Computational Prediction of New Hydrocarbon Materials: The

Aug 2, 2012 - Large-surface-area films of a new allotropic form of carbon, graphdiyne, were synthesized in 2009.(1) The semiconducting material consis...
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Computational Prediction of New Hydrocarbon Materials: The Hydrogenated Forms of Graphdiyne George M. Psofogiannakis and George E. Froudakis* Department of Chemistry, University of Crete, P.O. Box 2208, Voutes, Heraklion, Crete 710 03, Greece ABSTRACT: Periodic density functional theory calculations were used to assess the thermodynamic stability of the hydrogenated forms of graphdiyne. The sp carbons of graphdiyne films in principle can be hydrogenated to generate extended twodimensional hydrocarbons that consist of benzene rings linked with either sp2 or sp3 carbon chains. Free energy calculations for the addition of H2 to graphdiyne suggest that the fully hydrogenated form of the material (“graphbutane”) is thermodynamically favored up to very high temperatures. This prediction opens up the possibility of tailoring the electronic properties of graphdiyne via hydrogenation.

arge-surface-area films of a new allotropic form of carbon, graphdiyne, were synthesized in 2009.1 The semiconducting material consisted of multilayers of one-atom-thick pure carbon layers grown on the surface of copper via a crosscoupling reaction using hexaethynylbenzene. These graphdiyne layers consisted of an extended network of benzene rings linked by butadiyne (diacetylene) units to form a strongly bonded conjugated carbon layer with an extensive delocalized π system (see Figure 1A). The synthesis of the new carbon structure followed previous attempts directed toward the preparation of oligomeric substructures of graphdiyne2−5 and was aimed at applications in electronic and optoelectronic devices. Following its discovery, computational methods were used to characterize the electronic and physicochemical properties of the graphdiyne structure.6,7 In the last two decades, there has been a tremendous interest in the synthesis and isolation of new carbon structures for a wide variety of applications in nanotechnology, electronics, materials science, gas-storage and separation, etc. New allotropic forms of carbon, such as fullerenes, carbon nanotubes, and two-dimensional graphene, have been synthesized and isolated.8−10 In 2007, Sofo et al.11 predicted via firstprinciples calculations the existence of graphane, the hydrogenated form of graphene, which is characterized by a wide band gap. Graphane, consisting of a two-dimensional network of sp3 carbons, was predicted to be a thermodynamically stable hydrocarbon material (energetically favorable with respect to a mixture of graphene and hydrogen gas). In 2009, Elias et al.12 showed that graphene can react with atomic hydrogen via plasma treatment to transform the highly conductive graphene layer into an insulator, thus providing evidence for a graphanelike structure. The discovery of graphane manifested a possible route for tailoring the electronic and magnetic properties of graphene via hydrogenation.13 Influenced by the discovery of a computationally predicted form of hydrogenated graphene, we examined via firstprinciples calculations the thermodynamic stability of the two-dimensional hydrogenated forms of graphdiyne. In analogy

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to the graphene/graphane pair, the existence of a hydrogenated form of graphdiyne would offer the possibility of a material with controllable properties. In the present work, we examined the effect of replacing the butadiyne linkers of graphdiyne by either butadiene or butane linkers. The aim was to examine the structure of the resulting two-dimensional hydrocarbons as well as the thermodynamic conditions under which the hydrogenated forms of the material are energetically favorable with respect to a mixture of graphdiyne and H2. The latter involves a calculation of the free energy for the reaction of H2 addition to graphdiyne. Furthermore, an estimate of the electronic energy barrier for H2 addition to the triple bonds of the butadiyne linkers was needed to comment on the kinetics of a possible reaction of graphdiyne with hydrogen gas. Two-dimensional periodic density functional theory (DFT) calculations were performed using the default periodic algorithm in Gaussian 03.14 The gradient-corrected DFT functional of Perdew, Burke, and Ernzerhof (PBEPBE)15 was used in conjunction with 3-21G* basis sets for the C and H atoms. Figure 1 shows images of 3 × 3 supercells of the optimized periodic structures of graphdiyne and the new proposed hydrocarbons graphdiene and graphbutane. Graphdiene is formed by hydrogenation of the triple bonds of the graphdiyne linkers and contains two double bonds in each linker, as manifested by the characteristic bond lengths. Graphbutane is formed by complete hydrogenation of the triple bonds of the linkers of graphdiyne to single bonds. The bond orders, which are not strictly defined quantummechanically, are not depicted in the structure graphics but are shown in the top-left insets in Figure 1. Figure 1 also shows the dimensions of the optimized unit cells. All of the atoms in graphdiyne are coplanar. In graphdiene, the carbon chains are inclined with respect to the rings. The unequal size of the unit cell dimensions manifests the fact that the two-dimensional symmetry has been lost. This Received: July 6, 2012 Published: August 2, 2012 19211

