Hydrogen Storage on Metal-Coated B80 Buckyballs with Density

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Hydrogen Storage on Metal-Coated B80 Buckyballs with Density Functional Theory Guangfen Wu, Jinlan Wang,* Xiuyun Zhang, and Liyan Zhu Department of Physics, Southeast UniVersity, Nanjing, 211189, People’s Republic of China ReceiVed: December 24, 2008; ReVised Manuscript ReceiVed: March 4, 2009

We investigate the feasibility of bare and metal-coated boron buckyball B80 with M ) Li, Na, K, Be, Mg, Ca, Sc, Ti, and V for hydrogen storage using density functional theory approach. We find that M ) Ca or Sc are best candidates for hydrogen storage with moderate adsorption energy of H2 and with clustering of Sc or Ca on B80 surface avoided. We further address that an isolated cluster Ca12B80 (Sc12B80) can bind up to 66 (60) H2 molecules with an average binding energy of 0.096 (0.346) eV/H2, leading to a hydrogen storage capacity of 9.0 wt % (7.9 wt %). Two adsorption mechanisms, charge-induced dipole interaction and the Dewar-Kubas interaction, are demonstrated, and they are responsible for high hydrogen storage capacity of Ca12B80 and Sc12B80. Most interestingly, the hydrogen loaded B80Sc12-48H2 complex can further adsorb 12 H2 through charge-induced dipole interaction. In other words, these two mechanisms dominate the adsorption of different parts of H2 in the same cluster of Sc12B80-60H2. I. Introduction Materials suitable for hydrogen storage must meet some rigid requirements, such as high gravimetric and volumetric density (6 wt %, at 2010, specified by US Department of Energy (DOE)), fast kinetics, and favorite thermodynamics.1 Traditionally, hydrogen is stored through compression, liquefaction, or adsorption in metallic compounds and complex hydrides, to name a few methods.2-6 Although hydrogen capacity can be high through those traditional methods, it consumes a lot of energy in liquefaction and low temperature storage and also causes safety problems. In hydrides, hydrogen atoms are held by strong covalent bonds, and their dissociation requires high temperatures. On the other hand, the binding of hydrogen molecules on materials via physisorption7 is so weak that storage at ambient conditions is not feasible. In general, materials that bind hydrogen molecularly with an adsorption energy intermediate between physisorbed and chemisorbed states (0.2-0.6 eV) are good candidates to tailor the above requirements.8-10 Recently, many studies have been performed on metal-coated carbon fullerenes11-15 as well as their boron-, nitrogen-, and beryllium-substituted nanostructures.16-18 For instance, Sun et al.12 identified that Li12C60 can bind 60 H2 molecules (9 wt %) with binding energy of 0.075 eV/H2. Yoon et al.14 revealed that Ca32C60 can bind at least 92 H2 (8.4 wt %) with binding energy of ∼0.4 eV/H2 within local density approximation and ∼0.2 eV/H2 within generalized gradient approximation (GGA). Chandrakumar et al.15 found Na8C60 can adsorb 48 H2 molecules (9.5 wt %) with binding energy in the range of 0.149-0.159 eV/ H2. Most recently, Zhao et al.18 demonstrated several boronbased organometallic nanostructures are promising hydrogen storage materials, for example, a metal carboride buckyball Sc12B24C36 adsorbs 72 H2 (10.5 wt %) with a binding energy of 0.40 eV/H2. Boron and carbon, neighbors in the periodic table, both possess very rich physical and chemical properties. In a recent theoretical study, Szwacki et al.19 predicted an unusually stable boron cage B80 with icosahedral (Ih) symmetry; it may be the * To whom correspondence [email protected].

