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1 DOI: /NCHEM.2315 Ultrahigh-throughput exfoliation of graphite into pristine single-layer graphene using microwaves and molecularly engineered ionic liquids Michio Matsumoto 1, Yusuke Saito 1, Chiyoung Park 1, Takanori Fukushima 2 and Takuzo Aida 1,3 * * To whom correspondence should be addressed. aida@macro.t-tokyo.ac.jp (T.A.) Table of Contents 1. General... S2 2. Synthesis and Characterization... S3 (Supplementary Figs. 1 14, Supplementary Table 1) 3. Effects of Microwaves... S14 (Supplementary Figs ) 4. Characterization of Graphene Sheets... S17 (Supplementary Figs ) 5. Intercalation of HF into Graphite... S23 (Supplementary Fig. 24) 6. Redispersion of Graphene Sheets in IL2PF 6... S24 (Supplementary Figs. 25 and 26) 7. Supplementary References... S25 NATURE CHEMISTRY 1

2 1. General Unless otherwise noted, all reagents were purchased from Kanto Chemical, Aldrich, Tokyo Chemical Industry (TCI) and Wako Pure Chemical Industries, and used as received without further purification. Graphite supplied by Wako Pure Chemical Industries, Ltd was used after sieving with mesh #35. Microwave irradiation was performed with a CEM model Discover microwave reactor. 1 H and 13 C NMR spectra were recorded at 25 C on a JEOL model JEOL α-500 spectrometer, operating at 500 and 125 MHz, respectively, where chemical shifts (δ in ppm) were determined with respect to non-deuterated solvent residues as internal references. Electrospray ionization (ESI) mass spectrometry was performed on a Thermo Scientific model Exactive mass spectrometer. Transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM) were carried out with JEOL model JEM-1400 and JEM-2100 electron microscopes operating at 120 kv, respectively. Samples were deposited on a specimen Cu grid covered with thin polymer and carbon support films manufactured by Okenshoji Co., Ltd. Tapping-mode atomic force microscopy (AFM) was performed in air at 25 C on a SII NanoTechnology model NanoNavi S-image (scan range: µm) with silicon cantilevers (SI-DF20S, SII NanoTechnology). Scanning electron microscopy (SEM) images were collected with a JEOL model JSM-7001F scanning electron microscopy. Differential scanning calorimetry (DSC) was performed on a Mettler Toledo model DSC 822e differential scanning calorimeter, where the heating profile was recorded and analyzed using a Mettler Toledo STAR e software system. Electronica absorption and laser Raman spectra were recorded on a Molecular Devices SpectraMax Paradigm multi-mode microplate detection platform and a JASCO model NRS-5100 laser Raman spectrometer, respectively. Rheological properties were measured by using an Anton Paar model MCR-301 rheometer, with a 25-mm diameter parallel plate attached to a transducer. X-ray diffraction (XRD) measurements were carried out with a Rigaku model RINT-ULTIMA3 X-ray diffractometer. Small angle X-ray scattering (SAXS) was collected with a Rigaku model NANO-Viewer X-ray diffractometer using a Rigaku PILATUS 2D detector at a detection length of 120 cm. Surface tension measurements were carried out with a DataPhysics model OCA35 automatic video-based optical contact angle and drop contour analysis system. Electronic conductivity measurements were carried out with a Solartron model 1260 impedance/gain-phase analyzer. NATURE CHEMISTRY 2

