Synthesis of graphene oxide sheets with controlled sizes from sieved graphite flakes

Accepted Manuscript Synthesis of graphene oxide sheets with controlled sizes from sieved graphite flakes Ji Chen, Fengyao Chi, Liang Huang, Miao Zhang, Bowen Yao, Yingru Li, Chun Li, Gaoquan Shi PII:

S0008-6223(16)30751-5

DOI:

10.1016/j.carbon.2016.08.096

Reference:

CARBON 11282

To appear in:

Carbon

Received Date: 24 July 2016 Revised Date:

29 August 2016

Accepted Date: 31 August 2016

Please cite this article as: J. Chen, F. Chi, L. Huang, M. Zhang, B. Yao, Y. Li, C. Li, G. Shi, Synthesis of graphene oxide sheets with controlled sizes from sieved graphite flakes, Carbon (2016), doi: 10.1016/ j.carbon.2016.08.096. 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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Synthesis of graphene oxide sheets with controlled sizes from sieved graphite flakes Ji Chen, Fengyao Chi, Liang Huang, Miao Zhang, Bowen Yao, Yingru Li, Chun

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Li, and Gaoquan Shi*

Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of

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China.

—————— *Corresponding author: Tel: +861062773743; Fax: +861062771149. E-mail address: [email protected] (G. Q. Shi). 1

ACCEPTED MANUSCRIPT Abstract

_____________________________________________________________________ Graphene oxide (GO) sheets with desired sizes and narrow size distribution were

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synthesized via a modified Hummers method by using sieved graphite flakes as the starting materials. This method outweighs previous post-synthesis fractionation

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methods in efficiency and scalability; thus it is more promising for large-scale synthesis of size-controlled GO. The as-prepared GO suspensions exhibited size-dependent

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liquid crystal behavior. The importance of hydrolysis step on exfoliation and preserving the sizes of GO sheets, and the removal of their sulfate species was also

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discussed.

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1.

Introduction

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Chemically modified graphenes (CMGs) mainly include graphene oxide (GO) and reduced graphene oxide (rGO) [1]. They have a variety of applications in electronics [2], catalysis [3], sensors [4], and energy conversion and storage [5] because of their

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two-dimensional structures and excellent properties. The lateral dimensions of CMG sheets play an important role in controlling the microstructures and properties of

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CMG-based materials [6]. For example, the lateral dimensions of CMG sheets determine their aspect ratios, and consequently their assembly behaviors [7]. Either small or large CMG sheets have their advantages. Small graphene sheets are highly

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desirable for sensing and biological applications because of their strong electrocatalytic activity, good dispersibility and biocompatibility [8, 9]. On the other hand, large CMG sheets are favorable for fabricating two-dimensional layered

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architectures [10], and three-dimensional graphene-based networks [11]. In these

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cases, larger CMG sheets induced fewer inter-sheet contacts, thus endowing better mechanical properties. Furthermore, the sizes of CMG sheets also strongly affect the electrical and thermal conductivities of graphene materials [12, 13]. rGO is usually prepared by the reduction of GO. Therefore, the sizes of GO sheets determine those of CMG sheets. GO can be cheaply produced in large scale by oxidative exfoliation of natural graphite via Hummers or modified Hummers methods [14, 15]. Unfortunately, in these cases, GO sheets were usually cut into small pieces upon excessive oxidation, agitation, or sonication, resulting in a wide size distribution [16]. Consequently, a 3

ACCEPTED MANUSCRIPT cheap, convenient, and highly-efficient method for the size-control of GO sheets is important for both fundamental researches and practical applications. In fact, size fractionation of GO can be achieved either chemically or physically;

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each of them has its pros and cons. The chemical approaches usually involve adding additional chemical agents (e.g., acids) [17] or organic solvents (e.g., ethanol) [18] to selectively precipitate the larger GO sheets. The main issue of these methods is the

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introduction of foreign components. Physical strategies mainly include centrifugation

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[12] or membrane separation [6]. Centrifugation is an effective approach for the size separation, however they were restricted by high energy consumption and low efficiency. Membrane separation is both effective and energy-saving; unfortunately, it needs special membranes and complex equipment. Moreover, these post-synthesis

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size fractionation methods have the following drawbacks: 1) additional cost, time- and energy-consumptions; 2) low efficiency because of the low GO concentration required for effective fractionation.

