Tuning oxygen clusters on graphene oxide to synthesize graphene aerogels with crumpled nanosheets for effective removal of organic pollutants

Accepted Manuscript Tuning oxygen clusters on graphene oxide to synthesize graphene aerogels with crumpled nanosheets for effective removal of organic pollutants Xiaoxiao Chen, Dengguo Lai, Baoling Yuan, Ming-Lai Fu PII:

S0008-6223(18)31125-4

DOI:

https://doi.org/10.1016/j.carbon.2018.12.006

Reference:

CARBON 13709

To appear in:

Carbon

Received Date: 16 July 2018 Revised Date:

19 November 2018

Accepted Date: 1 December 2018

Please cite this article as: X. Chen, D. Lai, B. Yuan, M.-L. Fu, Tuning oxygen clusters on graphene oxide to synthesize graphene aerogels with crumpled nanosheets for effective removal of organic pollutants, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2018.12.006. 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.

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Tuning oxygen clusters on graphene oxide to synthesize graphene aerogels with crumpled nanosheets for effective removal of organic

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pollutants

Xiaoxiao Chen †, Dengguo Lai †, Baoling Yuan ‡, Ming-Lai, Fu †,

Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese

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College of Civil Engineering Huaqiao University, Xiamen 361020, China

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Academy of Sciences, Xiamen, 361021, China

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Corresponding author.

E-mail: [email protected] (M.-L. Fu) Tel: +86 592 6190762 Fax: +86 592 6190977

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Graphical Abstract

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ABSTRACT Previously available synthesis of crumpled graphene is always based on external substrates

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or triggers, and it is still a daunting challenge to construct three-dimensional (3D) porous

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graphene aerogels with crumpled nanosheets without additional corrugation-inducers. Here,

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we report a novel and facile method to fabricate crumpled dispersive nanosheets in water

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directly by tuning oxygen clusters on graphene oxide itself through general alkali-treatment at

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70oC. Given this, 3D graphene aerogel with crumpled nanosheets has been successfully

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constructed in aqueous via a common sol-gel process. In this kind of 3D architecture, the

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self-restacking of graphene nanosheets is thoroughly inhibited, and abundant diffusion paths

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and voids are created. The thus fabricated crumpled aerogel exhibits high elasticity and

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hydrophobicity with superior absorption capacity of organic pollutants, which is 224%-406%

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higher than that of conventional graphene aerogels assembled by flat and stiff plates,

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outperforming most of the pioneering reported graphene aerogels synthesized by sol-gel

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method. Intriguingly, this architecture also presents excellent recyclability. The suggested

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facile fabrication of crumpled aerogel with attractive 3D configuration opens a door to

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promote the performance of graphene-based devices by tuning oxygen clusters and graphitic

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domains on nanosheets for specific energy storage and sensors applications.

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Keywords: graphene aerogels, oxygen clusters, crumples, metastable nanosheets, phase

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separation, organic pollutants removal

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1. INTRODUCTION Three-dimensional (3D) interwoven porous graphene aerogel (GA), prepared from

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two-dimensional (2D) graphene nanosheets (GN), not only maintains the intrinsic excellent

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properties of GN, but also possesses a variety of fascinating properties of ultralow density,

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high surface area, tunable porosity and easy functionalization, thus has been considered as a

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“miracle material”.

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derivatives show greatly improved uptake of organic pollutants from water and easy

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reusability compared to the existing functionalized graphene powders.

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mechanical property for recyclability as well as the superior adsorption performance of GA is

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extremely anticipated for practical application. Sol-gel process is the most common and

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convenient method to prepare interwoven GA.

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one-atom-thick sheets suffer from serious face-to-face restacking due to the strong van der

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Waals attraction between parallel flat nanosheets during reduction, which inevitably result in

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the inferior surface areas and fragile frameworks with limited recoverable deformation of GA.

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12, 13

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units.

As a promising material for environmental remediation, GA and its

Good elastic

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However, by using this method, the

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Consequently, it is really necessary to fabricate GA with restacking-resistant construction

Crumple on graphene, a peculiar corrugation for 2D crystals, has attracted intense scientific

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interest since it gives rise to the easy stretching and compression property of graphene, which

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is rather different from conventionally stiff and flat sheets. 14, 15 Due to the mismatch between

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deformed nanosheets, the crumpled graphene sheets possess remarkably self-avoiding

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restacking property, which make them tightly packed without significant reducing of the

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accessible surface areas.

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heterogeneity and endow nanosheets with superhydrophobic / super-oleophilic properties,

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thus presenting great applications in the removal of organic pollutants from water.

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nanosheets are crumpled and assembled into 3D architectures, the restacking of nanosheets

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can be greatly prevented, which may not only preserve the original active sites but also

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provide abundant diffusion paths. In addition, these 3D architectures may inherit the elastic

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and superhydrophobic properties of the crumpled “building blocks”, and show excellent

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recyclability of organic pollutants through simple mechanical compression. More significantly,

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Furthermore, these corrugations also increase the surface

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If the

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moieties. However, as a main precursor for GA, graphene oxide (GO) normally appears as

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single extended layers in water or other common polar solvents, rather than crumpled

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structure, which would easily restack into rigid graphite plates during the assemble process

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using sol-gel method. 16, 17 19

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Considering that the morphologies of 2D GO sheets are extremely sensitive to external 14, 20

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involving the formation of crumples on graphene nanosheets have been reported. By far, there

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are two main techniques to create out-of-plane crumpled surface patterns from flat sheets,

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which are wrapping or crumpling the nanosheets on given substrates or adding triggers into

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GO suspensions.

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most popular scalable continuous nanomanufacturing process and has been widely used. 17, 21

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However, it still suffers from the special equipment and significant energy cost for

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evaporating and drying suspensions.

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present as individual crumpled balls with little cross-link sites, which are not suitable for the

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next construction of 3D interwoven architectures. On the other hand, the introduction of

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external agents, especially organic solvents, is environmentally unfriendly, and may occupy

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the active sites and hamper the further surface modifications of graphene nanosheets. To our

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best knowledge, it is still a huge challenge to obtain well-dispersed solvated crumpled

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nanosheets in water solution and directly be used to construct 3D architectures.

