High-throughput, direct exfoliation of graphite to graphene via a cooperation of supercritical CO₂ and pyrene-polymers

Abstract

A facile and green approach has been developed using supercritical CO₂ as penetrant, expanding agent and antisolvent, and pyrene-polymers as a molecular wedge and modifier, leading to high-throughput graphene dispersions. Herein, two specially designed pyrene-polymers with a large planar aromatic group and two substituted polymer dangling chains have been employed to stabilize the graphene sheets, which show excellent solubility in aqueous and organic solvents. The morphology and quality of the exfoliated graphene sheets are studied by transmission electron microscope (TEM), atomic force microscopy (AFM), Raman, FTIR, and wide-scanning X-ray photoelectron spectroscopy (XPS), which reveal that large scale and high quality graphene flakes are obtained by the facile process. The supercritical CO₂, pyrene-polymers and the solvent system have a significant influence on the exfoliation results. The pyrene-polymers attached to the graphene can further integrate graphene with other polymers to form new functional nanocomposites. This solution-based method combines high-throughput production and functionalization of graphene in one step, providing nanoscale building blocks for practical applications.

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Introduction

Graphene has attracted enormous interest recently due to its fascinating electronic properties such as unusual charge-carrier mobility and quantum Hall effect,¹ as well as excellent mechanical, thermal and optical properties.² Owing to its distinct properties, graphene sheets are attractive as atomically thin yet robust carbon components for applications in optoelectronics, sensors, nanocomposites and energy-storage materials.³ To fabricate useful graphene-based materials, graphene sheets must be available in high-quantity. To date, several approaches have been explored to obtain graphene by means of “micromechanical cleavage”,⁴ bottom-up organic synthesis,⁵ chemical vapor deposition (CVD) or thermal decomposition,⁶ chemical reduction of graphene oxide (GO),⁷ and intercalation and solution-phase exfoliation methods.⁸

Solution-phase exfoliation methods are widely considered to be a promising route for large-scale graphene production due to it is versatility and ability to deposit graphene sheets onto arbitrary substrates. However, graphene's intrinsic insolubility and easy to introduce irreversible defects are still major obstacles in developing its wide applications. Non-covalent functionalization of graphene is capable of overcoming both of these limitations. For example, amphiphilic surfactants and conjugated structure-containing molecules have been used to produce stable graphene dispersions via hydrophobic or π–π non-covalent interactions with graphene sheets.^8a,8f,9 Adsorption of pyrene derivatives on graphene through robust π–π stacking has been well-established by the work of various researchers, which afford highly uniform and stable dispersions of graphene without any damages to their graphite surface.¹⁰ Compared with the simple pyrene-derivatives, the advantage of pyrene-polymers as a modifier for functionalization of graphene is (i) the possibility to endow the graphene with special performances such as solubility,¹¹ stimuli-sensitivity,¹² biosensing,¹³etc, and (ii) their good compatibility with different polymer hosts, which make graphene as versatile nanofillers in nanocomposites.¹⁴

So far, successful non-covalent functionalization of graphene with pyrene-derivatives has only been reported on pre-exfoliated graphene sheets through interaction with pyrene-derivative stabilizers.^10–14 Exploiting available techniques for easy processing and functionalization of graphene in one step have so far remained a challenge and reported solution-phase exfoliation methods are time-consuming and suffer from inefficient solvent recovery. Therefore, the development of a facile and green route to obtain high-yield graphene dispersion is warranted. Supercritical fluids, which exhibit both “gas-like” and “liquid-like” properties above their critical temperatures and pressures, have been utilized to intercalate and delaminate tightly-stacked layered materials such as silicates and graphite.¹⁵ Because of their tunability, along with low interfacial tension, excellent wetting of surfaces, and high diffusion coefficients, they are expected to accelerate the penetration of the precursors into the nanoscale pores relative to liquid solvents, which in turn reduces reaction times from days to minutes.¹⁶ Supercritical CO₂ (SC CO₂) is the most widely studied supercritical fluid because it is nonflammable, essentially nontoxic, inexpensive, and environmentally benign with an easily accessible critical point (T_C = 31.1 °C and P_C = 7.38 MPa).¹⁷ Using SC CO₂ to help pyrene-polymer act as a molecular wedge to impregnate and exfoliate the graphite is worth investigating.

