Reconstruction of Inherent Graphene Oxide Liquid Crystals for Large

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Reconstruction of Inherent Graphene Oxide Liquid Crystals for Large-Scale Fabrication of Structure-Intact Graphene Aerogel Bulk toward Practical Applications Hongsheng Yang,† Zengling Li,† Bing Lu,† Jian Gao,† Xuting Jin,† Guoqiang Sun,† Guofeng Zhang,† Panpan Zhang,‡ and Liangti Qu*,†,‡ †

Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, Key Laboratory of Cluster Science, Ministry of Education, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P.R. China ‡ Key Laboratory for Advanced Materials Processing Technology, Ministry of Education of China, State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, P.R. China S Supporting Information *

ABSTRACT: The inherently formed liquid crystals (LCs) of graphene oxide (GO) in aqueous dispersions severely restrict the fabrication of large-size and structure-intact graphene aerogel bulk by an industry-applicable method. Herein, by developing a surfactant-foaming sol−gel method to effectively disrupt and reconstruct the inherent GO LCs via microbubbles as templates, we achieve the large-size and structure-intact graphene hydrogel bulk (GHB). After simple freezing and air-drying, the resulting graphene aerogel bulk (GAB) with a structure-intact size of about 1 m2 exhibits a superelasticity of up to 99% compressive strain, ultralow density of 2.8 mg cm−3, and quick solar-thermal conversion ability. The modified GAB (GABTP) shows a high decomposition temperature (Tmax) of 735 °C in air and a low heat storage capacity. These excellent performances make the GABs suitable for many practical applications, as proven in this work, including as high compressive force absorbers, high absorption materials for oils or dangerous solvents, superior solar-thermal management materials for rapid heater or controlled shelter, and high-efficiency fire-resistant and thermal insulation materials. The whole preparation process is easily scalable and cost-effective for mass production of structureintact multifunctional graphene aerogel bulk toward practical applications. KEYWORDS: graphene aerogel bulk, structure-intact, ultralight, superelasticity, fire-resistant, solar-thermal management freeze-drying,4,13 sol−gel method,14−18,23 template-mediated solution assembly,20−22,31,32 and 3D printing,33,34 have been developed to prepare ultralight graphene aerogels. Among previous studies, the sol−gel method followed by air-drying is regarded as one of the most promising approaches for realizing the cost-effective and large-scale commercial production of graphene aerogels with superelasticity (>90% strain) and ultralow density (30 cm).

Herein, we develop a surfactant-foaming sol−gel method to effectively disrupt and reconstruct the GO LCs in dispersions via microbubble templates to obtain large-size and structureintact graphene hydrogel bulk (GHB). After simple freezing and air-drying, the resulting graphene aerogel bulk (GAB) exhibits a structure-intact size of about 1 m2, superelasticity of up to 99% compressive strain, ultralow density of 2.8 mg cm−3, and quick solar-thermal conversion ability. In addition, the GAB modified by hexachlorocyclotriphosphazene (HCTP), named GABTP, shows a high decomposition temperature (Tmax) of 735 °C in air and a temperature reduction rate much

tional nematic or lamellar phase when no special control exists.37,38,40−42 Unfortunately, the nematic or lamellar GO LCs are microscopically ordered but macroscopically disordered severely, especially on a large scale (e.g., meter size), which will severely destroy the homogeneity and integrity of graphene hydrogels for large-size samples and further hinder the successful preparation of graphene aerogel bulk after airdrying. It is still a major challenge to fabricate large-size and structure-intact graphene aerogel bulk via an industryapplicable method. B

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Figure 2. (a) Photographs of GAB prepared from GHB after simple freezing, air-drying, and annealing process in ambient atmosphere. (b) Photographs of a large-size and structure-intact GAB with an area of about 1 m2. (c) X-ray diffraction spectra of GO, U-GAB, and GAB. (d) High-magnification POM image of FGO solution. (e) POM images of FGO dispersion with different foaming expansion times. (f) POM images of FGO dispersions prepared from the 8 and 16 mg mL−1 GO dispersions with same foaming expansion ratio. (g) Scanning electron microscopy (SEM) images of the microstructures of GABs prepared from the 8 and 16 mgm L−1 GO dispersions. (h) SEM images of microstructures of a GAB (density, 3.2 mg cm−3) with increased magnification.

