Endohedral BN Metallofullerene M@B36N36 Complex As Promising

J. Phys. Chem. C 2008, 112, 12195–12200

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Endohedral BN Metallofullerene M@B36N36 Complex As Promising Hydrogen Storage Materials Shu-Hao Wen,†,‡ Wei-Qiao Deng,*,‡ and Ke-Li Han*,† State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, People’s Republic of China 116023, and DiVision of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological UniVersity, Singapore 637616 ReceiVed: March 4, 2008; ReVised Manuscript ReceiVed: May 24, 2008

By using first-principles calculations within the frame of the density functional theory, we have investigated the encapsulation of metal atoms (Li, Na, Be, Mg, and Ti) in a B36N36 cage and the effects of endohedral metal atoms on hydrogen storage in the B36N36. The calculations showed that the chemisorption energies of the H on the B36N36 cage shell can be modified by the charged endohedral metal atom in the B36N36 cage, due to the electrostatic interaction and polarization of the cage electrons. Endohedral metal atoms also exert influences on H2 molecules residing inside the B36N36 cage. For Ti@B36N36, we predict a high hydrogen content (8 wt %) structure with a full H coverage on the outer cage shell and with the Kubas complex in the inner cavity. 1. Introduction Developing safe and reliable hydrogen storage materials that can meet performance and cost requirements is critical in achieving a future hydrogen economy, as proposed by the Department of Energy (DOE), USA. The target of the DOE is that the hydrogen storage amount of the desired materials is larger than 6.5 wt %, when operating at ambient temperature and pressure.1 Currently, carbon-based materials, metal hydrides, as well as chemical hydrogen storage materials are being extensively studied.2 The carbon-based materials target at high surface area sorbents such as hybrid carbon nanotubes, aerogels, nanofibers, as well as metal-organic frameworks and conducting polymers.3–9 Metal hydride studies are aimed at the development of advanced metal hydride materials, including lightweight complex hydrides, destabilized binary hydrides, intermetallic hydrides, and modified lithium amides.10,11 The chemical storage approaches are focusing on the borohydridewater system, novel boron chemistry, and innovations beyond boron.12–14 Significant progress involving the above-mentioned materials has been made during the past decade. Unfortunately, however, no hydrogen storage materials have satisfied the DOE targets thus far. B-N nanostructures with good heat resistance and structural stability in air are considered one of the promising candidates for hydrogen storage.15–19 It has been reported that BN nanotubes (BNNT) can take up 1.8-2.6 wt % of hydrogen under ∼10 MPa at room temperature,15 while collapsed BN nanotubes exhibit an even higher hydrogen storage capacity (4.2 wt %).16 Several authors have also reported the hydrogen uptake of the fullerene-like BN nanocage B36N36.20–22 The experimental studies15,16 indicated that the hydrogen uptakes of BN nanostructures are attributed primarily to chemisorption. However, recent theoretical18 studies indicated that chemisorption of H2 molecules on pristine BNNT is endothermic. It is believed that * Corresponding authors: E-mail: [email protected] and wqdeng@ ntu.edu.sg. † Dalian Institute of Chemical Physics, Chinese Academy of Sciences. ‡ Nanyang Technological University.

the high hydrogen storage capacity reported in ref 16 was attributed to the dangling bonds and the platinum cluster contaminants in defect BNNT, which offered strong binding sites and facilitated the dissociation of H2 on the BNNT.18 Consequently, to make the BN nanostructure a good material for hydrogen storage, an approach that can modify the binding energies for hydrogen adsorption is needed. Doping with metal atoms in C and BN nanotube, fullerenes has been investigated by previous work.4–7,19 Ciraci et al.4 predicted that up to 8 and 5.7 wt % of hydrogen storage can be achieved in titanium-decorated SWCNTs and SWBNT. Moreover, Zhang et al.6 predicted that some organometallic buckyballs, such as C48B12[ScH]12, can store up to 9 wt % of hydrogen under mild conditions. Sun et al.7 predicted that coating C60 with isolated transition metal atoms (e.g., Sc and Ti) can lead to hydrogen storage capacities up to 8 wt % of hydrogen. Recently, doping with metal atoms in B36N36 has been used as a means to synthesize new cluster endohedral B-N metallofullerenes. For example, Fe@B36N36, La@B36N36,23 and Y@B36N3624 have been reported from high-resolution electron microscope (HREM) studies. Koi et al.25 have used semiempirical calculations to investigate the effects of endohedral elements Li, Na, and K on the hydrogenation of the B24N24 cage. They suggested that the hydrogen storage in the Li@B24N24 system is up to 3.86 wt %. In this study, using first-principle calculations we investigate the possibility of modifying hydrogen binding energies by doping with metal dopants (Li, Na, Be, Mg, and Ti) inside the B36N36 cage. A high hydrogen content (8 wt %) structure based on the Ti@B36N36 nanocage is also proposed theoretically at last. 2. Theoretical Method Spin-polarized calculations of total energies and geometry optimizations were carried out with use of a plane-wave basis set with the projector augmented plane wave (PAW) method, as implemented in the Vienna ab initio Simulation Package (VASP).26,27 The structure optimization is symmetry unrestricted and uses a conjugate gradient algorithm. We adopted the PW91 form for the exchange-correlation functional.28 The clusters were

