highly compressible macroporous graphene monoliths via an improved hydrothermal process

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Highly Compressible Macroporous Graphene Monoliths via an Improved Hydrothermal Process

Yingru Li , Ji Chen , Liang Huang , Chun Li , Jong-Dal Hong , and Gaoquan Shi*

Y. R. Li, J. Chen, L. Huang, Dr C. Li, Prof. G. Q. Shi Department of ChemistryTsinghua University Beijing 100084 , P. R. China E-mail: [email protected] Prof. J.-D. Hong Department of ChemistryIncheon National University 406–772 , Incheon , South Korea

DOI: 10.1002/adma.201400657

n mg mL −1 ( n = 2, 3, 4, 5, or 7.5; volume ratio = 1:2) to form a graphene gel containing hexane ( Figure 1 , and Figure S1 and S2 in the Supporting Information). Then, the graphene gel was immersed in 80 °C de-ionized water to evaporate hexane. At the same time, water was fi lled into the pores of graphene monolith to keep its volume and the resulting hydrogel was freeze dried to form an aerogel. Finally, the graphene aerogel was annealed at 40 °C in an environment with 100% humidity and successively dried in air. During the annealing process, the walls of graphene pores became more wrinkled because of capillary effect (Figure S3a and S3b in the Supporting Informa-tion), while the d -space between graphene sheets did not show apparent changes according to X-ray diffraction measurements (Figure S3c, Supporting Information). The treated macropo-rous graphene monoliths are refered to as MGM- n (where n is the concentration of GO in mg mL −1 ).

The formation of macroporous structures of MGMs was mainly induced by the soft template of hexane droplets. Hexane droplets with diameters in the range of several tens of micrometers to about 200 µm can be stably dispersed in GO dispersion homogeneously [ 30 ] ( Figure 2 a). During hydro-thermal processing, GO sheets were reduced to conductive reduced graphene oxide (rGO) and formed a well-defi ned and interconnected 3D network around the hexane droplets. Here, we take MGM-5 as an example for morphological study. The pores of MGM-5 nearly kept the shapes of the hexane droplets and the resulting monolith was constructed by closed-cell dis-torted spherical pores like those of a polymeric foam (Figure 2b and c). The pore walls of MGM-5 are made of thin wrinkled rGO membrane (Figure 2 d). In comparation, the freeze-dried graphene hydrogel made from a pure aqueous dispersion of

Graphene is an atom-thick two-dimensional carbon material with excellent properties including high mechanical strength, high electrical and thermal conductivities, large specifi c surface area, and excellent stability. [ 1–3 ] Hence, it is a unique building block to construct three-dimensional (3D) macroscopic mate-rials for practical applications. [ 4–11 ] A variety of techniques, such as chemical vapor deposition over a porous template, [ 12,13 ] dip-coating on a porous framework, [ 14,15 ] and sol–gel reac-tions, [ 4,5,16–20 ] have been developed to fabricate graphene-based porous bulk materials. However, these materials are usually brittle with low compression resilience ratios. [ 12,16 ] Meanwhile, some elastic graphene materials are composites with skeletons of elastomeric polymers (e.g., polyurethane) or carbon nano-tube aerogel; thus, their elasticity is mainly attributed to their fi llers rather than graphene itself. [ 14,15 ] Elastic graphene mate-rials have also been prepared by using small molecules or ions as binding agents to enhance the strengths of graphene frame-works. [ 21,22 ] Recently, Li et al. successfully fabricated an elastic all-graphene monolith by directional freeze-drying an aqueous dispersion of partially reduced graphene oxide (PrGO). [ 23 ] In this case, the size and structure of PrGO sheets need to be carefully controlled to optimize their interactions. Following the idea of this excellent work, several graphene monoliths have been pre-pared. [ 24–26 ] However, the method developed by Li et al. involves complicated and critical procedures. Thus, it still remains a big challenge to fabricate elastomeric 3D porous graphene mate-rials via facile and scalable processes. Here, we report a modi-fi ed hydrothermal method to prepare macroporous graphene monoliths (MGMs) from an aqueous emulsion of graphene oxide (GO) containing hexane droplets. Similar to the processes of synthesizing porous materials with emulsion templates, [ 27–29 ] in our case, GO sheets were reduced and assembled around hexane droplets to form a 3D network. The resulting monoliths possess low density, good electrical conductivity, and excellent elasticity, exhibiting a potential application in pressure sensing.