GO LCs in the whole surface of dispersions with a diameter of 40 cm could be observed clearly by the naked eye (Figure 1b). These results indicate that the inherent nematic GO LCs were actually regionally ordered but severely macroscopically disordered. The GO aqueous dispersion can be reduced into the prereduced GO hydrogel (PRGH) by reducing agents such as ascorbic acid.43 However, the wrinkles and cracks are always inevitable when the sample size becomes larger, for instance, over 30 cm in diameter (Figure 1c and Figure S1). This problem hinders the successful preparation of uniform and structure-intact PRGH with a huge size. Therefore, the elimination and reconstruction of the macroscopically heterogeneous GO LCs become a crucial step to solve this problem. In order to obtain large-size uniform GO LCs for entire dispersions, we introduced the microbubbles as templates to locate all of the GO sheets in the micron-size narrow spaces among them. An environment-friendly surfactant of alkyl polyglucoside was selected to foam the GO aqueous dispersion by rapid stirring to produce a mass of microbubbles. The POM image of the foamed GO (FGO)

faster than that of metal (e.g., nickel, copper) foams from the same high temperature in air. These excellent performances give GABs potential in many practical applications, which are proven as high compressive force absorbers, high absorption materials for oils or dangerous solvents, superior solar-thermal management materials for rapid heater or controlled shelter, and high-efficiency fire-resistant and thermal insulation materials. The whole preparation process is easily scalable and cost-effective for mass production.

RESULTS AND DISCUSSION GO LCs usually form spontaneously in the aqueous dispersion.37−39 Most of them show a conventional nematic or lamellar phase with orientation if without any special control.37,38,40−42 Figure 1a shows the microscopic image of a 4 mg mL−1 GO aqueous dispersion between the crossed polarizers by polarized-light optical microscopy (POM). It displays a stable birefringence and vivid schlieren texture characteristic of nematic GO LCs.37,38 The colorful textures of C

