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. 2017 Oct 9;2(10):6492–6499. doi: 10.1021/acsomega.7b01057

Efficient Graphene Production by Combined Bipolar Electrochemical Intercalation and High-Shear Exfoliation

Emil Tveden Bjerglund †, Michael Ellevang Pagh Kristensen †,⊥, Samantha Stambula ‡, Gianluigi A Botton ‡,§,∥, Steen Uttrup Pedersen †,*, Kim Daasbjerg †,*
PMCID: PMC6645314  PMID: 31457250

Abstract

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In this study, we demonstrate that bipolar electrochemistry is a viable strategy for “wireless” electrochemical intercalation of graphite flakes and further large-scale production of high-quality graphene suspensions. Expansion of the graphite layers leads to a dramatic 20-fold increase in the yield of high-shear exfoliation. Large graphite flakes, which do not produce graphene upon high shear if left untreated, are exfoliated in a yield of 16.0 ± 0.2%. Successful graphene production was confirmed by Raman spectroscopy and scanning transmission electron microscopy, showing that the graphene flakes are 0.4–1.5 μm in size with the majority of flakes consisting of 4–6 graphene layers. Moreover, a low intensity of the D peak relative to the G peak as expressed by the ID/IG ratio in Raman spectroscopy along with high-resolution transmission electron microscopy images reveals that the graphene sheets are essentially undamaged by the electrochemical intercalation. Some impurities reside on the graphene after the electrochemical treatment, presumably because of oxidative polymerization of the solvent, as suggested by electron energy loss spectroscopy and X-ray photoelectron spectroscopy. In general, the bipolar electrochemical exfoliation method provides a pathway for intercalation on a wider range of graphite substrates and enhances the efficiency of the exfoliation. This method could potentially be combined with simultaneous electrochemical functionalization to provide graphene specifically designed for a given composite on a larger scale.

Introduction

Following the first micromechanical cleavage of graphene from graphite,1 applications of this two-dimensional (2D) material have been proposed within diverse areas such as polymer composites,2,3 energy storage,4 and nanoelectronics.5,6 All these applications aim to utilize the exceptional mechanical7 and electronic1,8 properties of graphene. Development of economic and high-yield bulk graphene production methods have become crucial to enable the further progress toward these applications.9

Currently, several strategies exist for large-scale top-down production of graphene from graphite, including chemical exfoliation,10 electrochemical exfoliation,11−14 and liquid-phase exfoliation using ultrasound15 or high shear.16 The high-shear exfoliation method developed by Paton et al. produces oxygen-free graphene of good quality.16 Unfortunately, the yield is low (<1%), and the procedure would become slightly cumbersome in a scaled-up version, where the requirement of graphite removal and recycling between exfoliation steps would be necessary to produce the material more efficiently. Recently, electrochemical exfoliation has attracted substantial interest as a method for the production of high-quality graphene in bulk quantities. Exfoliation was demonstrated in both aqueous5,6,12 and nonaqueous11,14 solvents under either anodic or cathodic conditions. Both positive and negative ions of varying sizes have already been used for graphite intercalation,13,17,18 making it possible to fine-tune the properties of the intercalated product. A limitation of electrochemical exfoliation pertains to the need for electrical contact with graphite to drive the intercalation process.5,6,14 Attempts to remedy this include placing the working electrode at the bottom of the electrochemical cell11 or using liquid NaK alloys as an electron source.19 A versatile and easily scaled-up bulk electrochemical method using graphite flakes or powder as the source material would be highly desirable.

In this respect, the “wireless” bipolar electrochemistry becomes particularly interesting.20,21 By polarizing an electrolyte containing a conducting substrate (the bipolar electrode, BE) in an electric field between two feeder electrodes (FEs), simultaneous electrochemical oxidation and reduction can be driven at the extremities of the BE (Figure 1a).22 Previous examples of this technique include modification of metal substrates,23,24 functionalization of carbon substrates such as glassy carbon,24,25 carbon nanotubes,26−28 graphite,29 and graphene,30,31 and exfoliation of 2D materials such as WS232,33 and black phosphorus.34

Figure 1.

