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Article Cite This: J. Am. Chem. Soc. 2017, 139, 17446−17456

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Two-Step Electrochemical Intercalation and Oxidation of Graphite for the Mass Production of Graphene Oxide Jianyun Cao,*,† Pei He,† Mahdi A. Mohammed,† Xin Zhao,† Robert J. Young,†,‡ Brian Derby,† Ian A. Kinloch,*,†,‡ and Robert A. W. Dryfe‡,§ †

School of Materials, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. National Graphene Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. § School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K. ‡

S Supporting Information *

ABSTRACT: Conventional chemical oxidation routes for the production of graphene oxide (GO), such as the Hummers’ method, suffer from environmental and safety issues due to their use of hazardous and explosive chemicals. These issues are addressed by electrochemical oxidation methods, but such approaches typically have a low yield due to inhomogeneous oxidation. Herein we report a two-step electrochemical intercalation and oxidation approach to produce GO on the large laboratory scale (tens of grams) comprising (1) forming a stage 1 graphite intercalation compound (GIC) in concentrated sulfuric acid and (2) oxidizing and exfoliating the stage 1 GIC in an aqueous solution of 0.1 M ammonium sulfate. This two-step approach leads to GO with a high yield (>70 wt %), good quality (>90%, monolayer), and reasonable oxygen content (17.7 at. %). Moreover, the as-produced GO can be subsequently deeply reduced (3.2 at. % oxygen; C/O ratio 30.2) to yield highly conductive (54 600 S m−1) reduced GO. Electrochemical capacitors based on the reduced GO showed an ultrahigh rate capability of up to 10 V s−1 due to this high conductivity.

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INTRODUCTION Graphene oxide (GO) is a two-dimensional, single-layer of carbon atoms that is covalently functionalized with oxygen containing groups (hydroxyl, epoxide, carboxylic, and carbonyl).1,2 These oxygen functional groups allow GO to be processed easily in solution and also give it the capability to act as a building block for the assembly of various macroscale graphene architectures (fiber, paper, hydrogel, aerogel, etc.).3−5 Moreover, upon reduction by either thermal or chemical means, the graphitic network can be restored, making reduced GO (rGO) that is electrically conductive.3,6 These unique features of GO and its derivatives have led to research into applications including transparent conductive films,7,8 electrochemical energy storage,9−11 printed electronics,12,13 water desalination,14,15 and polymer composites.16,17 Currently, the majority of the GO in the literature is produced by the chemical oxidation of graphite flakes, using either the Brodie,18 Staudenmaier,19 or Hummers’20 methods. All these chemical oxidation approaches involve the use of strong acids and oxidants, despite their extensive optimization, leading to environmental and safety issues particularly when scaled up.21,22 The use of strong oxidants, such as potassium permanganate, not only complicates the removal of the metal ions from the as-made GO, but leads to permanent defects that cannot be restored upon reduction.23,24 © 2017 American Chemical Society

Most recently, electrochemical oxidation has been explored as an attractive approach to produce GO due to its environmental friendliness and potential low cost.25−29 During electrochemical oxidation, oxygen groups are formed by the reaction between the positively charged graphene layers and nucleophilic water molecules.30 However, despite efforts to optimize both electrolyte composition27,29 and electrode configuration,28,29 these electrochemical processes are still not ideal for large scale GO production with high yield and a reasonable degree of oxidation. Direct electrochemical exfoliation of graphite in dilute acidic or neutral aqueous electrolyte leads to exfoliated graphene; this approach is scalable, but the exfoliated graphene behaves more like pristine graphene rather than GO due to its low oxygen content.31,32 Graphite intercalation compounds (GICs) are a class of materials formed by insertion of atomic, ionic, or molecular layers of a different chemical species (intercalants) between the layers of a graphite host material. The stage index, n, is defined as the number of graphene layers between two adjacent intercalant layers and the corresponding material is called a stage n GIC.33,34 It is believed that during the chemical oxidation in the H2SO4/KMnO4 route (i.e., the Hummers’ method),35 a pure Received: August 10, 2017 Published: November 1, 2017 17446

