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Biomimetic Architectured Graphene Aerogel with Exceptional Strength and Resilience Miao Yang,†,§ Nifang Zhao,†,§ Ying Cui,† Weiwei Gao,‡ Qian Zhao,† Chao Gao,‡ Hao Bai,*,† and Tao Xie† †
State Key Laboratory of Chemical Engineering, College of Chemical and Biological Engineering and ‡Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: Materials combining lightweight, robust mechanical performances, and multifunctionality are highly desirable for engineering applications. Graphene aerogels have emerged as attractive candidates. Despite recent progresses, the bottleneck remains how to simultaneously achieve both strength and resilience. While multiscale architecture designs may offer a possible route, the difficulty lies in the lack of design guidelines and how to experimentally achieve the necessary structure control over multiple length scales. The latter is even more challenging when manufacturing scalability is taken into account. The Thalia dealbata stem is a naturally porous material that is lightweight, strong, and resilient, owing to its architecture with threedimensional (3D) interconnected lamellar layers. Inspired by such, we assemble graphene oxide (GO) sheets into a similar architecture using a bidirectional freezing technique. Subsequent freeze-drying and thermal reduction results in graphene aerogels with highly tunable 3D architectures, consequently an unusual combination of strength and resilience. With their additional electrical conductivity, these graphene aerogels are potentially useful for mechanically switchable electronics. Beyond such, our study establishes bidirectional freezing as a general method to achieve multiscale architectural control in a scalable manner that can be extended to many other material systems. KEYWORDS: aerogel, biomimetic, plant stem, bidirectional freezing, mechanical performance
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hierarchical architectures ranging from nano/micro to macroscopic levels, leading to outstanding mechanical robustness despite being porous and made of weak constituents. For example, the stem of Thalia dealbata has an architecture with lamellar layers interconnected by bridges, which makes it strong, flexible, and lightweight, for surviving in frequent wild wind. Here, inspired by the Thalia dealbata stem, we applied a bidirectional freezing technique19,20 to use ice as a template in assembling GO sheets into a biomimetic architectured 3D aerogel. Such aerogels not only mimic the architectural features of the plant stem but also are exceptionally strong and resilient at the same time. Specifically, the graphene aerogel monolith can support more than 6000 times its own weight with around 50% strain, indicating its high specific strength. More importantly, it shows robust compressive properties. After 1000 compressive cycles at 50% strain, it still retains around 85% of its original compressive strength. Besides the graphene aerogel itself, we are proposing a general strategy for improving mechanical performance of porous materials, particularly
erogels have a wide range of applications in the areas such as thermal insulation, mechanical damping, pollution control, and catalyst support, owing to their ultralow density/thermal conductivity, high energy absorbability, and so on.1,2 To this end, exceptional strength and resilience are greatly demanded by aerogels, despite they are mutually exclusive and challenging to simultaneously achieve. Graphene aerogels, in particular, are suffering from such difficulty for their potential applications in actuator, sensor, battery, supercapacitor and other flexible electronic devices.3,4 Some techniques, including three-dimensional (3D) printing, chemical vapor deposition (CVD), hydrothermal process, and freeze-drying,5−15 were developed to tackle this problem. However, very few attempts were successful in fabricating graphene aerogels with highly ordered architectures,6,10 with most of them having only random porous structures. As we are still incapable of efficiently and precisely assembling graphene oxide (GO) sheets into architectures which are more carefully designed at multiple scales, it still remains a grand challenge to fabricate robust graphene aerogels with exceptional strength and resilience. Nature always provides a rich source of inspirations for materials design and development.16,17 Many natural porous materials, such as plant stems, are slender but strong,18 which can be attributed to their structures arranged in sophisticated © 2017 American Chemical Society
Received: March 15, 2017 Accepted: June 6, 2017 Published: June 21, 2017 6817
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Figure 1. Architecture of the Thalia dealbata stem and biomimetic graphene aerogel. (a) Optical image of a Thalia dealbata stem. (b and c) Optical and SEM images showing the multiscale architecture, where oriented lamellar layers (thickness: ∼100 μm) are connected by interlayer bridges (length: ∼1 mm). (d) In a bidirectional freezing technique, a low thermal conductive wedge is first placed between the GO/PVA suspension and cooling stage. When freezing, it simultaneously generates horizontal (ΔTH) and vertical (ΔTv) temperature gradients. (e) The schematic of the as-prepared graphene aerogel with plant stem-like architecture. (f) SEM image showing the detailed architecture of the biomimetic graphene aerogel, where lamellae are connected by interlayer bridges, observing from the cross-section parallel to the cooling stage after bidirectional freezing and thermal reduction. (g) Optical images showing a cubic aerogel sample (10 × 10 × 10 mm) (h) which supports >6000 times of its own weight with around 50% strain. (i) Full recovery with no obvious permanent deformation when unloading. (j) Representative stress−strain curves and (k) strength recovery ratio of an aerogel compressed (strain = 50%) and recovered after 1000 cycles, indicating its high resilience.
