Monolithic Crystalline Swelling of Graphite Oxide: A Bridge to

Mar 7, 2018 - The large-scale preparation of ultralarge graphene oxide (ULGO) is urgently needed for developing advanced devices and high-performance ...
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Article Cite This: Chem. Mater. 2018, 30, 1888−1897

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Monolithic Crystalline Swelling of Graphite Oxide: A Bridge to Ultralarge Graphene Oxide with High Scalability Jiajia Zhang,†,‡ Qiangqiang Liu,‡ Yingbo Ruan,†,‡ Shan Lin,† Ke Wang,†,‡ and Hongbin Lu*,†,‡ †

State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Collaborative Innovation Center of Polymers and Polymer Composites, Fudan University, 2005 Songhu Road, Shanghai 200438, China ‡ Shanghai Xiyin New Materials Corporation, 135 Guowei Road, Shanghai 200438, China S Supporting Information *

ABSTRACT: The large-scale preparation of ultralarge graphene oxide (ULGO) is urgently needed for developing advanced devices and high-performance nanocomposites. However, it is extremely difficult to produce ULGO in an industrially viable, high-efficiency manner because of the inevitable sheet fragmentation and significant gelation behavior occurred in existing methods. We propose a stationary oxidation-monolithic crystalline swelling strategy that can completely convert graphite to ULGO. This new stationary oxidation method minimizes the sheet fracture and prevents the exfoliation of oxidized layers without sacrificing the oxidation rate, resulting in oxidized flakes with high crystalline and lateral sizes the same as raw graphite. The oxidized graphite flakes undergo a monolithic crystalline swelling during the purification, leading to the formation of a three-dimensional ordered structure without peeling. This enables graphite oxide to be purified by spontaneous sedimentation within 1 h as gelation is avoided and to be exfoliated exhaustively into single-layered ULGO sheets through mild mechanical shaking, with an average size of 108 μm and the largest size of 256 μm. These ULGO sheets can form liquid crystals at a record dispersion concentration (as low as 0.2 mg/mL). The ULGO papers show outstanding mechanical properties and electrical conductivities (after HI reduction) that outperform the reported results.

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agitation generates the drag force through fluid flow and interparticle collision, which make ULGO sheets hard to survive in the resulting exfoliated products. In addition, the input of external energy during exfoliation, such as sonication or shear, further decreases the lateral sizes of GO. As a result, even though large flake graphite (e.g., flake size >200 μm) is employed, the resulting GO sheets always contain large amounts of small fragments, several to dozens of microns in the lateral size and the average size is typically below 50 μm.18,19 Also, it is very difficult to separate large GO from small fragments in practical GO production. Although a variety of fractionation methods, including density gradient centrifugation,20−22 filtration through track-etched membranes,23 liquid crystal (LC) selection24 and pH sedimentation,25 have been proposed, limited contents of ULGO sheets and energy/timeconsuming operations make them hard to apply in large-scale production. The other one is the severe gelation phenomenon occurring in the purification process.26,27 Graphite oxide undergoes layerto-layer gallery swelling with the deintercalation of H2SO4 from the interlayer space. Typically, exfoliation accompanies the

INTRODUCTION Graphene oxide (GO), with oxygen-containing functional groups on its basal plane and the edge, is a promising building block for constructing diverse macroscopic materials and an important precursor of graphene.1−4 Graphene and graphene oxides (GOs) with outstanding mechanical, electrical, and optical properties are promising for future electronics.5−9 As a two-dimensional (2D) macromolecule, the lateral size of GO plays a crucial role in its inherent properties, solution behavior and performance of resulting materials. Different from small GO, ultralarge GO (ULGO) has extremely high aspect ratios (sometimes up to 105), few intersheet junctions, strong alignment propensity, and enhanced intersheet interactions,10 which enable the resulting macrostructures and nanocomposites to exhibit superior performance.11−15 Nevertheless, it is quite difficult so far to produce ULGO sheets in an industrially viable, high-efficiency manner (Table S1). Two key factors restrict the scalability and production efficiency of ULGO. One of them is the size reduction or fragmentation of GO sheets occurred in the oxidation and exfoliation processes. During the transition from graphite to graphite oxide, fragmentation is sometimes hard to be avoided due to the buildup of elastic strain energy when oxygencontaining functional groups are introduced to the basal plane of graphite in the oxidation process.16,17 At this time, vigorous © 2018 American Chemical Society

Received: October 24, 2017 Revised: March 7, 2018 Published: March 7, 2018 1888

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Chemistry of Materials

Figure 1. Schematic illustrating the preparation process of ULGO.