DOI: 10.1021/jacs.7b08515 J. Am. Chem. Soc. 2017, 139, 17446−17456

Article

Journal of the American Chemical Society

Figure 1. Electrochemical production of GO: Schematic illustration of (a) two-step electrochemical intercalation and oxidation process for production of EGO; (b) cell setup for intercalation of graphite; (c) cell setup for oxidation of stage 1 GIC; and photographs of (d) a piece of graphite foil (2.5 cm in width and 0.5 mm in thickness) being charged to 2.2 V, forming a blue in color stage 1 GIC, (e) freeze-dried EGO (∼10.5 g) powder with a gray-green color, (f) aqueous dispersion of EGO in an ink with concentration 1 mg mL−1, and (g) dilute EGO aqueous dispersion (0.05 mg mL−1) showing a brown color.

stage 1 GIC (C(21−28)+·HSO4−·2.5H2SO4) is formed prior to the formation of graphite oxide.35 This behavior indicates the crucial role of the stage 1 GIC as an intermediate in the synthesis of GO, because it allows the diffusion of oxidizing agents into the interlayer gallery of graphite, by either replacing the existing intercalants (HSO4− anions and H2SO4 molecules) or by inserting between them. In electrochemical approaches, the formation of GICs with a low stage index (n ≤ 2) is inhibited by the large amount of water in the dilute electrolyte;30,36 thus, poor intercalation together with surface oxidation and oxygen evolution occurs leading to graphite exfoliation.31,37 However, although concentrated acid electrolytes can lead to the formation of stage 1 GICs, the lack of water molecules in these systems results in the long times (∼48 h) being required to form graphite oxide, as there are few oxygen radicals to accelerate the oxygen functionalization of graphite.29 This conflicting need for different water concentrations for the two processes, a high concentration for rapid oxygen-functionalization/exfoliation and a low concentration for GIC (n ≤ 2) formation, inhibits the electrochemical oxidation process, potentially limiting its further application. In this work, we demonstrate a simple two-step electrochemical intercalation and oxidation process for fast, high-yield mass production of GO. Briefly, commercial graphite foil was first charged in concentrated sulfuric acid (>95%) to form pure stage 1 GIC, and then the stage 1 GIC was oxidized and exfoliated in 0.1 M (NH4)2SO4 aqueous solution. This two-step approach separates the intercalation and oxygen-functionalization processes and thereby avoids the dilemma of water

concentration in the electrolyte, leading to the fast production of GO (less than 30 min) with a high yield of 71 wt. %. The asprepared electrochemical GO (EGO) was readily produced on a tens of grams scale with a reasonable oxygen content of 17.7 at. %. More importantly, we found that the stage index of the GIC formed by the first charging step plays a crucial role in determing the final yield of EGO. The EGO has an average flake size of 3.1 μm, and >90% of the flakes are monolayer with a thickness of around 1.0 nm. As few unrestorable oxygen functionalities (CO, −COO−) are formed on the EGO, the reduced EGO (REGO) shows a high C/O ratio of 30.2 (oxygen content 3.2 at. %) with an extraordinary electrical conductivity of 54 600 S m−1. The high solution processability of EGO allows inkjet printing of transfer-free transparent graphene film (83.5% transparency) on a glass substrate with a sheet resistance of 4.2 kΩ sq−1 after reduction. An electrochemical capacitor fabricated with a REGO gel membrane was found to exhibit an ultrahigh rate capability of up to 10 V s−1.

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RESULTS AND DISSCUSSION Electrochemical Production of GO. As shown in Figure 1a (left) and b, stage 1 H2SO4 GIC was first synthesized by galvanostatic charging a strip of graphite foil in a two-electrode cell with a platinum mesh as the counter electrode (cathode) and concentrated sulfuric acid (>95%) as the electrolyte. The time needed to form the stage 1 GIC (cell voltage reached 2.2 V) was about 10 min for a 1 cm × 2.5 cm, 0.5 mm thick graphite foil with a charging current of 100 mA (20 mA cm−2, 0.8 A g−1). The cell voltage was kept at 2.2 V for a further 10 17447


