Compressible, amphiphilic graphene-based aerogel using a molecular glue to link graphene sheets and coated-polymer layers

    Compressible, amphiphilic graphene-based aerogel using a molecular glue to link graphene sheets and coated-polymer layers Yu Xiang, Libin Liu, Ting Li, Zhao Dang PII: DOI: Reference:

S0264-1275(16)31120-0 doi: 10.1016/j.matdes.2016.08.057 JMADE 2210

To appear in: Received date: Revised date: Accepted date:

3 June 2016 17 August 2016 18 August 2016

Please cite this article as: Yu Xiang, Libin Liu, Ting Li, Zhao Dang, Compressible, amphiphilic graphene-based aerogel using a molecular glue to link graphene sheets and coated-polymer layers, (2016), doi: 10.1016/j.matdes.2016.08.057

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ACCEPTED MANUSCRIPT Compressible, amphiphilic graphene-based aerogel using a molecular glue to link graphene sheets and coated-polymer layers

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Yu Xiang, Libin Liu*, Ting Li, Zhao Dang

Shandong Provincial Key Laboratory of Fine Chemicals, School of Chemistry of

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Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, China

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Corresponding authors: Fax: +86 531 89631208, E-mail: [email protected] (L. B. Liu)

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Abstract

A universal method to fabricate compressible graphene-based aerogel has been

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developed by using a molecular glue strategy, such as -oxo-1-pyrenebutyric acid

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(OPBA). The OPBA could link the graphene skeleton sheets and dip-coated polymer layers, where the graphene skeleton could be obtained by hydrothermal or chemical

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reduction. In contrast to the brittle pristine graphene aerogel, the resulting polymer-coated graphene aerogel demonstrates high elastic properties. The improved compressible properties are attributed to the uniform coating of polymer on the

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graphene sheet and the effective stress transfer between the graphene sheet and polymer layers, which are enabled by OPBA molecular glue. In addition, this type of strong integrated graphene-based aerogel could be used in practical applications, such as hydrophilic and oleophilic intelligence and compressible electrical sensor.

Keywords: Aerogel, Graphene, Compressibility, Amphiphilicity, Sensor

1. Introduction Aerogels, as three-dimensional (3D), lightweight, highly porous structures, have recently attracted great attention due to their broad applications [1-4]. Consequently, various materials such as graphene [5-7], carbon nanotubes [8-11], silica [12,13], cellulose nanofibrils [14,15], boron nitrides [16], polymers [17,18], nanoparticales [19] 1

ACCEPTED MANUSCRIPT have been utilized to fabricate 3D aerogels that retain any or all of the properties of these nanoscale components. Among these materials, graphene, a well-defined two dimensional sp2 conjugated structure of carbon atoms, was recognized as one of the

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most important candidates for the construction of 3D porous structures due to its extraordinary mechanical, electrical, and thermal properties [20-22].

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Up to now, a lot of efforts have been made on the assembly of graphene into 3D aerogel [23-25]. However, most pristine graphene aerogels tend to become deformed and lose intrinsic 3D network structures after being compressed because of its brittle

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skeletons and strong intersheet attractive force. Development of ultralight graphene-based aerogel with high compressive and electrical properties has great

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promise for energy dissipation, vibration damping, conductive sensor and recyclable absorbent for oil [26-30]. To improve their elastic properties, a number of different

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approaches have been developed to prepare graphene-based aerogels, including

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hydrothermal/freeze-drying [27,28,31], ice template [26], wet chemistry assembly [27, 30,32,33], and 3D printing [34]. These methods are complicated or not well suited for

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mass production of 3D porous networks due to the involvement of complex and multiple synthesis processes. The fabrication of these 3D porous structures by simple and versatile methods is highly demanded. In this paper, we have developed a