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second example in nature after C60. Later, Gopakumar et al.20 reported that B80 with Th symmetry is more stable and that the valence orbital structure of B80 is almost identical to that of C60. Szwacki21 further predicted the existence of the boron fullerenes family computationally. Therefore, it is expected that B80 fullerene will be synthesized experimentally. Since it has even larger surface and boron is lighter than carbon, B80 fullerene may also be a good candidate for hydrogen storage. To our knowledge, little work has been done on the hydrogen storage properties of boron nanostructures,22 with the exception of a few on the mixtures of boron and other elements.16-18,23-29 In this work, we investigate hydrogen storage capability of light metal and transition metal (TM) coated B80 buckyballs by means of the gradient-correct density functional theory (DFT) approach. Most recently, Li et al.30 published a similar work on hydrogen storage of M@B80, M ) Li, Na, K. Different from their work, we find alkali metal elements are not such good dopants to enhance hydrogen storage capacity with small adsorption energy to H2. Instead, calcium in the alkalinesearth metals as well as scandium in the transition metals coated B80 buckyballs are good candidates with moderate adsorption energy of H2. Besides, we address that the clustering of Sc or Ca on B80 surface can be prevented, which is critical for large amount hydrogen storage. We further find metal coated B80 complex Ca12B80 (Sc12B80) can bind up to 66 (60) H2 molecules with an average binding energy of 0.096 (0.346) eV/H2, leading to a hydrogen storage capacity of 9.0 wt % (7.9 wt %). Moreover, charge-induced dipole and Dewar-Kubas interactions are demonstrated to be the underlying factors impacting the hydrogen storage capacity of Ca12B80 and Sc12B80, respectively. II. Computational Methods We used Perdew and Wang (PW91)31 parametrization of the exchange-correlation functionals as well as double numerical basis sets including d-polarization functions (DNP), as implemented in the DMol3 package.32 Relativistic semicore pseudopotential (DSPP)33 for TM atoms and all electron basis sets for alkaline and alkaline earth metals were adopted. All structures were fully optimized with no symmetry restriction. Spinpolarized calculations were applied for open-shell systems. For

10.1021/jp8113732 CCC: $40.75  2009 American Chemical Society Published on Web 04/02/2009

Hydrogen Storage on Metal-Coated B80 Buckyballs

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TABLE 1: Average Bond Length of B-M (RBsM), Binding Energy (Eb) of M to B80, and Highest-Occupied Molecular Orbital (HOMO)-Lowest-Unoccupied Molecular Orbital (LUMO) Gap (∆) of MB80 (M ) Li, Na, K, Be, Mg, Ca, Sc, Ti, V) system

LiB80

NaB80

KB80

BeB80

MgB80

CaB80

ScB80

TiB80

VB80

RB-M (Å) Eb (eV) ∆ (eV)

2.236 2.220 0.108

2.652 1.578 0.103

3.053 2.154 0.103

1.857 2.668 0.246

2.413 0.622 0.123

2.603 2.444 0.188

2.369 3.661 0.228

2.210 3.330 0.251

2.217 3.650 0.279

TABLE 2: Average Adsorption Energy per H2 and Consecutive Adsorption Energy (in Parentheses) of H2 to MB80 (M ) Li, Na, K, Be, Mg, Ca, Sc, Ti, V) per H2 system Li Na K Be Mg Ca Sc Ti V

MB80-H2 0.133 0.156 0.037 0.506 0.045 0.171 0.327 0.501 0.440

MB80-2H2

MB80-3H2

MB80-4H2

MB80-5H2

MB80-6H2

MB80-7H2

0.146 (0.113) 0.261 (-0.015) 0.391 (0.033)

0.120 (-0.034)

0.096 (0.059) 0.088 (0.021) 0.269 (0.032) 0.166 0.352 0.638 0.580

(0.161) (0.377) (0.775) (0.720)

0.159 0.344 0.516 0.543

(0.143) (0.329) (0.272) (0.469)