Figure 1

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(a) Illustration of the bipolar electrochemical cell with corresponding cathodic (−) and anodic (+) poles at the stainless steel FEs and graphite BEs. Photographs of the (b) pristine graphite foil and (c) Bu4N+-intercalated graphite foil. SEM images of the edge of the (d) pristine graphite foil and (e,f) Bu4N+-intercalated graphite foil. (g) Average Raman spectra showing the D and G peaks of the pristine and Bu4N+-intercalated graphite foil.

In this paper, we propose a new method for the bulk production of pristine graphene flakes, in which high-shear exfoliation is combined with bipolar electrochemistry. We demonstrate that with bipolar electrochemistry an efficient graphite intercalation process can be accomplished to significantly increase the exfoliation efficiency upon high-shear exfoliation (yield = 16.0%). Our method bypasses the need for an electrical contact to the graphite source material and may thus be applied to graphite powder or flakes in a simple setup. This demonstrates a true bulk process for high-yield production of graphene, in a manner that should be suitable to scale-up. Future work on this method should allow for scalable production of functionalized graphene sheets by combining the production with a direct functionalization step using diazonium salts30 or a simultaneous functionalization and exfoliation.35,36

Results and Discussion

First, to demonstrate that bipolar electrochemistry is a viable method for electrochemical graphite intercalation, the setup shown in Figure 1a was used to intercalate tetrabutylammonium tetrafluoroborate (Bu4NBF4) into a 1 cm piece of graphite foil (Figure 1b) in N-methyl-2-pyrrolidone (NMP). Other salts could also have been used for graphite intercalation.13,17,18Figure 1c shows a significant expansion of the cathodic end (left side) of the graphite foil. The expansion is expected to occur from the intercalation of Bu4N+ at the cathodic end (left side) of the electrode with the oxidation of NMP taking place at the anodic end.

Figure 1d shows the scanning electron microscopy (SEM) image of the edge of the pristine graphite foil, whereas Figure 1e,f illustrates the expansion of the layers as a result of the intercalation of Bu4N+ at the cathodic end of the foil.18 A clear separation of the graphite layers is observed. The graphite was also analyzed using Raman spectroscopy (Figure 1g). The intercalated edge exhibits an additional G peak at 1596 cm–1, which is commonly observed for intercalated graphite, because of the different environment of the graphite layers situated next to intercalated ions rather than neighboring graphite layers.37,38 The D peak is very small in the pristine graphite and does not change with intercalation, showing that this process does not damage the graphene sheets. Furthermore, this rules out the possibility that the shoulder on the G peak in the intercalated graphite could be a defect-related D′ peak.39,40

The theoretical potential difference between the anodic and cathodic ends of the BE can be calculated from eq 1.

graphic file with name ao-2017-01057n_m001.jpg 1

In this expression, ΔE is the voltage between the FEs, L is the distance between the FEs, ΔV is the potential difference across the BE, and l is the length of the BE.30,41 A ΔV of 14.3 V can be achieved in our experimental setup, considering ΔE = 50 V, L = 3.5 cm, and l = 1 cm. As observed in Figure 1, this voltage was sufficient to drive a cathodic intercalation and an anodic oxidation of NMP for the graphite foil BE.

For the purpose of graphene production from flakes or powders, it is reasonable to assume that a similar ΔV can be utilized for exfoliation; however, adjustments will be necessary due to the decrease in the BE size. A minimum ΔE of 950 V can be expected for flakes of ∼150 μm in size, if the FEs are positioned closer together with a distance L = 1 cm. Previous studies have shown that bipolar electrochemistry using high voltages is possible if the temperature and conductivity of the electrolyte are carefully controlled.25,28L values smaller than 1 cm were tested but led to repeated short circuit of the FEs.