Figure 2. (a) XRD patterns of PGO-S and PGO-C. (b) Evolution of shear viscosity with shear rate of graphite oxide-S and graphite oxide-C with a concentration of 10 mg mL−1. (c) Frequency sweep measurements for graphite oxide-S and graphite oxide-C with a concentration of 10 mg mL−1. (b−d) Conducted using 60 mm parallel plate geometry. (d) OM image of PGO-S. Scale bar = 200 μm. (e) PGO-S suspension before and after standing for 5 min. (f) Schematic illustration of PGO-S. (g) Schematic illustration of PGO-C.

achieving ULGO with high scalability and at low cost to achieve the practical applications of ULGO. Herein, inspired by the above considerations, we propose a stationary oxidation-monolithic crystalline swelling strategy to prepare ULGO sheets with a conversion rate of 100%. Different from the existing methods, pristine graphite oxide (abbreviated as PGO, referred to oxidized graphite without purification) with high crystalline and flake morphology similar to that of raw graphite was first synthesized through a stationary oxidation strategy. PGO undergoes a monolithic crystalline swelling during the purification process (no exfoliation occurs), resulting in the formation of a three-dimensional ordered structure, that is, crystalline monoliths. This method not only prevents the size reduction of GO sheets , but also largely enhances the purification efficiency as gelation is prevented; PGO can be exhaustively purified within 1 h through spontaneous

swelling because of the weakened attractions between the adjacent layers,28 which produces viscous slurries consisted of thin graphite oxide layers and exfoliated GO nanosheets. For example, the viscosity of the slurries is larger than 400 Pa.s (2.25 wt % in water, with the lateral size slighter larger than 1 μm) at small shear rates,26 making traditional purification methods, such as filtration, centrifugation and dialysis, quite tedious and difficult. For ULGO synthesis, this phenomenon becomes more prominent because ULGO can form gel at low concentrations because of its large excluded volume.29,30 To overcome this obstacle, excess acid and organic solvents were used to purify graphite oxide to minimize swelling.31,32 Nevertheless, additional washing with water is needed in order to remove residual impurities while the purity of obtained GO is low.26,33 Therefore, it is still demanding challenges in 1889

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Chemistry of Materials

Figure 3. (a) XRD patterns of the samples by stationary oxidation taken from the synthesis process at the reaction times = 0.5, 1, 2, 4, and 6 h. (b) XRD patterns of the samples by conventional oxidation taken from the synthesis process at the reaction times = 0.5, 1, 2, 4, 6, and 7 h. (c) The ratios of graphite and graphite oxide about the same samples as shown in a and b. (d) TGA curves of the samples by stationary oxidation taken from the synthesis process at the reaction times = 2, 4, and 6 h. (e) TGA curves of the samples by conventional oxidation taken from the synthesis process at the reaction times = 2, 4, and 6 h. (f) Comparison of weight losses in e and d.