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universal method to fabricate compressible graphene-based aerogel by using a molecular glue strategy via a simple solution-processed dip-coating method. The hypothesis was proposed that the uncompressible graphene skeleton was dip-coated by polymer layer, which was linked by a molecular glue, such as -oxo-1-pyrenebutyric acid (OPBA). In the fabrication process, OPBA plays a role of glue in a molecular lever to link the graphene sheets and polymer layers, in which the pyrene ring could be anchored on graphene sheet through π-π interaction. The carboxyl group of the OPBA could interact with polymers, like poly(vinyl alcohol) (PVA) or poly(acrylamide) (PAM), via hydrogen bonding. Upon compression, the stress could be released through the molecular glue and coated polymer layer. As expected,

the

resulting

polymer-coated

graphene

aerogel

exhibited

high

compressibility and multifunctionality, such as hydrophilic and oleophilic intelligence 2

ACCEPTED MANUSCRIPT and compressible electrical sensor. 2. Materials and Methods 2.1 Materials

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Natural graphite flakes (8000 mesh, purity 99.95%) and poly(vinyl alcohol) (Mowiol® PVA-117, Mw ~145,000 g/mol; Mowiol® PVA-203, Mw ~31,000 g/mol)

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were purchased from Aladdin. Concentrated sulfuric acid (95–98%), concentrated hydrochloric acid (36–38%) and potassium permanganate were analytically pure and purchased from Beijing Chemical Factory (China). Hydrogen peroxide (H2O2) and

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sodium nitrate were supplied by LaiYang Shi Kant chemical company. Poly(acrylamide) (PAM) was supplied by Shanghai Macklin Biochemical Company.

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γ-Oxo-1-pyrenebutyric acid and albumin from bovine serum (BSA) was supplied by Sigma-Aldrich Chemical Co., USA.

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2.2 Fabrication of RGO-OPBA-Polymer aerogel by Hydrothermal Reduction

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Graphene oxide (GO) was prepared from natural graphite by the Hummers method and was described in our previous report [35]. The RGO-OPBA aerogel can

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be easily prepared by heating a homogeneous aqueous mixture of GO (5 mg/ml) and OPBA (2 mg/ml) with different mass ratios (GO : OPBA = 2:1 4:1 8:1 10:1 16:1) in Teflon-lined stainless-steel autoclave at 180

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C for 12 h and freeze-drying.

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RGO-OPBA aerogel was dipped into the polymer (PVA, Mw ~145,000 g/mol and Mw ~31,000 g/mol; PAM) aqueous solution with different concentration of 0.5, 1, 3, 5, 10, 30 mg/ml for 5 hours, and subsequent freeze-drying to get compressible aerogel. 2.3 Fabrication of RGO-BSA-OPBA-Polymer aerogel by Chemical Reduction The RGO-BSA-OPBA aerogel was obtained by heating a homogeneous aqueous mixture of GO (5 mg/ml), BSA (2 mg/ml) and OPBA (2 mg/ml) with different mass ratios (GO : BSA : OPBA = 2:1:1, 4:1:1, 8:1:1, 10:1:1, 16:1:1) in glass vial for 18 h at 100 oC and subsequent freeze-drying. Next, the resulting compressible aerogel was fabricated by dipping into polymer (PVA or PAM) aqueous solution for 5 hours and subsequent freeze-drying. 2.4 Characterization 3

ACCEPTED MANUSCRIPT UV-Vis absorption spectra were recorded on a UV-2600 UV-Vis spectrometer (Shimadzu, Japan). Fourier-transform infrared spectroscopy (FT-IR) spectra were obtained

on

a

FT-IR

spectrometer

IR

Prestige-21

(Shimadzu,

Japan).

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Photoluminescence emission spectra were measured in a HITACHI F-4600 fluorescence spectrometer. X-ray diffraction (XRD) analysis was carried out on a D-8

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ADVANCE X-ray diffractometer (Bruker AXS, Germany). The SEM images were characterized by a QUANTA 200 (FEI, America). Freeze-drying was measured by Freeze-dryer FD-1-50 (Beijing Boyikang Laboratory Instruments Co., Ltd).

