A general and facile method for preparation of large-scale reduced graphene oxide films with controlled structures

A general and facile method for preparation of large-scale reduced graphene oxide films with controlled structures

Accepted Manuscript A general and facile method for preparation of large-scale reduced graphene oxide films with controlled structures Hengchang Bi, S...

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Accepted Manuscript A general and facile method for preparation of large-scale reduced graphene oxide films with controlled structures Hengchang Bi, Shu Wan, Xiehong Cao, Xing Wu, Yilong Zhou, Kuibo Yin, Shi Su, Qinglang Ma, Melinda Sindoro, Jingfang Zhu, Zhuoran Zhang, Hua Zhang, Litao Sun PII:

S0008-6223(18)31024-8

DOI:

https://doi.org/10.1016/j.carbon.2018.11.007

Reference:

CARBON 13623

To appear in:

Carbon

Received Date: 16 August 2018 Revised Date:

18 October 2018

Accepted Date: 4 November 2018

Please cite this article as: H. Bi, S. Wan, X. Cao, X. Wu, Y. Zhou, K. Yin, S. Su, Q. Ma, M. Sindoro, J. Zhu, Z. Zhang, H. Zhang, L. Sun, A general and facile method for preparation of large-scale reduced graphene oxide films with controlled structures, Carbon (2018), doi: https://doi.org/10.1016/ j.carbon.2018.11.007. 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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Reduced graphene oxide (rGO) films with controlled structures can be easily prepared through spray coating plus heating. The surface morphology and microstructure of rGO films could be controlled by simply changing heating temperature, which could fulfill various applications such as oil sorption, electrical conductive films, sensors, supercapacitor, thermal conductive films, and transparent flexible electrodes.

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A General and Facile Method for Preparation of Large-scale Reduced

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Graphene Oxide Films with Controlled Structures

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Hengchang Bi1, Shu Wan1, Xiehong Cao2,4, Xing Wu3, Yilong Zhou1, Kuibo Yin 1, Shi Su1,

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Qinglang Ma2, Melinda Sindoro2, Jingfang Zhu1, Zhuoran Zhang5, Hua Zhang2*, Litao Sun

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** SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of Ministry of Education,

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Collaborative Innovation Center for Micro/Nano Fabrication, Device and System,

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Southeast University, Nanjing 210096, P. R. China

2. Center for Programmable Materials School of Materials Science and Engineering,

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Nanyang Technological University, Singapore 639798, Singapore. E-mail address:

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[email protected]

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3. Key Laboratory of Polar Materials and Devices, Ministry of Education and Department of Electronic Engineering, East China Normal University, Shanghai 200241, China 4. College of Materials Science and Engineering, Zhejiang University of Technology,

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Hangzhou 310014, China

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Changzhou Carbon Star Technology Company Limited, Changzhou 213149, China

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Center for Advanced Carbon Materials Southeast University and Jiangnan Graphene

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Research Institute, Changzhou 213100, China Abstract:

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Graphene or reduced graphene oxide films (rGOFs) can be prepared by a number of

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methods including chemical vapor deposition (CVD), filtration, and spin-coating for a variety

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of applications. However, controlling their surface morphologies and microstructures to meet

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the requirements of specific applications is still a great challenge. Here, controlled

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microstructure of large-size rGOF with good electrical and thermal conductivities as well as

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high sorption ability is produced through a heating-assisted spray method. By simply tuning

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the heating temperature, the smooth surface and close-packed layered structure of rGOF can

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be changed to rough surface and porous structure. Impressively, the rapid preparation of

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*Corresponding author. E-mail: [email protected] (Litao Sun) E-mail: [email protected](Hua Zhang)

ACCEPTED MANUSCRIPT rGOF with area as large as ~216 cm2 in only 6 h has been successfully achieved, which is

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significant since normally it takes several days to prepare a rGOF with small area of ~10 cm2

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by using conventional filtration method. More importantly, our rGOFs show promising

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applications in oil sorption, supercapacitors, and thermally/electrically conductive films.

