Real-Time, In Situ Monitoring of the Oxidation of Graphite: Lessons Learned

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1 Supporting Information Real-Time, In Situ Monitoring of the Oxidation of Graphite: Lessons Learned Naoki Morimoto, a Hideyuki Suzuki, b Yasuo Takeuchi, a Shogo Kawaguchi, c Masahiro Kunisu, d Christopher W. Bielawski, e,f and Yuta Nishina b,g * a Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences, Division of Pharmaceutical Sciences, Okayama University, Tsushimanaka, Kita-ku, Okayama , Japan b Research Core for Interdisciplinary Sciences, Okayama University, Tsushimanaka, Kita-ku, Okayama , Japan c Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo, Hyogo , Japan d Toray Research Center, Inc., Surface Science Laboratories, 2-1 Sonoyama 3-chome, Otsu, Shiga, , Japan e Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea f Department of Chemistry and Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea g Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama , Japan 1

2 1. Materials Graphite (SP-1 grade) was purchased from BAY CARBON Inc. Graphite (XD-150 and CNP-7) were purchased from Ito Graphite Co., Ltd. H2SO4 (>95% wt.%), KMnO4, 30% wt.% aq. H2O2 and NaNO3 were purchased from WAKO CHEMICAL PURE INDUSTRIES Ltd. H2SO4 (>99.999% wt.%) was purchased from Sigma-Aldrich Japan K.K. 60% wt.% nitric acid was purchased from NACALAI TESQUE, INC. 2. Experimental Procedures 2-1. Formation of GICs with NaNO3 or KMnO4 Graphite (500 mg) was stirred in 95% wt.% H2SO4 (25 ml). KMnO4 (50 mg) or NaNO3 (250 mg) was added and stirred at room temperature for 1 min. The resulting mixtures were independently centrifuged, and the resulting paste-like solid products were studied using XRD analysis. Since similar GICs were formed in both cases, we concluded that using 10 wt% of KMnO4 gives a similar outcome as when using 50 wt% NaNO3 (Figure S1a) Exploring the Effect of NaNO3 on the Formation of GO Synthesis of GO using NaNO3 Graphite (500 mg) and NaNO3 (250 mg) was stirred in 95% wt.% H2SO4 (25 ml). KMnO4 (1.5 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with water (25 ml) under vigorous stirring and cooling so that the temperature did not exceed 50 C. The suspension was further treated with 30% wt.% aq. H2O2 (1.25 ml). The resulting suspension was purified by repeated centrifugation with water. The sample was labeled as GO-NaNO Synthesis of GO without NaNO3 Graphite (500 mg) was stirred in 95% w/w H2SO4 (25 ml). KMnO4 (1.5 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with water (25 ml) under vigorous stirring and cooling so that the temperature did not exceed 50 C. The suspension was further treated with 30% wt.% aq. H2O2 (1.25 ml). The resulting suspension was purified by repeated centrifugation with water. The sample was labeled as GO-std. 2

3 Characterization of the GO-NaNO3 and GO-std Samples While the C1s region of the XPS data revealed that NaNO3 enhanced the formation of C O bonds (Figure S1b), both GO-NaNO3 and GO-std were exfoliated via sonication to afford single layered materials as determined by SEM (Figure S1c). No remarkable differences were observed from the elemental analysis data collected for the samples. Figure S1. (a) XRD spectra of graphite that was treated with (i) 10 wt% of KMnO4, (ii) 50 wt% of NaNO3, or (iii) H2SO4. (b) XPS and (c) SEM images of GO that was prepared (i) with NaNO3 or (ii) without NaNO3. (d) Elemental compositions measured for each sample. 3

4 2-3 Determining the Origin of the Oxygen Found in GO Exploring the Effect of Water during the Oxidation of Graphite Graphite (250 mg) was stirred in H2SO4 (12.5 ml) containing various quantities of water. KMnO4 (750 mg) was added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with water (12.5 ml) under vigorous stirring and cooling so that the temperature did not exceed 50 C. The suspension was treated with 30% wt.% aq. H2O2 (1.25 ml) and purified by repeated centrifugation. XPS analysis revealed that a signal derived from oxygen-containing functional groups gradually increased as the amount of water increased (Figure S2a). However, the oxygen content of GO synthesized using 20 vol.% of water was measured to be relatively low when characterized by elemental analysis (Figure S2b). The difference suggested to us that surface oxidation became dominant when a large quantity of water was added to the reaction mixture. The addition of water also affected the morphology of resulting GO as unexfoliated graphitic domains were observed even after sonication (Figure S2c-iv). 4