Herein, SC CO₂ as an effective medium for preparation of pyrene-polymers functionalized graphene sheets with good solubility in water and organic solvent. Two specially designed pyrene-polymers, pyrene-polyethylene glycol (pyrene-PEG) and pyrene-polycaprolactone (pyrene-PCL), are used to exfoliate and stabilize graphene. With the assistance of SC CO₂, pyrene-polymers can not only act as molecular wedges to cleave graphite to obtain graphene, but also as a modifier to functionalize exfoliated graphene to form stable dispersion in water and organic solvent, depending on the dangling polymer chains. In addition, pyrene-polymers attached on graphene endow graphene with good compatibility with other polymers, which is helpful for further applications in nanocomposites and nano-electronic devices. We will start with a description of the preparation process and solubility of pyrene-polymers functionalized graphene sheets with the assistance of SC CO₂, followed by detailed investigation of their morphology, quality and yield. Finally, the high-concentration graphene solutions are integrated with host polymer and the thermal stability of the functional nanocomposites will be studied.

Experimental

Materials

The graphite powder used in all experiments was purchased from Acros Organics (Catalog 385031000). 1-Pyrenebutyric acid (PBA) (≥97%), 1-pyrenemethanol (98%), and poly(ethylene glycol) methyl ether (mPEG, M_n = 2,000 and 5,000) were purchased from Sigma-Aldrich. ε-Caprolactone (ε-CL) was purchased from Acros Organics. 1-Pyrenecarboxyric acid (PCA) was purchased from Tokyo Chemical Industry Co., LTD. Poly(ε-caprolactone) (PCL) (M_n = 53 557 and M_w = 76 598) was supplied by CSIRO CMIT in Australia. Polystyrene (PS) (PG-383 M) was purchased from Zhenjiang Chimei Chemical Co., LTD. Polypropylene (PP) filter membrane (Millipore-Omnipore membrane, 50 mm, 10 μm) and polyvinylidene fluoride (PVDF) membrane (Millipore-Omnipore membrane, 50 mm, 0.1 μm) were purchased from Haining Yatai pharmaceutical Co., LTD. All other reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without further purification. Aqueous solution was prepared with double-distilled water from a Millipore system.

Characterization

Attenuated total reflectance FTIR spectra were taken on a BRUKER TENSOR27 instrument with 32 scans at a resolution of 2 cm⁻¹ intervals. ¹H NMR spectra were obtained using a Bruker DPX-400 NMR spectrometer with CDCl₃ as solvent at 25 °C. The chemical shifts were relative to tetramethylsilane (TMS) at δ = 0 ppm for protons. Transmission electron microscope (TEM) images and associated electron diffraction (ED) patterns were taken with a JEM-2100 at an accelerating voltage of 200 kV. The TEM samples were prepared by drying a droplet of the graphene suspension (stage 3) on a lacy carbon grid. Raman spectra were carried out at 514.5 nm laser excitation on a Renishaw Microscope System RM 2000 at room temperature. The laser power density was kept less than 3 mW with a resolution of 1.5–2.0 cm⁻¹ over the spectral window. Spectra were collected at various locations on each sample studied to determine reproducibility. X-ray photoelectron spectroscopy was performed using a Thermo ESCALAB 280 system with Al/K (photon energy = 1,486.6 eV) anode mono X-ray source. A Digital Instruments MultiMode scanning probe microscope with a Nanoscope IIIA controller in tapping mode was used for the AFM measurements. Ultraviolet-visible (UV-vis) absorption spectral measurements were carried out with a Shimadzu UV-240/PC spectrophotometer. Differential Scanning Calorimetry (DSC) was carried out using TA-Q100 at a heating and cooling rate of 10 °C min⁻¹ under a nitrogen atmosphere. Samples with a typical mass of 5 mg were encapsulated in sealed aluminum pans.

