Self-Assembled Free-Standing Graphite Oxide ... - Wiley Online Library

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By Chengmeng Chen, Quan-Hong Yang,* Yonggang Yang,* Wei Lv, Yuefang Wen, Peng-Xiang Hou, Maozhang Wang, and Hui-Ming Cheng As a one-atom-thick two-dimensional crystal, graphene has been considered a basic building block for all sp2 carbons including fullerene, carbon nanotubes, and graphite.[1] Since Geim et al. peeled a few graphene sheets from highly crystalline graphite by a ‘scotch tape’ method in 2004,[2] some unique electronic properties of this conceptual matter, such as chiral quantum Hall effects[3] and charge-carriers independent conductivity,[4] have been found, which indicate potential applications in quantum devices,[5–7] nanocomposites with various matrixes,[8,9] and ultrathin membrane materials.[10,11] Many approaches have been developed to prepare graphene materials, mainly including micromechanical cleavage of highly oriented pyrolytic graphite (HOPG),[2] sublimation of silicon from silicon carbide,[12] and some chemical methods.[8–11] Since stable suspension of graphene oxide can be obtained by ultrasonic treatment of graphite oxide (GO) in water,[13,14] much effort has been made to assemble these welldispersed oxidized or chemically reduced (under controlled conditions, still water-dispersible) graphene nanosheets into membrane-shaped ordered macrostructures, mainly through flow-directed assembly by filtration.[10,11,15,16] Recently, molecular templates,[17] Langmuir–Blodgett assembly,[18,19] and direct chemical vapor deposition[20] have also been employed to obtain graphene-based or graphene-oxide-based membranes on selected substrates. In this communication, we propose a facile selfassembly approach to prepare macroscopic GO membranes at a liquid/air interface by evaporating the hydrosol of graphene oxide. This method is easy to scale up, and the free-standing

membranes obtained show excellent mechanical and optical properties. In a typical preparation, GO was synthesized from natural graphite powder by a modified Hummers method.[21] The X-ray diffraction (XRD) patterns of the parent graphite and GO (Fig. S1, Supporting Information (SI)) indicate the transformation of the interlayer spacing (d002 spacing) from 0.335 to 0.776 nm, which, together with the Fourier-transform infrared (FTIR) spectroscopy results (Fig. S2, SI), is a clear indication of the complete transformation from graphite to GO. The molecular formula of GO prepared was deduced to be C3.62O3.10H1.94 from the elemental analysis (Table S1, SI) and thermogravimetric (TGA) results (Fig. S3, SI). The graphene oxide hydrosol (Fig. 1a) was prepared by ultrasonic peeling of GO in aqueous suspension. The stable hydrosol of graphene oxide was heated to 353 K for a short period in a thermostatted water bath, during which a smooth and condensed thin film was formed very rapidly at the liquid/air interface (Fig. 1b). Such a stable membrane was easily separated from the parent graphene oxide suspension by decanting the residual suspension into another beaker. After drying, a flexible, semi-transparent and free-standing membrane was obtained and through this ultrathin membrane we can clearly identify the

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Self-Assembled Free-Standing Graphite Oxide Membrane

[*] Prof. Q.-H. Yang, Prof. Y. G. Yang, C. M. Chen, Y. F. Wen, Prof. M. Z. Wang Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology, Tianjin, University of Tianjin, 300072 (P. R. China) E-mail: [email protected]; [email protected] Prof. Q.-H. Yang, W. Lv Key Laboratory for Green Chemical Technology of Ministry of Education School of Chemical Engineering and Technology Tianjin University Tianjin 300072 (P. R. China) C. M. Chen Graduate University of Chinese Academy of Sciences Beijing 100049 (P. R. China) Dr. P.-X. Hou, Prof. H.-M. Cheng Shenyang National Laboratory for Materials Science Institute of Metal Research Chinese Academy of Sciences Shenyang 100016 (P. R. China)

DOI: 10.1002/adma.200803726

Adv. Mater. 2009, 21, 3007–3011

Figure 1. Digital images that demonstrate the self-assembly process and the resulting GO membrane. a) Colloid suspend of GO (2 mg mL1) after remaining stable for two weeks. b) A condensed film self-assembled at the liquid/air interface after heating at 80 8C for 15 min. c) A flexible and semitransparent GO membrane (15 mm 30 mm). d) A large-area (about 60 mm 60 mm) substrate-free membrane obtained at the liquid/air interface.