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graphite layers are bonding layers, leading to a single, enhanced G band at positions 1634 and 1617 cm−1, respectively.33,34,38 Typical Raman spectra of the GICs formed by charging to various cell voltages are displayed in Figure 2. At a cell voltage

min to allow the homogeneous formation of a stage 1 GIC both at the surface and in the bulk of the graphite electrode. The sulfuric acid electrolyte can be reused until it has been fully consumed, as no contamination is caused by the intercalation step. Figure 1d shows a photograph of a freshly charged graphite foil with a width of 2.5 cm, the blue color indicates the formation of a stage 1 H2SO4 GIC, and the charged graphite foil is still mechanically robust, allowing movement from cell to cell for further treatment. Figure 1a (right) and c illustrate the second electrochemical oxidation and exfoliation process. A two-electrode cell was used with the stage 1 GIC as the anode, platinum as the cathode, and 0.1 M (NH4)2SO4 aqueous solution as the electrolyte. Since stage 1 GICs deintercalate automatically in water, a high constant, two terminal cell voltage of 10 V was applied on the stage 1 GIC before and during immersion in the electrolyte. The voltage was applied until full exfoliation and dissociation of the electrode had occurred, which required 5−10 min. The exfoliated product was washed by centrifuging at least 5 times. Ultrasonication (5 min) in a sonic bath was used to redisperse the product after each cycle of centrifugation. Alternatively, filtration or dialysis could be used for scaled up production. The final paste-like product obtained was then freeze-dried and dispersed in water (1 mg mL−1) by bath sonication for 30 min after which ammonia solution (30 wt %) was added (3.5 μL for each 1 mL dispersion) to further stabilize the dispersion. The dispersion was maintained for 24 h to precipitate any hydrophobic materials, and the yield of EGO was then obtained by drying and weighing the precipitates. A high yield, of 71 wt % of the total exfoliated product, was obtained with this two-step approach. Moreover, this method could be easily scaled up for mass production. Through using graphite foils with dimensions of 2.5 cm × 10 cm, we produced ∼10.5 g of EGO powder (Figure 1e). Figure 1f displays the stable aqueous dispersion of EGO with a concentration of 1 mg mL−1. Dilute EGO aqueous dispersion (0.05 mg mL−1) shows a typical yellow brown color (Figure 1g). In order to investigate the effects of the GIC stages on the composition of the as-oxidized and -exfoliated products (yield of EGO), we synthesized H2SO4 GICs with various stages by setting the cell voltage limits at 1.0, 1.2, 1.4, 1.6, 1.8, 2.0, and 2.2 V in the first charging step. These typical cell voltage limits were selected according to the linear sweep voltammetry (LSV) curve of the graphite foil in sulfuric acid at a slow scan rate of 0.5 mV s−1 (Figure S1a). In addition, the basic electrochemistry of H2SO4 GIC synthesis was also studied by using LSV and galvanostatic charging, with both a two-electrode and a threeelectrode configuration. These electrochemical data and related discussion are available in the Supporting Information (Figure S1). Raman spectroscopy was used to identify the stage index number, as the G band position is sensitive to the staging phenomena according to previous research.34,38,39 For raw graphite (graphite foil), the Raman spectrum (Figure S2) shows the characteristic G band at 1581 cm−1 and 2D band at 2688 cm−1, with a weak D band at 1337 cm−1 and G* band at 2455 cm−1.34 For acceptor type GIC with stage n > 2, the G band splits into two distinct bands: the graphite interior layer mode (lower frequency component) and the graphite bounding layer mode (upper frequency component).33 Both mode frequencies upshift with the decrease of stage index number due to lattice strain associated with stage-dependent changes in the lattice parameters.33 For stage 1 and stage 2 GIC, all

Figure 2. Raman spectra of GICs formed by charging to various cell voltage limits: (a) whole spectra; (b) high resolution G band. GIC sample for Raman were sealed between a microscope glass slide and a coverslip, even the stage 1 GIC was stable for >10 min in this configuration.

