Exfoliation of Non-Oxidized Graphene Flakes for Scalable Conductive

Letter pubs.acs.org/NanoLett

Exfoliation of Non-Oxidized Graphene Flakes for Scalable Conductive Film Kwang Hyun Park,† Bo Hyun Kim,† Sung Ho Song,† Jiyoung Kwon,† Byung Seon Kong,‡ Kisuk Kang,§ and Seokwoo Jeon*,† †

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡ KCC Central Research Institute, Gyunggi-do, 446-912, Republic of Korea § Department of Materials Science and Engineering, Seoul National University (SNU), Seoul 151-742, Republic of Korea S Supporting Information *

ABSTRACT: The increasing demand for graphene has required a new route for its mass production without causing extreme damages. Here we demonstrate a simple and costeffective intercalation based exfoliation method for preparing high quality graphene flakes, which form a stable dispersion in organic solvents without any functionalization and surfactant. Successful intercalation of alkali metal between graphite interlayers through liquid-state diffusion from ternary KCl− NaCl−ZnCl2 eutectic system is confirmed by X-ray diffraction and X-ray photoelectric spectroscopy. Chemical composition and morphology analyses prove that the graphene flakes preserve their intrinsic properties without any degradation. The graphene flakes remain dispersed in a mixture of pyridine and salts for more than 6 months. We apply these results to produce transparent conducting (∼930 Ω/□ at ∼75% transmission) graphene films using the modified Langmuir−Blodgett method. The overall results suggest that our method can be a scalable (>1 g/batch) and economical route for the synthesis of nonoxidized graphene flakes. KEYWORDS: Graphene, graphite intercalation compound, conductive thin film, eutectic point, display

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raphite, which consists of numerous layers of the sp2 hexagonal carbon lattice, has been widely used as an anode material for secondary ion batteries due to its large charge density (372 mAhg−1) and cost-effectiveness.1 Accordingly, graphite intercalation compounds (GICs), which are formed by the insertion of atomic or molecular species in graphite interlayers,2 have been studied to understand the fundamental intercalation mechanism in crystals as well as to achieve maximum electrical storage of charge for high performance batteries from the early 1970s.3 For example, LiC6, a highly ordered intercalation compound pioneered by D. Guerard,4 helped expand the Li-ion battery business. Its success is mostly attributed to the small expansion of the interlayer distance (3.354 → 3.61 Å) compared to that of compounds made from cheaper monovalent alkali metals such as potassium, sodium, etc. The smaller expansion leads to a more stable charging/discharging performance even after >1000 cycles without undergoing exfoliation caused by self-decomposition or intercalation of additional elements.5 Because of the growing interests in graphene from having extraordinary mechanical,6 electrical,7,8 and thermal9 properties, GICs were reilluminated recently. The larger interlayer distance provided by GICs may offer a novel synthetic route to generating graphene flakes from graphite without any oxidation.10 In general, the binding energy between adjacent © 2012 American Chemical Society

graphene layers (∼43 meV) is about 160 times smaller than the in-plane C−C binding energy (∼7 eV) in graphite.11 This causes the anisotropy in mechanical strength of graphite although the binding energy between numerous interlayers is not small enough for easy exfoliation. According to the research of Spanu et al.12 and Charkarova-Kack et al.,13 the van der Waals (vdW) force between π−π stacked graphene sheets annihilates when the distance between the interlayers exceeds 5 Å. This explains the easy exfoliation of graphite oxide (GO) whose interlayer distance is ∼8 Å14,15 where the vdW force closely approaches zero in aqueous solution. KC8, a well-known GIC formed by the intercalation of potassium in a graphite crystal, has interlayer distance of ∼5.4 Å.2,16−18 Consequently, its large expanded interlayer can be utilized to produce highquality graphene flakes unless the potassium damages the inplane C−C bonds of graphene as oxidation does. The Kaner16−18 and Penicaud19,20 groups have conducted the exfoliation of the KC8 compound derived from direct insertion of potassium metal in graphite. The intrinsic properties of the graphene flakes produced by this method are comparable to those of reduced graphene oxide (RGO).15,21−27 More recently, Received: February 4, 2012 Revised: May 15, 2012 Published: May 22, 2012 2871

