Ultrathin

Article pubs.acs.org/JPCC

Preparation of Pristine Graphene Sheets and Large-Area/Ultrathin Graphene Films for High Conducting and Transparent Applications Yue Lin,† Jie Jin,† Olga Kusmartsevab,‡ and Mo Song*,† †

Department of Materials, Loughborough University, Loughborough, LE11 3TU, U.K. Department of Physics, Loughborough University, Loughborough, LE11 3TU, U.K.

‡

ABSTRACT: An effective and scalable exfoliation route has been developed to prepare high quality pristine graphene sheets. A remarkable yield of 55 wt % was achieved at a high solvent concentration of 20 mg/ mL, with the application of only 1 h ultrasonication in total. The exfoliation process was achieved by synergistic effect of intercalation, selfassemblage, and further expansion. The successful exfoliation was proved by means of X-ray diffraction (XRD) patterns, Raman spectra, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Furthermore, preparation of large-area and ultrathin pristine graphene films were demonstrated by a facile, rapid, and scalable electrophoretic deposition approach. The pristine graphene sheets which are extremely sensitive to the applied electric field were arrayed in strict order to form the high-quality graphene films. The resultant graphene films exhibit a high electrical conductivity of ∼1 × 105 S/m. Raman spectroscopy analysis reflected bilayer morphology of the graphene film. The simple and rapid processing route and the high transparency and conductivity of the graphene films suggest their potential applications in electrical and optical fields.

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INTRODUCTION Since 2004, graphene, a new two-dimensional nanomaterial, has attracted plenty of attention due to its fascinating electrical,1 mechanical,2 and thermal properties.3 Owing to all these exceptional properties, graphene, the rising star in both science and engineering, is intensively drawing wide interest in exploring its application. Currently, the potential applications of graphene are in polymer composites,4 energy storage devices, 5−7 sensors, 8 electronic devices, 9,10 and optical devices.9,11 In order to develop commercial graphene-based materials, graphene sheets should be available in a large quantity. Currently, the focus of public concern has been attracted into the liquid-phase exfoliation of graphite into graphene due to the benefits such as relatively low cost, industrially scalable, and quality guaranty.12 Hernandez et al.13 fabricated high-quality graphene sheets through simple dispersion and exfoliation of pristine graphite in certain organic solvents by ultrasonication. With concentrations up to 0.01 mg/mL, the monolayer yield of graphene dispersion in Nmethyl-2-pyrrolidone (NMP) solvent was around 1 wt %. The yield could be potentially improved to 7−12 wt % with further treatment. Qian et al.14 demonstrated a solvothermal-assisted exfoliation process to produce monolayer and bilayer graphene sheets from expandable graphite in acetonitrile (ACN). The yield of the monolayer and bilayer graphene sheets can reach around 10 wt %. An et al.15 exfoliated graphite into single-, few-, and multilayered graphene flakes with assistance of 1pyrenecaboxylic acid and long-time ultrasonication (more than 1 day). Bose et al.16 also achieved stable aqueous © 2013 American Chemical Society

dispersion of graphene from graphite by noncovalent functionalization of 9-anthracenecarboxylic acid. All this research showed the potentiality of the liquid-phase exfoliation methods to fabricate graphene in scalable and facile way. Although a great deal of exciting progress has been made for the liquid-phase exfoliation, there remains some key problems: (1) the yield of graphene sheets remains very low, (2) some methods, such as solvothermal-assisted exfoliation process, may be too dangerous to operate in industry, and (3) the procedures developed so far consumed extremely high energy, which results in unaffordable costs. More recently, Kian Ping Loh et al.17 used an electrochemical route to exfoliate graphite to graphene flakes. Although a high yield of 70% was achieved, there are some inherent drawbacks such as requirement of electrochemical instrument, complicated preparation of electrode, and usage of high concentration lithium-based solvents. These fatal flaws limited its application in environmental friendly and scale-up production. Xuping Sun et al.18 described a rapid exfoliation method to produce few-layer graphene flakes from graphite with the use of chlorosulfonic acid and H2O2 as exfoliating agents. An amazing yield of 95% was achieved within seconds. However, the usage of acid damaged quality of the graphene flakes, limiting the applications of the flakes in advanced electronic engineering. Furthermore, uncontrollable heat released in large amount and rapid volume expansion Received: April 19, 2013 Revised: July 24, 2013 Published: July 24, 2013 17237