agitation oxidation) shows no obvious diffraction peak. Rheological characterization of purified PGO-S and PGO-C samples unveils the difference between stationary and conventional agitation oxidations. To distinguish the samples before and after purification, we denote the purified PGO-S as GO-S and the purified PGO-C as GO-C. As shown in Figure 2b, for two dispersions of 10 mg/mL, GO-S displays a steady viscosity of 17.9 Pa.s at 0.01 s−1, which is 1 order of magnitude lower than that of GO-C (868.2 Pa s). The latter approaches the steady viscosity of polyoxymethylene melt (260−1000 Pa.s). The high viscosity of GO-C arises from the exfoliation of GO sheets, which is also reflected on the dynamic rheological curves. At the constant strain of 0.1%, GO-S dispersion (10 mg/mL) has a gradually increased storage (G′) and loss (G″) moduli with increased alternative frequencies and G′ is always larger than G″, showing a typical solid-like behavior (Figure 2c). For GO-C dispersion, G′ is also larger than G″ in the same frequency range but over 20-fold higher than that of GO-S dispersion (195.5 Pa vs 8.9 Pa at 1 Hz), implying the occurrence of a gelation phenomenon.34 This could cause significant difficulty in purification, sometimes needing a few days to obtain a well-purified GO-C sample.33 The stationary oxidation is a determinant step to achieve the rapid purification of PGO-S. Due to its tightly packed structure, PGO-S can precipitate out from the suspension within 5 min (Figure 2e, Figures S2 and S3), which is in sharp contrast with the conventional method (PGO-C remains suspended after standing for 24 h). Optical microscopy (OM) images show that PGO-S has the lateral size (250−500 μm) similar to that of raw graphite (Figure 2d and Figure S1) and appears uniform lightyellow-pearl from edge to center, besides a few dark wrinkles. This implies that the oxidation occurred in every graphene sheet of graphite under the stationary condition. Although intuitively mechanical stirring or external agitation can prompt diffusion and intercalation of the oxidant, our results show that such external agitation is unnecessary and even negative, especially for the preparation of ULGO sheets, given possible fragmentation of GO sheets.

sedimentation. The purified graphite oxide can be exfoliated to ULGO sheets by mild agitations, with an average lateral size of 108 μm and the largest size of 256 μm. Because of the huge aspect ratio (over 105), the ULGO dispersions can form LC at a concentration of 0.2 mg/mL and show a typical physical gelation behavior at low concentrations, which enables them to be assembled into long-range ordered macroscopic graphene materials by hydrothermal reduction. The ULGO papers obtained by vacuum filtration show outstanding mechanical properties and high conductivity after HI reduction. This work not only points to new opportunities for ULGO synthesis but also deepens the understanding of GO oxidation, which has use for reference to synthesize other 2D nanomaterials.

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RESULTS AND DISCUSSION Synthesis of ULGO. The preparation process of ULGO is illustrated in Figure 1. Graphite was first oxidized in the mixture of H2SO4 and KMnO4 at 35 °C without external mechanical agitations. After quenching in ice water followed by the addition of H2O2, PGO-S (PGO obtained by stationary oxidation method) with flake morphology similar to that of raw graphite was obtained (Figure S2), which rapidly settled down to the bottom of the container after standing for 5 min. Then, PGO-S were subsequently purified and swelled with deionized water by natural sedimentation. The repeated washing (4−5 times) enables the supernatant to approach neutral within 1 h and no obvious exfoliation occurs. The product was then exfoliated into single-layer ULGO sheets by a mild manner, either manual shaking or mechanical agitation. Stationary Oxidation of Graphite Oxide and Its Mechanism. Stationary oxidation is the critical step for rapid, high-yield preparation of ULGO sheets. PGO-S in wetting state exhibits a sharp (001) diffraction peak centered at 11.63° (Figure 2a), corresponding to the interlayer space of 0.76 nm, due to the introduction of oxygen-containing groups and the intercalation of the oxidant and H2SO4. Its full-width at half-maximum (fwhm) is only 0.13°, indicating that the crystallinity was well retained during the oxidation process. In contrast, PGO-C (referred to PGO by the conventional 1890

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Figure 4. (a−c) Photograph showing the changes in macroscopic volume of graphite oxide with different washing times. Scale bar = 300 μm. (d−f) The swollen graphite oxide observed via crossed polarizers. Scale bar = 300 μm. (g) XRD patterns of graphite oxide with different washing times. (h) Viscosity plotted against shearing rate of graphite oxide prepared by common method and stationary oxidation method. The tests were carried out using concentric cylinder geometry with sample volume of 1.9 mL.

