High Yield Controlled Synthesis of Nano-Graphene Oxide by Water

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High Yield Controlled Synthesis of Nano-Graphene Oxide by Water Electrolytic Oxidation of Glassy Carbon for Metal-Free Catalysis Qinwei Wei, Songfeng Pei, Guodong Wen, Kun Huang, Zhaohong Wu, Zhibo Liu, Wei Ma, Hui-Ming Cheng, and Wencai Ren ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b04447 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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High Yield Controlled Synthesis of Nano-Graphene Oxide by Water Electrolytic Oxidation of Glassy Carbon for Metal-Free Catalysis Qinwei Wei,†,‡, Songfeng Pei,†, Guodong Wen,† Kun Huang,† Zhaohong Wu,†,‡ Zhibo Liu,† Wei Ma,†,‡ Hui-Ming Cheng,†,‡,§ Wencai Ren*,†,‡ †Shenyang

National Laboratory for Materials Science, Institute of Metal Research, Chinese

Academy of Sciences, Shenyang 110016, P. R. China. ‡School

of Materials Science and Engineering, University of Science and Technology of China,

Shenyang 110016, P. R. China. §Tsinghua-Berkeley

Shenzhen Institute (TBSI), Tsinghua University, 1001 Xueyuan Road,

Shenzhen 518055, P. R. China. *Correspondence

to: [email protected].

ABSTRACT: The strong quantum confinement effect as well as abundant edges and oxygen functional groups enable nano-graphene oxide (NGO) a variety of intriguing applications such as catalysis, bioimaging, drug delivery and photovoltaic devices. However, the development of NGO is severely hindered because of the difficulty in controlled mass production. Here, we report the efficient synthesis of NGO with a high yield of ~40 wt% by water electrolytic

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oxidation of glassy carbon (GC). The NGO shows a high oxidation degree (C/O atomic ratio, ~1.4) and excellent dispersion stability. Moreover, its size can be easily tuned by the graphitization degree of GC, which enables the controlled synthesis of NGO with average size of 4 nm, 8 nm and 13 nm and different oxygen functional groups. As metal-free catalysts, the 13nm-sized NGO is found to be beneficial for the oxidative coupling reaction of benzylamine, while the 4-nm-sized NGO shows a conversion rate of 88 times higher than 13-nm-sized NGO for the oxidation reaction of benzene. In addition, the water electrolytic oxidation mechanism of graphitic materials is systematically studied. It is found that sulfuric acid has a protective effect on the graphite electrode during the water electrolytic oxidation process, and 50 wt% sulfuric acid solution well balances the protection and oxidation processes, leading to the highest oxidation efficiency and production rate.

KEYWORDS: glassy carbon, graphene oxide, quantum dot, electrochemical method, oxidation mechanism, metal-free catalyst.

Graphene oxide (GO), as an important derivative of graphene, has shown a great potential in various applications because of the presence of oxygen functional groups, good dispersion in water, and easy functionalization.1-6 When the size of GO sheets is decreased to nanometer scale, more edges are generated and the amount of edge-attached carbonyl and carboxyl groups are increased. First, the strong quantum confinement and edge effects lead to exceptional fluorescence in the visible and infrared regions.7 Second, the increased carbonyl and carboxyl groups make nano-GO (NGO) better dispersion stability than GO in a wider range of solutions including biological solutions and easier functionalization.8, 9 Third, the abundant edges impart


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NGO higher chemical reactivity.10 These properties enable NGO a promising material for optoelectronic devices,11 photovoltaic devices,12 cellular imaging,13,