of 1.0 V, two distinct G band peaks appear at 1584 and 1608 cm−1 with a more intense upper frequency component, corresponding to stage 3 GIC.38 Further charging to 1.2 V gives a mix of stages of 2 and 3. Pure stage 2 could be obtained at a cell voltage of 1.4 V with a single band at 1618 cm−1. Mixes of stages of 1 and 2 were formed at 1.6 V. A single enhanced G band at 1634 cm−1 found at a cell voltage range of 1.8 to 2.2 V suggests the formation of pure phase stage 1 GIC. Further raising the cell voltage to 2.4 V and above resulted in the amorphization/oxygen functionalization of stage 1 GIC, confirmed by the appearance of a broad D band at 1370 cm−1 and a lower frequency component of the G band at 1608 cm−1 (Figure S3).35 Thus, the pure stage 1 GIC formed at 2.2 V without any oxygen functionalization was selected for further experiments. For comparison, the stage 2 GIC formed at 1.4 V, the stage 3 GIC formed at 1.0 V, and the pristine graphite foil was also selected for the second step oxidation and exfoliation. It should be noted that the stages determined by Raman spectroscopy for GICs formed at various cell voltages correlated well to those shown in the LSV response (Figure S1a). X-ray diffraction (XRD) further confirmed the formation of pure stage 1, 2, and 3 H2SO4 GIC at cell voltage limits of 2.2, 1.4, and 1.0 V, respectively (Figure 3a). As shown in Figure 3a, the intense diffraction peak at 22.9° for GIC formed at 2.2 V corresponds to the (002) Bragg peak of stage 1 GIC, and peaks at 34.4°, 46.4°, and 58.5° are related to the (003), (004), and (005) peaks, respectively. The results are consistent with previously reported data.39,40 For stage 2 GIC, characteristic diffraction peaks of (003) at 23.9°, (004) at 31.7°, (006) at 48.6°, and (007) at 57.1° are present. The diffraction lines at 24.7°, 29.9°, and 56.1° can be attributed to (004), (005), and (009) peaks and suggest the formation of stage 3 GIC at the cell voltage of 1.0 V.40 In addition, pristine graphite foil shows a sharp (002) peak at 26.7°. The XRD patterns from as-oxidized and -exfoliated products of the stage 1, 2, and 3 GICs, as well as pristine graphite, are 17448


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the diffraction patterns of the products from the stage 3, 2, and 1 GICs all show a clear peak related to EGO at around 11°. For the products from the stage 2 and 1 GICs, the intensity of the EGO diffraction peak becomes dominant relative to the graphite and/or exfoliated graphene peak at ∼26.6°. The diffraction peak at ∼22° is due to weakly oxidized graphene with a small degree of functionality leading to a slightly larger interlayer spacing than graphite and/or exfoliated graphene.41 The XRD peak intensity ratio of the EGO (∼11°) to the graphite and/or exfoliated graphene in the as-oxidized product of stage 3, 2, and 1 GICs is 0.4, 4.0, and 9.1, respectively, suggests oxidation and exfoliation of stage 1 GIC leads to the highest yield of EGO. The yield of EGO was measured by precipitating the hydrophobic graphite and/or exfoliated graphene in aqueous dispersion for 24 h, with results consistent with the XRD characterization. The yield of EGO from oxidation of graphite, stage 3, 2, and 1 GIC is 7.5, 18.5, 52.0, and 71.0 wt %, respectively. For comparison, stage 1 GIC was also directly quenched with water; the washed and dried product shows a dominant (002) peak of graphite at 26.6° in the XRD pattern (Figure S4), suggesting negligible oxygen functionalization. Detailed XRD characterization and yield are summarized in Table S1. These results suggest the crucial role of low stage GICs (n ≤ 2) as intermediates in the formation of EGO, which is consistent with the formation of GO from the stage 1 GIC in the H2SO4/KMnO4 system.35 The mechanism for electrochemical oxidation and exfoliation of the stage 1 GICs is proposed as follows (Figure S5): (1) intercalation of nucleophilic water molecules occurs immediately once the stage 1 GIC is immersed into 0.1 M (NH4)2SO4 aqueous electrolyte with a high cell voltage bias of 10 V. The positively charged graphene layers react quickly with nucleophilic water molecules to form hydroxyl groups with a

Figure 3. XRD characterization of the GICs and the corresponding oxidized and exfoliated products: (a) XRD patterns of the raw graphite and GICs; (b) the corresponding products from the second oxidation and exfoliation step. XRD patterns for GICs were taken with samples sealed between commercial cling film and zero background Si wafer. Samples for oxidized and exfoliated products were prepared by dispersing washed and dried sample in ethanol, followed by drop coating the dispersion on zero background Si wafer.