progress of the whole oxidation process. The diffusion of oxidants includes diffusion in the reaction medium and diffusion within the interlayer gallery of graphite. Although mechanical agitations can accelerate the diffusion of oxidant in the reaction medium, they do not significantly affect the interlayer diffusion rate. More importantly, agitations induce fragmentation or partial exfoliation of oxidized flakes that largely increases the viscosity of the reaction medium, which may in turn reduces the diffusion rate of oxidant. In this sense, it is reasonable that stationary oxidation shows comparable or higher oxidation rate, relative to agitation oxidation. During the oxidation of graphite, the generation of cracks is usually inevitable due to the buildup of elastic strain energy on introducing oxygen-containing groups in graphene sheets.16 External agitation prompts the propagation of cracks and even probably induces new cracks. These would be the primary reason that external agitations result in less effective oxidation compared to the stationary oxidation. As explained above, powder graphite is expected to be oxidized faster due to the small size. To confirm this, we oxidized power graphite (Figure S4) with 3 wt equiv of KMnO4. As shown in Figure S5, graphite powder with particle size ranging from several micrometers to 26 μm turns into graphene oxide in 40 min without any black particles. XRD patterns of the samples (Figure S6) taken from the synthesis process demonstrate that the diffraction peak of graphite disappears in just 25 min. Monolithic Crystalline Swelling of Graphite Oxide. In the process of purification, the volume of graphite oxide increases dramatically. Interestingly, even though the supernatant became neutral, the swollen graphite oxide reveals no sign of breakage or exfoliation into separate GO sheets. OM was used to monitor the morphology evolution of graphite oxide during the purification. As shown in Figure 4a−c and Figures S7−S9, after first washing, graphite oxide still retains

To further elucidate the advantage of the stationary oxidation, we use XRD and thermogravimetric analysis (TGA) to explore the structural evolution of graphite oxide under two conditions. Figure 3a, b present the results obtained after different oxidation times. Whether in stationary (no stirring) or in conventional oxidation (mechanical stirring), 30 min are sufficient to introduce some oxygen functional groups on graphene sheets of graphite, resulting in the appearance of a (001) diffraction at 11.63° (0.76 nm) and a drastically weakened, broadened graphite diffraction peak (25.4°). With the progress of oxidation, the graphite diffraction peak nearly vanishes after 6 h for the stationary oxidation and 7 h for the conventional oxidation, only the diffraction peak at 9.5° can be observed. To quantify the oxidation rates under two conditions, we use the intensity ratio of (001) diffraction of graphite to graphite oxide to describe the reaction progress. As shown in Figure 3c, no significant difference appears prior to 4 h, but the stationary oxidation is slightly faster in the subsequent oxidation. This is indeed different from the expectation of accelerating oxidation by external agitations. TGA provides further evidence for this (Figure 3d, e). In the first 4 h, the products of two oxidation routes reveal nearly identical weight losses; however, after 6 h, the stationary oxidation resulted in a larger weight loss (41 wt %) relative to that of the conventional oxidation product (38 wt %), as shown in Figure 3f. This implies that within the same oxidation time, the stationary route could be more effective to complete the oxidation of graphite as opposed to the conventional agitation route. Essentially, the oxidation of graphite involves two primary steps, that is, the conversion of graphite to graphite intercalation compounds (GICs, stage I GICs for a KMnO4− H2SO4 system) and formation of graphite oxide.35,36 The latter step involves the diffusion of oxidant within the interlayer space and is much slower than the former, thus determining the 1891

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Figure 5. (a) SEM image of ULGO. (b) SEM image of the largest GO sheet. (c) Size distribution histograms of ULGO sheets from SEM images. (d) Area distribution histograms of ULGO sheets from SEM images. (e-f) OM image of ULGO. (g) AFM image of ULGO. (h) AFM image of GO after bath sonic for 30 min.

