Exfoliation of Graphite into Graphene in Aqueous. Solutions of Inorganic Salts

Transcription

1 Supporting Information Exfoliation of Graphite into Graphene in Aqueous Solutions of Inorganic Salts Khaled Parvez, Zhong-Shuai Wu, Rongjin Li, Xianjie Liu, Robert Graf, Xinliang Feng,,,,* Klaus Müllen,* Max Planck Institute for Polymer Research, Ackermannweg 10, Mainz 55128, Germany Department of Physics, Chemistry and Biology, Linköping University, SE Linköping, Sweden School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai, P. R. China *Address correspondence to Experimental Section: Electrochemical exfoliation of graphite. Natural graphite flakes were used as a carbon electrode (i.e., anode) for electrochemical exfoliation of graphite. The graphite flakes adhered to a conductive carbon tape, forming a pellet. A Pt wire was used as a cathode. The electrolyte for the exfoliation was prepared by dissolving 1.06 g (NH 4 ) 2 SO 4 in 80 ml H 2 O (i.e., 0.1 M). The distance between the graphite and the Pt electrode was ~2 cm and was kept constant throughout the electrochemical process. Electrochemical exfoliation S1

2 was carried out by applying positive voltage (10 V) to the graphite electrode. After the graphite exfoliation was completed, the product was collected through a PTFE membrane filter with 0.2-µm pore size and washed several times with deionised water by vacuum filtration. The resultant EG was then dispersed in DMF by sonication at low power for 10 min. The dispersion was maintained for 48 h to precipitate un-exfoliated graphite flakes or particles. The top part of the dispersion was used for characterisation and device fabrication. For large-scale production of EG, graphite foil (Alfa Aesar) was used instead of flakes. Material Characterisations. The morphology and structure of the samples were investigated by SEM (Gemini 1530 LEO), AFM (Veeco Dimension 3100), HRTEM and SAED (Philips Tecnai F20), and X-ray diffraction (Bruker D4 X-ray scattering systems with Ni-filtered Cu Kα radiation). Raman spectra and mapping were recorded with a Bruker RFS 100/S spectrometer (laser wavelength 532 nm). The XPS measurements were obtained using a Scienta ESCA 200 spectrometer in an ultrahigh vacuum (base pressure mbar). The measurement chamber is equipped with a monochromatic Al (Kα) x-ray source. The work function of EG was measured by ultraviolet photoelectron spectra (ESCA 200 spectrometer) with a He discharge lamp providing ev photon energy and applying a bias of -3 V to the sample. The work function was calculated using the equation Φ = hv E F + E cutoff, where hv, E F, and E cutoff are the photon energy of the excitation light, the Fermi level edge, and the measured secondary electron cut-off, respectively. Both the XPS and ultraviolet photoelectron spectra measurements were carried out on ~10 nm thick EG film prepared on Au (30 nm)-coated SiO 2 substrates at S2

3 room temperature. The sheet resistances of EG films were measured with a four-point probe system using a Keithly 2700 multimeter (probe spacing: mm, R s = V/I). Solid-State NMR measurements. Studies of the obtained graphene samples were performed at a Bruker Avance III console operating at MHz 13 C Larmor frequency under fast MAS conditions with a sample rotation frequency of 30 khz, using a commercial double-resonance MAS probe supporting zirconia rotors with a 2.5-mm outer diameter. Due to the strong paramagnetic properties and the high electric conductivity of the graphene sample, approximately 3 to 4 mg of graphene was placed in 2.5-mm MAS rotors such that they were located in the centred of the NMR detection coil when the rotor was spinning in the probe, while both ends of the rotor were filled with KBr. Significantly higher bearing and drive pressures had to be applied to spin the rotors in the magnetic field due to substantial eddy currents in the highly conducting samples, which also caused heating of the sample. Despite the paramagnetic perturbation, the samples showed a slow T 1 relaxation behaviour and the spectra were recorded with a 45 excitation pulse of 2 µs and 8000 transients with a recycle delay of 30 s. Fabrication of FETs. The EG films for FET fabrications were prepared using a Langmuir-Blodgett assembly method. Briefly, the EG dispersions (~0.2 mg/ml) in a 1:3 DMF/chloroform mixture were carefully dropped onto the water surface using a 100-µL glass syringe. Drop-wise addition of 3 ml EG dispersion resulted in a faint blackcoloured film on the water surface. The film was then compressed by Langmuir-Blodgett trough barriers and the surface pressure was monitored using a tensiometer. The film was collected by vertically dip-coating SiO 2 (300 nm) substrates. The resulting samples were annealed at 200 C for 30 min under vacuum to evaporate residual solvents, especially DMF. To fabricate single EG flake-based FETs, 100 nm thick Pt was deposited by S3