displayed in Figure 3b. These patterns were taken with washed and dried samples without separation of any unoxidized and/or unexfoliated graphite; thus, they can be used to qualitatively evaluate the yield of EGO in the product and the oxidation efficiency of the method used. For pristine graphite (stage index ∞), the XRD pattern of exfoliated product shows a dominant and broadened peak at 26.6°, corresponding to electrochemically exfoliated graphene;32 weak peaks at 7.1° and 22.4° are also detected, suggesting that the product contains a small amount of EGO and weakly oxidized graphene. Interestingly,

Figure 4. Comparison of physical and chemical characteristics of EGO and CGO: (a) XRD patterns; (b) UV−Vis spectra, the inset is photograph of EGO (left) and CGO (right) dispersions with same concentration of 0.05 mg mL−1; (c) absorbance at 660 nm as a function of concentration for the EGO and CGO dispersions, respectively; (d) FT-IR and (e) XPS C 1s spectra; (f) TGA curves. 17449


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absorption over the whole spectra region for the EGO. These phenomena are evidence for the reduced disruption of the electronic conjugation structure in the EGO compared to the CGO.3 In addition, the absorption peak of EGO at 243 nm is much lower than that of reduced CGO (270 nm),3 confirming the successful oxygen functionalization of graphite by the twostep approach. In order to further understand the relative degree of disruption in the lattice structure, a series of EGO and CGO dispersions with concentrations varying from 0.02 to 0.1 mg mL−1 were characterized by UV−Vis absorption spectroscopy (see Figure S7 for the spectra). A plot of the absorbance at 660 nm versus the concentration is shown in Figure 4c. The data are found to follow the Lambert−Beer law with the extinction coefficient (α) at 660 nm for EGO and CGO as 868 and 49 mL mg−1 m−1, respectively. αEGO is higher than αCGO due to the less disrupted conjugated structure of EGO compared to CGO. However, it should be noted that αEGO is still much smaller than that of the liquid phase exfoliated fewlayer graphene (α = 2460 mL mg−1 m−1), which has a pristine graphene structure.43 Fourier transform infrared (FT-IR) spectroscopy was used to identify the types of functionality on the EGO. In Figure 4c, the spectrum of EGO exhibits characteristic bands of C−O groups at 1403, 1224, and 1025 cm−1,28,44 similar to that of the CGO. The major differences between EGO and CGO are (1) the much weaker band in the 1700−1740 cm−1 range related to the CO in carbonyl and carboxyl groups and (2) the appearance of the CC stretching band at 1570 cm−1.44 The band at 1620 cm−1 is due to the vibration of the adsorbed water molecules.44 X-ray photoelectron spectroscopy (XPS) was used to quantify the chemical composition and functionalities of the as-prepared EGO. The oxygen content of EGO obtained from XPS (Figure S8) is 17.7 at. %, slightly lower than that of CGO at 24.5 at. %. The carbon/oxygen (C/O) ratio of EGO determined by XPS is 4.6, which is higher than both that of CGO (3.0) and the typical literature values for CGO (from 2.0 to 2.7) determined by either XPS or elemental analysis.6,22,45,46 Nevertheless, the C/O ratio of EGO is much lower than that of the oxygen functionalized graphene (8.9)47 and weakly oxidized graphite oxide (9.9),48 which have a low degree of functionalization. Detailed comparison of the elemental composition and C/O ratio of EGO with values from literature is available in Table S2. Deconvolution of the XPS C 1s spectra of EGO and CGO (Figure 4d) suggests that the concentration of the CO and −COO− groups (287.9 eV)6 in EGO (3.3 at. %) is much lower than that of CGO (8.1 at. %) and that the dominant oxygen functionality for EGO is C−O at a binding energy of 286.5 eV (32.0 at. %).44 The low content of CO and −COO− in EGO determined by XPS is consistent with the previously reported results from both XPS and solid-state nuclear magnetic resonance (NMR) spectroscopy on electrochemically produced GO.28,29 In addition, the existence of a π to π* component (290.5 eV) in the C 1s spectrum corresponds to the remaining conjugated structure in EGO.44 Detailed data from the deconvolution of the C 1s spectra are listed in Table S3. The XPS results are consistent with the UV−Vis and FT−IR characterization, suggesting less damage to the graphitic structure in the EGO compared to the CGO. This difference is because the majority of the oxygen atoms exist in EGO as C− O, including hydroxyl and epoxide groups. The formation of these groups will not break the C−C bond (Figure S6). On the contrary, the formation of CO and −COO− groups breaks

