Reactivity-Controlled Preparation of Ultralarge Graphene Oxide by

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Reactivity-Controlled Preparation of Ultralarge Graphene Oxide by Chemical Expansion of Graphite Lei Dong, Zhongxin Chen, Shan Lin, Ke Wang, Chen Ma, and Hongbin Lu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03748 • Publication Date (Web): 09 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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Chemistry of Materials

Reactivity-Controlled Preparation of Ultralarge Graphene Oxide by Chemical Expansion of Graphite Lei Dong,1,2 Zhongxin Chen,1 Shan Lin,1,2 Ke Wang,1,2 Chen Ma,1 Hongbin Lu1,2* 1

State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite, Materials and Department of Macromolecular Science, Fudan University, Shanghai, 200433, China, 2

Shanghai Xiyin New Materials Corporation, 135 Guowei Road, Shanghai, 200438, China

Abstract: The production of ultralarge graphene oxide (ULGO) is hindered by sluggish diffusion process of the oxidizing agents into graphite layers, as well as sheet fracture resulted from inhomogeneous oxidation. Previous methods rely on an excess amount of oxidants or multiple oxidation to overcome large diffusion resistance, but at the cost of ULGO yield and environmental risk. Here, we discover the chemical expansion of graphite (CEG) with high solvent-accessible surface areas can effectively boost mass diffusion and facilitate exhaustive oxidation at low oxidant dosage (2 wt equiv.). The oxidizing reaction is therefore controlled by the chemical reactivity of graphite with oxidant rather than the diffusion of oxidant, resulting in a ~ 100% yield of ULGO nanosheets with an area-average size of 127 µm. The worm-like structure of CEG and its oxide provides a chance to recover excess sulfuric acid using a 100-mesh filter, where subsequent exfoliation to ULGO nanosheets is achieved by mild agitation or shaking in several minutes. The ULGO paper prepared by blade casting exhibits superior mechanical properties (Young’s modulus of 11.9 GPa and tensile strength of 110.8 MPa) and electrical conductivity (~613 S/cm after HI reduction).

Keywords: ultralarge graphene oxide, chemical expansion, oxidant diffusion, reactivity-control, graphene paper

Introduction Graphene oxide (GO) is an important building block that exhibits intriguing application potential in nanoelectronics1, multifunctional fibers2, conductive inks3, nanocatalysis4, and energy storage devices5. As a 2D macromolecule, its physical properties, e.g., electric/thermal conductivity2,6,7, mechanical strength8,9, and solution behaviors10,11, critically depend on its lateral size and structural uniformity. Ultralarge GO sheets (ULGO, over 50 µm in lateral size) contain fewer edges and thus lower inter-sheet contact resistance than small GO sheets, which make ULGO possess better reinforcing effect in electrical/thermal conductivity and mechanical strength.7,12 However, highly efficient, scalable production of ULGO remains quite challenging since the fracture of GO sheets always occurs inevitably during the process of harsh oxidation13,14 and exfoliation13,15. Although the modified Hummers method may generate ULGO sheets with tens of microns, the majority of the product is still on micrometer or sub-micrometer scale.16 To obtain ULGO, various size fractionation methods have been established on the base of the differences in physical or chemical properties of large and small GO sheets, such as sedimentation rate17,18, pH12, liquid crystal behavior19 and membrane filtration20. Density gradient centrifugation has been employed to extract large GO sheets from the small