10.1021/jp801893f CCC: $40.75  2008 American Chemical Society Published on Web 07/17/2008

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TABLE 1: Dopant Position (D-P, in Å) and Embedding Energy (EE, in eV) for Metal Atom Encapsulation into the B36N36 and C60 Cage Li@B36N36 Li@B36N36a Be@B36N36 Na@B36N36 Mg@B36N36 Ti@B36N36 Ti@B36N36a Li@C60 Na@C60 Ti@C60

D-P

EE

1.41 0.0003 0.0008 0.0001 0.0003 1.43 0.0011 1.53 0.87 1.70

-0.04 +0.08 -0.06 +0.17 +0.09 -1.11 -0.26 -1.56 -1.31 -3.55

a Metastable equilibrium structures of Li@B36N36 and Ti@B36N36 with the endohedral metal atom located at the center of the B36N36 cage after optimization.

placed in a cubic cell with 15 Å vacuum spaces along the x, y, and z directions to make the dispersion effects negligible. We used the Γ point to represent the Brillouin zone due to the large cell, and the energy cutoff was set to 400 eV. The accuracy of our method was tested by computing the structure and properties of B36N36. The average binding energy per atom is 8.46 eV, and the highest occupied and lowest unoccupied (HOMO-LUMO) energy gap is 5.18 eV. The average B-N bond length is 1.465 Å, and the average cage radius is 3.940 Å. The symmetry of the optimized B36N36 structure is Td. The results agreed well with the previous ab initio calculations.29 The calculated bond length (1.913 Å) of the Ti2 dimer also agreed with the experimental value of 1.91 Å. To store hydrogen with high gravimetric density the materials must consist of light elements, so we performed computer experiments on the encapsulation of metal atoms M in the B36N36 cage, starting with Li, Na, Be, Mg, and Ti. Putting the M atoms in the center of the cage and shifting the M atoms from the cage center by 1.0 and 2.0 Å are used as the three initial structures for optimization. 3. Results and Discussion Encapsulation of Metal Atoms in the B36N36 Cage. Table 1 shows the dopant position (D-P) and the embedding energy (EE) for the encapsulation of a metal atom. The D-P refers to the distance between the center of the cage and the optimized location of the endohedral metal atom. The EE is defined as the difference between the total energy of the endohedral complexes M@B36N36 and the sum of the total energy of the isolated M atom and the B36N36 molecule. Our calculations showed that if the initial position of M is at the cage center, it will still be located at the center of the cage after optimization. However, the optimized Li@B36N36 and Ti@B36N36 with the M atoms located at the cage center are the metastable equilibrium structures, and the structures with energy minima are attained by displacing the Li and Ti atoms by ∼1.4 Å from the cage center, which are similar to the La@B36N36, as reported previously by Wang et al.30 The minus values of EE for Ti, Li, and Be encapsulation, -1.11, -0.04, and -0.06 eV, respectively, indicated that the encapsulation process is exothermic and the production of Ti@B36N36, Li@B36N36, and Be@B36N36 is energetically favorable. The positive values of EE for Na and Mg encapsulation suggest the possible existence of Na@B36N36 and Mg@B36N36 as a metastable complex. For comparison, we have also studied three C endohedral metallo-

Figure 1. Optimized structures and contour plots of the electron density for chemisorption of one hydrogen atom on N (a) and B (b) sites of B36N36. The plots are obtained from the plane containing the H atom and the nearest neighboring B and N atoms.

