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Solid-Phase Coalescence of Electrochemically Exfoliated Graphene Flakes into a Continuous Film on Copper Li Li,†,⊥,‡ Xinming Li,*,†,‡,¶ Mingde Du,†,‡ Yichuan Guo,† Yuanchang Li,† Hongbian Li,† Yao Yang,§ Fakhr E. Alam,∇ Cheng-Te Lin,∇ and Ying Fang*,†,∥ †

National Center for Nanoscience and Technology, Beijing, 100190, P.R. China Key Laboratory of Marine Materials and Related Technologies, Zhejiang Key Laboratory of Marine Materials and Protective Technologies, Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, P.R. China § Department of Materials Science and Engineering, University of California, Los Angeles, California 90095, United States ∥ CAS Center for Excellence in Brain Science and Intelligence Technology, 320 Yue Yang Road, Shanghai 200031, P.R. China ⊥ Sino-Danish Center for Education and Research, Sino-Danish College of UCAS, Beijing, 100190, P.R. China ∇

S Supporting Information *

ABSTRACT: The ability to directly synthesize high-quality graphene patterns over large areas is important for many applications such as electronic and optoelectronic devices and circuits. Here, we report a facile and scalable approach to coalesce and recrystallize electrochemically exfoliated graphene flakes into a continuous film by thermal annealing on copper foils. The underlying growth mechanism involves defectmediated decomposition of electrochemically exfoliated graphene flakes into active polycyclic carbon species, followed by coalescence of the active carbon species into a continuous, monolayer film of high material quality. First-principles calculations confirm that the enhanced affinity of the polycyclic carbon species with copper effectively prevents their surface desorption at elevated temperatures, which is distinct from graphene growth based on the decomposition of solid carbon sources into gaseous hydrocarbons. Significantly, the localized supply of active carbon species in our approach enables spatially confined growth of graphene. Combined with stencil printing of the exfoliated flakes, transparent and conductive graphene circuits have been directly synthesized over large areas. Subsequent lithographic patterning of graphene films is required for device integration, which is time-consuming and can cause damage and contamination to graphene.12−16 Thus, direct growth of high-quality graphene patterns over large areas is highly desirable. Recently, electrochemically exfoliated graphene (EEG) flakes have attracted attention due to their fast and environmentally friendly route to produce.27−29 Similar to chemically exfoliated graphene oxide (GO) based on the Hummers method, EEG flakes have the advantages of mass production, on a scale of tens of grams, and solution processability. In addition, EEG flakes have a relatively low density of oxygen-containing groups and preserve a large fraction of the defect-free hexagonal carbon lattice, which is distinct from overoxidized GO flakes. Here, we demonstrate that high-quality, large-area graphene patterns can be directly synthesized by the recrystallization and coalescence of EEG flakes on copper foils. At elevated temperatures, the oxygen-containing groups in EEG flakes can be removed

1. INTRODUCTION Graphene, owing to its exceptional electrical and mechanical properties and high transparency, holds significant potential as a functional material for electronics,1−4 sensor,5−7 and energy applications.8−10 The development of facile and scalable growth techniques is of central importance for exploiting the full potential of graphene. Among current preparation methods, mechanical cleavage1 and epitaxial growth on silicon carbide11 provide the highest-quality graphene essential for fundamental studies, but in very limited quantities. Chemical vapor deposition (CVD) on catalytic metals, especially polycrystalline copper foils, produces large-area and high-quality graphene films.12−21 Both gaseous hydrocarbons, such as methane, and solid carbon sources that decompose into gaseous hydrocarbons at elevated temperatures, such as poly(methyl methacrylate) (PMMA)22 and amorphous carbon (a-C),23 have been employed in the CVD growth of graphene. Polycyclic aromatic hydrocarbons (PAHs) have been shown to dehydrogenate and coalesce into graphene films at elevated temperatures.24−26 However, precise position control is generally difficult to achieve during CVD graphene growth due to the highly diffusive nature of the gaseous precursors. © 2016 American Chemical Society