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1. Introduction

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Because of its excellent thermal, electrical, and mechanical properties[1-3], graphene

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has been intensively used for a wide range of applications, such as flexible sensors[4,5], heat

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spreaders[6], energy storage devices[7,8], and anti-pollution agents[9]. Recently, many

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methods have been developed for the preparation of graphene films or reduced graphene

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oxide films (rGOFs). For example, chemical vapor deposition (CVD), one of the most

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promising methods, was used to produce high-quality graphene films with high transparency

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and electrical conductivity which are comparable with the mechanically cleaved graphene

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films[10]. Although graphene films prepared by CVD have been widely used for stretchable

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transparent electrodes[2,10], gas or ion filtration[11] and anti-corrosion coatings[12], the cost

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of CVD process is high[13]. On the other hand, the scaled-up preparation of rGOFs for real

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applications is feasible through the reduction of graphene oxide (GO) films prepared by

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various methods, such as spray coating[13,14], Langmuir–Blodgett film[15], dip-coating[16],

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spin-coating[17], filtration[11], continuous centrifugal casting [18] and inkjet printing[19,20].

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As known, there are still limitations and disadvantages in the aforementioned methods. For

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instance, spin-coating method is capable to fabricate rGOFs with smooth surface and uniform

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thickness used as transparent electrodes for solar cells[17]. However, these smooth rGOFs are

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not hydrophilic, which are not suitable for applications with hydrophobic surface, such as

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anti-corrosion coating[12] and oil sorption[21]. Moreover, it is still a great challenge to

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prepare graphene or rGO films with controlled surface morphologies and microstructures in

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order to fulfill the requirements of different applications[22].

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ACCEPTED MANUSCRIPT Here, we report a general and facile strategy in which a heating-assisted spray method was

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used to fabricate rGOFs with controllable microstructure, large film area, good electrical and

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thermal conductivities, as well as ultra-high oil sorption ability. In addition, the as-fabricated

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rGOFs with layered structure can be used as transparent thin-film electrodes and heat

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spreaders. By simply tuning the heating temperature during the synthesis, the rGOFs can be

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converted to the porous films with hydrophobic surface, which could be used for promising

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applications in the selective oil sorption and supercapacitors.

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2. Experimental

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2.1 Characterizations

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The photographs were taken using a Panasonic LX 5 digital camera (Japan). The

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optical microscopy images were recorded on a MV 6100 microscope (China). The X-ray

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powder diffraction (XRD) spectra were collected by a X'TRA ARL diffractometer

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(Switzerland) (Cu Kα X-radiation at 40 kV and 40 mA). Scanning electron microscopy

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(SEM) was performed on a FEI QUANTA 200 (USA). Fourier transform infrared spectra

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were collected using a NEXUS 870FT Infrared Spectrometer (NICOLET, USA). X-ray

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photoelectron spectroscopy (XPS) data were collected by an electron spectrometer (Japan,

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PHI 5000 VersaProbe). Raman spectra were collected on a Laser Raman spectrometer

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(France, JY HR800).

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2.2 Fabrication of rGOFs

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First, a colloidal dispersion of GO was first prepared by sonication of GO (300 mg),

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prepared by a modified Hummers method[23], in ethanol (600 mL) for 45 min. Second, glass

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plates were washed with acetone, ethanol and distilled water in sequence, followed by drying

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in an oven. Third, the glass plates were placed on a heating stage with a given temperature

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(25, 50, or 70 °C). Fourth, the GO dispersion was then sprayed on the glass plates. Fifth, the

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glass plates coated with GO were placed in an autoclave with hydrazine monohydrate and

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heated at 95 °C for 2 h. A sharp blade was used to separate the GF and the glass plate to

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obtain freestanding rGOFs. The transparent electrically conductive rGOFs and thermally conductive rGOFs were

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first prepared by heating-assisted spray method. Then, the fabricated GO films were

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chemically reduced by immersing them in a hydroiodic acid solution (55.0-58.0%, Nanjing

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Chemical Reagent Co., Ltd.) for 2 h. After that, the rGOFs were washed several times using

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ethanol.