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drug delivery,15

biosensors,16-20 and metal-free catalysis.21 Currently, NGO is mainly synthesized either by modified Hummers method using graphitized materials with tiny grains as raw materials, or by cutting the GO sheets synthesized by Hummers method with hydrothermal treatment, sonication or further acidic oxidation.8, 22-24 Although high oxidation degree can be achieved, these Hummers method based approaches suffer from complex synthesis process, explosive risk, serious environmental pollutions, long reaction time, and low yield. For example, the yield of NGO (5-13 nm) is only 5 wt% after taking over 25 h for acidic oxidation and hydrothermal treatment of GO.25 In addition, the unavoidable metal ion contamination is detrimental to biological and biomedicine applications. The recently rapidly growing electrochemical (EC) methods have been widely used for efficient and green synthesis of graphene and GO.26-28 However, the NGO obtained by EC methods has low oxidation degree and suffers from low yield as well.12, 29 Moreover, the control on the size of NGO is still very challenging for both methods in particular when the size is smaller than 20 nm, which is essentially important to tailor and optimize the properties of NGO in various applications. Here we report efficient synthesis of NGO with a high yield by one-step water electrolytic oxidation of glassy carbon (GC) in sulfuric acid solution. These electrochemically derived NGO (ENGO) show high oxidation degree, and the size of ENGO sheets can be easily controlled by changing the crystallinity of GC. Moreover, the ENGO samples with different size have various contents of oxygen-containing functional groups. Using these ENGO as catalysts, the size effect of NGO on the catalytic activity in different reactions was investigated. In addition, we systematically studied the EC oxidation mechanism of graphitic materials, in which the origin of


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oxygen in ENGO, the current change and structure evolution of graphitic materials during EC reactions, the influence of the structure of graphitic materials, the role of sulfuric acid, and the optimized sulfuric acid concentration were analyzed. RESULTS AND DISSCUSION One-step water electrolytic oxidation synthesis of ENGO. GC is a disordered graphitic material, which was synthesized by carbonization and graphitization of phenolic resin. Highresolution transmission electron microscopy (HRTEM) images show that GC is compactly intertwined by nanosized graphite sheets with random orientations (Figure 1a and Figure S1).30 Such structure makes GC unable be intercalated deeply in concentrated sulfuric acid solution (98 wt%) even under a high potential (3.5 V) after a long time EC reaction (Figure 1b), which is different from the typical graphitic materials.31 Moreover, during the EC synthesis process under a constant potential (3.5 V) in 50 wt% sulfuric acid solution, except for gradual thinning, GC anode shows no obvious volume expansion and the current under a constant potential remains almost the same with the EC reaction time (Figure S2). This indicates that the EC oxidation and exfoliation of GC anode proceed gradually from the surface to the core as shown in Figure 1c, Figure S2 and Figure S3. As a result, the amount of the exfoliated product gradually increases with prolonging the reaction time, as evidenced by the darkening of the electrolyte solution (Figure 1d). For comparison, we also studied the one-step EC reaction of three typical ordered graphitic materials including highly oriented pyrolytic graphite (HOPG), expanded graphite paper (EGP) and pyrolytic graphite plate (PGP) under the same conditions (Figure S2). Different from GC, severe expansion occurs soon after EC reaction starts, and such structural change cuts off the


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current supply and consequently terminates the EC reactions. It can be seen that the current first increases rapidly due to the increased reaction area caused by the expansion of the electrode, and then decreases rapidly to 0 within 1-12 minutes depending on the graphitic materials used because of the cracking and destruction of the electrodes. As a result, no oxidation is detected for the exfoliated products. These results suggest that such ordered graphitic materials are not suitable for the one-step EC synthesis of ENGO or GO. For characterization, the exfoliated product from GC was collected from the electrolyte followed by purification and freeze-drying, and deep-brown solid powders were achieved (Figure 2a). Interestingly, these powders are very easy to be re-dispersed in water and ethanol within 40 seconds to form a uniform solution as shown in Figure 2b and Figure S4. The Tyndall effect proves the colloid nature of the solution (Figure S5a). In contrast, the GO powders synthesized by traditional Hummers methods (HGO) show many visible particles that have not been well dispersed even after 10 min. Zeta potential measurements show that the colloid particles synthesized by our EC method are more negatively charged than the HGO sheets (Figure S5b), which explains their better redispersibility and leads to excellent dispersion in aqueous solution for more than two months in a wide range of pH values from 2 to 7 (Figure S5c). Moreover, no visible precipitation was observed for our samples even after high speed centrifugation at 12,000 RPM, while the HGO dispersion already showed obvious precipitation after centrifugation at 3,000 RPM (Figure S6). This result further confirms the excellent dispersion stability of our samples synthesized by EC oxidation and exfoliation of GC. The structure and chemical composition of our exfoliated product were then systematically characterized by various methods. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) measurements show that they mostly have thickness less than 2 nm (~90%)