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Thermogravimetric analysis (TGA) was conducted on a SDT Q600 (TA, America). The Raman spectra were performed using a LabRAM HR800 Raman spectrometer

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(HORIBA JY, France). The pore-size distribution was measured at 77 K using a Quantachrome Autosorb-6b static volumetric instrument. Compression experiment

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was performed using a TA Instruments Q800 Dynamic Mechanical Analyzer

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operating in a compression mode. The elasticity-dependent electrical conductivity (Digital Multimeter, Victor V9800) was measured using a two-probe method. To

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optimize the electrical contact between copper wires and aerogel, both ends of the cylindrical aerogel were carefully coated with a thin layer of silver paste.

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3. Results and discussion

To confirm our hypothesis, two methods for the reduced graphene oxide (RGO) skeleton formation were carried out: hydrothermal reduction and chemical reduction by albumin from bovine serum (BSA), as shown in Fig. 1a. The resulting graphene aerogels were obtained by dip-coating PVA (referred to PVA with high molecular weight of 140,000, unless specifically mentioned therein) on the graphene skeleton and subsequent freeze-drying. Our fabrication process does not involve toxic agents, complex synthesis procedure and the facile solution-processed dip-coating method can be easily scaled up for practical application. During the hydrothermal treatment, the partially reduced graphene oxide was coalesced and 3D graphene networks were formed with the assistance of van der Waals' forces, π–π interaction and abundant hydrogen bonds of water [36]. This 4

ACCEPTED MANUSCRIPT strategy has been proven to be powerful for the synthesis and application of graphene-based 3D materials. However, the obtained graphene aerogel become collapsed after being compressed (Fig. 1b-i). In dramatic contrast, when OPBA was

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added into GO aqueous solution, during the hydrothermal or chemical reduction, RGO-OPBA hydrogels were formed, in which the pyrene ring of the OPBA were

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anchored on the RGO skeletons by π–π interaction. The subsequent PVA coating

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demonstrates the elastic properties of the aerogel (Fig. 1b-ii and iii).

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ACCEPTED MANUSCRIPT Fig. 1. (a) Illustration of the preparation process of the polymer-coated reduced graphene oxide (RGO) aerogel. Graphene oxide (GO) solution was firstly mixed with -oxo-1-pyrenebutyric acid (OPBA). Then, GO-OPBA solution was treated by

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hydrothermal reduction or albumin from bovine serum (BSA) reduction. The obtained aerogels were immersed into PVA aqueous solution and subsequent freeze-drying to

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get the polymer-coated RGO aerogels. (b) Digital images of the aerogels. (i): the aerogel without polymer-coating shows no compressibility; PVA coated RGO aerogels reveal elastic properties, where the RGO aerogel could be obtained by

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hydrothermal reduction (ii) or BSA reduction (iii).

PVA

coated

RGO-OPBA

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The scanning electron microscopy (SEM) revealed that the RGO aerogel and (RGO-OPBA-PVA)

aerogel

showed

the

same

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microstructures and similar pore structures with the pore size ranging from several

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hundred nanometers to several micrometers, indicating that the PVA coating does not destroy the RGO skeleton (Fig. S1). The high magnification of SEM images showed

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that the surface of the RGO sheets was clean and flat (Fig. 2a). After dip-coating PVA, the surface of the graphene sheet became much rougher (Fig. 2b). Transmission electron microscopy (TEM) images revealed monolayer graphene sheet of RGO with

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wrinkled and folded surface and selected area electron diffraction (SAED) patterns showed the multi-sets of hexagonal spots (Fig. 2c and insets). For RGO-OPBA-PVA aerogel, the graphene sheet became much thicker and no any SEAD patterns were observed, as the polymer coatings are only a few nanometers thick and quite uniform, which is a feature of successful solution processed dip-coating method (Fig. 2d). It is noted that, during the long time ultrasonication of the TEM specimen preparation, PVA layers in the RGO-OPBA-PVA aerogels are still tightly attached to the surfaces of graphene sheets, suggesting the strong interaction between PVA layers and graphene sheet. The FTIR also revealed that a broad, large peak was observed at 3370 cm-1 and attributed to O–H stretching. These bonds are involved in intermolecular hydrogen bonding between PVA and OPBA [37], indicating that PVA was coated on the surface of RGO sheets (Fig. S2). 6