5 Figure S2. (a) XPS analysis of GO prepared in H2SO4 containing (i) 20 vol.%, (ii) 10 vol.%, (iii) 5 vol.%, or (iv) <0.001 vol.% of water. (b) A summary of elemental analysis data collected for samples of GO prepared in the presence of various quantities of water (indicated). (c) Optical microscope images of (i) (iii) as prepared GO, and (iv) (vi) GO after sonication. The water content used to prepare the samples shown was (i) and (iv) 20 vol.%, (ii) and (v) 10 vol.%, and (iii) and (vi) < vol.% Effect of Water on Various Samples of GO Proton-Deuterium Exchange Experiments Dried GO-std (25 mg) was mixed with D2O (50 L) under an atmosphere of argon and left overnight. The dispersion was dried under reduced pressure and TGA-MS analysis was performed. DHO was detected at >90 C which suggested to us that H/D exchange 5

6 occurred at oxygen-containing functional groups (e.g., hydroxyl groups and/or carboxyl groups) on GO O and 18 O Exchange Experiments Dried GO-std (25 mg) was mixed with H2[ 18 O] (50 L) under an atmosphere of argon and left overnight. The dispersion was dried under reduced pressure and TGA-MS analysis was performed. Relatively low quantities of H2[ 18 O] were detected TGA-MS Analysis of GO Prepared by Using H2[ 18 O] Graphite (20 mg) was stirred in H2SO4 (99.999% purity, 0.5 ml) under an atmosphere of argon. KMnO4 (60 mg) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with H2[ 18 O] (0.5 ml) under vigorous stirring and cooling such that the temperature did not exceed 50 C. The suspension was further treated with 30% wt.% aq. H2O2 (50 L). The resulting suspension was purified by centrifugation with H2O. The sample was labeled as GO-H2[ 18 O]. The GO was dried under reduced pressure and TGA- MS analysis was performed. Species containing [ 18 O], such as carbon monoxide, carbon dioxide, water, oxygen, various sulfur oxides, or other by-products were not detected. Figure S3. TGA (red) and MS (black) data collected for (a) GO-std and (b) GO-H2[ 18 O] Time-Course and In Situ Analyses of Graphite Oxidation Reactions Time-Course Analysis of the Concentration of Mn in the Liquid Phase via Atomic Absorption Spectroscopy Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually 6

7 added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C. After 120 min, water (150 ml) was added under cooling and stirred for an additional 130 min. The reaction suspension was treated with 30% wt.% aq. H2O2 (7.5 ml) and stirred for 20 min. After the KMnO4 was added, aliquots of the reaction mixture were removed at 10, 20, 30, 45, 60, 90, 120, 125, 160, 190, 220, 250, and 270 min. The collected samples were centrifuged, and the resulting supernatants were collected and then diluted with water. The concentrations of manganese in the samples were measured using atomic absorption spectroscopy Time-Course Analysis of Mn(VII) Using UV-Vis Spectroscopy Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C. After the KMnO4 was added, aliquots of the reaction mixture were removed at 10, 20, 30, 45, 60, 90 and 120 min, and centrifuged. The resulting supernatants were diluted with water and the concentration of the residual Mn(VII) species was measured using UV-Vis spectroscopy In Situ XRD Analysis A solution of KMnO4 (90 mg) dissolved in H2SO4 (0.3 ml) was added to a dispersion of graphite (30 mg) in H2SO4 (0.6 ml) with cooling by using a stream of nitrogen gas at - 10 C with a cold N2 gas flow apparatus. The oxidation reaction was initiated by heating the sample using N2 gas at 35 C. The in situ XRD measurements were performed before heating (0 min) and 1, 30, 60 and 120 min after the addition of KMnO In Situ XAFS Analysis To explore how various manganese containing species change during the oxidation of graphite, a series of XAFS analyses was performed with MnO, Mn2O3 and KMnO4 as standards. The spectra derived from KMnO4 showed a pre-edge signal at 6540 ev and a main absorption region was observed between 6550 to 6560 ev. Lower valence manganese species do not exhibit a pre-edge signals and primary absorptions are typically observed at lower photon energy regimes (Mn +2 : 6540 to 6553 ev, Mn +3 : 6543 to 6555 ev). 7