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perturbation, numerous theoretical simulations

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Spray drying, crumpling 2D GO sheets on aerosol water droplets, is the

Furthermore, the obtained crumpled graphene sheets

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and experimental approaches

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It is reported that GO produced by Hummers’ method is a metastable material whose

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randomly distributed oxygen groups can be adjusted into oxygen clusters with enlarged

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graphitic domains on the nanosheets by external stimulus, such as temperature and pH. 26-29 It

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is well known that GO nanosheets appear as extended state in water due to the uniform

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H-bond networks mediated by randomly distributed oxygen-containing groups and water

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molecules. The phase separation of hydrophilic oxygen clusters and hydrophobic graphitic

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domains on GO nanosheets lead to a destruction of the uniform H-bond networks. As a result,

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the extended GO sheets might evolve to collapsed states. Inspired by this, we realize a novel

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and facile approach to synthesis highly elastic and crumpled GA (annealed graphene aerogel:

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A-GA) with strong hydrophobicity by tuning the phase separation of GO nanosheets without

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found that more physical cross-linked sites are generated in the phase separation process,

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which can interconnect adjacent nanosheets tightly and preserve the porous architectures

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during freezing-thawing. After reduction, the resultant crumpled A-GA exhibits not only

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excellent recycling stability due to its outstanding elasticity but also an extremely high oil

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absorption capacity of 154-325 g/g for various oils and organic solvents due to the enlarged

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effective hydrophobic surface area. This absorption capacity of A-GA is 224%-406% higher

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than that of conventional graphene aerogels (C-GA) with flat sheets, outperforming most of

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the previously reported graphene-based macrostructures synthesized at similar conditions. To

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sum up, we provide a novel and facile strategy to fabricate crumpled graphene aerogels by

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taking advantages of graphene material itself, and present broad implications for the

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application and further surface-modification of graphene-based aerogels.

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2. EXPERIMENTAL SECTION

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2.1 Synthesis of conventional graphene oxide (C-GO) and annealed graphene oxide

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(A-GO)

Graphene oxide (GO) was synthesized by a modified Hummers’ method from natural

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graphite flakes. More details were present in Supporting Information (SI). The GO dispersion

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used in their as-synthesized form without any further treatment was labeled C-GO. The

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annealed graphene oxide dispersion (A-GO) was obtained by alkali-treatment at 70oC. 28 In a

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typical procedure, the C-GO dispersion was diluted to 2 mg/mL, and excess ammonia (18.1

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mol/L) was added. It was then brought to reflux for 30 min at 70oC. After cooling, the mixture

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was centrifugated at 9000 rpm for 3 h, resulting in the separation of a black solid and a

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pale-yellow supernatant. The black solid obtained by high-speed centrifugation was

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re-dispersed with diluted HCl (0.01 M) and refluxed at 70oC for 30 min again. After cooling,

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the mixture was centrifugated at 9000 rpm for 3 h and washed with water for 3 times, and the

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black dispersion obtained was designated as A-GO (annealed graphene oxide).

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2.2 Preparation of conventional graphene aerogel (C-GA) and annealed graphene

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aerogel (A-GA)

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Two different kinds of nanosheets, C-GO and A-GO, were used as building blocks to

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mg/mL, 3 mg/mL, and 4 mg/mL) was poured into desired containers and added with 0.2 mL

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of 1.5 wt% glutaraldehyde solution (G) at 50oC for 12 h, followed by being frozen in a

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refrigerator (-18oC) for 24 h. Then ascorbic acid and HI were added to the GO dispersion with

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a weight ratio of 2: 10: 1 after natural thawing. The dispersion was allowed to react at 60oC

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for 8 h to reduce the GO, washed by alcohol / water mixture for 24 h to remove residual

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ascorbic acid and HI, finally freeze-dried for 48 h. The hydrogels of C-GO and A-GO could

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also be obtained by adding ascorbic acid only. The aerogels obtained by the reduction of

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C-GO or A-GO dispersions were labeled C-GA or A-GA, respectively.

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The G solution was prepared by mixing 1.2 mL of 25% glutaraldehyde, 1 mL of methyl

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alcohol, 0.2 mL of acetic acid, 0.04 mL of sulfuric acid and diluted to 20 mL with deionized

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water.

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2.3 Characterization and test

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The surface element and chemical composition of the nanosheets were analyzed thoroughly

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by ultraviolet-visible (UV-vis) absorption spectra, Raman, X-ray photoelectron spectroscopy

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(XPS), Fourier Transform Infrared Spectroscopy (FTIR). UV-vis absorbance was measured

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using a UV-2550 (Shimadzu) with respect to a water (blank) baseline. Raman spectra were

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conducted by a LabRAM Aramis (Horiba Jobin Yvon S. A. S) excited by a 532

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nm-wavelength laser. The crystallite size (La) were determined according to the

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Tuinstra-Koening relation30,

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excitation wavelength). The XPS data were obtained using a Thermo Scientific Escalab

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250XI with Al Kα source. The XPS core-level spectra were analyzed using the XPS Peakfit

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software, and the C 1s and O 1s spectra are fitting with Gaussian-Lorentzian Waveforms after

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performing a Shirley background subtraction. FTIR were recorded in the 4000-400 cm-1 by a

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Thermo Nicolet FTIR spectrophotometer (iS10) using pellets with KBr powder. For each

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spectrum, 64 scans were collected with a resolution of 4.0 cm-1. The surface topography and

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assemble process were characterized by X-ray diffraction (XRD) and scanning electron

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microscopy (SEM). XRD spectra and SEM images of all the samples were obtained by an X’

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Pert Pro diffractor (PANalytical, Holland) with Cu Kα radiation (λ = 0.15406 nm) and an

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S-4800 (Hitachi, Japan), respectively. The specific surface area (SSA) and pore size

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, La (nm) = (2.4×10-10)⋅λ4⋅(ID/IG)-1 (where λ is the Raman

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distribution of graphene aerogels were measured by N2 adsorption-desorption in liquid N2

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temperature with NOVA-2000E surface area analyzer. The SSA were determined by a

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multipoint Brunauer-Emmett-Teller (BET) analysis of adsorption data with relative pressure

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(P/P0) between 0.05 and 0.3. The compressive tests of the aerogels were carried out by using a homemade universal

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testing machine fitted with a 50 N load cell. Compressive stress-strain response was measured

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at a strain rate of 0.05 mm/s. The water contact angles were tested by contact angle

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goniometer at ambient temperature. The volume of the water droplet was fixed at 6.0 µL.

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In a typical sorption test, a weighted aerogel was immersed into organic liquids for 5 min to

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reach equilibrium, followed by second weight measurement. The weight of a piece of aerogel

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before and after sorption was recorded for calculating the weight gain. The absorption

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recyclability of C-GA and A-GA were carried out in the same way. Considering the relatively

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low boiling point of n-hexane, distillation at 70 oC for 3 h was repeatedly performed for ten

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times. As for absorption-squeezing cycles, an aerogel after absorption was squeezed to half of

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its height by a glass slide, and repeatedly performed for ten times.

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Graphene aerogels used in all characterization, mechanical tests, absorption and

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absorption-squeezing/distillation cycles (unless explicitly stated) are all synthesized with a

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concentration of 2 mg/mL.

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3. RESULTS AND DISCUSSION

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3.1 Phase transformation on GO nanosheets

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To begin, the distribution and transformation of oxygen-containing structures on GO

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nanosheets is discussed. As shown in Figure 1a, the as-synthesized C-GO with randomly

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distributed oxygen containing groups is metastable in nature. After processing with

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alkali-treatment at 70oC, 26, 32 a remarkable phase transformation into distinct oxidized phases

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(oxygen clusters) and enhanced graphitic domains with a detachment of highly oxidized

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debris (OD) was achieved. Utilizing this phase transformation process, we tried tuning the

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distribution of oxygen clusters to create crumples on GO nanosheets by regulating the

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conditions of annealing temperature and solution pH. Subsequently, intriguing 3D graphene

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aerogels were constructed through the assembled process of these crumpled GO sheets via

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Figure 1. Illustration of the distribution and transformation of oxygen clusters on GO sheets

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by annealing treatment (a); The synthesis of C-GA and A-GA through sol-gel method (b). The

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gray and green colors in GO sheets and OD both stand for carbon and were set to make the

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scheme more distinctive. The concentration of GO suspensions was 2 mg/mL.