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ones;18 however, its yield and efficiency are rather limited by the low ULGO percentages in the exfoliated product (the content of GO with the lateral size > 25 µm is typically < 20 %, see Table S1) and also, the production process is time/energy intensive. In the existing preparation strategies of ULGO, large quantities of oxidant, sometimes as high as 10 wt equiv. of graphite (see Table S2), is frequently used to prompt the oxidation (i.e., to overcome large diffusion resistance), which not only significantly reduces the lateral size of GO sheets, but also brings about serious environmental risks and cost increment.21-28 In addition, structurally uniform ULGO has never been achieved due to the inherent diffusion-controlled kinetics in the oxidation of graphite.29 Large sheet size and high structural uniformity, although they are hardly attained simultaneously, are two critical factors that dominate the charge transfer efficiency and thermal, mechanical properties of GO and also correlated with each other. 7,8,12 Especially when a high dosage of oxidant is employed, the oxidation-leaded defects usually distribute unevenly in GO sheets, as occurred mostly in the (modified) Hummers method.29 The oxidation always preferentially takes place at the edge, boundary of crystalline domains or defect sites of graphene due to their higher reactivity, as opposed to that of those sp2 carbon atoms within crystal domains. The use of high-dosage oxidant can drive the oxidant into the interior interlayer galleries of graphite;29 however, this also exacerbates the structural homogeneity and reduces the lateral size of GO, even without exerting external disturbance such as ultrasonication or vigorous stirring.13,14 Although the use of thermally or microwave expanded graphite (TEG or MEG) contributes to diminish the diffusion resistance of oxidants to certain extent due to increased interlayer distances, their close pore structures and relatively low specific surface areas (typically 800 m2/g) can effectively boost the diffusion of oxidant into graphene galleries and thus makes the oxidation of graphite is no longer primarily diffusion-controlled, but more decided by the chemical reactivity itself (named as reactivity-controlled). As a result, we find that a very low dosage of oxidant (2 wt equiv. relative to graphite) fulfill the exhaustive oxidation of CEG in 4 hours. In particular, the use of low dosages of oxidant allows to retain the worm-like structure of CEG even after oxidation, which not only minimizes the size reduction of GO, but also greatly enhances the solid-liquid separation efficiency of the oxide of CEG (CEGO) ---- only several minutes are sufficient to complete filtration with a 100-mesh filter. Such CEGO particles are readily exhaustively exfoliated under mild conditions to form ULGO dispersion, and the average size (127 µm) of ULGO sheets is far superior to the reported results.21-28 The obtained ULGO sheets exhibit an outstanding film-forming capability, giving large-area GO papers with mechanical properties and exceptional electrical conductivity (after HI reduction).

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Figure 1. Fabrication process of ULGO. (a) CEG, as an oxidation precursor, is firstly prepared by a room-temperature intercalation-expansion method, showing enormous c-axis expansion.31 (b) Synthesis of CEGO under 2 wt equiv. of KMnO4, left; the optical microscopy (OM) image of the accordion-like CEGO particles, right. (c) CEGO particles can be separated from acidic solution with a 100-mesh metal filter, left; the digital image of 100 g CEGO with a concentration of 25 mg/ml, right. (d) Exfoliation of 100 mg CEGO in 1L water by 5 min of slight stirring to form a homogeneous aqueous ULGO dispersion.

Results and discussion Preparation of ULGO The preparation process is schematically illustrated in Figure 1. CEG is first prepared from flake graphite (+100 mesh) employing the room-temperature intercalation-expansion method that we previously reported, with a huge volume expansion (Fig. 1a and Fig. S1, S2).31 XRD patterns show that the intensity of the characteristic peak at 26.5o for CEG is over three orders of magnitude lower than that of flake graphite (Fig. S3), suggesting significantly weakened π-π stacking. Since water still occupies ~95% of the whole weight of CEG particles after filtration, CEG suffers from sharp decrease in the specific surface area, from 847 to 99.2 m2/g after drying,31 which severely interferes with the oxidation of CEG and also causes uneven structures in the resulting CEGO. To address this issue, wet CEG after filtration is re-soaked in a concentrated H2SO4 to substitute the water residuals in CEG galleries. Subsequently, 2 wt equiv. of KMnO4, relative to graphite, is added into the mixture for the oxidation of CEG at 35 °C. No agitation is applied in this process of 4 hours to prevent possible fracture of GO sheets. The obtained CEGO particles are loosely stacked in the bottom of the bottle and exhibit an accordion-like structure (Fig. 1b), which enables them to be readily filtered out from the solution in half a minute by single washing/filtration cycle with a 100-mesh filter (Fig 1c, Fig. S4, Fig. S5 and Video S1). This is dramatically different from those traditional GO production method, where exfoliated GO sheets generally undergo gelation at a relatively low concentration when the dispersion pH becomes neutral.32 Gelation severely retards the washing and purification of GO sheets and makes the whole process quite tedious and difficult, sometimes it even takes several days to complete the purification process.21,32 By comparison, the filtration of CEGO 3



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particles is much faster since the aggregated structure can be retained well even after several washing/filtration cycles, which allows the sulfuric acid residuals and soluble salts in CEGO galleries to be removed exhaustively. In this purification process, the breakage of CEGO is possible but the weight loss is found to be less than 5 wt%. As a result, such CEGO particles are readily produced on the large-scale and stored at a concentration of 25 mg/mL (Fig. 1c-right). After 5 min of mild mechanical agitation or 10 s of shaking, these CEGO particles can be fully exfoliated into individual GO sheets (yield: nearly 100%, Fig. 1d, Fig. S7 and Video S2), forming a homogeneous, stable ULGO aqueous dispersion (zeta potential: -53 mV, Fig. S8).