ACCEPTED MANUSCRIPT To prove that our hypothesis was valid in presence of chemical reducing agent, bovine serum albumin (BSA), an environment friendly reducing agent [38], was used to fabricate RGO skeleton. The X-ray diffraction (XRD) patterns revealed that the

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typical diffraction peak of graphite oxide at 12.5° was not observed in diffraction peak for BSA reduced graphene (RGO-BSA-OPBA) aerogel, indicating that almost all the

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oxygen functional groups of graphite oxide were removed. The Raman results exhibit two signature bands around 1349 cm-1 and 1583 cm-1, which are assigned to the D-band and G-band of carbon, respectively (Fig. S3). The ID/IG value was calculated

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to be 0.81 for GO, which gradually changed to 1.08 and 1.11 for RGO-OPBA and RGO-BSA-OPBA aerogel when treated by hydrothermal and BSA reduction,

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respectively, further conforming that the GO sheets were reduced and their conjugated

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structures were partly restored [35, 39].

Fig. 2. High resolution SEM images of (a) RGO aerogels and (b) RGO-OPBA-PVA aerogels. TEM of RGO aerogel (c) and RGO-OPBA-PVA aerogel (d) and the 7

ACCEPTED MANUSCRIPT corresponding SEAD patterns inserted in (c) and (d), respectively, indicating PVA was coated on the RGO sheets.

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The BSA reduced RGO aerogel also revealed 3D microporous structures and the surface of graphene sheet became more smooth compared to that of hydrothermal

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treated RGO sheet, which may be due to that BSA attached to RGO sheet [38] (Fig. S4). After dip-coating PVA, the surface of the sheet also became much rougher. The BSA reduced

RGO

aerogel

with

an

overall

uniform

coating

of

PVA

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(RGO-BSA-OPBA-PVA) could also recover its initial state within one second under compression, indicating the elastic properties of RGO-BSA-OPBA-PVA aerogel,

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similar to that of the hydrothermal reduced graphene (RGO-OPBA-PVA) aerogel (Fig. 1b, Movie S1).

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GO sheets are assumed to carry their carboxy groups at the edges, while the

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epoxy and hydroxy groups and graphitic domains reside in the basal plane [40]. Upon hydrothermal or chemical reduction treatment, the epoxy and hydroxyl group

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were removed, the interaction force between GO sheets changed from electrostatic effect to π–π stacking [41], leading to the self-assembly of graphene sheets into the sponge skeletons, as discussed above. Simultaneously, the pyrene rings of the OPBA

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were anchored on the RGO basal plane by π-π interaction. In order to demonstrate the interaction of OPBA with RGO, Fluorescence spectra were performed. Excitation of pyrene chromophore of OPBA at 340 nm showed a broad, reasonably intense monomer emission at 514 nm which was attributed to the π–π emitting state [42]. The emission of OPBA at 514 nm decreased significantly and almost disappeared in the presence of RGO or RGO-BSA (Fig. 3a). This result indicates that the OPBA could be better adsorbed onto the RGO surface via π–π stacking interaction between pyrene chromophore and the sp2 hybridized atoms of the RGO nanosheet with reduced alkoxy group. In addition, the normalized fluorescence intensity showed that the main fluorescence emission peak of RGO-OPBA or RGO-BSA-OPBA red-shifted 25 nm compared to that of RGO and blue-shifted 14 nm compared to that of OPBA, further demonstrating the π-π interaction of OPBA and RGO (Fig. 3b). 8

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Fig. 3. Interaction of OPBA with RGO sheets. (a) Fluorescence intensity and (b) normalized fluorescence intensity of OPBA, RGO, RGO-OPBA, RGO-BSA-OPBA, respectively, showing the π-π interaction of pyrene ring with RGO sheets. (c) Thermogravimetric analysis of OPBA, indicating the high stability of OPBA in our hydrothermal treatment. UV-vis spectra (d) and digital images (e) of the RGO-OPBA hydrogel solution after hydrothermal treatment of GO and OPBA with different molar ratio.