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Preparation of Pristine Graphene Sheets. A simple liquid-phase exfoliation approach has been developed to fabricate graphene from UF-4 graphite. Calculated amount of UF-4 graphite (G) was first sonicated in a 5 mL mixture of phenol and methanol (ratio: 5:1) for about 30 min. With addition of 10 mg of CTAB, the resultant graphite (IG-1) was sonicated for another 30 min and was then left to soak in the mixture for 1−2 days. Afterward, the resultant graphite (IG-2) was separated by centrifugation and was transferred into 100 mL mixture of water and methanol, followed by stirring for 2 h. Finally, exfoliated graphene (EG) was carefully separated and dried at 60 °C and can be stored for a long time. The EG powder can be further dispersed into different solvents such as water and dimethylformamide (DMF) for long term. For characterizations and applications, the EG powder was repeatedly washed for 3 times by water to remove all the small molecules attached. The cleaned graphene (EG-1) was then dried at 60 °C for 8 h. EG-1 can be stably dispersed in water for 1 day. Figure 1 explains the exfoliation mechanism of this approach.

during the process led to potential explosive risks in scale-up production. In order to bring graphene from science to engineering and industry, exploring a simple, effective, and environmental friendly method for the large scale manufacturing of high-quality graphene sheets becomes of critical importance. Besides, developing effective techniques for fabricating highquality single-layer or several-layers graphene films should also be taken into adequate consideration. Graphene films with properties that include large area, ultrathin, highly conductive, transparent, and transferable are extremely desirable for enhancing and widening its electrical and optical applications, such as transistors, soft touch screen, and solar cells.7,9,10 Fabricating the graphene films through top-down routes keeps on triggering public’s interest due to the dramatic advantages of unbeatable low cost, potentially large-scale production, and relaxed processing conditions. Another benefit of the top-down routes is that the produced graphene sheets can be conveniently transferred to any substrate with a simple process.7 Currently, most of the research is focusing on the the steps oxidative exfoliation of graphite to graphene oxide, subsequent chemical reduction, and rearrangement of reduced graphene oxide sheets to the reduced graphene oxide films. Eda et al.19 reported a solution-based method to produce uniform and thin reduced graphene oxide films. The thicknesses of the films are ranged from a single monolayer to several layers over large area. The thinnest films exhibit graphene-like ambipolar transistor characteristics, and the thicker films behave as graphite-like semimetals. Tung et al.20 demonstrated a versatile solution-based process to prepare single-layer chemically converted graphene oxide sheets. They dispersed graphene oxide paper in pure hydrazine to remove oxygen functionalities and to restore the planar geometry of the single sheets. The size of the reduced graphene oxide films can be up to 20 × 40 μm. All this research proves the reliability of fabrication of graphene films through top-down routes. However, a fatal weakness of the graphene oxide films is the defective plane structure, which leads to poorer performances than that of pristine graphene films. Up to date, there is little progress for fabricating ultrathin, transparent, and highly conductive graphene films through an effective top-down route without damage of the C-sp2 plane structure of graphene, for example, the liquid-phase exfoliation process. The development of this kind of route is extremely urgent and will no doubt create a new path for both research and the promising applications of graphene films as electrical devices (e.g., electrodes and transistors), optical devices (e.g., touch screen and solar cells), and electrochemical devices. Here, we reported an effective, energy-saving, and high yield strategy to exfoliate graphite to graphene. We also focused on developing an effective technique for constructing high-quality and ultrathin pristine graphene films by means of electrophoretic deposition (EPD), which will no doubt enhance the application prospects of graphene in the electrical and optical fields.

Figure 1. Schematic of the exfoliation mechanism of the UF-4 graphite to grpahene. G is UF-4, IG-1 is phenol intercalated graphite, IG-2 is phenol and CTAB intercalated graphite, EG is exfoliated graphene, and EG-1 is cleaned graphene.

Preparation of Ultrathin Graphene Films. Preparation of large-area, ultrathin graphene films was achieved by means of electrophoretic deposition (EPD). First, a calculated amount of EG-1 was dispersed in deionized water by sonication for 1 h to get a stable graphene suspension. PH value of the graphene suspension was then adjusted to about 3 by addition of hydrochloric acid. After this treatment, the graphene sheets became positively charged. The suspension was loaded in a glass beaker as electrolyte and two gold-coated conductive glasses were used as electrodes, with the distance of 5 mm between them. The deposition time ranged from 1 to 10 min. Under the applied electric field, the positively charged graphene sheets migrated toward the negative electrode and were finally deposited onto the surface of the negative electrode.

