Self-Assembled Multilayer Graphene Oxide Membrane and Carbon

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Self-Assembled Multilayer Graphene Oxide Membrane and Carbon Nanotubes Synthesized Using a Rare Form of Natural Graphite A. R. Kumarasinghe,*,† Lilantha Samaranayake,† Federica Bondino,‡ Elana Magnano,‡ Nilwala Kottegoda,† Elvio Carlino,‡ U. N. Ratnayake,† A. A. P. de Alwis,† Veranja Karunaratne,† and Gehan A. J. Amaratunga†,§ †

Sri Lanka Institute of Nanotechnology (SLINTEC), Zone 1, FTZ, Biyagama, Colombo, Sri Lanka Laboratorio Nazionale TASC, S.S. 14 Km. 1635, Basovizza, I-34149, Trieste, Italy § Centre of Advanced Photonics and Electronics, Department of Engineering, University of Cambridge, 9 J. J. Thomson Avenue, Cambridge, CB 3 0 FA, United Kingdom ‡

S Supporting Information *

ABSTRACT: The fabrication of flexible multilayer graphene oxide (GO) membrane and carbon nanotubes (CNTs) using a rare form of high-purity natural graphite, vein graphite, is reported for the first time. Graphite oxide is synthesized using vein graphite following Hummer’s method. By facilitating functionalized graphene sheets in graphite oxide to selfassemble, a multilayer GO membrane is fabricated. Electric arc discharge is used to synthesis CNTs from vein graphite. Both multilayer GO membrane and CNTs are investigated using microscopy and spectroscopy experiments, i.e., scanning electron microscopy (SEM), atomic force microscopy (AFM), high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), core level photoelectron spectroscopy, and C K-edge X-ray absorption spectroscopy (NEXAFS), to characterize their structural and topographical properties. Characterization of vein graphite using different techniques reveals that it has a large number of crystallites, hence the large number of graphene sheets per crystallite, preferentially oriented along the (002) plane. NEXAFS and core level spectra confirm that vein graphite is highly crystalline and pure. Fourier transform infrared (FT-IR) and C 1s core level spectra show that oxygen functionalities (−C−OH, −CO,−C− O−C−) are introduced into the basal plane of graphite following chemical oxidation. Carbon nanotubes are produced from vein graphite through arc discharge without the use of any catalyst. HRTEM confirm that multiwalled carbon nanotube (MWNTs) are produced with the presence of some structure in the central pipe. A small percentage of single-walled nanotubes (SWNTs) are also produced simultaneously with MWNTs. Spectroscopic and microscopic data are further discussed here with a view to using vein graphite as the source material for the synthesis of carbon nanomaterials. from a geologic fluid that is traversing through an emplaced rock. The first experiments on Ceylon vein graphite were performed by Brodie which date back to 1859.10 Brodie was exploring the structure of graphite by investigating its reactivity by treating graphite with strong acids. When graphite is reacted with a strong acid in the presence of a strong oxidant, it converts into graphite oxide,12 which is a material that contains carbon (C), oxygen (O), and hydrogen (H) in variable ratios. The method developed by Brodie to oxidize graphite was later modified and improved by Staudenmaier13 and Hummers and Offeman.11 The structure and properties of graphite oxide depend on the particular synthesis method and the degree of oxidation of graphite.14 The original sp2 hybridized carbon

1. INTRODUCTION Vein graphite (VG), also known as crystalline graphite, is a naturally occurring form of pyrolytic carbon (solid carbon deposited from a fluid phase).1−9 It has the highest degree of crystalline perfection,6−9 thermal and electrical conductivities, and cohesive energy of all natural graphite materials.7 Vein graphite is probably the most difficult form of all natural graphite material to describe geologically, and many theoretical approaches have been made to understand its origin.4,8,9 As the name suggests, it is a true vein mineral as opposed to a seam mineral (amorphous graphite) or a mineral that is disseminated throughout the ore rock (as in flake graphite (FG)).7 Because of the natural fluid-to-graphite deposition process, deposits of vein graphite are typically over 90% pure with some deposits reaching as high as 99.5% graphitic carbon in the “as found” state.7−9,28 This level of purity is possible because the deposition of carbon occurs as a precipitation of solid carbon © 2013 American Chemical Society