3 2. Synthesis and Characterization 2.1. Synthesis of IL2OTs, IL2PF 6 and IL2TFSI IL2OTs: To an MeCN solution (20 ml) of triethylene glycol bis(p-toluenesulfonate) (50.5 g, mol) under Ar was added 1-butylimidazole (33.1 g, mol), and the mixture was stirred for 16 h at 60 ºC. The reaction mixture was evaporated to dryness under reduced pressure, and to the viscous residue was added CH 2 Cl 2 (10 ml). The resulting solution was washed 3 times with AcOEt (50 ml). The lower phase separated was collected and evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL2OTs as pale yellow viscous liquid (74.9 g, mol, 95%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.16 (s, 2H), 7.79 (t, J = 1.8 Hz, 2H), 7.72 (t, J = 1.8 Hz, 2H), 7.47 (d, J = 8.0 Hz, 4H), 7.11 (d, J = 8.0 Hz, 4H), 4.32 (t, J = 5.0 Hz, 4H), 4.16 (t, J = 7.1 Hz, 4H), 3.72 (t, J = 4.9 Hz, 4H), 3.50 (s, 4H), 2.27 (s, 6H), 1.73 (tt, J = 7.3, 7.3 Hz, 4H), 1.21 (qt, J = 7.5, 7.5 Hz, 4H), 0.86 (t, J = 7.5 Hz, 6H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 145.7, 137.6, 136.3, 128.0, 125.4, 122.8, 122.2, 69.3, 68.1, 48.7, 48.5, 31.3, 20.7, 18.7, 13.2; ESI-MS: m/z ([M OTs] +, calcd. for C 27 H 43 N 4 O 5 S ). IL2PF 6 : To an MeCN/water (40 ml/80 ml) solution of IL2OTs (73.8 g, mol), was added KPF 6 (43.2 g, mol), and the mixture was stirred for 2 h at 25 ºC. The reaction mixture spontaneously separated into an aqueous phase and an organic phase as the reaction proceed, and was decanted to remove the upper aqueous phase. CH 2 Cl 2 (20 ml) was added to the organic phase collected, and the mixture was washed 3 times with distilled water (50 ml). The lower phase separated was collected and evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL2PF 6 as pale yellow viscous liquid (58.4 g, mol, 85%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.09 (s, 2H), 7.75 (t, J = 1.8 Hz, 2H), 7.69 (t, J = 1.8 Hz, 2H), 4.32 (t, J = 5.0 Hz, 4H), 4.17 (t, J = 7.3 Hz, 4H), 3.74 (t, J = 4.8 Hz, 4H), 3.52 (s, 4H), 1.76 (tt, J = 7.5, 7.5 Hz, 4H), 1.24 (qt, J = 7.4, 7.4 Hz, 4H), 0.89 (t, J = 7.3 Hz, 6H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 136.3, 122.8, 122.2, 69.4, 68.1, 48.8, 48.6, 31.4, 18.8, 13.2; ESI-MS: m/z ([M PF 6 ] +, calcd. for C 20 H 36 F 6 N 4 O 2 P ). NATURE CHEMISTRY 3

4 IL2TFSI: To an MeCN solution (20 ml) of IL2OTs (36.8 g, mol) was added LiTFSI (35.0 g, mol), and the mixture was stirred for 2 h at 25 ºC. The reaction mixture spontaneously separated into an aqueous phase and an organic phase as the reaction proceed, and was decanted to remove the upper aqueous phase. CH 2 Cl 2 (20 ml) was added to the organic phase, and the mixture was washed 3 times with distilled water (50 ml). The lower phase separated was collected and evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL2TFSI as pale yellow viscous liquid (44.5 g, mol, 92%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.13 (s, 2H), 7.78 (t, J = 1.8 Hz, 2H), 7.72 (t, J = 1.8 Hz, 2H), 4.34 (t, J = 4.8 Hz, 4H), 4.19 (t, J = 7.3 Hz, 4H), 3.76 (t, J = 5.0 Hz, 4H), 3.54 (s, 4H), 1.78 (tt, J = 7.5, 7.5 Hz, 4H), 1.24 (qt, J = 7.5, 7.5 Hz, 4H), 0.91 (t, J = 7.5 Hz, 6H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 136.5, 123.5, 122.9, 122.4, 121.0, 118.4, 115.8, 69.6, 68.3, , 31.5, 18.9, 13.2; ESI-MS: m/z ([M TFSI] +, calcd. for C 22 H 36 F 6 N 5 O 6 S ) Synthesis of IL1 and IL1PF 6 IL1 : 2-(2-(2-Methoxyethoxy)ethoxy)ethyl 4-methylbenzenesulfonate was synthesized according to ref. S1. IL1PF 6 : To an MeCN solution (10 ml) of IL1 (22.9 g, mol) under Ar was added 1-butylimidazole (8.97 g, mol), and the mixture was stirred at 60 ºC for 48 h. To the reaction mixture was added an aqueous solution (20 ml) of KPF 6 (15.17 g, mol), and the mixture was stirred for 2 h at 25 ºC. The reaction mixture was extracted 3 times with CH 2 Cl 2 (30 ml), and the lower phase separated was collected and washed 3 times with distilled water (90 ml). The lower phase separated was collected and evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL1PF 6 as transparent liquid (28.3 g, mol, 94%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.10 (s, 1H), 7.77 (t, J = 1.8 Hz, 1H), 7.74 (t, J = 2.0 Hz, 1H), 4.33 (t, J = 4.8 Hz, 2H), 4.17 (t, J = 7.3 Hz, 2H), 3.76 (t, J = 5.3 Hz, 2H), (m, 2H), (m, 4H), (m, 2H), 3.23 (s, 3H), 1.76 (tt, J = 7.5, 7.5 Hz, 2H), 1.24 (qt, J NATURE CHEMISTRY 4