Received: January 30, 2016 Revised: April 30, 2016 Published: May 3, 2016 3360

DOI: 10.1021/acs.chemmater.6b00426 Chem. Mater. 2016, 28, 3360−3366

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Chemistry of Materials

Figure 1. (a) Schematic illustration of the coalescence process of electrochemically exfoliated graphene (EEG) flakes into a continuous graphene pattern on copper foil and SEM image of a patterned letter “G” synthesized on copper foil. Scale bar, 200 μm. (b) SEM images of well dispersive EEG flakes (i) and EEG-derived graphene (ii) on copper foil. Scale bars, 20 μm. (c) TEM image of EEG-derived graphene. The inset shows the SAED pattern obtained from the EEG-derived graphene. Scale bar, 5 nm. (d) Representative Raman spectra of EEG flakes and EEG-derived graphene, respectively. (e) Typical transmittance spectrum of EEG-derived monolayer graphene on quartz. The inset shows transmittance versus sheet resistance of EEG-derived graphene. °C for 1 h in a tube furnace under H2 (8 sccm) and Ar (20 sccm) flow at 1.3 × 10−1 Torr. Stencil Printing of the EEG Flakes. A copper foil covered with a hollow patterned mask was placed underneath water. Then, EEG flakes (1 mg/mL) were injected onto the water surface, which spontaneously formed a uniform film.37 As the copper foil was lifted up, the EEG flakes covered the exposed area of the copper foil. The mask was removed after the sample was dried at 120 °C for 3 min. Transfer of EEG-Derived Graphene. PMMA (2%) was spincoated on the copper foil with EEG-derived graphene. The sample was placed in 0.5 mol/L FeCl3 for copper etching. The floating PMMA/ graphene layer was thoroughly rinsed with water and then transferred to a desired substrate. PMMA was removed by acetone. Characterization. SEM was taken by a Hitachi S4800. A Raman spectrum was obtained with a micro-Raman spectrometer (Renishaw at 514.5 nm laser). Tecnai G2 F20 U-TWIN was used for HRTEM. XPS was collected in ESCALAB 250Xi.

through the release of gaseous CO, CO2, and H2O in hydrogen atmosphere,30 which leaves behind active polycyclic carbon species on the copper surface. Subsequently, surface-mediated migration and coalescence of the carbon species lead to the formation of a continuous graphene film. Due to the localized supply and self-seeding effect of the carbon species, spatially confined graphene growth is achieved in our study, which is distinctly different from the uncontrolled growth based on the gaseous decomposition of other solid carbon sources. The newborn graphene shows high optical transmittance and electrical conductivity, highlighting its potential as transparent electrodes. In addition, EEG flakes offer the advantage of being solution processable, so large-scale EEG patterns can be readily prepared by a cheap and fast printing technique such as stencil printing31 or inkjet printing.32−36 In this way, transparent and conductive graphene circuits have been directly synthesized over large areas.

3. RESULTS AND DISCUSSION In the electrochemical exfoliation process, the composition of electrolyte for electrochemical exfoliation plays a key role to control the aspect ratio of EEG flakes. For example, a lower concentration of KOH leads to thinner but smaller flakes.38 In our experiment, 30% KOH was used to obtain EEG flakes with a layer number from 2 to 4 and lateral size from 3 to 7 μm. More details about EEG flakes preparation were given in the Experimental Section. Figure 1a shows a schematic for the direct growth of graphene patterns from EEG flakes. First, EEG flakes are printed on a rinsed copper foil.37 The samples are then thermally annealed at 1000 °C for 1 h in a tube furnace under hydrogen and argon flow. Accordingly, a patterned letter “G” has been directly synthesized on the copper foil as presented. The scanning electron microscopy (SEM) image in Figure 1b(i) illustrates the EEG flakes well-dispersed on the copper surface before annealing. The lateral size distribution of

2. EXPERIMENTAL SECTION Electrochemical Exfoliation Process. A natural graphite rod was used as the anode and a platinum wire as the cathode.27,28 Both were placed in an electrolyte solution consisting of a mixture of H2SO4 and KOH (2.4 g of H2SO4 and 11 mL of 30% KOH in 100 mL of deionized water).The spacing between two electrodes is 3 cm. A static bias of 1 V was initially applied to the electrochemical system for 5 to 10 min, followed by increasing the static bias to 10 V for 1 min. After the electrochemical exfoliation, the EEG flakes were collected by a filter paper and thoroughly washed with deioned (DI) water to remove the residual electrolyte. The EEG flakes were then redispersed in ethanol with mild bath sonication for 30 s. Synthesis of EEG-Derived Graphene. A 2 cm × 3 cm copper foil (99.8%, 25 μm, Alfa Aesar) was rinsed sequentially with acetone, ethanol, acetic acid, and DI water for 10 min, respectively. The treated copper foil was printed with EEG flakes37 and dried at 120 °C for 3 min to evaporate ethanol and water. The sample was annealed at 1000 3361


