Feasible Catalytic Strategy for Writing Conductive Nanoribbons on a

Sep 12, 2014 - Anhui, P. R. China ... ifications. Compared with graphene fluoride (GF), graphene oxide (GO) has many more oxygen-rich functional group...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCC

Feasible Catalytic Strategy for Writing Conductive Nanoribbons on a Single-Layer Graphene Fluoride Xinrui Cao,† Yongfei Ji,† Wei Hu,†,‡ Sai Duan,† and Yi Luo*,†,‡,§ †

Department of Theoretical Chemistry and Biology, School of Biotechnology, Royal Institute of Technology, S-106 91 Stockholm, Sweden ‡ Department of Chemical Physics, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, 230026 Anhui, P. R. China S Supporting Information *

ABSTRACT: An accessible method for local reduction of graphene fluoride catalyzed by the Pt-coated nanotip with the assistance of a mixture of hydrogen and ethylene atmosphere is proposed and fully explored theoretically. Detailed mechanisms and roles of hydrogen and ethylene molecules in the cyclic reduction is discussed based on extensive first-principles calculations. It is demonstrated that the proposed cyclic reduction strategy is energetically favorable. This new strategy can be effectively applied in scanning probe lithography to fabricate electronic circuits at the nanoscale on graphene fluoride under mild conditions.

■

fluoropolymer-covered graphene sheet with a laser,19 and a lowdamage CF4 plasma treatment in a plasma-enhanced chemical vapor deposition (PECVD) system.20 The latter may be realized by electron beam irradiation,21 thermochemical nanolithography (TCNL),19 and an electrostatically biased scanning probe.23 Very recently, Zhang et al.24 reported a new catalytic scanning probe lithography (cSPL) method to locally reduce GO with a heated Pt-coated tip of an atomic force microscope (AFM) in the presence of hydrogen at low temperature (≤115 °C) in which hydrogen plays a vital role to accelerate the reduction of GO. This novel technique is more feasible for practical application. Similar phenomena were also found in the reduction of GF, and the reduction could be enhanced efficiently under H2/Ar atmosphere.15,16,19 Given the aforementioned results, a question that arises here is whether it is possible to use the local reduction scheme to partially reduce GF with the Pt-coated AFM tip with good performance. In this study, extensive calculations to explore the nanoscale reduction of graphene fluoride induced by the Pt-coated tip have been carried out. Three types of ambient gases have been considered in the catalytic reduction, including pure hydrogen atmosphere, pure ethylene atmosphere, and the hydrogen/ethylene mixture. Our calculations indicate that the GF can be easily reduced under the three above-mentioned gas environments. The use of the mixture gases is found to be the most feasible way because it can avoid the harmful damages caused by hydrogen fluoride on

INTRODUCTION Graphene research has experienced an explosive development since its discovery in 2004.1,2 The unique properties, such as high carrier mobility,3−5 extraordinary thermal conductivity,6 and superior mechanical properties7 of graphene make it a high potential material for modern applications in electronics.8−12 However, many challenges still remain in dealing with the high leakage currents caused by the zero band gap of graphene.13 To address these hindrances, chemical functionalization methods are employed to open its bandgap while retaining its high conductivity. Oxidation13,14 and fluorination15−17 are the two most effective methods among widely used chemical modifications. Compared with graphene fluoride (GF), graphene oxide (GO) has many more oxygen-rich functional groups that can often destroy the graphene lattice when they are desorbed.18 Additionally, the reduction temperature required for GO is higher than that for GF.13,19 Moreover, GF could be readily obtained by exposing graphene under XeF2 gas with a relatively homogeneous surface.15,16 In general, GF is a better material for some practical applications than GO. To form an electronic circuit on graphene, a unique heterojunction structure consisting of semimetal−semiconductor−insulator should be constructed on the same graphene.20−22 Taking GF as an example, the insulating part is fluorinated graphene itself, which acts as a host material, and the conductive/semiconductive graphene channels separated by the insulating part should be fabricated. Recently, several approaches have been proposed to produce this kind of allgraphene device based on fluorinated graphene, including the selective fluorination of graphene and the direct reduction of graphene fluoride at the nanoscale. The former could be achieved via thermal dip-pen nanolithography,22 irradiating a © 2014 American Chemical Society

Received: July 25, 2014 Revised: September 10, 2014 Published: September 12, 2014 22643

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648

The Journal of Physical Chemistry C

Article

the devices. A practicable catalytic cycle with assistance of C2H4/H2 mixture atmosphere has been determined based on calculations.

