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Jul 11, 2015 - Heterocyclic aramid nanoparticle-assisted graphene exfoliation for fabrication of pristine graphene-based composite paper. Yao Mo . Qi Liu .
J Nanopart Res (2015) 17:297 DOI 10.1007/s11051-015-3099-x

RESEARCH PAPER

Heterocyclic aramid nanoparticle-assisted graphene exfoliation for fabrication of pristine graphene-based composite paper Yao Mo . Qi Liu . Jinchen Fan . Penghui Shi . Yulin Min . Qunjie Xu

Received: 27 April 2015 / Accepted: 27 June 2015 / Published online: 11 July 2015 Ó Springer Science+Business Media Dordrecht 2015

Abstract Mechanically strong, electrically conductive, and flexible pristine graphene-based composite paper was prepared based on heterocyclic aramid nanoparticle-assisted liquid-phase exfoliation of graphite. The macroscopic heterocyclic aramid yarns were split and assembled into heterocyclic aramid nanoparticles with the size of *30 nm by deprotonation in dimethylsulfoxide in the presence of potassium hydroxide. The obtained heterocyclic aramid nanoparticles dimethylsulfoxide dispersion was used as good medium solvent for highly efficiency liquid-phase exfoliation of graphite. The results demonstrated that the concentration of exfoliated graphene can facile reaches *2.72 mg/mL after direct sonication of 7 h with assist of heterocyclic aramid nanoparticles. After exfoliation, the self-assembled pristine graphene-

Electronic supplementary material The online version of this article (doi:10.1007/s11051-015-3099-x) contains supplementary material, which is available to authorized users. Y. Mo  Q. Liu  J. Fan (&)  P. Shi  Y. Min (&)  Q. Xu Shanghai Key Laboratory of Materials Protection and Advanced Materials in Electric Power, College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai 200090, People’s Republic of China e-mail: [email protected] Y. Min e-mail: [email protected]

based composite paper was fabricated by vacuumassisted filtration. Due to the introduction of heterocyclic aramid nanoparticles, the self-assembled pristine graphene/heterocyclic aramid nanoparticles composite paper exhibited good mechanical property with tensile strength of *129.7 MPa, meantime, has a high electrical conductivity of *1.42 9 104 S/m. Keywords Graphene  Paper  Aramid  Liquidphase exfoliation  Nanoparticles  Self-assembling

Introduction Graphene, single layer of graphite, has an ideal 2D structure with sp2 carbon atoms packed into honeycomb crystal plane (Stankovich et al. 2006). As a fascinating nanomaterial, graphene aroused great interest to sever as new nanoscale building blocks to create unique macroscopic materials, such as aerogel, fiber, and paper, for practical application (Compton and Nguyen 2010). In recent years, due to its flexibility, outstanding mechanical and electrical properties, graphene-based papers have been widely studied and applied in Li-ion battery, supercapacitors, biosensor, etc. (Wang et al. 2009, 2012; Xiao et al. 2011). All the time, graphene oxide (GO), obtained from oxidization of graphite, was commonly used as nanoscale building blocks for fabrication of graphene-based papers (Putz et al.

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2011; Valle´s et al. 2012; Park et al. 2012; Compton et al. 2011). In fact, GO is conceptually different from pristine graphene (Hernandez et al. 2008). The existent structural defects, introduced from oxidation of graphite, are continued to disrupt the structural integrity even for reduced GO (RGO) (Lotya et al. 2009, 2010). As a consequence, the mechanical and electrical properties of graphene-based papers, originated from GO, were affected and restricted greatly. In order to solve this problem, the direct liquid-phase exfoliation (LPE) method was utilized for preparing defect-free pristine graphene nanosheet (Hernandez et al. 2008; Coleman 2009). Previously, Coleman’s group reported a method for preparing graphene dispersions at high concentrations of *1.2 mg/mL through N-methyl-pyrrolidone (NMP)-exfoliation with low-power sonication of long time (Khan et al. 2010). Successively, by vacuum filtering, the self-assembly pristine graphene paper exhibits a high electrical conductivity of *18,000 S/m. However, toward the mechanical property, the tensile strength of pristine graphene paper was only around 18 MPa (Khan et al. 2010). The low mechanical property still hindered the further application of pristine graphene-based papers compared to GO and RGO. Guided by the mechanical enhancement methods for layered structural GO and RGO-based papers, the surface functionalization of graphene and introduction of adhesive agents in paper-layered structure can significantly improve the interaction between graphene layers and result mechanically graphene-based papers (Luong et al. 2011; Tian et al. 2013; Chow et al. 2014; Compton et al. 2010). Therefore, based on this consideration, high-concentration LPE of graphene, accompany with surface active functionalization, could realize the construction of high-performance pristine graphene-based paper. Here, we found that the heterocyclic aramid containing imidazole ring macroscale yarns can be split and formed into heterocyclic aramid nanoparticles (HANs) in dimethylsulfoxide (DMSO) and potassium hydroxide (KOH) solvent system. The obtained HANs/DMSO dispersion can be used for high-concentration LPE of graphene. The self-assembled pristine graphene/HANs (GHN) composite paper fabricated by vacuum-assisted filtration exhibited high mechanical performance and excellent electrical conductivity.