A two-step procedure was used for graphene production: (1) intercalation of Bu4N+ via bipolar electrochemistry (Scheme 1a) and (2) high-shear exfoliation of intercalated graphite to produce graphene (Scheme 1b). These two steps can be performed consecutively and require no midway workup. Briefly, 1 mM Bu4NBF4 in NMP was mixed with graphite flakes (1 mg mL–1). Two stainless steel FEs set 1 cm apart were immersed in the solution (Scheme 1a). A voltage of 1100 V was applied between the FEs for 1 h, while vigorously stirring the solution. The graphene was then exfoliated for 1 h at a shear rate of 33 000 s–1 (Scheme 1b). Finally, the solution was centrifuged to remove large flakes of residual graphite, and the supernatant was collected.

Scheme 1. Illustration of the Experimental Setups for (a) Graphite Intercalation Using High-Voltage Bipolar Electrochemistry and (b) High-Shear Exfoliation of Intercalated Graphite To Produce Graphene Flakes.

Scheme 1

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Figure 2 shows (a) the graphene solution produced by our method compared to control experiments where (b) no electrochemistry was applied or (c) no Bu4NBF4 was added prior to electrochemistry and high-shear exfoliation. The yield of graphene in each suspension was measured by filtration of a known volume on poly(tetrafluoroethylene) (PTFE) membrane filters, and the results are reported in Table 1.

Figure 2.

Figure 2

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Solutions of graphene in NMP, where graphene was produced by the (a) combined bipolar intercalation and high-shear exfoliation, (b) high-shear exfoliation with no electrochemical preactivation, and (c) combined bipolar electrochemistry and high-shear exfoliation without using Bu4NBF4 in the electrochemical procedure.

Table 1. Concentrations and Yields of Graphene Using the Combined Bipolar Intercalation and High-Shear Exfoliation Procedure.

sample concentration (μg mL–1) crude yield (%)a corrected yield (%)b
graphene 211.0 ± 3.4 21.0 ± 0.3 16.0 ± 0.2
graphenec 8.4 ± 1.1 0.8 ± 0.1  
graphened 5.3 ± 0.7 0.5 ± 0.1  
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a

Inline graphic, where m denotes mass and V denotes volume.

b

See text.

c

High-shear exfoliation with no electrochemical preactivation.

d

Combined bipolar electrochemistry and high-shear exfoliation without Bu4NBF4.

An application of extreme electrochemical conditions for graphene production has the potential to generate organic residues from side reactions at the FEs, which would add to the overall yield. To assess this and evaluate the quality of the graphene produced, a number of characterization techniques such as Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), high-resolution transmission electron microscopy (HR-TEM), and scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) were employed. Figure 3 shows the TGA of the graphene sample from 30 to 1100 °C at a heating rate of 10 °C min–1. Foremost, it reveals a mass loss at 600 °C, which potentially could originate from the decomposition of organic NMP residues on graphene.42 This is supported by performing XPS analysis of the samples before and after TGA, which shows an increase in the C/N ratio from 66 to 393, respectively. The total mass retained after the loss at 600 °C is 75.9%. A detailed analysis of the atomic ratios (see the Supporting Information) concludes that 84.5% of the collected mass is from graphene, which matches the results from TGA. From this, we estimate a corrected graphene yield of 16.0 ± 0.2%, as reported in Table 1.

Figure 3.

Figure 3

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TGA of the graphene produced by the combined bipolar intercalation and high-shear exfoliation showing the (a) mass loss as the temperature is increased and (b) first derivative of the mass loss with respect to temperature.

Obtaining a yield of 16.0% is a remarkable improvement compared with our control experiments and other studies on bulk production of graphene. Previous studies have shown yields of <0.1% for high-shear exfoliation16 and ∼1% for ultrasonication of graphite,15 with possibilities of recycling the remaining graphite to obtain a higher yield. The combined bipolar electrochemical intercalation and high-shear exfoliation thus provides a significant improvement without even considering recycling residual graphite.