■

COMPUTATIONAL DETAILS The density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP)25,26 was employed to perform all the calculations. The electron−ion interaction was described by the projector augmented wave (PAW) pseudopotentials.27 The GGA-PBE functional28 and a 400 eV cutoff energy for the plane-wave basis set were used in the calculations. The lattice parameters of the tetragonal supercell are a = 26.104 Å, b = 17.172 Å, and c = 26.000 Å in all calculations, as used in the previous study.24 The Brillouin zone was sampled by 1 × 3 × 1 k-points within the Monkhorst−Pack scheme.29 The Pt-coated AFM tip was held at ∼3 Å above the F atom of the GF surface. A four-layer cluster model consisting of 20 Pt atoms was represented for the Pt-coated AFM tip, and the atoms at the top two layers were fixed during the optimization. Meanwhile, we also fixed the heights of the edge atoms of the underlying graphene nanoribbon comprising 16 carbon and 16 hydrogen atoms in total. All the atoms were allowed to relax until the maximum force became less than 0.03 eV/Å. The climbing-image nudged elastic band (CI-NEB) method30 was used to explore the transition states and the minimum energy pathway (MEP) of the reduction reactions.

Figure 1. Computational models and the MEP for catalytic reduction of GF by Pt-coated tip under hydrogen atmosphere. The labels a, b, and c refer to the initial state, transition state, and final state, respectively. The blue, white, gray, and azure spheres represent Pt, H, C, and F atoms, respectively.

possible hazards arising from HF, such nanoscale reduction of GF under hydrogen atmosphere is less practicable despite its high potential for the direct writing of electronic devices on GF. Reduction under Ethylene Atmosphere. Because the platinum hydride will produce corrosive HF gas in reduction of GF, what other alternative atmosphere can be used for the catalytic reduction at the nanoscale? We note that ethylene has unsaturated carbon atoms and they can accommodate F atoms. The addition of F to ethylene will create a covalent C−F bond, which is much stronger than that in GF, and presumably, this process should be thermodynamically favorable. Additionally, the ethylene molecule can interact with a Pt cluster,38 and its adsorption energy highly depends on the cluster size and shape. Accordingly, the ethylene-adsorbed Pt tip may serve as a potential acceptor to abstract F from GF. To test this hypothesis, we introduced a simple model in which only one ethylene molecule is adsorbed on the abovementioned Pt20 cluster, to estimate the reduction activity of GF by the Pt-coated tip under ethylene atmosphere in calculation. Considering that the top two layers of the Pt cluster are fixed, four possible initial configurations are taken into account in this work, as illustrated in Figure 2. Here, configuration b (in Figure