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Figure 1. Schematic of graphene fabrication process. (a) Mixture with eutectic compositions of three compounds (KCl/NaCl/ZnCl2 = 0.2:0.2:0.6 in mole fraction) and forming of eutectic salt (ES) from melting of the mixed salts at 250 °C for 30 min in Teflon vessel. (b) Mixture of ES (90 wt %) and pristine graphite (10 wt %) and insertion process of eutectic salts in vacuum with varying temperatures (210, 250, 300, 350 °C) for 10 h in autoclave vessel. (c) Schematic of eutectic graphite intercalation compound (EGIC) formed by insertion of salts, left. Optical spectroscopy (OM) image and digital image (inset) of EGIC, right. (d) Schematic of dissolution process of the EGIC by Pyridine, left. Digital image of graphene flakes dispersed in pyridine solution after sedimentation for 24 h, right.

Figure 2. Characterizations of EGIC. (a) Comparison of XRD patterns for KC8 and EGICs. Main peaks related with c-axis distance between graphene−graphene interlayers (d-spacing: d002 and d004 planes) is significantly changed. (b,c) C1s and K2p XPS spectra of graphite, KC8, ES, and EGIC. C1s peaks of EGIC and KC8 show more broadened binding energy than that original graphite (284.5 eV). K2p3/2 and K2p1/2 peaks for ES observe at 293 eV (K−Cl) and 296 eV (K+) while the peaks of KC8 show at 294.6 eV (K metal) and 297.6 eV. The EGIC has two types of K2p doublets containing ES and KC8.

graphene flakes prepared from insertion of halogen compounds or salts not only showed improved electrical properties without any noticeable signs of damage.28−31 However, yield still needs to be improved for large-scale application. Here, we propose a novel GIC approach to generate graphene flakes by using a eutectic system of ternary KCl− NaCl−ZnCl2 salts. Through the intercalation process minimal damage and stable diffusion of intercalants in graphite are achieved because of a lower temperature (350 °C) than that necessary for single or bi salts, which is comparable to synthetic temperature of graphene recently reported by Kwak et al.32 The phase transition caused from intercalation is accurately proven from X-ray diffraction (XRD) and X-ray photoelectric spectroscopy (XPS). Also, high qualities of the graphene flakes are confirmed by high resolution-transmission electron microscopy

(HR-TEM), Raman spectroscopy, and XPS. By forming a conductive transparent film using modified Langmuir−Blodgett (mLB) method33 and mild thermal treatments (below 300 °C), we observe that the sheet resistance is ∼930 Ω/□ at a transparency of 75%, which is to our knowledge, the highest reported value from graphene flakes.15,34,35 The proposed mLB process suggests a simple and effective extraction method of graphene flakes from salts without leaving any residues and undergoing degradation from oxidation. The method also enables fabrication of graphene-based thin films with favorable electrical properties. Ternary salts (KCl, NaCl, and ZnCl2) were selected in order to synthesize a graphite intercalation compound (GIC) at low temperature through the formation of a eutectic system. The overall processes are illustrated in Figure 1. The initial steps 2872

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Figure 3. Characterizations of graphene flakes. (a) HR-TEM image of single layer graphene flake, left. Edge image and diffraction pattern of the single layer graphene flake, right. (b) Raman spectra of graphene flakes depending on the number of layers (D and G; left, 2D; right). (c) AFM image of single layer graphene flake (height profile, top; topology, left; friction, right). (d) Histogram of graphene flakes plotted as a function of thickness and AFM topology image (inset, 5 μm × 5 μm). The graphene flakes are mainly distributed in the range of 0.4−10 nm thickness (∼75%). (e) XPS spectra of graphene and graphite. Inset: C1s peak of the graphene flakes (284.5 eV (C−C) and 285.9 eV (C−O)).