8 Figure S4. XANES spectra recorded for various manganese standards. Graphite (1.0 g) was stirred in 95% wt.% H2SO4 (25 ml). KMnO4 (3.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C. The reaction mixture was measured after 30, 60 and 120 min. Afterward, water (25 ml) was added to the solution, stirred for 10 min, and then analyzed. The mixture was further treated with 30% wt.% aq. H2O2 (2.5 ml) and then analyzed. To separately measure liquid and solid phases, a portion of the mixture was taken up and centrifuged. The supernatant liquids and solid residues were independently measured Time-Course Analysis of the Oxygen Content in GO Using Elemental Analysis Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C. After KMnO4 was added, aliquots of the reaction mixture were removed after 10, 20, 30, 45, 60, 90 and 120 min, and treated with excess amount of cold water followed by 30% wt.% aq. H2O2, and then purified by centrifugation. The resulting products were freeze-dried and the oxygen contents were determined by CHNS elemental analysis Time-Course Analysis of Potassium Content Using Atomic Absorption Spectroscopy Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C and stirred for 120 min. After KMnO4 was added, aliquots of the reaction mixture were removed after 10, 20, 30, 45, 60, 90 and 120 min, and centrifuged. The resulting supernatants were diluted, and the concentrations of potassium 8

9 were measured using atomic absorption spectroscopy. Figure S5. Time-course analysis of the quantity of potassium in the liquid phase of a reaction mixture that was prepared as described above Exploring Graphite Size Effects Analysis Using In Situ XRD Graphite flakes of different sizes (50 mg) were independently mixed with 95% wt.% H2SO4 (0.5 ml) in a PTFE bag, loaded in an X-ray irradiation instrument, and then cooled with a stream of nitrogen gas at -10 C. KMnO4 (150 mg) in 95% wt.% H2SO4 (1.0 ml) cooled to 0 C was quickly added to the graphite dispersion and an XRD measurement was conducted. When graphite with a relatively small particle size (<75 m) was used, a signal assigned to a GIC was observed after 1 min (Figure 2). In contrast, when graphite with a relatively large particle size (150 m) was used, signals assigned to a GIC as well as graphite were observed after 1 min. The GIC peak gradually decreased over time (Figure S6b) Analysis Using Atomic Absorption Spectroscopy and UV-Vis Spectroscopy Different-sized graphites (3.0 g) were independently stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C. After the addition of KMnO4, aliquots of the reaction mixture were removed after 10, 20, 30, 45, 60, 90 and 120 min. After centrifugation, the supernatant was diluted and measured by atomic absorption spectroscopy as well as UV-Vis spectroscopy to determine the concentration of Mn in the liquid phase. The use of graphite with a relatively large particle size (150 9

10 m) exhibited relatively slow intercalation and consumption of Mn(VII) when compared to samples prepared from graphite with a relatively small particle size (<75 m) (Figures S6c and S6d). Figure S6. (a) SEM images collected for graphites with different particle sizes. (b) In situ XRD analysis using graphite with a relatively large particle size (150 m) after (i) 0 min, (ii) 1 min, (iii) 30 min, (iv) 60 min, and (v) 90 min. (c) Time-course analysis of the Mn concentration in the liquid phase using atomic absorption spectroscopy. (d) Time-course analysis of the consumption of Mn(VII) in the liquid phase using UV-Vis spectroscopy Analysis of Surface Adsorbed Water on GO TGA of GO After Treatment with N2 and Air TGA was conducted on GO-std after freeze drying (Figure S7(i)), after exposure to a stream of N2 (Figure S7a(ii)), after exposure to a stream of air (Figure S7a(iii)), and reexposure to a stream of N2 (Figure S7a(iv)). The N2 treatment was performed at a flow rate of 300 ml/min at 70 C for 2 h. The air (humidity: 28%) treatment was performed at a flow rate of 300 ml/min for 2 h. 10