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the chemical reagents, such as ascorbic acid and alkyl polyglucoside, are almost nontoxic and pollution-free for the environment. The transformation from GO to the final GAB was investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The typical sharp diffraction peak of GO in XRD appears at around 10.28°, corresponding to the interlayer space (d) of 0.863 nm (Figure 2c). In contrast, a wide diffraction peak of the unannealed GAB (U-GAB) is located at 23.44° due to the restoration of the conjugation of sp2 regions and π−π stacking interactions during graphene self-assembly. The wide diffraction peak of GAB is at 24.21°, indicating that the d space further decreases from 0.387 to 0.376 nm by thermal treatment. U-GAB shows a new diffraction peaks at 11.26° (d = 0.789 nm), probably corresponding to the directional stacking alkyl polyglucoside molecules at the gas−liquid interface, and the diffraction peaks of GAB disappeared after thermal degradation of alkyl polyglucoside at 200 °C in air. After chemical reduction and annealing treatment, most of the oxygen functional groups are removed (Figure S8). In the Raman spectra (Figure S9), GO shows an ID/IG ratio of 1.15, whereas the value of GAB increases to 1.62 after reduction, further indicating that the π−π conjugated structure of graphene is partially restored in the as-prepared GAB. GO dispersions with alkyl polyglucoside surfactants were stirred vigorously to form homogeneous micron-size air bubbles. All GO sheets were settled around the microbubbles owing to the surfactant-induced relatively stable gas−liquid interfaces (Figure 2d).12−14 The volume (V) of the FGO dispersion was significantly larger than the initial GO dispersion (V0) because of the introduced air bubbles (Figure S10). The foaming expansion ratio (V/V0 from 1.5 to 2.5) and bubble sizes (from 200 to 50 μm) can be easily controlled by changing the volume of GO dispersion when the other conditions such as the dosage of alkyl polyglucoside, rotation speed of stirring, and vessel volume are kept constant (Figure 2e and Figure S10). Actually, the foaming expansion ratio depends on the introduced air volume, and the volume of the introduced air depends on the stirring intensity for the whole GO dispersion. Therefore, the foaming expansion and bubble sizes are easily tunable by this way (Figure S10). The concentration of the initial GO dispersion was controlled from 8 to 16 mg mL−1, and the foaming expansion ratio was adjusted to about 2.0 for the purpose of a robust GHB framework and ultralow density. The GO dispersion with low concentration (6 mg mL−1) was not suitable for the gelation process, and the 3D framework easily collapsed (Figure S11). The foaming expansion ratio was controlled according to the GO concentration (Figure S11). As is known, the air bubbles are not stable due to the Ostwald ripening. The chemical reduction of GO introduces the π−π stacking and agglomeration of graphene layers to form the 3D structures, which restrict the motion of bubbles. The high density and relatively low foaming expansion ratio will strengthen the 3D structures to enclose the bubbles for successful preparation of the hydrogels. Therefore, the uniform GHBs with the smaller pores and thinner graphene walls can be obtained by carefully adjusting the densities and foaming expansion of the GO dispersions (Figures S12 and S13). If the foaming expansion ratio is similar (2.0), the size of bubbles in FGO from 8 mg mL−1 GO dispersion is smaller than that from 16 mg mL−1 GO dispersion (Figure 2f). As shown in Figure 2g, the GABs have typical bubble-filled porous structures (diameter of ca. 100−300 μm), which is very homogeneous for

dispersion (Figure 1d) shows a homogeneous color with abundant circle patterns, indicating the formation of uniform GO LCs and existence of homogeneous microbubbles within the FGO dispersion. The formation of homogeneous FGO dispersions on a large scale with a diameter of 40 cm is shown in Figure 1e. The FGO dispersion was thoroughly reduced by ascorbic acid at 80 °C for 12 h to form the GHB (Figure 1f). The smooth and intact GHB (Figure S2) demonstrates that the large-scale reconstruction of GO LCs by the surfactantfoaming strategy indeed effectively avoided the occurrence of wrinkles and cracks in GHBs. Observing the surfaces of GO aqueous dispersions with diameters of 5, 10, 20, 30, and 40 cm, we found the colorful texture patterns become increasingly obvious and heterogeneous with the increase of dispersion area (Figure S3a). Different orientations of GO LCs usually reflect different visual colors because of their special optical response.37,38,44 Stripelike patterns usually mean the different orientations of GO LCs.37,38 Thus, the GO dispersion with larger size has more macroscopically disordered GO LCs. A vital boundary effect may significantly influence the GO sheet arrangement, and the GO sheets would be more free to arrange in different directions for the GO dispersion with larger size.40−42 As is known, LC is the mesomorphic ordered state of anisotropic particles that bears liquid-like fluidity as well as crystal-like ordering.44 The liquid-like fluidity of GO LCs can be proven by the color-varied textures of GO LCs after every oscillation for the dispersions placed between crossed polarizers (Figure S4). This feature can also be observed clearly by the naked eye without the crossed polarizers when the size of the GO dispersion is large enough (e.g., diameter >30 cm). Figure S5 shows the texture varied with the glass rod stirring. When the GO dispersion was dropwise added into the stripe-like surface of a large-area GO dispersion, the circle-like patterns existed in the stripe-like surface changed the orientation of GO LCs (Figure S6). These results demonstrate that the liquid-like fluidity of GO LCs are amplified on a large scale, which significantly enhances the macroscopic disorder of GO LCs (Figure S3a) and shows a strong correlation with the different wrinkled and cracked PRGHs (Figure S1). The gelation process of GO dispersion can be described as the self-assembly of prereduced GO sheets due to π−π interaction and hydrogen bonding effect.45 Therefore, the gelation process of GO dispersions for a large size is illustrated in Figure 1g. The wrinkles and cracks of hydrogels most likely occurred in the borders between two distinctly different domains of GO sheet arrays (Figure 1g and Figure S7). Indeed, large cracks are observed with the size increase of GO dispersions due to the macroscopic heterogeneity of GO LCs (Figure S3b). For the FGO dispersion, all GO sheets were confined within the microbubble gaps, forming the large-scale uniform GO LCs, as illustrated in Figure 1h. As a result, the GHBs with large size (e.g., diameter >30 cm) are very smooth and intact. The inherent heterogeneous GO LCs are reconstructed to form the large-size homogeneous dispersion of GO, which really benefits the preparation of large and structure-intact GHBs. After simple freezing (−18 °C), air-drying (60 °C), and annealing (200 °C) in air, the GHB was further turned into the smooth and intact GAB without any obvious shrinkage (Figure 2a). Through this method, the GAB is easily enlarged with a huge size of about 1 m2, which is the largest 3D graphene reported in literature so far (Figure 2b and Table S1). All the processing techniques are easily scalable and cost-effective, and D

