Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids

Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids

Accepted Manuscript Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids T...

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Accepted Manuscript Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids Tao Liu, Meiling Huang, Xiaofeng Li, Chongjie Wang, Chen-Xi Gui, Zhong-Zhen Yu PII:

S0008-6223(16)30038-0

DOI:

10.1016/j.carbon.2016.01.038

Reference:

CARBON 10669

To appear in:

Carbon

Received Date: 15 November 2015 Revised Date:

31 December 2015

Accepted Date: 11 January 2016

Please cite this article as: T. Liu, M. Huang, X. Li, C. Wang, C.-X. Gui, Z.-Z. Yu, Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids, Carbon (2016), doi: 10.1016/j.carbon.2016.01.038. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Highly compressible anisotropic graphene aerogels fabricated by directional freezing for efficient absorption of organic liquids

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Tao Liua, Meiling Huanga, Xiaofeng Lia*, Chongjie Wanga, Chen-Xi Guia, Zhong-Zhen Yua,b* State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China

Beijing Key Laboratory on Preparation and Processing of Novel Polymer Materials, Beijing

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University of Chemical Technology, Beijing 100029, China

Abstract: Highly compressible three-dimensional graphene aerogels with anisotropic porous structure are fabricated by directional-freezing of graphene hydrogel using anisotropically grown ice crystals as templates followed by freeze-drying. The directional-freezing approach

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endows the graphene aerogel with a high compressive strength in the axial direction and good compressibility in both axial and radial directions. The anisotropic graphene aerogel also

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exhibits ultralow density, excellent flexibility in liquids, satisfactory fire-resistance, and strain-sensitive electrical conductivity. After absorbing organic liquids, the aerogel can be

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well recycled by burning, distilling, or squeezing, which makes it promising for oil absorption with a good recyclability.

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*Corresponding author. Tel/Fax: +86-10-64428582. E-mail: [email protected] (X. Li); [email protected] (Z.-Z. Yu)

ACCEPTED MANUSCRIPT 1. Introduction Graphene has emerged in recent years as a unique and important class of carbon nanomaterials [1] due to its extraordinary mechanical [2,3], electrical and thermal properties

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[4-6], and is being used in many fields including electronics [7-10], conductive nanocomposites [11], films [12], electromagnetic interference shielding [13] and sensors [14,15]. Graphene sheets could be constructed to lightweight and three-dimensional (3D)

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porous structures for special applications in the fields like catalyst support, energy storage

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and environmental cleaning [16-19]. The construction of 3D graphene aerogels (GAs) [20-23] not only avoids the restacking of individual sheets and maintains the intrinsic properties of graphene sheets such as high conductivity and large specific surface area, but also makes the 3D monolith structure with ultralow density and high porosity [24-27].

microstructures

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Many efforts have been devoted to the fabrication of 3D graphene materials with various properties,

including

self-assembly

[28-35],

template-assisted

preparation [24,36,37] and chemical vapor deposition [38-41]. Among these approaches,

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self-assembly of graphene is commonly used with graphene oxide (GO) as its precursor.

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Firstly, 3D graphene hydrogels are fabricated by hydrothermal process or chemical reduction method, during which GO is converted to graphene by thermal treatment or chemical reducing agents, such as NaHSO3, Na2S, hydroiodic acid, hydrazine, hydroquinone, and Vitamin C [42,43]. Subsequently, GAs can be obtained by freeze drying or supercritical fluid drying of the graphene hydrogels. When prepared by freeze drying approach, the porous microstructures including pore size and orientation of GAs can be controlled by changing the conditions including freezing temperature and freezing direction [31]. 2

ACCEPTED MANUSCRIPT However, in most cases the obtained GAs are fragile, which limits their practical applications, organic additives are thus added to improve their flexibility. For instance, Qiu et al. [44] reported ultralight and compressible GAs with an ethylenediamine (EDA)-mediated

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process, leading to the simultaneous functionalization and reduction of GO and the assembly of the reduced GO sheets into hydrogels, and subsequent microwave irradiation after freeze drying endowed the GAs with high compressibility. Li et al. [45] prepared highly