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2.3 Dynamic imaging of oil sorption

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Photographs of oil floating on the surface of artificial seawater were taken and

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analyzed to characterize the dynamic process of oil sorption. The artificial seawater was

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prepared by mixing sodium chloride (23.926 g), sodium sulfate (4.008 g), potassium chloride

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(0.667 g), sodium bicarbonate (0.196 g), potassium bromide (0.098 g), boric acid (0.026 g)

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and sodium fluoride (0.003 g) in water (1 Kg) [24]. Dodecane (1 mL) was stained with Sudan

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red 5B (1 mg) to facilitate the evaluation of oil sorption. The same method was conducted for

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toluene. Photos were taken every 20 s.

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2.4 Electrochemical measurements

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All electrochemical measurements were performed in a conventional three-electrode

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system. 1 mol L-1 H2SO4 aqueous solution was used as the electrolyte. rGOFs, Pt wire and

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Ag/AgCl electrode were used as the working, counter and reference electrodes, respectively.

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Cyclic voltammetry (CV) and galvanostatic charge/discharge tests were performed in

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Solartron analytical equipment (Model 1470E, AMETEK, UK).

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2.5 Thermal and electrical measurements

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Thermal conductivity was measured using NETZSCH LFA 447 NanoFlash (Germany).

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In this method, the sample was cut into round shape with a diameter of 12.7mm. First, the

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sample was heated by a light pulse, and then the resulting temperature rise at four different

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positions is measured using infrared detector. The thermal conductivity (K) is calculated

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using the following equation: K=ρCp0.1388d (t50) 2

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In Equation, ρis the density of rGOFs, which is obtained according to ρ= mV-1 (m and

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V are the mass and volume of the sample, respectively). Cp is the specific heat capacity

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obtained from differential scanning calorimetry (DSC) at 25 OC. d is the thickness of the

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sample, which is measured by SEM at six different sites in the round sample with a

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diameter of 12.7 mm. t50 is half of the diffusion time.

Electrical conductivity measurement was performed in a standard four-point probe

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station (ST 2253 digital four-point probe, Suzhou Jingge electronic Co. Ltd. China) at

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room temperature. ECG waveform was measured using a vital signs monitor (PHILIPS,

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Neitherland) by a conventional three-electrode method.

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3. Results and Discussion

Fabrication of reduced graphene oxide films (rGOFs). Figure 1a shows the schematic

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illustration for the fabrication of rGOFs. After graphene oxide (GO) sheets were prepared

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(see the Methods for details), a heating-assisted spray method was used to deposit them onto

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the glass plate, placed on a heating stage (see Methods for details). It is noteworthy that the

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concentration of GO dispersion in ethanol should be less than 0.5 mg/ml in order to obtain

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uniform GO dispersion (Figure S1 in SI). The morphologies and structures of formed GO

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films can be tuned by changing the heating temperature on the stage. The obtained GO films

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were then chemically reduced by immersing them in hydrazine in an autoclave at 95 °C for 2

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h. After the reduction, the color of GO films changed from dark yellow to black, indicating

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that most of the functional groups on GO sheets were removed and the rGOFs were obtained

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ACCEPTED MANUSCRIPT (Figures S2-5 in SI). It is worthy to mention that it only took 6 h for the whole preparation

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process to obtain the rGOF with area as large as ~216 cm2 (Figure S6 in SI). In contrast, it

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took several days to prepare a small rGOF (~10 cm2) using the conventional filtration

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method[25,26]. Therefore, our developed method is highly efficient for preparation of large-

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area rGOFs. More importantly, our rGOF is freestanding and flexible comparable to the

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graphene paper[26], beneficial to a wide range of applications.

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The effect of heating temperature on the surface morphologies and microstructures of

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rGOFs was investigated by scanning electron microscope (SEM) and optical microscope. As

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shown in the Method, rGOFs prepared with heating temperature of 25, 50, and 70 °C are

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denoted as rGOF-1, rGOF-2, and rGOF-3, respectively. The optical microscopy images

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indicate that the surface morphologies of rGOFs obtained at different heating temperature are

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distinct (Figure S7 in SI). SEM images indicate that rGOF-1 consists of unevenly distributed

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rGO particles with size ranging from 20 to 50 µm (Figure 1b). In contrast, rGOF-2 and