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Experimental Materials Heterocyclic aramid (HA) yarns were obtained from Guangdong Charming Co. Ltd. Natural graphite powder (100 meshes, 99.9995 %) was purchased from Alfa-aesar Co. Ltd. KOH and DMSO were supplied by Sinopharm Co., Ltd. Preparation of HANs/DMSO dispersion The HANs/DMSO dispersion was obtained by the method for preparation of aramid nanofibers reported by Kotov’s group. In brief, 0.5 g of HANs yarns and 1.5 g of KOH were added into 500 mL of DMSO. After magnetic stirring for 1 week at room temperature, a dark red HANs/DMSO dispersion (1 mg/ mL) was finally obtained after removing remained KOH. Direct LPE of graphite with assist of HANs Five samples of natural graphite powders (0.3 g) were severally added into 10 mL of HANs/DMSO dispersion (1 mg/mL). Successively, the graphite and HANs/DMSO mixture dispersions were sealed in the glass vials, followed with sonication of 1, 2, 3, 5, and 7 h at below 40 °C, respectively. For investigating the effect of concentration of HANs/DMSO dispersion on the final LPE of graphite, HANs/DMSO dispersion (1 mg/mL) was first diluted to a certain concentration with DMSO. Then, 0.3 g of natural graphite powders was added into 10 mL of HANs/DMSO dispersion with different concentrations (0.1, 0.3, 0.5, 0.6, 0.7, and 1 mg/mL), respectively. In the next, the graphite and HANs/DMSO mixture dispersions were sealed in the glass vials, followed with sonication of 7 h at below 40 °C. After sonication, the above mixture dispersions were centrifuged at 10,000 rpm for 30 min to sediment un-exfoliated graphite and thick graphite flakes. Meantime, the as-resulted top supernatants HAN-stabilized graphene nanosheets/DMSO dispersions were collected. Fabrication of GHN composite paper The GHN composite paper was fabricated by freestanding method with vacuum-assisted filtration.

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Briefly, the as-resulted HAN-stabilized graphene nanosheets/DMSO dispersion was vacuum-assisted filtrated through PVDF filter membrane (47 mm in diameter and 0.22 lm pore in size), followed by extensive washing with DMSO. After peeling off from the filter membrane, as-received HANs/graphene nanosheets composite paper was dried under vacuum at 100 °C for 12 h between two pieces of compacted glasses. Characterization and instruments UV–Vis absorption spectra of the HAN-stabilized graphene nanosheets/DMSO dispersion were recorded by UV-2550 spectrophotometer (Shimadzu, Japan). Fourier transform infrared (FT-IR) spectra were recorded on a Perkine-Elmer Paragon 1000PC spectrometer. Transmission electron microscopy (TEM) images were obtained by JEOL2100F. Atomic force microscope (AFM) images were recorded by digital E-Sweep AFM in tapping mode. Raman spectra were taken with Jobin-Yvon microRaman spectroscopy (RamLab-010), equipped with a holographic grating of 1800 lines/mm and a He– Ne laser (532 nm) as an excitation source. The equilibrium contact angles were measured by SL200B optical contact-angle meter (Kino industry, USA) at room temperature. Thermo-gravimetric analysis (TGA) was performed with a Perkin-Elmer TGA 2050 instrument at a heating rate of 20 °C/min. X-ray powder diffraction (XRD) spectra were recorded on a D/max-2200/PC (Japan Rigaku Corp.) ˚ ). The tensile using CuKa radiation (k = 1.5418 A properties of the composite papers were measured with an Instron 4465 instrument at room temperature with a humidity of about 25 % at a crosshead speed of 2 mm/min for PMMA composite films and their initial gage lengths were 20 mm. The samples were cut into strips of 40 mm 9 4 mm with a razor blade and five strips were measured for each sample. The fracture surfaces were also characterized by scanning electron microscopy (SEM) (Nova NanoSEM NPE218). The electric conductivity of composite papers was determined by a four-probe method (SB100A/21A, SH-METER) under a sample platform (SB120/2, SH-METER) equipped with parallel probes (probe spacing: 1 mm).