fullerenes:31,32 Li@C60, Na@C60, and Ti@C60. The values of D-P showed that, for C endohedral metallofullerenes, the M atom shifted from the C60 cage center after optimization, and the EE of M@C60 are significantly higher than those of the corresponding BN endohedral complexes [email protected] It is well-known that C60 doped with metal atoms involves charge transfer between the C60 cage and the metal atoms due to the existence of a strong interaction.31,32,34,35 The much Lower embedding energies of M@B36N36 than those of M@C60 indicate the weak interactions33 between the B36N36 cage and the endohedral metal atom, and there is no charge transfer between them,30 which is consistent with a recent report that used BNNT as ideal sheaths of nanocables due to weak interaction between the BNNT and the encapsulated conducting metallic nanowires.36 From parts d and e of Figure 3 we can compare the difference between electron density contours of Ti@C60 and Ti@B36N36. Furthermore, our spin-polarized calculations also show that Ti@B36N36 carries a high magnetic moment of 4.0 µB without spin transfer from the metal atom to the B36N36 cage, but the magnetic moment of Ti@C60 is 0 with a complete spin transfer to C60. It is not surprising because electrons from metal atom can populate on the low-lying electron acceptor states (e.g., t1µ) of C60; however, the BN cage has a large bandgap and charge transfer between endohedral metal atoms and the B36N36 cage is not preferable. This indicates that the B36N36 cage can work as a chemical Faraday cage, and even paramagnetic transition metal atoms can retain their atomic character after they are encased, which indicated its potential application in magnetic applications37 and information storage.38 Effect of the Endohedral Metal Atom on Chemisorption. We have investigated the effect of the endohedral metal atoms on hydrogen chemisorption on the B36N36 cage shell. We first considered the chemisorption of H on pristine B36N36. The B36N36 cage shell consists of tetragonal and hexagonal BN rings, so there are four kinds of B and N sites for H chemisorption on the outer shell of the B36N36 cage. When one H atom is adsorbed on the B site of the hexagonal ring, the B-H distance (r ) 1.30 Å) is substantially longer than that of a typical B-H bond (1.19 Å). The change of the NBN angles at the boron center (from 120.0°, 120.0°, and 117.7° to 114.1°, 114.1°, and 115.0°,

Endohedral BN Metallofullerene M@B36N36

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Figure 2. Chemisorption energies of one hydrogen atom on M@B36N36 as a function of the endohedral element M. Both H on B and N sites of hexagonal (b) and tetragonal (9) ring are considered. “None” means that there is no metal atom encapsulated in B36N36.

respectively) indicates a modest deformation toward tetrahedral. For the H adsorbed on the N atom, the length of the N-H bond is 1.05 Å, and the BNB angle changes (to 105.4°, 105.4°, and 113.0°) are toward tetrahedral. The changes in structural features for the H on the tetragonal rings are similar to those of the hexagonal rings, and these structural changes imply the occurrence of hybridizations of the boron and nitrogen atoms being converted from sp2 to sp3. The H chemisorption energy (EC) on the B and N sites of the hexagonal ring are -0.41 and +0.24 eV, respectively, and for the tetragonal rings the EC are -0.61 and -0.18 eV, respectively. These results are similar to the cases of H adsorbed on the (8, 0) BN SWNT19 and the h-BN (001) plane,39 which is determined by the essential ionic character of the B-N bonding. The slight difference in EC between them is due to the curvature difference of different BN nanostructures, and the EC difference of the H adatom on tetragonal and hexagonal rings on the B36N36 cage surface is also due to the same reason.19 The minus value of the chemisorption energy means that the adsorption of the H atom on the boron site is exothermic. The positive binding energy of the N-H bond on the hexagonal ring indicates that the formation of the N-H bond is endothermic.19 The energy barrier for the H atom adsorption is due to the repulsion between the electrons of the H and the π-electrons of the B36N36 cage, which are mostly populated on the electronample N atoms (Figure 1a). Electron density contours (Figure 1b) show that the B sites are electron-deficient and more electrons can be transferred easily, implying that the B sites are more favorable than the N sites for H chemisorption. Figure 2 shows the chemisorption energies of the H on B and N sites of the tetragonal and hexagonal rings all decreased as the metal atom is encapsulated in the cage, and the EC decrease for the B sites is more remarkable than that for the N sites. Noticeably, the endothermic adsorption of H (EC ) +0.24 eV) on the N sites of hexagonal rings becomes exothermic (EC ) -0.03 to -0.8 eV). The bond lengths of B-H (∼1.21 Å) and N-H (∼1.02 Å) for M@B36N36 also decreased, as compared to the B-H (1.30 Å) and N-H (1.05 Å) bond lengths for the pristine B36N36. As has been discussed above, there is little charge transfer between the B36N36 cage and metal atoms in the cage. However, when a H atom is adsorbed on the B36N36 cage, the impurity state will be introduced in the band gap. The impurity state is near the valence-band edge (VBE) for the B-H adsorption and near teh conduction-band edge (CBE) for the N-H adsorption.19 The presence of the impurity states in the band gap makes charge transfer to the B36N36 cage more favorable. The endohedral

