Functionalized Single Graphene Sheets Derived from

Transcription

1 Schniepp et al. - Graphene Sheets 16/03/ :01:35 1/13 Supporting Information (SI) for Functionalized Single Graphene Sheets Derived from Splitting Graphite Oxide Hannes C. Schniepp, *, Je-Luen Li, *, Michael J. McAllister, Hiroaki Sai, Margarita Herrera-Alonso, Douglas H. Adamson, Robert K. Prud homme, Roberto Car, Dudley A. Saville, and Ilhan A. Aksay,# Department of Chemical Engineering, Department of Chemistry, and Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey 08544, USA * These two contributed equally to this work. Department of Chemical Engineering, Princeton University. Department of Chemistry, Princeton University. Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University. # Corresponding author. iaksay@princeton.edu.

2 Schniepp et al. - Graphene Sheets 16/03/ :01:35 2/13 SI1: Production of Functionalized Graphene Sheets. Natural flake graphite, nominally sized at 45 µm, was provided by Asbury Carbons (P.O. Box 144, 405 Old Main St., Asbury, NJ 08802). Fuming nitric acid (> 90%), sulfuric acid (95-98%), potassium chlorate (98%) and hydrochloric acid (37%) were purchased from Sigma-Aldrich and used as received. Oxidized graphite samples were examined by simultaneous thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) (STA 449 C Jupiter, Erich NETZSCH GmbH & Co. Holding KG, D-95100, Selb, Germany). The thermal analysis unit was coupled with Fourier-transform infrared (FTIR) spectroscopy for evolved gas analysis (Thermo Nicolet Nexus 670, Thermo Electron Corporation, Waltham, MA 02451). Graphite oxide (GO) was prepared according to the Staudenmaier method. 1 A reaction flask containing a magnetic stir bar was charged with sulfuric acid (87.5 ml) and nitric acid (45 ml) and cooled by immersion in an ice bath. The acid mixture was stirred and allowed to cool for 15 min, and graphite (5 g) was added under vigorous stirring to avoid agglomeration. After the graphite powder was well dispersed, potassium chlorate (55 g) was added slowly over 15 min to avoid sudden increases in temperature. The reaction flask was loosely capped to allow evolution of gas from the reaction mixture and allowed to stir for 96 h at room temperature. On completion of the reaction, the mixture was poured into 4 L of deionized water and filtered. The GO was redispersed and washed in a 5% solution of HCl. The filtrate was tested with barium chloride for the presence of sulphate ions. The HCl wash was repeated until this test was negative. The GO was then washed repeatedly with deionized water until the ph of the filtrate was neutral. The GO slurry was spray-dried and stored in a vacuum oven at 60 C until use. Graphite oxide (0.2 g), prepared as described above where the oxidation step was carried out for 96h, was placed in an alumina boat and inserted into a 25-mm ID, 1.3-m long quartz tube that was sealed at one end. The other end of the quartz tube was closed using a rubber stopper. An argon (Ar) inlet and thermocouple were then inserted through the rubber stopper. The sample was flushed with Ar for 10 min, then the quartz tube was quickly inserted into a Lindberg tube furnace preheated to 1050ºC and held in the furnace for 30 s. 1. Staudenmaier L. Ber. Dtsch. Chem. Ges. 1898, 31, 1481.

3 Schniepp et al. - Graphene Sheets 16/03/ :01:35 3/13 To determine the effect of water on the exfoliation process, three different sample treatments were used. One sample was used dry, directly out of the vacuum oven. The other two samples were placed in a saturated water atmosphere, one at room temperature and the other at 80 C. The water content of each sample was determined by TGA. The samples were placed at the end of a quartz tube which was then purged with argon. The quartz tube was then inserted quickly into a furnace preheated to 1050 C to exfoliate the graphite oxide. A typical TGA/DSC scan is shown in Figure SI1-1. Water is lost during the initial heating stage. At 200 C, there is a sharp mass loss accompanied by an exothermic reaction. These features correspond to the decomposition of oxygen-containing groups in the graphite oxide. The main product of this decomposition as determined by FTIR is carbon dioxide, as well as small amounts of water (Fig. SI1-2). The mass lost during the thermal decomposition is approximately 30% of the total GO mass. Using X- ray photoelectron spectroscopy (XPS) composition data (SI4) before and after exfoliation and assuming all oxygen is lost as CO 2, we estimate a weight loss of 36% which is in reasonably close agreement with that observed. Discrepancies may be due to small amounts of water that are produced. X-ray diffraction (XRD) patterns of graphite, graphite oxide, and the exfoliated (graphene) powders are shown in Figure SI1-3. SI2: Surface Area Measurements. Surface areas were first measured using the BET method with nitrogen gas adsorption (Micromeritics Gemini V, One Micromeritics Drive, Norcross, GA 30093); areas ranged from 600 to 1500 m 2 /g. These are below the theoretical limit of 2600 m 2 /g. Variations observed within a single batch, suggested that BET surface areas determined on dry compacts are affected by the state of agglomeration of the powder. Accordingly, to check the BET data, a surface area was measured in an ethanol suspension with methylene blue dye as a probe. Methylene blue has been used as an indicator of graphitic material surface areas in previous studies, with each milligram of adsorbed methylene blue representing 2.45 m 2 of surface area. 2 A known amount of a standard methylene blue solution in ethanol was added to a graphene suspension, agitated, left to settle, and centrifuged to remove suspended material. From the concentration of methylene blue in the supernatant, as determined by UV-vis 2. Boehm, H.P.; Clauss, A.; Fisher, G.O.; Hofmann, U. Zeitschrift für anorganische und allgemeine Chemie 1962, 316, 119.