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Results and discussion Preparation of HANs/DMSO dispersion As a kind of aromatic polyamide, HA yarns have been used for a variety of applications, including bulletproof vests, protective clothing, and high-performance materials similar to Kevlar R (Bin and Jiantuan 2006; Dobb et al. 1977). The HA can be considered as a block copolymer containing poly(para-phenylene terephthalamide) (PPTA) and poly(amide-benzimidazole) (PABI) chains (Fig. 1a). The existence of imidazole rings in relatively large PABI chains reduced the degree of molecular structural regularity. As shown in Fig. 1b, like production of aramid nanofibers from aramid yarns reported by Kotov’s group (Yang et al. 2011; Cao et al. 2013), the HA yarns were similarly treated by adding HA yarns (0.5 g) in DMSO (500 mL) and KOH (1.5 g) followed with stirring for about 1 week until the brown HA macroscopic fibers were all disappeared. The red-brown transparent and homogeneous HANs/DMSO dispersion was finally obtained. By abstraction of mobile hydrogen from[NH groups and substantial reduction of the strength of hydrogen bonds between HA polymer chains (Yang et al. 2011; Cao et al. 2013), the HA macroscopic fibers can be split into the individual HA polymer chains. Then, the individual block copolymer chains of HA can be shrink and twined into the HANs through the hydrophobic attraction and p–p stacking. TEM and AFM images were both used to demonstrate the formation of HANs. From Fig. 1c, d, there are no macroscopic HA fibers, replaced by irregularly spherical HANs with the size of *30 nm across whole of the TEM and AFM images. From inset figure in Fig. 1c, the size distribution of HANs was in the range from 25 to 45 nm, mainly concentrated on size of *30 nm. Direct LPE of graphite with assist of HANs Through deprotonating process, the macroscopic HA fibers were split and assembled into HANs in DMSO and KOH solvent system. Afterward, the HANs/ DMSO dispersion was used as exfoliation media for LPE of graphite. The LPE of graphite was carried out by bath sonicating the mixture dispersion of pristine graphite and HANs/DMSO dispersion at room

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Fig. 1 a Chemical structure of HA. b Schematic diagram for preparation of HANs/ DMSO dispersion. c TEM image of HANs (inset statistical histogram of particle size distribution). d AFM image of HANs deposited on mica

temperature. After sonication, the final mixture dispersion was centrifuged at 8000 rpm for 30 min to sediment un-exfoliated graphite particles and ultrathin graphite flakes, and the top supernatants were collected as HAN-stabilized graphene dispersion. From the photograph in Fig. 2, the obtained HAN-stabilized graphene dispersion appears light black and very stable without sediment even after 1 month in a sealed bottle. The colloidal nature and high stability of the HAN-stabilized graphene dispersion were confirmed by the presence of Tyndall effect (Li et al. 2008; Younga´Choi and Seoka´Seo 2010). The results indicated that graphite is exfoliated into graphene flakes in HANs/DMSO dispersion and the stable colloidal graphene dispersion is achieved. For further demonstrating the graphene exfoliation with

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assist of HANs, TEM and AFM images offer the direct evident information of graphene exfoliation. As shown in Fig. 3a–d, the exfoliated graphene flakes exhibit good transparent and sharp-edged with size of several square hundred nanometres. The cross-sectional selected area electron diffraction (SAED) pattern (inset of Fig. 3c) demonstrates the typical six-fold symmetry characteristic diffraction with an ordered well-crystallized graphene structure. Due to the attached HANs, the surface of exfoliated graphene flakes was not smooth and clean, which is replaced by densely decorating with black dots which belong to the anchored HANs. AFM images were also used as proof of HANs-assisted graphene exfoliation. From Fig. 4, in selected AFM images of exfoliated graphene flakes, there are two distinguished exfoliated graphene flakes