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The construction process of C-GA and A-GA is depicted in Figure 1b. The synthesized

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C-GO showed well dispersion in water, while a darker dispersion of A-GO was obtained after

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subjected to alkali-treatment at 70 oC to accelerate the phase transformation of oxidized (sp3)

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and boost the graphitic (sp2) domains (Figure 1a). The dispersions of C-GO and A-GO then

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underwent an ice-template freeze casting process. After natural thawing, GO was reduced to

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form hydrogels, and further freeze dried into corresponding aerogels (Figure S1, Supporting

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Information). Interestingly, C-GO tended to re-dissolve in water after thawing while A-GO

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still preserved the architecture of frozen GO (Figure 1b), indicating the generation of more

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cross-linked sites in A-GO nanosheets.

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volume shrinkage of the hydrogel obtained from A-GO, but significant volume contraction

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was observed for C-GO hydrogel.

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By subsequent reduction, there was no obvious

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3.2 Structure and morphology characterizations

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To further clarify the transformation of oxygen clusters on GO nanosheets and the assemble

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mechanism of 3D porous architectures, UV-vis, Raman, XPS, FTIR and XRD measurements

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were performed. UV-vis absorption spectra of C-GO and A-GO are displayed in Figure 2a.

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The typical absorption peak of C-GO dispersion is obtained at about 230 nm, attributed to the

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π-π* transitions of C=C in amorphous carbon systems.

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absorption peak of A-GO remains at 230 nm without a red shift to 260 nm as reduced GO. 33,

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the spectra of C-GO and A-GO, which will disappear almost immediately after exposure to

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hydrazine and reduction.

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ID/IG intensity ratio and crystallite dimensions (La) were calculated in Figure 2b-f. There are

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two characteristic peaks on Raman spectra. The D band is related to the defects participated in

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the double-resonance Raman scattering near K point of the Brillouin zone, whereas the G

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band is activated by the in-plane stretching of C-C bonds. The relative intensity ratio of D and

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G band of GO nanosheets decreases from 1.090 (C-GO) to 1.067 (A-GO), which is rather

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different from variation trend of reduced GO. 35, 36 The removal and decomposition of oxygen

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containing groups covalent binding on GO nanosheets would loss carbon and cause cracking

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on the in-plane C=C, resulting in the generation of more defects and increase of ID/IG.

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The La of A-GO (17.99 nm) is larger than that of C-GO (La = 17.64 nm), suggesting an

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increase in the average size of in-plane graphitic domains of A-GO during the annealing

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treatment. Both UV-vis spectra and ID/IGs prove the preservation and rearrangement of the

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covalent binding oxygen containing groups on GO nanosheets in thermal-base annealing,

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which is coincident with literatures.

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ID/IGs of C-GA and A-GA were measured to be 1.333 and 1.146, respectively, thus their Las

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were calculated to be 14.43 and 16.78 nm, correspondingly. The obviously increased ID/IGs of

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C-GA and A-GA compared to C-GO and A-GO (ID/IGs are 1.090 and 1.067, respectively)

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reveal that the oxygen functional groups have been effectively removed and the in-plane

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graphitic domains became smaller but prevalent during the reduction process.

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much larger crystalline graphitic domains were generated on A-GA (La = 16.78 nm) than

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C-GA (La = 14.43 nm).

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In comparison, the main

Raman and XPS analysis were conducted and the corresponding

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In addition, there is a broad shoulder around 320 nm assigned as n-π* transitions of C=O in

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When GO nanosheets were reduced to aerogels, the

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Similarly,

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The XPS analysis is in great consistent with previous Raman data. After reduction, the O/C

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atomic ratios decrease to 0.16 and 0.21 for C-GA and A-GA, respectively. As is reported, the

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more thorough removal of oxygen covalent binding to the nanosheets, the more defective sites

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may formed. 36 Compared with C-GO (O/C = 0.41), the O/C atomic ratio of A-GO is reduced

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to 0.30, attributing to the effective detachment of adsorbed OD under alkaline condition

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(Figure 1a) and the ignorable removal of covalent linked oxygen clusters.

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fitting results of C-GO and A-GO and more information about the C 1s and O1s spectra are

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present in Figure 2e, f, Figure S2 and Table S1. The C 1s spectra of C-GO and A-GO can be

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deconvolved into three peaks. The binding energy at 284.4-284.8 eV is assigned to C=C, and

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the chemical shifts of +2.0, and +4.0 eV are assigned to C-O, and C=O,

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Clearly, the amount of C-O present in C-GO is comparable to that of C=C (Figure 2e), while

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there is a significant decrease of C-O peak of A-GO along with an increase in the contents of

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C=C (Figure 2f), demonstrating the formation of more graphitic domains on A-GO and the

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detachment of highly oxidized OD. Therefore, the phase transformation via annealing

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generates more graphitic domains with enhanced sizes, which plays important roles in π-π

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interactions of nanosheets and crumples. These results give a good interpretation of the hold

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of 3D architecture of A-GO during the freezing-thawing process (Figure 1b). Simultaneously,

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A-GO nanosheets still showed great dispersibility in water due to the well preserved covalent

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binding oxygen containing groups.

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The C 1s

respectively.

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Figure 2. UV-vis absorption spectra of C-GO and A-GO dispersions and the corresponding

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photographs shown in insets (left: C-GO, right: A-GO) (a); Raman spectra of samples (b);

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crystallite dimensions (La) and ID/IGs of all samples derived by analyzing Raman data (c);

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XPS spectra of samples (d-f).

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The phase transformation on GO nanosheets during annealing and reduction is further

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evidenced by FTIR measurements (Figure 3a). The absorption peak at 1625 cm-1 of C-GO,

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which is assigned to C=C skeletal vibrations of graphitic domains and deformation vibration

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of intercalated water (scissor mode), remains unchanged even after annealing, demonstrating

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the transformation rather than removal of oxygen containing groups on A-GO sheets. By 10

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appearance of a new peak at 1580 cm-1, ascribed to the formation of prominent graphitic

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domains in the nanosheets. These results are in good agreement with Raman and XPS spectra,

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suggesting the oxygen transformation of A-GO and effective reduction of GO nanosheets in

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C-GA and A-GA.