the flake morphology similar to PGO. After third washing, the axial length of graphite oxide (vertical to the graphene plane) increases to ∼300 μm, showing an extended lamellar structure. After fifth washing, the axial length further increases and some of them approach ∼600 μm. Under the crossed polarizer, the swollen structures show characteristic birefringence (Figure 4d−f and Figures S10 and S11), a typical long-range order character. Because no obvious exfoliation occurs, we call it monolithic crystalline swelling, similar to that observed in inorganic layered structures.37,38 This phenomenon has never been observed in graphite oxide. The extent of swelling depends on the degree of water washing, along with the removal of impurities such as H2SO4. As shown in Figure 4g, after first washing, the diffraction peak of PGO-S at 11.5° shifts down to 7.13°, an interlayer distance of 2.1 nm. Some impurity diffraction peaks appeared in PGO-S become weaker or vanish. After second washing, the signals of the impurities have become invisible, besides a diffraction peak around 4°, indicating an increased extent of swelling and the validity of water washing for purification. Further water washing results in larger extent of swelling, as seen in Figure 4b, c, e, f, the corresponding interlayer distance already exceeds the testing range of XRD. However, rheological characterization can afford supplementary information to the repeatedly washed samples. For GO-C and GO-S dispersions, as shown in Figure 4h, the former reveals much higher steady viscosity than that of the latter, because of the occurrence of exfoliation. Because of the high solid content (10 mg/mL), GO-S also exhibits some shear thinning at low shear rates. Different from the situation of GO-C, the viscosity of GO-S dispersion (obtained after 5-time washing) increases with increasing shearing rates when the shearing rate exceeds 850 s−1, which can be attributed to the exfoliation of GO sheets. This exfoliation condition is quite mild and beneficial to preparation of ultralarge GO. During the purification, both H2SO4 and impurities deintercalate from the interlayer space driven by osmotic pressure between the interlayer gallery and the bulk solution, resulting in deprotonation of oxygen functional groups. This in turn enlarges the electrostatic repulsion between ULGO sheets as GO sheets become negatively charged.39 Meanwhile, water molecules enter into the interlayer gallery driven by osmotic pressure and hydration with the functional groups.40 The intercalated water molecules can form hydrogen bonds with oxygen functional groups, which enables more water molecules

to accumulate within the interlayer space, enlarging the interlayer spacing. Eventually, the GO sheets are connected through the interlayer H-bonded water molecules, resulting in the occurrence of monolithic crystalline swelling. Because of the ultralarge dimension and high flexibility of ULGO sheets, such swollen graphite oxide monoliths are stable, except for exerting external agitations. Exfoliation and Characterization of ULGO. The swollen graphite oxide monoliths are able to be exfoliated into singlelayered GO sheets by mild manual or mechanical shaking, which avoids energy-intense operations such as sonication, shearing and ball milling. We employed three microscopic techniques, that is, scanning electron microscopy (SEM), optical microscopy (OM) and atom force microscopy (AFM), to analyze and characterize the exfoliated GO sheets. All the samples were prepared utilizing the as-exfoliated GO suspension and no fractionation operation was implemented, which enables us to observe the real result of the exfoliated GO sheets and their size distribution. In SEM images, some overlaps and wrinkles are observed (Figure 5a), due to the van der Waals intersheet interaction and good flexibility of ULGO. On the basis of the statistics of 100 GO sheets, the exfoliated sheets reveal an average size of 108 μm and 80% of them are larger than 80 μm (Figure 5c). The largest GO sheet has a lateral size up to 256 μm (Figure 5b). Figure 5d presents a lateral size distribution histogram, giving an area-averaged size of 117 μm. More SEM images are presented in Figure S12. This ultralarge character is in agreement with the OM observation, where ULGO sheets were deposited on SiO2/Si substrates, as shown in Figure 5e, f. Because of the presence of a large amount of oxygen-containing functional groups, the monolayer GO sheets exhibit larger thicknesses than that of graphene sheets (0.335 nm). Typically, they have a thickness range from 0.8 to 1.2 nm and most of them are ∼1 nm thick (Figure 5g), a single-layer character.11 In addition, these ULGO sheets can be tailored by changing exfoliation condition. When sonicating for 30 min, for example, the lateral size of the resulting GO sheets reduces to several microns, as shown in Figure 5h. We next characterized the chemical structure of ULGO and compare it with small GO (SGO, the synthesis method is supplied in the Supporting Information) using Raman, XRD, thermogravimetric analysis (TGA), ultraviolet−visible (UV− vis) and X-ray photoelectron spectra (XPS). In Raman spectra (Figure 6a), both ULGO and SGO exhibit characteristic D 1892

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Figure 6. (a) Raman spectra of ULGO and SGO. (b) XRD patterns of ULGO and SGO. (c) TGA curves of ULGO and SGO. (d) UV spectra of ULGO and SGO. (e) XPS spectra of ULGO and SGO. (f) C 1s XPS spectrum of ULGO.