Figure 3. Optimized structures of Ti@C60 (a), Ti@B36N36 (b), and Ti@B36N36-H (c), as well as the corresponding contour plots of the electron density for Ti@C60 (d), Ti@B36N36 (e), and Ti@B36N36-H (f). Plots d, e, and f are obtained from the plane containing the Ti atom and the nearest neighboring C or B and N atoms.

metal atom becomes positively charged after charge transfer to the B36N36 cage. From paanels e and f of Figure 3, we can compare the difference of electron density contour before and after one H atom adsorbed on Ti@B36N36. The presence of charged endohedral metal atoms in the cage implies that two effects have to be taken into account. In the B36N36, owing to the large ionicity of the B-N bond, the B atoms are cations, while the N atoms are anions. The first effect is the existence of a strong electrostatic interaction40 between the charged metal atom in the cage and the B cations or N anions on the cage shell. The second effect is the polarization35 of the cage electrons by the electrostatic potential, which occurs mainly on electron-ample N atoms. The electrostatic interaction between the positively charged endoheral M atom and the B cation is repulsive, the direction of which is the same as the direction of the hybridization of a B-H bond in the optimized structure (from sp2 to sp3, as discussed above). Although the influence of electron polarization on the electron-deficient B sites is slight, these two effects can facilitate the adsorption of H atoms on the B sites. As for the electron-ample nitrogen sites, the electron polarization effect is remarkable, which can weaken the repulsion between the cage π-electrons and the electrons of the adsorbed hydrogen atoms. However, the electrostatic interaction between the positively charged endoheral metal atom and the N anion is attractive, and its direction is contrary to the direction of the sp3 hybridization for the optimization of N-H bonds. The trading off between the two effects results in a relatively small decrease in EC for N-H adsorption. These analyses

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TABLE 2: Insertion Energy (IE, in eV) for One H2 Molecule Inserted Inside B36N36 and M@B36N36 cluster IE

B36N36

Li@B36N36

Na@B36N36

Be@B36N36

Mg@B36N36

Ti@B36N36

0.00

+0.70

+0.76

+0.88

+0.94

-0.32

suggest that the charged endohedral metal atom is required for enhanced chemisorption, and the substantial ionicity of the B-N bonds causes the different decreases in EC for H being adsorbed on B and N sites. The electrostatic interaction between the charged endohedral metal atom and ionicity B and N is the prime cause of the modification of the H adsorption energies. As shown in Table 1 (the values of D-P), the endohedral Li and Ti have a bigger displacement of 1.41 and 1.43 Å from the cage center relative to endohedral Be, Mg, and Na, which means a closer distance and a stronger electrostatic interaction between endohedral Ti and Li and the ionicity B and N on the cage shell. Thus, the

adsorption energy changes (Figure 2) for endohedral Ti and Li are greater than that of Be, Mg, and Na. In a previous study, Jhi et al. suggested that a systematic increase of the binding energy for molecular hydrogens on the B-N nanotube can be achieved by modifying the B-N sp2 bonding, such as substitutional doping and structural defects.41 In M@B36N36, the charged endohedral metal atoms will lead to the modification of the local charge, electric field distribution, and sp2 bonding of the B-N on the cage shell, thus we believe there will be an increase for molecular hydrogen binding on the metal doped inside B36N36.

Figure 4. (a) Optimized structure of TiH2@B36N36 and the approach direction for four H2 entering into TiH2@B36N36. (b) Optimized structure of the Kubas@B36N36 complex. (c) The PDOS of the endoheral TiH2-4H2 structure at the Γ point. (d) Optimized structure of Kubas@B36N36-72H with full coverage on the outer shell and the Kubas complex in the inner cavity.