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and lateral size less than 20 nm (Figure 2c-f). Different from GO powders, the X-ray Diffraction (XRD) pattern of ENGO powder shows almost no peak (Figure S7), indicating that the nanosized ENGO is hard to restack even after drying, which is one reason for the excellent dispersion stability shown above. The combustion elemental analyzer (EA) analysis shows an elemental composition (wt%) of C (47.6 %), O (46.1 %), H (3.6 %), N (0.4 %) and S (0.6 %), corresponding to a C/O atom ratio of 1.4. Note that this value is lower than the typical values (~2.0) of HGO and reported NGO,32, 33 indicating the higher oxidation degree of ENGO. The higher content of oxygen-containing functional groups is the main reason for the more negative Zeta potential and better dispersion stability observed above. In addition, despite the much smaller lateral size and more functional groups, our samples still show strong Raman D peak (~1348 cm-1) and G peak (~1598 cm-1) (Figure 2g). This indicates that our ENGO samples still have a large number of graphitic sp2 carbon regions.34,

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Therefore, we can easily transform

these ENGO to crystalline graphene quantum dots by a simple hydrothermal treatment to the solution, which shows clear blue fluorescence (Figure S8). We further analyzed the functional groups in our ENGO samples by using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). XPS spectra (Figure 2h) show the presence of chemical bonds of C=C/C-C (284.6 eV), C-O (287.0 eV), C=O (288.0 eV) and O-C=O (289.0 eV).36 More importantly, it is worth noting that the C=O and O-C=O peaks are more pronounced than those of HGO. FTIR spectrum of ENGO shows the same functional groups with HGO (Figure 2i), which includes O-H stretching vibration (3427 cm-1), C=O vibration (1736 cm-1), C=C vibration of sp2 bond (1630 cm-1), O-CO vibration (1260 cm-1) and C-O vibration (1087 cm-1). Consistent with the XPS results, ENGO shows a stronger C=O vibration peak than HGO. These results suggest that our ENGO has


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higher percentage of carbonyl and carboxyl groups than GO sheets. This is attributed to the higher edge-to-area ratio of ENGO. It has been reported that C=O and O-C=O bonds are mainly formed with the carbon atoms at the graphene edges.37, 38 In addition, we found that the efficiency of ENGO synthesis is closely related with the concentration of electrolyte. As shown in Figure 3a-c, the electrolyte containing 50 wt% sulfuric acid exhibits the highest efficiency for ENGO synthesis, which means the highest production rate (7.7 g/m2h) and oxidation degree (C/O ratio, 1.4). The corresponding yield of ENGO is ~40 wt%, which is significantly higher than those of acidic oxidation and hydrothermal treatment method (~5 wt%).25 When the concentration of sulfuric acid is less than 38 wt%, the GC electrode is destroyed, producing only weakly oxidized black particles in the bottom of the electrolyte, which cannot be exfoliated even after 20 minutes of sonication (Figure 3c). At high sulfuric acid concentration (such as 98 wt%), nearly no visible exfoliated product is obtained (Figure 3c). It is important to note that the influence of sulfuric acid concentration on the synthesis of ENGO is almost the same as that on the synthesis of GO by the two-step EC method.26 Water electrolytic oxidation mechanism of graphitic materials. In order to understand the oxidation and exfoliation mechanism of GC, we performed