4 focused ion-beam to connect the isolated flakes to Au source/drain electrodes. The hole carrier mobility was calculated from the linear regime of the transfer curves using the following equation 1 : µ = (L/WC i V d ) ( I d / V g ) Where C i is the dielectric capacitance (11 nf/cm 2 ), and L and W are channel length and width, respectively. The sheet resistance (R s ) of a single EG flake was measured using a two-point probe method on the same device and the value was obtained using the equation R s = RW/L, where R is the resistance at 0.5 V, W is the width of the EG flake, and L is the channel length. All the device measurements were obtained with a Keithly SCS 4200 semiconductor characterization system inside a glove box filled with nitrogen. Preparation of graphene-ink and conductive paper. To create graphene ink, 1.0 g of EG powder was dispersed in 100 ml DMF (i.e., conc. 10 mg/ml) followed by sonication for 20 min. The prepared EG dispersion was then applied to commercial A4-size paper using a paintbrush. The paper was dried at 120 C for 40 s. The paintbrush application and drying process were repeated several times. The sheet resistance of the conductive paper was measured using a four-point probe system and a bending test was performed using a home-built system. Fabrication of graphene paper-based supercapacitors. Graphene paper supercapacitors were prepared by drop-casting polyvinyl alcohol/h 2 SO 4 gel on EGcoated paper (EG loading ~0.60 mg cm -2 ) and solidified overnight. The active area of the supercapacitor device was 2 cm 2 cm. Two pieces of EG-coated paper electrodes were then integrated together without an additional current collector. Thus, an all-solid-state flexible supercapacitor was obtained and used for electrochemical characterization. The S4

5 area capacitance value was calculated from cyclic voltammetry data according to the following equation: Where C 1 V f area = I( V dv v V V A ). ( ) Vi rate (V s -1 ), f i C area is the area capacitance based on graphene electrodes (F cm -2 ), v is the scan V f and V i are the integration potential limits of the voltammetric curve, and I (V ) is the voltammetric discharge current (A), and A refers to the total area (cm 2 ) of the device. S5

6 Figure S1. Photographs of large-scale synthesis of EG. (a) and (b) graphite foil before and after electrochemical exfoliation. The dimension of graphite foil is 11.5 cm 2.5 cm. (c) Exfoliated product in aqueous (NH 4 ) 2 SO 4 electrolyte, majority of the product is suspended at the bottom of solution. (d) EG dispersion in DMF (total volume of 2 liters) with a concentration of ~ 3 mg/ml. S6

7 Figure S2. Photographs of graphite electrodes during electrochemical exfoliation in various electrolyte systems. (a) Graphite electrode before electrochemical exfoliation. (b) and (c) expanded graphite electrode after electrochemical treatment in 0.1 M NaNO 3 and NaClO 4, respectively. After applying +10 V for 10 min, the graphite electrode shows only expansion without any obvious exfoliation. (d) and (e) exfoliated graphene in 0.1 M Na 2 SO 4 and K 2 SO 4 electrolytes, respectively. S7

8 Figure S3: SEM images of EG obtained from K 2 SO 4 and Na 2 SO 4 electrolytes. (a) and (b) SEM images show thin EG flakes exfoliated in 0.1 M aqueous K 2 SO 4 and Na 2 SO 4 electrolytes, respectively. The inset of (a) and (b) shows the largest flakes observed in the corresponding samples. S8

9 Figure S4. Morphology of graphite foils during the electrochemical process. (a) Photographs and (b-f) optical microscopic (OM) images of graphite foil exfoliated at different time intervals, showing morphology changes on the electrode surface. The graphite electrodes with different morphology were obtained by applying a DC bias of +10 V in (NH 4 ) 2 SO 4 electrolyte. As indicated in (b-f), the voltage was switched off after a certain period of time and subjected to OM analysis to monitor the morphological changes. S9

10 Figure S5. SEM images of graphite foil during the electrochemical process. (a), (d) surface and, (b), (e) edge morphology of the graphite foil after applying a bias voltage of + 10V for 5 and 30s in aqueous (NH4)2SO4 electrolyte, respectively; (c), (f) are the magnified SEM image of (b) and (e) respectively. S10