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Figure 2. SEM images of EEG flakes on copper foils after annealing at 800 °C (a), 900 °C (b), and 1000 °C (c) for 1 h, respectively. Scale bars, 10 μm. (d) Corresponding Raman spectra of EEG flakes and EEG-derived graphene samples that were synthesized at 800, 900, and 1000 °C for 1 h. (e) Corresponding ID/IG and I2D/IG ratios as a function of growth temperature.

the EEG flakes is mainly between 3 and 7 μm. The typical thickness of the EEG flakes is around 2−3 nm, as determined by atomic force microscopy (AFM) (Figure S1a). Transmission electron microscopy (TEM) characterization further reveals that the EEG flakes consist mostly of 2 to 4 layers (Figure S1b). After annealing at 1000 °C, the disconnected EEG flakes convert into a continuous graphene film, as shown in Figure 1b(ii). TEM characterization of the EEG-derived graphene film is summarized in Figure 1c, which indicates that the EEGderived graphene is of monolayer thickness. The selected area electron diffraction (SAED) pattern displays the typical hexagonal crystalline structure of monolayer graphene. The material quality of EEG-derived graphene is further characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. XPS results (Figure S2) show that the C 1s peak of the original EEG flakes can be fitted by four components at 284.5 eV (CC), 286.2 eV (C−O), 287.3 eV (CO), and 288.7 eV (COOH), respectively.39 The oxygencontaining groups in the EEG flakes arise from the oxidization of graphite by acid during the electrochemical exfoliation process. In contrast, the (C−O), (CO), and (COOH) peaks in the EEG-derived graphene are dramatically decreased due to the removal of the oxygen-containing groups. In addition, the Raman spectrum of the EEG-derived graphene in Figure 1d reveals a greatly enhanced 2D band that can be fitted by a single Lorentzian peak. Furthermore, the intensity ratio of the 2D band and G band (I2D/IG) increases to ∼2.6 for the EEGderived graphene, indicating that the EEG-derived graphene is predominantly a monolayer.40 In addition, the D band intensity is very weak, revealing a low density of defects in the EEG-

derived graphene. We further characterized the electrical conductivity and optical transmittance of the EEG-derived graphene. As shown in Figure 1e and Table S1, EEG-derived graphene exhibits high transmittance of 94.7% at 550 nm and low sheet resistance of 4.5 kΩ/sq. We note that the thickness of the EEG-derived graphene depends on the surface density of the original EEG flakes on copper foils (Figure S3). Large-area graphene films with controllable thickness from 2 to 9 nm have been grown by varying the initial concentration of the EEG flakes solutions from 1 to 8 mg/mL. As a result, the electronic and optical properties of the EEG-derived graphene can be easily tuned by changing its thickness (Figures 1e and S3), which makes it a versatile transparent electrode material of interest. We systematically investigated the effects of the annealing temperature on the growth of the EEG-derived graphene monolayer. Figure 2 summarizes the evolution of the EEG flake surface morphology at different annealing temperatures for a fixed growth time of 1 h. Below 800 °C, no graphene growth can be observed on the copper surface (Figure 2a). The EEG flakes are morphologically indistinguishable from those at room temperature, indicating the thermal stability of the EEG flakes. However, as the annealing temperature is increased to 900 °C, active carbon species are formed through the dissociation of the EEG flakes, and newborn graphene starts to appear surrounding the EEG flakes (Figure 2b). As the annealing temperature is further increased to 1000 °C, the surface density of the active carbon species increases, and disconnected EEG flakes are converted into a continuous, monolayer graphene film (Figure 2c). 3362


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Figure 3. (a) Two rinsed bare copper foils were placed upstream and downstream of a EEG-coated copper foil and annealed at 1000 °C for 1 h. (b) Corresponding Raman spectra of middle, upstream, and downstream copper foils after annealing. (c) SEM images of the same EEG flake before (i) and after (ii) annealing at 1000 °C for 10 min. Scale bars, 4 μm.

Figure 4. (a) Schematic drawing of the growth mechanism of EEG-derived graphene on the copper surface. (b) The affinity energies of carbon species C13H9, C16H10, C19H11, and C24H12 on Cu (111) surface.