18O

isotopic tracing experiments to

identify the origin of oxygen in ENGO. The results show that the oxygen in ENGO dominantly comes from water in the electrolyte (Figure 4a). This means that the synthesis of ENGO is based on water electrolytic oxidation mechanism, which is the same as the two-step EC synthesis of GO.26 If the water electrolysis occurs inside the graphitic electrode, the produced oxygen gases can easily destroy the electrode, which leads to broken circuit, ineffective oxidation and consequently a low oxidation degree.31, 39 Therefore, the prerequisite for uniform and sufficient


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oxidation of graphite electrode is to ensure the EC reactions occur gradually from the surface to the core of the electrode. In this case, the graphite layers that participate in EC reaction can always connect with the power supply until they are fully oxidized. We then studied the current change as a function of EC reaction time at a constant potential of 3.5 V for three different sulfuric acid concentrations (10 wt%, 50 wt% and 98 wt%). As shown in Figure 4b, the concentration of sulfuric acid solution has a significant influence on the current. The current shows only a small decrease after more than two hours EC reaction in 50 wt% sulfuric acid solution and almost no change in 98 wt% sulfuric acid solution, indicating that the electrodes remain good stability during EC reaction. Moreover, no delamination of GC was observed even further increasing the potential to 4 and 4.5 V. In contrast, the current decreases rapidly in 10 wt% sulfuric acid solution after about 5500 s and eventually becomes 0 after about two hours, indicating that the GC electrode is destroyed. This is consistent with the production of weakly oxidized black particles (Figure S9). The above results indicate that sulfuric acid has a protective effect on GC electrode during the water electrolytic oxidation process. The protection effect of sulfuric acid can be understood with the electric double layer theory (Figure 4c-e).40 In general, H2SO4 is dissociated into SO42- and HSO4- in water. In the case of GC, no intercalation occurs (Figure 1) and therefore, these SO42- and HSO4- will attracted onto the surface of positively charged GC electrode by electrostatic interaction. Due to the dipole moments, the water molecules will compete with ions for sites on the electrode surface. Therefore, in a low concentration of sulfuric acid solution, only a small part of electrode surface is covered by ions (Figure 4c). With increasing the concentration of sulfuric acid solution, the coverage of SO42- and HSO4- increases and finally a compact layer of SO42- and HSO4- will be formed on the electrode surface (Figure 4d, e). We suggest that such compact ion layer acts as a


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protective layer to prevent the water molecules from penetrating into the core of the electrode, which ensures that the EC reactions are only restricted on the electrode surface. This is the reason why the electrode can be easily destroyed in low concentration of sulfuric acid solution but remains a high stability at high concentration of sulfuric acid solution. Different from GC, graphitic materials with long range ordered layered structure, such as graphite flakes and HOPG, are easy to be intercalated by sulfuric acid. Therefore, besides surface adsorption, intercalation occurs as well for such materials (Figure S10a), which impedes the formation of the compact protective layer of SO42- and HSO4- on the electrode surface. As a result, water molecules will penetrate into the core of graphitic materials, which results in rapid production of a large amount of gases and consequently destroy the electrode as observed before26 and in Figure S2. One way to solve this problem is to pre-intercalate the graphite electrode by sulfuric acid before water electrolytic oxidation as shown in our previous work, which inhibits the intercalation and ensures the formation of compact protective layer of SO42and HSO4- on the electrode surface during water electrolytic oxidation process (Figure S10b).26 In addition to the prerequisite of stable electrode, the oxidation and exfoliation of ENGO should strongly depend on the current and the content of water in the electrolyte, i.e., concentration of sulfuric acid solution, because of the water electrolytic oxidation mechanism. Note that under the same constant potential, the current for 50 wt% sulfuric acid solution is over 2 times larger than that for 10 wt% sulfuric acid solution and ~8 times larger than that for 98 wt% sulfuric acid solution (Figure 4b). Although the GC electrode is very stable at 98 wt% sulfuric acid solution, nearly no materials are synthesized because of the negligible water and very small current (Figure 4b, e). In contrast, because of the presence of sufficient water as well as stable and large current supply for 50 wt% sulfuric acid solution, the highest efficiency for