Figure 4 illustrates a representative Raman spectrum, recorded from the exfoliated graphene by the combined bipolar intercalation and high-shear exfoliation deposited on SiO2. Raman spectroscopy is a strong tool in the quality assessment of graphene because a qualitative description of the defect density and the number of layers is easily derived. The spectrum displays the three characteristic peaks that would be expected from graphene flakes, that is, the D, G, and 2D peaks as well as the minor D + D′, 2D′, and D + D″ peaks.40 These peaks are all commonly fitted to Lorentzian functions. However, to make a good fit, it was necessary to add an additional Gaussian peak at the base of the G peak. We hypothesize that this could be from amorphous carbon (am-C) in the form of an NMP residue from the electrochemical procedure.43

Figure 4.

Figure 4

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Representative Raman spectrum of the produced graphene by the combined bipolar intercalation and high-shear exfoliation.

hrough examination of the intensity, I, from the Raman spectrum, we find ID/IG = 0.24 ± 0.10 and I2D/IG = 0.53 ± 0.08, and for full width at half maximum (Γ) of the 2D peak, Γ2D = 69.8 ± 5.4 cm–1. On the basis of the 2D peak shape,16 we deduce that the majority of flakes contain 4–6 graphene layers (see the Supporting Information). These results are similar to the work of Paton et al.16 wherein they showed that the D peak in this type of graphene was related to the flake size and mainly caused by edge defects. Graphene with comparable or slightly lower ID/IG values than the graphene produced in this paper was demonstrated in recent works of Cooper et al.,14 Yang et al.,12 and Ossonon and Bélanger.35 The lower ratios in these works is most likely due to a larger average flake size. Importantly, in those cases, a direct electrochemical intercalation procedure was employed in contrast to the more versatile wireless procedure used in this work.

Figure 5 shows the size distribution of graphene flakes (maximum length) measured from STEM images containing 412 flakes (see the Supporting Information). As seen, it approximately follows a log–normal distribution, as has previously been described for fragmentation processes in 2D materials.44 The mean flake size is 0.42 μm, which is smaller than the graphene flakes of 1–5 μm described by Yang et al.12 and Ossonon and Bélanger.35 The graphene flakes produced in this work are probably smaller because of the high shear forces employed, cutting the sheets apart as they are exfoliated.

Figure 5.

Figure 5

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Size distribution of the graphene flakes produced by the combined bipolar intercalation and high-shear exfoliation with a fitted log–normal distribution (blue curve) and mean size [red error bar, 0.42 μm, 95% confidence interval = (0.38, 0.46) μm]45 based on 412 flakes.

The electrical properties of graphene films deposited on a nylon membrane were examined. The films are found to be electrically conducting, as would be expected from the low ID/IG in Raman spectroscopy. However, a large internal resistance is measured which we ascribe to poor electrical contact between individual flakes (see the Supporting Information for details). In an attempt to resolve this matter, we are performing ongoing work using bipolar electrochemistry to connect individual graphene flakes with silver nanowires.

Figure 6 shows further structural examination of the graphene flakes using monochromated HR-TEM. A low-magnification image was acquired on a 1.5 μm flake with an overall appearance of a thin sheet with many surface impurities, which were later examined with EELS. Upon observation of exfoliated flakes at a higher magnification, Figure 6b reveals that the impurities consist of amorphous residues, but more importantly, clean graphene areas are easily identifiable. Furthermore, Figure 6c illustrates the rough structure of the graphene edge, potentially originating from damage from the production method, chemical functionalities located along the graphene edge, and/or electron-induced beam damage. The insets in Figure 6 show the Fourier transforms acquired from different areas containing the graphene sheets.46,47

Figure 6.

Figure 6

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(a) TEM image showing the general appearance of the exfoliated graphene flakes and (b,c) HR-TEM images of different areas in the graphene flake with the corresponding Fourier transforms (insets).