0.156 0.369 0.514 0.530

calcium coated complexes, a thermal smearing of 0.001 Hartree was employed to make the electronic structure convergent. Electrostatic potential calculations were made on the fully relaxed structures in order to get the distribution of electric field (the electric field is the differential of the potential with respect to the distance). As an approximation, only the radial component of the electric field strength was considered with the radial axis being defined as the line between the center of the structure and one of the metal atoms. We then performed an interpolation procedure to get the radial component of the electric field strengths, because the data from DMol3 are on a XYZ grid. III. Results and Discussion We first investigate hydrogen adsorption on the bare buckyball B80. As in the case of C60,34 hydrogen adsorption on B80 is through the weak van der Waals force. For example, the adsorption energy of a single H2 to the outer wall of B80 is only 0.017 eV. Moreover, only one H2 molecule can be held inside the cage exothermally with a formation energy of 0.035 eV, whereas the adsorption of a second H2 is found to be endothermic. This means that pure B80 is not a good candidate for hydrogen storage directly. Second, we study hydrogen adsorption capabilities of light and transition metal (Li, Na, K, Be, Mg, Ca, Sc, Ti, V) coated B80. Although metal atoms can occupy different sites of B80, we consider TM adsorbed to the pentagonal ring only. The geometry and energy information of MB80 are presented in Table 1. For alkaline and alkaline earth metals, the larger the radius of M atom, the longer the M-B distance becomes. However, in the case of transition metals, the M-B distances are insensitive to atomic radius because of their close atom radius of TM. Interestingly, we find that, for each group of the periodic table (Li, Na, K; Be, Mg, Ca; Sc, Ti, V), the binding energy of the second element (Na, Mg, and Ti) on B80 is the smallest. We compute the average adsorption energy per H2, Eave ) {E[MB80] + nE[H2] s E[MB80-nH2]}/n, and consecutive adsorption energy, Er ) E[MB80 - (n - 1)H2] + E[H2] E[MB80 - nH2], with E[...] being the total energies of relaxed MB80, MB80-nH and H2, and present them in Table 2. The binding energy of H2 on alkaline metals coated B80 is relatively weak. For LiB80 and NaB80, only one H2 can be adsorbed with adsorption energy larger than 0.1 eV. In the case of KB80, the adsorption energy for one H2 is only 0.037 eV. Alkaline earth metals Be-, Mg-, and Ca-coated B80 tell different stories.

(0.149) (0.442) (0.507) (0.491)

0.153 0.316 0.462 0.427

(0.138) (0.103) (0.257) (0.016)

Although the adsorption energy of the first H2 on BeB80 reaches up to 0.506 eV, that of the second H2 is only 0.032 eV. The stronger binding of H2 to B80Be and the relatively larger HOMO-LUMO gap of B80Be-H2 imply that this structure is of high stability. Thus, the second H2 is hard to adsorb as also observed in the CO molecule adsorbed in Scn clusters.35 As for MgB80, even the first H2 adsorbed on it only has small adsorption energy of 0.045 eV. The situation for CaB80 gets better with regards to higher gravimetric/volumetric density and moderate adsorption energies. One isolated Ca site can bind six H2 with relatively constant consecutive binding energies of 0.113-0.171 eV. For TM, both Sc and Ti sites on B80 can bind five H2, while VB80 can bind only four H2 with average adsorption energies per H2 ranging from ∼0.3 to ∼0.6 eV. The number of H2 adsorbing on TMB80 can easily be understood by the 18-electron rule.36 On the basis of the above discussion, Ca-, Sc-, and Ticoated B80 might be good candidates to attract more H2 with moderate adsorption energies. However, considering the increase of the atomic mass and the computational costs, we only focus our attention on Ca- and Sc-coated B80 and analyze them in detail below. The optimized structures of CaB80-nH2 and ScB80-nH2, for n ) 1-5, are rather similar. Therefore, we only take ScB80-nH2 as an example and present the optimized structures in parts a-e of Figure 1. The locations of H2 relative to Sc are more symmetric for ScB80-nH2 (n ) 2 and 4) than those for n ) 1, 3, and 5. The structures of ScB80-nH2 (n ) 1-3 and 5) can be resembled on the basis of ScB80-4H2 by removing three, two, or one H2 or by adding one H2 on the top of Sc. When the sixth H2 is put on, it flees away after relaxation as shown in Figure 1f. Moreover, we can see from Table 2 that the consecutive adsorption energies of H2 in ScB80-nH2 (n ) 1-5) show oddseven oscillation with size n and reach the peak at n ) 4. This suggests that the higher symmetry of H2 location corresponds to larger adsorption energy and ScB80-4H2 is rather stable. As for CaB80-6H2, five H2 are on the side and one on the top of Ca atom, and the seventh H2 moves a little farther from CaB80-6H2. Previous studies revealed that many TM atoms are easy to cluster on carbon fullerenes or nanotubes.37,38 So we further consider two configurations: 12 Sc/Ca atoms clustering or coating on the top of the center of pentagons of the B80 buckyball as shown in Figure 2. We find that the coating structures are more stable than the clustering ones by 2.472 and 1.464 eV