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rGOF-3 consist of crumpled rGO sheets, in which rGOF-3 has denser crumpled rGO sheets

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ACCEPTED MANUSCRIPT Fig. 1. Preparation of rGOFs for various applications. (a) Schematic illustration of heating-

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assisted spray process for fabrication of rGOFs with different surface roughness and

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microstructures obtained at different heating temperature for various applications, such as

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supercapacitors, flexible sensors, transparent electrically conductive films, oil sorption,

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anticorrosion coating, and thermally conductive films. (b-d) Top-view and (e-g) cross-section

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SEM images of the rGOFs fabricated at different heating temperatures, e.g. 25 °C (rGOF-1),

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50 °C (rGOF-2), and 70 °C (rGOF-3), showing the different surface morphologies and

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internal structures. Insets in (b-d): their high-magnification SEM images.

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(Figure 1c, d). High-magnification SEM images indicate that the rGOF-1 has a smoother

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surface compared to rGOF-2 and rGOF-3 (insets of Figure 1b-d). It is worth to mention that

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the crumpled rGO sheets can effectively prevent the restacking of rGO sheets and, hence,

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increase the specific surface area of rGOFs (Table S1 in SI). The cross-section SEM image of

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rGOF-1 confirms its layered structure (Figure 1e, Figure S8a in SI), similar to the rGOFs

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obtained from the filtration method[11]. The loosely stacked rGO sheets in rGOF-1 may be

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attributed to the leavening process when hydrazine was used as the reduction agent[21]. In

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contrast, rGOF-2 and rGOF-3 showed the porous structures (Figure 1f,g, and Figure S8b,c in

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SI) which may be ascribed to their rapid evaporation rates of ethanol at higher heating

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temperature. The as-prepared rGOFs with layered and porous structures have been

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successfully prepared by tuning the heating temperature.

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3.1 Wettability of rGOFs

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Surface roughness and low surface energy are two essential factors for water

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repellence[27,28]. In this work, the prepared rGOFs have similar chemical compositions

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(Figure S2-5 in SI). To further investigate the effect of surface roughness of rGOFs on their

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wettability, the water contact angles (WCAs) and surface rougnness of rGOFs were examined

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(Table S1 in SI). Figure 2 indicates that the rGOFs prepared at different heating temperatures

ACCEPTED MANUSCRIPT exhibited different WCAs. The measured WCAs on rGOF-1, rGOF-2, and rGOF-3, are ~82°,

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105°, and 137°, respectively (Figure 2a-c), indicating that rGOFs with various wettability can

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be easily prepared by tuning the surface roughness (Table S1 in SI) through adjusting the

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temperature on the heating stage in our heating-assisted spray process .

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Fig. 2. Wettability and adhesive properties of rGOFs. (a-c) The water contact angles of

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rGOF-1, rGOF-2, and rGOF-3 are 82°, 105°, and 137°, respectively. Insets: Photographs of

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the water droplets (100 µL) placed on the surfaces of corresponding rGOFs. (d) A water

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droplet (550 µL) was steadily hung from the surface of rGOF-3. (e) Advanced liquid front of

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rGOF-3 with an advancing contact angle of ~146°. (f) Receding liquid front of rGOF-3 with a

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receding contact angle of ~41°. (g) Photographs of the transfer process of a water droplet,

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transported from a hydrophobic polyterafluoroethene substrate to a hydrophilic glass slide

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with the assistance of rGOF-3.

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Figure 2d shows a drop of water with volume of ~550 µL hanging from the surface of

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rGOF-3, indicating the strong adhesion between rGOF-3 and water. To better evaluate the

ACCEPTED MANUSCRIPT adhesive performance of rGOFs, advancing and receding WCA measurements were

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performed, in which the WCA was determined by the low-bond axisymmetric drop shape

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analysis technique. Briefly, a water droplet (~5 µL) was first brought into contact with the

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surface of rGOF. The size of droplet increased and then decreased to advance and retract the

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liquid front, subsequently. This process was repeated 5 times and tested on several batches of

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rGOFs to ensure the reproducibility of the result. Figure 2e-f show the advancing and

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receding conditions of rGOF-3 with WCAs of ~146° and 41° for advancing (Figure 2e) and