■

RESULTS AND DISCUSSION Reduction under Hydrogen Atmosphere. In general, the normal C−F bond in a molecule is very strong and its bond dissociation energy (BDE) is 513.8 ± 10.0 kJ/mol,31,32 indicating that removing the F atom is difficult. Here, the predicted C−F bond length in the fluorinated graphene is 1.61 Å, which is within the length range of so-called semi-ionic C−F bonds with BDE less than 350 kJ/mol.33 In particular, the BDE of C−F bond in C2F sheet is determined to be 44.3 kJ/mol by previous calculations.34 Similar low BDE values were found in fluorinated single-wall carbon nanotubes34 and fullerenes.34,35 Obviously, the C−F bond in π-conjugation systems is labile and the F is easily removed from GF, restoring its electrical conductivity. In the reduction simulation of GF, the Pt tip was first introduced under hydrogen atmosphere, and this technique was successfully mimicked to locally catalyze the reduction reaction of GO with high performance. The computational models and the corresponding MEP for the catalytic reduction process are shown in Figure 1. The dissociation of hydrogen molecules on the Pt tip was believed to be a facile process.36,37 Therefore, hydrogen molecules will dissociate at the Pt20 cluster spontaneously as the Pt tip placed under hydrogen environment until the tip was fully saturated by hydrogen. Here, the bottom two layers of the Pt20 cluster are completely hydrogenated with eight hydrogen atoms. As demonstrated in Figure 1, the reduction process is exothermic and nearly barrier free, and this defluorination could be considered a one-step reaction. Under induction of the hydrided Pt tip, the C−F bond is activated and elongated gradually at first. Followed by C−F bond cleavage and hydrogen abstraction from the Pt tip by the newly generated active F radical, the hydrogen fluoride (HF) molecule is formed around the tip. This entire process needs to overcome only a very small energy barrier of 0.03 eV. In consideration of

Figure 2. Four possible adsorption configurations and their relative energies for one ethylene molecule adsorbed on the Pt-coated AFM tip. The most stable adsorbed state (configuration b) is chosen as the zero-point of energy.

2) shows relatively higher stability and is lower in energy than configurations a, c and d by 0.22, 0.57, and 0.24 eV respectively. However, if only the topmost layer is fixed, the energy difference between configurations b and a decreases to 0.09 eV. Therefore, configuration a with ethylene in the near-attack reactive conformation for F abstraction is also possible in reality, and it was considered in the following reduction reaction. The Pt cluster with ethylene adsorbed at the tip site can easily remove one fluorine from GF, and this process is exothermic and barrierless, as shown in Figure 3. With the 22644

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648

The Journal of Physical Chemistry C

Article

Figure 4. Possible configurations and their relative energies for the coadsorption of H2 and C2H4 at the Pt-coated AFM tip. The total energies of configuration a and two isolated ethylene molecules are defined as the zero reference point. Therefore, the total energies of a, b, c, d, e, and f are referred to Pt20(H)6(C2H4)(a) + 2 C2H4, Pt20(H)6(C2H4)(b) + 2 C2H4, Pt20(H)6(C2H4)(c) + 2 C2H4, Pt20(H)4(C2H4)2 + H2 + C2H4, Pt20(H)2(C2H4)2 + 2 H2 + C2H4, and Pt20(H)2(C2H4)3 + 2 H2, respectively.

Figure 3. Computational models and the MEP for catalytic reduction of GF under ethylene atmosphere. The configurations of the initial state and the finial state are named a and b and are inset in the energy profile.

formation of C2H4F, the carbon atom of C2H4F is chemically bonded to the tip, which may hinder the adsorption of other ethylene at the tip site, and the reduction reaction cannot proceed further as a consequence. Reduction under Hydrogen/Ethylene Mixture Atmosphere. As mentioned above, the Pt tip can effectively reduce the graphene fluoride under hydrogen atmosphere, but the reduction process releases corrosive and toxic HF. With the assistance of ethylene, the initial removal of F from GF is facile by the Pt tip, and unfortunately, the Pt tip cannot be regenerated. It is thus highly desirable to find a new strategy for preventing the creation of HF and recycling the reduction process of GF. From a chemical point of view, the initial product of C2H4F bound at the tip in Figure 3 could be released by hydrogenation. Previous studies show that hydrogenation of CH2CH2 and CH3CH2 adsorbed on the Pt surface to C2H6 generally experience relatively higher barriers with respect to dehydrogenation, but the high-index surface can enhance the hydrogenation reactivity.39−41 Here, the possibility for the reduction of GF induced by the Pt-coated AMF tip under H2/ C2H4 mixture atmosphere has been explored. To have an insight into interactions of the mixture gases with the Pt tip, plausible patterns for the coadsorption of H2 and C2H4 at the Pt-coated AFM tip, as shown in Figure 4, have been investigated. Calculations indicate that the adsorption of C2H4 prefers the tip and edge sites under the mixture atmosphere. For the coadsorption with involvement of single ethylene, configuration a is the most stable structure, 0.11 and 0.71 eV lower than configurations b and c, respectively. Furthermore, we note that the total energy of pattern a and one isolated H2 is 0.70 eV lower than that of the separated hydrogen-saturated Pt tip and ethylene, showing that the substitution of C2H4 for H2 is energetically favorable. The consecutive substitution processes have been evaluated, and the substitution of C2H4 for one or two H2 molecules to configuration d or e in Figure 4 is predicted to be endothermic by 0.03 or 2.06 eV, respectively. The further substitution of C2H4 for one H2 molecules to the configuration f based on configuration d at least requires an energy of 0.43 eV. The details for configuration d, e, and f are given in Figure S1 of the Supporting Information. These results lend support to