noticeably in EGICs with increasing temperature whereas its relative peak intensity at 15.5° proportional to the portion of fully intercalated EGIC increases. It confirms that the intercalation by ES is predominantly influenced by temperature. This suggests that highly ordered GICs can be fabricated by controlling the operation temperature. More interestingly, the d004 peak of the EGIC (350 °C) is observed at a similar location to that of KC8 whereas the d002 peak at 26.4° entirely disappears. The result suggests that the alkali salt (dominantly potassium) derived from eutectic salts is fully inserted into the graphite interlayers and its intercalation mechanism is similar to that of potassium metal.2 Figure 2b,c shows that XPS results identify the similar characteristics between the EGIC and KC8. XPS can detect any changes of electronic charge density in the valence π-orbitals, which are related to the Fermi level.37 Therefore, carbon (C1s) and potassium (K2p), which are considered the most discriminable and predominant elements predicted from XRD analysis, are carefully observed to study the changes of their binding energy. The C1s peaks of KC8 and the EGIC are broadened toward higher binding energy relative to that of the graphite (284.5 eV). Besides, the position and shape of the C1s peak from EGIC are very similar to that of KC8. These results indicate that the amounts of electronic charge in the valence πorbitals of EGIC and KC8 are identically changed by intercalation. The K2p peaks of all the samples are discretely explored by peak separation for K2p3/2 and K2p1/2. The characteristic K2p3/2 (K−Cl) and K2p1/2 (K+) peaks for ES are discovered at 293 and 296 eV, respectively, while the K2p3/2 (K) and K2p1/2 peaks are observed at 294.6 and 297.6 eV,38 respectively. Interestingly, the peak locations of K2p3/2 for ES and KC8 are significantly changed and the EGIC, which contains ES (90 wt %) and graphite (10 wt %), consists of a similar K2p doublet with ES and KC8. This indicates that the

start from the mixing, melting, and grinding of alkali salts according to the mole fraction ratio (KCl/NaCl/ZnCl2 = 2:2:6) of the eutectic composition (Figure 1a).36 Then, the grinded homogeneous eutectic salts (ES, 9 g, 90 wt %) are wellmixed with graphite powder (1 g, 10 wt %, SP-1, Bay Carbon Inc.) and transferred to an autoclave vessel. After the removal of air and moisture from the vessel, the compound is kept in vacuum for 10 h with varying reaction temperatures (210, 250, 300, and 350 °C) while being mechanically mixed to intercalate the alkali salts into graphite (Figure 1b). Further details of the experimental method are described in the Supporting Information (S1.2). Figure 1c illustrates the eutectic GIC (EGIC) formed by the crystallographic alloy of intercalants in the graphite crystal (left) and the change of colors from graphite powder to EGIC under reflective optical microscopy (OM, right). The inset shows a digital image of the EGIC. Figure 1d schematically describes the dissolution process of the EGIC in pyridine solution (left) and successful dispersion of the exfoliated graphene (right). The total yield of graphene flakes dispersed in pyridine excluding the sedimented graphene flakes after 24 h is ∼60%. In our work, pyridine has the best dissolution property of both eutectic salts (ES) and exfoliated graphene. This result is confirmed with dispersion tests in various solvents based on the polarity index (Supporting Information Figure S2.1). XRD is a powerful tool to globally identify the phase transition of intercalated graphite crystals. The XRD patterns of KC8, prepared by direct evaporation of K metal,2 and EGICs formed at various temperatures (210, 250, 300, and 350 °C) are shown in Figure 2a. The main peaks related to the c-axis distance (d-spacing) between interlayers of KC8 (∼5.56 Å), EGIC (∼5.71 Å), and graphite (∼3.34 Å) are observed at 16.1, 15.5, and 26.4°, respectively. The peak intensity at 26.4° corresponding to the state of unintercalated graphite decreases 2873