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receding angles (Figure 2f), respectively. Since rGOF-3 has a large WCA hysteresis (~105°),

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it is highly adhesive to water[29], arising from the defects and residual hydrophilic functional

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groups in rGO[30,31]. This hydrophobic nature and high adhesion with water make rGOF-3

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an ideal candidate for transportation of water. As shown in Figure 2g and Movie S1 in SI,

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rGOF-3 can be used to transport small amount of water or glycerol from a hydrophobic

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surface (polyterafluoroethene substrate) to a hydrophilic one (glass slide), indicating its

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potential to transport small quantity of aqueous sample for the microsample analysis (e.g. the

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construction of mechanical hand)[32,33].

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3.2 Sorption of organic solvents and oils

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A series of sorption experiments was performed to investigate the sorption abilities of

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rGOF-3 for organic solvents and oils. The sorption processes are shown in Figure 3a,b and

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Movie S2,3 in SI. When the rGOF-3 was brought into contact with a layer of dodecane

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(stained with Sudan red 5B), it quickly absorbed dodecane and repelled the artificial seawater

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(Figure 3a). Similarly, a layer of toluene stained with Sudan red 5B on artificial seawater was

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also quickly absorbed by rGOF-3 (Figure 3b). The aforementioned results arise from the

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porous structure and hydrophobic nature of rGOF-3. Due to its high affinity towards organic

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solvents and oils, the rGOF-3 can be an excellent sorbent for the selective oil/water

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separation, especially used for cleaning the leakages of crude oil, petroleum products, and

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ACCEPTED MANUSCRIPT toxic organic solvents, just as other sorbents[9].

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Fig. 3. Sorption capability and recyclability of rGOFs. Photographs showing the sorption of

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(a) dodecane and (b) toluene in rGOF-3. Both dodecane and toluene, stained with Sudan red

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5B for clarity, float on the artificial seawater. These sorption processes are very quick and

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complete within 20 s. (c) Sorption capacities of rGOF-1, rGOF-2 and rGOF-3 for motor oil

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and chlorobenzene. The weight gain is defined as the ratio of the mass of absorbate to the

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weight of dried rGOFs. (d) Sorption capacities of rGOF-3, rGO foam, and nanowire

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membrane for various organic liquids. The rGOF-3 shows higher sorption capacities

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compared to rGO foam and nanowire membrane. The recyclability of rGOF-3 for (e)

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cyclohexane and (f) toluene. The sorption capacity of rGOF can be determined from its weight gain (wt%) after

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sorption, defined as the weight of the sorbed oil or solvent per unit weight of dried rGOF. As

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shown in Figure 3c, the sorption capacities of rGOF-3 for motor oil and chlorobenzene

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(~75.3 and 37.5 times, respectively) are much higher than those of rGOF-1 (~27 and 10 times

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respectively) and rGOF-2 (~64.3 and 16.5 times, respectively). The differences of sorption

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capacities in these rGOFs may be due to their different surface morphologies and internal

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porous structures, the latter factor being more dominant. Moreover, the rGOF-3 also showed

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excellent sorption capacities for other organic liquids, including cyclohexane, toluene, and

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chloroform (Figure 3d). In all the aforementioned sorption measurement, rGOF-3 is able to

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absorb liquids between 26 to 75 times its dried weight, which are higher than those of rGO

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foam prepared by the leavening process[21] or inorganic nanowire membrane[34]. For

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instance, the sorption capacity of rGOF-3 for motor oil, cyclohexane, chlorobenzene, and

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toluene is ~75.3, 26.5, 37.5 and 29.5 times, respectively, while ~36, 21, 26 and 17 times for

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the rGO foam[21].

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Sorption rate is another important factor for the pollution control and environmental

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protection. In order to protect our living environment, the oil spill should be cleaned up in a

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short time. Otherwise, the pollution will become more serious as the oil spreads. As shown in

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Movies S2,3 in SI, liquids (dodecane and/toluene) can be quickly sorbed by the rGOF-3. The

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sorption rate is related to the molecular weight and viscosity of oil. Moreover, since some

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pollutants are either precious or toxic, proper recycling of the sorbents is very important.