configuration a as the most probable structure for the saturated Pt tip under the H2/C2H4 atmosphere, and configuration a was used in the following calculations on reduction of GF. As Figure 5 shows, the removal of the first F from the GF by the Pt-coated tip under the mixture atmosphere (configuration

Figure 5. Computational models and the MEP for the first F removed from the GF by the Pt-coated tip under the hydrogen/ethylene mixture atmosphere. The corresponding configurations for the initial state, transition state, and final state are labeled a, b, and c, respectively.

a) is also a barrierless process, with an energy release of 1.04 eV. The subsequent hydrogen migration from the Pt-coated tip to the newly formed C2H4F yields C2H5F; this process is slightly exothermic, and the barrier for the rate-determining step is 0.31 eV, as shown in Figure 6. Because the first F abstraction is exothermic by 1.04 eV, the following hydrogen transfer with the relatively small barrier should be facile, and the excess energy may drive the release of C2H5F, leading to an unsaturated Pt-coated tip. With the release of C2H5F, the vacant sites in the Pt-coated tip are available for surrounding H2 and C2H4 molecules. What would happen next? Two possibilities, the dissociation chemical adsorption of 3/2 H2 molecules regenerating the hydrogensaturated Pt tip and the adsorption of ethylene initiating a new reduction process of GF, are considered. The hydrogenation 22645

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648

The Journal of Physical Chemistry C

Article

Figure 6. Computational models and the MEP for the adsorbed C2H4F proceeds to the first C2H5F by hydrogenation on Pt-coated tip under the hydrogen/ethylene mixture atmosphere. The initial state (a), the transition state (b), and the final state (c) are given in the right-hand side.

process, Pt20(H)5 + 3/2 H2 → Pt20(H)8, was predicted to be exothermic by 1.44 eV, and the adsorption of ethylene, Pt20(H)5 + C2H4 → Pt20(H)5(C2H4), releases an energy of 1.76 eV. Accordingly, the adsorption of C2H4 is a dominant process upon desorption of C 2 H 5 F. With the formation of Pt20(H)5(C2H4), the Pt-coated tip with one vacant site could either dissociate and adsorb 1/2 H2 molecule to regenerate the most stable configuration for the saturated Pt tip under the mixture gases or reduce the GF directly. The further hydrogenation process, Pt 20 (H) 5 (C 2 H 4 ) + 1/2 H 2 → Pt20(H)6(C2H4), has an energy release of 0.41 eV, and the direct reduction of GF to yield the C2H4F-bound at the Ptcoated tip is exothermic by 1.01 eV. Obviously, the further hydrogenation process is less favorable than the F abstraction by ethylene. Considering that the F transfer from GF to ethylene-bound at the Pt-coated tip is barrier free (see Figure S2 of Supporting Information), here we focus on only the following reactions starting from C2H4F-bound at the Pt-coated tip. We note that with the new C−F bond formation, the initial state can be rotated to a more stable configuration a, as shown in Figure 7, which is 0.13 eV lower in energy. The adsorbed C2H4F proceeds to the second C2H5F through its hydrogenation with a barrier of 0.72 eV, and this process is slightly endothermic. This hydrogenation desorption of C2H5F can be thermally driven or by the remarkable exothermicity in the initial F abstract from GF. Presumably, the final step can benefit from the entropy increase during the desorption of C2H5F. The reported best working temperature for reduction of graphene oxide by the platinum AFM tip under hydrogen environment is ≤115 °C;24 if this working temperature could be adopted for the reduction of GF, the energy barrier of 0.72 eV is accessible under these conditions. After the newly produced C2H5F is desorbed, the Pt-coated tip becomes unsaturated again. Calculations show that the following hydrogenation to the fully hydrogen-saturated Pt tip, i.e., Pt20(H)4 + 2 H2 → Pt20(H)8, is energetically more favorable, with an exothermicity of 0.37 eV, compared to adsorption of ethylene. A new cycle for the reduction of GF by the Pt-coated tip under the mixture atmosphere may begin from here, and the complete reaction cycle is schematically shown in Figure 8.