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compressive GAs by keeping GO suspension with EDA at 80 °C for 24 h followed by a

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freeze drying process. Although organic additives are effective in endowing the 3D structure with high flexibility, they may increase the density of GAs along with the involvement of other elements that may affect the applications in some extreme conditions [44]. Additive-free method is also developed to prepare GAs with excellent compressive

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behavior. Li et al. [46] fabricated GAs by mimicking the hierarchical structure of natural cork with a directional freezing method, which made the GAs recover rapidly from >80% compression. Shi et al. [47] reported a convenient emulsion modified hydrothermal method to

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obtain the compressive GAs. Barg et al. [48] combined the emulsion modification with

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directional freezing to assembly graphene sheets into complex cellular network, which could absorb oil and recover to its initial size after squeezing out the oil. By solvothermal treatment with ethanol as the solvent, Chen et al. [49] reported the scalable self-assembly of randomly oriented graphene sheets into additive-free and homogenous graphene sponge materials with both cork-like and rubber-like properties. Herein, highly compressible anisotropic graphene aerogels are prepared by reducing GO with ascorbic acid at 70 °C for four hours, followed by directional-freezing and freeze-drying. 3

ACCEPTED MANUSCRIPT Interestingly, the resultant anisotropic graphene aerogels (AGAs) exhibit high compressive strength in the axial direction (freezing direction) because of the anisotropic structure and good compressibility in both axial and radial directions with strain-sensitive electrical

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conductivities, similar to the homogenous graphene sponge prepared by solvothermal treatment [49]. In addition, the aerogels also show ultralow density, high porosity, fire-resistance and excellent flexibility in organic liquids. It is interesting that the AGAs after

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absorption of organic liquids can be easily recycled by burning, distilling and squeezing.

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2. Experimental 2.1 Materials

Graphite flakes with an average diameter of 13 µm were supplied by Huadong Graphite Factory (China). Ascorbic acid, concentrated sulfuric acid (95–98%), concentrated

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hydrochloric acid (36–38%), potassium permanganate (99.5%), sodium nitrate (99%) were purchased from Beijing Chemical Factory (China) and used as received. 2.2. Preparation of AGAs

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Graphite oxide was prepared by oxidizing graphite flakes based on the modified Hummers

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method [50]. 10 ml GO suspension (2-8 mg/ml) was mixed with ascorbic acid (1:1, w/w) in a 25 ml beaker, and the mixture was heated at 70 °C for 4 h to obtain graphene hydrogel. After the hydrogel was subjected to dialysis in deionized water to remove soluble species, it was put on a Cu disk which was half dipped in liquid nitrogen for directional freezing. The hydrogel was directionally frozen from the Cu/hydrogel interface to the top surface until it was totally frozen. The frozen hydrogel was then freeze-dried in a FD-1C-50 freeze drier (China) to obtain AGA. The AGA samples were defined as AGA-x, where x (mg cm-3) 4

ACCEPTED MANUSCRIPT represents the density of AGA. 2.3. Characterization GO and AGAs were characterized with a Thermo VG RSCAKAB 250X high resolution

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X-ray photoelectron spectroscopy (XPS) and a Nicolet Nexus 670 Fourier transform infrared spectroscopy (FT-IR), and a 65 Renishaw inVia Raman microscopy (Britain). Morphology and microstructure were observed by a Hitachi S4700 field emission scanning electron

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microscope (SEM) at 20 kV. X-ray diffraction (XRD) measurements were carried out using a

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Rigaku D/Max 2500 diffractometer with CuKα radiation (λ=1.54 Å) at a generator voltage of 40 kV and a generator current of 40 mA. Compressive properties of AGAs were tested with an Instron 1185 testing machine (Britain) at a constant rate of 10 mm/min and their electrical properties were measured by a Victor VC890C+digital multimeter.