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Actually, the ideal size control of GO sheets should be carried out during their

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synthsis process by selecting the raw material and optimzing the synthetic conditions. Indeed, the flake sizes of graphite precursor have great influence on the sizes of as-prepared GO sheets [19-21]. For example, Chen, et al. reported an effective method for the large-scale synthesis of size-controlled GO, however, the sheet sizes were mainly smaller than 1 µm [19]. Cheng and co-workers explored the experimental conditions of synthesizing large-size GO sheets for producing highly conductive rGO. Unfortunately, the yield of GO in this case was low (12.5 wt%) [20]. 4

ACCEPTED MANUSCRIPT High-yield (80%) synthesis of large-size (32.70±24.30 µm) GO has been realized by Kim, et al. [7]. However, this method required high-temperature pre-exfoliation of graphite and a large amount of acid for dispersing exfoliated graphite. In this paper,

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we report a method of size-controlled synthesis of GO by using sieved graphite flakes with a narrow size distribution as raw material and optimizing the preparation procedures. The as-prepared GO sheets have a much narrower size distribution

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compared with that of counterparts prepared by traditional methods.

2. Experimental 2.1. Synthesis of GO

Graphite was purchased from Qingdao Huatai Lubricant Sealing S&T Co. Ltd.,

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Qingdao, China. Sulfuric acid was purchased from Modern Oriental Fine Chmistry Co. Ltd. Other chemicals were supplied by Beijing Chemical Works. All reagents were used as received.

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Small-sized GO was prepared by the oxidation of natural graphite powder (12, 000

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meshes) according to a modified Hummers method developed by us [14, 15]. Typically, graphite powder (1.0 g) and concentrated sulfuric acid (23 mL) was added into a 250 mL flask. Under mechanical stirring (150 r.p.m.) in an ice bath, potassium permanganate (3.0 g) was added slowly to keep the temperature of the suspension lower than 20 oC. Successively, the reaction system was transferred to a 40 oC oil bath and stirred (150 r.p.m.) for 1 h. Then the reaction mixture was poured onto 300 mL ice/water mixture, the resultant GO was nominated as ‘SGO ice’. Alternatively, 50 5

ACCEPTED MANUSCRIPT mL of water was added, and the solution was stirred for 15 min at 95 oC, the resultant GO is called ‘SGO hydrolysis’. Medium-sized GO was prepared by the oxidation of sieved natural graphite

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powder (300−500 meshes, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) via a modified Hummers method developed by us [14, 15]. However, a larger volume of sulfuric acid (30 mL H2SO4 per gram graphite) was added to ensure the

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sufficient mass and heat transfer in this reaction system. Typically, graphite powder

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(1.0 g) and concentrated sulfuric acid (30 mL) were added into a 250 mL flask. Under mechanical stirring (150 r.p.m.) in an ice bath, potassium permanganate (3.0 g) was added slowly to keep the temperature of the suspension lower than 20 oC. Successively, the reaction system was transferred to a 40 oC oil bath and stirred (150

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r.p.m.) for 3 h. Then the reaction mixture was poured onto 300 mL ice/water mixture, the resultant GO is nominated as ‘MGO ice’. Alternatively, 90 mL ice-cooled of water was added dropwisely, keeping the temperature stable, and the solution was stirred for

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another 1 h at 40 oC, the resultant GO is named ‘MGO hydrolysis’.

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Large-sized GO was prepared by the oxidation of sieved natural graphite powder (>80 meshes, Qingdao Huatai Lubricant Sealing S&T Co. Ltd., Qingdao, China) via a modified Hummers method developed by us [14, 15]. In this case, the volume of sulfuric acid (60 mL H2SO4 per gram graphite) was futher increased to ensure the sufficient mass and heat transfer in the reaction system. Typically, graphite powder (0.5 g) and concentrated sulfuric acid (30 mL) was added into a 250 mL flask. Under mechanical stirring (150 r.p.m.) in an ice bath, potassium permanganate (1.5 g) was 6

ACCEPTED MANUSCRIPT added slowly to keep the temperature of the suspension lower than 20 oC. Successively, the reaction system was transferred to a 40 oC oil bath and stirred (150 r.p.m.) for 5 h. Then the reaction mixture was poured onto 300 mL ice/water mixture,

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the resultant GO is called ‘LGO ice’. Alternatively, 90 mL ice-cooled of water was added dropwisely, keeping the temperature stable, and the solution was stirred for another 1 h at 40 oC, the resultant GO is named ‘LGO hydrolysis’.