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The XRD patterns were used to investigate the assembly features of the nanosheets. As

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illustrated in Figure 3b, the XRD patterns of C-GO, A-GO, and C-GA show peaks centered at

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2θ = 11.5°, 10.8°, and 24.2° respectively, and their d-spacings are calculated to be 0.77, 0.82,

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and 0.37 nm correspondingly. The d-spacing values of C-GO and A-GO are much larger than

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that of graphite powder (0.34 nm) (Figure 3b), indicating the presence of large numbers of

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oxygen functional groups on GO sheets. 35, 36 Intriguingly, the d-spacing of A-GO (0.82 nm) is

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slightly larger than that of C-GO (0.77 nm). As the oxygen-containing groups on A-GO are

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less than those on C-GO (evidenced by XPS data), these results can be explained by the

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out-of-plane deformation of the extended nanosheets

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later. C-GA presents similar d-spacing with graphite powder (0.34 nm) in Figure 3b,

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manifesting the effective elimination of most oxygen functional groups on the GO plane by

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ascorbic acid and HI reduction and the face-to-face restack of nanosheets due to the strong

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π-π interactions. It’s noteworthy that there is no peak in the XRD pattern of A-GA, which is

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rather different from other graphene aerogels in literatures,

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not only show well dispersion in water,

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restacking during reduction.

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and verified by SEM characterization

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suggesting that A-GO sheets

but also present excellent self-avoiding

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Figure 3. FTIR spectra of C-GO, A-GO, C-GA and A-GA (a); XRD patterns of C-GO, A-GO,

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The surface topography of C-GO and A-GO sheets, and the assembly process of C-GA and

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A-GA were determined by SEM. Apparently, C-GO nanosheets show a smooth surface

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(Figure 4a and b) and fit with each other well to form regular stacking (rectangle shown in

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Figure 4b). It should be noted that some ice crystal structures can also be observed in the

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structure of C-GO (circle in Figure 4b) induced by the direct freezing-drying process. In

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contrast, A-GO sheets appear as a more flexural morphology with highly wrinkled borders

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(Figure 4c and d). The wrinkles of A-GO create local curvatures or distortion and disturb

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regular restacking of the nanosheets, matching well with above XRD analysis (Figure 3b).

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The assembly principles and porous architectures of C-GA and A-GA are further studied.

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When assembled into 3D architectures, C-GA network exhibit a typical well-defined

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honeycomb-like structures (Figure 4e-h), which is similar to regular self-assembled graphene

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aerogels.

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direction as the various SEM images of top-view and side-view of C-GA in Figure S3. In a

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magnified SEM images, we can clearly see that those stiff plates are consist of a stack of

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graphene nanosheets. The nanosheets restack with each other tightly to form graphite-like

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flakes and exhibit smooth morphology with much less wrinkles. Some raised edges of some

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stacked nanosheets can be observed clearly (Figure 4g and h). Of particular interest is that the

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architecture of A-GA is in sharp contrast to that of C-GA. The SEM images show more

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randomly oriented microstructure of A-GA. The plate interacted with each other in vertical or

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parallel manners through strong edge-to-edge cross-linking (Figure 4i-l). With high

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magnification in Figure 4k and l, the sheets are ultrathin, flexible and clean without any tilted

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edges. Notably, typical crumples can be observed clearly on all the nanosheets of A-GA. The

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structures appear in a well orderly microscale folding with the alignment perpendicular to the

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shrinkage direction, which is rather different from the disordered nanoscale wrinkles of GO

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sheets.

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orderly crumples by tuning oxygen clusters on nanosheets. What’s more, the ice crystal

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structures are clearly observed in the structure of A-GA (marked as circles in Figure 4k, l),

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which shall derive from the freezing-thawing process and provide more evidences about the

The lamellas are stiff and assembled with each other face to face in the same

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Figure 4k, l provides the direct and convincing images for the creating well

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ACCEPTED MANUSCRIPT generation of more cross-link sites on A-GO. The high crumpled, flexible graphene sheets

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with enough cross-link sites can disturb the regular face-to-face stacking and, as a result,

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bring little volume shrinkage, decreased density of dried A-GA and highly enhanced the

306

elastic property. More microscopic structures about the top-view and side-view of C-GA and

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A-GA are provided in Supporting Information.

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Figure 4. SEM images of C-GO (a, b), A-GO (c, d), C-GA (e-h), and A-GA (i-l) at different

310

magnifications (top-view).

To further investigate the specific areas and porosity of C-GA and A-GA, N2

312

adsorption-desorption measurements were performed (Figure S7). Both adsorption-desorption

313

isotherms represent unrestricted uptake over a range of high P/P0, attributed to type II, which

314

suggested that both aerogels were macro-porous structures. The specific surface areas (SSA)

315

of A-GA was 97.18 m2/g, much larger than that of C-GA (SSAC-GA=12.88 m2/g). The

316

corresponding pore size distributions (Figure S7b) also confirmed more micro and mesopores

317

existed in A-GA. It is an irrefutable fact that the π-π restacking of GO sheets in A-GA have

318

been significantly inhibited and abundant diffusion paths and voids created, according to the

319

increased SSA and higher micropore and mesopore volumes, which were beneficial for

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pollutant diffusion and adsorption.

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3.3 Crumpled and assemble process of C-GO and A-GO

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the experimental phenomena observed in Figure 1b and reveal crumpled and assemble

324

mechanisms of C-GA and A-GA. The change of the appearances of C-GO and A-GO, and the

325

formation of the corresponding aerogels are shown in Figure 5 based on the above results and

326

analysis. It is well known that GO appears in extended states in good solvents, such as water

327

due to the hydrophilic oxygen containing groups randomly attached to its molecular backbone.

328

The extended GO sheets tend to evolve to crumpled state when introduced into poor solvents,

329

which is ascribed to the weak interactions between nanosheets and solvent molecules.

330

this study, the interactions between nanosheets and water molecules can also be weakened and

331

the uniform H-bond networks mediated by oxygen-containing groups and water molecules are

332

interrupted, via the rearrangement of oxygen clusters to generate more prevalent and greater

333

hydrophobic graphitic domains on A-GO (Figure 5a). The chemically heterogeneous

334

distribution of hydrophilic oxygen clusters and hydrophobic graphitic domains, and the

335

removal of highly dispersive OD lead to dynamic unbalance between flat GO nanosheets and

336

water molecules and the pristine GO sheets become spontaneous crumpled. As the generation

337

of more and larger graphitic domains (cross-linked sites), 3D porous architectures of A-GO

338

can be formed directly through an ice-template in freezing-thawing process and well

339

preserved in further reduction (Figure 5b). The crumples on nanosheets could be further

340

intensified in the pre-built 3D architecture. Differently, the construction of C-GA is ascribed

341

to the self-assembly of reduced nanosheets, which inevitably lead to the face-to-face

342

restacking of the nanosheets. 12

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In this study, we revealed that the extended nanosheets could be crumpled and assembled

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into 3D porous architectures directly in water by tuning oxygen clusters on GO nanosheets

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without introducing any other toxic organic solvents, such as ethyl acetate, or acetone via a

346

common sol-gel method. As water is the most commonly used medium for the synthesis and

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decoration of graphene-based materials, it is highly significant to modify the morphology of

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GO nanosheets directly in water without any extra solvents.