MGMs were prepared by hydrothermal reduction of emulsions formed by shaking mixtures of hexane and an aqueous dispersion of GO at a concentration of

Adv. Mater. 2014, DOI: 10.1002/adma.201400657

Figure 1. Schematic illustration of the preparation of an MGM.

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5 mg mL −1 GO via hydrothermal reduction under the same condition (G-Aerogel-5) showed irregular pores (Figure 2 e). Its pore sizes and volume are much smaller than those of MGM-5 (Figure 2 e,f). The weight density of MGM-5 was measured to be about 8.6 mg cm −3 , and this value is much lower than that of G-Aerogel-5 (about 24.4 mg cm −3 ).

The morphology and weight density of an MGM can be modulated by the concentration of GO dispersion ( C GO ). When C GO = 2 mg mL −1 , the graphene gel has an irregular shape (Figure S4 in the Supporting Information); its top part is much thicker than its bottom part. This is mainly due to the number of GO sheets being insuffi cient to stabilize the O/W emulsion and most of the hexane droplets accumulating at the top part of the dispersion. As C GO increased to 3 mg mL −1 , a cylindrical graphene gel was obtained. However, this graphene gel showed a dramatic volume shrinkage upon freeze drying and mois-ture annealing to form MGM-3 (Figure S4 and S5 in the Sup-porting Information). If C GO ≥ 4 mg mL −1 , the resulting MGMs (e.g., MGM-4, -5, and -7.5) have volumes and shapes similar to those of their corresponding gel precursors (Figure S4), and their pores have similar sizes and diameter distributions

(Figure S5, Supporting Information). The specifi c surface areas (SSAs) of regular-shaped MGM- n ( n = 4, 5, 7.5) were measured to be in the range of 310 to 490 m 2 g −1 by the standard method of methylene blue absorption (Figure S6, Supporting Informa-tion). The gravimetric SSA decreases signifi cantly while the vol-umetric SSA increases sightly with the weight density of MGM- n . These results indicate that the morphologies of MGMs are mainly controlled by the soft hexane templates if C GO was suffi ciently high. These phenomena also suggest that the thicknesses of pore walls increased with the increase of C GO . MGM-4, -5, and -7.5 will be discussed in the following sections because of their regular shapes and stable microstructures; their weight densities increased in the same sequence ( Table 1 ).

The chemical structures of the MGMs were studied by Raman and X-ray photoelectron spectroscopy (XPS). The Raman spectrum of GO or MGM-5 has two remarkable bands at around 1340 and 1580 cm −1 and they are assigned to the D- and G-bands of carbon (Figure S7a in the Supporting Infor-mation). [ 31 ] The G-band is related to graphitic carbon and the D-band is associated with the structural defects or partially dis-ordered structures of graphitic domains. The intensity ratio of D- and G-bands for GO was calculated to be 0.89, while that of MGM-5 was increased to 1.13. These results indicate that the GO sheets were reduced to rGO and their conjugated structures were partly restored during the hydrothermal process. [ 32 ] The C 1s XPS spectra of GO and MGM-5 (Figure S7b, Supporting Information) refl ect the presence of four types of carbon bonds: C−C/C=C (284.6 eV), C−O (286.6 eV), C=O (287.8 eV), and O−C=O (289.0 eV). However, the bands associated with the oxy-genated groups in the spectrum of MGM are much weaker than those in the spectrum of GO. Actually, the C/O atomic ratio was increased from 2.55 for GO to 6.87 for MGM-5 (Table 1 and Figure S8, Supporting Information).