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Fig. 2 Photographs of a HANs/DMSO dispersion (0.6 mg/mL) and b Tyndall effect of HAN-stabilized graphene dispersion (0.1 mg/mL)

with the thinnest thicknesses of *1.79 and * 2.07 nm. Considering the thickness of single-layer graphene nanosheet for 0.34 nm, the graphene flakes were consisted of 5 * 6 layers of graphene nanosheets. By collecting the thickness data of 25 distinguishable exfoliated graphene flakes in AFM images, this

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thickness statistical histogram is illustrated in Fig. 4c. With 0.6 g of graphite and 20 mL of HANs/DMSO dispersion (30 mg/mL), after 7 h of sonication, the thickness of exfoliated graphene flakes obtained was mainly concentrated in the range of 2–5 nm. The achievement of HAN-assisted graphene exfoliation was further confirmed by using Raman spectroscopy. In Fig. 5, three bands are immediately clear: the D band around 1343 cm-1, the G band around 1573 cm-1, and the 2D band around 2696 cm-1 (Lotya et al. 2009). The D band is indicative of the presence of defects, which in graphene are generally divided into basal plane defects and edge defects. In LPE of graphite, it more probably reflects the formation of new edges as graphene flakes are cut during sonication (Lotya et al. 2009; Vadukumpully et al. 2009). Compared to pristine graphite, the D band becomes predominant and the intensity is obviously increased. The relatively low D band intensity observed accounts for low defect for the exfoliated graphene flakes obtained from LPE of graphite with assist of LPE. The intensity ratios of D to G band (ID/ IG) increased from *0.07 for graphite to *0.2858 for single-layer graphene, suggesting that very little defects or HANs functionality are presented. It is well

Fig. 3 Selected TEM images of HAN-assisted exfoliated graphene flakes (inset of c shows a selected area electron diffraction (SAED) pattern)

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Fig. 4 a, b Selected AFM images of exfoliated graphene flakes deposited on mica and c The thickness statistical histogram of exfoliated graphene flakes (exfoliation condition: 0.6 g of

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graphite, 20 mL of HANs/DMSO dispersion (30 mg/mL), and 7 h of sonication)

Fig. 5 Representative Raman spectra detected from pristine graphite and different samples of exfoliated graphene flakes with the assist of HANs in DMSO

known that the 2D band is originated from secondorder double resonant Raman scattering from zone boundary. It can reflect the number of layers and can be used to distinguish single-layer, few-layer, and multi-layer graphene flakes. When the layers [5, the 2D band of the graphene flakes strongly resembles the 2D band for the pristine graphite (Ferrari et al. 2006; Ferrari 2007). For the 2D band of the exfoliated graphene flakes, it is noted that the width and shape were distinguished from the pristine graphite. However, the results of Lorentz fitting really cannot demonstrate the single-layer graphene nanosheet. As shown in Fig. 5, all the 2D bands of exfoliated

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graphene flakes can be fitted into two distinct superimposed bands (2D1 and 2D2) located at 2659 and 2670 cm-1 by Lorentz fitting (Graf et al. 2007; Hao et al. 2010). Compared to pristine graphite, the positions of 2D1 and the intensity ratio of 2D1/2D2 for the samples of exfoliated graphene flakes are different (Fig. 5). It demonstrated that the pristine graphite can be successfully exfoliated by the assist of HANs in DMSO. Toward Sample 1, the 2D band was more symmetry than the other samples of exfoliated graphene flakes, suggesting that the graphite might be exfoliated into few-layered graphene flakes. These results indicate that the graphite can be directly

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Fig. 6 UV–Vis absorption spectroscopy for HANs/DMSO and HAN-stabilized graphene DMSO dispersions

exfoliated into graphene flakes with assist of HANs in DMSO by direct sonication and agree well with TEM and AFM images. As is well known that graphene commonly could be used as a platform for immobilizing organic and inorganic molecules due to its high surface area, high p-conjunction, and hydrophobic properties. In fact, the HAN-assisted LPE process is accompanied with noncovalent functionalization with HANs. From Fig. 6, the wavelength of maximum absorption for the UV– Vis spectroscopy of HANs/DMSO dispersion is located at *350 nm. After exfoliation, the wavelength of maximum absorption of HAN-stabilized graphene/DMSO dispersion moved to *343 nm with a blue-shift of 7 nm. It is attributed to the p–p

