolayer was further compressed. Instead, the soft and flexible GO sheets started to fold at the touching points along their edges (Fig. 4b) to accommodate the increased surface pressure, thus leaving their interior largely flat and free of wrinkling. At even higher compression, partial edge-overlapping was observed, leading to a nearly complete, interlocked GO monolayer (Fig. 4c). Upon further compression, the interlocked monolayer buckled like a whole piece of thin film, generating macroscopic wrinkles at the millimeter scale, which can be seen with the naked eye. This eventually led to the collapse
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Figure 4. LB assemblies of GO sheets. a–c) SEM images of a large-area assembly of GO monolayers collected on a silicon wafer at different stages of isothermal compression. The packing density was continuously tuned: a) monolayer of closely packed GO, b) over-packed monolayer with sheets folded at interconnecting edges, and c) over-packed monolayer with folded and partially overlapped sheets interlocking with each other. The assembly is uniform over macroscopic areas. d) Isothermal surface pressure–area plot showing the corresponding regions (a–c) at which the monolayers were collected. e) Transparent conducting thin film obtained by chemical reduction of an over-packed, interlocking GO layer, such as those collected from region (c). The film was reduced at 500 8C in argon environment. The film transmits 95% of light. Four-probe resistivity measurements give a sheet resistance of 4 MV & 1. The scale bar indicates 1 cm.
of the monolayer. The 2D assembly of GO sheets, shown in Figure 4, was reversible even after many cycles of compression and expansion. SEM study confirmed that the folds, wrinkles, and partial overlapping observed in Figure 4c completely disappeared when the film was opened. As shown above, large areas of GO monolayers can be collected at the desired surface pressure, yielding uniform coverage of different densities. The GO sheets can be reduced by known methods (hydrazine, hydrogen, or thermal annealing) to generate chemically modified graphene.[14,17] The closely packed monolayers (Fig. 4a) would readily produce single-layer graphene sheets in high yield for large-scale device fabrication. The interlocked monolayers (Fig. 4b and 4c) already constitute continuous electrical pathways that can be potentially useful for transparent-conductor applications.[17,22,28,40] As a proofof-concept experiment, we collected a monolayer at the overpacked region of the pressure–area plot (region c) on a glass slide. The film was then thermally reduced by annealing at 500 8C in an argon atmosphere. Four gold electrodes were patterned onto the film for electrical measurement (Fig. 4e). Transmission measurement showed that the film has an average of 95% transmittance in the visible region of the spectrum. The current–voltage plot obtained by four-probe measurements shows sheet resistance of 4 MV & 1, which is comparable to previous reports on reduced GO films.[17]
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The surface activity of GO is demonstrated and utilized to achieve controlled 2D assembly. GO is found to be amphiphilic and stable at air/water and oil/water interfaces. GO sheets can be transported to the water surface from its dispersion by flotation using gas bubbles and by spreading a volatile organic solvent. They can also stabilize oil droplets, forming Pickering emulsions. Surfactant-free LB monolayers of GO can be prepared at an air/water interface, creating large-scale 2D assemblies with controllable density. The strong edge-to-edge electrostatic repulsion effectively prevents stacking or aggregation that is often seen in other solution-processed films. With the advances in the reduction method of GO, the electrical properties of reduced GO have been continuously improved. Therefore, parallel progresses in controlled assembly of these 2D materials should lead to many exciting functional graphene-based materials.
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4. Conclusions
Acknowledgements
This work was supported by the National Science Foundation through a CAREER award (DMR 0955612) and the Northwestern University Materials Research Science & Engineering Center (NU-MRSEC, NSF DMR-0520513) through a capital equipment fund. L.J.C. gratefully acknowledges the National Science Foundation for a graduate research fellowship. We thank the NUANCE Center at Northwestern, which is supported by NU-NSEC, NU-MRSEC, Keck Foundation, the State of Illinois, and Northwestern University, for use of their facilities. This article is part of a Special Issue on USTC Materials Science.
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