Received: March 9, 2013 Revised: April 4, 2013 Published: April 4, 2013 9507

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water. In some attempts, graphite samples were subjected to repeated oxidation. Exfoliation of graphite oxide was achieved by adding deionized water into graphite oxide viscous slurry (1 mg/1 mL) followed by ultrasound (Elma, USA, 150 W, 135 kHz) sonication for 15 min and subsequently stirring the dispersion magnetically. This treatment results in a clear dispersion of graphite oxide which is stable for months. It may contain multiple layers of graphene oxide platelets. By keeping it moderately diluted, the viscous slurry of graphite oxide in a humidity-controlled (relative humidity ∼40%) environment for 24−48 h results in a multilayer GO membrane which is a paper-like material. Alternatively, placing a moderately diluted viscous slurry of graphite oxide in an oven at a moderate temperature (∼50 °C) for a sufficient length of time could also produce the same paper-like material. Irrespective of the method used, the quality of the resultant multilayer GO membrane appears to be the same except the surface created at the air/water interface is less glossy than that created at the substrate/water interface. When the exfoliated graphite oxide dispersion is allowed to vaporize slowly in an oven maintained at a temperature of 50 °C over a length of time, a semitransparent film of graphene oxide firmly attached to the substrate can be obtained. Further, by varying the concentration of the exfoliated graphite oxide dispersion, either a flexible semitransparent film or a semitransparent film firmly affixed to the substrate can be obtained. 2.2. Growth of Carbon Nanotubes (CNTs) Using Crystalline Vein Graphite. The CNT growth experiment was conducted in an Ar (99.99%) environment using a DC arcdischarge setup. The basic system consists of two graphite electrodes placed from 99%) samples were obtained from a commercial supplier.28 No attempts were made to further purify vein graphite samples; they were used as received. Carefully controlling the parameters, vein graphite was oxidized according to the procedure developed by Hummers and Offeman.11 All chemicals, conc. H2SO4 (>98.0%), KMnO4 (>99.9%), H2O2 (30%), and HCl (>99.8%), were purchased from reputed suppliers (e.g., Sigma Aldrich, St. Louis, MO). The Hummers’ method for oxidation of graphite is as follows. Natural vein graphite (10 g) was added gradually into H2SO4 (230 mL) maintained at 0 °C. Subsequently, KMnO4 (30 g) was added into the mixture while stirring and cooling. The addition of KMnO4 was such that the temperature of the mixture was not allowed to rise above 20 °C. The system was further stirred for another 2−4 h at 35 °C, and distilled water (410 mL) was added. In 15 min, the reaction was terminated by adding a large amount of distilled water (1.4 L) and 30% of H2O2 solution (30 mL), after which the color of the mixture becomes bright yellow. The mixture was filtered and washed with 1:10 HCl solution (2.5 L), in order to remove metal and SO42− ions. It was further washed with distilled water until the pH of the rinsewater becomes neutral (pH = 7), and then the resultant viscous graphite oxide slurry was dispersed in distilled 9508

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λ = 1.5406 Å, with θ/2θ configuration) was used to monitor the interlayer spacing of graphitic layers in vein graphite and multilayer GO membrane. XRD data were also recorded from flake graphite for comparison. The nature of the functional groups of multilayer GO membrane introduced by the chemical oxidation was probed using Fourier transform infrared spectroscopy (FT-IR, Bruker Vertex 80, ATR mode with ZnSe crystal). Atomic force microscopy (AFM, Park System XE-100, Suwon, Korea, noncontact (NC-AFM) mode, NCHR Si cantilevers with resonant frequency 330 kHz, force constant 42 N/m, tip radius of curvature 50 μm) and (b) for a thin (5 μm < t < 50 μm) membranes.