Figure 7. (a, b) POM images of ULGO at 0.2 and 2.5 mg/mL. (c) Evolution of shear viscosity with shear rate with different ULGO concentration. (d) Frequency sweep measurements for ULGO. (e, f) SEM images of long-range ordered microstructures by hydrothermal treatment with different magnifications. (g) Strain−stress curves of ULGO paper. (h) FESEM image of ULGO paper.

band and G band with ID/IG ratios of 0.96 and 1.01, respectively. The higher ID/IG ratio for SGO, relative to that of ULGO, is attributed to the presence of more edge defects. XRD patterns (Figure 6b) of ULGO and SGO show only one single peak centered at 10.7°. This implies a smaller interlayer spacing, compared to wetting graphite oxide, which would arise from leaving of the interlayer water molecules during drying. TGA curve of ULGO shows similar weight loss compared with that of SGO (Figure 6c). UV spectra (Figure 6d) display a main peak around 232 nm and a shoulder peak around 305 nm, corresponding to π−π* and n-π* transitions. According to XPS spectrum, the C/O ratio of ULGO is estimated to be 2.1 (Figure 6e). The C 1s peak of ULGO (Figure 6f) can be divided into four peaks centered at 284.6, 286.6, 287.8, and 289.0 eV,41 corresponding to CC/C−C, C−O, CO, and OC−O groups, respectively. Compared with GO synthesized by the conventional method, our ULGO shows a larger

C−O content (49.7%), indicating the validity of our stationary oxidation strategy.42 As mentioned above, the oxidation process of graphite is a diffusion-controlled process. In our experiments, the main factors affecting the reproducibility or yields are the amount of KMnO4 used and the reaction time. To demonstrate the reproducibility of our method, we repeated the experiments under the same reaction conditions as mentioned in the experimental section. Figure S13a−c exhibits the typical SEM image of three batches of samples. No significant difference can be observed, demonstrating the good reproducibility of our method. In addition, the yield is estimated to be 164 ± 3 wt % based on the weight of the raw graphite (the details about yield calculation are supplied in the Supporting Information). Therefore, our method shows good reproducibility and high yields. 1893

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Chemistry of Materials Properties of ULGO Dispersions. Because of its remarkable anisotropic character, ULGO sheets have strong orientation propensity to arrange themselves in solution phase.43,44 Figure 7a, b present the microscopic images of ULGO dispersions under crossed polarizers (transmission mode). It is found that even at the concentration as low as 0.2 mg/mL, a birefringence phenomenon can still be observed, a typical character of LC formation. When the concentration increases to 2.5 mg/mL, stable birefringence appears in the whole dispersion that shows a vivid schlieren texture. This is the lowest critical concentration (0.2 mg/mL, corresponding to 0.02 wt %) for LC formation among all reported GO LC.15,45,46 Note that our ULGO dispersions were prepared directly using as-exfoliated GO sheets (no any fractionation was exerted). The ultralow LC critical concentration would arise from the exhaustive exfoliation of GO sheets and their ultralarge lateral sizes, which is also reflected on their rheological behavior. Figure 7a presents the steady-state rheological curves of different concentrations of ULGO dispersions. The obvious shear thinning behavior is associated with the orientation of ULGO sheets under external force fields. Even at 0.25 mg/mL, a non-Newtonian (shear thinning) behavior can still be clearly visible in the tested shear rate range, indicating the strong orientation propensity of ULGO sheets.47,48 The corresponding dynamic rheology was characterized in their linear viscoelastic regime, with a constant strain amplitude of 0.1%. Notably, even at 0.25 mg/mL, the dispersion still reveals a considerable elastic modulus (G′ ≈ 1.2 Pa at 50 Hz). At 2.5 mg/mL, G′ is larger than G″ in the whole frequency range and both become frequency dependent at higher frequencies, suggesting a broken network structure. When the concentration of the dispersion increases to 5 mg/mL, G′ reaches 130 Pa, whereas both G′ and G″ become independent of frequency, a typical strong-gel behavior.34,49 Similar to lyotropic LC colloids, the LC behavior of GO dispersions arises from an entropy driven excluded volume effect. For concentrated GO dispersions, the degree of freedom of GO sheets becomes restricted because of overlap of the excluded volumes, which results in entropic loss. Thermodynamically, GO sheets tend to align themselves to minimize the free energy of the system. The orientational alignment of GO sheets increases the total entropy of the system because the arranged sheets retain translational freedom and the excluded volume of the residual ones is recovered. Meanwhile, the loss of orientational entropy is compensated by the gain in excluded volume entropy.15 In the Onsagers theory,50 the empirical concentration for LC formation can be described as Φ ≈ 4T/ W, where Φ is the volume fraction, T and W represent the thickness and lateral size. This implies that ULGO dispersion can form LC at quite a low concentration. Based on this theory, our ULGO dispersion would have a critical volume fraction of 0.004 vol % for LC formation, that is, 0.0088 wt % if assuming the mass density of ULGO sheets to be 2.2 g/cm3.20 However, our experimental result shows a lower value than the theoretical prediction, which would be associated with the large tendency of wrinkling of ULGO sheets and their size polydispersity. Macroscopic Materials from ULGO and Their Properties. The specific rheological behavior of ULGO dispersions makes them suitable for fabrication of ordered macroscopic materials. For instance, ULGO can be assembled into 3D reduced GO (rGO) foam with long-range-ordered microstructure by hydrothermal reduction. In our study, the concentration of ULGO dispersions was chosen to be 2.5