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Figure 4. Graphene flake thin film. (a) Schematic of modified Langmuir−Blodgett method used to fabricate graphene flake thin film from EGIC. (b) Schematic for glass modification by using 3-aminopropyltriethoxysilane (APTES). (c) Digital (left) and OM image (right) and XRD pattern (inset) of the graphene flake thin film transferred onto glass substrate. Its broaden XRD pattern centered at 21.4° (d-spacing: 0.42 nm) is observed. (d) Sheet resistance (red) and transparency (blue) of the graphene flakes thin film with annealing treatment at different temperatures (25, 150, 200, and 300 °C) for 30 min in H2 atmosphere.

the 2D peaks (∼2723 cm−1) in our work is changed, which might be due to the decreased size of the graphene flakes or edge doping and functionalization by slightly remaining salts or solvents. Atomic force microscopy (AFM) was used to measure the thickness of graphene flakes. A representative AFM image of a single layer graphene flake (thickness: ∼0.5 nm) is shown in Figure 3c. Additional AFM images are shown (Supporting Information Figure S2.7). For the statistical analysis, a histogram plotted as a function of thickness (Figure 3d) is obtained from the AFM image (5 μm × 5 μm). Approximately 45% of the graphene flakes have thicknesses in the range of 2− 10 nm while ∼18% of the flakes have single/bi layers. Most of the graphene flakes (∼75%) are located in the thickness range of 0.4−10 nm (average size: ∼500 nm) without any additional separation processes from pyridine solutions. Figure 3e shows the XPS spectra of the pristine graphite and the prepared graphene flake. The resulting elemental surface compositions of the graphene flake mainly consist of carbon (96.6%) and oxygen (2.9%), which is similar to pristine graphite. The C1s spectrum ascribed to the C−C binding at 284.5 eV and C−O binding at 285.9 eV are shown in the inset of Figure 3e. Overall, we confirm that the EGIC system is a valuable method to generate non-oxidized graphene flakes without any introduction of structural defects of graphene flakes. To further prove the quality of graphene flakes, a transparent graphene film was prepared by the modified Langmuir− Blodgett (LB) assembly as shown in Figure 4a. This simple process is an effective purification method to form graphene films from the pyridine solvent containing both salts and nonoxidized graphene flakes. The glass surface is modified by 3aminopropyltriethoxysilane (APTES) in order to improve the adsorption affinity between the graphene film and glass substrate (Figure 4b). The detailed substrate modification

formation of EGIC is consistent with the theory for intercalation of potassium atoms in between graphene− graphene layers by electron transfer.37,39 Additional XPS information for the introduction of oxygen into the EGIC system is shown in Supporting Information Figure S2.2. The optical and structural/chemical characterizations of potassium intercalated graphite compounds are illustrated in Supporting Information Figure S2.3. Furthermore, through thermal gravimetric analysis (TGA), the thermal stability of EGIC was shown to have improved compared to that of ES, clearly demonstrating an evidence for practical potassium intercalation (Supporting Information Figure S2.4). The properties of graphene flakes are investigated by highresolution transmission electron microscopy (HR-TEM), Raman spectroscopy, AFM, and XPS. Figure 3a shows a HRTEM image of a single layer graphene produced by our system. The single layer graphene retains the crystallite structure as can be seen from the clear diffraction patterns (intensity ratio of I(0−110)/I(1−210) is ∼1.5) as shown in the inset of Figure 3a.40 Additional HR-TEM images for few layers graphene flakes are given (Supporting Information Figure S2.5). Raman spectroscopy is utilized to assess the crystalline quality of the prepared graphene flakes and they are classified by the 2D band shape depending on the number of layers, as shown in Figure 3b.41 The D band (∼1350 cm−1) of a few layer graphene flakes is extremely low compared to the ones reported from GO or RGO.14,15 The D to G (∼1580 cm−1) ratio is ∼0.15 which suggests that, even though the flake size is small, disorder in the sp2 carbon lattice and the degree of functionalization at the edges are small. The intensity and location of the 2D band are sensitive to doping or interaction with impurities.30 Although no significantly traceable amount of impurities or doping is detected (Supporting Information Figure S2.6), the shape of 2874