Figure 7 displays a STEM-EELS analysis (using HyperSpy 1.1)48 based on a spectrum image of the graphene flake segment shown in Figure 7b, which is used to describe the chemical impurities present on the surface of the graphene sheets in greater detail. Ordinary elemental maps and extracted spectra from EELS are available in the Supporting Information. To eliminate noise in the data set, we performed principal component analysis (PCA), in which the first seven PCs were used based on the scree plot (see the Supporting Information). These seven components explain 97.8% of the variance in the data set. Simply utilizing the raw EELS spectra to map the location of the impurities in this data set is particularly difficult owing to the overlap of the C species originating from the crystalline C in the graphene flakes and the am-C residuals from the solvent. Furthermore, other edge overlaps were observed between C and Ca, O and Cr, and F and Fe. However, by performing independent component analysis (ICA) in HyperSpy,48 it was possible to separate the different impurity sources. We found that using five independent components (ICs) resulted in clearly separated component spectra that were also physically reasonable.

Figure 7.

Figure 7

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ICA of an EELS map from a graphene flake with the (a) independent component spectra IC1–5, (b) ADF image of a graphene flake, where the green rectangle highlights the area analyzed with EELS, and (c–g) spatial loading maps of IC1–5. All scale bars are 40 nm.

Figure 7a displays each component along with the energies of the edges for each element found in the sample. Figure 7c–g shows the corresponding loading maps for each IC. It should be noted that while principle components (PCs) are ranked by significance, ICs are randomly ordered, and thus, IC1 is not more significant in describing the sample than the other ICs.49 The graphene signal is captured in IC5 where the C-K edge displays sharp π* and σ* peaks.50 The corresponding loading map (Figure 7g) displays the presence of graphene covering the entire map, as the sheet is intact with the possible overlap of two different layers, displayed by the sharp boundary in the lower right part of the image.

An am-C signal on graphene is evident in IC2 and IC3,50 which is observed as clusters on the sample. IC3 clearly displays an O-K edge and a small N-K edge. Because the source of am-C is most likely originating from oxidized NMP during the bipolar electrochemistry, the simultaneous presence of C, O, and N is to be expected in IC3. A small amount of Fe is also found in IC3, which may be due to the spectral overlap with IC1. Both am-C and O signals are found in IC2. A literature investigation was completed for the potential compounds within our material to try to identify the spectral fingerprint of each molecule (NMP and Bu4N+), but the noise and uncertainty in the decomposition do not allow for an unambiguous assignment of the molecules.51−53

Upon further examination of the ICs, a Ca impurity, along with a small amount of F, is captured in IC4, which is probably a remnant of the CaCl2 used in the workup procedure. If a small quantity of CaF2 was present in the CaCl2, this could be the potential source of F on the graphene sheets as CaF2 is insoluble in water. Finally, IC1 shows the presence primarily of iron and chromium particles and trace amounts of C and O, indicating that the stainless steel FEs are, as expected, not entirely stable at the 1100 V applied during the bipolar electrochemistry.

In Table 2, the quantification of elements in the graphene sample using XPS is reported. In general, they agree with the EELS analysis, although the finding of 1.5 at % nitrogen is noteworthy. The nitrogen content was not clearly revealed by all EELS spectra; however, other EELS studies on organic materials similar to NMP have also been unsuccessful in detecting nitrogen.54 The fact that the analysis spot area in XPS is significantly larger makes the detection of low-concentration elements easier using this technique. The potential nitrogen sources are organic polymeric residues formed from the oxidation of NMP and/or remains of Bu4N+ ions intercalated between the layers. As already mentioned, the am-C detected with EELS and the 24% material loss observed in TGA until 600° would be consistent with the presence of such residues. The clear increase in the C/N ratio after TGA supports this statement.

Table 2. XPS Quantification of Elements in the Graphene Sample Deposited on SiO2 and Graphene Powder after TGA.