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Figure 1. (a-f) Optimized structures of ScB80-nH2 (n ) 1-6). (g) Bonding orbital of side H2 and Sc in ScB80-5H2. (h) Optimized structure of Sc12B80-48H2.

Figure 2. Optimized structures of coating and clustering M12B80 (M ) Ca and Sc) and energy differences with respect to that of the lowest energy structures.

lower in energy for Ca12B80 and Sc12B80, respectively. The HOMO-LUMO gap and the binding energy per M atom of M12B80 relative to an individual M atom and a relaxed B80 molecule are 0.775 and 2.237 eV for M ) Ca and are 0.486 and 4.380 eV for M ) Sc, respectively, indicating the coating complexes are rather stable. To estimate how far H2 from M can be counted in the storage capacity, before putting more H2 on M12B80, we performed single point energy calculations on MB80-H2 (M ) Ca, Sc) to determine the relationship between the H-M distance and the H2 adsorption energy. When the H-Ca and H-Sc distances are less than 3 Å, the adsorption energies are greater than 0.1 eV. Under this criterion, ultimately, we introduce 66 H2 molecules to Ca12B80 with the average adsorption energy of

0.096 eV/H2 and hydrogen capacity of 9.0 wt %. Since GGA often underestimates the binding strength,39 the realistic hydrogen adsorption energy is likely larger. All the Ca-H distances are in the range of 2.4-2.7 Å, and the H-H bond lengths are lengthened from 0.751 to 0.76-0.78 Å. The decrease in the number of H2 adsorbing from 72 (provided each Ca binding six H2) to 66 and in the adsorption energy compared with that on CaB80 may mainly be due to the steric effects.40 In the case of Sc12B80, sixty H2 can be stored with each Sc site binding five H2, with a hydrogen capacity of 7.9 wt %. At each Sc site, four H2 bind to the side of Sc atom symmetrically with Sc-H distances falling to ∼2.0 Å, and the H-H bond lengths are lengthened to 0.81-0.83 Å. The fifth H2 resides on the top with Sc-H distances of ∼2.5 Å and the H-H bonds are about 0.753-0.755 Å. The average adsorption energy of H2 in B80Sc12s60H2 is 0.346 eV/H2 which lies in the desirable range of 0.2-0.6 eV revealing the feasibility of storing and releasing H2 at near-ambient conditions. In addition, the big HOMO-LUMO gap (1.315 eV) implies the high stability of the complex B80Sc12-60H2. Now let us address the interaction mechanism between hydrogen molecules and metal (Ca and Sc) coated B80 buckyball. Local densities of states (LDOS) analysis on isolated TM, TMB80, TMB80-H2, and B80Sc-5H2 obtained from the PW91/ PAW method implemented in the Vienna ab initio simulation package (VASP) are displayed in Figure 3,41 because those in the DMol3 package are based on Mulliken population analysis, which often overestimates charge transferred.42 We can see from Figure 3a that close to the Fermi level the LDOS of H2 in B80Ca-H2 almost retains the same shape as that of free H2 even though being pushed to lower energy states. The d orbitals of isolated Ca atom near the Fermi level are highly degenerate. However, after loading on the B80 cage, the degeneracy of d orbitals is broken and the d orbitals are partly occupied (see parts d-f of Figure 3). This is consistent with charge population analysis. We find a large amount of positive charges ∼+0.41 e from Hirshfeld method and ∼+0.73 e from Mulliken method on each Ca atom in Ca12B80, as a net result of the electron donation from B to the empty d orbitals of Ca and the backdonation from Ca s orbitals to B. This is also manifested by