11 Desorption and Adsorption of Water on GO Under N2 or Air GO (7.191 mg) was weighed on a precision balance and the atmosphere (N2 vs. air at a humidity of 28%) were alternately exchanged over 2 h intervals. The weight of the sample gradually decreased upon treatment with N2 and gradually recovered upon exposure to air. From the data, > 7.8 wt% of water was calculated to be adsorbed on GO under air. Figure S7. (a) TGA data recorded for GO (i) as prepared, (ii) after exposure to a stream of N2, (iii) after exposure to a stream of air, (iv) after re-exposure to a stream of N2. (b) The weight change of GO under N2 or air flow. The numbers (i - iv) in Figure S7b correspond to the samples measured via TGA in Figure S7a Calculation of the Oxidant Efficiency In situ XANES analysis (Figure 3c) revealed that Mn(VII) species were reduced to their Mn(III) derivatives after the graphite oxidation reaction. Thus, 2 moles of oxygen were introduced to graphite per 1 mole of KMnO4 employed. When 1 g of graphite (83.3 mmol of C) and 3 g of KMnO4 (19.0 mmol) were used, 38 mmol of oxygen was theoretically incorporated into the graphite. Therefore, the theoretical oxygen content (wt.%) may be calculated as follows: Oxygen weight 38 mmol 16 g / mol wt % GO weight 1000 mg 38 mmol 16 g / mol Based on the results of the CHNS elemental analysis, the sample labeled GO-std was calculated to contain 48.3 wt% of oxygen, which is higher than the theoretical value and may be due to adsorbed water and/or the presence of sulfoxide-containing functional groups. The amount of adsorbed water was measured by TGA to be 7.8 wt.% (O:

12 wt%). The amount of sulfur was determined by CHNS elemental analysis to be 6.5 wt.%. Considering that the sulfur may stem from residual sulfuric acid, 1 we concluded that 9.8 wt.% of the oxygen was not derived from KMnO4. Based on these calculations, the oxidation efficiency of KMnO4 was measured to be 84%. Note: since 6 mg of CO2 was generated from 3 g of graphite (see below), the effect of CO2 was neglected in the calculation Measurement of Temperature Changes during the Graphite Oxidation Reaction Graphite (500 mg) was stirred in 95% wt.% H2SO4 (25 ml). KMnO4 (1.5 g) was gradually added to the solution, and the mixture was stirred at 35 C for 2 h. The temperature of the solution was measured at 5 min intervals (Figure S8). On this scale, external heating was not performed, but rather cooled using a water bath because the reaction mixture temperature exceeded 55 C within 15 min of adding KMnO4. Figure S8. Time-course analysis of the temperature of the reaction mixture Removal of the Residual Mn from GO Effect of H2O2 Graphite (500 mg) was stirred in 95% wt.% H2SO4 (25 ml). KMnO4 (1.5 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with water (25 ml) under vigorous stirring and cooling so that temperature did not exceed 50 C. The suspension was stirred overnight to precipitate the insoluble Mn species. The resulting suspension was purified by centrifugation from water, and then freeze-dried. An image taken using SEM-EDS showed the presence of Mn (Figure 4a). 12

13 Effect of Other Reductants Graphite (250 mg) was stirred in 95% wt.% H2SO4 (12.5 ml). KMnO4 (750 mg) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h. The resulting mixture was diluted with water (12.5 ml) under vigorous stirring and cooling so that the temperature did not exceed 50 C. The suspension was further treated with citric acid or 30% wt.% aq. HCl. The resulting suspension was purified by centrifugation from water, and then freeze-dried. To measure the residual Mn, GO (50 mg) was sonicated in a solution containing 60% wt.% aq. nitric acid (10 ml) and 30% wt.% aq. H2O2 (5 ml) for 90 min. The resulting solution was filtered, diluted and then analyzed using atomic absorption spectroscopy (Figure S9). Figure S9. (a) Residual Mn in GO as measured using atomic absorption spectroscopy. (b) The elemental composition of GO as prepared using citric acid (0.4 w/w) to quench the oxidation reaction Oxidation Degree and Defect Formation upon the Generation of CO Effect of the Amount of KMnO4 Added and the Reaction Time Graphite (1.0 g) was stirred in 95% wt.% H2SO4 (50 ml). KMnO4 was gradually added to the mixture while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 C for 2 h or 5 days, and then diluted with water (50 ml) under vigorous stirring and cooling so that the temperature did not exceed 50 C. The suspension was further treated by adding 30% wt.% aq. H2O2 (2.5 ml). The resulting suspension was purified by centrifugation with water. The oxygen content did not change when 500 wt.% of KMnO4 was used or when the reaction time was performed for 5 days. These results suggested to us that the oxygenation reaction was complete within 2 h and using more 13