11 Figure S6. Effect of electrolyte concentration on the electrochemical exfoliation. (a) Relationship between the exfoliation potential and concentration of (NH 4 ) 2 SO 4 solution and (b) photographs of the exfoliated product in (NH 4 ) 2 SO 4 electrolyte of different concentration. For all cases, both the applied DC voltage (i.e. +10 V) and exfoliation time (3 min) were kept the same. S11

12 Figure S7. SEM images of EG obtained from (NH 4 ) 2 SO 4 electrolyte. (a) EG flakes on silicon wafer, (b) the largest graphene flake (~ 44 µm) observed in SEM images. Figure S8. AFM images of EG flakes obtained from (NH 4 ) 2 SO 4 electrolyte. (a) bilayer and (b) multi-layer EG flakes on silicon wafer. The corresponding height profiles with the thickness are shown inside each AFM image. S12

13 Figure S9: HRTEM images of EG obtained from (NH 4 ) 2 SO 4 electrolyte. (a) 3-layer and (b) 4-layer graphene flakes. Figure S10: XPS spectra of EG obtained from (NH 4 ) 2 SO 4 electrolyte. The XPS survey spectra of EG recorded in the range of ev. S13

14 Figure S11. Solid-state NMR spectra of EG and GO. Solid-state 13 C MAS NMR spectra of GO and EG recorded at 30 khz MAS and 176 MHz 13 C Larmorfrequency. It is clearly revealed that, EG has broad peak centered at 122 ppm, indicating pure sp 2 hybridized carbon sites. Whereas, GO shows an additional peak in the range of 60 to 70 ppm, for sp 3 hybridized carbons bound to oxygen. S14

15 Figure S12: Powder XRD of graphite and EG obtained from (NH 4 ) 2 SO 4 electrolyte. The diffraction peak (002) of EG appears at 26.3, with a interlayer d-spacing of 3.48 Å; whereas, the peak of graphite appears at 26.5 d-spacing of 3.36 Å). The slightly lower 2θ angle of EG with large d-spacing compared to graphite suggests that, EG contains only a small amount of functional groups. S15

16 Figure S13. AFM image of the single layer EG based FET device. (a) AFM image shows both EG and platinum (Pt) electrodes. The Pt electrodes deposited by focused ionbeam were used to connect graphene sheet. The EG is obtained from (NH 4 ) 2 SO 4 electrolyte. (b) Height profile of EG, shows a typical thickness of 0.71 nm corresponding to a single layer. S16

17 Figure S14. Fabrication of transparent and conductive EG film. (a) Illustration of the fabricating transparent, conductive EG films on PET substrates. The fabrication process includes vacuum filtration of EG dispersion in DMF (0.1 mg/ml) through PTFE membrane followed by transfer of the film through mechanically pressing against a PET substrate. Vacuum filtration of 3 and 9 ml of EG dispersion results in ~ 6.0 and ~ 16.0 nm thick films, respectively. (b) and (c) transmittance and sheet resistance (R s ) of the asprepared EG films with different thickness. The thin films are fabricated by the EG obtained from (NH 4 ) 2 SO 4 electrolyte. S17

18 Figure S15: Sheet resistance of EG films after HNO 3 doping. Chemical doping of EG 6.0 and 16.0 nm EG (obtained from (NH 4 ) 2 SO 4 electrolyte) films with 65% HNO 3 for 2 h leads to sheet resistance of 0.87 and 0.33 kω sq. -1, respectively. Figure S16. SEM images of Xerox paper with and without EG coating. (a) Surface morphology of commercial Xerox paper showing several micro-meter sized pores and, (b) EG coated paper fabricated by brush painting of ~ 10 mg/ml in DMF EG dispersion (obtained from (NH 4 ) 2 SO 4 electrolyte). S18

19 Figure S17. Electrochemical characterizations of EG paper based conventional supercapacitor. Specific capacitance as a function of EG loading on paper. S19

20 The electrolytes containing sulphate (SO 2-4 ) anions exhibits pronounced exfoliation efficiency compared to other anions, such as Cl -, NO - 3, ClO - 4 etc. and thin graphene flakes are readily obtained in less than 5 min. This might be explained by the following redox reactions: Scheme S1: Schematic representation of electrochemical reduction potential of different anions. It is clear from above reactions that the SO 4 2- anions possess the lowest reduction potential (i.e V) and thus, are expected to be more favorable for generating gaseous species (i.e. SO 2 ) than NO 3 - and ClO 4 - ions to produce NO and Cl 2 gases. Therefore, the fast SO 2 gas formation in between the graphite layers is responsible for the rapid exfoliation. S20