viscosity, slope angle of the PDMS wedge, cooling rate, and so on.19,22,23 Bidirectional freezing technique is a time-efficient, environmentally friendly method to make highly ordered porous materials with different constituents: ceramic/metal particles, polymers, and their composites. In this work, we use GO and poly(vinyl alcohol) (PVA) as a proof of concept toward robust graphene aerogel. Figure 1 summarizes the architectures of the Thalia dealbata stem and corresponding biomimetic graphene aerogel that has exceptional strength and resilience. The optical image in Figure 1a shows the appearance and microstructure of Thalia dealbata, a perennial aquatic plant which belongs to Marantaceae and is native to South America and the tropics of Mexico. Its porous stem is typically ∼2 m tall and 6−10 mm thin. With such a large height/diameter ratio (∼200−350), the stem has to be strong and resilient to survive in the native environment with frequent wild wind. From the optical (Figure 1b) and SEM images (Figure 1c) of the stem, an architecture of ‘oriented lamellar layers parallel to the growth direction with interconnected bridges’ can be observed. The thickness of the lamellae was about 100−200 μm, while the length of the
strength and resilience, by mimicking highly ordered natural architectures.
RESULTS AND DISCUSSION Fabrication of Biomimetic Graphene Aerogel. Recently, Bai et al. have developed and applied a bidirectional freezing technique to fabricate a long-range (centimeter scale) ordered 3D structure,19 which provided a higher level of control over hierarchical architectures of materials and in turn excellent mechanical performances.10,20,21 This was achieved by introducing a polydimethylsiloxane (PDMS) wedge with a slope angle of around 15° between the cooling stage and precursor suspension, which can generate dual temperature gradients (both horizontal (ΔTH) and vertical (ΔTV)) during freezing. In the bidirectional freezing technique, the ice crystals serve as templates for the final structures of biomimetic graphene aerogels after sublimation and thermal reduction. The nucleation and growth of ice crystals and thus final architectures, e.g., microstructure, porosity, and orientation, can be tuned by parameters such as suspension concentration/ 6818
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Figure 2. Comparison between typical biomimetic and random graphene aerogels in their compression−recovery behaviors and microstructures. (a and b) When compressed to 50%, the maximum stresses of the biomimetic and random aerogels are 5.0 and 4.5 kPa, respectively. (c) The biomimetic and random aerogel retains 92% and 45% of its original strength after 100 and 10 cycles, respectively. (d and e) When compressed to 90%, the maximum stresses of the biomimetic and random aerogels are 134.1 and 123.1 kPa, respectively. (f) The biomimetic and random aerogel retains 77% and 36% of its original strength after 100 and 10 cycles, respectively. (g and l) SEM images of (g) biomimetic and (j) random graphene aerogels before compression, with oriented pores (thickness: about 1−2 μm; bridge length: 20−30 μm) and random pores (diameter: about 20−30 μm), respectively. Magnified SEM images showing the structure variation of the corresponding aerogels (h and k) before and (i and l) after 100 cycles of compression (strain = 50%) and recovery. Inserts show the corresponding schematics. All of these indicate that the biomimetic aerogels are exceptionally strong and resilient.