14 than 300 wt% of KMnO4 was unnecessary (Table S1). Note: the sulfur content in the GO product may be removed via dialysis (average pore size of dialysis tube: 50 nm, concentration of GO: 0.1 wt%, until ph becomes neutral). Table S1. Summary of elemental composition data GO prepared using various quantities of KMnO4 and after various periods of time. KMnO4/graphite oxidation time C wt.% H wt.% N wt.% S wt.% O wt.% h n.d h n.d h n.d days n.d days n.d a 2 h n.d. n.d a The elemental composition data were obtained after subjecting the GO product to dialysis for 5 days. n.d. = none detected CO2 Generation During Oxidation Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at 35 or 80 C. The gases generated during the reaction were trapped using a saturated aq. solution of Ba(OH)2, and the resulting solid BaCO3 was collected by filtration. The amount of generated CO2 during the oxidation was calculated from the weight of the BaCO Effect of Temperature Graphite (3.0 g) was stirred in 95% wt.% H2SO4 (150 ml). KMnO4 (9.0 g) was gradually added to the solution while keeping the temperature <10 C using an ice bath. The mixture was then stirred at <10 C. Any structural change of graphite was investigated by XRD (Figure S10a). After the KMnO4 was added, aliquots of the reaction mixture were taken after 10, 20, 30, 45, 60, 90 and 120 min. Each sample was centrifuged and the resulting supernatants were diluted. The concentrations of manganese in the solutions were measured using atomic absorption spectroscopy and the consumption of Mn(VII) was measured using UV-Vis spectroscopy. XRD analysis revealed that the graphite 002 signal at 26.5 disappeared within 10 min and a GIC signal appeared at 22. This result suggested to us that intercalation can occur at <10 C. However, amount of Mn in the liquid phase did not decrease to <80% (Figure S10b) and the consumption rate of Mn(VII) 14

15 was relatively slow (Figure S10c). Elemental analysis of the product, labeled as GO-low temp, indicated that the level of oxidation was relatively low (Table S2). Figure S10. (a) Time-course analysis of a low temperature synthesis of GO using XRD. (b) The change in manganese content in the liquid phase over time. (c) The consumption of Mn(VII) species in the liquid phase over time. Table S2. Elemental composition data collected for GO prepared at a low temperature. C wt.% H wt.% N wt.% S wt.% O wt.% GO-low temp Continuous Flow Synthesis of GO 6.0 wt% of KMnO4 in H2SO4 at a flow rate of 1.0 ml/min was mixed with 2.0 wt% of graphite in H2SO4 at a flow rate of 1.0 ml/min in a PTFE tube. The graphite stock dispersion was stirred using a magnet to prevent sedimentation (note: while sedimentation was not observed when the flow rate was 1 ml/min, partial sedimentation was observed when a lower flow rate (0.4 ml/min) was used). After mixing, the tube was heated to 35 C for 1.5 h. The eluted solution was treated with an aqueous solution of citric acid (1 wt%, 100 ml). The resulting GO was purified by centrifugation. The sample was labeled 15

16 as GO-flow. As a result of the CHNS elemental analysis (Table S3) and SEM analysis (Figure 5), we concluded that single-layer GO was formed after sonication. Table S3. Elemental composition data collected for GO-flow. C wt% H wt% N wt% S wt% O wt% GO-flow Reference 1. Dimiev, A.; Kosynkin, D. V.; Alemany, L. B.; Chaguine P.; Tour, J. M. J. Am. Chem. Soc. 2012, 134,