dx.doi.org/10.1021/jp306704b | J. Phys. Chem. C 2012, 116, 19211−19214

The Journal of Physical Chemistry C

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Figure 2. DFT-optimized structures of (A) 1,4-diphenylbutadiyne, (B) 1,4-diphenylbutadiene, and (C) 1,4-diphenylbutane. The vibrational frequencies of these structures were used to calculated approximate ZPE and entropy corrections for the hydrogenations of graphdiyne and graphdiene.

Figure 3. Calculated Gibbs free energy changes for the hydrogenation reactions of graphdiyne and graphdiene as functions of temperature. The graphbutane structure has the lowest free energy up to 860 K.

Figure 1. 3 × 3 supercells of the DFT-optimized periodic structures of (A) graphdiyne, (B) graphdiene, and (C) graphbutane. The selected view is from above the plane of the structures. The unit cell dimensions and selected bond lengths are shown. In each panel, the inset at the top left shows a simplified chemical structure of the linker between two rings, emphasizing the bond order.

happens because the energetically preferred structure has the rings in parallel planes rather than in a coplanar structure. The unit cell size is smaller than in graphdiyne as a result of buckling of the carbon chains, despite the fact that the C−C bonds become longer. In graphbutane, the unit cell size is about the same as in graphdiyne, as the effects of buckling of the carbon chains and lengthening of the carbon bonds due to saturation cancel each other out. Symmetry is restored, as all of the rings are coplanar and all of the carbon chains are sloped in the same direction, perpendicular to the plane of the rings. The bond lengths and angles of the linker carbons are characteristic of fully saturated hydrocarbon bonds, such as in butane.

Figure 4. Calculated Gibbs free energy changes for the hydrogenation reactions of graphdiyne and graphdiene as functions of H2 pressure. The graphbutane structure has the lowest free energy over the entire pressure range considered.

The energetic quantity of primary interest in these calculations was the energy of hydrogenation, defined as the energy of the following reactions: 19212

dx.doi.org/10.1021/jp306704b | J. Phys. Chem. C 2012, 116, 19211−19214

The Journal of Physical Chemistry C

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Gaussian 03, a simpler, chemically similar, cluster model was used to estimate the ZPE and thermodynamic quantities. The model is shown in Figure 2 and consists of the molecules 1,4diphenylbutadiyne, 1,4-diphenylbutadiene, and 1,4-diphenylbutane. The ZPE-uncorrected energies for the reactions diphenylbutadiyne + 2H 2 → diphenylbutadiene

(R3)