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ENGO synthesis is achieved (Figure 4b, d). Because of the same reason (Figure S11), the highest efficiency for GO synthesis by the two-step EC method is also achieved in 50 wt% sulfuric acid solution.26 The above analyses suggest the intertwined nanocrystalline structure of GC and a proper sulfuric acid concentration play key roles in one-step high efficiency EC synthesis of ENGO by water electrolytic oxidation. The intertwined structure makes GC unable be intercalated in sulfuric acid solution even after a very long reaction time under a high potential, which allows for the efficient formation of a protective layer of SO42- and HSO4- without any pre-treatment. The 50 wt% sulfuric acid concentration balances the protection and oxidation of GC electrodes, which provide enough water, large current supply and sufficient long EC reaction time to enable the gradually oxidation of GC and consequently the formation of ENGO. For the two-step EC method, the pre-intercalation of H2SO4 efficiently inhibits the formation of O2.26 The higher concentration of reactive oxygen radicals (*OH, *O and *OOH) together with the high current (Figure S11) enable the ultrafast synthesis of GO.26 Controlled synthesis of ENGO for catalysis application. Another important merit of our EC method is that the size of ENGO can be easily turned by using GC with different degrees of graphitization. It has been reported that the graphitization degree of GC can be controlled by the annealing temperature.41 We synthesized GC at three different annealing temperatures (2160, 1700 and 1200 ºC), which was named as GC2160, GC1700 and GC1200, respectively. TEM images, XRD patterns and Raman spectra clearly show that the graphitization degrees and mean crystalline size of GC decrease with decreasing the annealing temperature (Figure S12). The high resolution C1s XPS spectra show only one peak of C=C/C-C (284.6 eV) for the three GC samples, indicating fully carbonization of GC at these three annealing temperatures (Figure


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S12f). Figure 5a-c show the ENGO (ENGO2160, ENGO1700 and ENGO1200) synthesized with GC2160, GC1700 and GC1200 as raw materials, respectively, by one-step EC oxidation. It is clearly seen that the mean size of ENGO decreases with the graphitization degree and crystalline size of the GC used (Figure S12). The average size of the ENGO2160, ENGO1700 and ENGO1200 is 13 nm, 8 nm and 4 nm, respectively (Figure 5d-f). Moreover, it is worth noting that although all these samples show the similar oxidation degree with a C/O ratio of 1.3-1.4, the percentage of carbonyl and carboxyl groups increases with decreasing the size of ENGO (Figure 5g-i and Table S1), which is due to the larger edge-to-area ratio for the smaller NGO as discussed above. These samples provide the possibilities for investigating the effect of size and oxygen-containing functional groups on the properties of NGO in various applications. Recently, GO has been explored as a type of carbon catalyst owing to its large surface area, solid acid produced from oxygen functional groups, spin electrons at the edges/defect and conjugation of sp2 carbon network.42-47 However, the size effect of GO is still unclear. As the typical reactions of GO catalysis, the oxidative coupling reaction of benzylamine and the oxidation reaction of benzene were studied by using different sized ENGO. As shown in Figure 6a, b, the catalytic performance is improved with increasing the size of ENGO for the oxidative coupling reaction of benzylamine. ENGO2160 exhibits the best catalytic activity with a 65% conversion rate among the three ENGO studied, which is also better than that of HGO reported in literature.42 ENGO2160 has a relatively larger in-plane size than ENGO1700 and ENGO1200, which provide relatively abundant -conjugation system. Furthermore, ENGO2160 has more edges and carboxyl/carbonyl groups compared to HGO. These results suggest that both the conjugation system and edges (carboxyl/carbonyl) are essential for oxidative coupling reaction of benzylamine, which is consistent with the previous report on oxidative coupling of amines to