4 Schniepp et al. - Graphene Sheets 16/03/ :01:35 4/13 spectroscopy, the surface area was determined to be ~ 1200 m 2 /g as compared to a BET surface area of approximately 650 m 2 /g. The surface area determined for the dispersed graphene more likely reflects the actual surface area of the sheets; however, the actual area per methylene blue molecule may vary somewhat with the surface chemistry and topology of the surface. The ambiguities in surface areas determined by BET and methylene blue adsorption led us to rely on AFM mesurements to confirm the presence of single sheets. The AFM method presented in our work demonstrates a preponderance of single sheets. Figure SI2-1 shows the water content of the samples as determined by TGA and the resulting surface areas after exfoliation as determined by BET. These results clearly show the detrimental effect of increased water content on the final surface area. The mechanism of exfoliation is the thermal expansion of evolved gases trapped between the graphene sheets. While the decomposition of the functional groups is exothermic and therefore self-propagating, the vaporization of water is endothermic and delays the heating process. This allows gases to diffuse out of the galleries instead of heating up and creating sufficient pressure to overcome van der Waals interactions between graphene layers. The maximum pressures generated by both water and carbon dioxide can be estimated using the spacing of the layers from diffraction measurements, the mass of vapor generated from TGA, and assuming an isochoric heating process. The pressures were found to be in excess of 100 MPa for CO 2 and 60 MPa for water. Both of these pressures exceed the estimated van der Waals force between the graphite layers, which was calculated as 26 MPa using a Lennard-Jones potential pair-wise summation. However, it appears that only the CO 2 producing reaction occurs rapidly enough to effectively exfoliate the material. SI3: Sample Preparation for Atomic Force Microscopy. An 8 ml scintillation vial containing a magnetic stir bar was loaded with 5.5 mg of graphene (detailed in SI1) and 3.0 ml of 1-methyl-2-pyrrolidinone (NMP) as the dispersion medium. The vial was immersed in an ice bath while the suspension was sonicated (VirSonic 100, The Virtis Co., Gardiner, NY; with an output power 12 W for 5 min under continuous stirring). After allowing the suspension to sit at room temperature for 1 h, an aliquot (0.5 ml) was taken and added to a 1.5 ml flex tube for centrifugation. The samples were centrifuged for nine cycles at 10,000 RPM for 5 min each. The sedimented material was discarded after each cycle, finally yielding a light-brown suspension. The suspension was spin-coated at 5,000 RPM onto freshly cleaved, highly oriented pyrolytic graphite