Synthesis of pyrene-polymer

Poly (ethylene glycol) methyl ether (mPEG, M_n = 2,000 and 5,000) was dried in vacuum for 24 h before use. PBA (≥97%) and 1-pyrenemethanol (98%) were used without further purification. ε-Caprolactone (ε-CL) was purified with CaH₂ by vacuum distillation. Pyrene-terminated polyethylene glycol (pyrene-PEG_2K, pyrene-PEG_5K, 2 K and 5 K are the molecular weight of PEG, respectively) was formed by conjugating the hydroxyl group of mPEG with the carboxyl group of PBA according to the literature.¹⁸ Pyrene-terminated poly(ε-caprolactone) (pyrene-PCL₁₉ and pyrene-PCL₄₈, 19 and 48 are the number of repeat units of caprolactone (CL), respectively) was synthesized by ring-opening polymerization of ε-CL with 1-pyrenemethanol as initiator according to the method in the literature.¹⁹ The chemical structures of the synthesized pyrene-PEG and pyrene-PCL were confirmed by FT-IR and ¹H NMR spectra using CDCl₃ as a solvent. And the changes in molecular weight of pyrene-PEG and pyrene-PCL were evaluated by ¹H NMR (M_n, NMR) (Fig. S1 and S2 in the ESI†).

Graphene preparation

Fig. S3 in the ESI† schematically describes the process of direct exfoliation of graphite based on the cooperation of pyrene-polymers and SC CO₂. The graphene solution was prepared by sonicating a mixture of 20 mg graphite powder and 0.013 mmol pyrene-polymers in 10 ml dimethylsulfoxide (DMSO) for 3 h using a low-power sonic bath (Shanghai Shengxi DS-2510DTH bath sonicator, 40 kHz, 120 W), and a grey liquid with a large amount of macroscopic sedimentation was obtained. The dispersion was then quickly transferred into the SC CO₂ apparatus (a stainless steel autoclave (50 ml) with a heater and a temperature controller). The autoclave was heated to 40 °C and CO₂ was then injected into the autoclave until the pressure reached 16 MPa. The mixture was maintained in an environment of SC CO₂ and stirred with a Teflon-coated magnetic stir bar for 6 h. After the SC CO₂ treatment, the resulting dispersion was sonicated for an additional 2 h. After that, a dark grey homogeneous solution with some macroscopic sedimentation was obtained. The solution was allowed to settle overnight and then was centrifuged at 9000 rpm for 20 min to remove the clear supernatant liquid. The precipitates were re-dispersed in 20 ml fresh water (used for pyrene-PEG/graphene systems) or DMSO (used for pyrene-PCL/graphene systems) by sonicating for 30 min, form a dark gray suspension with some macroscopic sedimentation, as the “original solution” (stage 1, Fig. S3 in the ESI†).

To remove the excess (unmodified on graphene) pyrene-polymers, the original solution was centrifuged at 9000 rpm for 20 min, the clear supernatant was poured away, the precipitate was refilled with 20 ml water or DMSO, and the mixture was sonicated for 2 min. This process was repeated three times to obtain pyrene-polymers/graphene solutions with a minimal quantity of uncombined pyrene-polymers (stage 2, Fig. S3 in the ESI†). The final wash was made to pass through a PP filter membrane (Millipore-Omnipore membrane, 50 mm, 10 μm), to remove any un-exfoliated graphite microparticles. The resultant dark grey homogeneous solution consists mostly of single-, bi- or tri-, and multilayered graphene sheets (stage 3, Fig. S3 in the ESI†). In order to obtain the yield of the individual graphene sheets, we vacuum filtered the graphene dispersion (stage 3) through PVDF membrane (Millipore-Omnipore membrane, 50 mm, 0.1 μm). Carefully measuring the weight of the PDVF membrane before and after filtration of graphene sheets on it, accounting for the original graphite mass (20 mg) or residual solvent (∼20 ml), the yield and the concentration of dispersed graphene sheets were obtained (stage 3). The PDVF membranes before and after filtration were dried in a vacuum oven at 60 °C for 3 days until the membrane mass no longer changed.