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imines, and these two factors should be balanced to achieve a high catalytic performance. In contrast, ENGO1200 shows the best catalytic activity with a conversion rate of 8.8% among the three ENGO samples for the oxidation of benzene by using hydrogen peroxide, which is about 88 times higher than that of ENGO 2160 (Figure 6c, d). Such huge difference in conversion rate indicates that the edges and carbonyl/carboxyl groups of ENGO play a dominant role in its catalytic performance for oxidation of benzene. Although the mechanisms are not clear, these results provide useful information for guiding the structure design of NGO for different catalytic reactions. CONCLUSIONS In conclusion, we demonstrate efficient and controlled synthesis of ENGO by a one-step EC reaction based on water electrolytic oxidation mechanism in sulfuric acid solution with GC as anode. The special intertwined nanocrystalline structure of GC and a proper sulfuric acid concentration play key roles in high efficiency EC oxidation of GC. The ENGO obtained shows high oxidation degree (C/O ratio, 1.4) with a high yield (~40 wt%) and excellent dispersiblity in a wide range of pH values. Importantly, the size of NGO can be easily tuned by the graphitization degree and crystalline size of GC, which allows us to study the size effect of NGO in various applications. As an example, we found that ENGO with larger size shows improved catalytic activity in oxidative coupling reaction of benzylamine while that with small size shows much better catalytic activity for oxidation of benzene. Further studies show that sulfuric acid has a protective effect on graphitic electrode during water electrolytic oxidation process, and 50 wt% sulfuric acid solution can well balance the protection and oxidation processes. These findings not only provide more information on the understanding of EC synthesis of graphene


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materials but also will promote the applications of NGO in various fields such as catalysis, bioimaging, biological detection, drug delivery and optoelectronic devices. METHODS Materials. HOPG, PGP and HGO were purchased from Shenzhen Matterene technology Co. Ltd. EPG was purchased from Shenyang RuiYu Chemical Co. Ltd. Concentrated sulfuric acid (98 wt%), ammonia (28 wt%), n-Butanol and N,N-Dimethylformamide were purchased from Sinopharm Chemical Reagent Co. Ltd. Phenolic resin was purchased from Henan BoRun Material Co. Ltd. Heavy-oxygen water (97 atom%, H218O) was purchased from Shanghai Aladdin Bio-chemical Technology Co., Ltd. H2S16O318O was synthesized as reported previously.26 GC synthesis. The powders of phenolic resin were first dissolved by mixing with n-Butanol and N-N-Dimethylformamide with a ratio of 1:0.5:0.5 (wt%), The resulting mixture solution was casted into a square mould, and then cured by successively heating at 80 °C for 20 hours, 100 °C for 10 hours and 120 °C for 10 hours. After that, the material obtained was subjected to carbonization by slowly heating to 1200 °C at a rate of 0.1-0.2 °C/min. Finally, it was graphitized to form GC with different graphitization degrees and crystalline size by annealing at 2160 °C (GC2160), 1700 °C (GC1700) or 1200 °C (GC1200) for two hours. If not specified, the graphitization temperature was 2160 °C. ENGO synthesis. A GC panel and a platinum wire were inserted into an electrolytic cell (500 mL) containing 350 mL diluted H2SO4 solution (50 wt%) as anode and cathode, respectively, and then a constant potential of 3.5 V was applied. After the GC anode was fully oxidized and exfoliated to form dark yellow solution, a 1kDa dialysis bag was used to purify ENGO. The dry