It should be mentioned that OPBA is stable enough for the reduction process. OPBA began to degrade at 280 oC, which is much higher than the operation 9

ACCEPTED MANUSCRIPT temperature of RGO reduction (180 oC) (Fig. 3c). In addition, to evaluate the appropriate amount of OPBA, different ratio of GO and OPBA was used to fabricate hydrogels. As shown in Fig. 3e, hydrogels could not be formed by using low ratio of

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GO and OPBA (2:1 and 4:1). Increasing the ratio of GO and OPBA leads to the formation of hydrogels. UV-Vis absorption of OPBA in RGO-OPBA hydrogel

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solution demonstrated that the absorption of OPBA almost disappeared when the ratio of GO and OPBA was higher than 8:1 (Fig. 3d). This indicated that OPBA was almost anchored on the graphene sheet during RGO skeleton formation. To ensure the

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sufficient interaction of PVA with OPBA by hydrogen bonding, the hydrogels fabricated by GO and OPBA in the ratio of 8:1 were selected for the final fabrication

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of PVA coated aerogel.

To further illustrate the effect of OPBA and PVA on the compressibility of the

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aerogels, several kinds of graphene-based aerogels were tested. Without OPBA and

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PVA, the RGO aerogel was brittle. When OPBA was added to GO aqueous solution, the resulting RGO-OPBA aerogel displayed compressible properties with the recovery

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rate of 70% after 10 compression cycles at 50% strain (Fig. S5 and Table S1). When PVA was coated on the RGO aerogel, the elastic properties were also increased. This means that both OPBA and PVA are contributed to the elastic properties of the

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aerogels. Therefore, for RGO-BSA-OPBA-PVA aerogel, the recovery rate increased to 98% under compression strain of 50% after 10 compression cycles. When the graphene skeleton was obtained by chemical reduction, after addition of OPBA and PVA, similar results were obtained (Fig. S6, Table S2). Only OPBA or PVA has enhanced

the

elastic

properties

of

the

aerogel

to

some

extent.

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RGO-BSA-OPBA-PVA aerogel revealed the highest recovery rate compared to RGO-BSA-OPBA aerogel and RGO-BSA-PVA aerogel. Both OPBA and PVA are responsible for the elastic properties of the aerogel. Next, the effect of PVA concentration on the compressible RGO-OPBA-PVA aerogel was investigated. The elastic property of the resulting PVA coated aerogel was dependent on the amount of PVA, which was related to the PVA concentration during dip-coating process. The compressive stress ()-stain () curves revealed that the 10

ACCEPTED MANUSCRIPT aerogel by dip-coating PVA at low concentration of 0.5 mg/ml had no stress even compressed to the strain of 42% (Fig. 4b). This may be due to the nonuniform coating of PVA layers. As the concentration of PVA increased, the recovery rate increased.

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However, too much PVA loading leads to the increase in the density of RGO-OPBA-PVA aerogel (Fig. 4a). Simultaneously, the elastic ability of the aerogel

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reduced (Fig. 4c-d). It is found that the RGO-OPBA-PVA aerogel coated by PVA at the concentration of 1 mg/ml demonstrates the best elastic behavior (Fig. 4f). The density of this kind of aerogels is about 18.2 mg/cm3. For BSA reduced aerogel, the

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density of RGO- BSA-OPBA- -PVA aerogel is about 13.8 mg/cm3. The density value of the two kinds of polymer-coated aerogels are higher than that of carbon nanotube

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aerogels (5–10 mg/cm3) [43] and carbon nanofiber aerogels (10 mg/cm3) [44], and comparable to that of graphene-based aerogels (12±5 mg/cm3) [5]. When compared to

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the conventional carbon-based aerogel (100–800 mg/cm3) [45,46], its bulk density is

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one to two orders of magnitude lower. In addition, the specific surface area of RGO, RGO-OPBA-PVA, RGO-BSA-OPBA-PVA aerogels were estimated by nitrogen

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adsorption/desorption experiment, which is 73.8521 m2/g, 14.1955 m2/g, 10.5737 m2/g, respectively (Fig. S7). Considering the overall properties of PVA-coated aerogel,

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the PVA concentration was fixed at 1 mg/ml for the experiment below.