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(SEM) image of a typical GO membrane (heating period of the hydrosol: 40 min), which shows that this membrane is of uniform thickness (10 mm) and relatively smooth surface. The thickness can be precisely controlled in a range of 0.5–20 mm by adjusting the evaporating time of the hydrosol. Figure 3b shows a topsurface SEM image, from which the individual graphene oxide sheets can be clearly identified as ‘building blocks’ for the membrane. The cross-sectional image, as shown in Figure 3c, exhibits a compact layer-by-layer stacking of graphene oxide sheets. The layering of the GO membrane was further investigated by X-ray diffraction (XRD) analysis (Fig. 3f) and the result indicates that the interlayer distance is around 0.747 nm, almost the same as that of the parent GO. The above results suggest that water-dispersed graphene oxides re-assemble into a layer-by-layer macroscopic structure (layered GO membrane) as presented in Figure 3d. Accordingly, a membraneformation process is proposed as schematically presented in Figure 3e. During the heating process of the hydrosol, Brownian motion of graphene oxide sheets is being strengthened in the aqueous suspension, and the liquid level of the hydrosol gradually decreases. Thus, there are increased opportunities for graphene oxide sheets to collide and interact with each other and move up to the liquid/air interface with the water spilling out from the hydrosol. The liquid/air interface provides a smooth space for the two-dimensional graphene oxide sheets, and the nanosheets that reach the interface tend to aggregate along it. When new sheets move near to the interface, they are captured by the sheets that already reside there through interlayer van der Waals forces. Thereafter, perpendicular to the interface, the graphene oxide sheets begin to stack and, as a result, layer-by-layer nanostructures are formed. According to the XRD results, the mean dimension of an ordered stack of graphene oxide sheets that constitute the membrane is around 8.2 0.1 nm, which indicates that the stacking structure contains 10 to 11 roughly parallel graphene oxide sheets. These layer-by-layer nanostructures that arrange along the liquid/air interface further interact with each other and aggregate into a continuous membrane when the heating process continues. Finally, this solvent-evaporation-driven layer-by-layer self-assembly process results in a macroscopic membrane of a thickness of several micrometers that covers the whole liquid/air interface. Since the self-assembly process is promoted by water evaporation, the membrane formation is believed to be sensitive to the heating temperature and concentration of the graphene oxide suspension. Although GO membranes can be formed over relatively broad temperature and graphene oxide concentration ranges (temperature: 50–95 8C; graphene oxide: 0.2–3.0 mg mL1), higher heating temperatures and concentrations of graphene Figure 2. Microscopic characterization of graphene oxide sheets dispersed in the hydrosol. a) oxide result in faster membrane formation AFM ichnography and b) cross-section contour of the graphene oxide sheets. c) Optical and better quality (uniformity) of the obtained membranes. The optimum temperature microscopic image of a graphene oxide sheet.

characters on a background paper as indicated in Figure 1c. The size of the flexible membrane is solely determined by the area of the liquid/air interface, and a large-area membrane (Fig. 1d) could be easily obtained so long as a large container was employed for the assembly. An atomic force microscopy (AFM) image (Fig. 2a) of the hydrosol offers immediate evidence for peeled-off single graphene oxide sheets. The thicknesses of the graphene oxide layers we prepared are within a very narrow range of 1.1–1.2 nm, as indicated in Figure 2b. As reported previously,[22,23] these graphene oxide layers should be mostly monolayered, although these values are somewhat larger than the interlayer spacing (0.776 nm) of the parent GO. The sheets are a little more ‘bumpy’ than predicted, which is possibly due to the existence of abundant functional groups, such as epoxy and hydroxyl groups, bonded to both sides of the graphene sheets, which disrupts the original conjugation and introduces lattice defects to result in folds and distortions on the sheets.[24] The AFM (Fig. 2a) and optical microscopy observations indicate that most graphene sheets are of a relatively large size (several hundred nanometers to tens of micrometers). Figure 2c gives a typical optical microscopy image of a large-size graphene oxide sheet of lateral dimension over 10 mm, which is remarkably larger than those reported in most previous publications,[25,26] and slightly smaller than those recently reported by Tung et al.[27] Through self-assembly at the liquid/air interface, monolayered graphene oxide sheets aggregate into a semi-transparent GO membrane. Figure 3a presents a scanning electron microscopy