5 = 7.5, 7.5 Hz, 2H), 0.89 (t, J = 7.5 Hz, 3H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 136.3, 122.8, 122.2, 71.3, 69.6, 69.5, 69.5, 68.0, 58.1, 48.8, 48.6, 31.4, 18.8, 13.2; ESI-MS: m/z ([M PF 6 ] +, calcd. for C 14 H 27 N 2 O ) Synthesis of IL4, IL4, IL4OTs and IL4PF 6 IL4 : To an MeCN solution (60 ml) of triethylene glycol bis(p-toluenesulfonate) (52 g, mol) under Ar was added 1-butylimidazole (4.69 g, mol), and the mixture was stirred for 10 h at 60 ºC. The reaction mixture was evaporated to dryness under reduced pressure, and a CH 2 Cl 2 (15 ml) solution of the viscous residue was subjected to column chromatography on SiO 2 with CH 2 Cl 2 /MeOH = 9/1 as eluent, to allow isolation of IL4 as pale yellow viscous liquid (15 g, mol, 68%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.14 (s, 1H), (m, 4H), (m, 4H), 7.05 (d, J = 8.0 Hz, 2H), 4.32 (t, J = 4.9 Hz, 2H), 4.16 (t, J = 7.0 Hz, 2H), 4.08 (t, J = 4.6 Hz, 2H), 3.73 (t, J = 4.9 Hz, 2H), (m, 6H), 3.36 (s, 4H), 2.41 (s, 3H), 2.27 (s, 3H), 1.75 (tt, J = 7.5, 7.5 Hz, 2H), 1.24 (qt, J = 7.4, 7.4 Hz, 2H), 0.87 (t, J = 7.5 Hz, 3H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 145.8, 145.0, 137.6, 136.3, 132.3, 130.2, 128.1, 127.6, 125.5, 122.8, 122.2, 70.0, 69.5, 69.4, 68.1, 67.9, 48.8, 48.5, 31.3, 21.1, 20.8, 18.7, 13.3; ESI-MS: m/z ([M OTs] +, calcd. for C 20 H 31 N 2 O 5 S ). IL4 : 1,1 -[1,2-Ethanediylbis(oxy-2,1-ethanediyl)]bis(imidazole) was synthesized according to ref. S2. IL4OTs: To an MeCN (10 ml) solution of a mixture of IL4 (13.85 g, mol) and IL4 (2.97 g, mol) was stirred under Ar for 48 h at 60 ºC. The reaction mixture was evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL4OTs as pale yellow viscous liquid (16.82 g, mol, 100 %). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.17 (s, 2H), 9.14 (s, 2H), (m, 6H,), 7.47 (d, J = 7.5 Hz, 8H), 7.11 (d, J = 7.5 Hz, 8H), 4.34 (m, 16H), 3.73 (t, NATURE CHEMISTRY 5