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EXPERIMENTAL SECTION Materials. Ultrafine grinding graphite UF-4 was provided by the Graphit Kropfmuehl Group. All solvents were purchased from Scientific Fisher, UK. Cetyltrimethylammonium bromide (CTAB) was purchased from Sigma-Aldrich, UK. The molecular weight of the CTAB is 364 g/mol. Aggregation number of the CTAB is 170, and micellar average molecular weight is 62 000. 17238

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Characterization. In order to examine the exfoliation of the graphene, X-ray diffraction (XRD) patterns were obtained by using a Philip-X9 Pert X-ray diffractometer (anode 40 kV, filament current 35 mA) with nickel-filtered Cu Kα (λ = 0.1542 nm) radiation at a scan speed of 1° min−1. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical state of the exfoliated graphene. The analysis was performed on a VG ESCALAB 5 (VG Scientific Ltd., England) under 10−7 Torr vacuum with an Al Kα X-ray source using a power of 200 W. In order to find out whether the graphene remained undistorted, Raman spectra were recorded from 1200 to 2950 cm−1 on a HORIBA Jobin Yvon high-resolution LabRam 800 Raman microscope system, which contains an optical microscope adapted to a double grating spectrograph and a CCD array detector. The laser excitation was provided by a SpectraPhysics model 127 helium−neon laser operating at 35 mW of 488 nm output. Transmission electron microscopy (TEM) was applied to observe the morphology of the graphene sheets using a JEOL 2100 FX instrument. The graphene/water dispersion was dropped on a copper grid for TEM imaging directly. Furthermore, to observe the morphology of the graphene sheets and the graphene films deposited on a gold coated glass substrate, scanning electron microscopy (SEM) images were taken by field emission gun scanning electron microscopy (FEGSEM) (LEO 1530VP instrument). The electrical conductivity of the graphene films was determined by both 4-point probes measurement and 2-point probes measurement using a FLUKE PM6306 programmable automatic RCL meter. For 2-point probes measurement, one probe contacted with the graphene films and the other probe contacted with the gold film. Thus, the gold film acted as an electrode, and the resistance of the graphene films can be determined directly. For 4-point probes measurement, in order to determine the resistance of the graphene films, the resistance of the gold film (Rgold) and the graphene/gold bilayer (Rbilayer) films were measured first. The bilayer films acts as a parallel circuit. Hence, there is 1/R bilayer = 1/R gold + 1/R graphene

stable complex. This process expanded the gaps between the graphite layers and allowed free CTAB in the solvent to intercalate into the space between the graphite layers. Thus, updated intercalated graphite (IG-2) was formed. This process was proved by means of XRD, XPS, and SEM, which were discussed in detail in the further discussion. In the presence of water, CTAB will aggregate to a micellar structure. This nature was utilized to achieve final exfoliation of the graphite to the graphene. In step 3, due to change of media from phenol/ methanol to more polar water/methanol, the intercalated CTA+ ions rearranged themselves and aggregated on the surface of graphite layers. Because of the formation of aggregations, the gaps between graphite layers were further expanded. Meanwhile, due to the presence of water, a large number of disordered free CTAB molecules were wedged into the gaps between graphite layers, which is more hydrophobic phase compared to outer environment. These CTAB molecules further aggregated with the CTA+ ions to form the ordered micellar structure. Therefore, the van der Waals force between graphite layers was overcome, and the graphene sheets (EG) were exfoliated from graphite due to formation of the micelles. The detailed evidence for the exfoliation mechanism was provided in the following discussion. X-ray diffraction (XRD) was carried out for investigation of the exfoliation mechanism of the graphite to graphene through noncovalent functionalization. Figure 2A shows XRD patterns

(1)

Equation 1 can be transferred to the following form: R graphene = (R bilayerR gold)/(R gold − R bilayer)

(2)

Thus, the resistance of the graphene films can be calculated through eq 2. Afterward, the conductivity of the graphene films was determined.