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Figure 5. Schematic illustration of spontaneous crumpled process of GO sheets (a) and

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different synthetic routes of C-GA and A-GA (b). 3.4 Mechanical and absorption properties of graphene aerogels

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Based on the successful construction of crumpled graphene aerogels using sol-gel method,

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it is expected to possess enhanced elasticity, high absorption capacity of organic pollutants

355

and excellent recycle performance of A-GA. The mechanical testing of C-GA and A-GA was

356

carried out to study their compressive strength, compressibility, and recoverability (up to 50%

357

strain (ε)). The stress-strain curves show an initial linear elastic region (ε < 35%-40%) at

358

relatively low stress levels after second cycle (Figure 6), indicating a typical feature of porous

359

material with high porosity and softness. The curves become compaction and steep at ε >

360

35%-40% with the rapidly increased stress, which is mainly attributed to the densification of

361

aerogels. In the case of C-GA (Figure 6a and b), there is a large irrecoverable plastic

362

deformation of about >15% during the cycles as evidenced by the zero-stress response at ε ≤

363

15%. When loaded with a 100 g weight, C-GA can be compressed to 50% of original height.

364

It is worth noting that the bottom of C-GA is crushed during the compression, while the upper

365

part is almost unchanged (Figure 6c, SI Video 1). It is better to regard it as crush rather than

366

compression. The Young’s modulus of C-GA is much larger than A-GA. The thick and stiff

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sheets in constructing GA (in Figure 4) are responsible for the stiffness and fragile of C-GA.

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12, 45

369

decreases by only 6% during all the cycles for A-GA (2 mg/mL), while it tends to less than 3%

370

when the GO concentration increases to 4 mg/mL (Figure 6d and e). The hysteresis loop

371

shrinks for the second cycle but stay the same after third cycle. More significantly, A-GA can

372

be compressed to less than 10% by loading 100 g weight and completely recover to its

373

original height instantaneously after removing the load (Figure 6f, and SI Video 2).

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In contrast, A-GA exhibit an outstanding softness and compressibility. The peak stress

Figure 6. Compressive stress-strain curves of C-GA (a, b) and A-GA (d, e) prepared with

376

different GO concentrations; digital photographs of the compression-recovering process of

377

C-GA (c) and A-GA (f).

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Figure 7. Digital photographs showing the process of absorbing chloroform (dyed with oil 16

ACCEPTED MANUSCRIPT 380

red O) from the chloroform/water mixture with A-GA and the water contact angle

381

measurements of C-GA and A-GA (a); Comparison of organic liquids and oil absorption

382

capacities of C-GA and A-GA (b). Figure 7a reveals that the as-synthesized A-GA has excellent water repellency with highly

384

hydrophobic surface. It can float on water and a uniform silver mirror-like surface is observed

385

on the surface of A-GA when partly or totally immersed into water by an external force

386

(Figure 7a), similar to the previous superhydrophobic aerogels.

387

non-wetting behavior is the air layer between the surface of aerogel and water.

388

chloroform (dyed with oil red O) with higher density at the bottom of water can be selectively

389

absorbed by A-GA within 2 s, as demonstrated in Figure 7a and SI Video 3. The water contact

390

angle (CA) measurements further clarify the more hydrophobic surface of A-GA than C-GA,

391

making A-GA as promising candidates for selective absorption of organic liquid or oil in

392

aqueous. In Figure 7b, we choose six kinds of organic solvent liquids to study the absorption

393

capacity of C-GA and A-GA. Importantly, A-GA exhibits an absorption capacity of 154-325

394

g/g for various oils and organic solvents, which is 224%-406% larger than that of C-GA, and

395

much higher than most of the pioneering reported graphene-based macrostructures

396

synthesized at similar temperatures, such as chemical reduction or hydrothermal reduction

397

without high-temperature treatment (> 500oC), and cellulose fibers

398

with high temperature treatments (Table 1). Notably, our A-GA show comparable water

399

repellency and a much more excellent absorption capacity than those superhydrophobic and

400

superoleophilic GA modified with n-dodecyltriethoxysilane (DDTS)/cellulose (~80 g/g for

401

n-hexane)49, perfluorodecanethiol (PFDT) (~12 g/g for n-hexane)

402

(PVDF) (~65 g/g for chloroform) 40, and ι-phenylalanine (~225 g/g for chloroform) 51. Apart

403

from the comparable or slightly lower absorption capacity of A-GA compared with GA-based

404

materials treated under high temperatures (≥ 500oC, N2/Ar/H2), the fabrication method of

405

A-GA is much simpler and energy-saving without any special equipment.

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The reason behind the 46

Moreover,

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and CNT sponges

48

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406 407 408

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, polyvinylidene fluoride

ACCEPTED MANUSCRIPT Table 1. Summary of n-hexane and chloroform absorption capacities of various carbon

410

aerogels for comparison with our work.

A-GA

n-Hexane (g/g) 156.5

Chloroform (g/g) 325.1

80oC (EDA, 24 h)

EDA/GA

~120

\

52

60oC (HI, 8 h)

PVA/GA

\

~274

53

70oC (AA, 4 h, directional freezing) 80oC (HI, 8 h)

GA

~105

\

54

fluorinated GA

\

~112

55

~107

~225

51

Materials

60oC (HI/AA, 8 h)

95oC (ι-phenylalanine, 48 h) 90oC (12 h)

180oC (6 h) 160oC (10 h)

GA

~125

~235

56

GA

~90

~180

57

GA

~52

\

9

PVDF/GA

~28

~65

40

~110

\

58

180oC (24 h)

GA

~43

~86

59

750oC (N2, 3 h)

GA

~187

~350

56

CNT/GA

~140

~265

57

ultralight GA

~160

282.9

60

GA (melamine foam as sacrificial skeleton) MWCNT-PDA/ GA cellulose fibers aerogels CNT sponge

~176

~460

61

~225

~533

62

~80

~161

47

~88

~175

48

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CNT/GA

800oC (acetylene, Ar, 1 h) o 1000 C (N2, 2 h) 500oC (Ar/H2, 1 h)

750oC (N2, 3 h) 1000oC (N2, 4 h) 860oC (Ar/H2, 4 h) 411

Our work

120oC (12 h)

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High temperature treated graphene aerogels (GA)

120oC (12 h)

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Hydrothermal reduced graphene aerogels (GA)

GA

Refs.

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Temperature

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Synthetic method Chemical reduced graphene aerogels (GA)

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It is very important with excellent recyclability of absorbent and easy collection of 18

ACCEPTED MANUSCRIPT pollutants because of the costly and noxious of most solvents. Squeezing and distillation

413

methods were performed to evaluate the recyclability of absorbents in our work. For

414

recovering valuable solvents, squeezing is a simple and alternative method. The absorption

415

capacity and its integrated 3D shape of A-GA are retained even after 10 absorption-squeezing

416

cycles, while C-GA crushes into small pieces only after 6 cycles, as depicted in the inset of

417

Figure 8a. The other distillation process was carried out at 70 oC in oven to recovery the

418

absorbed n-hexane solvent. As displayed in Figure 8b, there is almost no change for the

419

absorption capacity of A-GA and C-GA after the 2nd cycle. These results demonstrate that the

420

fabricated A-GA exhibit excellent stability under both absorption-squeezing and distillation

421

cycles, which shows promising character of this material for organic pollutants removal

422

applications.