Graphene-based polymer composites

PCL, PS and corresponding graphene nanocomposites were prepared and investigated. The DMSO solution of pyrene-PCL₁₉ non-covalently functionalized graphene was initially prepared as described above. The obtained solution was filtrated through PP filter membrane directly and without centrifugation. The polymer was heated in DMSO (10–20 ml) by oil bath (PCL solution was heated to 65 °C, and the PS solution was heated to 180 °C). Depending on the wt% of the nanocomposites, a certain amount of graphene solution was added dropwise into the polymer solution and then the mixed solution was magnetically stirred vigorously for 2 h at the heated temperature. Upon completion, the coagulation of the polymer composites was accomplished by adding the DMSO solution dropwise into a large volume of vigorously stirred methanol (10 : 1 with respect to the volume of DMSO used). The coagulated composite powder was isolated via centrifugation; washed with methanol (200 ml); dried at 45 °C under vacuum for 2 days to remove residual solvent; crushed into a fine powder with a mortar and pestle, and then pressed in a hot press at a temperature of 100 °C for PCL and 240 °C for PS. The obtained films were 0.1–0.2 mm thick.

Results and discussion

Preparation process and solubility

Pyrene-PEG and pyrene-PCL with two different molecular weights, pyrene-PEG_2K, pyrene-PEG_5K, pyrene-PCL₁₉ and pyrene-PCL₄₈ were used as molecular wedges and modifiers to obtain stable graphene dispersion with the assistance of SC CO₂. Fig. 1(a) shows a schematic illustration of the overall process to obtain pyrene-polymer functionalized graphene sheets based on SC CO₂'s assistance. In step 1, graphite powder and pyrene-polymers were mixed in dimethylsulfoxide (DMSO). Pyrene-polymers are able to interact with the exposed graphite surface by a strong π–π stacking interaction while their dangling chains towards the solvent. However, the limited dangling polymer chains are not sufficient to dislodge the graphene sheets due to strong binding of graphite by a robust interlayer interaction. In step 2, when the solution was transferred into SC CO₂ system, intercalation and exfoliation of graphite occurs. First, the gas-like transport properties of SC CO₂, including low viscosity, high diffusivity, and near zero surface tension, are well suited for rapidly penetrating within the interlayers of graphite and delivering pyrene-polymers to the areas with high aspect ratios and poorly wettable surfaces.¹⁶ Second, pyrene-polymers that are stable in DMSO now are exposed to a SC CO₂/DMSO binary medium. The compatibility of SC CO₂ with DMSO is good,²⁰ whereas pyrene-polymers exhibit poor native solubility in CO₂ which causes the pyrene-polymers to become increasingly less soluble. This phenomenon is defined as the antisolvent effect of SC CO₂.²¹ Then pyrene-polymers attempt to minimize their interaction with CO₂, find their way to the graphite interlayers that act as molecular wedges, and form a large number of π–π stacking interactions with the conjugated π-network of graphene sheets. The out-of-plane dangling chains of the inserted pyrene-polymers increase the distance between adjacent graphene layers and prevent them from re-aggregating. Eventually, a large number of individual graphene sheets were prepared (step 3).

Fig. 1 (a) Schematic illustration of the preparation process of pyrene-polymers functionalized grapheme sheets based on SC CO₂'s assistance (from step 1 to step 3). (b) Photographs of pyrene-PEG_2K functionalized graphene aqueous dispersion (A), pyrene-PEG_5K functionalized graphene aqueous dispersion (B), pyrene-PCL₁₉ functionalized graphene dimethylsulfoxide (DMSO) dispersion (C), pyrene-PCL₄₈ functionalized graphene DMSO dispersion (D).