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2.2. Purification of GO

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The reaction mixture, containing graphite oxide (GrO), sulfuric acid, manganese species and water, was filtered and washed with 1:10 HCl aqueous solution (50 mL × 3) to remove metal ions. The resulting wet solid cake was diluted with deionized water to 200 mL for LGO or 400 mL for SGO (or MGO) and stirred for 2 h, forming a

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GrO aqueous dispersion. Finally, it was purified by dialysis for one week using a dialysis membrane (Beijing Chemical Reagent Co., China) with a molecular weight cut off of 8,000 to 14,000 g mol−1 to remove the remaining ionic species.

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The resultant GrO aqueous dispersion was then diluted to 300 or 500 mL, stirred

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for 1 h for further exfoliation. In the case of preparing SGO, additional sonication was applied. GO dispersion was then centrifuged at 3000 r.p.m. for 15 min twice to remove the unexfoliated graphite. 2.3. Measurements of GO yields. A certain volume of purified GO aqueous dispersion was freeze-dried for over 48 h, and the weight of dried GO was used to calculated the concentration of GO (CGO, mg mL−1). The yield of GO (YGO) was calculated by YGO = (CGO×VGO/mGr) × 100%, where VGO is 7

ACCEPTED MANUSCRIPT the total volume of purified GO dispersion (in mL), mGr is the weight of feeding graphite powder (in mg). 2.4. Characterizations

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X-ray photoelectron spectra (XPS) were carried out by the use of an ESCALAB 250 photoelectron spectrometer (ThermoFisher Scientific) with Al Kα (1486.6 eV) as the X-ray source at 150 W with a pass energy of 30 eV for high resolution scan. Raman

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spectra and optical images were recorded on a LabRAM HR Evolution (Horiba Jobin

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Yvon) with a 532-nm laser. Attenuated total reflectance-Fourier transform infrared spectra (ATR-FTIR) were taken out by using a Fourier transform infrared spectrometer (Bruker Vertex V70). For these characterizations (XPS, Raman, ATR-FTIR), freeze-dried GO was compressed to tablets and used as the samples. X-ray diffraction

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(XRD) was performed by the use of a D8 Advance X-ray diffractometer with Cu Kα radiation (λ=0.15418 nm, Bruker, Germany). In this case, GO films prepared by drying GO

dispersions

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Transmission-mode polarized-light optical microscope (POM) images were collected

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by using a Nikon LV-UEPI system (Nikon Corporation, Japan). GO suspensions were stirred to arrange the GO sheets and then deposited onto slide glass. Scanning electron micrographs (SEM) were performed by the use of a field-emission scanning electron microscope (Sirion-200, Japan). The samples used for SEM characterizations were prepared by depositing ethanol-diluted (5 µg mL−1) GO aqueous solutions onto 300 nm SiO2/Si wafers (washed by alternative sonication in acetone/ethanol for 3 cycles and

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3. Results and discussion

Graphite powders were fractionated into small, medium, and large fractions

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(nominated as SGr, MGr, and LGr) using standard sieves. The flake sizes of SGr, MGr, and LGr are mainly 1−4, 30−50, and 200−400 µm, respectively (Fig. 1a). The C

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1s XPS curves of graphite samples (Fig. 1b) indicate that the content of oxygen atoms slightly decreases with the increase of flake sizes (3.1% for SGr and 2.3% for LGr), mainly due to the decrease of physically adsorbed oxygen and oxygenated functional groups bonded to the defective sites. XPS spectra did not trace other elements,