Figure 7. Schematic diagram and the MEP for the formation of the second C2H5F on unsaturated Pt-coated tip under the hydrogen/ ethylene mixture atmosphere. The labels a, b, and c correspond to the initial state, transition state, and final state, respectively.

Figure 8. Schematic diagram of the possible reaction cycle for catalytic reduction of GF by Pt-coated tip under hydrogen/ethylene atmosphere. The energy barriers of the reduction of GF for different steps are shown along the arrows.

different atmospheres by first-principles calculations. The present results show that the tip-induced reduction of graphene fluoride with the assistance of pure hydrogen atmosphere is facile despite release of the hazardous HF. Ethylene was predicted to be an excellent acceptor for F abstraction from GF, but the corresponding defluorination cycle cannot be recycled. Under the H2/C2H4 mixture atmosphere, the Pt-coated tip can effectively and sequentially reduce graphene fluoride and with the release of relatively harmless reduction product, C2H5F, which opens an alternative avenue toward direct writing of electronic devices on graphene fluoride.

■

■

ASSOCIATED CONTENT

S Supporting Information *

CONCLUSIONS In summary, we have systematically investigated the reduction process of graphene fluoride induced by the Pt-coated tip under

Further details for configuration d, e, and f from different perspectives; computational models and the MEP for the second F removed from the GF by the Pt-coated tip under the 22646