Hydrogen Storage on Metal-Coated B80 Buckyballs

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Figure 3. LDOS on H2 and Ca in CaB80-H2, on the top-on and side-on H2, and Sc d orbitals in ScB80-5H2 together with PDOS of isolated H, Ca, and Sc atoms. The black dotted line is the Fermi level. (All the DOS are extracted from VASP calculations.)

Figure 4. Deformation electron density (left) and electric field (middle) of Ca12B80 and Sc12B80-48H2 and the optimized structure of Ca12B80-66H2 and Sc12B80-60H2. The deformation electron density is plotted at isovalue of 0.05 electrons/Å3. The dashed red line denotes the Ca and Sc location.

the accumulation of charges near Ca seen from the deformation electron density (the total density with the density of the isolated atoms subtracted) displayed in Figure 4. Furthermore, we compute the electric field, induced by the charge redistribution along the radial direction of Ca12B80 complex (see Figure 4). The electric field strength where hydrogen molecules lie are in the range of 1.6-3.4 × 1010 V/m, which is large enough to polarize H2.43 Charges on H atoms are about -0.02 to ∼+0.02 e from Hirshfeld charge analysis and are about -0.05 to ∼+0.05 e from Mulliken charge analysis, indicating charges transfer from one H atom to another, which eventually induces a dipole moment on the H2 molecules. We computed the dipole moments of MB80 (Be, Mg, Ca) using a finite field (FF) approach44 and identified a rather big value of 15.633 Debye for CaB80. This large dipole moment in combination with the net charge distribution on Ca generates the electric field around the complex. In contrast, for BeB80 and MgB80 the dipole moments are only 2.767 and 4.242

Debye, respectively, which can explain why they do not readily adsorb more H2. Therefore, we assume that the electric field produced by Ca12B80 polarizes the H2 molecules and the dipole-dipole as well as the charge-dipole interaction, consequently, binds H2 around Ca12B80. Orbital analysis of CaB80-6H2 shows there is no bonding orbital between H2 and Ca, which also supports this argument. Similarly, Yoon et al. also revealed charge induced dipole interaction is responsible for the high hydrogen storage capacity of C60Ca32.14 In the case of Sc12B80-60H2, the interaction mechanism of B80, Sc, and the four side-on H2 can be ascribed to the renowned Dewar coordination and Kubas interaction.45 The TM atoms are chemically bonded onto different molecules or nanostructures through hybridization of the LUMO with TM d orbitals (Dewar coordination). The complex binds many H2 through hybridization between H2-σ* antibonding and TM d orbitals (Kubas interaction). As displayed in Figure 1g, the strong bonding