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powders of ENGO were collected by vacuum freeze-drying. ENGO samples with different sizes and structures were synthesized by using different GC materials as raw materials including GC2160, GC1700 and GC1200. If not specified, the ENGO was synthesized by GC2160. Characterizations of GC, ENGO and Reduced ENGO. The morphology of GC and ENGO were investigated by AFM (Bruker MultiMode 8), SEM (FEI Nova NanoSEM 430) and TEM (JEM 2010, 200 kV and FEI Tecnai F20, 200 kV). The chemical composition of ENGO was analyzed by EA (Elementar, vario MICRO cube), XPS (ESCALAB 250 using focused monochromatized Al Kα radiation of 1486.6 eV), and FTIR (Bruker Tensor 27). The microstructure of ENGO and GC was characterized by Raman spectroscopy (JY Labram HR 800 spectrometer using 532.08 nm laser) and XRD (Rigaku diffractometer with Cu Kα radiation). Zeta potential was measured with Malvern Zetasizer Nano-ZS90. The absorption spectra were obtained at room temperature by a UV-vis spectrophotometer (UV-vis-NIR, Cary 5000). The fluorescent spectra and fluorescence lifetime of reduced ENGO by hydrothermal treatment were measured at room temperature by an FLSP-920 Edinburgh Analytical Instrument apparatus with Xe-900 lamp and EPL-330 lamp. Electrochemical experiments were conducted by an electrochemical workstation (Metrohm, Autolab M204). Isotopic tracing experiments. To trace the transfer path of oxygen, ENGO samples were synthesized with different combinations of reagents, including Normal: H216O (50 wt%) and H2S16O4; 18O Aq: H218O (12.5 wt%), H216O (37.5 wt%) and H2S16O4; 18O Acid: H216O (50 wt%) and H2S16O318O. After purification and freeze-drying, the

18O

content in each ENGO sample

was measured by thermogravimetric mass spectrometry (TG-MS) with Netzsch STA 449C Jupiter/QMS 403C.


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Catalytic performance tests of ENGO. For the oxidative coupling reaction of benzylamine, 25 mg of ENGO and 0.5 mmol of benzylamine were reacted at 80 °C in CH3CN with continuous stirring and bubbling with oxygen for 5 h. For the synthesis of phenol from benzene using ENGO as catalyst, the ENGO (50 mg) was dispersed in acetonitrile (10 mL), in which 0.5 mL of hydrogen peroxide and benzene with a molar ratio of 13.5 were mixed. The reaction lasted for 6 hours at 60 °C.


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Figure 1. Synthesis of ENGO by EC oxidation of GC. (a) A typical HRTEM image of GC, showing intertwined nanocrystalline graphite structure. (b) XRD patterns of the GC electrode before and after EC reaction in concentrated sulfuric acid solution (98 wt%) for 2 hours, showing that no intercalation occurred. (c) Schematic of the EC oxidation and exfoliation process of GC. (d) Photos of the EC oxidation product of GC electrode in 50 wt% sulfuric acid solution after different reaction time: 0 hour, 2 hours, and 8 hours.


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Figure 2. Characterizations of ENGO. (a) Photo of the dried ENGO powder. (b) Dispersion of ENGO powder in aqueous solution by stirring for different time: 0 s, 5 s, 10 s and 40 s. (c-e) AFM image of ENGO on a mica substrate (c), the height profile along the yellow line in c (d), and statistical height histogram obtained based on AFM measurements (e). (f) TEM image of ENGO. (g-i) Raman spectra (g), high resolution C1s XPS spectra (h), and FTIR spectra (i) of ENGO and HGO.


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Figure 3. Effect of the concentration of sulfuric acid on the synthesis of ENGO. (a, b) Synthesis rate (a) and C/O atomic ratio (b) of ENGO synthesized by sulfuric acid solutions with different concentrations. (c) Photos of the electrolytes after two hours of EC reaction in sulfuric acid solutions with different concentrations, clearly showing the difference in synthesis rate and oxidation degree of the products.


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Figure 4. Water electrolytic oxidation mechanism of GC. (a) Comparison of 18O content in the ENGO samples synthesized by three different electrolyte combinations with the same sulfuric acid concentration of 50 wt%. Normal: H216O (50 wt%) and H2S16O4.