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648

The Journal of Physical Chemistry C

Article

(15) Robinson, J. T.; Burgess, J. S.; Junkermeier, C. E.; Badescu, S. C.; Reinecke, T. L.; Perkins, F. K.; Zalalutdniov, M. K.; Baldwin, J. W.; Culbertson, J. C.; Sheehan, P. E.; et al. Properties of fluorinated graphene films. Nano Lett. 2010, 10, 3001−3005. (16) Nair, R. R.; Ren, W.; Jalil, R.; Riaz, I.; Kravets, V. G.; Britnell, L.; Blake, P.; Schedin, F.; Mayorov, A. S.; Yuan, S.; et al. Fluorographene: A two-dimensional counterpart of Teflon. Small 2010, 6, 2877−2884. (17) Ribas, M. A.; Singh, A. K.; Sorokin, P. B.; Yakobson, B. I. Patterning nanoroads and quantum dots on fluorinated graphene. Nano Res. 2011, 4, 143−152. (18) Bagri, A.; Mattevi, C.; Acik, M.; Chabal, Y. J.; Chhowalla, M.; Shenoy, V. B. Structural evolution during the reduction of chemically derived graphene oxide. Nat. Chem. 2010, 2, 581−587. (19) Lee, W. K.; Haydell, M.; Robinson, J. T.; Laracuente, A. R.; Cimpoiasu, E.; King, W. P.; Sheehan, P. E. Nanoscale reduction of graphene fluoride via thermochemical nanolithography. ACS Nano 2013, 7, 6219−6224. (20) Ho, K.-I.; Liao, J.-H.; Huang, C.-H.; Hsu, C.-L.; Zhang, W.; Lu, A.-Y.; Li, L.-J.; Lai, C.-S.; Su, C.-Y. One-step formation of a single atomic-layer transistor by the selective fluorination of a graphene film. Small 2014, 10, 989−997. (21) Withers, F.; Bointon, T. H.; Dubois, M.; Russo, S.; Craciun, M. F. Nanopatterning of fluorinated graphene by electron beam irradiation. Nano Lett. 2011, 11, 3912−3916. (22) Lee, W.-K.; Robinson, J. T.; Gunlycke, D.; Stine, R. R.; Tamanaha, C. R.; King, W. P.; Sheehan, P. E. Chemically isolated graphene nanoribbons reversibly formed in fluorographene using polymer nanowire masks. Nano Lett. 2011, 11, 5461−5464. (23) Lee, W. K.; Tsoi, S.; Whitener, K. E.; Stine, R.; Robinson, J. T.; Tobin, J. S.; Weerasinghe, A.; Sheehan, P. E.; Lyuksyutov, S. F. Robust reduction of graphene fluoride using an electrostatically biased scanning probe. Nano Res. 2013, 6, 767−774. (24) Zhang, K.; Fu, Q.; Pan, N.; Yu, X. X.; Liu, J. Y.; Luo, Y.; Wang, X. P.; Yang, J. L.; Hou, J. G. Direct writing of electronic devices on graphene oxide by catalytic scanning probe lithography. Nat. Commun. 2012, 3, 1194. (25) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. J. Comput. Mater. Sci. 1996, 6, 15−50. (26) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. (27) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953. (28) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (29) Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188−5192. (30) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A climbing image nudged elastic band method for finding saddle points and minimum energy paths. J. Chem. Phys. 2000, 113, 9901−9904. (31) Haynes, W. M. CRC Handbook of Chemistry and Physics, 94th ed.; CRC Press/Taylor and Francis: Boca Raton, FL, 2004. (32) Luo, Y. Comprehensive Handbook of Chemical Bond Energies; CRC Press: Boca Raton, FL, 2007. (33) Nakajima, T.; Groult, H. Fluorinated Materials for Energy Conversion; Elsevier: Amsterdam, 2005. (34) Bettinger, H. F.; Kudin, K. N.; Scuseria, G. E. Thermochemistry of fluorinated single wall carbon nanotubes. J. Am. Chem. Soc. 2001, 123, 12849−12856. (35) Papina, T. S.; Kolesov, V. P.; Lukyanova, V. A.; Boltalina, O. V.; Lukonin, A. Y.; Sidorov, L. N. Enthalpy of formation and C-F bond enthalpy of fluorofullerene C60F36. J. Phys. Chem. B 2000, 104, 5403− 5405. (36) Chen, L.; Cooper, A. C.; Pez, G. P.; Cheng, H. Density functional study of sequential H2 dissociative chemisorption on a Pt6 cluster. J. Phys. Chem. C 2007, 111, 5514−5519. (37) Zhou, C.; Wu, J.; Nie, A.; Forrey, R. C.; Tachibana, A.; Cheng, H. On the sequential hydrogen dissociative chemisorption on small

hydrogen/ethylene mixture atmosphere. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +46-8-55378414. Fax: +46-855378590. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the Göran Gustafsson Foundation for Research in Natural Sciences and Medicine, the Swedish Research Council (VR), the Major State Basic Research Development Programs (2010CB923300), and the National Natural Science Foundation of China (20925311). The Swedish National Infrastructure for Computing (SNIC) is acknowledged for the supercomputer resources.