The MGMs are closed-cell cellular solids. Thus, their loading curves have similarities with those of usual closed-cell cellular solids such as cork and polymeric foams (see the Supporting Information). However, they possess unique features because of the high tensile strength of graphene sheets. The cyclic com-pression curves of MGM-5 with a maximum strain of 50% are shown in Figure 3 a and the pictures of the sample recorded during the loading/unloading process are demonstrated in Figure 3 b. In the fi rst cycle, the loading curve shows 3 regions: i) a nearly linear region from 0 to about 30%, where the com-pressive modulus was calculated to be about 28.3 kPa; ii) a successive short linear region with a steeper slope; and, iii) the slope of the loading curve showed a notable break down at the strain of about 36% followed by a linear increase. The fi rst linear region was much broader than that of a typ-ical closed-cell cellular solid (≤5%). [ 23 ] Moreover, the second region is a short steep line, not a broad and relatively fl at stress plateau as is shown by a usual closed-cell cellular solid. However, in this region, a densifi cation was not supposed to occur, because the maximum strain is far smaller than the theoretical ultimate strain: ε D (about 99%, see the Supporting Information). [ 33 ] These phenomena can be explained by the extraordinary high strength and modulus of graphene walls of the macropores. In the fi rst region, the large strain (about 30%) should induce heavily bending and buckling of gra-phene pore walls and the loading curve would be expected to


Figure 2. a) Optical microscope image of the emulsion made from the mixture of 2 mL hexane and 4 mL 5 mg mL −1 GO dispersion. b,c,d) Cross-sectional SEM images of MGM-5 at different magnifi cations; e) Cross-sectional SEM image of G-Aerogel-5. f) Photographs of MGM-5 (left) and G-Aerogel-5 (right).

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Nfl atten. However, the ultrahigh horizontal stress in gra-phene membrane made a signifi cant increase in compressive stress of MGM-5. The second steeper region is also possibly caused by the high membrane stresses of graphene pore walls. Although the tensile strength of graphene membrane is high, its elongation at break is smaller than that of an elastic polymer. Therefore, this region was short and followed by an infl ection point, indicating that the pore walls were torn and partially irreversibly damaged (insert of Figure 3 a). The tearing of pore walls reduced the stress of graphene walls; thus, the slope of region iii is smaller than that of region ii. Moreover, the irreversible damage also caused that the stress at a given strain in the second stress–strain cycle is lower than that of the fi rst cycle (Figure 3 c). The work used for carrying out the fi rst cycle was signifi cantly higher than that of each successive cycle (Figure 3 d). Tearing pore walls cost considerable energy, so the energy loss effi ciency in the fi rst cycle was also notably higher than that of the following every cycle (Figure 3 d). From the second cycle, the loading curves were nearly overlapped with each other and each of them showed two regions: a

quasi-linear region and another one with a steeper slope. This phenomenon indicates that no breakdown occurred in these cycles. The maximum stress decreased for only about 3%, and the total strain was lost for only about 4% and tended to be stable in the fi rst 10 cycles (Figure 3 c). All the unloading curves, including the fi rst cycle, were nearly identical, indicating that the energy stored in each cycle was unchanged. Energy absorp-tion is one of the key parameters of cellular solids. The MGM-5 showed excellent energy absorption ability. The energy loss coeffi cient of MGM-5 was measured to be 65% in the fi rst cycle and about 58% in each of following cycles (Figure 3 d).

The MGMs showed excellent mechanical elasticity. For example, MGM-5 can maintain its elastic property after 1000 loading/unloading cycles, while the stress at a given strain was decreased and the sample was also shortened for about 15% (Figure S9 in the Supporting Information). This result is surprising, because most existing carbon-based cellular solids exhibited brittle yield when subjected to a large deformation. [ 16 ] In addition, super-elasticity has not been observed in irregularly structured graphene foams or aerogels prepared by CVD or


Table 1. Properties of different MGMs.