Figure 2. Structural characterization of CEGO particles. (a) OM (left) and corresponding polarized optical microscopy (POM, right) images of the CEGO suspension. (b) The top-left is OM image of a typical CEGO particle. The brightness of POM images observed under two crossed polarizers (X and Y) depends on the included angel (denoted as a) between the particle long-axis A and the polarization axis Y.

Different from the “balloon structure” of TEG in which the majority of pores are closed,30 CEG has a homogeneous, open, structure, called “a perforate structure”.31 This enables more graphene surface in CEG to be exposed to oxidant, and thus more homogeneous oxidation and higher exfoliation yields can be achieved. In particular, under the controlled oxidation condition, namely, 2 wt equiv., rather than 4 wt equiv. or more, of KMnO4 to be employed, the resulting CEGO is not exfoliated spontaneously in this process (Fig. 2a, left) and still maintained a worm-like structure similar to that of the CEG unless fluid disturbance is applied. This offers an advantage to improve the solid-liquid separation efficiency and the structural uniformity in production of ULGO (Fig. 1c and Fig. S4). Owing to the controlled oxidation, CEGO retains large lateral dimensions ranging from 50 to 470 µm (Fig. S5) and reveals a clear birefringent phenomenon when observing under two orthogonal polarizers (Fig. 2a, right). This have never been reported before, which implies CEGO particles consist of a series of long-term ordered GO arrays, similar to the alignment-controlled GO or clay liquid crystals induced by high magnetic field33, directional liquid flow34, or confined space35. Further observation shows that the intensity of output light strongly depends on the included angle () between the particle long-axis A and the polarization axis Y (Fig. 2b and Fig. S9). When fixing the two crossed polarizers and rotating the sample table, the observed image of CEGO particles appeared and disappeared alternately. We assume that the GO layers in CEGO particle are parallelly arranged and neglect the light absorption from solution and CEGO. According to the Malus law, the imaging intensity (I) of optical microscopic images is written as,

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∆Φ

∆Φ

= ∗ 4cos ∗ cos (90 − ) ∗ cos () = ∗ cos ∗ sin (2)

[1]

, where is the intensity of incident light, ∆Φ is the phase retardation between ordinary light e and extraordinary light o and is defined as ∆Φ =

∗( )

, in which

is the length of CEGO along the long-axis, λ is the wavelength

of incident light, !" is the refractive index of ordinary light e, and !# is the refractive index of extraordinary light o (Fig. S9). Except the included angle , the other parameters are constant. Accordingly, the output light intensity I