The transformation from the EEG flakes to graphene was monitored with Raman spectroscopy measurements. It has been well accepted that the intensity ratio of the D band to G band (ID/IG) is associated with the density of defects in graphene. As shown in Figure 2d,e, no obvious change of ID/IG was observed for the EEG flakes after annealing at 800 °C. After annealing at 900 °C, I2D/IG of the EEG flakes was increased from 0.21 to 0.60, indicating the formation of graphene structure. When the annealing temperature was further increased to 1000 °C, the corresponding Raman spectrum exhibits the typical features of monolayer graphene. The low ID/IG ∼ 0.11 and high I2D/IG ∼ 2.6 are consistent with the high crystalline quality of monolayer graphene. We find that the formation mechanism of graphene from EEG flakes is distinctively different from other solid carbon sources such as PMMA and a-C. For example, Ji et al.23 used aC sputtered on the copper foil to grow graphene. In their study, graphene was also formed on two bare copper foils placed upstream and downstream of the a-C-coated copper foil. Similar results have also been reported by Kwak et al.22 when PMMA was used as the solid carbon source. The underlying growth mechanism has been attributed to the formation of gaseous hydrocarbon through the reaction of a-C or PMMA with hydrogen at high temperature. The gaseous hydrocarbon could then be transported several millimeters downstream and upstream to grow graphene on the bare copper foils. As a result, the graphene growth was not confined to the area originally covered by the a-C or PMMA. For comparison, similar experiments were carried out when EEG flakes were used as the

solid carbon source. As shown in Figure 3a, a copper foil coated with EEG flakes was placed in the middle of a furnace tube, and two bare copper foils were placed upstream and downstream of the EEG-coated copper foil. The gap from the bare copper foils to the EEG-coated copper foil is about several millimeters. After annealing at 1000 °C, graphene was only formed on the EEG-coated copper foil (Figure S4). Raman spectra further confirm the absence of graphene on the upstream and downstream copper foils (Figure 3b). This result indicates that the thermal annealing of EEG flakes produce little gaseous hydrocarbon. Instead, the activate carbon species from the decomposition of the EEG flakes remain attached to the original copper surface at the growth temperature. To investigate the growth mechanism of graphene from the EEG flakes, the reaction was interrupted after 10 min annealing at 1000 °C. Figure 3c illustrates the SEM images of the same EEG flakes before and after annealing, highlighting two important features. First, EEG-derived graphene was spatially confined around the original EEG flakes. In addition, the growing front of newborn graphene closely follows the shape of the EEG flakes at the initial growth stage (Figures S5 and S6). These results suggest that the original EEG flakes act as both the nucleus and precursor, and the active carbon species are captured once they diffuse to the edges of the EEG flakes. Second, the area of the original EEG flakes is 56 μm2 (Figure 3c(i)), whereas the area of the EEG-derived graphene has increased to 116 μm2 after 10 min annealing (Figure 3c(ii)). The doubled area of the EEG-derived graphene is consistent 3363


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Chemistry of Materials with transformation from bilayer or trilayer EEG flakes to monolayer graphene. On the basis of the experimental results, we propose a possible growth mechanism of graphene from the EEG flakes on the copper surface, as shown in Figure 4a. At stage I, the oxygen-containing groups in EEG flakes are removed through the release of gaseous CO, CO2, and H2O in the hydrogen atmosphere at elevated temperature,30 which generates holes and vacancies in EEG flakes. We note that the decomposition of the EEG flakes probably have started below 800 °C, as indicated by the enhanced 2D Raman band in Figure 2d. At stage II, due to the rapidly increased density of holes and vacancies at high temperature, the EEG flakes are decomposed into active carbon species consisting of fused hexagonal rings. At stage III, residual EEG flakes act as preplaced “seeds”, and the active carbon species diffuse on the copper surface until they are captured at the EEG flake edges.41 Therefore, when the growth temperature is below 1000 °C, there is insufficient thermal energy for thorough removal of oxygen-containing groups and dissociation of active carbon species from the EEG flakes. High growth temperature augments the amount and activity of active carbon species and enhances the quality of newborn graphene, which is in accordance with Raman results in Figure 2d. First-principles calculations were performed to investigate the affinity energies of active carbon species on copper. We chose four carbon species (C13H9, C16H10, C19H11, and C24H12) as examples to investigate with van der Waals density functionals (vdW-DF), and the affinity energy Ea was defined in eq 1:

Figure 5. (a) Schematic illustration of the preparation of the EEGderived graphene circuit using stencil printing. (b) Photo image of the patterned EEG circuit printed on copper foil. (c) EEG-derived graphene circuit transferred onto PDMS substrate.