6 J = 4.9 Hz, 12H), 3.51 (d, J = 1.2 Hz, 12H), 2.28 (s, 12H), 1.74 (tt, J = 7.5, 7.5 Hz, 4H), 1.22 (qt, J = 7.5, 7.5 Hz, 4H), 0.87 (t, J = 7.3 Hz, 6H); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 145.7, 137.7, 136.6, 136.3, 128.1, 125.5, 122.8, 122.6, 122.3, 69.3, 68.2, 68.1, 48.7, 48.5, 31.4, 20.8, 187, 13.3; ESI-MS: m/z ([M OTs] +, calcd. for C 59 H 87 N 8 O 15 S ). IL4PF 6 : To an MeCN solution (10 ml) of IL4OTs (16.82 g, mol) was added an aqueous solution (20 ml) of KPF 6 (9.97 g, mol), and the mixture was stirred for 2 h at 25 ºC. The reaction mixture spontaneously separated into an aqueous phase and an organic phase as the reaction proceed, and was decanted to remove the upper aqueous phase. The organic phase was washed 3 times with distilled water (90 ml). The lower phase separated was collected and evaporated by a rotary evaporator, and the residue was put overnight in a vacuum oven at 105 C over P 2 O 5, affording IL4PF 6 as yellow viscous liquid (14.78 g, mol, 94%). 1 H NMR (500 MHz, DMSO-d 6, 25 ºC) δ (ppm) 9.11 (s, 2H), 9.06 (s, 2H), 7.78 (t, J = 1.5 Hz, 2H), 7.70 (d, J = 1.5 Hz, 6H,), (m, 12H), 4.17 (t, J = 7.0 Hz, 4H), 3.73 (t, J = 4.5 Hz, 12H), 3.52 (t, J = 2.5 Hz, 12H), 1.77 (tt, J = 7.4, 7.4 Hz, 4H), 1.24 (qt, J = 7.5, 7.5 Hz, 4H), 0.89 (t, J = 7.3 Hz, 6H,); 13 C NMR (125 MHz, DMSO-d 6, 25 ºC) δ (ppm) 136.5, , 122.8, 122.6, 122.3, 69.3, 68.2, 68.1, 48.8, 48.6, 31.3, 18.7, 13.2; ESI-MS: m/z ([M PF 6 ] +, calcd. for C 38 H 66 F 18 N 8 O 6 P ). NATURE CHEMISTRY 6

7 H and 13 C NMR Spectroscopy Supplementary Figure 1. 1 H NMR spectrum of IL2PF 6 in DMSO-d 6 at 25 ºC. Asterisked signals at δ2.49 and 3.34 ppm are due to partially non-deuterated residues of DMSO-d 6 and water, respectively. Supplementary Figure C NMR spectrum of IL2PF 6 in DMSO-d 6 at 25 ºC. An asterisked signal at δ39.5 ppm is due to DMSO-d 6. NATURE CHEMISTRY 7

8 Supplementary Figure 3. 1 H NMR spectrum of IL2TFSI in DMSO-d 6 at 25 ºC. Asterisked signals at δ2.49 and 3.34 ppm are due to partially non-deuterated residues of DMSO-d 6 and water, respectively. Supplementary Figure C NMR spectrum of IL2TFSI in DMSO-d 6 at 25 ºC. An asterisked signal at δ39.5 ppm is due to DMSO-d 6. NATURE CHEMISTRY 8

9 Supplementary Figure 5. 1 H NMR spectrum of IL1PF 6 in DMSO-d 6 at 25 ºC. Asterisked signals at δ2.49 and 3.34 ppm are due to partially non-deuterated residues of DMSO-d 6 and water, respectively. Supplementary Figure C NMR spectrum of IL1PF 6 in DMSO-d 6 at 25 ºC. An asterisked signal at δ39.5 ppm is due to DMSO-d 6. S9 NATURE CHEMISTRY 9