reaches the maximum when is 45o or 135o and approaches zero when is 0° or 90°. This agrees with the experimental observation in Figure 2b and Figure S10. Apparently, irregular structural arrangement of GO layers would weaken this angle-dependent optical behavior, as observed in osmotically swollen layered materials (e.g. protonated titanate and vermiculite).36 Consequently, the long-term order observed here indicates that CEGO particles have well-separated interlayer spacings and highly accessible specific surface areas, the latter can be confirmed further by methylene blue (MB) absorption experiments (Fig. S11).37 Figure S11 shows the MB absorption results of CEGO particles, in which the data of exfoliated ULGO sheets are also included for comparison. For CEGO, its characteristic absorption band at ~665 nm decreases gradually with time and reached an equilibrium after 4 hours (Fig. S11a-b). In contrast, ULGO reaches the equilibrium in 10 minutes. The slower MB-absorption rate for CEGO arises from the limited diffusion space, relative to dispersed ULGO sheets. Nevertheless, the maximum absorption capacity (Qm, maximum MB-absorption mass per gram of samples) of CEGO is comparable to that of ULGO sheets, suggesting a well-separated structure of CEGO facilitating exfoliation. Characterization of ULGO The morphology of ULGO sheets is observed by atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Based on a statistics of ~50 sheets in AFM images (Fig. S13), the thickness of ULGO sheets is mostly ~0.9 nm, validating its monolayer character.17 TEM image shows an isolated ULGO sheet with an ultralarge lateral size (Fig. 3a) and the clear hexagonal pattern in selected area electron diffraction (inset in Fig. 3a) suggest that the crystalline sp2 clusters are extended and present in large amounts, which accord with the HR-TEM image in Figure S14. Compared to the (-2110) diffraction spot, the higher (-1010) diffraction intensity also indicates the monolayer nature of ULGO. To determine the lateral size of ULGO sheets, we deposited the ULGO aqueous dispersion on silicon substrates for SEM observation (Fig. 3b and Fig. S15). It is observed that even with controlled oxidation and limited disturbance, the resulting ULGO sheets still reveal a wide size distribution (Fig. 3c), which would closely relate to the original dimension of the CEG or CEGO particles, as shown in Fig S1d and Fig. S5. Given that the 2D nature of GO, we employ two methods, area-average and number-average lateral sizes, to describe the size distribution of ULGO sheets (See Materials and Methods). By counting 130 sheets (Fig. S15), the size distribution histogram of ULGO is shown in Figure 3c, giving an area-average size ($&% ) of 127.7 µm and a '''! ) of 83.4 µm, respectively (the largest ULGO sheet with a lateral size of 478 µm is presented number-average size ($ in Fig. S15a, over 2 times larger than the reported record values17,45). Considering that over 90% of our ULGO sheets

are single layer (0.86 nm, see Fig. S13 ), we obtain a record aspect ratio up to 1.5×105, which is far higher than those reported results (a detailed comparison can be found in Table S1 and S2). To further evaluate the present method, we choose three key factors (lateral size, exfoliation yield and oxidant dosage) for comparison with the reported methods, and the corresponding results are present in Figure 3d, Table S1 and S2. Based on such a survey, we find that no other 5



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studies can simultaneously achieve an exfoliation yield of > 50 % and a mean lateral size of > 40 µm.

Figure 3. Characterization of ULGO sheets. (a, b) TEM (a) and SEM (b) images of representative ULGO sheets. Inset of a shows the corresponding selected area electron diffraction pattern38, yielding d-spacing of 2.13 Å and 1.23 Å. (c) Number- (Ni) and area-average (Si) size distribution histograms of ULGO sheets from SEM images. The dot-lines are the corresponding Gaussian fitting curves. (d) Comparison of our work (red dots) with other representative modified Hummer’s oxidation (blue dots) and size fractionation (black dots) methods, where solid dots and open dots denote the lateral size of GO sheets and oxidant dosages, respectively. (e, f) Raman D band (e) and G band (f) mapping images of ULGO sheets, giving a ID/IG range from 0.92 to 0.96. Inset of f shows the corresponding OM image.

The composition and defect distribution of ULGO are characterized by Raman mapping (Fig. 3e, f) and X-ray photoelectron spectra (XPS, Fig. S17). We obtain the Raman spectra and the corresponding intensity mapping of D (~1350 cm-1) and G (~1600 cm-1) bands with a 532 nm excitation laser. The D band arises from the breathing mode of sp2 carbon atoms and the intensity ratio of D to G (ID/IG) reflects the degree of defects caused by oxidation, edges or structural disorder. Figure S16 shows a representative Raman spectrum of ULGO with an ID/IG of 0.93, a typical defect structure that is similar to the GO prepared by the Hummers method (Hummers-GO). The small color contrast in D band and G band mapping indicates the uniform distribution of defects in this ULGO sheet (Fig. 3e, f), along with a narrow ID/IG range from 0.92 to 0.96. XPS result reveals that ULGO has an oxygen content of 27.5 wt%, which is lower than that of the Hummers-GO (33.3 wt%, Fig. S17a). The XPS C1s spectrum indicates that ULGO has a stronger C=C bond peak (285.2 eV, Fig. S17b) than that of the Hummers-GO (Fig. S19). These facts (including large sheet size, uniform defect-distribution and low oxygen content) suggest that the oxidation of CEG mainly depends on the reactivity of carbon atoms rather than the diffusion process of oxidant.29 This is important for the preparation of ultralarge GO sheets in high-yield, scalable manner that prevents the inevitable sheet fracture from inhomogeneous and excess oxidation.21-28