Ea = E[Cu] + E[carbon species] − E[carbon species_Cu] (1)

where E [Cu] and E [carbon species] are the energies of the Cu (111) substrate and carbon species, respectively, and E [carbon species_Cu] is the total energy of carbon species on the Cu (111) surface. The optimized geometries and corresponding affinity energies are summarized in Figure 4b. The affinity energies were more than 2 eV, which effectively prevent their desorption from the surface and are in good consistence with our experimental results and previous work.41 Spatially confined growth of the EEG-derived graphene allows the direct synthesis of patterned graphene. We note that the line width of EEG-derived graphene depends on the lateral size of the initial EEG flakes, mask dimension, and the growth condition. In our study, the minimum line width of graphene patterns is ca. 50 μm (Figure S7). As an example, large-area, patterned EEG flakes were printed on copper foil with a stencil printing technique (Figure 5a). The samples were then annealed at 1000 °C to grow graphene. Figure 5b,c shows large-area EEG flakes patterns that have been printed on the copper foil and corresponding EEG-derived graphene that has been transferred onto a transparent polydimethylsiloxane (PDMS) substrate. Compared with a conventional lithographic process, our approach is suitable to fabricate large-area, patterned graphene with high-throughput and low cost. Owing to the high optical transmittance and conductivity, EEG-derived graphene can be applied as transparent and flexible electrodes. We thus further characterized their electrical behavior under mechanical bending. As shown in Figure 6a and Movie S1, the resistance of the EEG-derived graphene electrode shows negligible change up to a bending radius of

Figure 6. (a) Normalized resistance variation of graphene electrode with increasing bending curvature during the mechanical test. The inset shows a photograph of the bending graphene electrode on polyethylene terephthalate (PET). (b) Comparison of sheet resistance of graphene patterns prepared by printing techniques. The red spots were our samples while the black ones were summarized from the literature (See Table S2).

6.4 mm, illustrating the high mechanical stability of the EEGderived graphene. Former studies have applied graphene oxide (GO) and reduced graphene oxide (rGO) based inks for the scalable, lowcost fabrication of transparent, flexible electrodes. However, the 3364


Chemistry of Materials GO or rGO flakes in the electrodes are merely physically stacked together, and the interfaces between flakes act as tunneling barriers. In addition, GO and rGO have relatively high defect density. As a result, the thickness of the GO or rGO films has to be increased to achieve high electrical conductivity, which degrades the optical transmittance of the electrodes. Our EEG-derived graphene, on the other hand, is a continuous film with low defect density, offering simultaneously high transmittance and conductivity (Figure S8a). Figure 6b and Table S2 compare the thickness-dependent conductivity of the EEGderived graphene electrodes with printed electrodes based on GO and rGO, which shows that the EEG-derived graphene electrodes exhibit more than 1 order of magnitude higher conductivity than GO or rGO electrodes of the same thickness. Patterned EEG-derived graphene was further applied as a switch in the LED circuit (Figure S8b).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00426. Additional characterizations of EEG flakes and EEGderived graphene, demonstrations of EEG-derived graphene electrodes in LED circuits, and a brief summary of printing graphene based electronics. (PDF) Resistance of the EEG-derived graphene electrode showing negligible change up to a bending radius of 6.4 mm. (AVI)

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CVD, chemical vapor deposition; PMMA, poly(methyl methacrylate); a-C, amorphous carbon; EEG, electrochemical exfoliated graphene; rGO, reduced graphene oxide; SAED, selected area electron diffraction; XPS, X-ray photoelectron spectroscopy; vdW-DF, van der Waals density functionals; PET, polyethylene terephthalate; PDMS, polydimethylsiloxane; LED, light emitting diode

4. CONCLUSIONS In summary, we found a novel growth mechanism of graphene based on the recrystallization and coalescence of EEG flakes. Combined experimental results and theoretical simulations suggest that polycyclic carbon species from the decomposition of the EEG flakes, instead of gaseous hydrocarbons, yield graphene on copper foil at high annealing temperature. The unique solid-phase process allows the direct growth of graphene patterns from prepatterned EEG flakes. The ability to directly synthesize graphene patterns over large areas opens up new opportunities for various applications, ranging from devices to transparent electrodes.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.L.). *E-mail: [email protected] (Y.F.). Present Address ¶

X.L.: Department of Electronic Engineering, The Chinese University of Hong Kong, Hong Kong SAR, China.

Author Contributions ‡

L.L., X.L., and M.D. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51402060, 21322302) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB02050008). 3365


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