and diphenylbutadiene + 2H 2 → diphenylbutane

were calculated with DFT (PBEPBE/6-31++G**) and found to be E(R3) = −2.04 eV/H2 and E(R4) = −1.22 eV/H2, in sufficient agreement with the analogous reactions R1 and R2. On the basis of the structural and energetic similarities, the vibrational frequencies of the structures in Figure 2 were calculated and used to estimate the ZPE and the vibrational thermal energy, enthalpy, and entropy contributions to the free energy of reactions R1 and R2. Standard statistical thermodynamic relations were used to calculate these quantities.16 On the basis of the standard view of the adsorption of gases on solids, the translational and rotational degrees of freedom of H2 are transformed into additional vibrational degrees of freedom of the final product of the reaction. This results in loss of entropy when adsorption takes place (ΔSR < 0) and a corresponding increase in the free energy change for the adsorption reaction (ΔGR = ΔHR − TΔSR) that increases with temperature. The ZPE-corrected reaction energies for reactions R1 and R2 were E(R1) = −1.81 eV/H2 and E(R2) = −1.00 eV/ H2. The ideal gas assumption was used to calculate the enthalpy term (PV = RT) and the translational and rotational contributions to the internal energy and entropy of molecular H2. For T > 200 K and P < 100 atm, the compressibility factor for H2 is less than 1.1, and therefore, the ideal gas assumption is sufficient.17 Only vibrational contributions were used for the calculations of internal energies and entropies of the molecules shown in Figure 2, as the molecules represent the solid state, as discussed before. In Figure 3, the calculated free energies for the hydrogenation reactions are plotted as functions of temperature at atmospheric pressure. The free energy of reaction R1 is negative for temperatures less than ∼1330 K, while the free energy of reaction R2 is negative for temperatures less than ∼860 K. Thus, at any temperature lower than ∼860 K, the fully hydrogenated state of the material, graphbutane, is thermodynamically preferred with respect to a state composed of graphdiyne and molecular H2 or graphdiene and molecular H2. Although this temperature is based on approximate assumptions, it clearly indicates that graphbutane is the thermodynamic minimum up to very high temperatures. In Figure 4, the free energy changes for the reactions are plotted as functions of H2 pressure; these plots clearly show that graphbutane is the thermodynamic minimum even at very low pressures, such as 10−8 atm. These results indicate that graphbutane is a very stable structure over a wide range of temperature and pressure conditions. Although graphbutane is predicted to be a stable structure, the synthesis of this material from graphdiyne would require the addition of H2 to the carbon−carbon triple bonds. Hydrogenation of unsaturated carbon bonds is an activated process. An estimate of the energy barrier associated with the first step of this reaction was obtained via DFT (PBEPBE/631++G**) using the 1,4-diphenylbutadiyne model shown in Figure 5A. A constrained optimization (energy scan)

Figure 5. DFT-calculated reaction coordinate scan, presented as a plot of the energy as a function of the distance between the H atoms of H2, during the addition of the first H2 molecule to one of the triple bonds of 1,4-diphenylbutadiyne (structure A, a simple model for graphdiyne). The reaction proceeds though the intermediate state B to the final structure C.

graphdiyne + nH 2 → graphdiene

(R1)

and graphdiene + nH 2 → graphbutane

(R4)

(R2)

where in both cases, for the unit cell shown in Figure 1, n = 6. The zero-point-energy (ZPE)-uncorrected DFT energies (PBEPBE/3-21G*) for these reactions per H2 molecule added were E(R1) = −2.15 eV/H2 and E(R2) = −1.35 eV/ H2. The ZPE-uncorrected reaction energies suggest that hydrogenation is exothermic. Nevertheless, the free energy change for these two reactions is needed to verify the thermodynamic propensity for hydrogenation of graphdiyne. To calculate the ZPE corrections for the reaction energies as well as the enthalpy and entropy corrections at finite temperatures, vibrational frequencies were needed. As frequencies are not available for periodic calculations with 19213