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Figure 4. The effect of internal structure of graphite on oxidation and exfoliation. (a-d) OM images (a, c) and the corresponding in-situ Raman spectra (b, d) of CEGO (a, b) and the oxide of dry-CEG particles (c, d), normalized by D bands. The top surface of the oxide of dry-CEG is removed by scotch-tape method for convenience of observation or avoiding excessive oxidation. (e) Schematic illustration of reactivity-controlled oxidation and diffusion-controlled oxidation. The two different strategies are decided by the structure of precursor graphite. Scale bars, 100 µm (a, c).

Critical factors to prepare ULGO The extraordinary properties of GO based materials are highly dependent on the lateral size of GO sheets. Research efforts have therefore been directed to understand the critical factors for dimensional control in the preparation of GO. In particular, Aksay and coworkers have demonstrated periodic cracking of platelets occurs during the inhomogeneous oxidation of graphite flake.13 Tour el al. show that the diffusion of the oxidizing agent into the graphite interlayer spacing is the rate-determining step,29 that is, inhomogeneous oxidation is inevitable for graphite flake because of the very limited d-spacing (0.336 nm). Therefore, graphite with increased interlayer galleries is a precondition for preparing ULGO sheets in high yield manner. However, the exfoliation of the oxide of TEG to ULGO nanosheets is not really successful with a yield of 10 ~ 20%,16 which implies the internal structure of expanded graphite also plays an important role. TEG exhibits a typical multilayer stacking structure, in which the “balloon” wall in worm structures usually consists tens of graphene sheets to reveal a close, expanded appearance, with a low specific surface area.30 Consequently, a portion of graphene surface in TEG are inaccessible by sulfuric acid and oxidant, leading to non-uniform oxidation and mechanical weakpoints. We assume that the well-separated, solvent-accessible structure of CEG is necessary for the high-yield preparation of ULGO. To elucidate this, we choose these expanded graphite materials to study the effect of internal structures on oxidation, including CEG (847 m2/g), dried CEG (dry-CEG, 99.2 m2/g) and TEG (128.7 m2/g).31 As aforementioned, CEG has an open, highly accessible graphene surface, where TEG and dried CEG becomes solvent inaccessible due to shrinkage

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and re-stacking of graphene layers. We record the Raman spectra of these compounds at 8 locations to examine the defect formation under oxidation. As shown in Figure 4a-d and Fig. S20, dry-CEG and TEG are only oxidized partially after 4 h of agitation-free oxidation. The ID/IG ratios at the black, internal region (0.21-0.63) are lower than those at the light yellow edges (0.92~0.98). This structural difference can also be directly observed with optical microscope, from which the luminous edges are easily distinguished from the un-oxidized, dark internal region (Fig. S21), indicating that the diffusion of oxidant in the internal region is rather difficult. In contrast, CEGO exhibits uniform, light yellow color along with slightly varied ID/IG ratios (0.92-0.97) (Fig. 4a), validating the homogeneous oxidation of CEG. In addition, it is found that the dark-green color of MnO3+ disappears at the end of oxidation and the suspension becomes grey black. This implies that for CEG, 2 wt equiv. of KMnO4 is needful but sufficient to achieve uniform oxidation of all graphene layers. For dry-CEG and TEG, however, it is impossible to achieve similar oxidation and exfoliation due to the significant oxidant diffusion resistance (Fig. 4e). Even after 10 minutes of sonication, both oxidized dry-CEG and oxidized TEG only reveal an exfoliation yield of < 10% (Fig. S22). Another critical factor is minimizing fluid disturbance. In the previous studies, sonication or mechanical stirring is frequently used to exfoliate GO in water.13,17-21 However, such disturbance can remarkably reduce the size of GO sheets, especially for those large-sized GO with high strain buildup. To demonstrate this, we examine the size dependence of as-prepared ULGO sheets on mechanical agitation, where ULGO is dispersed in aqueous solution at 0.1 mg/mL with magnetic stirring at 800 rpm with a period of time. As shown in Figure S23, both area- and number-average sizes reveal '''' an obvious reduction with increasing shear times; that is, > 50% of decreases in ''' $( and $ in 60 minutes. Such a decrease actually is not surprising, given that the oxidant is apt to preferentially react with the high-reactivity carbon atoms located at the crystal domain boundaries in graphene sheets. The defects formed thereby make them quite fragile and fracture even at small fluid disturbance such as slow agitation, shear and low-power sonication. This suggests that whether in the oxidation of CEG or in the exfoliation of CEGO, vigorous disturbance should be avoided, in order to acquire ULGO sheets in a high-yield manner.