Information, Figure S2), a graphene aerogel (GO/PVA = 8/8 mg/mL) with lamellar layers (thickness: 1−5 μm) and interconnected bridges (length: 10−30 μm) was obtained after freeze-drying and thermal reduction, mimicking the architectural features of the Thalia dealbata stem (Figure 1e− f). As shown by the SEM images, the graphene aerogel contained large amounts of large pores, which had little contribution to the specific surface area measurement based on
bridges was 1−1.2 mm. We then used the bidirectional freezing technique19 to duplicate such hierarchical architecture in graphene aerogels. As shown in Figure 1d, ice crystals first nucleated at the lowest line of the PDMS wedge and then grew under both vertical and horizontal temperature gradients. By carefully adjusting the slope angle of PDMS wedge (0°, 5°, 10°, and 15°) (Supporting Information, Figure S1) and composition/viscosity of the GO/PVA suspension (Supporting 6819
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Figure 3. Comparison of microstructures and compressive behaviors of biomimetic graphene aerogels with different numbers of lamellae and bridges. (a−d) SEM images show the cross sections parallel to the cooling stage of biomimetic aerogels prepared with suspensions containing 5, 8, 10, and 15 mg/mL GO (with fixed GO/PVA ratio). All the samples show lamellar structures with interconnected bridges. (e−h) Representative stress−strain curves for 100 cycles of compressive tests at 50% strain. The maximum stresses are 2.0, 5.0, 8.4, and 25.6 kPa, respectively. Inserts show the corresponding schematics for different architectures. (i) The number of lamellae first increases and then decreases with density, while the number of bridges increases monotonously. (j) The strength recovery ratio first increases and then decreases with density, while the strength increases monotonously. All the samples were prepared at the same freezing and reduction condition.
the Brunauer−Emmett−Teller (BET) equation. The specific surface area was measured to be 45.57 m2/g. According to the Barrett−Joyner−Halenda (BJH) method, much of the pore volume (0.11 cm3/g) lies in the 3.5−100 nm range for the pore size distribution, with a sharp peak at 3.8 nm and a broad peak at 30.2 nm (Supporting Information, Figure S3). The chemical composition of the reduced graphene aerogel was also measured by energy dispersive spectrometer (EDS) (Supporting Information, Table S1). The microstructure of the aerogel barely changed before and after thermal reduction (Supporting Information, Figure S4). Note that the dimension of microstructure in the biomimetic aerogel was much smaller compared to that of the plant stem. As the same with plant stems, a long-range ordered (centimeter scale) lamellar structure was beneficial for the mechanical performance. A cubic biomimetic graphene aerogel monolith (10 × 10 × 10 mm, limited by the mold size) is shown in Figure 1g. It can support more than 6000 times of its own weight with around 50% strain, indicating its high specific strength (Figure 1h). When unloading, the aerogel fully recovered with no obvious
permanent deformation (Figure 1i). More importantly, it shows robust compressive properties. After 1000 compressive cycles at 50% strain (Figure 1j), it still retains around 85% of its original compressive strength (Figure 1k). These primarily show that exceptional strength and resilience can be simultaneously obtained in the graphene aerogel with biomimetic architecture fabricated by the bidirectional freezing technique. Comparison between Graphene Aerogels with Biomimetic and Random Architecture. To further investigate the origin of the excellent mechanical robustness of the biomimetic graphene aerogel, we compared it with aerogel with random architecture (i.e., random pores), simplified as “biomimetic” and “random” respectively in Figure 2. Biomimetic aerogel was made by bidirectionally freezing an aqueous suspension of GO/PVA (8:8 mg/mL) at −90 °C, while the random aerogel was made by conventional unidirectional freezing with a 15 mg/mL GO suspension under the same cooling condition. Despite similar density (6.5−7.5 mg/cm3), these two types of graphene aerogels have distinct compressive behaviors (Figure 2). For the biomimetic aerogel, it shows a 6820
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Figure 4. Comparison among the biomimetic graphene aerogels with other previously reported porous materials. (a) Plot of strength recovery ratio and strength showing biomimetic graphene aerogels are highly resilient even when they are relatively strong. Numbers in parentheses represent relevant references. (b) Plot of strength and strain recovery ratio showing biomimetic graphene aerogels are highly resilient, indicated by both high strength and strain recovery ratio. The compressive strain for every data point is indicated beside the corresponding dots in the chart. Numbers in parentheses represent compressive cycles.