5 Schniepp et al. - Graphene Sheets 16/03/ :01:35 5/13 (HOPG) substrates. To show that centrifugation does not alter the thickness distribution, we also prepared alternate set of samples where a smaller concentration of graphene (1.4 mg per 4 ml NMP) was sonicated and spun directly onto HOPG. The sheet thicknesses were equivalent. SI4: Compositional Analysis. Elemental analysis of a GO sample (Atlantic Microlab, Inc, Norcross GA) oxidized for 96 h indicates a C/H/O ratio of 55.7/2.1/40.1 by weight and 4.6/2.1/2.5 by mole. The elemental analysis of graphene shows a C/H/O ratio of 86.4/0.8/11.3 by weight and 7.2/0.8/0.7 by mole. Thus the molar C/O ratio of the material goes from nearly 2/1 in GO that has been oxidized for 96 h to almost 10/1 after thermal treatment at 1050ºC. GO and thermally exfoliated GO samples were also characterized by XPS. Powder samples were pressed into pellets, stored in clean scintillation vials, and dried at room temperature under vacuum (20 mtorr) prior to analysis. XPS spectra were recorded on a Physical Electronics Quantum 2000 Scanning ESCA Microprobe with AlK α excitation at 15 kv acceleration voltage and 50 W for a probing size of 200 µm. The chamber pressure was maintained at 10-8 Torr. Spectra were acquired at 15º and 75º take-off angles (between the plane of the surface and the entrance lens of the detector optics). Determination of atomic composition and curve-fitting analysis were performed using Multipak software. The effect of exfoliation temperature on the surface atomic composition of graphene was determined by XPS and the results are presented in Table SI4-1. In general, increasing exfoliation temperatures resulted in materials with a lower content of oxygen-containing functional groups. The carbon/oxygen ratio increased from 2.6/1 for graphite oxide to 9.7/1 for graphene at 1050ºC. Figure SI4-1 shows the C 1s core level photoemission spectra of graphite oxide and graphene. The photoelectron peak of graphite oxide was curve-fitted with two peaks at and ev, assigned to graphitic carbon (C-C) and carbon singly bound to oxygen (C-O-C and C-O-), respectively. 3-5 We were unable to observe the signal associated with carbonyl groups (288.2 ev). It is important to mention that the shake-up satellite (π π* at ev), characteristic of aromatic systems, is not observed in this sample due to its high degree of oxidation, in agreement with previous findings. 3 The loss of oxygen-containing species during thermal 3. Hontoria-Lucas, C. et al. Carbon 1995, 33, Xing, Y. et al. Langmuir 2005, 21, de la Puente, G. et al. J. Anal. Appl. Pyrolysis 1997, 43, 125.

6 Schniepp et al. - Graphene Sheets 16/03/ :01:35 6/13 treatment, responsible for exfoliation, is evidenced in the C 1s spectrum of graphene (Fig. SI4-1). In this case, only one predominant peak is observed (284.5 ev) and attributed to graphitic carbon. However, the high binding energy tail of this signal suggests that small amounts of oxygen-containing functional groups remain after the thermal treatment. The aromatic character of graphene is demonstrated by the appearance of a shake-up satellite at 291 ev, confirming that reduction occurred during the thermal treatment. The O 1s spectrum of graphite oxide shows only one peak at ev. However, this signal has contributions from three different types of oxygen-containing species: oxygen in carboxyl groups (531.3 ev), oxygen bound to aliphatic carbons (532.6 ev), and oxygen bound to aromatic carbons (533.1 ev). 3 After exfoliation the intensity of the oxygen signal decreases considerably and the principal peak position shifts to ev. The shift in peak position from oxygen bound to aliphatic carbons in graphite oxide to oxygen bound to aromatic carbons in graphene also supports the idea of chemical reduction during exfoliation. Table SI4-1 XPS atomic composition (at 75º take-off angle) of graphite oxide and thermally exfoliated graphite oxide samples processed at different exfoliation temperatures. Exfoliation temperature (ºC) Atomic composition (%) C O Graphite oxide The data obtained by elemental analysis compares reasonably well with the data obtained by XPS considering the fact that while elemental analysis gives the bulk composition whereas XPS is a surface analysis technique. For GO, XPS gave a C/O molar ratio of 2.6/1, while elemental analysis showed 2/1. The thermally treated graphene material showed a C/O molar ratio of 9.7/1 while elemental analysis showed a molar ratio of 10/1.