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Fig. 4. The effect of PVA concentration on the compressible RGO-OPBA-PVA aerogel. (a) A function of PVA concentration and the density of aerogel. The stress-strain curves of RGO-OPBA-PVA aerogel obtained by dip-coating PVA at the concentration of (b) 0.5 mg/mL, (c) 1 mg/mL, (d) 3 mg/mL, (e) 5 mg/mL, respectively. (f) PVA concentration versus recovery rate of the aerogel.

The porous structure of RGO-OPBA-PVA aerogel was investigated by in-situ SEM observation. When compressed at different strain, its 3D network was not destroyed and the RGO sheets were not exfoliated from the skeletons (Fig. 5a). The aerogel could completely recover its original shape at different compression strain of 20%, 40%, 70%, respectively (Fig. 5b). The starting point for each cycle is the same 12

ACCEPTED MANUSCRIPT and equal to the initial thickness of the sample. Importantly, this property keeps unchanged after 100 cycles of compression tests (Fig. S8a), confirming its excellent mechanical stability. The loading process of RGO-OPBA-PVA aerogel exhibits three

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distinct deformation stages: a linear-elastic region for ε < 25% with an elastic

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modulus of 6.8 KPa, a plateau region, for 25 < ε < 55%, and a steep slope region for

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ε > 55%. The unloading curves almost return to the initial points, suggesting complete shape recovery without plastic deformations. The dynamic mechanical analysis (DMA) measurement also revealed that the storage modulus (E’) of the

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RGO-OPBA-PVA aerogel was always much higher than loss modulus (E’’) over the measured range from 0.1 to 10 Hz (Fig. 5c), indicating a bulk elastic response with

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slight dependence on the frequency. The small and stable loss tangent less than 0.2 over the entire tested range indicates that the aerogel has a good elastic recovery

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property in accordance with the compressible test results.

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In addition, the effect of the molecular weight (MW) of PVA on the elastic properties was also checked. Fig. 5d showed the compressible strain-stress curves of

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the aerogel coated PVA with low-MW at different strain of 20%, 40% and 70%, respectively. The maximum stress (at = 70%) is 56 KPa, smaller than that of aerogel coated by PVA with high MW (78 KPa), which may be due to the higher modulus of

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PVA with longer chains. The fatigue test with 100 loading/unloading cycles indicated that the stress-strain curve in the second cycle was far more compliant than that observed in the first cycle, which was referred to as softening behavior [47] (Fig. S8b). The elastic modulus and loss modulus are similar to that of high MW PVA coated aerogel, indicating the good recovery property (Fig. 5e).

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Fig. 5. (a) In-situ SEM observation of RGO-OPBA-PVA at compression strain of 0%, 20%, 40%, 60%, respectively. Stress-strain curves (b, d) and the frequency dependence of storage modulus, loss modulus and loss tangent (c, e) of RGO-OPBA-PVA aerogel by dip-coating PVA with high molecular weight (b, c) and low molecular weight (d, e), respectively. (f) The proposed compression mechanism of the PVA coated aerogel.

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ACCEPTED MANUSCRIPT The concept demonstrated here is not restricted to the current system. To further prove that our hypothesis is versatile for other system, poly(acrylamide) (PAM) has been coated on the RGO-OPBA aerogel instead of PVA. The resulting

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RGO-OPBA-PAM or RGO-BSA-OPBA-PAM aerogel revealed that the RGO sheet featured a rougher surface compared to that of aerogel without PAM coating, similar

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to the results of PVA coated aerogel (Fig. S9). As is expected, The PAM coated aerogels were also subjected to a cyclic compression test with 100 loading/unloading fatigue cycles at a large strain of 60%, highlighting their elastic behavior (Fig. S10,

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movies S2 and S3).