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RESULTS AND DISCUSSION As is demonstrated in Figure 1, the exfoliation of G was achieved by cooperation of intercalation, self-assemblage, and further expansion. In step 1, phenol was intercalated into gaps between the graphite layers and was then attached onto the surface of individual graphite layers to form intercalated graphite (IG-1) via a noncovalent π−π stacking aromatic interaction, which does not disrupt the sp2 hybridization of graphite.21 In step 2, cetyltrimethylammonium bromide (CTAB) was added in. CTAB consists of two parts, which are hydrophobic macromolecularly olefinic chain and hydrophilic ammonium bromide end. The hydrophobic chain of CTAB drove CTAB to intercalate into the graphite layers which are also hydrophobic. Afterward, CTAB self-assembled with IG-1 via exchange of cetyltrimethylammonium cations (CTA+) with hydrogen cations from phenol in order to form

Figure 2. XRD patterns of (A) G, IG-2, and EG-1 and (B) IG-2 with different preparation concentrations. For comparison, the graphitic peaks of IG-2 in (B) were calibrated to the same intensity.

of G, IG-2, and EG-1. For G, a sharp peak appears at 26.5°, which is relative to (001) crystalline plane reflection of G. The distance between the planes is 0.345 nm. For IG-2, two peaks are shown at 26.5° and 19°, respectively. The broad peak at 19° is an indication of amorphous structure, which proves the successful intercalation of phenol and CTA+ ions into the gaps 17239

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between the graphite layers. It is noticeable that the (001) graphitic plane reflection is present as well. This observation indicates that the intercalation between some graphite layers were not strong enough to transform all crystalline graphite (G) to intercalated amorphous graphite. For the EG-1, there is no any peak present, which indicates the successful exfoliation. The EG-1 powders prepared under different concentrations were weighted to calculate the yield of G to EG-1. The results are listed in Table 1. The yield increased with the decreasing Table 1. Yield of G to IG-2 and G to EG-1 versus Concentration concn (mg/mL)

ratio (intercalated amorphous graphite in IG-2) (%)

yield (G to EG-1) (%)

35 20 2

37.5 55.6 57.1

36.5 55 54

concentration and reached a platform at a concentration of 20 mg/mL. A critically high output of 55% was achieved at concentration as high as 20 mg/mL. XRD experiment was carried out to examine IG-2 prepared with different concentrations of G in mixture of phenol and methanol. The ratio of intercalated amorphous graphite in IG-2 was calculated by integration of the amorphous peak and the crystalline graphitic peak, followed by dividing the total area of the two peaks by that of the amorphous peak. This method is a common route to calculate the crystallinity of semicrystalline material. The results are listed in Table 1 as well. Figure 2B shows XRD patterns of IG-2 with different concentrations. It is clearly seen that the higher intensity of the intercalated amorphous graphite peak (19°) is achieved with the lower concentration, which indicates higher yield. As is shown in Table 1, the ratio of intercalated amorphous graphite in IG-2 and the yields of G to EG-1 are almost the same, which implies that the intercalated amorphous graphite was fully exfoliated into EG. Thus, it is concluded that successful formation of intercalated amorphous graphite in IG-2 is the key point for fabrication of EG-1. Figure 3A shows the overview for XPS spectra of UF4 graphite powder (G) and exfoliated grpahene powder after wash (EG-1). Table 2 lists their elemental composition. The elemental composition of EG-1 is almost the same as that of G. This result confirms that the exfoliation route does not have any chemical effect on the graphene, such as oxidation. Figure 3B,C shows XPS spectra of the C 1s peaks of the graphite powder (G) and the exfoliated graphene (EG-1). Table 3 lists their bond compositions. The shape and intensity of the graphitic peak of EG-1 remain the same as that of pristine graphite. This observation proves that the C-sp2 network of the graphene was undistorted after the exfoliation. The peak at 290.5 eV corresponds to a typical π−π* shake-up satellite, which is indicative of aromaticity in graphite. It is remarkable that the intensity of π−π* transition peak significantly decreased more than 95% after exfoliation. The decrease appears as a proof of deshybridization of some carbon atoms in graphene sheets. Furthermore, the intensity of C−O peak increased dramatically. Considering that the elemental composition of oxygen and the shape and intensity of graphitic peak are almost not changed after exfoliation, this increase should be related to strong interaction of oxygen atoms with the carbon atoms of graphene sheets due to sp2 deshybridiza-

Figure 3. XPS spectra of (A) G and EG-1, (B) the C 1s peaks of G, and (C) the C 1s peaks of EG-1.