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Distillation method to recycle the sorbed organics is preferred, compared to solvent

ACCEPTED MANUSCRIPT extraction treatment or combustion process, since it is simple, efficient, and low-cost. The

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sorbed organics were released after heating the sample followed by collecting the condensate.

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The recyclability of rGOF-3 was tested by using cyclohexane and toluene with boiling points

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of 80.7 and 110.6 °C, respectively. In a typical sorption-release process, at the standard

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atmospheric pressure, the liquid-filled rGOF-3 was heated to 75 °C to evaporate cyclohexane

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or 105 °C to evaporate toluene. Then the heated rGOF-3 was weighed to determine the

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weight of residual liquid. As a result, both the residual weights of cyclohexane and toluene in

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rGOF-3 are less than 4% in each sorption-release cycle (Figure 3e,f), indicating an excellent

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recycling performance of rGOF-3. Moreover, no physical damage to the rGOF-3 was

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observed after 10 cycles of sorption-desorption tests (Figure S9 in SI).

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3.3 Supercapacitor applications

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The electrochemical performance of rGOF electrodes was characterized by cyclic

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voltammetry (CV) and galvanostatic charge/discharge measurements in 1 mol L-1 H2SO4

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aqueous solution at the potential window of 0-1 V (Figure 4). Figure 4a shows the CV curves

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of rGOF-1, rGOF-2, and rGOF-3 at scan rate of 50 mV s-1. The rGOF-1 exhibited a much

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larger integrated area of CV curve, indicating its larger specific capacitance than those of

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rGOF-2 and rGOF-3. Therefore, rGOF-1 was chosen for the further studies on its

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electrochemical performance. Figure 4b shows the CVs of rGOF-1 at different scan rates

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from 10 to 100 mV s-1. The near rectangular shapes with no obvious redox peaks (Figure 4b

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and Figure S10 in SI) indicate the electrochemical double-layer capacitor (EDLC) behavior

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of rGOF-1. Figure 4c shows the mass specific capacitances of rGOF-3 at different scan rates.

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The highest specific capacitance is ~135 F g-1 at scan rate of 10 mV s-1, higher than those of

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previously

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charge/discharge test was performed at different current densities from 2 to 10 A g-1 (Figure

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4d). As shown in Figure 4e, the highest specific capacitance for the rGOF-1, based on the

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supercapacitors[35,36].

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Fig. 4. Characterization of rGOF-based supercapacitors. (a) CV curves of rGOF-1, rGOF-2

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and rGOF-3 at 50 mV s-1 in 1 mol L-1 H2SO4 electrolyte. (b) CV curves of rGOF-1 at

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different scan rates. (c) Specific capacitances of rGOF-1 at different scan rates. Inset: Specific

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capacitances of rGOF-1 at scan rate from 10-100 mV s-1. (d) Galvanostatic charge/discharge

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curves of rGOF-1 at different current densities. (e) Specific capacitances of rGOF-1 at

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different current densities. (f) Cycling performance of rGOF-1 at current density of 2 A g-1 for

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1000 cycles.

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by ~25 % in the first 250 cycles, which could be due to the improved wettability of the

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electrode surface and the activation of the electrode material[37,38]. More importantly, no

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obvious loss of capacitance was observed after 1000 charge/discharge cycles, indicating the

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good cycling stability of rGOF-1.

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3.4 Electric and thermal properties

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By varying the spray time, the thickness of rGOFs can be controlled. Here, two rGOF-1

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with thickness of 100 nm, referred to as thin rGOF-1, and 10 µm, referred to as thick rGOF-1,

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were prepared and then used as transparent conductive films and thermal conductive films

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(Figure 5). With a relatively short spray time (~10 s), the thin rGOF-1 with high transparency

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was obtained on a flexible polyethylene terephthalate (PET) substrate (Figure 5a and Figure

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S11 in SI). The conductivity of the as-prepared film is ~100 S/cm measured by the standard

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four-point probe method at ambient conditions (Table S1 in SI). Such a good conductivity of

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thin rGOF-1 surpasses most conductive polymer films, including polypyrrole (PPy)[39],

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suggesting that it has great potential as transparent flexible electrode. As a proof-of-concept

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application, the as-prepared thin rGOF-1 electrode was used for the real-time

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electrocardiogram (ECG) measurement. Compared to the rigid and complicated traditional

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Ag/AgCl electrode[40], the flexible thin rGOF-1 is more conformable with human skin.