Figure 5. Physical properties of ULGO dispersion and ULGO papers. (a) ULGO and Hummers-GO dispersions under the same concentration of 5 mg/ml, showing distinctly different flow behaviors. (b) Storage modulus (G′, solid dots) and loss modulus (G′′, open dots) of ULGO (red) and Hummers-GO (black) dispersions, under the same concentration of 5 mg/ml. (c) ULGO disersion can be evenly deposited on PET base through simple blade casting method (inset). And after drying at 50 oC for 10 h, the formed film is teared off directly,

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giving a continous, flexiable ULGO paper. (d, e) Cross-section SEM images of ULGO paper (d) and Hummers-GO paper (e). (f) Strain-stress curves of ULGO paper (red) and Hummers-GO paper (black). Scale bar, 1 cm (c), 4 µm (d), 2 µm (e).

Figure 6. Characterization of ULGO paper after HI reduction. (a) The reduced ULGO paper exhibits uniform structure and hydrophobic nature (inset). (b) Multipoints (7× 9 points) conductivity measurement of reduced ULGO paper using four-point probe method, showing an average value of 613 S/cm and a narraw-range distribution of 582-633 S/cm. The survey points evenly locate in a 3 cm× 4 cm region.

ULGO dispersion and paper The ULGO aqueous dispersion reveals a typical physical gelation behavior at 5 mg/ml (Fig. 5a). The storage modulus (G′) of the dispersion is greater than the loss modulus (G″) in the tested frequency range (Fig. 5b), with a much higher G′ value (167-214 Pa) than that of Hummers-GO dispersion (0.4-2.4 Pa), and also one order of magnitude higher than that (15-77 Pa at 4.5 mg/ml) of the reported ultra-large GO (~36 µm) dispersion due to the large aspect ratios of ULGO.39,40 This remarkable gelation tendency at low concentrations also prevent us from obtaining higher concentration (e.g. 10 mg/ml) of ULGO dispersions by traditional centrifugation method. The rheological characterization of ULGO dispersions at different concentrations allows us to determine the critical gel concentration. As shown in Fig. S24 and Fig. S25, this value is found to be 1 mg/ml far below the reported values (> 4 mg/ml) due to the size-related excluded volume effect.41 Below 1 mg/ml, ULGO dispersions exhibited a liquid-like response (G′< G′′). Beyond this point, both G′ and G′′ increase monotonically with ULGO concentration, suggesting homogeneously dispersed structure in ULGO dispersion. Such a rheological feature makes ULGO dispersions especially suitable for the preparation of large-area graphene papers. As shown in Figure 5c, uniform ULGO paper is obtained via a blade casting method with a 5 mg/ml ULGO aqueous dispersion. In this case, the thickness of papers is controlled by tuning the gap of casting blade. SEM observations reveal that ULGO paper has close-packed feature that distinctly different from the Hummers-GO counterpart (Fig. 5d, e). Such stacking behavior should arise from the excluded volume interaction of ULGO sheets, that is, better flake alignment as a result of high aspect ratio (~1.59×105).23 The obtained ULGO papers are mechanically strong with a Young’s modulus of 11.9 GPa and a tensile strength of 110.8 MPa, corresponding to 197.3% and 267.1% of increases relative to those of the Hummers-GO papers, respectively (Fig. 5f). In addition, ULGO paper shows obvious hydrophobicity after reduction by HI, similar to that of the pristine graphene (Fig. 6a).42 We measure the electrical conductivity of reduced ULGO paper at 63 points in a 3×4 cm2 region. As shown in Figure 6b, the paper has a relatively uniform conductivity distribution (from 582 to 633 S/cm) and a mean value of 613 S/cm. In addition, we found that the electrical conductivity of reduced ULGO paper might be further enhanced through optimizing interlayer stacking. For example, a higher conductivity (1063 S/cm) was obtained when vacuum filtration was used to form the ULGO paper; the latter was also reduced by HI before electrical conductivity measurements. Due to lowered inter-sheet contact resistance, the electrical conductivity of our ULGO papers is far higher than the majority of reported results;9,18,20,27,43 e.g., HI-reduced Hummers-GO papers (409 S/cm),20 thermally reduced GO papers at 1000