3.1. Morphology of AGAs

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3. Results and discussion

Fig. 1a illustrates the synthesis procedure of AGA with a cylindrical morphology. The GO

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and ascorbic acid suspension is ultrasonicated for 30 min and then heated at 70 °C. After two

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hours, the aqueous suspension starts to turn from brown to dark but is still homogeneous, marking the occurrence of the chemical reduction. At early stage of the reduction, the reduced GO sheets become hydrophobic due to the removal of their oxygen-containing groups, and the increasing hydrophobicity and π-π conjugated structure result in the compact 3D hydrogel. With the reduction proceeds, the hydrogel shrinks and floats onto the surface of the medium. It is then subjected to dialysis in water followed by directional-freezing and freeze-drying to obtain AGA (Fig. 1b). The density of AGAs, calculated by simply dividing 5

ACCEPTED MANUSCRIPT the mass by the volume, increases with the initial GO concentration in the suspension, and is in the same order of magnitude as that of air (1.29 mg cm-3). The AGA stands well on a green

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bristlegrass, demonstrating its ultra-lightness (Fig. 1c and d).

Fig. 1. (a) Images showing the synthesis procedure of graphene hydrogel, where GO is reduced at 70 °C for 4 h; (b) AGA is prepared by directional freezing following by freeze drying of the anisotropic hydrogel; (c) standing of an AGA-5.4 cylinder on a green bristlegrass; and (d) plot of density of AGAs versus the initial GO concentration. It has been demonstrated that, when an aqueous suspension of nanoparticles is frozen, phase separation can result in the exclusion of solid nanoparticles from the ice and the 6

ACCEPTED MANUSCRIPT nanoparticles are thus aggregated between the growing ice crystals [51,52]. Subsequent sublimation of ice crystals leads to the formation of a porous material. In the present study, when the graphene hydrogel is directionally frozen, the ice crystals prefer to grow along the

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vertical direction and expel the reduced GO sheets, which are entrapped between neighboring ice crystals to form a 3D network. After freeze-drying, the ice crystals disappear and AGA

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with anisotropic porous structure is thus obtained (Fig. 2a).

Fig. 2. (a) A schematic illustrating the directional-freezing and freeze-drying; (b) top-view and (c) side-view SEM images of the anisotropic porous structure of AGA-8.7. SEM observation reveals that the reduced GO sheets in the cell walls are oriented in nearly parallel manner (Fig. 2b and c). The sizes of the pores are in tens of microns and 7

ACCEPTED MANUSCRIPT decrease with the increase of the density of AGAs (Fig. S2). Li et al. [46] prepared similar GAs by a multi-step method consisting of directional-freezing of an aqueous suspension of partially reduced GO, thawing, further reduction, freeze drying and finally annealing at

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200 °C. The directional-freezing of partially reduced GO was necessary to obtain the cork-like structure, and the further reduction of the thawed sample was also conduced to maintain the stability of this cork-like structure. Zhang et al. [53] modified the multi-step

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method that AGAs with large pores were obtained by drying under ambient conditions. In the

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present work, AGAs are easily prepared by directional-freezing and drying of the one-step reduced graphene hydrogel, which it is simple and suitable for large-scale production compared to hydrothermal or solvothermal methods.

3.2. Chemical evolution of GO during the reduction process

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XRD patterns of GO and AGA are shown in Fig. 3a. The peak of GO at 12° disappears in the XRD pattern of AGA, suggesting there is no regular stacking of the reduced GO sheets. Fig. S1 shows FT-IR spectra of GO and AGA. The stretching vibration band of C=O at 1731 cm-1

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and the stretching vibration band of C-O in epoxide at 1230 cm-1 and alkoxy at 1050 cm-1

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demonstrate that GO has abundant oxygen-containing groups. After reducing with ascorbic acid, the peaks for the oxygen containing groups are weakened obviously. Furthermore, a new broad peak at 1574 cm-1 is associated with the vibration of C=C band. The reduction of GO is also confirmed by comparing the XPS spectra of GO and AGA (Fig. 3b). The two peaks at 286 and 533 eV are related to C and O elements, respectively. The elemental ratio of C/O increases from 2.3 for GO to 4.1 for AGA, implying that most of the oxygen-containing groups are removed. This is evident by comparing the intensities of the peaks for 8

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oxygen-containing groups in GO and AGA (Fig. 3c and d).