mg/mL and no pretreatment (e.g., fractionation) was conducted. As shown in Figure 7e, after hydrothermal reduction and freeze-drying, the ULGO dispersion turns into a foam that exhibits an orderly organized porous morphology, in sharp contrast to the disordered structure obtained under the same conditions.51 Owing to the high flexibility of ULGO sheets, some wrinkles can be discerned on the rGO walls in which the rGO sheets bridged with each other prevent the volume shrinkage during the hydrothermal process. The density of the resulting foam is found to be 12.8 mg/mL, which is smaller than early work.52 ULGO papers were also fabricated by vacuum filtration to show the superiority of the ultralarge sheets. ULGO papers reveal a closely packed structure (Figure 7h), with a Young’s modulus of 11.2 GPa and a tensile strength of 182 MPa (Figure 7g), which are among the highest value in reported results10,23,53−55 and even comparable with that of GO papers cross-linked/doped with polymers, ions.56−59 Under external force, GO papers undergo two forms of deformation: deformation of GO sheets themselves and intersheet deformation. The former needs to stretch CC/ C−C covalent bonds whereas the latter stretches the weak intersheet hydrogen bonds. Thus, the intersheet deformation is the main form that determines the mechanical property of GO paper. Generally, two factors can influence the intersheet deformation, that is, the alignment of GO sheets and the interaction energy between GO sheets. ULGO sheets have larger aspect ratios, leading to enhanced excluded volume effect in solutions. The large excluded volume effect is beneficial for the alignment of ULGO sheets, which is reflected by the low critical concentration for the formation of liquid crystals (0.2 mg/mL, shown in Figure 7a). In addition, ULGO sheets have less carboxyl groups (Figure 6f), which is also beneficial for their alignment. Therefore, GO papers made from ULGO sheets show more compact and aligned structures (Figure 7h). Due to the ultralarge feature of our GO sheets, more hydrogen bonds are formed between the adjacent sheets, which improves the interaction energy. Thus, the intersheet deformation in ULGO papers is significantly hindered, revealing improved mechanical properties. After chemical reduction by HI, the reduced ULGO papers exhibit a conductivity up to 936 S/cm, which is much higher than those of the reported results evaluated under the similar conditions23,60−62 and even higher than other samples, for example, thermally reduced GO paper at 1000 °C,22 exfoliated graphene films.63 One of the main reasons is that there exist less intersheet junctions in reduced ULGO papers, and thus smaller intersheet contact resistance. Also, the compact and aligned structure contributes to improve the conductivity. In addition, the reduced ULGO papers exhibit less edge defect sites, which are hard to be removed. For comparison, we measured the conductivity of small-sized rGO paper. Smallsized GO was obtained by ultrasonic treatment of ULGO for 30 min and the preparation method for rGO paper was the same as that for reduced ULGO paper. The corresponding conductivity of small-sized rGO paper was 407 S/cm. ULGO papers show more compact, aligned structures and improved intersheet interaction energy. Since the intersheet deformation in ULGO papers is significantly hindered, the as-obtained rGO papers exhibit good bending property. Even bended for 100 times, the conductivity of the rGO paper still retains at 925 S/ cm. We believe that such ULGO sheets with outstanding 1894