Within the two pyrene-polymers, the pyrene group can form robust π–π interactions with graphene sheets, while the dangling hydrophilic PEG chains can facilitate graphene to disperse in aqueous solution and the hydrophobic PCL chains lead to good solubility of graphene in various organic solvents, thus endowing graphene with great promise for applications in biotechnology and nanocomposites.²² The stable dispersions of pyrene-polymers non-covalently functionalized graphene were successfully prepared (stage 3, see Fig. S3 in the ESI†). Digital photographs of such graphene solutions dispersed in two different solvents are shown in Fig. 1(b). The hydrophilicity of PEG and hydrophobicity of PCL allow graphene sheets to form homogeneous dispersions in water and DMSO, respectively. DMSO is found to be a better solvent for graphene than water in our method, which agree with the results reported by Hernandez et al.^8e and Shih et al.^8h

Morphology and the exfoliation effect

The morphology of the exfoliated graphene sheets was first studied by transmission electron microscope (TEM). Fig. 2 shows the TEM images and corresponding electron diffraction (ED) patterns of typical graphene sheets (stage 3). It should be noted that after intense centrifugation, almost all of the excess pyrene-polymers were removed and only a small amount of functionalized pyrene-polymers on the suspended graphene sheets remained, and the relatively naked monolayer graphene sheets are presented in Fig. 2(a–d). The presence of monolayer graphene flakes could be confirmed by analysis of the ED patterns. The typical six-fold symmetry pattern and the intensity difference between {1100} and {2110} (I_{1100} > I_{2110}) are the unique characteristics of monolayer graphene.^8e,23 The corresponding ED patterns (Fig. 2(e–h)) show the {1100} spots clearly, and the {2110} spots are less intense, confirming the presence of monolayer graphene. In addition, TEM analysis reveals a large quantity of graphene sheets of different types, which generally fall into three classes. The first class, as shown in Fig. 2, contains monolayer graphene. Second, large number of sheets has few-layer graphene sheets, including some bilayers and trilayers (Fig. 3(a–d)). Third, a number of rather disordered sheets formed by re-stacking of few-layer graphene are found (Fig. 3(e–h)). We also found some graphene sheets showed diffraction patterns with two six-fold patterns rotated slightly from each other (the inset in Fig. 3(h)), indicating that the sheets were formed by the random stacking of two sheets with different layers.²⁴ Atomic force microscopy (AFM) and Raman spectroscopy are also powerful nondestructive tools for probing the number of layers of graphene,²⁵ from which we find that the obtained graphene solutions consist mostly of single-, bi- or tri-, and multilayered (<5 layers) graphene sheets (see Fig. S4 and S5 in the ESI†).

Fig. 2 TEM images of (a) pyrene-PEG_2K functionalized graphene sheets, (b) pyrene-PEG_5K functionalized graphene sheets, (c) pyrene-PCL₁₉ functionalized graphene sheets, and (d) pyrene-PCL₄₈ functionalized graphene sheets. (e–h) Electron diffraction (ED) patterns of the exfoliated graphene sheets corresponding to (a–d), respectively.

Fig. 3 TEM images of (a–d) bilayer and trilayler graphene sheets and (e–h) disordered multilayer graphene sheets prepared by non-covalent functionalization of pyrene-PEG_2K, pyrene-PEG_5K, pyrene-PCL₁₉ and pyrene-PCL₄₈ with assistance of SC CO₂, respectively. The inset in Fig. 3(h) is the corresponding ED pattern.

The success of this method was further confirmed by showing that without pyrene-polymer or SC CO₂ treatment, the exfoliated effect was extremely poor. Control experiments were performed by producing graphene aqueous dispersions while intentionally avoiding one of the constituents or steps (Fig. 4). Fig. 4(a) visually compares the concentration of regular graphene sheets dispersion in water against control experiments at stage 3. The concentrations of graphene sheet dispersions obtained without pyrene-polymer or SC CO₂ (Fig. 4a(B, C)) are much lower than that in Fig. 4a(A). Almost no dispersed graphene sheets were obtained if both pyrene-polymer and SC CO₂ (Fig. 4a(D)) were removed. The resultant solutions of the control experiments were analyzed by TEM. Fig. 4(b–e) show typical examples of TEM images from Fig. 4a(B, C), respectively. As seen in Fig. 4(b,c), rather disordered thicker graphene sheets are formed compared with the sheets shown in Fig. 3(a,e). And the vast majority of the flakes observed in Fig. 4(b,c) are rather disordered and quite a lot of small thin flakes aggregate on the bigger flakes, indicating significant re-aggregation of multilayer graphene sheets occurs in the dispersion phase. This indicates that only SC CO₂ is insufficient to obtain completely exfoliated graphene sheets due to the lack of pyrene-polymer as molecular wedge and stabilizer. Likewise, in Fig. 4(d,e), more thicker graphene layers are obtained than the graphene sheets shown in Fig. 3(a,e) and Fig. 4(b,c), which indicates that the exfoliation is quite inefficient due to the lack of SC CO₂ as penetrant, expanding agent and antisolvent. Hence the pyrene-polymer can be hardly carried into the graphite layers owing to the robust van der Waals energy stored in π–π stacked layers. The two groups of control experiments clearly establish that both pyrene-polymer and SC CO₂ play essential roles in obtaining stable solution of graphene sheets.