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o

C (760 S/cm),18 and even small-sized pristine graphene films (400 S/cm).43 Although the present result is still lower than that of GO papers reduced by Joule heating,44 which can generate a local high temperature of 2750 K and reveal an extremely high electrical conductivity (3112 S/cm), it is more than 2 times higher than that of the reported HI-reduced GO papers.20,45 Apparently, this high conductivity would originate from the uniform, controlled oxidation, large lateral size of GO sheets, as well as closely stacking structure in ULGO papers. We believe that such a high-yield, high-efficiency preparation method of ULGO benefits the application of many fields, for example, 3D printing, anti-corrosion coatings, transparent optical films etc. Conclusions We demonstrate a rapid, high-yield (~100%) exfoliation method to produce ULGO nanosheets, in which chemically expanded graphite (CEG) with highly accessible (> 800 m2 g-1), worm-like structure is employed to facilitate the mass diffusion process of oxidants into graphene layers. With much enlarged interlayer galleries, the oxidation is accelerated and 2 wt equiv. of oxidant KMnO4 is sufficient to completely oxidize CEG in 4 hours, far less than those in the literature (3-10 wt equiv.). As-obtained CEG oxide can be filtered with a 100-mesh filter in several minutes, and exhaustively exfoliated into ULGO sheets via mild agitation. The ULGO nanosheets possess an area-average size of 127.7 µm, over one order of magnitude larger than those from modified Hummer’s method. ULGO aqueous dispersion shows unique rheological properties due to its ultrahigh aspect ratio (1.59×105), which can be conveniently transformed into ULGO paper by blade casting. ULGO paper reveals superior mechanical improvements (Young’s modulus of 11.9 GPa and tensile strength of 110.8 MPa) together with excellent electrical conductivity (~613 S/cm after HI reduction). The present method offers a new approach to produce ULGO in a low-cost, highly efficient and scalable manner, which can inspire us for other high performance GO-based materials and applications, such as graphene papers/fibers, nanofiltration membranes, optoelectronic devices and functional 3D networks.

Materials and Methods The synthetic procedure was performed based on our previous report.31 In a typical experiment, 1 g of natural flake graphite (+100 mesh, Aldrich) and 8.5 g of CrO3 was added into 7 mL of hydrochloric acid (37 wt%). The mixture was stirred at room temperature for 2 h, giving graphite intercalation compound (GIC) flakes. The as-prepared GIC was washed by deionized water repeatedly to remove unreacted CrO3 and then immersed in 40 mL of H2O2 (30%) at room temperature for 20 h. Subsequently, the obtained product was washed with deionized water three times to remove the residual H2O2 and chromium salt. Then wet CEG cake was obtained through filtration. Preparation of ULGO sheets Before oxidation, the interlayer water in CEG was removed by immersing the flesh CEG into concentrated sulfuric acid (98%, 200 mL) for 10 min and followed by filtration to remove the excess sulfuric acid. KMnO4 (2 g, 2 wt equiv.) was added in batches into 40 mL of concentrated H2SO4 (40 mL, 98 %) in ice-water bath during 30 mins. Then the ice-water bath was removed and the resulting CEG was added. The system was kept at 35 °C for 4 h without any mechanical agitation (e.g. stirring, shaking or ultrasonic). After reaction, the grey black mixture was filtered through a 100-mesh metal filter. Instantly, the filter cake was poured into ice water (100 ml) and 5 mL of H2O2 was added to decompose the insoluble manganese salts and then yellow graphite oxide (CEGO) particles suspended in solution were obtained. Different from the traditional acid-removing method by time-consuming high-speed centrifugation or dialysis, we here removed the residual acid and ions by a simple water-washing (200 ml) and mesh-filtration (100-mesh) method for 6-9 times, or by natural sedimentation method (standing for 20 min and then pouring out the supernatant colorless water) for 5-7 times. Note: water should be