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Figure 8. Recyclability of C-GA and A-GA for absorption of n-hexane under

425

absorption-squeezing cycles (a), and absorption-distillation cycles (b).

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4. CONCLUSIONS

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In summary, by tuning oxygen clusters on GO nanosheets, we have successfully

428

synthesized 3D porous graphene aerogels assembled with orderly crumpled nanosheets via a

429

facile sol-gel method. The created crumples greatly inhibit the tight restacking of GO

430

nanosheets, thus create sufficient inner channels and preserve the available of the active

431

adsorption sites on both sides of nanosheets. More cross-linked sites on GO are also generated

432

during annealing process, which not only enhance the mechanical stability of graphene

433

aerogels, but also further intensify the crumples on the reduced building blocks. The crumpled 19

ACCEPTED MANUSCRIPT nanosheets endow A-GA with high elasticity and hydrophobicity, which show high absorption

435

speed and capacity, and excellent recyclability in water pollutant treatments. These results

436

demonstrate that it is a promising way to enhance the performance of graphene aerogels

437

through creating crumples on the sheets via controlling oxygen clusters. Our work emphasizes

438

the importance to exercise any regulations over the physical or chemical structure of this

439

“miracle” 2D nanosheets itself before further device applications, rather than being employed

440

or modified in their originally synthesized form.

442

ASSOCIATED CONTENT

443

Supporting information

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Preparation process of graphene oxide (GO) nanosheets, photos of synthesized C-GA and

445

A-GA (Figure S1), distribution of C 1s species of C-GA and A-GA (Figure S2, Table S1),

446

typical top-view and side-view SEM images of C-GA (Figure S3), A-GA (Figure S4), FTIR

447

spectrum of OD (Figure S5) and photos of synthesized C-GA and A-GA with different

448

reductants (Figure S6) and N2 adsorption-desorption isotherms and pore size distributions of

449

C-GA and A-GA (Figure S7) were presented.

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Video 1 showed the compression and recovering process of C-GA.

451

Video 2 showed the compression and recovering process of A-GA.

452

Video 3 showed A-GA picking up non-polar chloroform dyed with oil red O from the

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chloroform/water mixture.

454

Notes

455 456 457

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The authors declare no competing financial interests.

ACKNOWLEDGEMENTS

458

This work was financially supported by National Natural Science Foundation of China

459

(51478449, 21806163 and 51778598), Science and Technology Program of Xiamen

460

(3502Z20172022) and China-Japanese Research Cooperative Program (2016YFE0118000).

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REFERENCES

463

1.

Gorgolis, G.; Galiotis, C., Graphene aerogels: a review. 2D Mater. 2017, 4, (3), 032001-1-032001-21.

464

2.

Chabot, V.; Higgins, D.; Yu, A.; Xiao, X.; Chen, Z.; Zhang, J., A review of graphene and graphene oxide

465

sponge: Material synthesis and applications to energy and the environment. Energy Environ. Sci. 2014, 7, (5),

466

1564-1596.

467

3.

468

Applications in Sustainable Energy and Environment. Chem. Mater. 2016, 28, (22), 8082-8118.

469

4.

470

macrostructures: adsorption, transformation, and detection. Environ. Sci. Technol. 2015, 49, 67-84.

471

5.

472

water purification: from the nanoscale to specific devices. Environ. Sci.: Nano 2018, 5, 1264–1297.

473

6.

474

Architectures: Biointeractions, Fabrications, and Emerging Biological Applications. Chem. Rev. 2017, 117, (3),

475

1826-1914.

476

7.

477

based adsorbents for high-efficient water treatment via the promotion of biopolymers. J. Hazard. Mater. 2013,

478

263, 467-78.

479

8.

480

polydopamine-layer on graphene: Mechanisms, versatile 2D and 3D architectures and pollutant disposal. Chem.

481

Eng. J. 2013, 228, 468-481.

482

9.

483

low density graphene aerogels. J. Mater. Chem. A 2017, 5, (44), 23123-23130.

484

10. Huang, H.; Chen, P.; Zhang, X.; Lu, Y.; Zhan, W., Edge-to-Edge Assembled Graphene Oxide Aerogels with

485

Outstanding Mechanical Performance and Superhigh Chemical Activity. Small 2013, 9, (8), 1397-1404.

486

11. Fang, Q.; Shen, Y.; Chen, B., Synthesis, decoration and properties of three-dimensional graphene-based

487

macrostructures: A review. Chem. Eng. J. 2015, 264, 753-771.

488

12. Qiu, L.; Liu, J. Z.; Chang, S. L.; Wu, Y.; Li, D., Biomimetic superelastic graphene-based cellular monoliths.

489

Nat. Commun. 2012, 3, 1241-1-1241-7.

490

13. Gao, L.; Wang, F.; Zhan, W.; Wang, Y.; Sui, G.; Yang, X., Viable synthesis of highly compressible,

RI PT

462

Gong, X.; Liu, G.; Li, Y.; Yu, D. Y. W.; Teoh, W. Y., Functionalized-Graphene Composites: Fabrication and

SC

Shen, Y.; Fang, Q.; Chen, B., Environmental applications of three-dimensional graphene-based

M AN U

Yang, K.; Wang, J.; Chen, X.; Zhao, Q.; Ghaffar, A.; Chen, B., Application of graphene-based materials in

Cheng, C.; Li, S.; Thomas, A.; Kotov, N. A.; Haag, R., Functional Graphene Nanomaterials Based

TE D

Cheng, C. S.; Deng, J.; Lei, B.; He, A.; Zhang, X.; Ma, L.; Li, S.; Zhao, C., Toward 3D graphene oxide gels

EP

Cheng, C.; Li, S.; Zhao, J.; Li, X.; Liu, Z.; Ma, L.; Zhang, X.; Sun, S.; Zhao, C., Biomimetic assembly of

AC C

Hu, K.; Szkopek, T.; Cerruti, M., Tuning the aggregation of graphene oxide dispersions to synthesize elastic,

21

ACCEPTED MANUSCRIPT ultra-light graphene-carbon nanotube composite aerogels without additional reductant and their applications for

492

strain-sensitivity. Chem. Commun. 2017, 53, (3), 521-524.

493

14. Lui, C. H.; Liu, L.; Mak, K. F.; Flynn, G. W.; Heinz, T. F., Ultraflat graphene. Nature 2009, 462, (7271),

494

339-341.

495

15. Nicholl, R. J.; Conley, H. J.; Lavrik, N. V.; Vlassiouk, I.; Puzyrev, Y. S.; Sreenivas, V. P.; Pantelides, S. T.;

496

Bolotin, K. I., The effect of intrinsic crumpling on the mechanics of free-standing graphene. Nat. Commun. 2015,

497

6, 8789-1-8789-7.