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reflecting the high purity of graphite samples. The XRD pattern of SGr, MGr or LGr (Fig. 1c) exhibits a sharp (002) reflection peak at 2θ = 26.50, 26.56, or 26.60o with a

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full width at half maximum (FWHM) of 0.36, 0.27, or 0.21o, corresponding to a d-spaces of 0.335−0.337 nm. The decreasing in the d-space and FWHM with the sizes

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of graphite flakes are attributed to the increase in their crystallinity. Raman spectroscopy is a powerful tool for studying the structure of graphene related materials. The G-band (~1590 cm−1) is associated with graphitic carbons and the D-band (~1350 cm−1) is related to the structural defects or partially disordered structures of graphitic domains [22, 23]. The Raman spectra of graphite samples (Fig. 1d) show a strong G-band accompanying with a weak D-band with intensity ratios of ID/IG = 0.110, 0.065, and 0.020 for SGr, MGr or LGr, respectively, indicating a higher 9

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content of structural defects in SGr flakes [24].

Fig. 1 – (a) Typical SEM images, (b) C 1s XPS spectra, (c) XRD patterns, and (d)

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532-nm excited Raman spectra of SGr (red), MGr (blue), and LGr (black), respectively.

Size-controlled synthesis of GO sheets can only be realized under optimized

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experimental conditions. Practically, the conversion of graphite to GO can be divided

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into four consecutive steps: oxidation of graphite to GrO, hydrolysis of sulfur containing species, purification of GrO, and exfoliation of GrO to GO. The optimization of experimental conditions for producing size-controlled GO sheets are discussed as follows.

In the oxidation process, the volume of sulfuric acid used for dispersing one gram graphite was increased with the size of graphite flakes (23, 30, and 60 mL g−1 for SGr, MGr, and LGr, respectively) to provide the reaction system with a low viscosity for 10

ACCEPTED MANUSCRIPT achieving efficient mass and heat transfer. The mechanical stirring rate was controlled to be as slow as 150 r.p.m. to avoid tearing up GrO flakes under high-rate shearing. The oxidation time (1, 3, and 5 h for SGr, MGr, and LGr, respectively) was determined by

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the time required for changing the color of reaction system from dark green accociated

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with MnO3+ ions to bright grey [25], as demonstrated by Fig. 2.

Fig. 2 − (a, b) Photographs of reaction mixture (a) at the beginning and (b) at the

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end of 40 oC oxidation.

Hydrolysis is a crucial step for subsequent exfoliation of GrO to GO. Furthermore, the second oxidation step of Hummers method in diluted sulfuric acid at an elevated

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temperature (95 oC) can greatly reduce the sizes of GO sheets [20, 26]. Thus, the 95 oC

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second oxidation step was only applied in the case of synthesizing SGO. For synthesizing MGO and LGO, an additional 40 oC 1 h hydrolysis in 1:3 (by volume) diluted sulfuric acid solution was applied to hydrolyze the sulfur containing species (mainly organosulfates). It was reported that GO sheets can be covalently bonded by organosulfates, leading to insufficient exfoliation of GrO [27]. In our cases, the hydrolysis process is crucial for fully exfoliating GrO to GO (Fig. 3a-b, S1a-b, and S2a-b). The hydrolysis step slightly changed the oxidation degree of GO (Fig. 3c, S1c, 11

ACCEPTED MANUSCRIPT and S2c). The partial removal (~50%) of sulfur species of GrO upon hydrolysis was verified by the increase in its C/S atomic ratio (around 30 before to 50−60 after hydrolysis, based on XPS results). The typical ATR-FTIR spectrum of GO has the

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following characteristic absorptions [28, 29]: C−O−C (~1000 cm−1), C−O (1230 cm−1), C=C (~1620 cm−1), and C=O (1740−1720 cm−1) bonds. The O−H stretching vibrations in the region of 3600−3300 cm−1 correspond to the hydroxyl and carboxyl groups of

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GO and residual water between GO sheets. On the other hand, the S=O stretching

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vibration peaks of sulfates are at 1225 and 1165 cm−1 and they are partially overlapped with the vibrations of epoxy groups. The peak centered at 850 cm−1 is associated with S−O bonds [30]. After hydrolysis (Fig. 3d, S1d, and S2d), the absorption at 1225 cm−1 weakened and the 1165 and 850 cm−1 peaks disappeared,