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Figure 3. (a) GAB sample (4.6 mg cm−3) with a diameter of 34 cm can support a person with weight of 51 kg at 40% strain and quickly recover to its original height. (b) Stress−strain curves of the U-GAB at 50% strain for three cycles. (c) Stress−strain curves of the GAB (2.8 mg cm−3) at 99% strain for three cycles. Inset: Enlarged stress−strain curves of GAB at low strain. (d,e) Stress−strain curves (d) and stress− strain recovery ratio (e) of GAB (4.3 mg cm−3) at 70% strain for 1000 cycles. (f) Compressive force curve of GAB (4.3 mg cm−3) as a function of sample diameters. (g) Electrical resistance variation of GAB when repeatedly compressed up to 60% strain for 20 cycles. (h) Images of the fast removal process of cyclohexane (dyed with oil red O) floating on water using the as-prepared GAB. (i) Adsorption capacities of GAB in terms of weight gain. The inset image shows the burning recycle process of GAB after absorbing combustible oils.

the whole vertical section of the GAB (Figure S14). Moreover, the walls of GAB prepared from 16 mg mL−1 GO dispersion were obviously thicker than that derived from 8 mg mL−1 (Figure 2g). The ultralow density of GAB is tunable from 2.8 to 6.3 mg cm−3 with initial GO concentration from 8 to 16 mg mL−1 (Figure S15a). More importantly, the density of GAB prepared from FGO is only about 40% of that prepared from GO with the same concentration because the reduced FGO hydrogel has a larger volume than the reduced GO hydrogel, which is attributed to the hindering effect of air bubbles during the volume shrinkage of hydrogels (Figure S15b). Enlarged

views of scanning electron microscopy (SEM) images (Figure 2h) clearly demonstrate that reduced graphene sheets are densely assembled around the air bubbles, forming the closely packed interconnected graphene walls with a thickness of ∼40 nm, which possess a BET surface area of 14.5 m2 g−1 and the pore volume of 0.03 cm3 g−1 (Figure S16). In addition, there are some holes on the graphene walls owing to the ice-template recasting effect.14−16,46 Therefore, the continuous open-cell porous microstructures with large pores and strong walls enable the GHBs to be dried without detectable shrinking in ambient conditions.14−16 E