498

16. Luo, J. Y.; Jang, H. D.; Huang, J. X., Effect of Sheet Morphology on the Scalability of Graphene-Based

499

Ultracapacitors. ACS nano 2013, 7, (2), 1464-1471.

500

17. Luo, J.; Jang, H. D.; T., S.; Xiao, L.; He, Z.; Katsoulidis, A. P.; Kanatzidis, M. G.; Gibson, J. M.; Huang, J.,

501

Compression and Aggregation-Resistant Particles of Crumpled Soft Sheets. ACS nano 2011, 5, 8943-8949.

502

18. Feng, C.; Yi, Z.; She, F.; Gao, W.; Peng, Z.; Garvey, C. J.; Dumee, L. F.; Kong, L., Superhydrophobic and

503

Superoleophilic Micro-Wrinkled Reduced Graphene Oxide as a Highly Portable and Recyclable Oil Sorbent.

504

ACS Appl. Mater. Interfaces 2016, 8, (15), 9977-9985.

505

19. Kim, J.; Cote, L. J.; Huang, J., Two dimensional soft material: New faces of graphene oxide. Acc. Chem.

506

Res. 2012, 45, 1356-1364.

507

20. Fasolino, A.; Los, J. H.; Katsnelson, M. I., Intrinsic ripples in graphene. Nat. Mater. 2007, 6, (11), 858-861.

508

21. Ma, X. F.; Zachariah, M. R.; Zangmeister, C. D., Crumpled Nanopaper from Graphene Oxide. Nano Lett.

509

2012, 12, (1), 486-489.

510

22. Thomas, A. V.; Andow, B. C.; Suresh, S.; Eksik, O.; Yin, J.; Dyson, A. H.; Koratkar, N., Controlled

511

Crumpling of Graphene Oxide Films for Tunable Optical Transmittance. Adv. Mater. 2015, 27, (21), 3256-3265.

512

23. Deng, S. K.; Berry, V., Wrinkled, rippled and crumpled graphene: an overview of formation mechanism,

513

electronic properties, and applications. Mater. Today 2016, 19, (4), 197-212.

514

24. Chen, Z.; Wang, J.; Umar, A.; Wang, Y.; Li, H.; Zhou, G., Three-Dimensional Crumpled Graphene-Based

515

Nanosheets with Ultrahigh NO2 Gas Sensibility. ACS Appl. Mater. Interfaces 2017, 9, 11819-11827.

516

25. Chen, P. Y.; Liu, M.; Wang, Z.; Hurt, R. H.; Wong, I. Y., From Flatland to Spaceland: Higher Dimensional

517

Patterning with Two-Dimensional Materials. Adv. Mater. 2017, 1605096-1-1605096-16.

518

26. Kumar, P. V.; Bardhan, N. M.; Tongay, S.; Wu, J.; Belcher, A. M.; Grossman, J. C., Scalable enhancement

519

of graphene oxide properties by thermally driven phase transformation. Nat. Chem. 2014, 6, (2), 151-158.

520

27. Kim, S.; Zhou, S.; Hu, Y.; Acik, M.; Chabal, Y. J.; Berger, C.; de Heer, W.; Bongiorno, A.; Riedo, E.,

AC C

EP

TE D

M AN U

SC

RI PT

491

22

ACCEPTED MANUSCRIPT Room-temperature metastability of multilayer graphene oxide films. Nat. Mater. 2012, 11, (6), 544-549.

522

28. Rourke, J. P.; Pandey, P. A.; Moore, J. J.; Bates, M.; Kinloch, I. A.; Young, R. J.; Wilson, N. R., The real

523

graphene oxide revealed: stripping the oxidative debris from the graphene-like sheets. Angew. Chem. Int. Ed.

524

2011, 50, (14), 3173-3177.

525

29. Liu, Z.; Jiang, L.; Sheng, L.; Zhou, Q.; Wei, T.; Zhang, B.; Fan, Z., Oxygen clusters distributed in graphene

526

with “Paddy Land” structure: Ultrahigh capacitance and rate performance for supercapacitors. Adv. Funct. Mater.

527

2017, 1705258-1-1705258-11.

528

30. Moon, I. K.; Yoon, S.; Chun, K.-Y.; Oh, J., Highly Elastic and Conductive N-Doped Monolithic Graphene

529

Aerogels for Multifunctional Applications. Adv. Funct. Mater. 2015, 25, (45), 6976-6984.

530

31. Xin, G.; Yao, T.; Sun, H.; Scott, S. M.; Shao, D.; Wang, G.; Lian, J., Highly thermally conductive and

531

mechanically strong graphene fibers. Science 2015, 349, (6252), 1083-1087.

532

32. Thomas, H. R.; Day, S. P.; Woodruff, W. E.; Vallés, C.; Young, R. J.; Kinloch, I. A.; Morley, G. W.; Hanna,

533

J. V.; Wilson, N. R.; Rourke, J. P., Deoxygenation of graphene oxide: reduction or cleaning? Chem. Mater. 2013,

534

25, (18), 3580-3588.

535

33. He, G.; Chen, H.; Zhu, J.; Bei, F.; Sun, X.; Wang, X., Synthesis and characterization of graphene paper with

536

controllable properties via chemical reduction. J. Mater. Chem. 2011, 21, (38), 14631–14638.

537

34. Eda, G.; Lin, Y. Y.; Mattevi, C.; Yamaguchi, H.; Chen, H. A.; Chen, I. S.; Chen, C. W.; Chhowalla, M., Blue

538

photoluminescence from chemically derived graphene oxide. Adv. Mater. 2010, 22, (4), 505-509.

539

35. Fang, Q.; Zhou, X.; Deng, W.; Liu, Z., Ordered self-assembly of amphipathic graphene nanosheets into

540

three-dimensional layered architectures. Nanoscale 2016, 8, (1), 197-203.

541

36. Chen, J.; Li, Y.; Huang, L.; Jia, N.; Li, C.; Shi, G., Size Fractionation of Graphene Oxide Sheets via

542

Filtration through Track-Etched Membranes. Adv. Mater. 2015, 27, (24), 3654-60.

543

37. Eigler, S.; Dotzer, C.; Hirsch, A., Visualization of defect densities in reduced graphene oxide. Carbon 2012,

544

50, (10), 3666-3673.

545

38. Bardhan, N. M.; Kumar, P. V.; Li, Z.; Ploegh, H. L.; Grossman, J. C.; Belcher, A. M.; Chen, G. Y.,

546

Enhanced cell capture on functionalized graphene oxide nanosheets through oxygen clustering. ACS nano 2017,

547

11, (2), 1548-1558.

548

39. Chen, X.; Chen, B., Direct observation, molecular structure, and location of oxidation debris on graphene

549

oxide nanosheets. Environ. Sci. Technol. 2016, 50, (16), 8568-8577.

550

40. Li, R.; Chen, C.; Li, J.; Xu, L.; Xiao, G.; Yan, D., A facile approach to superhydrophobic and

AC C

EP

TE D

M AN U

SC

RI PT

521

23

ACCEPTED MANUSCRIPT superoleophilic graphene/polymer aerogels. J. Mater. Chem. A 2014, 2, (9), 3057.