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Figure 5b shows the ECG waveform measured using our thin rGOF-1 electrode, exhibiting a

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very clear pattern of heartbeats with heart rate of ~90 min-1.

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Recently, the ultrahigh thermal conductivity of graphene (2000 W m-1 K-1~5300 W m-1 K-

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fabricated rGOF-1. As shown in Figure 5c and Table S1, the prepared thick rGOF-1 (10 µm)

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exhibited the highest in-plane thermal conductivity of 613 ± 27 W m-1 K-1 at room

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temperature among rGOFs, which is larger than that of graphene laminate (40-90 W m-1 K-1)

) has been reported[1,41,42]. Here, we also examined the thermal conductivity of our

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1.56 and 2.57 times those of Cu

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Fig. 5. Electrical and thermal properties of rGOFs. (a) Photograph of a highly flexible,

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transparent and conductive rGOF-1 coated on a PET substrate. (b) The rGOF-1/PET in (a)

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showing good electrical conductivity (100 S cm-1), which can be used as a conductive

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electrode to detect the electrocardiogram (ECG) waveform in order to replace the

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traditionally rigid Ag/AgCl electrode. (c) Photograph of a flexible and thermally conductive

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rGOF-1. The rGOF-1 can be used as a heat spreader due to its a high in-plane thermal

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conductivity of 613 ± 27 W m-1 K-1 at room temperature. (d) Plot of the temperatures of

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rGOF-1 and PET film versus the heating stage temperature. Inset: After the rGOF-1 with

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circular shape was placed on a PET film, they were placed on a heating stage. It is clearly

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seen that the temperature of rGOF-1 is lower than that of the PET film, indicating the poor

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thermal conductivity of rGOF-1 along the direction vertical to films.

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(393.6 W m-1 K-1) and Al (238.6 W m-1K-1) at room temperature, respectively, which are the

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commonly used materials in commercial heat spreaders. It indicates that our thick rGOF-1 is

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promising as the thermally conductive film for heat spreaders. According to the previous report[45], our rGOF-1 should have a low inter-plane thermal

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conductivity at room temperature and show a strong anisotropy of the thermal conductivity.

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In order to confirm it, a simple experiment was designed and carried out. A thick rGOF-1

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with circular shape was placed on a PET film, which was then placed on a heating stage

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(Figure 5d). A non-contact infrared thermometer was used to measure the temperature of PET

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film and the thick rGOF-1 upon specific heating stage setting (Figure 5d). It is clearly seen

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that the temperature of the thick rGOF-1 is lower than that of the PET film at the same

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temperature of heating stage. For example, the temperature difference of 31 °C between the

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PET film and the thick rGOF-1 was observed when the temperature of the heating stage was

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set at 90 °C. Therefore, the rGOF-1 is a poor thermal conductor along its inter-plane

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direction.

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3.5 Discussion

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The mechanism for the GO films formed with different surface morphologies and

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microstructures were further investigated by a series of experiments. Figure 6a schematically

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illustrates the formation process of GO films by our heating-assisted spray process, which

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includes the evaporation of ethanol, agglomeration of GO sheets, and the formation of

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crumpled films. As shown in the photograph and optical microscopy image (Figure S1 in SI),

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GO sheets can be well dispersed in ethanol. The droplets containing GO sheets and ethanol

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were ejected from an airbrush gun at ~20 cm high above the heating stage with an initial

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speed of 0.46 m/s (see the detail calculations for evaporation process in SI). The evaporation

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rate of ethanol droplet, depending on the heating temperature, affects the final volume of

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ethanol droplet (Figures 6b,c). Figure 6d is the plot of V20s/V0 vs. distance at heating

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temperature of 70 °C, in which V20s is the volume of droplet after heating for 20 s and V0 is

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ACCEPTED MANUSCRIPT the original volume just ejected from the airbrush gun. Figure 6d indicates that the volume of