Table 2. Elemental Composition of the UF-4 Graphite and Exfoliated Graphene atom percentage (%) sample

C

O

Br

G EG-1

94.9 95.5

5.1 4.5

0 0

tion, rather than oxidation of graphene sheets.22 These oxygen atoms can belong to both the existing oxygen atoms from the UF-4 graphite and −OH groups from aqueous molecules attached on the surface of EG-1. This bonding can be effective using π electron, but without distortion of the aromatic cell and break of σ bond in graphene. The appearance of this bonding effect fixed the oxygen atoms on the surface of graphene. When dispersed in a solvent, such as water and DMF, the oxygen atoms on the surface of graphene will interact with the solvent, 17240

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aqueous dispersion changed with concentration. With the increase of concentration, the color changed from yellow to dark green and finally black. This change may be due to light interference effect. When light passed through the solution, the interference happened between the reflection paths which originate from the water to EG interfaces and EG to EG interfaces. Interfering paths will experience relative phase shifts depending on their distance. Because of the increasing concentration of EG, the distance between the EG sheets dispersed in water became different. Thus, the color shifts were observed by eye for the EG aqueous dispersion with different concentration due to distance variation of a fraction of wavelength. Similar interference phenomena were reported by a few researchers.23−25 Besides the fabrication of pristine graphene, preparation of large-area, ultrathin, and highly conductive graphene films through an effective top-down route is another challenging issue. Figure 5 shows the SEM images of the graphene sheets and the graphene films deposited on the gold-coated glass. Table 4 shows the conditions of the EPD routes corresponding to Figure 5. With the concentration of 0.1 mg/mL and deposition time of 1 min, the individual graphene sheets were deposited onto the surface of the glass (Figure 5A). Figure 5B shows a typical high quality and transparent graphene sheet. The right part of the graphene sheet flatly clings to the surface, resulting in the boundary difficult to distinguish. The left part of the graphene sheet is relative rough, so that the boundary can be easily observed. Furthermore, the geography of the background under the graphene sheet can be easily distinguished. These observations claim that the graphene sheet is highly transparent. When the deposition time increased to 5 min, more graphene sheets were orderly deposited onto the surface to form a uniform graphene film, as shown in Figures 5C,D. It is remarkable that the graphene film is quite

Table 3. Bond Composition of C 1s Peak for the UF-4 Graphite Powder (G) and the Exfoliated Graphene Powder (EG-1) sample G

EG-1

peak

position (eV)

area

percentage (%)

graphitic C−O π−π* graphitic C−O π−π*

284.4 286.5 290.5 284.5 286.2 290.5

1845 347 537 1715 700 61

67.6 12.7 19.7 69.3 28.3 2.5

which prevents the graphene from restacking. Thus, the EG-1 was stable dispersed in water for 1 day. Figure 4A shows a TEM image of EG dispersed in water by sonication for 30 min. Comparing to the cleaned graphene (EG-1) shown in Figure 4B,C, the EG sheet was covered by threadlike objects, which is CTAB aggregations. Figures 4B,C are images of EG-1 dispersed in water, which indicates the successful exfoliation of the graphite into graphene. EG-1 is the product that EG after repeated water washes with assist of sonication. As is shown in Figures 4B,C, the attached small molecules were fully removed after 3 times washes. Thus, it is proved that the molecules are noncovalently attached onto the surface of graphene. Figure 4C shows a typical TEM image of a graphene sheet held at the edge of copper grid. The graphene sheet is observed to be folded several times, indicating the extremely high toughness. Figure 4D is a digital picture of graphite aqueous dispersion (1 mg/mL) and EG dispersed in water with concentration of 0.01, 0.1, and 1 mg/mL (left to right) after 3 months, It is shown clearly that the parent graphite had deposited to the bottom after static placement for 3 months. In contrast, all EG was stable dispersed in its aqueous dispersion. It is interesting that the color of EG

Figure 4. TEM image of EG (A), EG-1 (B, C), and digital image of graphite and graphene aqueous dispersion (D). 17241

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Figure 5. SEM images of graphene sheets and graphene films prepared under different EDP conditions. The pictures at the right side are the enlarged images for the highlighted squares of the pictures at the left side.