552

41. Hu, H.; Zhao, Z.; Wan, W.; Gogotsi, Y.; Qiu, J., Ultralight and highly compressible graphene aerogels. Adv.

553

Mater. 2013, 25, (15), 2219-23.

554

42. Xu, X.; Zhang, Q.; Yu, Y.; Chen, W.; Hu, H.; Li, H., Naturally Dried Graphene Aerogels with

555

Superelasticity and Tunable Poisson's Ratio. Adv. Mater. 2016, 28, (41), 9223-9230.

556

43. Lee, W. K.; Kang, J. M.; Chen, K. S.; Engel, C. J.; Jung, W. B.; Rhee, D.; Hersam, M. C.; Odom, T. W.,

557

Multiscale, Hierarchical Patterning of Graphene by Conformal Wrinkling. Nano Lett. 2016, 16, (11), 7121-7127.

558

44. Xiao, Y.; Xu, Z.; Liu, Y.; Peng, L.; Xi, J.; Fang, B.; Guo, F.; Li, P.; Gao, C., Sheet Collapsing Approach for

559

Rubber-like Graphene Papers. ACS Nano 2017, 11, (8), 8092-8102.

560

45. Zhao, X.; Yao, W.; Gao, W.; Chen, H.; Gao, C., Wet-Spun Superelastic Graphene Aerogel Millispheres with

561

Group Effect. Adv. Mater. 2017, 29, (35), 1701482-1-1701482-9.

562

46. Zhu, Q.; Pan, Q.; Liu, F., Facile Removal and Collection of Oils from Water Surfaces through

563

Superhydrophobic and Superoleophilic Sponges. J. Phys. Chem. C 2011, 115, (35), 17464-17470.

564

47. Li, L.; Hu, T.; Sun, H.; Zhang, J.; Wang, A., Pressure-Sensitive and Conductive Carbon Aerogels from

565

Poplars Catkins for Selective Oil Absorption and Oil/Water Separation. ACS Appl. Mater. Interfaces 2017, 9,

566

(21), 18001-18007.

567

48. Gui, X.; Wei, J.; Wang, K.; Cao, A.; Zhu, H.; Jia, Y.; Shu, Q.; Wu, D., Carbon nanotube sponges. Adv. Mater.

568

2010, 22, (5), 617-21.

569

49. Mi, H.-Y.; Jing, X.; Politowicz, A. L.; Chen, E.; Huang, H.-X.; Turng, L.-S., Highly compressible

570

ultra-light anisotropic cellulose/graphene aerogel fabricated by bidirectional freeze drying for selective oil

571

absorption. Carbon 2018, 132, 199-209.

572

50. Cao, N.; Lyu, Q.; Li, J.; Wang, Y.; Yang, B.; Szunerits, S.; Boukherroub, R., Facile synthesis of fluorinated

573

polydopamine/chitosan/reduced graphene oxide composite aerogel for efficient oil/water separation. Chem. Eng.

574

J. 2017, 326, 17-28.

575

51. Xu, L.; Xiao, G.; Chen, C.; Li, R.; Mai, Y.; Sun, G.; Yan, D., Superhydrophobic and superoleophilic

576

graphene aerogel prepared by facile chemical reduction. J. Mater. Chem. A 2015, 3, (14), 7498-7504.

577

52. Li, J.; Li, J.; Meng, H.; Xie, S.; Zhang, B.; Li, L.; Ma, H.; Zhang, J.; Yu, M., Ultra-light, compressible and

578

fire-resistant graphene aerogel as a highly efficient and recyclable absorbent for organic liquids. J. Mater. Chem.

579

A 2014, 2, (9), 2934-2941.

580

53. Ye, S.; Liu, Y.; Feng, J., Low-Density, Mechanical Compressible, Water-Induced Self-Recoverable

AC C

EP

TE D

M AN U

SC

RI PT

551

24

ACCEPTED MANUSCRIPT Graphene Aerogels for Water Treatment. ACS Appl. Mater. Interfaces 2017, 9, (27), 22456-22464.

582

54. Liu, T.; Huang, M.; Li, X.; Wang, C.; Gui, C.-X.; Yu, Z.-Z., Highly compressible anisotropic graphene

583

aerogels fabricated by directional freezing for efficient absorption of organic liquids. Carbon 2016, 100,

584

456-464.

585

55. Hong, J.-Y.; Sohn, E.-H.; Park, S.; Park, H. S., Highly-efficient and recyclable oil absorbing performance of

586

functionalized graphene aerogel. Chem. Eng. J. 2015, 269, 229-235.

587

56. Wang, F.; Wang, Y.; Zhan, W.; Yu, S.; Zhong, W.; Sui, G.; Yang, X., Facile synthesis of ultra-light graphene

588

aerogels with super absorption crossManc capability for organic solvents and strain-sensitive electrical

589

conductivity. Chem. Eng. J. 2017, 320, 539-548.

590

57. Wang, C.; Yang, S.; Ma, Q.; Jia, X.; Ma, P., Preparation of carbon nanotubes/graphene hybrid aerogel and

591

its application for the adsorption of organic compounds. Carbon 2017, 118, 765-771.

592

58. Wan, W.; Zhang, R.; Li, W.; Liu, H.; Lin, Y.; Li, L.; Zhou, Y., Graphene-carbon nanotube aerogel as an

593

ultra-light, compressible and recyclable highly efficient absorbent for oil and dyes. Environ. Sci.: Nano 2016, 3,

594

(1), 107-113.

595

59. Bi, H.; Xie, X.; Yin, K.; Zhou, Y.; Wan, S.; He, L.; Xu, F.; Banhart, F.; Sun, L.; Ruoff, R. S., Spongy

596

Graphene as a Highly Efficient and Recyclable Sorbent for Oils and Organic Solvents. Adv. Funct. Mater. 2012,

597

22, (21), 4421-4425.

598

60. Li, L.; Li, B.; Zhang, J., Dopamine-mediated fabrication of ultralight graphene aerogels with low volume

599

shrinkage. J. Mater. Chem. A 2016, 4, (2), 512-518.

600

61. Li, C.; Jiang, D.; Liang, H.; Huo, B.; Liu, C.; Yang, W.; Liu, J., Superelastic and Arbitrary-Shaped

601

Graphene Aerogels with Sacrificial Skeleton of Melamine Foam for Varied Applications. Adv. Funct. Mater.

602

2017.

603

62. Zhan, W.; Yu, S.; Gao, L.; Wang, F.; Fu, X.; Sui, G.; Yang, X., Bioinspired Assembly of Carbon Nanotube

604

into Graphene Aerogel with "cabbagelike" Hierarchical Porous Structure for Highly Efficient Organic Pollutants

605

Cleanup. ACS Appl. Mater. Interfaces 2018, 10, (1), 1093-1103.

AC C

EP

TE D

M AN U

SC

RI PT

581

25