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droplet is also related to the distance between the droplet and the heating stage. As the droplet

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ACCEPTED MANUSCRIPT Fig. 6. Mechanism for the formation of GO films with different surface morphologies and

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microstructures. (a) Schematic illustration of formation of GO films at different heating

3

temperatures. The droplets containing GO sheets and ethanol are ejected from the airbrush

4

gun at an original distance (d0) with an initial speed of 0.46 m s-1. The volume of these GO-

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ethanol droplets at a distance (d), which is between original distance (d0) and heating

6

substrate (d=0), is determined by the evaporation rate of ethanol which is related to heating

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temperature. The volume decrease of the GO-ethanol droplet induces the compression of GO

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sheets, resulting in the GO film with different crumpled morphologies on the heating

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substrate (d=0) at different heating temperatures. (b) Plot of the evaporation rate of GO-

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ethanol droplet vs. the distance (d) between the droplet and glass substrate on the heating

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stage. (c) Plot of the evaporation rate of GO-ethanol droplet vs. the heating temperature on

12

the heating stage.

13

glasssubstrate at heating temperature at 70 °C. V20s is the volume of GO-ethanol droplet after

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heating for 20 s, and V0 is the original volume of GO-ethanol droplet. (e) Plot of Vc/V0 vs.

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heating temperature. Vc is the volume of the GO-ethanol droplet contacting the substrate.

16

This ratio, Vc/V0, decreases with the increase of heating temperature. The original volume

17

(V0) of GO-ethanol droplet in (b-e) is ~3.33×10-8 L.

18

reaches the heating stage, it becomes much smaller due to the evaporation of ethanol. Our

19

calculations indicate the size of droplet is 18%, 14.4%, and 5.7% of its original size at the

20

heating temperatures of 25, 50, and 70 °C, respectively, when the GO-ethanol droplets

21

reached the heating stage (Figure 6e). Therefore, the capillary compression resulting from the

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shrinking of droplets [46] leads to the deformation of GO sheets to form rGOFs with different

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surface morphologies and microstructures, controlled by simply varying the heating

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temperature.

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(d) Plot of V20s/V0 vs. the distance (d) between the droplet and

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4. Conclusion In summary, we have developed an efficient, low-cost, and convenient heating-assisted

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spray method for the fabrication of rGOFs with different surface morphologies and

4

microstructures by simply tuning the heating temperature. The as-prepared rGOFs are

5

multifunctional and can be used for various applications. Due to its hydrophobicity and

6

porosity, the rGOF-3 can be used as an efficient sorbent for organic solvents and oils,

7

showing promising applications in oil/water separation and environmental protection. In

8

addition, the excellent electrochemical performance of rGOF-1 electrode was also studied,

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indicating its promising application as supercapacitor electrode material. Moreover, the as-

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prepared rGOFs demonstrated potential applications in the ECG measurement and heater

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spreader.

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Acknowledgements

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H.B. and S.W. contributed equally to this paper. This work was supported by the

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Fundamental Research Funds for the Central Universities (2242017K41006, and

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2242016R20013), the National Natural Science Foundation of China (Nos 61274114,

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51420105003, and 113279028), China Postdoctoral Science Foundation funded project (Nos

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2017M611653), and the “Qianjiang Scholars” program and “Thousand Talent Program” of

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Zhejiang Province. This work was also supported by MOE under AcRF Tier 2 (ARC 19/15,

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No. MOE2014-T2-2-093; MOE2015-T2-2-057; MOE2016-T2-2-103) and AcRF Tier 1

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(2016-T1-001-147; 2016-T1-002-051), NTU under Start-Up Grant (M4081296.070.500000),

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and NOL Fellowship Programme Research Grant in Singapore. This research grant is

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supported by the Singapore National Research Foundation under its Environmental & Water

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Technologies Strategic Research Programme and administered by the Environment & Water

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Industry Programme Office (EWI) of the PUB (project No.: 1301-IRIS-47). This research is

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supported by the National Research Foundation, Prime Minister’s Office, Singapore under its

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Campus for Research Excellence and Technological Enterprise (CREATE) programme.

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