5E,F that the graphene film becomes less transparent, as the geography of the background under the graphene film is hard to distinguish now. Furthermore, more edges of the individual sheets are emerged. It is interesting that graphene films were no longer formed when the concentration of graphene aqueous

smooth; thus, the boundaries of individual graphene sheets are hard to distinguish. Some edges of the individual graphene sheets can still be observed due to the localized film roughness (Figure 5D). With 10 min deposition, the graphene film became thicker and rougher. It is clearly observed from Figure 17242

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Table 4. Deposition Conditions label

concn (mg/mL)

deposition time (min)

EPD-A EPD-B EPD-C EDP-D

0.1 0.1 0.1 1

1 5 10 5

morphology graphene graphene graphene graphene

sheets film film sheets

dispersion increased to 1 mg/mL. It can be observed clearly from Figures 5G,H that the graphene sheets prefer to stack together to form clusters, rather than formation of a graphene film. On the other hand, with change in deposition voltage, the graphene films were no longer formed as well. Thus, it is believed that the pristine graphene sheets are highly sensitive to the applied electric field and can be arrayed in order to form the high-quality graphene film. The concentration of dispersion, deposition time, and deposition voltage are key points which need to be carefully controlled in order to fabricate high-quality graphene films. Raman spectroscopy is known to be a powerful tool for characterization of nanocarbons, such as graphene and carbon nanotubes. The Raman spectrum of both UF-4 graphite powder and EPD-B was excited at 488 nm. In terms of the UF-4 graphite, a very small D band at 1360 cm−1 and a sharp G band at 1579 cm−1 are shown, which implies its nearly defectfree state. For the EPD-B, similar features for D band and G band are observed, which prove its undistorted C-sp2 graphitic network. The ratio of G band to D band increases from 3.13 for UF-4 graphite to 4.51 for the graphene film. This increase is due to the ordered feature of the graphene film, which contains fewer edges than that of the UF-4 graphite in the same area. Furthermore, the 2D band is upshifted from 2737 cm−1 for the UF-4 graphite to 2722 cm−1 for the graphene film, which is also a strong evidence of exfoliation. A Lorentzian peaks fitting was done for 2D band of both UF-4 graphite and the EPD-B. Distinguished with the 2D band of the UF-4 graphite best fitted with two peaks, the 2D band of the EPD-B is best fitted with four Lorentzian peaks, which correspond to four resonant transitions.26 This result is a strong indication of bilayer graphene structure, which reflects the two layers morphology of the EPD-B.26−28 More importantly, the measured electrical conductivity of the EPD-B transparent graphene film is 0.9 × 105 S/m. It is believed that the high electrical conductivity is contributed by both strictly ordered arrangement of the graphene sheets in the graphene films and the undistorted C-sp2 graphitic network of the graphene sheets. The simple and rapid processing route to produce the large-area and ultrathin graphene films suggests its potential for low cost and industrially scale production. The high transparency and incredible conductivity of the graphene films will no doubt secure their potential applications in electrical and optical fields, especially for soft touch screen and transparent conductor.

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deposition voltage (V)

corresponding images in Figure 5

6 6 6 6

A, B C, D E, F G, H

Figure 6. Raman spectra of (A) G and EG film prepared under concentration of 0.1 mg/mL and deposition time of 5 min.

devices, and optical devices. The exfoliation was achieved through several steps: intercalation of phenol and cetyltrimethylammonium bromide into graphite, self-assemblage of the graphite layers with the small molecules, expansion, and further exfoliation of the graphite into the graphene. The exfoliation was proved by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. X-ray photoelectron spectroscopy and Raman spectroscopy analysis indicated that the C-sp2 network of the graphene remains undistorted. Furthermore, preparation of large-area and ultrathin pristine graphene films was demonstrated by a facile, rapid, and scalable electrophoretic deposition approach. Raman spectroscopy analysis reflected bilayer morphology of the graphene film formed under controlled conditions. The resultant graphene films exhibit a high electrical conductivity of ∼1 × 105 S/m. This effective and scalable top-down route to fabricate large-area pristine graphene films will no doubt enhance both research and application prospects of graphene.

CONCLUSIONS

In summary, a novel, simple, effective, and scalable liquid phase exfoliation route has been developed for preparation of highquality pristine graphene sheets. A remarkable yield of 55 wt % was achieved with the preparation concentration of 20 mg/mL. This approach enables the scalable fabrication of graphene to allow researchers in exploring its properties and to ensure its promising applications in different fields, such as polymer nanocomposites, energy storage devices, sensors, electronic

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.S.). Notes

The authors declare no competing financial interest. 17243

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dx.doi.org/10.1021/jp403903k | J. Phys. Chem. C 2013, 117, 17237−17244