maximum compressive stress of 5.0 and 134.1 kPa when undergoes 50% and 90% compression (Figure 2a and d), and still retains 92% and 77% of the maximum stress after 100 cycles (Figure 2c and f). Although there are no obvious differences on maximum stresses between biomimetic and random aerogels as they have similar density, random one is much less robust and resilient. After only 10 compressive cycles at 50% and 90% strains (Figure 2b and e), the strength of the aerogel has already sharply decreased, retaining only 45% and 36% of its maximum stress (Figure 2c and f). In order to associate the mechanical behaviors of the aerogels with their microscale architectures, we traced the microstructures of biomimetic and random graphene aerogels before and after compressive cycles at 50% strain. In the SEM images, the biomimetic aerogel has a long-range ordered lamellar structure with lamellae thickness of 1−2 μm and bridge length of 20−30 μm (Figure 2g−h). For better illustration, schematics depicting their architectural features are also shown in the insets (Figure 2h−i and 2k−l). During compression, the stress could spread over the entire lamellar layer to avoid stress concentration. The interconnected bridges function as numerous “springs” between each lamellar layer, and elastic deformation dominates during the compressive cycles. After compression, the aerogel could recover to its original state entirely with no obvious damage (Figure 2i). In the SEM images, the random aerogel has randomly arranged pores with diameters of 20−30 μm (Figure 2j−k), similar to the bridge length of the biomimetic aerogel. As indicated by the schematics in the inset of Figure 2k−l, the load-bearing area is more localized and concentrated, compared to the biomimetic aerogel. As a result, the final property depends on the weakest connections in the random porous architecture. During compression, it tends to crack in the weakest parts, resulting in inelastic deformation and poor resilience (Figure 2f). All of these SEM images and compressive tests indicate that biomimetic graphene aerogels are much more resilient than random ones because of their architectures of ‘lamellar layers with interconnected bridges’. Control over Architectures and Properties. To further study the effect of microstructure on the final mechanical performance of biomimetic graphene aerogels, we prepared a series of precursor suspensions which have different concentrations and viscosities to adjust the number of lamellae and
bridges, two main features according to the SEM images (Figure 3). All the samples were fabricated under the same cooling rate, at a fixed GO/PVA ratio, but with different GO concentrations, i.e., 5, 8, 10, 12, and 15 mg/mL (Supporting Information, Table S2). Four typical SEM images (Figure 3a− d) all show plant stem-like structures but with different numbers of lamellae and bridges. As also shown in the insets of Figure 3e−h, the schematics summarize their microstructural features in detail. Figure 3i summarizes the variation of the numbers of lamellae and bridges with the aerogel density. For better comparison, we converted concentrations to the final densities of the aerogels. The number of bridges increases monotonously from about 3 to 18 with density within a unit observing area (around 100 × 80 μm), while that of lamellae increases at first and then decreases with a maximum at the density of 10 mg/cm3. This could be mainly attributed to the viscosity effect on the ice nucleation and growth.22,23 We have conducted a series of experiments to obtain a phase diagram for the guidance of structural control (Supporting Information, Figures S5−S7). When the concentration/viscosity of the suspension is high, particles are more likely to be trapped when ice crystals grow,22 resulting in more dendrites/bridges attached to the lamellar layers (Supporting Information, Figure S2d and g). A higher viscosity also raised the nucleation density of ice crystals on the cooling stage, resulting in more lamellae in the final aerogel. However, when the viscosity is high enough (above 10 mg/mL in our experiments), most initiated ice crystal nuclei would diminish or amalgamate, instead of growing into stable ice crystals.24 As a result, the amount of ice crystals would be greatly decreased, resulting in lower lamellae density. The above-mentioned discussion indicates that the properties of the precursor suspensions, particularly viscosity, indeed modify the nucleation and growth kinetics of ice crystals in the bidirectional freezing technique. In this way, the microstructures of biomimetic graphene aerogels were subtly tuned for studying and optimizing their mechanical performances. To compare the robustness of different biomimetic aerogels, compressive tests were performed at 50% strain for 100 cycles (Figure 3e−h). The strength and its recovery ratio are plotted with the density of the aerogels (Figure 3j). The strength increases monotonously with density (up to 16.5 mg/cm3) from 2 to 25.6 kPa. It is noteworthy that 25.6 kPa at 50% strain 6821
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Figure 5. Pressure-sensitive conductivity. (a−e) Pressure-sensitive behavior of a representative biomimetic graphene aerogel, with corresponding strain of 0%, 25%, 50%, 25%, and 0% from (a) to (e). (f) Stress−strain curve of biomimetic graphene aerogel at strains from 10% to 80%. The initial curves overlap well for each cycle, indicating high resilience. (g) The corresponding conductivity−compressive strain profile, measured by a two probe digital conductivity tester. (h) Apparent resistance cycling test profile of the biomimetic graphene aerogel (50% strain, 2 mm/min). Inset shows the maximum and minimum resistance cycling test profile. All of these demonstrate the potential of the biomimetic graphene aerogel in flexible electronics.
strength recovery ratio is also plotted with the strain recovery ratio (Figure 4b). For dense graphene aerogels with high strength, microfactures in the cell walls and edges usually happen during compression, resulting in brittle collapse and unrecoverable deformation, which is reflected by the relatively low strain recovery ratio (Figure 4b). By contrast, the current biomimetic graphene aerogels overcome such limitations, achieving both high strength and strain recovery ratios. Conductivity under Compressive Cycles. As strong and highly resilient graphene aerogels are important for sensors and other flexible electronics, 32−36 we further studied the conductivity of biomimetic graphene aerogel and its variation under compression. Before compression, the electrical conductivity increased monotonously with density, which was comparable to other physical cross-linking graphene aerogels with similar density (Supporting Information, Figure S8).37 A light-emitting diode (LED) is connected with the graphene aerogel in a circuit. The LED becomes brighter and darker during a compression−recovery cycle at 50% strain, suggesting strain-sensitive conductivity of the aerogel (Figure 5 and Supporting Information, Movie S1). The compressive stress− strain curves of the aerogel are plotted in Figure 5f to show its recoverability. The conductivity is tested and plotted with compressive strain from 10 to 80%, showing its good strain dependence (Figure 5g). To show the robustness of the biomimetic graphene aerogel, its electrical resistance profile under compression (50% strain) was recorded for 20 cycles (Figure 5i). The highest resistance value without compression is about 1000 Ω, while the lowest resistance when compressed to 50% is approximately below 10 Ω. All of these demonstrate the potential of the aerogels in flexible electronics such as sensors.