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sustaining source for the membrane-formation process, and the process is repeatable when the residual suspension is heated, which renders it possible to achieve a large-scale production of large-area membrane materials. Third, the thickness and size of this membrane can be well controlled by adjusting the assembly period and the liquid/air interface area provided by the container for membrane formation, since this method is based on a selfassembly process at the liquid/air interface. Fourth, the obtained membrane can be partly reduced by heat treatment and transformed into an ultrathin functionalized graphite membrane. The systematic characterization of the ultrathin graphite membrane is ongoing. Further effort is also being made to extend the present approach for the self-assembly of graphene sheets, since one can obtain a very good hydrosol suspension of graphene chemically reduced under controlled conditions.[15] The optical transmittance of this semi-transparent membrane was measured and is shown in Figure 3g. The membrane discourages all the ultraviolet light from transmitting, while the optical transmittance of the membrane increases sharply with the wavelength in the visible region and retains a high value from 30 to 82% in the nearinfrared region. The special optical properties indicate potential applications in beam splitting, UV shielding, optical switching, and as a protective layer. The obtained GO membrane strips of different membrane thickness (5–10 mm) were employed for mechanical measurements. The preliminary results (Table S3, SI) indicate that the self-assembly membrane is characterized by slightly lower modulus (average value of 12.7 GPa) but similar tensile strength (average value of 67.7 MPa) compared with the membranes prepared by filtration method.[10] The mechanical performance of these membranes is apparently higher than that of the flexible graphite foils composed of stacked expanded graphite.[28] The stress–strain curve for an individual membrane is presented in Figure 4a, and it is indicated that the deformation process of the membrane contains two steps: i) a selfreinforcement step, and ii) an elastic deformation step. In the first step, the modulus increases rapidly with the stress, which has been explained as the self-orientation of tile-like lamellae structures under tensile loading.[10] In the second step, a linear relationship between stress and strain exists, and the elastic modulus and tensile strength are evaluated to be 13.8 GPa and 75.9 MPa, respectively. The SEM image of the fracture section of the membrane strip (Fig. 4b) indicates that there is no ‘pulling-out’ of graphene sheets during the tension loading process, while the fracture section Figure 3. Microstructural analyses, proposed self-assembly process, and optical transmitshows a ‘necking-down’ phenomenon (decreases tance of a GO membrane. a–c) SEM images of the GO membrane. d) SEM image showing layer-by-layer structures. e) Schematic representation of a proposed self-assembly process from 7.0 to 1.5 mm after fracturing). Note that in of the GO membrane at the liquid–air interface. f) XRD patterns and g) optical transmit- the evaluation of the mechanical properties, the change of the stress plane was not taken into tance curve of the GO membrane. (80–95 8C) and graphene oxide concentration ranges (1.0–3.0 mg mL1) are suggested by the experimental results. The present self-assembly approach to obtain large-area freestanding GO membranes is very facile and easy to scale up. First, this method is time saving and low energy consuming. In the present case, the membrane was obtained by self-assembly within 10–40 min, while for the filtration method it takes much longer to obtain membranes of similar thickness. Second, the hydrosol is a

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Figure 4. Mechanical properties of the GO membrane. a) Stress–strain curve of a typical membrane sample. b) Fracture section of the membrane sample after tensile testing.

consideration and, therefore, the real stress will be higher than the experimental value. The excellent mechanical properties indicate many potential applications in composites and other mechanical areas. In conclusion, free-standing GO membranes can be produced through a facile self-assembly process at the liquid/air interface, and the membranes are thickness controlled and area adjustable. Such macroscopic membranes are constructed by individual graphene oxide sheets through layer-by-layer stacking, and show excellent mechanical and optical performances. The repeatable process of the membrane formation from the same graphene oxide hydrosol indicates a scaled-up approach for GO and graphite membranes, which will provide high-quality, microstructure-controllable macroscopic samples for the exploration of novel properties and development of new applications. Systematic investigations are underway to further clarify the nature of the self-assembly, and to find unique properties and potential applications of the GO and ultrathin graphite membrane (after reduction).