18O

Aq: H218O (12.5


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wt%), H216O (37.5 wt%) and H2S16O4. 18O Acid: H216O (50 wt%) and H2S16O318O. (b) Current change with reaction time at a constant potential of 3.5 V in electrolytes with sulfuric acid concentration of 10 wt%, 50 wt%, and 98 wt%. (c-e) The schematic of the oxidation and exfoliation mechanism of GC in 10 wt% (c), 50 wt% (d) and 98 wt% (e) sulfuric acid solutions.


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Figure 5. Characterizations of the ENGO synthesized from GC with different graphitization degrees. (a-c) TEM images of ENGO2160 (a), ENGO1700 (b) and ENGO1200 (c), which were synthesized from the GC annealed at 2160 °C (GC2160), 1700 °C (GC1700) and 1200 °C (GC1200), respectively. (d-f) Size distributions of ENGO2160, ENGO1700 and ENGO1200 obtained from TEM measurements. (g, h) C1s XPS spectra of ENGO1700 (g) and ENGO1200 (h). (i) The peak area ratio of O-C=O to C-O and C=O to C-O for ENGO2160, ENGO1700 and


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ENGO1200 obtained based on their C1s XPS spectra, clearly showing the difference in the percentage of carbonyl and carboxyl groups.

Figure 6. Catalytic performance of ENGO with different sizes. (a, b) Schematic representation (a) and conversion (b) of the oxidative coupling reaction of benzylamine by using ENGO as catalyst. (c, d) Schematic representation (c) and conversion (d) of the oxidation reaction of benzene by using ENGO as catalyst.


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ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information. Photo and electron diffraction pattern of GC; Photos of electrode and electrolyte as well current change vs EC reaction time curve for different carbon materials as anodes; XPS spectra and SEM image of GC surface after EC reaction; Photos showing the dispersion of ENGO and HGO; Tyndall effect of ENGO aqueous solution, Zeta potential of ENGO and HGO and photos of dispersion state of ENGO; SEM images of HGO and S-HGO; XRD patterns of ENGO, HGO and S-HGO powders; Photos and UV-vis absorbance of ENGO and HGO solution; TEM image, XPS spectra, photos, UV-vis spectra, PL emission spectra, PL excitation spectra and time-resolved PL spectra of the reduced ENGO; SEM image of black particles obtained by using dilute sulfuric acid solution; Schematic of the oxidation and exfoliation mechanism of GC; Schematic of the oxidation and exfoliation mechanism of EGP and GICP; Current change vs EC reaction time for the H2SO4 intercalated graphite paper in different concentration of sulfuric acid solution; TEM images, XRD patterns, Raman spectra and XPS spectra of GC synthesized with different graphitization temperatures; Peak fitting results of XPS spectra of ENGO2160, ENGO1700 and ENGO1200. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID


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Wencai Ren: 0000-0003-4997-8870. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Qinwei Wei and Songfeng Pei contributed equally to this work. ACKNOWLEDGMENT We thank W. Geng, L. Qi and X. Wei for TEM Characterizations. This work was financially supported by the National Key R&D Program of China (No. 2016YFA0200101), National Science Foundation of China (Nos. 51325205, 51290273, 51521091, 51861135201, and 21503241), the Strategic Priority Research Program of Chinese Academy of Sciences (No. XDB30000000), Chinese Academy of Sciences (No. 174321KYSB20160011), Liaoning Revitalization Talents Program (No. XLYC1808013), and Open Fund Program of Material Corrosion and Protection Key Laboratory of Sichuan Province (No. 2014CL04). REFERENCES 1.

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TABLE OF CONTENTS GRAPHIC Nano-graphene oxide (ENGO) is efficiently synthesized with a high yield (40 wt%) by one-step electrochemical oxidation of glassy carbon (GC). It shows a high oxidation degree and excellent dispersibility. Importantly, the size of ENGO can be tuned (4, 8 and 13 nm) by the graphitization degree of GC, and size effect on the catalytic activities in different reactions is studied.

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High Yield Controlled Synthesis of Nano-Graphene Oxide by Water

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