7 Schniepp et al. - Graphene Sheets 16/03/ :01:35 7/13 Due to the elimination of water and some oxygen functional groups (vide supra) during the rapid heating, elemental analysis showed an increase in C/O ratio from 2/1 in GO to 10/1 in graphene. At this point, the nature of the oxygen-containing groups left on the surface of the graphite oxide sheets is an important issue as this determines the chemistry of the functionalized graphene surface. Our attempts to acquire 13 C NMR spectra for graphene failed, presumably due to the strong absorption of the radio frequency arising from the relatively high electrical conductivity (SI6) of the sample. SI5: Modelling and Defect Formation on Graphite. For the pseudopotentials used in this calculation, we employ ultrasoft pseudopotentials for all three atomic species (C, H, and O). The computational package and instructions to generate these pseudopotentials can be found in In the calculations, the plane wave cutoff for the augmented charge within ultrasoft pseudopotential is 200 Ry. In the supercell calculation, only the gamma point of the Brillouin zone is used. The (rectangular) unit cell of the simulation in Fig. 3(c) is 46.1 x x in atomic unit. This ensures there is negligible overlap between replicas within periodic boundary conditions. A defect is very stable 6 and sustains its structure at 3000K. Regarding the diffusion rate of a single vacancy defect on graphene, the energy barrier of single vacancy diffusion is found 6 to be 0.94 ev, comparable to the energy barrier of an epoxy group hopping on graphite. 7 Two nearby single vacancies can then coalesce into a double vacancy after overcoming an energy barrier of 1.52 ev, while the more stable defects 8 can not be formed at low (< 3000K) temperatures 6 due to a rather high energy barrier of 5.17 ev. Experimentally, Hashimoto et al. 9 observed diffusion of single vacancies under high-resolution transmission electron microscopy within a few hundred seconds at room temperature which also indicates that the diffusion barrier of single vacancy should be about 1 ev. Therefore, even at room temperature, a single carbon vacancy and an isolated epoxy group can diffuse in a time frame of a few hundred seconds. This supports our assertion that in our functionalized graphene sheets, single carbon vacancies can coalesce into line defects and form wrinkled structures. 6. Lee, G.-D.; Wang, C.Z.; Yoon, E.; Hwang, N.-M.; Kim, D.-Y.; Ho, K.M. Phys. Rev. Lett. 2005, 95, Li, J.-L.; Kudin, K.N.; McAllister, M.J.; Prud homme, R.K.; Aksay, I.A.; Car, R. Oxygen-driven unzipping of graphitic materials. Submitted to Phys. Rev. Lett. (January 16, 2006). 8. A defect is composed of three pentagons and three heptagons; see Footnote Hashimoto, A.; Suenaga, K.; Gloter, A.; Utita, K.; Iijima, S. Nature 2004, 430, 870.

8 Schniepp et al. - Graphene Sheets 16/03/ :01:35 8/13 SI6: Conductivity Measurements. Resistances were measured with a Keithley 6514 electrometer in the range of 10 mω to 210 GΩ. Thin films were formed by evaporating the solvent (water) from a suspension in a rectangular cell equipped with rectangular electrodes. The resistances were then converted to conductivities using the dimensions of the films. Three sorts of measurements were made to establish that the functionalized graphene sheets are highly conducting. Functionalized graphene (1 mg/ml) dispersed in a water-soluble polymer (Pluronic F-127 surfactant, Mw=12,600 g/mol, BASF) yielded films with dc conductivities ranging from 1x10 3 to 2.3x10 3 S/m. This value compares favorably with conductivities of compressed graphite monoliths measured by Celzard et al. 10 at similar bulk densities (~ 0.3 g/cm 3 ). For comparison purposes, conductivities of graphite oxide films and dried polymer were measured in the same apparatus. The conductivity of a graphite oxide film was ~ 6x10-5 S/m; for the polymer film the measured value was below the level of detection of the instrument. 10. Celzard, A.; Marech, J.F.; Furdin, G.; Puricelli, S. J. Phys. D: Appl. Phys. 2000, 33, 3094.

9 Figure SI1-1. Decomposition behavior of graphite oxide. TGA/DSC scan of wet GO. This sample was stored at 80 C for 48 h and accumulated approximately 10% water by weight. DSC shows an endothermic signal during the water evaporation. The second abrupt mass loss corresponds to the decomposition of oxygen-containing groups and is accompanied by an exothermic DSC signal. Schniepp et al. - Graphene Sheets 16/03/ :01:35 9/13

10 Figure SI1-2. FTIR absorbance data of evolved gas. This scan is acquired at a furnace temperature of 220 C, when the vapor generation rate is greatest. The CO 2 peaks are clearly visible at 2350 cm -1 and 670 cm -1. The regions from cm -1 and cm -1 correspond to water vapor. Schniepp et al. - Graphene Sheets 16/03/ :01:35 10/13

11 Figure SI1-3. X-ray diffraction patterns of graphite, graphite oxide, and graphene. XRD patterns of pristine graphite, GO produced by 24- and 96-h acid treatment, and graphene produced through the thermal expansion of GO after 120-h acid treatment. Schniepp et al. - Graphene Sheets 16/03/ :01:35 11/13

12 Figure SI2-1. Surface area as a function of water content in graphite oxide. The detrimental effect of water on exfoliation can be seen as a reduction in the surface area. Schniepp et al. - Graphene Sheets 16/03/ :01:35 12/13

13 Figure SI4-1. XPS analysis. C 1s core-level spectra of graphite oxide (GO, top) oxidized for 120 h and graphene (bottom) processed at 1050ºC. Schniepp et al. - Graphene Sheets 16/03/ :01:35 13/13