  C (%) O (%) N (%) Fe (%) Cr (%) other (%)a C/N
grapheneb 96.6 ± 1.2 0.2 ± 1.1 1.5 ± 0.1 0.3 ± 0.0 0.2 ± 0.0 1.2 66
after TGAc 88.3 ± 0.6 7.1 ± 0.9 0.2 ± 0.1 0.5 ± 0.0 0.2 ± 0.0 3.6 393
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a

Contains Ca, F, and Cl.

b

Graphene deposited on SiO2. The amount of oxygen was corrected for SiO2 as Ocorr. = Omeas. – 2 × Simeas.

c

Graphene powder after TGA up to 1100 °C.

Figure 8 displays the high-resolution C 1s XPS spectrum to give information on the chemical state. From the deconvolution of the peak, the major contribution is found to be C=C (sp2) from graphene at 284.5 eV, with additional peaks from C–C (sp3) at 285.1 eV, C–N at 286.0 eV, and O=C–N at 288.0 eV. Overall, this corresponds well to graphene with impurities consisting mainly of NMP residues.16,55

Figure 8.

Figure 8

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High-resolution XPS spectrum of the C 1s peak from graphene.

Conclusions

In conclusion, we have developed an electrochemical pretreatment for the exfoliation of graphene from graphite that increases the yield of high-shear production of graphene substantially to as much as 16.0 ± 0.2%. This method exploits the wireless capabilities of bipolar electrochemistry as a pretreatment of the graphite to intercalate and expand the graphite structure. The graphene produced by this method is structurally intact. Chemical impurities are present to some extent and, depending on the application, additional cleaning of graphene might be necessary. Future experiments with different solvents, intercalants, and using membranes to separate FEs and BEs could provide optimized systems that help remedy the impurities. The wireless nature of the method makes it versatile, and thus, it can be applied to other graphite sources or other conducting materials in a scalable manner.

Experimental Section

Graphite Intercalation Using Bipolar Electrochemistry

A 1 cm piece of graphite foil was placed between two stainless steel FEs (6 cm × 1.5 cm) 3.5 cm apart in a solution of 0.1 M Bu4NBF4 in NMP. A voltage of 50 V was applied between the FEs for 2 min. The sample was rinsed with NMP and high-performance liquid chromatography (HPLC) acetone before analysis.

Graphene Production with Bipolar Electrochemistry and High-Shear Exfoliation

Bu4NBF4 (100 mL, 1 mM) was mixed with 100 mg of graphite flakes in a large beaker. The solution was placed in an ethylene glycol/dry ice cooling bath and allowed to cool to −10 °C. A magnetic stir bar was added, and two freshly polished stainless steel electrodes (6 cm × 1.5 cm) were mounted 1 cm apart. The electrodes reaching ∼1 cm into the solution were connected to the power supply. Using a Heinzinger PNC 6000-300 power supply, 1100 V was applied for 1 h under vigorous stirring, while monitoring the temperature. The voltage was turned off if the temperature raised above 50 °C, and the system was then allowed to cool to −10 °C before again applying the voltage. The voltage, current, and solution temperature were monitored while applying a voltage (see Figure S2). After 1 h of applied voltage, the solution turned black. Following bipolar electrochemical intercalation, the cooling bath, the stirrer bar, and the electrodes were removed. Graphene was exfoliated by applying a shear rate of 33 000 s–1 (6000 rpm) for 1 h using a Pro Scientific PRO250 rotor–stator mixer with a 20 mm shear generator. The solution was centrifuged at 750g for 20 min to remove residual graphite and collect the supernatant. The yield of graphene was measured by filtering 10 mL of suspension on 0.45 μm PTFE membrane filters (Sterlitech), washing with 30 mL of ethanol, and drying at 80 °C overnight. The filters were all weighed with 0.01 mg precision five times before and after deposition, and the yields were calculated relative to the graphite starting concentration of ∼1.00 mg mL–1 (exact masses recorded for each experiment was used in calculations). The majority of graphene in the solution was collected by mixing the NMP suspension 1:1 with a 1 wt % CaCl2 aqueous solution to induce precipitation overnight. The following day, the solution was centrifuged, and the sediment was filtrated, washed with deionized water, and oven-dried.