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interaction between HANs and exfoliated graphene flakes. Through HAN-assisted LPE of graphite, the HANs can be continually attached onto the surfaced of exfoliated graphene flakes through p-conjunction. In the HAN-assisted LPE of graphite, the HANs were modified on the surfaces of exfoliated graphene flakes. The obtained HAN-functionalized graphene flakes (HANGS) were characterized by X-ray diffraction (XRD). In the pattern of pristine graphite (Fig. 7), the diffraction peak at 26.2° is attributed to the (002) reflection of a hexagonal graphite structure (Becerril et al. 2008; Lu et al. 2012). After exfoliation, the diffraction peak at 26.2° stills exists in the HANGS. It is worth noting that there is an evident decrease of the intensity value of the (002) diffraction peak from *16,000 to *3000. It indicated that the HANGS were restacking to form graphite-like structure in sample preparation. Meanwhile, the HANGS show no characteristic peak of graphene oxide around 10°. The broad peak at 2h * 20° is assigned to the crystallization peak of HANs. After exfoliation and functionalization, the HANGS can be considered as composites of HANs and pristine graphene. Due to introduction of HANs, the characteristic peaks of HANs can be observed in FT-IR Spectroscopy of HANGS (Fig. 8). The peak at 3440 cm-1 was attributed to the stretching vibration of N–H bonds. The peak for carbonyl groups of C=O (Amide I) appeared at 1651 cm-1. The wavenumbers of absorption bands for Amide II and Amide III were severally located at 1549 and 1246 cm-1, which were attributed to the transformation-coupled vibration of C–N

Fig. 7 XRD patterns for pristine graphite and HANGS

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the wavelength range from 200 to 800 nm. From Fig. 6, the UV–Vis absorption spectroscopy of HANs/ DMSO dispersion approached zero at 500 nm and above. Such observation allowed using the absorbance of the obtained exfoliated graphene flakes dispersions at 660 nm for calculating the graphene concentration for LPE of graphite. Then, the graphene concentration was calculated as follows: A660nm ¼ a C L;

Fig. 8 FT-IR spectroscopy of pristine graphite and HANGS

stretching vibration and N–H bending vibration (Cao et al. 2013; Yang et al. 2011). In addition, the vibration peak at 1593 cm-1 was derived from the linked benzene and benzimidazole rings (Zhou et al. 2006). It demonstrated the existence of imidazole groups in the structure of HANGS. It is considered that the graphene flakes can be obtained by HAN-assisted LPE of graphite with sonication. Previously, the graphene flakes can be hardly prepared by LPE of graphite in pure DMSO, even if added NaOH (Liu and Wang 2011; Shih et al. 2010; Du et al. 2013). For comparison, the same amounts of pristine graphite (0.3 g) were severally added into 10 mL of DMSO with addition of KOH and HANs/DMSO dispersion (1 mg/mL) followed with sonication of 7 h. Through centrifugal separation, the color of the exfoliated graphene dispersions from HANs/DMSO dispersion is opacity and jet black, deeper than the graphene dispersion exfoliated in DMSO with addition of KOH (Inset of Fig. 10). Evidently, the existence of HANs in DMSO effectively improved the efficiency of exfoliation of graphite. For further quantitative analyzing of the efficiency for graphene exfoliation, the concentration of exfoliated pristine graphene was calculated from the absorbance of 660 nm with Lambert–Beer law by UV–Vis analysis according to the method presented by Coleman’s group. The obtained HAN-stabilized graphene dispersion was diluted a number of times, and their UV–Vis absorption spectra were recorded by a double-beam Hekios a spectrophotometer (Shimadzu, UV-2550) in