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conductivity of graphene thin film. This material is available free of charge via the Internet at http://pubs.acs.org.

procedures are illustrated in Supporting Information S3.1. Figure 4c (left) shows a digital image of the graphene flake thin film transferred onto a modified glass substrate (mGS). We observe that graphene flakes are homogenously stacked on the mGS from the OM image of graphene film (Figure 4c, right). However, the broadened XRD peak of graphene film centered at 21.4° (d-spacing: 0.42 nm) indicates that the graphene flakes restacked during the film formation are not fully recovered to the Bernal stacking of graphite (inset of Figure 4c, right). It is valuable to notice the correlation between sheet resistance and transparency plotted with annealing treatment at different temperature (Figure 4d). The sheet resistance of the graphene film is decreased and its transparency is steadily increased for increasing annealing temperature. This is because of enhanced contact between graphene flakes through the extraction of remaining moisture or solvent. In particular, the average value of sheet resistance after the annealing treatment at 300 °C is ∼930 Ω/□ at a transparency of ∼75%. Optical transmittance, surface roughness, and sheet resistance of the graphene thin films are statistically measured and presented in Supporting Information Figures S3.2 and S3.3. Considering the thickness of the graphene film, the maximum electrical conductivity of ∼91 000 Sm−1 (Supporting Information Figure S.3.4) is the reported best value in conducting graphene films fabricated from graphene flakes. As a result, our proposed eutectic-based exfoliation method allows scalable synthesis of high quality graphene flake without any degradation at low cost. We expect its potential use in various fields such as electronics, capacitors, batteries, and composites. In summary, we introduce a novel eutectic based method for fabrication of high-quality graphene flakes. The proposed technique is simple and cost-effective. We confirm that the alkali metal (potassium) between graphite interlayers is successfully inserted at the optimized operation condition. The resulting graphene flakes preserve the unique properties of graphene. These high quality graphene flakes are stably dispersed (>6 months) in pyridine solution without additional functionalization and surfactant stabilization. Transparent conducting graphene films from well-dispersed graphene flakes with high yield (∼60%) are produced by the modified Langmuir−Blodgett assembly. The resulting graphene film exhibits a sheet resistance of ∼930 Ω/□ at a transparency of ∼75% and a high conductivity (∼91 000 Sm−1). The overall results suggest that the eutectic-based method to graphene production is a scalable and low-cost route that brings graphene-based electronics and composite fields closer to practical applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-42-350-3342. Fax: +82-42-350-3310. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (No. 2011-K000231, No. 2011K000623) and Basic Science Research Program through the National Research Foundation of Korea (CAFDC-20110001137).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The contents of Supporting Information include the following: (S1.1) Materials, (S1.2) Methods, (S1.3) Equipment and techniques, (S.2.1) Dispersion of graphene flakes in solvents with varying polarity index, (S2.2) XPS depth Profiling analysis of oxygen, (S2.3) Characterizations of potassium intercalated graphite compound, (S2.4) TGA analysis of graphite, EGIC, and ES, (S2.5) HR-TEM images of graphene flakes, (S2.6) XPS analysis of graphene flakes, (S2.7) AFM images of graphene flakes according to different thicknesses, (S.3.1) Modification of glass surface, (S3.2) Optical and morphological characterizations of graphene thin film, (S3.3) Electrical characterizations of graphene thin film, and (S3.4) Electrical 2875

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