Fig. 4 (a) Digital photographs of graphene dispersion and control experiments. (A) Regular graphene sheets dispersion in water obtained by non-covalent functionalization of pyrene-PEG_2K with the assistance of SC CO₂. (B) Dispersion in water obtained without pyrene-PEG_2K. (C) Dispersion in water obtained without SC CO₂. (D) Dispersion in water obtained without pyrene-PEG_2K and SC CO₂. (b,c) TEM images of resultant dispersion in water obtained without pyrene-PEG_2K. (d,e) TEM images of resultant dispersion in water obtained without SC CO₂.

Quality and yield of the exfoliated graphene

The presence of defects is fatal to the properties of graphene, so it is critical to determine whether this exfoliation method results in defects. Raman, infrared, and X-ray photoelectron spectroscopy (XPS) were carried out to characterize the quality of the exfoliated graphene sheets. Fig. 5(a) shows Raman spectra of four deposited films with a spectrum of the starting graphite powder. These measured films were prepared by vacuum filtration onto PTFE filters. The G-band (∼1580 cm⁻¹) and 2D-band (∼2700 cm⁻¹) are clearly visible in all cases. The intensity of D-band of the four deposited films is almost unchanged, which demonstrate that our process does not introduce significant defects. In addition, Raman spectra of individual graphene sheets cast on holey TEM carbon grids were performed, which not only allow the identification of single-, bi- or tri-, and multilayer graphene sheets from the shape of the 2D band but also examination of the quality of exfoliated graphene sheets. In the case of the spectra associated with the individual graphene sheets, a D-band is observed (see Fig. S6 in the ESI†). However, the intensity of these D-bands are low, and the ratio of the intensities of D-band and G-band peaks (R = I_D/I_G) are presented (see Table S1 in the ESI†). The low values can be compared to that of highly purified single wall carbon nanotubes and is mainly due to edge effects.²⁵

Fig. 5 (a) Raman spectra of graphite powder and four vacuum filtered graphene films, which were prepared by non-covalent functionalization of pyrene-PEG_2K (A), pyrene-PEG_5K (B), pyrene-PCL₁₉ (C) and pyrene-PCL₄₈ (D), respectively. (b) ATR-FTIR spectra of pure PTFE membrane and four vacuum filtered graphene films (A–D), which are the same as Fig. 5a(A–D). (c) Carbon 1s core-level XPS spectra for four vacuum filtered graphene films (A–D). The Shirley background has been removed. Fit lines represent graphitic carbon (C–C ∼284.6 eV), and C–O (∼286.3 eV), respectively.