7056 J. Phys. Chem. C, Vol. 113, No. 17, 2009 between the four side-on H2 and Sc comes from the interaction between the σ* antibonding orbital of H2 and the dxy orbital of Sc. This can also be seen from parts g-i of Figure 3. The Sc dxy orbital is degenerate with the dx2-y2 and dxz located above the Fermi level in the isolated Sc atom. When it loading on the B80 cage, this orbital is pushed below the Fermi level and interacts with the antibonding orbital of H2. Meanwhile, the Sc dz2 orbital which could interact with the σ* antibonding orbital of H2 is pushed above the Fermi level. So, no bonding orbital between the top-on H2 and the Sc atom is seen. Therefore, one may expect a different interaction mechanism of the binding of the top-on H2. Actually, the average adsorption energy of topon H2 to the rest B80Sc12-48H2 is 0.086 eV, which is much smaller than that of the four side-on H2 of 0.410 eV/H2 in the relaxed B80Sc12-48H2 and is comparable to that of B80Ca12-66H2. This may imply that the possible adsorption mechanism for the top-on H2 molecules is the same as that in B80Ca12-66H2; more precisely, polarization effect with large electric field around the top-on H2. To verify this point, we also compute the deformation electron density of Sc12B80-48H2 and the corresponding electric field of the radial component and present them in Figure 4 together with the optimized structure of Sc12B80-60H2. Like the case of Ca12B80, the electric field strength produced by Sc12B80-48H2 ranges from 1.0 to 1.5 × 1010 V/m at the optimal position of H2. This electric field then polarizes H2 (top-on one) and thus the dipole-dipole as well as the charge-dipole interaction absorbs 12 H2 around Sc12B80-48H2. Thus, we can argue that Dewar-Kubas interaction makes the Sc12B80 adsorb 48 H2 molecules and the polarization effect induced by the electric field of Sc12B80-48H2 attracts another 12 H2 molecules, which eventually leads to the high hydrogen storage capacity of Sc12B80. In a word, the high hydrogen storage capacity of Ca12B80 mainly comes from the polarization effect, while that of Sc12B80 is codetermined by both the Dewar-Kubas interaction and the polarization effect. IV. Conclusion In summary, we have studied the hydrogen adsorption capabilities of B80 buckyball as well as alkaline metals (Li, Na, K), alkaline-earth metals (Be, Mg, Ca), and transition metals (Sc, Ti, V) coated B80 systematically. Calcium and scandium coated B80 buckyballs are demonstrated to be good candidates with moderate adsorption energy of H2. The consecutive adsorption energies of H2 vary in a small range on calcium coated B80. All hydrogen bonded on TM coated B80 are in molecular form different from those on TM coated C60 with part of hydrogen bonded atomically. However, alkali metals (Li, Na, K) may not be good dopants with small adsorption energy of H2 and fewer number of H2 to be attracted. Besides, the calcium and scandium atoms on B80 are not suffering from clustering. The coating structures are more stable than the clustering ones by 2.472 and 1.464 eV lower in energy for Ca12B80 and Sc12B80, respectively. We find that Ca12B80 with 12 Ca atoms coating on the pentagonal rings of B80 can adsorb 66 H2. The high hydrogen storage capacity of Ca12B80 comes from the influences of the electric field (resulted from the redistribution of the electron density after loading the Ca atoms) surrounding the coated fullerenes. Meanwhile, Sc12B80 can attract 48 H2 through the well-known Dewar-Kubas interaction and absorb another 12 H2 through the polarization effect found in B80Ca12. The hydrogen storage capacity reaches up to 9.0 wt % and 7.9 wt % for B80Ca12 and B80Sc12, respectively, which satisfies the DOE target of 6 wt % in the year of 2010. By consideration of the similarities in boron fullerenes and carbon