21 Scheme S2: Schematic representation of the electrochemical oxidation process of graphite electrode. As shown in Scheme S2, the electrochemical oxidation process i.e. oxide bond formations can be summarized as follows: when a positive bias voltage is applied to the graphite electrode, nucleophilic OH - ions generated from water, attack on sp 2 carbon introducing C-OH groups followed by the formation of vicinal OH groups (reaction 1). As the electrochemical process continues, epoxy rings might form (reaction 2). Furthermore, the diol can be further oxidized via C-C cleavage to form carbonyl groups (reaction 3). The C-O covalent bond formation can be accompanied by evolution of CO 2 (reaction 4) and self-oxidation of water, which produces O 2 (reaction 5). 2-4 The formation of these oxygen containing groups on EG is supported by the XPS analysis discussed in the main text. S21

22 Table S1: Summary of the inorganic salt based electrolytes investigated in the electrochemical exfoliation of graphite. Electrolyte Electrolyte concentration (M) Voltage Time Results NH 4 Cl V 5 to 10 min No exfoliation Na 2 SO V 3 to 5 min Efficient exfoliation NaNO V 5 to 10 min Poor exfoliation. Product yield is very low. Graphite electrode expands due to intercalation K 2 SO V 3 to 5 min Efficient exfoliation NaClO V 5 to 10 min No exfoliation. Intercalation of graphite electrode occurs as evident by the expansion of the electrode (NH 4 ) 2 SO V 3 to 5 min Efficient exfoliation. As described in the text * DI water was used as a solvent to dissolve the inorganic salts and ph of the electrolyte ranges from 6.5 to 7.0. S22

23 Table S2. Comparison of the elemental analysis of graphene prepared by different methods. Types of graphene Reduction process O content C/O ratio Reference Reduced graphene oxide (rgo) 55% HI acid, 100 C, 1 h Not given 12 rgo HI-AcOH, 40 C, 40 h Not given rgo Phenylhydrazine, 24 h rgo Na-NH 3, 30 min Carbon 2010, 48, 4466 Nat. Commun. 2010, 1, 73 Chem. Commun. 2010, 46, 4375 Nat. Commun. 2013, 4, 1539 rgo Hydrazine:GO = 700: Nat. Nanotechnol. 2008, 3, 101 rgo i) Hydrazine reduction at 80 C ii) additional thermal annealing at 1100 C ~ Adv. Funct. Mater. 2009, 19, 2577 Electrochemically exfoliated in H 2 SO 4 electrolyte Exfoliated graphite in LiOH No chemical/thermal reduction N/A NG 8.6 ACS Nano 2013, 7, 3598 J. Mater. Chem. 2012, 22, Electrochemically exfoliated in (NH 4 ) 2 SO 4 electrolyte No chemical/thermal reduction S This work

24 Table S3 Comparison of the mobility of graphene produced by chemical exfoliation. Types of graphene Sample type Hole mobility (cm 2 V -1 s -1 ) Electron mobility (cm 2 V -1 s -1 ) Reference Reduced GO (thermal annealing at 1000 C) Film rgo (hydrazine vapor reduction at 80 C, 24 h) Sheets Liquid-phase exfoliated graphene Film 95 - Nano Lett. 2010, 10, 92 Adv. Mater. 2012, 24, 2299 ACS Nano 2012, 6, 2992 Electrochemically exfoliated graphene in H 2 SO 4 electrolyte Film ACS Nano 2013, 7, 3598 Bi-layer sheet Electrochemically exfoliated graphene in H 2 SO 4 + KOH electrolyte rgo (Na-NH 3 reduction at -33 C, 30 min) Single graphene sheet Single sheet ACS Nano 2011, 5, 2332 Nat. Commun. 2013, 4, 1539 Electrochemically exfoliated graphene in (NH 4 ) 2 SO 4 electrolyte Film Single layer sheet This work S24

25 References: (1) Parvez, K.; Li, R. J.; Puniredd, S. R.; Hernandez, Y.; Hinkel, F.; Wang, S. H.; Feng, X. L.; Müllen, K. ACS Nano 2013, 7, (2) Beck, F.; Jiang, J.; Krohn, H. J. Electroanal. Chem. 1995, 389, 161. (3) Noel, M.; Santhanam, R. J. Power Sources 1998, 72, 53. (4) Alsmeyer, D. C.; Mccreery, R. L. Anal. Chem. 1992, 64, S25