Furthermore, attenuated total reflectance (ATR) FTIR spectra of these four deposited films and the PTFE membrane were also measured to show the absence of oxidization typically associated with graphene oxide.^7b All these spectra show features at ∼1200 and ∼1140 cm⁻¹, which are the characteristic adsorption peaks assigned to the C–F stretching vibration (Fig. 5(b)). A key feature of the spectra for the vacuum filtered graphene films is the absence of peaks attributed to C–OH (∼1340 cm⁻¹) and –COOH (∼1700 cm⁻¹) groups. Again it further proves the production of a large amount of defect-free graphene sheets. XPS is also performed on the four vacuum filtered graphene films. Four carbon 1s core-level XPS spectra of the four vacuum filtered graphene films are shown in Fig. 5(c). These spectra are all dominated by a feature around 284.6 eV, which associates with graphitic carbon. And the main C–C peak makes up from 81% to 93% of the spectrum. An additional relatively weak C–O peak is found at 286.3 eV. The residual pyrene-polymer functionalized on graphene sheets can contribute to the traces of C–O. XPS survey scans of the four vacuum filtered graphene films and elemental analysis can also confirm the low levels of oxidation of the exfoliated graphene sheets (see Fig. S7 and Table S2 in the ESI†). From the Raman, FTIR and XPS results, it can be found that there are no significant defects for the exfoliated graphene sheets.

Many promising applications of graphene require the development of effective routes to produce high-yield of graphene sheets. Solvent sonication is a very useful approach for the preparation of high-quality liquid-phase graphene.⁸ However, the liquid-phase dispersions have relatively low yield (8.3 wt% in benzyl benzoate;^8e 1 wt% in water^8g). The yields of graphene sheets using our present method were obtained by vacuum filtering the graphene dispersions (stage 3, Fig. S3 in the ESI†) through polyvinylidene fluoride (PVDF) membrane (Millipore-Omnipore membrance, 50 mm, 0.1 μm). The yields of graphene sheets prepared by different pyrene-polymers and solvents were listed in Table 1. The yields of graphene sheets reach as high as 10.2 wt% in water, and 51.8 wt% in DMSO. We find that the molecular weight of pyrene-polymer play an important role in the exfoliation of graphite, where the pyrene-polymer with higher molecular weights leads to a lower yield of graphene sheets. The pyrene-polymer with a higher molecular weight may be hard to insert into the graphite interlayers due to the limited space within the interlayer. In order to indicate the advantage of pyrene-polymers, another control experiment using 1-pyrenecarboxylic acid (PCA) to exfoliate the graphite was carried. The yield of graphene sheets is only 2 wt% (Table 1(e)). It indicates that the exfoliation of graphite using pyrene-polymers with suitable molecular weight is ideal due to the dangling polymer chains can increase the distance between adjacent graphene layers and facilitate their stable dispersion in solution.

Table 1 The yields of graphene sheets prepared using pyrene-polymers and PCA

Sample	Pyrene-polymer	Solvent	wt% remaining after filtration	mg ml^−1 of stage 3
a	pyrene-PEG2K	water	10.2	0.102
b	pyrene-PEG5K	water	0.9	0.009
c	pyrene-PCL19	DMSO	51.8	0.518
d	pyrene-PCL48	DMSO	14.9	0.149
e	PCA	water	2.0	0.020

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Graphene-based polymer composites

High-concentration graphene solutions are attractive for nanocomposites. The thermal stability of nanocomposites, poly(ε-caprolactone) (PCL)-graphene and polystyrene (PS)-graphene nanocomposites was investigated. The DSC traces of pure polymer and with different percentages of graphene are shown in Fig. 6. Table 2 and Table 3 summarize the DSC measurement results for these samples. For PCL system, the crystalline temperature (T_c), the melting temperature (T_m), and degree of crystallinity (X_c) are all increased (Fig. 6(a) and Table 2). Especially for the T_c and X_c, where the T_c increases 10 °C at only 1 wt% of the nanofiller and X_c increases nearly 10% at 5 wt% of the nanofiller. Likewise, for PS system, the T_g data for the graphene-PS nanocomposite are prominent: a shift of nearly 10 °C occurs at only 1 wt% of the nanofiller (Fig. 6(b) and Table 3).

Fig. 6 (a) Non-isothermal DSC scans of (A) pure PCL, (B) 1 wt% graphene-PCL nanocomposite and (C) 5 wt% graphene-PCL nanocomposite. (b) Non-isothermal DSC scans of (A) pure PS and (B) 1 wt% graphene-PS nanocomposite. Inset of (a) and (b) show photographs of the corresponding hot-pressed composite films.