10 Supplementary Figure 7. 1 H NMR spectrum of IL4PF 6 in DMSO-d 6 at 25 ºC. Asterisked signals at δ2.49 and 3.34 ppm are due to partially non-deuterated residue of DMSO-d 6 and water, respectively. Supplementary Figure C NMR spectrum of IL4PF 6 in DMSO-d 6 at 25 ºC. An asterisked signal at δ39.5 ppm is due to DMSO-d 6. NATURE CHEMISTRY 10

11 2.5. Electrospray Ionization Mass (ESI-MS) Spectrometry Supplementary Figure 9. ESI-MS spectrum of IL2PF 6. Obsd. m/z ([M PF 6 ] +, calcd. for C 20 H 36 F 6 N 4 O 2 P ). Supplementary Figure 10. ESI-MS spectrum of IL2TFSI. Obsd. m/z ([M TFSI] +, calcd. for C 22 H 36 F 6 N 5 O 6 S ). NATURE CHEMISTRY 11

12 Supplementary Figure 11. ESI-MS spectrum of IL1PF 6. Obsd. m/z ([M PF 6 ] +, calcd. for C 14 H 27 N 2 O ). Supplementary Figure 12. ESI-MS spectrum of IL4PF 6. Obsd. m/z ([M PF 6 ] +, calcd. for C 38 H 66 F 18 N 8 O 6 P ). NATURE CHEMISTRY 12

13 2.6. Surface Tensions of Ionic Liquids Supplementary Table 1. Surface tensions of ionic liquids at 25 ºC. Density (g cm 3 ) Surface Tension (mn m 1 ) BMIPF * IL1PF IL2PF IL4PF 6 IL2TFSI * Data reported in ref. S Differential Scanning Calorimetry (DSC) Analysis Supplementary Figure 13. DSC trace of IL2PF 6 on second heating (red) from 50 C and second cooling (black) from 150 C at a rate of 10 ºC min 1. NATURE CHEMISTRY 13

14 2.8. X-ray Photoelectron Spectroscopy (XPS) Analysis Supplementary Figure 14. XPS spectrum of IL2PF 6 placed on an indium-foil sheet. 3. Effects of Microwaves 3.1. Heating Effect of Microwaves on Ionic Liquids Supplementary Figure 15. Temperature change profile of an IL2PF 6 suspension of graphite (25 mg/ml, 0.5 ml) upon 30-min exposure to microwaves at 30 W. NATURE CHEMISTRY 14

15 3.2. Heating Effect of Microwaves on Decomposition of IL2PF 6 Supplementary Figure H NMR spectra in DMSO-d 6 at 25 ºC of IL2PF 6 (a) before and (b) after 30-min exposure to microwaves at 30 W. NATURE CHEMISTRY 15

16 3.3. Heating Effect of Microwaves on Decomposition of IL2TFSI Supplementary Figure H NMR spectra in DMSO-d 6 at 25 ºC of IL2TFSI (a) before and (b) after 30-min exposure to microwaves at 30 W. NATURE CHEMISTRY 16

17 4. Characterization of Graphene Sheets 4.1. Powder X-ray Diffraction (XRD) Analysis Supplementary Figure 18. (a, b) Powder XRD profiles of precursor graphite (a) and graphene sheets (b). The exfoliation and isolation conditions are essentially identical to those described in Fig. 1. NATURE CHEMISTRY 17

18 4.2. Scanning Electron Microscopy (SEM) Analysis SEM a b c 1 µm 1 µm 2 µm d e f 10 µm 25 µm 25 µm Supplementary Figure 19. (a d) SEM micrographs of graphene sheets. The samples were prepared by casting from MeCN onto a silicon substrate. The exfoliation and isolation conditions are essentially identical to those described in Fig. 1. S18 NATURE CHEMISTRY 18