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Fig. 3. (a) XRD patterns and (b) XPS wide scan spectra of GO and AGA-3.4; C 1s XPS spectra of (c) GO and (d) AGA-3.4.

3.3. Compressive properties of AGAs

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Fig. 4a shows that the AGA with a density of 8.3 mg cm-3 can support a 500 g weight, which

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is 9,000 times higher than its own weight. After the weight is removed, the AGA recovers to nearly its initial height, indicating its good mechanical strength and flexibility. However, the GA with similar shape and density, prepared by conventional freeze-drying (without directional-freezing), cannot support the 500 g weight, and it is fragile after compression (Fig. 4b). Additionally, the compressive stress of GA is much less than that of AGA, suggesting that directional-freezing has a great effect on the mechanical properties of the aerogels (Fig. S3). 9

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Fig. 4. (a) Digital images show that AGA-8.3 (height=18.6 mm, diameter=21.3 mm, mass=55

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mg) can support 500 g weight and exhibits a good compressive performance; (b) GA-8.3 (height=18.6 mm, diameter=21.3 mm, mass=55 mg) prepared by conventional freeze-drying fails to support the 500 g weight; (c) compressive stress-strain curves of AGA-8.3 at the maximum strain of 50% for 10 cycles in the axial direction; (d) plots of compressive stress in the radial direction versus strain for AGA-9.8 at the maximum strain of 50% for 10 cycles; (e) compressive stress-strain curves of AGAs with different densities at the maximum strain of 50% in the axial direction. 10

ACCEPTED MANUSCRIPT Compression rebound tests in the axial direction of the AGA with a density of 8.3 mg cm-3 reveal an excellent resilience when released from compression (Fig. 4c). During the first cycle, the stress increases rapidly and linearly when the strain is less than 20%, resulting from

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the elastic bending and shearing deformation of graphene walls. Then, the stress increases slowly but still linearly due to the increasing densification [46,54]. From the second cycle, there are three deformation regions in the stress-strain curve: nearly linear elastic region (ε <

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18%) corresponding to the bending of cell walls; relatively flat stress plateau (18% < ε <

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38%) corresponding to the elastic buckling of the cell walls; and abrupt stress increasing region (38% < ε < 50%) due to the densification of the cells. The AGA can return to 97% of its original height after 10 cycles, which is attributed to its hierarchical structure. But the irreversible destroy of the cell walls in the first circle prevents it return to its original height

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[55] and the maximum stress decreases from 21.5 to 20.4 kPa.

In the radial direction, which is perpendicular to the axial direction, AGA also shows good compression recovery properties and can return to 95% of its original height after

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10-cycle compression (Fig. 4d), indicating that AGA with the anisotropic structure can also

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own highly compressive properties in both axial and radial directions like the homogenous graphene sponge prepared by solvothermal treatment [49]. The AGA shows an obvious mechanical anisotropy that the compressive stress in the axial direction is higher than that in the radial direction (Fig. 4c and d). The different mechanical properties result from the cork-like anisotropic structure, where the oriented graphene sheets in the aerogel afford the high compressive stress in the axial direction [56]. Fig. 4e shows the compression curves of three AGAs with different densities ranging 11

ACCEPTED MANUSCRIPT from 3.4 to 8.3 mg cm-3 in the axial direction. Both compressive stress and Young’s modulus increase with increasing the density. This is because the larger density contributes to stronger interactions between the graphene sheets. The stress at 50% strain for the AGA with a density

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reported in the literature with a similar density (Table S1).

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of 6.1 mg cm-3 is about 14.1 kPa, which is among the highest in all graphene aerogels

Fig. 5. (a) Digital images showing the good compressibility of AGA-6.6 in n-heptane; and (b) the recovery of AGA-6.6 height at the maximum strain of 50% in n-heptane. In addition to the good compressibility of AGA in air, its compressibility in organic liquids such as n-heptane is also investigated. Fig. 5a shows digital images of AGA 12

ACCEPTED MANUSCRIPT compressions in n-heptane. AGA is recovered to nearly its initial height after suffering a compressive strain of up to 50%. Interestingly, even after 20 compression-recovery cycles, the height of the AGA still recovers to 95.8% of its original height (Fig. 5b), implying its

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excellent flexibility in both air and organic liquid.