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reflecting the partial removal of sulfate species. The removal of organosulfate also lowered the d-spaces of GO samples (see XRD spectra in Fig. 3e, S1e, and S2e), because sulfate species usually locate between adjacent GO layers. For

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graphene-based materials, the intensity ratio ID/IG in Raman spectrum can reflect the

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average distance between defects (LD) in graphene. The value of ID/IG initially increases with increasing LD (1−3 nm, stage 2), followed by a decrease (> 3 nm, stage 1) [24]. Usually, GO and rGO belong to stage 2 [31]. Raman spectra (Fig. 3f, S1f, and S2f) show that the hydrolysis resulted in lowering the ID/IG ratio of GrO, suggesting partial structural destruction of graphene sheets. This phenomenon can be attributed to the further oxidation of graphene sheets by different oxidative species (MnO3+ or MnO4−) in the solution of concentrated or diluted sulfuric acid [26, 32]. 12

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Fig. 3 – (a, b) SEM images of (a) SGO ice and (b) SGO hydrolysis, scale bar = 20 µm. (c) XPS C 1s spectra, (d) ATR-FTIR spectra, (e) XRD patterns, and (f) Raman spectra of SGO ice and SGO hydrolysis.

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Purification is also important for GO preparation. During the purification of GrO, 1:10 (by volume) diluted HCl aqueous solution was used to remove the salts from GrO.

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Completely drying of GrO filter cake was usually carried out for evaporating HCl as much as possible. However, this drying process increased the difficulty in the

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exfoliation of GrO to GO and led to the formation of tiny GO sheets (Fig. 4). Experimentally, dried GrO flakes have strong interlayer binding force, making them unable to exfoliated into large intact GO sheets. To address this problem, we collected the wet GrO cake by washing it from the filter paper with a water flow (scratching a wet GrO cake would result in destroy the filter paper to induce impurities), and then stirred for exfoliation. Finally, the suspension was dialysed for more than 7 days.

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in each of the following days.

Fig. 4 – SEM image of the GO sheets synthesized from LGr and purified by overnight drying.

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Exfoliation of GrO to GO can be achieved via either sonication or agitation. However, sonication and long-time agitation can induce severe fragmentation of GO sheets. Consequently, sonication was only applied for preparing SGO until the

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suspension is transparent (monitered by observing an object set behind 1 cm-thick

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cuvette containing 0.5 mg mL−1 SGO). For synthesizing MGO and LGO, only magnetic agitation was applied before and after the dialysis. Experimentally, GO samples were synthesized via our previous reported

improved Hummers method [15] with some modifications to enhance the hydrolysis and removal of sulfur species [30], and the details of these modifications are discussed above. The yields (the weight of freeze-dried GO divided by the weight of graphite powder) of SGO, MGO, and LGO were measured to be 152±3% 113±2%, 14

ACCEPTED MANUSCRIPT and 96±2%, respectively. These results indicate the successful conversion of graphite to GO. The C 1s spectra of GO samples (Fig. 5a) demonstrate four types of carbon bonds: C−C/C=C (284.8 eV), C−O (286.8 eV), C=O (287.8 eV), and O−C=O (289.0

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eV) [15]. The peak intensity ratios of oxygenated carbon atoms are calculated to be 52.0%, 55.0%, and 57.6%, correspondingly, indicating that all the GO samples are well functionalized with oxygen functional groups. The XRD patterns of SGO, MGO,

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and LGO give (001) reflection peaks at 2θ = 10.95, 10.46, and 10.34o (Fig. 5b),

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corresponding to a d-space of 0.808−0.856 nm, and this value is in consistent with that typical values for GO. The larger interlayer spacing of larger GO sheets can be attributed to more oxygenated functional groups introduced on their basal planes by elongated oxidation treatment [28]. Raman spectroscopic studies (Fig. 5c) indicate

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that the ID/IG of SGO, MGO, and LGO are 0.887, 0.949, and 0.926. These ID/IG are

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among the range of reported values for well functionalized GO [26].