19 4.3. Transmittance Electron Microscopy (TEM) Analysis Supplementary Figure 20. (a d) TEM and (e h) HR-TEM micrographs of graphene sheets. The samples were prepared by casting from MeCN onto a carbon-grid substrate. The exfoliation and isolation conditions are essentially identical to those described in Fig. 1. NATURE CHEMISTRY 19

20 4.4. Atomic Force Microscopy (AFM) Analysis Supplementary Figure 21. Tapping-mode AFM images of graphene sheets. The sample was prepared by casting from MeCN onto a mica substrate. The exfoliation and isolation conditions are essentially identical to those described in Fig. 1. NATURE CHEMISTRY 20

21 4.5. Electronic Conductivity Analysis Supplementary Figure 22. (a, b) Schematic illustrations of a micro-gap electrode for electronic characterization, (c) optical micrograph and (d) I V profile of a graphene sheet on the micro-gap electrode (R S : an average of those obtained with four separately prepared micro-gap electrode devices) Methods Micro-gap Au electrodes with a gap of ~2 µm were fabricated on silicon substrates. Samples were prepared by casting an MeCN dispersion of graphene sheets onto the micro-gap electrodes. Conditions for the exfoliation of graphite and isolation of the resultant graphene sheets were essentially identical to those described in Fig. 1. Four separately prepared micro-gap electrode devices were used for the I V profile evaluation of a single graphene sheet with a two-point probe method in conjunction with a Solartron model 1260 impedance/gain-phase analyzer. From the resultant I V profiles, resistance values R at 0.2 V were obtained and converted into sheet resistance values R S using the equation R S = RW/L, where W and L represent graphene sheet width and electrode gap length, respectively. NATURE CHEMISTRY 21

22 4.6. XPS Analysis Supplementary Figure 23. (a) C1s and (b) N1s XPS spectra of precursor graphite, IL2PF 6 and isolated graphene sheets. NATURE CHEMISTRY 22

23 5. Intercalation of HF into Graphite Supplementary Figure 24. (a c) Time-dependent XRD profiles in 0 (a), 12 (b), and 36 hours (c) of graphite immersed at 25 C in HF-containing IL2TFSI (for preparation, see the Method section). The diffractions at 2θ = 26.5º and 15.4º are due to the interlayer distances of graphite without and with intercalated HF, respectively, while a broad peak centered at 2θ = 20.0º is due to an amorphous halo of IL2TFSI. NATURE CHEMISTRY 23

24 6. Redispersion of Graphene Sheets in IL2PF Electronic Absorption Spectroscopy Supplementary Figure 25. Electronic absorption spectrum of graphene sheets redispersed in IL2PF 6 (1.0 mg/ml). The exfoliation and isolation conditions are essentially identical to those described in Fig Dynamic Viscoelastic Analysis Supplementary Figure 26. (a) Strain (γ) dependencies of storage moduli (G, solid circles) and loss moduli (G, open circles) of IL2PF 6 containing graphene sheets (25 mg/ml, blue), together with those of its reference samples; graphite in IL2PF 6 (25 mg/ml, grey) and IL2PF 6 alone (orange) at 35 ºC. The applied angular frequency (ω) was rad s 1. The S24 NATURE CHEMISTRY 24

25 γ sweep was made from lower to higher. (b) Storage moduli (G, solid circles) and loss moduli (G, open circles) of IL2PF 6 containing graphene sheets (25 mg/ml) at different temperatures. Applied γ and ω were 0.1% and rad s 1, respectively. The exfoliation and isolation conditions are essentially identical to those described in Fig Supplementary References S1. Gooding, J. J. et al., J. Am. Chem. Soc. 134, 844 (2012). S2. Bara, J. E. Ind. Eng. Chem. Res. 50, (2011). S3. Countinho, J. A. P. et al., J. Colloid Interface Sci. 314, 621 (2007). NATURE CHEMISTRY 25