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(4) Wan, W. B.; Haley, M. M. Carbon Networks Based on Dehydrobenzoannulenes. 4. Synthesis of “Star” and “Trefoil” Graphdiyne Substructures via Sixfold Cross-Coupling of Hexaiodobenzene. J. Org. Chem. 2001, 66, 3893−3901. (5) Marsden, J. A.; Haley, M. M. Carbon Networks Based on Dehydrobenzoannulenes. 5. Extension of Two-Dimensional Conjugation in Graphdiyne Nanoarchitectures. J. Org. Chem. 2005, 70, 10213−10226. (6) Long, M.; Tang, L.; Wang, D.; Li, Y.; Shuai, Z. Electronic Structure and Carrier Mobility in Graphdiyne Sheet and Nanoribbons: Theoretical Predictions. ACS Nano 2011, 5, 2593−2600. (7) Jiao, Y.; Du, A.; Hankel, M.; Zhu, Z.; Rudolph, V.; Smith, S. C. Graphdiyne: A Versatile Nanomaterial for Electronics and Hydrogen Purification. Chem. Commun. 2011, 47, 11843−11845. (8) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. C60: Buckminsterfullerene. Nature 1985, 318, 162−163. (9) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354, 56−58. (10) 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. (11) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Graphane: A TwoDimensional Hydrocarbon. Phys. Rev. B 2007, 75, No. 153401. (12) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M.; Morozov, S. V.; Blake, P.; Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Control of Graphene’s Properties by Reversible Hydrogenation: Evidence for Graphane. Science 2009, 323, 610−613. (13) Novoselov, K. S. Beyond the Wonder Material. Phys. World 2009, 22, 27−30. (14) Frisch, M. J.; et al. Gaussian 03, revision D.01, Gaussian, Inc.: Wallingford, CT, 2004. (15) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (16) Gokhale, A. A.; Kandoi, S.; Greeley, J. P.; Mavrikakis, M.; Dumesic, J. A. Molecular-Level Descriptions of Surface Chemistry in Kinetic Models Using Density Functional Theory. Chem. Eng. Sci. 2004, 59, 4679−4691. (17) Zhou, L.; Zhou, Y. Determination of Compressibility Factor and Fugacity Coefficient of Hydrogen in Studies of Adsorptive Storage. Int. J. Hydrogen Energy 2001, 26, 597−601. (18) Psofogiannakis, G. M.; Froudakis, G. E. Fundamental Studies and Perceptions on the Spillover Mechanism for Hydrogen Storage. Chem. Commun 2011, 47, 7933−7943. (19) Lin, Y.; Ding, F.; Yakobson, B. I. Hydrogen Storage by Spillover on Graphene as a Phase Nucleation Process. Phys. Rev. B 2008, 78, No. 041402.

calculation was used to probe the reaction coordinate for the addition of H2 to the triple bond. The reaction coordinate variable was chosen to be the distance between the H atoms of the dissociating H2 molecule. As shown in Figure 5, the reaction was found to be a two-step process. A high-energy intermediate state was found in which hydrogen has been added to the same carbon atom of the triple bond (Figure 5B). The energy barrier for the first dissociation step was found to be 2.1 eV. The relatively unstable intermediate state is transformed into the final structure (Figure 5C) via a 0.5 eV energy barrier. The net reaction energy (−1.98 eV) manifests a highly exothermic addition reaction, in conjunction with the exothermicity of reaction R1. Nevertheless, the very high activation energy barrier shows that hydrogenation of graphdiyne would require a catalytic pathway. Such a catalytic pathway could be provided by the mechanism of hydrogen spillover, wherein nanoparticles of Pt or Pd metal serve as the H 2 dissociation catalyst and the generated H atoms subsequently hydrogenate a carbon (or other) material.18 The spillover process has also been discussed in association with the hydrogenation of graphene to graphane.19 In summary, we have used first-principles calculations to predict the existence of a new two-dimensional hydrocarbon material. This material has been named graphbutane and consists of benzene rings interconnected with butane linkers. This material could be theoretically synthesized starting from graphdiyne, since hydrogenation of graphdiyne is thermodynamically favored up to very high temperatures, as predicted by free energy calculations. There also exists a hydrogenation state of the material intermediate between graphdiyne and graphbutane, which we have named graphdiene, that has butadiene linkers between the aromatic rings. Despite the thermodynamic propensity for hydrogenation, the synthesis of graphbutane would require intelligent ways to overpass high hydrogenation barriers without destroying the material structure.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support through a European Union Marie-Curie International Reintegration Grant (IRG) is greatly appreciated. This research has also been cofinanced by the European Union (European Social Fund, ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF) - Research Funding Program: THALES.

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REFERENCES

(1) Li, G.; Li, Y.; Liu, H.; Guo, Y.; Li, Y.; Zhu, D. Architecture of Graphdiyne Nanoscale Films. Chem. Commun. 2010, 46, 3256−3258. (2) Zhou, Q.; Carroll, P. J.; Swager, T. M. Synthesis of Diacetylene Macromolecules Derived from 1,2-Diethynylbenzene Derivatives: Structure and Reactivity of the Strained Cyclic Dimer. J. Org. Chem. 1994, 59, 1294−1301. (3) Haley, M. M.; Bell, M. L.; English, J. J.; Johnson, C. A.; Weakley, T. J. R. Versatile Synthetic Route to and DSC Analysis of Dehydrobenzoannulenes: Crystal Structure of a Heretofore Inaccessible [20]Annulene Derivative. J. Am. Chem. Soc. 1997, 119, 2956− 2957. 19214

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