Fig. 5 − (a) C 1s XPS spectra, (b) XRD pattern, and (c) 532-nm excited Raman spectra, of SGO, MGO, and LGO samples; their curves are shown in red, blue and black lines, correspondingly, in panels b, c. 15

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The lateral dimensions of the GO sheets depend strongly on the sizes of their graphite precursors. The corresponding size distribution histograms (Fig. 6) show that

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SGO, MGO, and LGO sheets have sizes of 1.7±1.0, 14.9±8.3, and 38.0±16.3 µm, respectively. SGO sheets have an extremely narrow size distribution and LGO contains GO sheets with lateral dimensions up to 200 µm (Figure S3). These results

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confirm that the sizes of GO sheets can be well controlled by using sieved graphite

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precursors and properly optimized preparation conditions. It should be noted here that the GO sheets are mostly monolayers (>90 %) in the as-prepared samples, indicating

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the well exfoliation of GrO.

Fig. 6 − Typical SEM images and corresponding histograms of GO size distributions (right to the SEM image) of SGO, MGO, and LGO samples (up to 16

ACCEPTED MANUSCRIPT down); Scale bar = 100 µm. The histograms of GO size distributions were obtained by counting more than 300 sheets for each sample. Lateral sizes have a direct impact on the liquid crystal (LC) behaviors of GO

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sheets. Usually, increasing the sizes of GO sheets results in decreasing the critical concentration of GO for the formation of nematic LC phase. Theoretically, the formation of GO LC is related to the aspect ratio of its building blocks (the ratio of

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width to thickness) [33, 34]. The average aspect ratio of GO sheets is mainly decided

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by their average sizes and size distribution. Consequently, larger GO sheets should have higher level of orientation at lower concentrations compared with those of small GO sheets [35]. The arrangements of SGO, MGO, and LGO sheets in their suspensions with different GO concentrations were monitored by using a polarized

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optical microscope (POM, Fig. 7). SGO sheets were tested to be unable to form a nematic phase even at a high CGO of 10 mg mL−1 becasue of their small aspect ratios [36]. In comparison, the image of MGO showed connected birefringence domains as

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CGO increased to 2.0 mg mL−1. For LGO suspensions, ordered texture with aligned

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bands was formed even at a low CGO of 1.0 mg mL−1; this critical concentration is among the lowest values reported for forming GO LCs [7]. Moreover, the better fluidity (low viscosity) in the low-concentration nematic phases, can facilitate the alignment of GO sheets in a specific direction upon external force [35]. Aligned self-assembly of nanomaterials can also be achieved in this nematic phase. Such properties can be exploited to fabricate self-aligned CMG-based composite materials with a high degree of orientation. 17

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Fig. 7 − POM images of SGO, MGO, and LGO suspensions, the concentrations of

4. Conclusions

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GO (in mg mL−1) are depicted in the images. Scale bar = 400 µm.

We developed an effective method for synthesizing size-controlled GO sheets by

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using sieved graphite flakes with narrow size distributions as raw materials, and

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optimizing the contition of graphite oxidation, and those for the hydrolysis, purification and exfoliation of GrO. The condition optimizations include 1) more sulfuric acid was used in the step of oxidizing larger graphite flakes; 2) hydrolysis of GrO to remove its sulfate species; 3) avoiding fully drying GrO during purification; and 4) exfoliating the GrO from large graphite flakes by mechanical stirring without sonication. This method is promising for large-scale synthesis of GO sheets with tuanble sizes and narrow size distributions, and it can also be integrated with 18

ACCEPTED MANUSCRIPT post-synthesis fractionation methods to achieve even better size control. Moreover, the suspensions of larger GO sheets exhibited lower critical concentrations for the

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formation of nematic LC phases.

Acknowledgements

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This work was supported by the National Basic Research Program of China (973 Program, 2012CB933402, 2013CB933001), MOST (2016YFA0200200) and the

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Natural Science Foundation of China (51433005, 21674056 ).

References

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[1] Bai H, Li C, Shi G. Functional Composite Materials Based on Chemically Converted Graphene. Adv Mater 2011; 23(9): 1089−115.