reflection mode (Cu Ka radiation, l ¼ 0.15406 nm, D8 Advance, BRUKER/ AXS, Germany). Elemental analysis was conducted for GO powder predried at 353 K for 6 h (Vario EL, Germany). The GO samples were characterized by simultaneous thermoanalysis (STA 409PC luxx, Netzsch, Germany, 5 K min1, N2) and FTIR spectroscopy (IR200, Thermo Nicolet, US). AFM observation was conducted for the graphene oxide sheets (Tip mode, frequency 0.803 Hz, Veeco NanoScope IIIa Multimode, DI, USA) and the samples for AFM analyses were precisely prepared by depositing the hydrosol of graphene oxide on freshly cleaved mica surfaces. The graphene oxide sheets dispersed in the hydrosol were also observed using optical microscopy (Nikon Microscope Alphaphot 2 YS2 Mint, USA) and the GO membrane was observed using SEM (Nova NanoSEM 430, FEI). A single fiber tensile tester (YG003A, Taicang, China) equipped with an X–Y plotter was employed to evaluate the mechanical performance of the GO membranes. Before the tensile test, the membranes were predried at 353 K for 6 h and then cut into approximately 5 mm 30 mm strips. The measurements were carried out under the following conditions: room temperature, relative humidity of 30%, initial tensile length of 20 mm, and drawing speed of 2 mm min1. Optical transmittance of the GO membrane was evaluated using a UV-vis-NIR spectrophotometer (UV-3600, Shimadzu, Japan, scanning rate 5 nm s1) and the membrane specimen with a thickness of 10 mm was preloaded on a solid sample holder before the measurements.

Acknowledgements The authors thank Prof. F. Li and L. B. Gao for their help with AFM observations, and Prof. J. Fan for her help in SEM observations. Q. H. Y. and the T. J. U. group also appreciate the financial support from the National Natural Science Foundation of China (No. 50842060), the NSF of Tianjin, China (No. 07JCYBJC15200), and the Program for New Century Excellent Talents in University (NCET-07-0607), Ministry of Education, P. R. China. Supporting information is available online from Wiley InterScience of from the authors. Received: December 17, 2008 Revised: February 7, 2009 Published online: April 20, 2009

Experimental GO was prepared by a modified Hummers method. While maintaining agitation, graphite powder (carbon content >98 wt%, 10 g) and sodium nitrate (5 g) were mixed with sulfuric acid (230 mL, 98 wt%) in an ice bath, and potassium permanganate (30 g) was slowly added to prevent the temperature from exceeding 293 K. The reaction was kept at 308 2 K for 30 min with gas release, and then deionized water (460 mL) was gradually added, bringing about violent effervescence. The temperature of the water bath was increased to 371 K and the reaction was maintained for 40 min in order to increase the oxidation degree of the GO product. The resultant bright-yellow suspension was diluted and further treated with a H2O2 solution (30 mL, 30%), followed by centrifugation and careful washing to clean out remnant salt. The wet GO was dewatered by vacuum drying (323 K). A GO suspension in water (2 mg mL1, 200 mL) was treated in an ultrasonic cleaner (KQ-100, frequency 40 kHz, output power 100 W) for 30 min, followed by high-speed centrifugation (5000 rpm, 20 min) to remove impurities, which only resulted in a slightly precipitation. The stable graphene oxide hydrosol was heated at 353 K for a short period (around 20 min for a 5 mm thick membrane and 40 min for a 10 mm thick membrane) in a thermostatted water bath, during which time a smooth and condensed thin film was formed very rapidly at the liquid/air interface. The undersurface suspension was then decanted into another beaker, and the membrane left at the bottom was dried at 353 K for 8 h and torn off. XRD measurements were performed for pristine graphite powder, GO powder, and membrane samples at room temperature using specular

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