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Figure 4. (a,b) Infrared images recording the temperature variation of the GAB with a density of about 6 mg cm−3 (a) and Cu foam (b) under 1 sun solar illumination as a function of time. The surface temperature of GAB was up to 89 °C, and the Cu foam was only 40 °C after 60 s 1 sun solar illumination. (c) Infrared images recording the temperature variation of a Cu foam when being put on the GAB surface under 1 sun solar illumination as a function of time. The temperature of the Cu foam increased to 50 °C from 40 °C as a result of the GAB heating effect. (d) Infrared images recording the temperature variation of the Cu foam as it was exposed to solar light for 60 s and then covered by the GAB for 60 s under 1 sun solar illumination. The temperature of the Cu foam decreased to 30 °C from 40 °C as a result of the GAB sheltering effect. (e−g) Infrared images recording the temperatures of the surfaces covered by a 1.5, 2.5, and 5.5 cm height GAB, respectively, for 60 s under 1 sun solar illumination. The temperatures of the surfaces covered by the GAB decreased when the height of the GAB increased.

diameter at 50% compressive strain. It can be found that the compressive force of a large GAB with 30 cm diameter is up to 700 N. The GAB also shows a cyclic stable relationship between electrical resistance and deformation (Figure 3g), which is potential as a pressure sensor.20−22 Consequently, this superelastic GAB can be used as the high compressive force absorber or sensor in practical applications. The ultralight and highly porous GAB possesses hydrophobic nature (Figure S20) and solvent resistance, which make it an excellent absorber for most organic solvents.4,13,47−50 Figure 3h and Movie S2 show the cyclohexane dyed by oil red O floating on the water can be quickly and completely absorbed by a GAB sample to achieve the oil−water separation. As illustrated in Figure 3i, the ultralow density (2.8 mg cm−3) and continuous open-cell microstructure endow the GAB with high capacity (260−570 times its own weight) for collection of oils or organic solvents such as DMF, methylbenzene, THF, pump oil, etc. This capability is higher than that of many graphene-based aerogels and close to the best capacity reported previously.13,14,16,47 The absorbed oil or organic solvents can be removed by burning (inset of Figure 3i) or recycled by distilling or squeezing based on the robust and elastic framework of the GAB.4,14,47 The morphology and structure of the GABs are still maintained after burning the adsorbed oil (Figure S21). In addition, the intact graphene aerogel with large size is not only an excellent absorber but also

Figure 3a−g shows the excellent mechanical properties of GAB. The residual alkyl polyglucoside and ascorbic acid restrict the recovery of U-GAB, and the elasticity of GAB can be fully restored just by a thermal treatment at 200 °C in air (Figure S17 and Movie S1). As illustrated in Figure 3a, the asprepared GAB can even support a person (ca. 51 kg) with only about 40% compressive strain with a good recovery. The three cyclic stress−strain curves of U-GAB at 50% strain show its poor elasticity (Figure 3b).16 In contrast, the GAB even compressed to 99% strain can completely come back, as illustrated in Figure 3c and Figure S13c, which is the largest value compared with other works (Figure S18 and Table S1). Moreover, the durability test shows that 99.3% of height and 96% of maximum stress could be kept after 1000 consecutive compression cycles at 70% strain (Figure 3d,e). The superelasticity of GAB can be attributed to its interconnected porous elastic graphene nanowalls. The stress−strain curve of the GAB exhibits a typical viscoelastic behavior with a hysteresis loop between the loading and unloading curves. Therefore, it can be used as a mechanical energy absorption material.14,15,23 Generally, the GAB samples with increased diameters can support the same force at decreased compressive strain (Figure S19). The GAB with a large size is more suitable for the high force energy absorption than the small one even though they have the same stress. Figure 3f shows the corresponding compressive force as a function of the GAB F

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Figure 5. (a) Mass loss as a function of the burning time for GAB and GABTP on the flame of alcohol lamp. (b) TGA curves of HCTP, GAB, GABTP, and GABTPF conducted under air atmosphere. (c) GABTP protects the paper crane from the flame of the alcohol lamp for 10 min. (d) Photographs of a GABTP, Ni foam, and Cu foam on the alcohol flame (left) and infrared images (right) recording the temperatures of GABTP, Ni foam, and Cu foam when heated by the alcohol flame for the same time and then cooled to room temperature in air atmosphere.