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3.4. Absorption properties of AGAs

Fig. 6. (a) Digital images showing the removal of n-heptane that is stained with Sudan Red 5B and floating on water. (b) The ability of AGA-4.2 to absorb different organic liquids. 13

ACCEPTED MANUSCRIPT Because of the highly porous structure and hydrophobicity, AGAs exhibit a high ability to absorb organic liquids. Fig. 6a shows the process of removing n-heptane, which is stained with Sudan Red 5B and floating on the surface of deionized water. When AGA-4.2 is put in,

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the red liquid is absorbed completely in about 4 s. The general absorbing rate is 40 g n-heptane per gram of AGA per second, which is faster than reported values (0.57 g g-1 s-1 [34] and 27 g g-1 s-1 [45]). Similarly, white oil stained with Oil Red O and floated on the top

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of water is easily cleaned by AGA (Fig. S4). Fig. 6b exhibits the absorption ability of AGA

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with a density of 4.2 mg cm-3 to different organic liquids including ethanol, acetone, white oil, pump oil, vegetable oil, n-heptane and n-hexane. Q0 is the mass of original AGA, and Q is the total mass of AGA with absorbed solvent. AGA has a good ability of absorbing liquids by 120 to 200 times than its own mass.

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It is interesting that, after the absorption of n-hexane, the height of AGA changes little (Fig. 7a). In addition, the AGA absorbed with n-hexane can be ignited and burns quietly and stably until n-hexane is burnt out. After the combustion, AGA still maintains its original

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appearance except for the color change from black to gray, indicating a good stability under

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combustion, which is because the high porosity of AGA helps remove the generated heat quickly during the combustion [45]. Compression rebound tests of the AGA with a density of 8.3 mg cm-3 after burning reveal that AGA exhibits an excellent resilience even after burning (Fig. 7b), demonstrating the excellent fire-resistance. Fig. 7c shows XPS broad scan spectra of AGA before and after burning. The peak intensity of O elements slightly decreases although the intensity of C peak changes little. The C/O ratio increases from 4.1 of AGA to 5.4 after burning once, and increases further to 7.3 after burning twice, implying the 14

ACCEPTED MANUSCRIPT decomposition of some organic groups of AGA and further reduction of AGA during the

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combustion.

Fig. 7. (a) Snapshots of filling AGA-8.3 with n-hexane and lighting it, and when the inner organic liquid burns up, the AGA still maintains nearly to its original appearance except for

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its color change from black to gray; (b) compressive stress-strain curves of AGA-8.3 after

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burning at the maximum strain of 50% for 10 cycles; (c) XPS wide scan spectra of AGA-8.3 before and after burning.

Raman shift (Fig. S5) further elaborates on the order/disorder degree in the sp2 network that two main characteristic peaks are well known for graphene materials: G mode due to first order scattering of the E2g photon of sp2 C atoms (~1580 cm-1), and D mode resulting from a breathing mode of j-point photons of A1g symmetry (~1350 cm-1). AGA exhibits D-band as a result of the oxidation-induced defects. As to the intensity ratio of D-band to G-band (ID/IG), 15

ACCEPTED MANUSCRIPT it ranges from 1.09 to 0.97 after burning twice, which suggests heat treatment contributes to

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the decrease in the density of defects in sp2 carbon atoms [57,58].