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

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Chemistry of Materials mechanical, electrical, and optical properties are promising for future electronics.

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CONCLUSIONS We propose a stationary oxidation-monolithic crystalline swelling strategy that can completely convert graphite to ULGO sheets. The PGO generated by the stationary method undergoes monolithic crystalline swelling during water-washing purification, resulting in an ordered three-dimensional crystalline structure. Such swollen structures can be exfoliated into single-layer ULGO sheets by mild agitations, which have an average lateral size of 108 μm and the largest size of 256 μm. This stationary strategy not only minimizes the size reduction of GO sheets but also inhibits the occurrence of gelation phenomenon, so that the resulting graphite oxide can be purified in a simple, fast manner. Meanwhile, the oxidation rate retains well. The exfoliated ULGO sheets can form LC at a record concentration as low as 0.2 mg/mL, which makes them particularly suitable to construct long-range ordered macrostructures. In addition, the ULGO papers exhibit excellent mechanical properties (tensile strength of 182 MPa and Young’s modulus of 11.2 GPa) and high conductivity (936 S/cm) after HI reduction. This strategy addresses the critical issue regarding the preparation of ULGO sheets, which is expected to significantly prompt the development of GO and graphene-based functional materials. What’s more, the swollen crystalline structures open new opportunities for exploring a variety of wet-chemical reactions confined in nanoscale spaces, and engineering rationally 2D nanofluidic channels.

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MATERIALS AND METHODS

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ASSOCIATED CONTENT

Additional preparation methods, characterization methods, figures and tables as described in the text (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hongbin Lu: 0000-0001-7325-3795 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by 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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REFERENCES

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Stationary Oxidation of Flake Graphite. In an ice bath, 55.9 g of KMnO4 was added into 520 mL of H2SO4 slowly to control the temperature below 10 °C and stirred for more 2 h to dissolve KMnO4 completely. The solution was transferred to a 35 °C water bath and heated for 15 min followed by the addition of 13 g of flake graphite. After stirred for 30 min, the mixture was heated in the 35 °C water bath for 16 h with no agitations. Subsequently, the mixture was added into ice water followed by the addition of H2O2 until no bubbles were observed to obtain pristine graphite oxide. Purification and Exfoliation of Graphite Oxide. Pristine graphite oxide was purification by a natural sediment method. In detail, after standing for 5 min, the supernatant of the above mixture became clear and colorless, indicating complete PGO-S separation. The supernatant was poured out and distilled water was added into the precipitate. After 5 min of standing, the supernatant became colorless again. Adding distilled water into the precipitate, standing for graphite oxide separation and pouring out of the supernatant were regarded as 1 washing cycle. After 4−5 times’ washing, the supernatant became neutral. The whole purification process took less than 1 h. During purification process, water should be added slowly to avoid exfoliation by external forces. For exfoliation, the purification graphite oxide was exfoliated by hand shaking to obtain ULGO dispersion. Characterization. The morphology of ULGO sheets were characterized on Ultra 55 FESEM and multimode 8 AFM. The swelling structures were recorded on recorded on DM2500P. The crystal structures were recorded using X’pert PRO. TGA analysis was performed on Pyris 1 TGA. Rheological behaviors were investigated using a HAAKE MARS III under stress control mode.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04458. 1895

DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897

Article

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DOI: 10.1021/acs.chemmater.7b04458 Chem. Mater. 2018, 30, 1888−1897