GO membranes has been observed in a recent experiment for films having thickness 289 eV.60 For all highly ordered graphite, these transitions give rise to peaks at 285.4 and 291.7 eV, respectively.61,58 However, one or two additional resonances in the energy range of 286−289 eV are sometimes observed. The origin of these peaks is not universally agreed.62−64 Recent theoretical work however has suggested that the peaks in the range of 287−288 eV can be attributed to the presence of defects such as atomic vacancies in graphite.65 Given that, the sharp peak seen at a photon energy of 285.3 eV is due to transition from the C 1s orbital to the unoccupied π* orbitals principally originating from sp2 (CC) sites.66 Any defect in the sp2 coordinated benzene ring sites giving rise to 5 member or 7 member rings which introduce concave/convex curvature

occur due to structural disorder that accompanies the oxidation of graphite.14,17 Shown in Figure 10 is the C K-edge NEXAFS spectra recorded from precursor vein graphite and flexible multilayer GO membrane. For vein graphite sample, the spectra were recorded with incoming photon beam at normal and at gracing incidence to the sample surface. At normal incidence, the electric vector (E) of the incoming photon beam is in-plane with respect to the plane of the sample surface and it is out-ofplane at grazing incidence. Soft X-ray spectroscopy, in particular NEXAFS, is a powerful tool to investigate materials and it has been performed on man-made graphite samples, generally thin films.57−59 NEXAFS can provide information on different aspects of the structure of graphite, for example, the orientation of the graphite planes to the X-ray beam, the purity of the sample, in particular the presence of hydrogenated and oxygenated groups, and the presence or absence of ordering and the relative size of their planner ring systems.58 In C Kedge NEXAFS recorded for vein graphite, there is a sharp peak at photon energy of 285.3 eV. In addition, there are several 9515

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eV where the C 1s to σ* transitions occur. However, the C Kedge NEXAFS spectra of vein graphite agree with those recorded from man-made HOPG samples.68 The dependence of the spectral features in NEXAFS with the polarization of the electric filed vector E is well understood. Bonds and the associated molecular orbitals are highly directional and the spatial orientation of an orbital, i.e., the direction of maximum orbital amplitude on the excited atom, determines the angular dependence of the K-shell spectra.60,69 Therefore, the transition intensities depend on the orientation of the electric field vector relative to the orientation of the molecules or bond axis. σ*orbitals have maximum orbital amplitudes along the bond axis, while π*-orbitals have maximum amplitude normal to the bond direction.69 Therefore the intensity of the 1s to σ* transitions will be maximum when the electric vector is in-plane and will be minimum when the electric vector is out-of-plane. In contrast, the 1s to π* transitions will behave in opposite way, which is what is essentially seen in the C K-edge NEXAFS of vein graphite in Figure 10. The C K edge NEXAFS spectrum for multilayer GO membrane recorded with electric field vector E of the incident beam at out-of-plane is also shown in Figure 10. The line shape of the spectrum agrees with previous observations made with few layers of graphene oxide membrane.68 The variation in the intensity of σ* (at ∼291 eV) and π* (at ∼284.5 eV) orbitals seen here (compared to the observation in ref 68) is due to the difference in the polarization angle in our case. When the electric field vector is nearly perpendicular with respect to the plane of the sample, the amplitude of the π* orbital will be at its maximum dominating the spectrum. The first absorption peak exists at the photon energy of 284.5 eV. We observe a shift in the spectral position toward lower photon energy in multilayer GO membrane compared to precursor vein graphite. Above 295 eV, the NEXAFS spectrum of the multilayer GO membrane is featureless. The suppression (featurelessness) of the high energy features due to the polarization of light in the NEXAFS spectrum for graphene has been observed.68 This peculiarity of the NEXAFS spectrum of graphene can be explained by the multiple scattering theory, namely, considering the scattering of the excited electron wave function by neighboring atoms.68 Due to an open cage around the absorbing atom when the polarization of light would select atoms above and below the graphene plane, high energy features of the C K edge spectrum are suppressed.67 NEXAFS spectra of graphene and few layer graphene showed that high energy features of the C K edge spectrum are fully recovered above 5 layers.68 In our experiment, in spite of the number of layers, the same behavior is found for GO membrane; i.e., the spectrum becomes featureless above 295 eV. This reflects the much higher interlayer distance (0.89 nm) of GO compared to vein graphite (0.335 nm) and that multilayer GO membrane behaves as a material composed of strongly decoupled layers. This observation shows that the electronic structure of GO is largely insensitive to the number of layers in the multilayer membrane. The ratio between 1s−π* and 1s−σ* transition peaks provide information on the sp2/sp3 carbon ratio. In the GO membrane it can vary locally within the plane akin to defects in graphite.