Fig. 8. Recyclability of AGA for absorption of n-hexane under (a) absorbing-burning cycles,

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and (b) absorbing-distilling cycles. (c) Digital images showing that AGA has a good

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compression-recovery performance after absorbing n-heptane. (d) Recyclability of AGA for absorption of n-heptane under absorbing-squeezing cycles. As an ideal material for absorption of oil and organic liquids, the recyclability of AGA determines if it is suitable for practical application. Since burnt AGA still keeps a good appearance,the recyclability of AGA-5.6 for absorbing n-hexane is tested by burning off the absorbed n-hexane and the results are shown in Fig. 8a. It is interesting that after ten absorbing-burning cycles, the absorption efficiency of AGA decreases by 10% only, which 16

ACCEPTED MANUSCRIPT results from the slight volume shrinkage during the burning process. In reality, the absorbing and recycling of organic liquids will be more economic and eco-friendly than just burning. Another approach for recycling AGA is to distract the oil by distilling at a temperature near

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its boiling point. Due to its good thermal stability and 3D structure, AGA-5.6 remains a highly stable absorption ability and its shape changes little even after ten absorbing-distilling cycles (Fig. 8b). Inspired by the fact that AGA has a good compression-recovery

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performance in the organic liquid of n-heptane, the approach of squeezing is utilized to

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collect the absorbed organic liquid in AGA. As shown in Fig. 8c, after compressing the AGA filled with n-heptane to 50% strain, its height can recover to nearly 100%. The recyclability of AGA-6.6 for collecting and recycling of n-heptane is tested (Fig. 8d). AGA still keeps a good

performance

after

ten

absorbing-squeezing

cycles.

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clear

that

the

approaches.

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absorbing-squeezing approach is much more efficient than both burning and distilling

3.5. Electrical properties of AGAs

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To demonstrate the good electrical conductivity of AGA, it is connected with a battery and a

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light emitting diode (LED) in series along with two copper plates to clamp the AGA-6.2 for compressing it (Fig. S6). In the absence of AGA, the LED shines brightly by connect two copper plates directly to form a closed circuit, and there is no light when the two plates are separated. By inserting AGA into the circuit to connect the two plates, dim light is observed, implying that AGA is an electrical conductor instead of an insulator. When a compression is loaded on the AGA, the LED turns bright and the brightness depends upon the compressive strain. The brightness reaches to a peak at the maximum compressive strain. When we stop 17

ACCEPTED MANUSCRIPT compressing the AGA and let it recover by itself, the light turns dim again. The relationship between the electrical resistance of AGA and compression strain is measured (Fig. 9). The electrical resistance of the aerogel is strongly dependent on the

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deformation that it varies from 3400 to 82 Ω as the strain varies from 0 to 83.7%. Higher compressive strain means better contact between the graphene sheets due to the enhanced density, which results in greater electrical conductivity. The strain-sensitive electrical

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conductivity of AGA contributes to some applications like pressure sensor and

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stimulus-responsive graphene systems.

Fig. 9. Plot of electrical resistance as a function of compressive strain for AGA-6.2 with a height of 18.8 mm and a diameter of 23.8 mm. 4. Conclusion We demonstrate a facile directional-freezing approach to prepare three-dimensional additive-free AGAs with directionally grown ice crystals as the template. The anisotropic 18

ACCEPTED MANUSCRIPT structure allows AGAs to afford high weight in axial direction and AGAs exhibit good compressibility in both axial and radial directions. Due to its high porosity, excellent flexibility in both air and liquids and satisfactory fire-resistance, AGAs are efficient

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absorbents with good recyclability for absorption of organic liquids under absorbing-burning, absorbing-distilling, and absorbing-squeezing cycles. Interestingly, AGA exhibits a strain-sensitive electrical conductivity.

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Acknowledgements

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Financial support from the National Natural Science Foundation of China (51125010, 51403016, 51533001) and the Fundamental Research Funds for the Central Universities (YS201402) is gratefully acknowledged. Appendix A. Supplementary data

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Supplementary data associated with this article can be found in the online version. References

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Highlights 

Highly compressible graphene aerogels with anisotropic porous structure are fabricated



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by directional-freezing

The directional-freezing approach endows the aerogel with an excellent compressibility



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in both axial and radial directions

The anisotropic aerogel is promising for oil absorption with good recyclability by

The aerogel exhibits ultralow density, excellent flexibility in liquid and a strain-sensitive

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electrical conductivity

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combustion, distilling and squeezing

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