■

REFERENCES

(1) 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 thin carbon films. Science 2004, 306, 666−669. (2) Novoselov, K. S.; Jiang, D.; Schedin, F.; Booth, T. J.; Khotkevich, V. V.; Morozov, S. V.; Geim, A. K. Two-dimensional atomic crystals. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 10451−10453. (3) Morozov, S. V.; Novoselov, K. S.; Katsnelson, M. I.; Schedin, F.; Elias, D. C.; Jaszczak, J. A.; Geim, A. K. Giant intrinsic carrier mobilities in graphene and its bilayer. Phys. Rev. Lett. 2008, 100, 016602. (4) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayou, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; et al. Electronic confinement and coherence in patterned epitaxial graphene. Science 2006, 312, 1191−1196. (5) Wu, Y. Q.; Lin, Y. M.; Bol, A. A.; Jenkins, K. A.; Xia, F. N.; Farmer, D. B.; Zhu, Y.; Avouris, P. High-frequency, scaled graphene transistors on diamond-like carbon. Nature (London, U.K.) 2011, 472, 74−78. (6) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior thermal conductivity of single-layer graphene. Nano Lett. 2008, 8, 902−907. (7) Lee, C.; Wei, X.; Kysar, J. W.; Hone, J. Measurement of the elastic properties and intrinsic strength of monolayer graphene. Science 2008, 321, 385−388. (8) Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Chemically derived, ultrasmooth graphene nanoribbon semiconductors. Science 2008, 319, 1229−1232. (9) Geim, A. K.; Novoselov, K. S. The rise of graphene. Nat. Mater. 2007, 6, 183−191. (10) Kosynkin, D. V.; Higginbotham, A. L.; Sinitskii, A.; Lomeda, J. R.; Dimiev, A.; Price, B. K.; Tour, J. M. Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons. Nature (London, U.K.) 2009, 458, 872−876. (11) Lin, Y. M.; Dimitrakopoulos, C.; Jenkins, K. A.; Farmer, D. B.; Chiu, H. Y.; Grill, A.; Avouris, P. 100-GHz transistors from wafer-scale epitaxial graphene. Science 2010, 327, 662−662. (12) Wu, J.; Pisula, W.; Mllen, K. Graphenes as potential material for electronics. Chem. Rev. (Washington, DC, U.S.) 2007, 107, 718−747. (13) Wei, Z.; Wang, D.; Kim, S.; Kim, S.-Y.; Hu, Y.; Yakes, M. K.; Laracuente, A. R.; Dai, Z.; Marder, S. R.; Berger, C.; et al. Nanoscale tunable reduction of graphene oxide for graphene electronics. Science 2010, 328, 1373−1376. (14) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S. T.; Ruoff, R. S. Preparation and characterization of graphene oxide paper. Nature (London, U.K.) 2007, 448, 457−460. 22647

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648

The Journal of Physical Chemistry C

Article

platinum clusters: A density functional theory study. J. Phys. Chem. C 2007, 111, 12773−12778. (38) Nieminen, V.; Honkala, K.; Taskinen, A.; Murzin, D. Y. Intrinsic metal size effect on adsorption of organic molecules on platinum. J. Phys. Chem. C 2008, 112, 6822−6831. (39) Chen, Y.; Vlachos, D. G. Hydrogenation of ethylene and dehydrogenation and hydrogenolysis of ethane on Pt(111) and Pt(211): A density functional theory study. J. Phys. Chem. C 2010, 114, 4973−4982. (40) Anghel, A. T.; Wales, D. J.; Jenkins, S. J.; King, D. A. Pathways for dissociative ethane chemisorption on Pt110 (1 × 2) using density functional theory. Chem. Phys. Lett. 2005, 413, 289−293. (41) Anghel, A. T.; Wales, D. J.; Jenkins, S. J.; King, D. A. Theory of C2Hx species on Pt110 (1 × 2): Reaction pathways for dehydrogenation. J. Chem. Phys. 2007, 126, 044710.

22648

dx.doi.org/10.1021/jp507469r | J. Phys. Chem. C 2014, 118, 22643−22648