Considering the unique intrinsic structures, the possible mechanism of the

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compressible polymer-coated aerogel has been proposed (Fig. 5f). Two main reasons should be responsible for the significant improvement in elastic properties, including

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the uniform coating of the polymer layers and the effective stress transfer between

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graphene skeleton and polymers via OPBA molecular glues. Our solution processed dip-coating method allows the entire surface of the graphene skeleton to be coated by

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polymer layers while maintaining the original network structure. During the graphene skeleton formation, OPBA as the molecule glues should be anchored both on the graphene sheet planar and in the junctions between graphene sheets (Fig. 5f). For all

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the cyclic stress-strain curves of the polymer-coated aerogel, no any yield stress can be detected. This indicated good stress transfer between the graphene sheets and polymer layers [47]. OPBA provides the strong interfacial interaction between the graphene sheet and polymer layers, which can bear enough stress and avoid rupturing between RGO sheets during compression [48].

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Fig. 6. (a) Photo of absorption of water and oil (dyed with Rhodamine B and Sudan respectively)

by

RGO-OPBA-PVA aerogel

simultaneously.

(b)

Water

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III,

absorption-squeezing behavior after 10 cycles. (c) The plot of sorbed water mass and recovered mass of the aerogel after squeezing. (d) The weight gain of aerogel after absorption of different organic solvents. (e) Absorption-squeezing behavior after 10 cycles using dichloromethane as an example.

Typically, the intrinsic properties of the materials that make up aerogels dictate the properties and their applications of the aerogel. The improved overall mechanical properties, such as high compressibility, rapid shape recovery and good cycling stability, render our RGO-OPBA-PVA aerogels suitable as a recycle adsorbent for water and organic solvents by manual squeezing. Most recently, Song [49] reported a super-amphiphilic 3D graphene-based aerogel through hydrothermal treatment of GO 16

ACCEPTED MANUSCRIPT and phytic acid. The resulted product shows both hydrophilic and oleophilic intelligence. However, this kind of graphene-based aerogel does not show any compressibility. In our RGO-OPBA-PVA aerogel, hydrophilic PVA layer endows the

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hydrophobic graphene skeleton with hydrophilic. When pushing pieces of RGO-OPBA-PVA aerogel over drops of water and hydrophobic oil (dyed with

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Rhodamine B and Sudan III for better visibility, respectively), it rapidly adsorbed both water and oil (Fig. 6a and Movie S4). Our results contrast with previously reported graphene-based 3D aerogel, which were either mono-hydrophobic [5, 50-52] or

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mono-hydrophilic [53].

Fig. 6b showed the sorption capacities ((weight after saturated sorption – initial

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weight)/initial weight) of the RGO-OPBA-PVA aerogel. It can absorb 43 times its weight of water in 5 seconds. The absorbed water can be readily released by

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mechanical squeezing without destroying the porous structures. After reabsorption of

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water, the regenerated aerogels still keep their original shape and ultrahigh absorption capability after more than 10 cycles. It was observed that the aerogel (12 mg) could

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adsorb water up to 516 mg in the first cycle. However, 140 mg of water remained in the aerogel after squeezing (Fig. 6c). Such phenomenon is due to the interaction between PVA molecule and water; resulting part of water is trapped into the aerogel.

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The mass of the trapped water does not increase after the second recycle, indicating the good absorption-squeezing behavior (Fig. 6b). For a typical absorption of oil measurement, common organic solvents were chosen as absorbates. The RGO-OPBA-PVA aerogel exhibited excellent sorption capacities ranging from 23 to 90 times its own weight for different organic solvents (Fig. 6d). This sorption capacity was higher than that of many previously reported sorbents, such as PDMS sponge (4–11 times) [54], poly(orthocarbonate)s sponge (5–25 times) [55], chitin sponge (30–60 times) [56], and comparable to that of spongy graphene (20–86 times) [5]. To evaluate the recyclability of aerogel, dichloromethane was chosen as a model organic compound. The cycle test was performed through manual squeezing. As shown in Fig. 6e, after 10 cycles of adsorption-squeezing tests, no obvious deterioration in the absorption capacity was observed, indicating its 17

ACCEPTED MANUSCRIPT excellent reusability. The high absorption ability not only was related to the properties of the materials but also was ascribed to the high porosity. Nitrogen adsorption measurements revealed similar pore size distribution before and after dip-coating PVA

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layers with the pore size in the range of several nanometers to several micrometers (Fig. S11). This means that the surface wettability could be changed by PVA coating

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on the 3D graphene skeleton, but this was not accompanied by dramatic decrease in their porosity.