Raman Spectroscopy

For Raman analysis, graphene powder from a PTFE filter was suspended in ethanol by ultrasonication for 30 min. Graphene was then self-assembled into thin films, by the method developed by Shim et al.,56 and collected onto wafers of 285 nm SiO2 on Si. The samples were dried at 80 °C overnight before analysis. A Renishaw inVia Raman Microscope equipped with a 150 mW 514 nm laser was operated at an intensity of ∼5 mW and using a 150× lens. For each experiment, 441 spectra were acquired using a 5 s acquisition time in the Streamline HR mode. Wire 4.3 was used for the analysis. A polynomial background was subtracted before fitting to Lorentzian functions and a Gaussian function.

X-ray Photoelectron Spectroscopy

XPS analysis was performed on graphene deposited on SiO2, as in Raman spectroscopy. A Kratos Axis Ultra-DLD spectrometer was used. A monochromatic Al Kα X-ray source at a power of 150 W with an analysis area of 300 × 700 μm2 was used as the X-ray source. To record survey spectra, two acquisitions were performed in the 0–1350 eV range at a pass energy of 160 eV. Four acquisitions at a pass energy of 20 eV were used for high-resolution scans. CasaXPS was used for data processing. Atomic surface concentrations were determined by subtracting a Shirley background, using the default sensitivity factors. All spectra were calibrated against the C=C sp2 peak for C 1s at 284.5 eV.

Thermogravimetric Analysis

The amount of amorphous carbon in the sample was determined from TGA on a Netzsch STA 449 instrument. The sample was heated from 30 to 1000 °C at a rate of 10 °C min–1 in an argon atmosphere. A smoothing spline was used on the mass–temperature curve before calculating the first derivative.

High-Resolution TEM, STEM, and STEM-EELS

The graphene powder was suspended in HPLC methanol by ultrasonication, and two drops were deposited on a 200-mesh copper grid with a lacey carbon support structure. Prior to analysis, the samples were baked at 100 °C in vacuum (<10–5 mbar) overnight. HR-TEM and annular dark-field (ADF) imaging were performed on a FEI Titan 80-300 Cubed TEM equipped with a monochromator, a hexapole-based aberration corrector (corrected electron optical systems GmbH) for the image and probe lenses, and a high brightness field-emission gun (XFEG). An 80 kV electron source was used to limit knock-on damage to the graphene. The monochromator was used to achieve an energy resolution of 0.26 eV based on the full width at half maximum of the zero-loss peak for negative Cs monochromated HR-TEM imaging. EELS spectra were acquired in STEM mode using Gatan DigitalMicrograph software. The spectrum image was treated in Gatan DigitalMicrograph to align and calibrate the spectra to a C-K π* energy of 285.0 eV, and X-rays were removed using automated procedures. PCA and ICA were performed in HyperSpy 1.1 (hyperspy.org).48 The STEM images for measuring graphene flake sizes were obtained on a FEI Talos F200X analytical STEM with an XFEG at 200 kV.

Acknowledgments

HR-TEM and STEM-EELS were performed at the Canadian Center for Electron Microscopy, a national facility supported by the Canada Foundation for Innovation under the MSI program, the Natural Sciences and Engineering Research Council, and McMaster University. The authors would like to thank Dr. Aref Hasen Mamakhel at the Inorganic Chemistry Department, Aarhus University for help with TGA. The authors would also like to thank the Innovation Fund Denmark and the National Initiative for Advanced Graphene Coatings and Composites (NIAGRA) for financial support. Also, the authors are deeply appreciative of the generous financial support from the Danish National Research Foundation (grant no. DNRF118).

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01057.

  • Materials and methods, measurement of flake sizes, PCA scree plot, EELS elemental maps, estimation of the number of graphene layers from the Raman data, calculation of the mass fraction of graphene from XPS data, high-resolution N 1s spectrum, and electrochemistry with graphene (PDF)

Author Contributions

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

The authors declare no competing financial interest.

Supplementary Material

ao7b01057_si_001.pdf (685.5KB, pdf)

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