is one of the largest compressive strengths reported for graphene aerogels, which can support more than 6000 times its own weight (Figure 1h). While the strength of the biomimetic graphene aerogels increases with density as in other porous materials,1 their strength recovery ratio increases with density at first and decreases thereafter. This could be understood by the elastic energy storage and recovery during compression related to their microstructures, mainly numbers of lamellae and bridges. The compressive stress could uniformly spread over more lamellar layers to avoid stress concentration. The compressive energy could be favorably stored within more layers of interconnected springs/bridges, leading to elastic deformation and higher strength recovery ratio. With the same number of lamellae, the structure becomes more rigid with more bridges and more likely to generate cracks during compression, making the aerogel less resilient. This is demonstrated by aerogels in Figure 3b−c, which have similar numbers of lamellae but different numbers of bridges. With more bridges, the aerogel in Figure 3c shows higher strength but lower strength recovery ratio than that in Figure 3b. From the above discussion, it indicates that the key to obtain a strong and resilient aerogel is to optimize its architecture at high density, i.e., numbers of lamellae and bridges in our experiments. To further demonstrate the advantage of such biomimetic architecture, we compared our graphene aerogels prepared by the bidirectional freezing technique with other reported porous materials (Figure 4).5−12,25−31 The strength recovery ratio is first plotted with strength (Figure 4a). Although there are some aerogels with a high recovery ratio reported in literature, their strengths are usually modest. The biomimetic aerogels show remarkable recovery behavior (>80% of its maximum stress) even when they are strong (>60 kPa), i.e., an excellent combination of exceptional strength and resilience. The 6822
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samples as the compressive tests. The conductivity can be calculated by the equation: σ = 1/ρ. At least five samples were tested for each condition to obtain statistically reliable values.
CONCLUSIONS In summary, we have successfully fabricated graphene aerogels with hierarchical architectures mimicking the plant stem. Such a biomimetic architecture composed of lamellar layers with interconnected bridges gives the aerogel exceptional strength and resilience, which is usually mutually exclusive and challenging to achieve. We systematically studied the effect of different architectures on the robustness of the biomimetic graphene aerogels and illustrated their potential in flexible electronics. Although we used a GO/PVA suspension as a proof of concept, the material design strategies and fabrication techniques can be readily expanded to produce strong and resilient aerogels with other types of materials (e.g., pure polymer or polymer/inorganic particle composites), which are ongoing in our lab. Our study provides insights to mimic natural porous architectures in synthetic aerogels, to improve their mechanical properties and functions, especially when a higher level of control over structure is required.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01815. Figure S1: Architectural control over graphene aerogels by varying the angles of PDMS wedge in the bidirectional freezing technique. Figure S2: Three kinds of microporous graphene aerogels. Figure S3: The pore size distribution of the graphene aerogel. Figure S4: The bidirectional freezing technique and resulted graphene aerogels. Figure S5: Effect of GO concentration on the architecture of graphene aerogels. Figure S6: Effect of PVA + GO concentration on the architecture of graphene aerogels; GO/PVA ratio remains constant. Figure S7: Phase diagram of architectural control with respect to PVA and GO concentrations. Figure S8: The electrical conductivity of the graphene aerogel as a function of density. Table S1: EDS result of the graphene aerogel. Table S2: Concentration, proportion, number of lamellae, number of bridges, density, maximum stress, and strength recovery ratio for samples prepared with different concentrations (PDF) Movie S1: Pressure-sensitive conductivity (AVI)