Considering the conductivity of RGO, the RGO-OPBA-PVA aerogel as an

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electrical sensor was also investigated. The device was shown in Fig. 7a. To avoid contacting resistance, two ends of the aerogel were pasted with silver adhesives. As

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the RGO-OPBA-PVA aerogel was compressed to the strain of 60%, the electrical resistance decreased dramatically to 10% of its initial state (Fig. 7b). This

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phenomenon may be due to that the compression created numerous new temporary

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contacts among graphene sheets. Although insulating polymer layer was coated on the graphene sheet, the contacted edges of the graphene sheets could also build the

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conduction paths throughout the sample, thus decreasing the electrical resistance of the aerogel. To test the effect of compression cycle on the electrical resistance, resistance change (ΔR/R0 = (R0-R)/R0, where R0 represents the initial resistance) of

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the aerogel at the strain of 60% upon 100 compression cycles was shown in Fig. 7c. The compression/release curves are symmetrical and sharp and the resistances changes maintain almost at 92% in every compressed state, meaning excellent recovery property and high sensitivity of pressure sensor. In the released state, the resistance changes reached to about 1% and kept quite constant after the first 30 compression/release cycles, demonstrating the excellent mechanical stability and elasticity (Fig. 7d). A light-emitting diode was illumined using a 3V circuit when connected with the RGO-OPBA-PVA aerogels, and its brightness fluctuated as the aerogels were compressed and released (Fig. 7e, Movie S5). The change in the electrical resistance is reproducible and the electrical resistance is only demined by the compressive strain. These results support the use of RGO-OPBA-PVA aerogels as light, pressure responsive sensors for various applications. 18

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Fig. 7. (a) The device for electrical resistance measurement. (b) R/R0 decreases with

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an increase in strain up to 60%. (c) Electrical resistance change when repeatedly compressed up to 60% of strain for 100 cycles. (d) The electrical resistance change of the aerogel in the released state for 100 compression/release cycles. (e) Illumination of a 3V light emitting diode under compression and release.

4. Conclusions In summary, we have demonstrated the fabrication of a new kind of graphene-based aerogel by using OPBA as molecular glues to link graphene skeleton and coated polymer layers. In this work, PVA and PAM were used as coating polymers on the graphene skeleton for the proof-of-concept, where the graphene skeletons could be obtained by hydrothermal or chemical reduction. The polymer-coated aerogels exhibit extraordinary compressibility, in marked contrast to 19

ACCEPTED MANUSCRIPT the brittle nature of traditional graphene aerogel. By exploiting the hydrophilic polymer layers and hydrophobic aromatic graphene skeleton, the compressible graphene-based aerogel displays unique amphiphilic properties. Moreover, the

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compressible aerogels could also be used as pressure-responsive electrical sensor.

Acknowledgement

The authors acknowledge the support by Program for Scientific Research Innovation

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Team in College and Universities of Shandong Province, the National Natural Science Foundation of China (21204044 and 21276149), the Natural Science Foundation of

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Shandong Province for Excellent Young Scholars (ZR2015JL009) and Ji’nan

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Overseas Students Pioneer Plan (20120202).

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights (1) γ-Oxo-1-pyrenebutyric acid (OPBA) as a molecular glue could link graphene sheet through π−π interactions and water-soluble polymer layer (poly(vinyl

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alcohol) or poly(acrylamide)) by hydrogen bonding.

(2) The compressible graphene-OPBA-polymer aerogel exhibits high elastic

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properties with recovery rate up to 83% after hundreds of compressive cycles at 60% strain.

(3) The as-fabricated compressible aerogel diplays excellent recoverability and

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superior absorption capacity for both organic solvents and water. (4) The electrical conductivity sensitive to compressive strain makes the aerogel

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high potential to be used as pressure-responsive electrical sensor.

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