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where A660 nm is the absorbance of HAN-stabilized graphene dispersion in the wavelength of 660 nm, a is the extinction coefficient which is related to the absorbance, C is the concentration, and L (value: 10-2 m) is the path length. For LPE of graphite in pure DMSO, the extinction coefficient of the exfoliated graphene flakes DMSO dispersions was 3620 mL mg-1 m-1, determined in pure solvents (Du et al. 2013). For determining the extinction coefficient of HAN-assisted exfoliated graphene flakes dispersion, the HAN-functionalized graphene flakes (HANGS) were collected by high-speed centrifugation (20,000 rpm, 30 min), followed with vacuum dry at 80 °C for 24 h. Then, the different amounts of HANGS were re-dispersed in DMSO with sonication of 30 min. The UV–Vis absorption spectra of HANGS/DMSO dispersion were recorded. Additionally, the pristine graphene content of HANGS was also calculated from the TGA analysis (Fig. 9a). Judging from the weight loss at the plateau region around 700 °C, the amount of pristine graphene content accounts for *76.8 wt% in the HANGS. Based on this, the relationship between graphene concentration and absorbance (660 nm) for the HANGS/DMSO dispersion is illustrated in Fig. 9b. By linear fitting, the extinction coefficient of HAN-assisted exfoliated graphene flakes dispersion can be obtained from the slope of fitting line, *457.8 mL mg-1 m-1. Holding the concentrations of graphite and HANs at 30 and 1 mg/mL, the graphene concentration (Cgraphene) increased with the increase of sonication time. As shown in Fig. 10, the yield of graphene for DMSO with the addition of KOH is relatively quite low, which is only achieved at *0.019 mg/mL with the fixed Cgraphite of 60 mg/mL after sonication of 7 h. After the introduction of HANs, the efficiency of graphene exfoliation significantly increased. By contrast, with the fixed Cgraphite of 30 mg/mL and CHANs of 1 mg/mL, the Cgraphene increased with the

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Fig. 9 a TGA curves for pristine graphite, HANGS, HAN. b The relationship between graphene concentration and the absorbance at 660 nm for the HANGS/DMSO dispersion

(Fig. 11c). It is maybe ascribed to the high concentration of HANs which is out of the critical micelle concentration (CMC). With the fixed Cgraphite of 30 mg/mL and CHANs of 0.6 mg/mL, the Cgraphene can facile reaches *2.72 mg/mL after sonication of 7 h. Fabrication of GHN composite paper

Fig. 10 Effect of sonication time on the concentration of exfoliated graphene flakes in DMSO with addition of KOH (Cgraphite: 60 mg/mL) and HANs/DMSO dispersion (Cgraphite: 30 mg/mL, CHANs: 1 mg/mL)

sonication time and can facile reaches *2.58 mg/mL after sonication of 7 h, increased by almost 136 times compared to DMSO with addition of KOH. Obviously, the introduction of HANs significantly enhanced the yield of graphene with LPE of DMSO. It may be attributed to the existence of HANs that adjusted the surface tension of DMSO so as to reduce the interfacial tension and improve the wettability on the surface of pristine graphite from the change of contact angles (Fig. 11a, b). Additionally, the CHANs also plays an important role in the efficiency of graphene exfoliation. In fact, when the CHANs exceeds 0.6 mg/mL, the Cgraphene begins to decrease

Through deprotonating process, the macroscopic HA fibers were split and assembled into HANs in DMSO and KOH solvent system. Then, the HANs not only facilitated the high-efficiency LPE of graphite, but also functionalized the rigid surfaces of exfoliated graphene flakes simultaneously. Based on this premise, the macroscopic pristine graphene-based composite papers with high mechanical and electrical performances can be expected by combining HANs and exfoliated graphene flakes. As shown in Fig. 12a, the pristine graphene/HANs composite (GHN) papers were prepared by self-assembly with vacuum filtration. For fabrication of pristine GHN papers, 0.6 g of graphite was added into 20 mL of HANs/DMSO dispersion (30 mg/mL), followed with 7 h of sonication. After sonication, HAN-stabilized graphene dispersion was obtained by centrifugal separation from the final mixture dispersion with 8000 rpm for 30 min. Investigated from a number of TEM images of 80 distinguishable exfoliated graphene flakes, the distributions of the lateral size were mainly concentrated at the range of *0.08 lm2 (Fig. 13).