secondary chemical reaction possibly leading to epoxides C− O−C. Over-oxidation of the vicinal diol leads to the formation of carbonyl CO functionalities (see schematic illustration in Figure S6).30 (2) Oxidation of the intercalated water produces an oxygen gaseous product, leading to a volume expansion and hence dissociation/exfoliation of EGO from bulk GIC. The expansion and dissociation could also be attributed to the hydrolysis of EGO in water, which disrupts the ordering in the c-axis direction.35 In contrast, for pristine graphite, oxygen evolution and surface oxidation at the edges and grain boundaries are dominant at the beginning, followed by the intercalation of sulfate anions into the gallery of graphite. Cointercalation of water molecules can occur during this stage, and the subsequent decomposition of the intercalated water to an oxygen gaseous product results in the exfoliation of the graphite.32 The reason for the oxygen functionalization not being observed for the conventional electrochemical exfoliation is because of the following: (1) the high concentration of water in the electrolyte (0.1 M (NH4)2SO4) stops the formation of GICs with stage index n ≤ 2;30,36 (2) based on the literature results,29,35,41 GICs (n ≤ 2) always play a role as intermediate in the formation of graphite oxide, in both chemical or electrochemical approaches. Hence, in the conventional electrochemical exfoliation, the cointercalated water molecules tend to decompose to oxygen gaseous products, which expand and exfoliate the graphite. Oxidation and exfoliation of higherstage GICs (n ≥ 2) will follow the combined mechanisms discussed above, leading to a mixture of EGO and exfoliated graphene. Evidence supporting the proposed mechanism is the observation of gas (oxygen) evolution during the initial stage of the oxidation/exfoliation. Instead of vigorous gas evolution, which is seen when pristine graphite is immersed into the electrolyte with a cell voltage of 10 V, only a few gas bubbles appeared during the oxidation and exfoliation of the stage 1 GIC, even though the stage 1 GIC still expanded and dissociated from the bulk electrode. Another interesting observation is that the exfoliated products of the stage 1 GIC tended to precipitate to the bottom of the electrolyte immediately after dissociation from the bulk electrode, indicating few trapped gas bubbles in the expanded EGO; in contrast, the exfoliated products of graphite tended to spread over the electrolyte’s urface as reported previously,31 which is probably due to the large amount of trapped gas product between the expanded layers. Physical and Chemical Characterization. The small amount of unoxidized but exfoliated graphene and/or remaining graphite in the product from the stage 1 GICs can be removed by precipitation due to their hydrophobicity. As shown in the XRD pattern in Figure 4a, the disappearance of the diffraction peak at ∼26.5° confirmed the removal of exfoliated graphene and/or graphite from the EGO. The extra peak at 22.6° in the diffraction pattern of EGO compared to the chemical GO (CGO) made by the modified Hummers’ method21 is due to the existence of weakly oxidized graphene in EGO. The aqueous dispersion of EGO shows a dark brown color, and the color is much deeper than that of CGO dispersion with the same concentration (inset of Figure 4b), probably due to the size of the conjugated π-domains in EGO being larger than that of CGO.42 UV−Vis absorption spectra of the EGO and CGO at the same concentration are displayed in Figure 4b; there is a slight red shift of the absorption peak from 230 nm (CGO) to 243 nm (EGO) and an enhanced 17450


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Figure 5. Morphology and structural characterization of EGO: (a) SEM image of EGO sheets; (b) flake sized distribution of 100 EGO flakes; (c) AFM image of EGO sheets; (d) statistical thickness analysis of EGO based on 100 flakes; (e) TEM image of an EGO sheet; (f) SAED pattern from EGO.