Wu et al. ones, we expect the synthesis of B80 and even the application of hydrogen storage on metal coated B80 and C60 systems experimentally with the development of techniques. Acknowledgment. The work is supported by the National Nature Science Foundation of China (No.10604013, 20873019), the Program for New Century Excellent Talents in the University of China (NCET-06-0470), and the Project sponsored by SRF for ROCS, SEM, the Qinglan Project in the University of Jiangsu Province, and the Teaching and Research Foundation for the Outstanding Young Faculty and Peiyu Foundation of Southeast University. The authors would like to thank the computational resource at Department of Physics, Southeast University. The author would thank Prof. X. C. Zeng and Prof. M. L. Yang for valuable discussions. References and Notes (1) http://www.eere.energy.gov/hydrogenandfuelcells/storage/. (2) Bogdanovic´, B.; Brand, R. A.; Marjanovic´, A.; Schwickardi, M.; To¨lle, J. J. Alloys Compd. 2000, 302, 36. (3) Grochala, W.; Edwards, P. P. Chem. ReV. 2004, 104, 1283. (4) Orimo, S.; Fujii, H. Appl. Phys. A 2001, 72, 167. (5) Seayad, A. M.; Antonelli, D. M. AdV. Mater. 2004, 16, 765. (6) Ritter, J. P. Mater. Today 2003, 6, 18. (7) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (8) Chen, P.; Wu, X.; Tan, K. L. Science 1999, 285, 91. (9) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; Keeffe, M. O.; Yaghi, O. M. Science 2003, 300, 1127. (10) Han, S. S.; Goddard, W. A. J. Am. Chem. Soc. 2007, 129, 8422. (11) Zhao, Y. F.; Kim, Y. H.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2005, 94, 155504. (12) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 307. (13) Weck, P. F.; Dhilip Kumar, T. J. J. Chem. Phys. 2007, 126, 094703. (14) Yoon, M.; Yang, S. Y.; Hicke, C.; Wang, E. G.; Geohegan, D.; Zhang, Z. Y. Phys. ReV. Lett. 2008, 100, 206806. (15) Chandrakumar, K. R. S.; Ghosh, S. Nano Lett. 2008, 8, 13. (16) Sun, Q.; Wang, Q.; Jena, P. Nano Lett. 2005, 5, 1273. (17) Kim, Y. H.; Zhao, Y. F.; Williamson, A.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2006, 96, 016102. (18) Zhao, Y. F.; Lusk, M. T.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Nano Lett. 2008, 8, 157. (19) Szwacki, N. G.; Sadrzadeh, A.; Yakobson, B. I. Phys. ReV. Lett. 2007, 98, 166804. (20) Gopakumar, G.; Nguyen, M. T.; Ceulemans, A. Chem. Phys. Lett. 2008, 450, 175. (21) Szwacki, N. G. Nanoscale Res. Lett. 2008, 3, 49. (22) Cabria, I.; Lo´pez, M. J.; Alonso, J. A. Nanotechnology 2006, 17, 778. (23) Wu, X. J.; Yang, J. L.; Zeng, X. C. J. Chem. Phys. 2006, 125, 044704. (24) Seung, H. J.; Young, K. K. Phys. ReV. B 2004, 69, 245407. (25) Chen, X.; Gao, X. P.; Zhang, H.; Zhou, Z.; Hu, W. K.; Pan, G. L.; Zhu, H. Y.; Yan, T. Y.; Song, D. Y. J. Phys. Chem. B 2005, 109, 11529. (26) Seung, H. J. Phys. ReV. B 2006, 74, 155424. (27) Shevlin, S. A.; Guo, Z. X. Appl. Phys. Lett. 2006, 89, 153104. (28) Sankaran, M.; Viswanathan, B. Carbon 2006, 44, 2816. 2007, 45, 1628. (29) Durgun, E.; Jang, Y.-R.; Ciraci, S. Phys. ReV. B 2007, 76, 073413. (30) Li, Y. C.; Zhou, G.; Li, J.; Gu, B. L.; Duan, W. H. J. Phys. Chem. C, in press. (31) Perdew, J. P.; Wang, Y. Phys. ReV. B 1992, 45, 13244. (32) DMOL is a density functional theory program distributed by Accelrys, Inc. Delley, B. J. Chem. Phys. 1990, 92, 508. (33) Delley, B. Phys. ReV. B 2002, 66, 155125. (34) Pupysheva, O. V.; Garajian, A. A; Yakobson, B. I. Nano Lett. 2008, 8, 767. (35) Wu, G. F.; Wang, J. L.; Lu, Y. M.; Yang, M. L. J. Chem. Phys. 2008, 128, 224315. (36) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 3rd ed.; Wiley Interscience: New York, NY, 2001. (37) Sun, Q.; Wang, Q.; Jena, P.; Kawazoe, Y. J. Am. Chem. Soc. 2005, 127, 14582. (38) Krasnov, P. O.; Ding, F.; Singh, A. K.; Yakobson, B. I. J. Phys. Chem. C 2007, 111, 17977. (39) Fuchs, M.; Da Silva, J. L. F.; Stampfl, C.; Neugebauer, J.; Scheffler, M. Phys. ReV. B 2002, 65, 245212.

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