a good barrier for large-scale spill accidents of hypertoxic or corrosive solvent in practical applications. Graphene aerogels with high porosity, outstanding light absorption, excellent photothermal transduction, and super stability in many extreme conditions are one some of the most attractive candidates for efficient solar-thermal conversion materials.28,51,52 Herein, the structure-intact GAB with large size has good solar-thermal management property. Thermographic recordings (Figure 4a and Movie S3) show that GAB has a high surface temperature of 89 °C after only 60 s solar illumination under 1 sun, confirming the highly efficient solarthermal conversion ability.28 The solar converted heat of GAB remains dynamically stable after 60 s (Figure S22). The thermally reduced GO aerogels with small pores prepared by a liquid nitrogen freeze-drying method are introduced to compare the solar-thermal conversion ability of GABs with large pores (Figure S23). It seems that the pore size might not be the major factor for solar-thermal conversion. The temperatures on different surfaces of the GAB sample are different, as shown in Figure S24. The parallel graphene layers

on the bottom surface with metallic luster might reflect the light and reduce the solar absorption ability. Thus, the blackness of graphene aerogels is also important for the solar-thermal conversion, which are tunable by controlling the orientation of graphene sheets. The surface of GAB with a large area can be used as the rapid solar-thermal heater to reach a high temperature. For instance, the average temperature of Cu foam was about 40 °C after 60 s solar illumination of 1 sun (Figure 4b and Movie S3), and it increased to about 50 °C when the Cu foam was put on the surface of a GAB sample (Figure 4c and Movie S3). Because of the large-size and structure-intact characteristics of GABs with ultralow density, it can even be used as the shelter to protect something from much solar radiant heat. For example, the average temperature (ca. 40 °C) of Cu foam under 1 sun solar illumination decreased to about 30 °C when a 2.5 cm thick GAB covered on the Cu foam for 60 s, although the surface of the GAB had a high temperature (88 °C) (Figure 4d and Movie S4). This result reveals that the GAB with centimeterlevel thickness can effectively dissipate the solar heat into the G

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thinner 3D walls (Figure S30). This result reveals that the prepared GABTP with the ultralow density and highly porous framework only stores very little heat and rapidly disperses in air, even though its initial temperature is very high. This largesize and structure-intact GABTP can be applied in practical situations, requiring ultralight and high-efficiency fire-resistant and thermal insulation materials.