METHODS Materials. Graphene oxide (GO, 99%, Nanjing XFNANO Materials Tech Co., Ltd., China), Poly(vinyl alcohol) (PVA, Mw = 205,000, 99%, Aladdin Chemistry Co., Ltd., China), and polydimethylsiloxane (PDMS, silicone elastomer base and agent, Sylgard 184, Dow corning, USA). Preparation of PDMS Wedge. Square Teflon tubes (10 × 10 × 20 mm) were sealed with a copper plate on one end and tilted to an angle of around 15°. PDMS precursor solution was then poured into the mold to completely cover the copper plate. PDMS wedges were obtained, after curing at 80 °C for 2 h. Preparation of GO Suspension. A certain amount of GO and PVA were dissolved in distilled water to form the precursor suspension with different concentrations. Then the solution was sonicated for 15 min by a noise isolating chamber (JY 92-IIN, Ningbo Scientz Biotechnology Co., Ltd., China) at 10% power and cooled to 4 °C before use. Bidirectional Freezing and Freeze-Drying. The precursor suspension was poured into a square tube with PDMS wedge mentioned above and then frozen using cryogenic ethyl alcohol. After the precursor suspension was frozen entirely, the sample was tapped out of the mold and freeze-dried for more than 48 h at −60 °C with a freeze-dryer under 0.05 mbar pressure (Labconco 8811, Kansas City, USA). Reduction. The freeze-dried sample was transferred into tube furnace (OTF-1200X, HeFei KeJing Materials Technology Co., Ltd., China) and reduced under 800 °C for 2 h with an atmosphere of 98% argon and 2% hydrogen. Characterization on Structural Features. The morphology of the graphene aerogels was observed by scanning electron microscopy (SEM, S-4800, Hitachi, Tokyo, Japan) at an acceleration voltage of 15 kV. The Structural parameters, such as lamellae thickness, bridge length, and number of lamellae and bridges were manually measured. The size was measured by Vernier caliper, and the weight (w) was measured by analytical balance. The density (ρ) equals weight (w) divided by volume (v). For each parameter, more than 10 measurements were performed. Compressive Tests of Samples. Compressive strengths were measured by performing uniaxial tests on blocks (10 × 10 × 10 mm) cut from specimens using a blade. Samples were then compressed in the direction perpendicular to the freezing direction in an electronic universal testing machine (UTM2102, ShenZhen Suns Technology Stock Co., Ltd., China) at a displacement rate of 0.2 mm/min. At least five samples were tested for each condition to obtain statistically reliable values. Conductivity Measurements. The surface resistance was measured by Keithley 2400 (Keithley, USA) under a current range of 2 mA, with a linear probe head (2.0 mm space) using the same
AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. ORCID
Hao Bai: 0000-0002-3348-6129 Tao Xie: 0000-0003-0222-9717 Author Contributions §
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 51603182, 51603183, and 21674098), the State Key Laboratory of Chemical Engineering (no. SKL-ChE-16T02), the Fundamental Research Funds for the Central Universities (no. 2017QNA4036), and the “1000 Youth Talents Plan” of China. REFERENCES (1) Gibson, L. J.; Ashby, M. F. Cellular Solids Structures and properties, 2nd ed.; Clarke, D. R., Suresh, S., Ward, I. M., Eds; Cambridge University Press: Cambridge, 1997; pp 1−14. (2) Lee, J. W.; Kim, J.; Hyeon, T. Recent Progress in the Synthesis of Porous Carbon Materials. Adv. Mater. 2006, 18, 2073−2094. (3) Nardecchia, S.; Carriazo, D.; Ferrer, M. L.; Gutiérrez, M. C.; del Monte, F. Three Dimensional Macroporous Architectures and Aerogels Built of Carbon Nanotubes and/or Graphene: Synthesis and Applications. Chem. Soc. Rev. 2013, 42, 794−830. (4) Huang, X.; Yin, Z. Y.; Wu, S. X.; Qi, X. Y.; He, Q. Y.; Zhang, Q. C.; Yan, Q. Y.; Boey, F.; Zhang, H. Graphene-Based Materials: Synthesis, Characterization, Properties, and Applications. Small 2011, 7, 1876−1902. (5) Liu, T.; Huang, M. L.; Li, X. F.; Wang, C. J.; Gui, C. X.; Yu, Z. Z. Highly Compressible Anisotropic Graphene Aerogels Fabricated by 6823
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DOI: 10.1021/acsnano.7b01815 ACS Nano 2017, 11, 6817−6824