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Fig. 11 Equilibrium contact angles of a DMSO, b HANs/DMSO dispersion (1 mg/mL) on the surface of pristine graphite, and c Effect of CHANs on the final graphene concentration (Cgraphene) for the HANassisted exfoliated graphene dispersion with sonication time of 7 h (Cgraphite = 30 mg/mL)

Fig. 12 a Schematic diagram for self-assembled GHN paper by vacuum filtration. b Photograph of the as-prepared GHN paper. c– e One time of bending cycle for the flexibility test of the GHN paper

Then, by vacuum filtrating of the HAN-stabilized graphene dispersion through a PVDF filter membrane (0.22 lm pore size), the GHN papers were prepared by extensive washing with DMSO and peeled off from the filter membrane. Afterward, the GHN papers were placed between two glass plates and died at 80 °C in a vacuum oven for 6 h. The density of obtained GHN papers was around 1.34 g/cm3. As shown in Fig. 12c– e, the GHN paper exhibited very flexible and could keep its integrity without any breakage and fragmentation. It may be attributed to the HANs of GHN paper that acted as a binder and enhanced the interactions between graphene flakes. From the SEM images, the self-assembled GHN paper obviously displayed the layered structure, and the tensile fracture surfaces of GHN paper were very compact and rough (Fig. 14).

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By vacuum filtration, the HANs can be inserted into the lamellar structure of GHN paper and formed the artificial nacre-like graphene-based paper. The intercalated HANs can significantly enhance the interaction between neighboring graphene flakes and hinder the slippage of the layered graphene flakes in GHN paper. Naturally, the introduction of HANs could tailor the mechanical performance of the pristine graphene paper. The tensile strain–stress curves of tensile tests for GHN paper are illustrated in Fig. 15. Obviously, the maximum tensile strength of GHN paper was achieved at *129.7 MPa, about seven times higher than the pristine graphene paper (*18 MPa) reported by Coleman’s group. The initial straightening stage for tensile deformation during the tensile loading is distinct. It may be attributed to the intercalated HANs

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Fig. 13 Selected typical TEM images of distinguishable exfoliated graphene flakes and histogram of size distribution (exfoliation condition: 0.6 g of graphite, 20 mL of HANs/DMSO dispersion (30 mg/mL), and 7 h of sonication)

Fig. 14 SEM images of tensile fracture surface for the GHN paper with different magnifications

in the layered structure of GHN paper. The polymer chains of HANs were stretched under the stress transfer due to the interaction between HANs and graphene flakes. The initial modulus for GHN paper was only *4.1 GPa. For the linear (‘elastic’) deformation, the modulus for GHN paper increased to *14.7 GPa. It demonstrated that the introduction of

HANs could effectively enhance the interaction between the neighboring graphene flakes and prevent the slippage of graphene layers which lead to brittle fracture. The electric conductivity of the GHN paper was determined by a four-probe method (Support information). The GHN paper showed remarkably high electrical conductivity of *1.42 9 104 S/m.

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J Nanopart Res (2015) 17:297 (No. 21271010) and Science and Technology Commission of Shanghai Municipality (No. 14DZ2261000). Additionally, the study was also sponsored by ‘‘Chenguang Program’’ supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG55).

References

Fig. 15 Typical tensile strain–stress curve for the GHN paper

This value is lower than the conductivity of pristine graphene paper (*1.8 9 104 S/m) (Khan et al. 2010). The reason for this is that the insulating HANs were inserted between the layered graphene flakes and reduced the contact surface of graphene flakes.

Conclusion In conclusion, the macroscopic HA yarns were totally split and assembled into HANs with the size of *30 nm by deprotonation in DMSO in the presence of KOH. The obtained HANs/DMSO dispersion can be used as good medium solvent for highly efficiency LPE of graphite. After adding pristine graphite in HANs/DMSO dispersion, the graphene concentration can facile reaches *2.72 mg/mL after direct sonication of 7 h. The introduction HANs not only enhanced the efficiency of graphene exfoliation, but also significantly improved the mechanical performance of pristine graphene-based paper. By vacuum filtration, the tensile strength of self-assembled GHN paper reached *129.7 MPa, meantime, with a high electrical conductivity of *1.42 9 104 S/m. We have reasons to believe that the mechanically strong and electrical conductive pristine graphene-based papers could be used in more exciting applications, such as energy storage, heat conductive, engineering, etc. Acknowledgments This research was supported by Leading talent program of Shanghai, Sailing program of Shanghai science and technology commission (15YF1404700), Startup Fund for New Talent, Shanghai University of Electric Power (K2014-046), National Nature Science Foundation of China

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