surrounding air, and the surface of the GAB without illumination (dark side) is much cooler than the exposed surface of GAB under light. Importantly, the thickness of GAB determines its sheltering or heating ability of the dark side under solar illumination, and the temperature of the surface covered by GAB gradually decreased with the thickness of GAB increasing, as illustrated in Figure 4e−g. Therefore, the structure-intact GAB with large size has excellent solar-thermal conversion ability and can be used as a rapid solar heater in the exposed surface or controlled solar heat shelter on the unexposed surface, displaying versatile practicability as solarthermal management material.53 Graphene aerogels have great potential to be used as multifunctional ultralight fire-retardant materials.25,54 For practical applications, it has been a great challenge until now to prepare large-size and structure-intact GABs with highly efficient flame resistance by industrialized methods.25,54 Here, we developed a simple method to functionalize the GABs by absorbing functional molecule dispersions and then air-drying (Figure S25). The large size and structure-intact properties of GAB modified by HCTP (GABTP) will endow it with superior practicability as ultralight fire-resistant materials (Figure S26). The thermal stability of GAB and GABTP was assessed by the mass loss as a function of the burning time on an alcohol flame (Figure 5a). After 60 min, GABTP still kept about 65% mass and most of the volume (Figure S27a), whereas GAB lost 100% of its initial mass in just 2 min. Figure 5b presents the thermogravimetric analysis (TGA) curves of HCTP, GAB, GABTP, and GABTP after flame burning (GABTPF) measured in air with a heating rate of 10 °C min−1. As can be seen, the GABTPF has a decomposition temperature higher than those of GAB and HCTP. The GABTP shows two distinct decomposition temperatures. The GABTP and GABTPF have the same Tmax (the temperature of maximum mass loss rate) of about 735 °C, which is much higher than that of GAB (454 °C) by ca. 280 °C (Figure S27b). These results indicate the GAB and HCTP, which compose the GABTP, would react and significantly enhance the heat resistance of GABTP when it was heated at a high temperature or burnt by a flame. The corresponding energy-dispersive spectroscopy (EDS) analysis and element mapping images of GAB, GABTP, and GABTPF are shown in Figures S28 and S29. The P and Cl elements coexist and are evenly distributed over the GABTP, revealing the HCTP molecules were uniformly introduced. After flame burning, the GABTPF lost almost all the Cl element, but most of P element was still existent. It reveals that the doping of P element can prevent the graphene sheets from serious oxygen etching, which has been widely regarded as the critical factor for flame resistance.25 The GABTP with a thickness of ca. 1.2 cm can effectively protect the paper crane from the burning flame for at least 10 min (Figure 5c and Movie S4). This indicates that the GABTP is not only a good fire-resistant material but also an excellent thermal insulation material due to its ultralow density and highly porous structure.54,55 As illustrated in Figure 5d and Movie S5, we compared the temperature reduction rate of GABTP in ambient atmosphere with that of nickel (Ni) and copper (Cu) foams by recording the used time of every material from the same high temperature to room temperature. It was found that the GABTP used much less time (just 3 s) than the Ni or Cu foam (ca. 10 s), indicating the GABTP has faster reduction rate for material than the common metal foams, which was attributed to its much lower density and

CONCLUSION In summary, a surfactant-foaming sol−gel method followed by air-drying technique has been developed to prepare structureintact GABs with a size of up to about 1 m2, which have a superelasticity (up to 99% strain), ultralow density (low to 2.8 mg cm−3), quick solar-thermal conversion ability, high thermal decomposition temperature (Tmax = 735 °C) in air, and low heat storage capacity. The effective reconstruction of GO LCs via microbubbles as templates is the key step to obtain largesize and structure-intact GHBs and GABs. The final GABs with superior performances and large sizes have been proven to have great potentials in practical applications as high compressive force absorbers (force = >700 N), high absorption materials for oils or dangerous solvents (capacity = 260−570 times of their own weight), outstanding solar-thermal management materials for rapid heating or controlled shelter, and ultralight fire-resistant materials. The whole preparation process is easily scalable and cost-effective for mass preparation of graphene aerogel bulk toward practical applications. EXPERIMENTAL SECTION Synthesis of GO. GO aqueous dispersion (8−16 mg mL−1) was prepared from natural graphite powder (325 mesh) according to a modified Hummers method as reported in a previous paper.14 Preparation of PRGH and GHB. Typically, 3000 mL of GO aqueous dispersion (4 mg mL−1) with 24 g of ascorbic acid was poured into the reaction vessel (diameter = 40 cm) with a cover and reacted at 80 °C for 1 h to obtain the large wrinkled and cracked PRGH. Fifteen hundred milliliters of GO aqueous dispersion (8 mg mL−1) with 24 g of ascorbic acid and 24 mL of alkyl polyglucoside (50 wt %) was foamed in a 7500 mL bucket under stirring at 2500 rpm for 3 min. The prepared FGO dispersion was poured into the reaction vessel (diameter = 40 cm) with cover and reacted at 80 °C for 12 h to obtain large smooth and intact GHBs with a diameter of about 34 cm. Preparation of U-GAB and GAB. The prepared GHB was frozen completely at −18 °C for about 4−6 h and then completely thawed at 60 °C in air. The GHB was washed with DI water 1−3 times to remove most residual ascorbic acid and then dried at 60 °C in air for about 12 h to obtain U-GAB. The U-GAB was annealed at 200 °C in air for 4 h to obtain the GAB. Preparation of GABTP and GABTPF. The GAB absorbed the HCTP (P3N3Cl6) alcohol solution (mass ratio, HCTP/GAB = 1:10) and then dried at 60 °C in air to obtain GABTP. The GABTP was burned on the alcohol flame for about 10 s to obtain the GABTPF. Characterizations. The POM observations were performed with the Axiocam 506 color, and the liquid samples were loaded between the glass slides. The SEM observations were performed with the JSM7500F at an accelerating voltage of 5 kV. The density of GAB was calculated according to the weight and volume of a cylindrical sample. The XRD patterns were conducted at room temperature using a Netherlands 1710 diffractometer (λ = 1.54 Å). The Raman spectra were recorded on a Horiba JY HR-800 Raman spectrometer with an excitation wavelength of 633 nm. The XPS data were obtained with an ESCALab220i-XL electron spectrometer. The mechanical compression test was conducted on a Shimadzu AGS-X. The resistance variation was measured by a Keithley 2612 sourcemeter. The temperature was measured using a Fluke IR camera. The TGA H