Density [mg cm −3 ]

Conductivity [S m −1 ]

C/O ratio Maximum stress at 50% strain [kPa]

Compressive modulus [kPa]

MGM-4 6.73 0.48 6.53 14.05 16.83

MGM-5 8.60 0.75 6.86 20.66 28.33

MGM-7.5 12.32 1.76 6.73 31.94 42.29

Figure 3. Compressive properties of MGM-5. a) Compressive stress-strain curves of the fi rst 10 cycles. The inset is the SEM image of a pore of MGM-5 after the fi rst compression to 50%, scale bar = 50 µm. b) A set of real-time images of a compressed sample showing the recovering process. c) Reten-tion of maximum stress at 50% strain and total strain loss during the fi rst 10 compression cycles (calculated from the stress-strain curves shown in panel (a). d) Work done by compression and energy loss coeffi cient during the fi rst 10 compression cycles (calculated from the stress–strain curves shown in panel (a).

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gelation. Consulting the elastic graphene monoliths prepared based on ice chemistry, [ 23–26 ] the highly compressible properties and excellent elasticity of our MGMs are attributed to their unique macroporous structures. The structure of closed-cell cellular solid can be changed from perfect-ordered honeycomb-like structure to completely disordered foam or 3D network. Graphene monoliths prepared by directional freeze-drying has an ordered honeycomb-like structure, [ 23 ] while our MGMs have a disordered microstructure. Both materials have good elas-ticity, suggesting that the arrangement of pores is not a decisive factor for their elasticity. However, the pore structures of these two types of monoliths have some similarities. The diameters of the pores were large and the distributions of pore diameters were narrow. The pore walls are composed of multilayer of face-to-face oriented graphene sheets. The tightly stacking of graphene sheets maximizes their π–π interactions, allowing the graphene pores to restore to their initial shape after deforma-tion. In comparison with previous methods, [ 23–26 ] the improved hydrothermal process developed by us does not need any pre-treatments of GO precursors or require critical experimental conditions. Furthermore, the volumes, shapes, and masses of the MGMs can be easily controlled by experimental parameters and the costs of the monoliths are relatively low.

The mechanical properties of MGMs depend strongly on their weight densities; this is mainly due to the thicknesses of pore walls increasing with the weight density. Thicker walls will provide pores with higher stiffness and membrane ten-sile strengths. As shown in Figure 4 , the stress–strain curves of MGM-4, -5, and -7.5 are similar in their shape. However, the compressive modulus increased from 16.8 kPa for MGM-4 to 42.3 kPa for MGM-7.5 according to the corresponding fi rst regions of loading curves (Table 1 and Figure S10a in the Sup-porting Information). The maximum stress at a strain of 50% increased from 14.1 kPa for MGM-4 to 31.9 kPa for MGM-7.5 (Table 1 and Figure S10b in the Supporting Information). According to Figure S10, the compressive modulus or the max-imum stress at a strain of 50% has a nearly linear relationship with the weight density of MGMs. In the fi rst ten cycles, the maximum stress of MGM-4 or -7.5 also decreased a little and the retention was >95% (Figure S11 and S12, Supporting Infor-mation). Moreover, the energy loss effi ciencies of all MGMs

are close to each other (Figure S11 and S12, Supporting Infor-mation). It should be noted here that the mechanical proper-ties of our MGMs are comparable to those of other graphene-based elastic materials (Table S1, Supporting Information) or those of various cellular solids, such as metallic microlattices [ 34 ] and carbon nanotube foams. [ 35 ] These excellent mechanical properties provide MGMs with potential applications in shock damping and energy cushioning. [ 21,33 ]

The conductivity of a MGM increases with its weight den-sity (Table 1 and Figure S13 in the Supporting Information). Typically, MGM-5 has a conductivity of about 0.75 S m −1 , and this value is comparable to those of the other graphene aerogels prepared by hydrothermal reduction of GO. [ 16 ] Combining good electric conductivity and excellent elasticity, the MGMs are sug-gested to be applicable for sensing pressure deformations. To demonstrate this potential application, an MGM-5 sample was pretreated by compressing with 30% strain for 100 cycles until its height remained unchanged. The resistance response of MGM-5 to repeated compression cycles is shown in Figure 5 . The curve of Δ R / R 0 –strain is nearly a perfect parabola, where Δ R and R 0 are, respectively, the resistance response induced by deformation and the initial resistance of the sample. The


Figure 4. Compressive stress–strain curves of MGM-4, -5, and -7.5.