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[2] Dua V, Surwade S P, Ammu S, et al. All-Organic Vapor Sensor Using

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Inkjet-Printed Reduced Graphene Oxide. Angew Chem Int Ed 2010; 49(12): 2154−7.

[3] Scheuermann G M, Rumi L, Steurer P, et al. Palladium Nanoparticles on Graphite Oxide and Its Functionalized Graphene Derivatives as Highly Active Catalysts for the Suzuki-Miyaura Coupling Reaction. J Am Chem Soc 2009; 131(23): 8262−70. [4] Liu Y, Dong X, Chen P. Biological and chemical sensors based on graphene materials. Chem Soc Rev 2012; 41(6): 2283−307.

19

ACCEPTED MANUSCRIPT [5] Sun Y, Wu Q, Shi G. Graphene based new energy materials. Energy Environ Sci 2011; 4(4): 1113−32. [6] Chen J, Li Y, Huang L, et al. Size Fractionation of Graphene Oxide Sheets via

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Filtration through Track-Etched Membranes. Adv Mater 2015; 27(24): 3654−60. [7] Aboutalebi S H, Gudarzi M M, Zheng Q B, et al. Spontaneous Formation of Liquid Crystals in Ultralarge Graphene Oxide Dispersions. Adv Funct Mater 2011;

SC

21(15): 2978−88.

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[8] Liu Z, Robinson J T, Sun X, et al. PEGylated nanographene oxide for delivery of water-insoluble cancer drugs. J Am Chem Soc 2008; 130(33): 10876−7. [9] Sun X, Liu Z, Welsher K, et al. Nano-Graphene Oxide for Cellular Imaging and Drug Delivery. Nano Res 2008; 1(3): 203−12.

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[10] Yousefi N, Gudarzi M M, Zheng Q, et al. Self-alignment and high electrical conductivity of ultralarge graphene oxide-polyurethane nanocomposites. J Mater Chem 2012; 22(25): 12709−17.

AC C

5549−63.

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[11] Li C, Shi G. Three-dimensional graphene architectures. Nanoscale 2012; 4(18):

[12] Lin X, Shen X, Zheng Q, et al. Fabrication of Highly-Aligned, Conductive, and Strong Graphene Papers Using Ultra large Graphene Oxide Sheets. ACS Nano 2012; 6(12): 10708−19.

[13] Zheng Q, Ip W H, Lin X, et al. Transparent Conductive Films Consisting of Ultra large Graphene Sheets Produced by Langmuir-Blodgett Assembly. ACS Nano 2011; 5(7): 6039−51. 20

ACCEPTED MANUSCRIPT [14] Hummers W S, Offeman R E. Preparation of Graphitic Oxide. J Am Chem Soc 1958; 80(6): 1339. [15] Chen J, Yao B, Li C, et al. An improved Hummers method for eco-friendly

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synthesis of graphene oxide. Carbon 2013; 64: 225−9. [16] Pan S, Aksay I A. Factors Controlling the Size of Graphene Oxide Sheets Produced via the Graphite Oxide Route. ACS Nano 2011; 5(5): 4073−83.

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[17] Wang X, Bai H, Shi G. Size Fractionation of Graphene Oxide Sheets by

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pH-Assisted Selective Sedimentation. J Am Chem Soc 2011; 133(16): 6338−42. [18] Zhang W, Zou X, Li H, et al. Size fractionation of graphene oxide sheets by the polar solvent-selective natural deposition method. RSC Adv 2015; 5(1): 146−52. [19] Zhang L, Liang J, Huang Y, et al. Size-controlled synthesis of graphene oxide

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sheets on a large scale using chemical exfoliation. Carbon 2009; 47(14): 3365−8. [20] Zhao J, Pei S, Ren W, et al. Efficient Preparation of Large-Area Graphene Oxide Sheets for Transparent Conductive Films. ACS Nano 2010; 4(9): 5245−52.

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[21] Zhou X, Liu Z. A scalable, solution-phase processing route to graphene oxide and

AC C

graphene ultralarge sheets. Chem Commun 2010; 46(15): 2611−3. [22] Ferrari A C. Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects. Solid State Commun 2007; 143(1-2): 47−57.