average flake size of 3.12 ± 1.27 μm, and the majority of the flakes (∼50%) are 2−3 μm in size. The thickness (number of layers) of EGO was measured by AFM. A typical flake of EGO with a thickness ∼1.0 nm is shown in Figure 5c, corresponding to monolayer GO.3,8 Thickness analysis of 100 flakes suggests that >90% of EGO flakes are monolayer (thickness Hydrazine > VC > Thermal annealing. Nevertheless, simple annealing at 200 °C is still able to increase the conductivity of the EGO membrane to 25 300 S m−1. The HI reduced EGO membrane exhibits the highest conductivity of 44 000 S m−1. The relatively lower electrical conductivity of the hydrazine reduced EGO membrane (40 000 S m−1) compared to that of the REGO membrane discussed above is due to a different experimental procedure. The good dispersibility of EGO, as well as the high electrical conductivity after reduction, allows for the fabrication of transfer-free, inkjet printed, transparent conductive graphene films on glass substrates (Figure 7d). The transmittance of the as-printed film can be easily controlled by the concentration of the EGO ink and the number of overprinted layers. A mixed vapor of HI and acetic acid was used to reduce the as-printed EGO film. Figure 7e shows the reduced EGO film on a glass substrate with different transmittance values from 79.2% to 98.4%. Plots of transmittance versus sheet resistance of the reduced EGO film are shown in Figure 7f. The sheet resistance values of the reduced EGO film with a transmittance of 98.4%, 92.8%, 83.5%, and 79.2% are 224.7, 14.2, 4.2, and 2.8 kΩ sq−1, respectively.

is slightly decreased while a broad 2D band appears. This is probably because the structure of CGO cannot be restored effectively with the reduction method used. Hence, there is no significant change in the defect density for the Raman spectrometer to detect.24 According to the model proposed by Lucchese et al.54 and Cançado et al.,51 the as-measured ID/IG ratio of REGO (2.97 ± 0.09) can be correlated to a defect distance (LD) of 1.49 ± 0.02 nm (Figure S13a). By following the description of Englert et al.,53 the experimental ID/IG ratio can be further related to the degree of functionalization/density of defects (θ, defined as the ratio of C(sp3) to C(sp2)); the as-determined θ value of REGO is 1.32 ± 0.03% (Figure S13b). These values suggest that there is still a large number of defects in the REGO sheets compared to the pristine graphene.51,53 The cleavage of the C−C bond (formation of CO group) due to over-oxidation is responsible for these permanent hole defects which are impossible to heal. Besides, the LD and θ values for EGO, CGO, and RCGO cannot be determined by the method discussed above because the Raman spectra show no significant changes when the θ value is larger than ∼3%.24,55 Nevertheless, based on the XPS results (oxygen content: 17.7 at.%), and assuming equal amounts of hydroxyl and epoxy groups (neglecting the 3.3 at.% of CO and −COO− groups), the estimated θ value of EGO is ∼32%, which is lower than the literature value for CGO (50%).55 The behavior of EGO is more like that of oxygen functionalized graphene as proposed by Eigler,24 rather than conventional CGO. Electrical Conductivity of Reduced EGO. The sheet resistance of the as-prepared REGO membrane formed by vacuum filtration of the REGO dispersion was measured by using the four-probe method. The REGO membrane with a 1 μm thickness has a sheet resistance of 18.3 Ω sq−1; thus, the ascalculated electrical conductivity is as high as 54 600 S m−1. The I−V curve obtained from the REGO membrane is shown in Figure 7a. The conductivity is superior to the previously reported data with reduced CGO,56 thermally annealed EGO,28 17453


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Figure 8. Electrochemical performance of a REGO gel membrane based supercapacitor: (a and b) CV curves of the REGO supercapacitor at various scan rates; (c) rate capability of REGO supercapacitor, the inset is charging−discharging curves at a large current density of 121 A g−1; (d) plots of specific capacitance versus frequency (the inset is Nyquist plot of the REGO supercapacitor).