Table 2 Crystalline temperature (T_c, °C), melting temperatures (T_m, °C), heats of fusion (ΔH_i, J g⁻¹), and weight fractional crystallinities (X_c, %) of (A) pure PCL, (B) 1 wt% graphene-PCL nanocomposite and (C) 5 wt% graphene-PCL nanocomposite

Sample	T c (°C)	T m(°C)	ΔHi (J g^−1)	X c (%)^a
a The weight fractional crystallinities of the samples were calculated by the equation: Xc (%) = ΔHi/Φi ΔHim ×100%, where ΔHi is the enthalpy of fusion of the prepared samples, directly obtained by DSC, and Φi is the mass fraction of polymer in the hybrids. ΔHim, the enthalpy of fusion of 100% crystalline polymer, are 81.6 J g^−1 for PCL.^26
A	29.0	54.5	67.5	82.7
B	39.3	55.7	67.0	82.9
C	41.3	56.2	70.8	91.3

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Table 3 The glass transition temperature (T_g) measurement results for (A) pure PS and (B) 1 wt% graphene-PS nanocomposite

Sample	T g (°C)
a Onset temperature of Tg. b Mid-point temperature of Tg. c End temperature of Tg.
	T ig ^a	T mg ^b	T eg ^c
A	89.5	93.1	96.1
B	100.3	103.5	105.3

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These experimental results of graphene-polymer nanocomposites suggest that the surface functionality of graphene afford it good interaction with the host polymer, thereby imparting highly enhanced thermal properties at low loadings. Considering the abundance of graphite, the obtained graphene will have good potential to revolutionize the use of nanocomposites.

Conclusions

In summary, we have demonstrated a facile and green method for preparation of stable graphene dispersions in different solvents by taking advantage of SC CO₂ and two specially designed pyrene-polymers. The novel exfoliation process involves multiple interactions among graphite, solvent, pyrene-polymers and SC CO₂, where SC CO₂ acts as penetrant, expanding agent and antisolvent, and the pyrene-polymer acts as molecular wedge and modifier. Morphology and quality studies reveal that large scale and high quality graphene flakes are obtained, primarily consisting of single-, bi- or tri-, and multilayered graphene sheets. On the other hand, yield studies illustrate that the molecular weight of pyrene-polymers and the solvent system have significant influence on exfoliation results. The high yields of graphene sheets are full of promise for the application in new functional nanocomposites.

Our strategy provides new opportunities for the direct exfoliation of graphite to large scale, high-quality graphene. Apart from pyrene-PEG and pyrene-PCL, other pyrene-derivatives can also be employed to exfoliate and modify graphene in a similar way. Through careful selection of appropriate molecules (bearing structural resemblance to or comprising pyrene units) and the SC CO₂'s assistance could further improve the quantity and quality of the graphene sheets, while the variation of the dangling chains of pyrene-derivatives, such as chain length and hydrophilic/hydrophobic properties, would further affect the solubility and the yield of the graphene. This study also illustrates that in addition to the pyrene-polymer, SC CO₂ can also be used as key technique for the preparation of high-throughput graphene sheets. The method of combining high-throughput production and functionalization of graphene in one step make it potentially for developing multifunctional high-performance applications of graphene and graphene-based materials in various fields.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (No. 20974102, 50955010, 51173170), the Program for New Century Excellent Talents in Universities (NCET), and the Program for Excellent Scientist from Henan province (No.114200510019).

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Footnote

† Electronic supplementary information (ESI) available: FTIR and ¹H NMR spectra of pyrene-PEG (pyrene-PEG_2K and pyrene-PEG_5K) and pyrene-PCL (pyrene-PCL₁₉ and pyrene-PCL₄₈) and their structure formula, process flow of intercalation and exfoliation of graphite to graphene with different steps, AFM of few-layer graphene on Si, Raman spectra of individual graphene sheets deposited from solution on holey carbon TEM grids and I_D/I_G intensity ratios, XPS survey scans of the four vacuum filtered graphene films and elemental analysis determined by XPS. See DOI: 10.1039/c2ra21316h

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