[23] Kudin K N, Ozbas B, Schniepp H C, et al. Raman spectra of graphite oxide and functionalized graphene sheets. Nano Lett 2008; 8(1): 36−41. [24] Lucchese M M, Stavale F, Ferreira E H M, et al. Quantifying ion-induced defects 21

ACCEPTED MANUSCRIPT and Raman relaxation length in graphene. Carbon 2010; 48(5): 1592−7. [25] Dimiev A M, Tour J M. Mechanism of Graphene Oxide Formation. ACS Nano 2014; 8(3): 3060−8.

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[26] Chen J, Zhang Y, Zhang M, et al. Water-enhanced oxidation of graphite to graphene oxide with controlled species of oxygenated groups. Chem Sci 2016; 7(3): 1874−81.

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[27] Dimiev A, Kosynkin D V, Alemany L B, et al. Pristine Graphite Oxide. J Am

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Chem Soc 2012; 134(5): 2815−22.

[28] Marcano D C, Kosynkin D V, Berlin J M, et al. Improved Synthesis of Graphene Oxide. ACS Nano 2010; 4(8): 4806−14.

[29] Eigler S, Dotzer C, Hirsch A, et al. Formation and Decomposition of CO2

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Intercalated Graphene Oxide. Chem Mater 2012; 24(7): 1276−82. [30] Eigler S, Dotzer C, Hof F, et al. Sulfur Species in Graphene Oxide. Chemistry-a European Journal 2013; 19(29): 9490−6.

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[31] Eigler S, Dotzer C, Hirsch A. Visualization of defect densities in reduced graphene

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oxide. Carbon 2012; 50(10): 3666−73. [32] Kang J H, Kim T, Choi J, et al. Hidden Second Oxidation Step of Hummers Method. Chem Mater 2016; 28(3): 756−64.

[33] Bates M A, Frenkel D. Nematic-isotropic transition in polydisperse systems of infinitely thin hard platelets. J Chem Phys 1999; 110(13): 6553−9. [34] van der Kooij F M, van der Beek D, Lekkerkerker H N W. Isotropic-nematic phase separation in suspensions of polydisperse colloidal platelets. J Phys Chem B 2001; 22

ACCEPTED MANUSCRIPT 105(9): 1696−700. [35] Yao B, Chen J, Huang L, et al. Base-Induced Liquid Crystals of Graphene Oxide for Preparing Elastic Graphene Foams with Long-Range Ordered Microstructures.

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Adv Mater 2016; 28(8): 1623−9. [36] Jalili R, Aboutalebi S H, Esrafilzadeh D, et al. Formation and processability of

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liquid crystalline dispersions of graphene oxide. Mater Horiz 2014; 1(1): 87−91.

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ACCEPTED MANUSCRIPT Figure captions: Fig. 1 – (a) Typical SEM images, (b) C 1s XPS spectra, (c) XRD patterns, and (d) 532-nm excited Raman spectra of the SGr (red), MGr (blue), and LGr (black),

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respectively. Fig. 2 − (a, b) Photographs of reaction mixture (a) at the beginning and (b) at the end of 40 oC oxidation.

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Fig. 3 – (a,b) SEM images of (a) SGO ice and (b) SGO hydrolysis, scale bar 20 µm.

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(c) XPS C 1s spectra, (d) ATR-FTIR spectra, (e) XRD patterns, and (f) Raman spectra of SGO ice and SGO hydrolysis.

Fig. 4 – SEM image of the GO sheets synthesized from LGr and purified by overnight drying.

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Fig. 5 − (a) C 1s XPS spectra, (b) XRD pattern, and (c) 532-nm excited Raman spectra, of SGO, MGO, and LGO samples; their curves are shown in red, blue and black lines, correspondingly, in panels b, c.

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Fig. 6 − Typical SEM images and corresponding histograms of GO size

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distributions (right to the SEM image) of SGO, MGO, and LGO samples (up to down); Scale bar, 100 µm. The histograms of GO size distributions were obtained by counting more than 300 sheets for each sample. Fig. 7 − POM images of SGO, MGO, LGO suspensions, the concentrations of GO (in mg mL−1) are depicted in the images. Scale bar: 400 µm.

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