Figure 5. Electrical resistance changes upon repeatedly compression up to 30% strain. a) The relationship between Δ R / R 0 and compressive strain. b) Electrical resistance changes recorded in the fi rst 10 loading/unloading cycles.

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loading and the unloading curves are symmetrical, indicating the recovery rate is as fast as that of compression. The cycling stability was also tested and the resistance response of MGM-5 to compression was nearly unchanged during 300 loading/unloading cycles (Figure S14, Supporting Information).

We have developed a facile method to fabricate a new type of MGM. These MGMs have low weight densities, good elec-trical conductivity, and excellent elasticity with a rapid rate of recovery. Thus, they have important potential applications in shock damping, energy cushioning, pressure sensing, and graphene-based fi xable devices. Furthermore, this technique can be extended to fabricate other macroporous monoliths or composites by using different organic droplets as templates or fi lling additional hydrophobic functional components into the organic phases.

Experimental Section Synthesis and Purifi cation of GO : GO was prepared and purifi ed by

the oxidation of natural graphite powder (325 mesh, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) according to a modifi ed Hummers’ method [ 12 ] (see the Supporting Information).

Preparation of MGMs: Typically, 4 mL of 2, 3, 4, 5, or 7.5 mg mL −1 GO aqueous suspension was mixed with 2 mL hexane followed by shaking fi rmly to form a homogeneous mixture. The mixture was sealed in a 12 mL Tefl on autoclave and maintained at 180 °C for 10 h. Then the autoclave was naturally cooled to room temperature and the obtained gel with hexane inside their pores was taken out. The gel was immersed in 80 °C de-ionized water for 1 h to evaporate hexane and lead water to fi ll into the graphene pores. The graphene hydrogel was rapidly frozen by quenching in liquid nitrogen and then freeze dried to form an aerogel. Finally, the aerogel was treated at 40 °C in a chamber with 100% humidity for a week and successively dried in air to form an MGM.

Characterization: GO dispersions were freeze dried and used for morphological and structural characterizations. Raman spectra were recorded on a Renishaw R2000 microscopic Raman spectrometer with a 514 nm laser at a power density of 4.7 mW. X-ray photoelectron spectra (XPS) were taken out by using an ESCALAB 250XI photoelectron spectrometer (Thermo Fisher Scientifi c, USA). SEM was performed on a fi eld-emission scanning electron microscope (Sirion-200, Japan). Optical microscope images were obtained on an inverted fl uorescence microscope (Olympus IX71, Japan). X-ray diffraction was carried out on a D8 Advance X-ray diffractometer with Cu Kα radiation ( λ = 0.15418 nm, Bruker, Germany).

Compressive and Electric Tests: The compressive tests were carried out by using a model 3342 Instron Universal Testing Machine at a rate of 100% strain min −1 . The electric test was performed on a CHI 440D potentiostat-galvanostat (CH Instruments Inc. Shanghai, China). The electrical conductivity was measured by a two-electrode method and two copper sheets were used as the current collectors. To optimize the electrical contact between copper sheets and MGM, each end of the MGM sample was carefully affi xed to copper sheet with a thin layer of silver paste.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgments This work was supported by the National Basic Research Program of China (973 Program, 2012CB933402), the Natural Science Foundation of China (51161120361, 21274074), and the Basic Science Research Program through the National Research Foundation of Korea

(NRF) funded by the Ministry of Education, Science and Technology (20110015807).

Received: February 11, 2014 Revised: March 28, 2014

Published online:

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highly compressible macroporous graphene monoliths via an improved hydrothermal process

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