Figure 9. High-resolution C 1s core level spectra recorded from crystalline vein graphite (lower panel) and multilayer GO membrane (upper panel) with photon energy 588 eV at gracing incidence (normal emission).

Figure 10. C K-edge NEXAFS spectra recorded from crystalline vein graphite (lower spectra) and multilayer GO membrane (upper spectrum).

in the graphene sheet can lead to an intermediate hybridization between sp2 and sp3 such as in C60.70 This can lead to a shift in the π* and σ* energies and could be what is seen as the sub peak and shoulder between the 1s to π* and 1s to σ* transitions attributed to standard graphite. The clear peak at 287 eV seen here has been proposed by Papworth et al.70 as originating from the molecular nature of sp2 bonded carbon in structures such as C60. That one sees this in vein graphite, but not HOPG,70 suggests that there are isolated pentagonal defects in the graphene plane which give a C60 like curvature and a pseudo molecular signature. The C K-edge NEXAFS spectrum of vein graphite is different from those observed with other graphite,66 also in the higher photon energy range >289

4. CONCLUSION We have successfully fabricated a flexible multilayer GO membrane and CNTs using a rare form of precursor graphite, vein graphite. We also have investigated structural and other 9516

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(7) (a) Asbury Carbons. Technical report on vein graphite. http:// www.asbury.com/vein-graphite.html; accessed Feb 10, 2010. (b) Balasooriya, N. W. B.; Touzain, Ph.; Bandaranayake, P. W. S. K. Lithium Electrochemical Intercalation into Mechanically and Chemically Treated Sri Lanka Natural Graphite. J. Phys. Chem. Solids 2006, 67, 1213−1217. (8) Dissanayake, C. B. The Origin of Graphite of Sri Lanka. Org. Geochem. 1981, 3, 1−7. (9) Wilbert, K. V. Epigenetic Vein Graphite Mineralization in the Granualite Terrein of Sri Lanka. Gondwana Res. (GNL) 1999, 2, 654− 657. (10) Brodie, B. C. Sur le Poids Atomique du Graphite. Ann. Chim. Phys. 1860, 59, 466−472. (11) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (12) Sorokina, N. E.; Khaskov, M. A.; Avdeev, V. V.; Nikol’skaya, I. V. Reactions of Graphite with Sulfuric Acid in the Presence of KMnO4. Russ. J. Gen. Chem. 2005, 7, 162−168. (13) Staudenmaier, L. Verfahren zur Darstellung der Graphitsaure. Ber. Dtsch. Chem. Ges. 1898, 31, 1481−1499. (14) Lee, D. W.; De Los Santos, V. I.; Seo, J. W.; Leon Felix, I.; Bustamante, D. A.; Cole, J. M.; Barnes, C. H. W. The Structure of Graphite Oxide: Investigations of Its Surface Chemical Groups. J. Phys. Chem. B 2010, 114, 5723−5728. (15) Park, S.; Ruoff, R. S. Chemical Methods for Production of Graphene. Nat. Nanotechnol. 2009, 4, 217−224. (16) Gao, W.; Alemany, L. B.; Ci, L.; Ajayan, P. M. New Insight into the Structure and Reduction of Graphite Oxide. Nat. Chem. 2009, 1, 403−408. (17) Lee, D. W.; Seo, J. W. sp2/sp3 Carbon Ratio in Graphite Oxide with Different Preparation Time. J. Phys. Chem. C 2011, 115, 2705− 2708. (18) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dekany, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxide. Chem. Mater. 