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ACS Nano was performed on a Netzsch STA 449C under atmospheric conditions at a heating rate of 10 °C min−1. The elemental mappings were obtained using a scanning transmission electron microscope unit with high-angle annular dark-field detector (Hitachi S-5500) operating at 30 kV.

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b06380. Photographs of the GO dispersions, FGO dispersions, PRGHs, GHBs with different size; microstructures of GABs, density curve, BET analysis, compression and recovery process, supporting the weight, hydrophobicity, temperature variation under solar illumination; modifying process, fire-resistant; DTG curves; EDS curves and mapping images; a table summarizing the typical 3D graphene (PDF) Movie S1: compress and recover (AVI) Movie S2: oil−water separation (AVI) Movie S3: solar−thermal management (AVI) Movie S4: protecting the paper crane from the burning (AVI) Movie S5: cooling process (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] or [email protected]. ORCID

Liangti Qu: 0000-0002-0161-3816 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the financial support from the National Key R&D Program of China (2017YFB1104300, 2016YFA0200200), NSFC (Nos. 51673026, 51433005), NSFC-MAECI (51861135202), Beijing Municipal Science and Technology Commission (Z161100002116022), and China Postdoctoral Science Foundation (2016M600077, 2017T100062). REFERENCES (1) Novoselov, K. S.; Geim, A. K.; Morozov, S.; Jiang, D.; Katsnelson, M.; Grigorieva, I.; Dubonos, S. V.; Firsov, A. A. TwoDimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197−200. (2) Li, D.; Kaner, R. B. Graphene-Based Materials. Science 2008, 320, 1170−1171. (3) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (4) Zhao, Y.; Hu, C.; Hu, Y.; Cheng, H.; Shi, G.; Qu, L. A Versatile, Ultralight, Nitrogen-Doped Graphene Framework. Angew. Chem., Int. Ed. 2012, 51, 11371−11375. (5) Yang, Y.; Han, C.; Jiang, B.; Iocozzia, J.; He, C.; Shi, D.; Jiang, T.; Lin, Z. Graphene-Based Materials with Tailored Nanostructures for Energy Conversion and Storage. Mater. Sci. Eng., R 2016, 102, 1− 72. (6) Hu, Q.; Liu, X.; Zhu, B.; Fan, L.; Chai, X.; Zhang, Q.; Liu, J.; He, C.; Lin, Z. Crafting MoC2-Doped Bimetallic Alloy Nanoparticles Encapsulated within N-Doped Graphene as Roust Bifunctional Electrocatalysts for Overall Water Splitting. Nano Energy 2018, 50, 212−219. I

DOI: 10.1021/acsnano.8b06380 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

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DOI: 10.1021/acsnano.8b06380 ACS Nano XXXX, XXX, XXX−XXX