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slowly added in order to avoid partial exfoliation of CEGO in this process. Through calculation, we found that the loss of CEGO during the acid-removing process was less than 5 wt%. Finally, the resulting CEGO particles were fully exfoliated in water by magnetic stirring at 800 rpm for 5 min, giving a yellow, stable ULGO dispersion. Methyl blue absorption technique In a typical MB absorption experiment, a CEGO particle suspension (solid content 26.5 mg) was added to 1 L of MB aqueous solution (0.0125 mg/ml), and UV-vis absorption spectra were recorded at different time intervals to monitor the adsorption process at the wavelength of 665 nm. Reduction of ULGO paper The as-prepared ULGO paper was immersed in HI (57%) and kept at 60 °C for 20 min. The reduced ULGO paper was then washed with ethanol and deionized water for 5 times. After vacuum-drying at 60 °C for 10 h, a black, completely reduced ULGO paper was obtained. Calculation of size distribution We assume that ULGO sheet is considered as a quadrangle with the side length of x (decided by SEM images in Fig. S15), corresponding to a sheet area of x2. In a unit-interval (0-5 µm, 5-10 µm, 10-15 µm… or 0-10 µm, 10-20 µm, 20-30 µm…), the number of ULGO sheets are counted as !) and the total area of these ULGO sheets are calculated, and written as %) . To express a number-average distribution, *) is

set as the ratio of !) to the total number of ULGO sheets in all intervals, defined by *) = ∑- + . Similarly, to express an area-average +./ +

(

distribution, we set 0) as the ratio of %) to the total area of sheets in all intervals, defined by 0) = ∑- + . +./ (+

Following the statistical method of number- and weight-average molecular weight in polymer science, we introduce number-average lateral '''' ''' size ($ ) and area-average lateral size ($( ) to characterize ULGO sheets, which were written as, '''' $ = ∑2 )3( '''( = ∑2 $ )3( where

)

is the middle value of unit-interval (e.g.

)

× *) ) =

)

)

× 0) ) =

∑+./(+ ×+ )

[1]

∑+./(+ ×(+ )

[2]

∑+./ +

∑+./ (+

is 2.5 µm, 7.5 µm or 12.5 µm for the unit-intervals 0-5 µm, 5-10 µm or 10-15 µm,

respectively) . Characterizations Transmission electron microscopy (TEM) was performed on a JEM-2100F electron microscope operating at 200 kV. TEM samples were deposited on holey copper grids and selected area electron diffraction (SAED) patterns were carried out as well. Atomic Force Microscopy (AFM) was conducted in the tapping mode on a Multimode 8 model scanning probe microscope, for which the GO dispersion was drop-casted onto freshly cleaved mica surfaces. Scanning electron microscopy (SEM) was carried out with an Ultra 55 mode electron microscope, for which the samples (~0.02 mg/ml) were drop-casted onto silicon wafers. Raman images was performed with an XploRA laser Raman spectrometer and a green (532 nm) excitation laser, for which the samples were drop-casted onto clean glass wafers. OM and POM images were taken with a Leica DM2500P model Optical microscopy. X-ray photoelectron spectra (XPS) was performed on an AXIS UltraDLD system, operating at 150 W with Al Kα radiation (1486.6 eV). Ultraviolet-visible spectra (UV-vis) was carried out with a Lambda 35 model UV-vis spectrophotometer. Electrical conductivity was measured with a SX1944 model four-probe instrument. Contact angles were tested on a JC2000DM model contact angle measuring instrument. Rheological properties were investigated using a HAAKE

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MARS III model rotary rheometer, operating at 25 °C using cone-plate or cylinder geometries. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Characterizations, comparison of preparation methods and properties of ULGO (DOCX) Video S1 related to the filtration of CEGO from acid solution via a 100-mesh filter (AVI) Video S2 related to the exhaustive exfoliation of CEGO with ~10 s shaking (AVI) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding The 973 project, the National Science Foundation of China, Shanghai key basic research project. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors are grateful for the financial support by the 973 project (2011CB605702), the National Science Foundation of China (51173027), and Shanghai key basic research project (14JC1400600).

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Reactivity-Controlled Preparation of Ultralarge Graphene Oxide by

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