2006, 18, 2740−2749. (19) (a) Enoki, T.; Suzuki, M.; Endo, M. Graphite Intercalation Compounds and Applications; Oxford University Press: New York, 2003. (b) Dresselhaus, M. S.; Dresselhaus, G. Intercalation Compounds of Graphite. Adv. Phys. 2002, 51, 1−186. (20) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (21) Compton, O. C.; Nguyen, S. T. Graphene Oxide, Highly Reduced Graphene Oxide, and Graphene: Versatile Building Blocks for Carbon-Based Materials. Small 2010, 6, 711−723. (22) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Synthesis of Graphene-based Nanosheets via Chemical Reduction of Exfoliated Graphite Oxide. Carbon 2007, 45, 1558−1565. (23) Zhu, B. Y.; Murali, S.; Cai, W.; Li, X.; Suk, J. W.; Potts, J. R.; Ruoff, R. S. Graphene and Graphene Oxide: Synthesis, Properties and Applications. Adv. Mater. 2010, 22, 3906−3924. (24) Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Adv. Mater. 2010, 22, 2392−2415. (25) Rao, C. N. R.; Subramanyam, K. S.; Ramakrishna Matte, H. S. S.; Abdulhakeem, B. A.; Govindaraj, A.; Das, B. A Study of the Synthetic Methods and Properties of Graphenes. Sci. Technol. Adv. Mater. 2010, 11, 054502−054517. (26) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric Field Effect in Atomically Thick Carbon Films. Science 2004, 306, 666−669. (27) Geim, A. K.; Novoselov, K. S. The Rise of Graphene. Nat. Mater. 2007, 6, 183−191. (28) Graphit Kripfmuhl AG Homepage. http://www.gk-graphite.lk; accessed Oct 12, 2009. (29) Zhamu, A.; Jang, B. Z. Mass Production of Pristine Nano Graphene Materials. USPTO July 2012, Patent No. 8,266,801 B2.

properties of vein graphite and vein graphite based nanomaterials using XRD, FT-IR, TGA, SEM, AFM, HRTEM, UPS, and NEXAFS. SEM clearly showed that vein graphite has characteristic needle-like shaped topographical features which arise from its natural formation. The structure present in the central pipe of MWNTs and the formation of SWNTs without metal catalyst are unique to vein graphite. C K-edge NEXAFS does not show signs for the presence of hydrogenated or oxygenated carbon species on vein graphite suggesting it is pure. However, angle-resolved NEXAFS studies on different locations of vein graphite sample are required to further confirm this. The results obtained here show that vein graphite can be used as a precursor to produce nanomaterials with distinctive features, opening a number of avenues to progress research on vein graphite based nanomaterials in future.

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ASSOCIATED CONTENT

S Supporting Information *

Raman studies of GO prepared using vein and flake graphite following the same oxidation procedure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +94-11-4650507. Fax: +94-11-4741995. E-mail: [email protected], [email protected], ark@ uwu.ac.lk. Notes

The authors declare no competing financial interest. Permanent Address: A.R.K.: Faculty of Science and Technology, Uva Wellassa University, 90000, Badulla, Sri Lanka.

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ACKNOWLEDGMENTS A.R.K. acknowledges the financial support received from ICTPELETTRA Users Program of the International Centre for Theoretical Physics (ICTP), Trieste, Italy to carry out photoemission experiments at beamline BACH at ELETTRA Synchrotron, Trieste, Italy. A.R.K. further acknowledges Bogola Graphite Lanka PLC for providing high quality vein graphite samples and Asbury Carbon, NJ, USA, for providing flake graphite samples for research.

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