High Power Supercapacitor Based on REGO. To further demonstrate the application of REGO in the area of electrochemical energy storage, a REGO supercapacitor was fabricated using a REGO gel membrane as the active material. Two pieces of free-standing REGO gel membrane were sandwiched between two pieces of titanium foil (current collector) and a PTFE membrane filter (separator); 1 M sulfuric acid aqueous solution was used as the electrolyte. The unique properties of the REGO gel membrane lead to a supercapacitor with ultrahigh power capability. First, the high electrical conductivity of REGO allow fast electron transfer in the electrode; second, the water molecules trapped between the REGO layers prevent graphene from restacking, allowing fast diffusion of the electrolyte ions. The cyclic voltammetry (CV) response of the REGO supercapacitor is displayed in Figure 8a and b. The CV curve shows a nearly perfect rectangular shape at a scan rate of 1.0 V s−1, and even maintains a good rectangular characteristic at a rapid scan rate of 10 V s−1. Galvanostatic charging−discharging was used to determine the specific capacitance of REGO. The evolution of specific capacitance with increasing discharge current density is shown in Figure 8c. The REGO has a specific capacitance of 40.3 F g−1 at a current density of 0.3 A g−1, and remains at 34.8 F g−1 (86.3%) when the current density increases to 300 A g−1. The charging−discharging curves at a large current density of 121 A g−1 (the inset of Figure 8c) show typical triangular shape without an obvious IR drop. The smaller specific capacitance of REGO when compared with previously reported reduced CGO is due to a lower density of oxygen functionalities and defects as confirmed by the statistical Raman analysis. Electrochemical impedance spectroscopy (EIS) was further conducted to analyze the frequency response of the REGO supercapacitor. Nyquist plots (inset of Figure 8d) indicate a small equivalent series resistance of ∼1 Ω. The

evolution of specific capacitance versus frequency (Figure 8d) suggests the REGO supercapacitor has a short time constant of 17.8 ms (determined by the frequency corresponding to 50% of maximum capacitance).

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CONCLUSION

In summary, a simple two-step electrochemical intercalation and oxidation approach for fast and high yield production of GO has been developed. The separation of the intercalation and oxygen-functionalization processes into two distinct steps in different electrolytes has addressed the dilemma of water concentration in the electrolyte, leading to fast intercalation to form stage 1 GIC and subsequent efficient oxidation and exfoliation of the stage 1 GIC into EGO. Thin layer EGO sheets (>90%, monolayer) with an oxygen content of 17.7 at. % were readily produced on a tens of grams scale, showing great potential for industrial mass production. Moreover, the asproduced EGO shows properties distinct from conventional CGO, including a low degree of oxidation, few carbonyl and carboxyl groups, and less disruption of the graphene honeycomb lattice structure. Hence, the REGO shows an ultralow oxygen content of 3.2 at. % (C/O ratio 30.2) and much fewer defects than reduced CGO, leading to a remarkable electrical conductivity of 54 600 S m−1. The good solution processability of EGO in both aqueous and organic systems allows inkjet printing of transparent films which can be subsequently reduced to make them electrically conductive. Electrochemical supercapacitors based on a REGO gel membrane exhibit ultrahigh power capability and are able to operate up to 10 V s−1 with a relaxation time constant of 17.8 ms. The ability to produce EGO with these unique properties potentially on a large scale and at low cost will enable its application in a 17454


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number of areas, such as polymer composite materials, printed electronics, and energy storage and conversion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08515. Experimental details; basic electrochemistry of H2SO4 GIC synthesis; Raman spectrum of pristine graphite foil and GICs formed at cell voltage ≥2.5 V; XRD pattern of water quenched stage 1 GIC; schematic illustration of EGO formation; schematic illustration of oxygen groups formation on graphene; UV−vis spectra of EGO and CGO for extinction coefficient; XPS survey spectra of CGO and EGO; SEM and AFM images used for flake size and thickness analysis, respectively; distribution of Γ2D for EGO and REGO; statistical Raman analysis of CGO and RCGO; Correlation of the ID/IG ratio to LD and θ; summary of phase composition of oxidized products and yield of EGO from pristine graphite, stage 3, 2, and 1 GICs; comparison of elemental composition and TGA weight loss of EGO and CGO with literature values; summary of XPS characterization of EGO and CGO; summary of statistical Raman analysis; comparison of physical and chemical properties of various types of graphene (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Jianyun Cao: 0000-0003-3437-9947 Brian Derby: 0000-0001-5753-0166 Ian A. Kinloch: 0000-0003-3314-6869 Robert A. W. Dryfe: 0000-0002-9335-4451 Notes

The authors declare no competing financial interest. All research data supporting this publication are directly available within this publication and the corresponding Supporting Information as well as available from the corresponding authors upon reasonable request.

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ACKNOWLEDGMENTS The authors are grateful to the Engineering and Physical Sciences Research Council (EPSRC), Grant Reference EP/ K016954/1 and EP/L020742/1 for supporting this work.

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