Endohedral BN Metallofullerene M@B36N36 Effect of the Endohedral Atom on the Hydrogen Molecules Residing in the B36N36 Cage. Previous reports have indicated that H2 molecules can enter into the B36N36 cage through the six-membered ring.17,21 Here we have also investigated the case of H2 molecules entering and residing in the M@B36N36. We define the H2 molecule insertion energy (IE) as the difference between the total energy of H2-M@B36N36 and the sum of the total energy of separated M@B36N36 and H2 molecules. Table 2 shows the insertion energies of one H2 molecule for the pristine B36N36 and five other M@B36N36 clusters. For the pristine B36N36, the insertion energy of one H2 is ∼0 eV, which agrees well with the results of Sun et al.21 For other M@B36N36, only Ti@B36N36 has a minus value for the insertion energy. We have also found that it is energetically favorable for five hydrogen molecules to enter the Ti@B36N36 through the six-membered ring and form a Kubas complex with the endohedral Ti atom. The first H2 molecule entered the Ti@B36N36 and formed two Ti-H bonds with the endohedral Ti atom (Figure 4a) (its insertion energy is -0.32 eV) and then the other four H2 molecules entered, forming the so-called Kubas complex42 (TiH2-4H2@B36N36) in the cage (Figure 4b) (with insertion energy of -0.17 eV). The attempts to insert the fifth hydrogen molecule will result in a positive IE, suggesting a limit of TiH2-4H2 in the cage, which is denoted as Kubas@B36N36. It is not surprising that the Kubas complex can be formed in the B36N36 cage, because the Ti atom retains its atomic character after it is encased and the B36N36 cage has a large enough cavity. The projected density of states (PDOS) (Figure 4c) shows that the binding states below the EF are a special hybridization of the Kubas complex.4 The peak around -2 to -1 eV corresponds to the hybridization of the s orbital of the H atoms with the Ti d orbital, and is responsible for the two Ti-H bonds due to the dissociated H2 molecule. The peaks between -10 and -4 eV indicate the molecular orbital of four H2 complex hybridizations with the Ti d orbital. Although the Kubas complex has formed in the B36N36 cage, the expansion of the cage is still very slight, indicating no charge transfer between the cage and the endohedral complex.35 However, the endohedral Ti atom will become charged owing to the formation of the Kubas complex, for which the Ti atom donates charge to dissociate the first H2 molecule and form the 4H2 complex.4 As has been discussed above, the chemisorption will be enhanced with the presence of the charged Ti atom in the cage. We then calculated the EC for H atom chemisorption on the shell of this Kubas@B36N36 cluster. The EC for the hexagonal ring (B-H, -1.65 eV, and N-H, -0.79 eV) and the tetragonal ring (B-H, -1.85 eV, and N-H, -0.45 eV) all decreased, as compared to that of the pristine B36N36. Both adsorptions of B-H and N-H become exothermic, indicating that a full coverage with 72 hydrogen atoms chemisorbed on the outer shell of Kubas@B36N36 is possible. The HOMO-LUMO gap of this compound is 2.21 eV (Figures S-2 and S-3 in the Supporting Information), which suggests that this structure is chemically stable. The calculated average EC43 for a full coverage of 72 H atoms on the outer shell is -2.35 eV, showing a decrease as compared to the case of 72 H atoms adsorbed on pristine B36N36 (-2.13 eV). The average bond lengths of B-H (1.22 Å) and N-H (1.02 Å) also decrease compared with the average bond lengths of B-H (1.30 Å) and N-H (1.05 Å) for the pristine B36N36. The average EC of -2.35 eV is close to half of the H2 binding energy (namely -2.27 eV), indicating that this level of coverage in a thermodynamic process has been achieved. In the structure with full coverage on the outer shell

J. Phys. Chem. C, Vol. 112, No. 32, 2008 12199 and a Kubas complex in the inner cavity (denoted as Kubas@B36N36-72H, Figure 4e), the endohedral Ti atom, acting as a catalytic active center, can enhance the total hydrogen storage in B36N36 up to 8 wt %. 4. Conclusion (i) It is energetically favorable for some metal atoms (such as Ti and Li) to be encapsulated into the B36N36 cage. The B36N36 cage works as a chemical Faraday cage, in which even the paramagnetic transition metal atoms can retain their atomic character after they are encased. Spin-polarized calculations showed that Ti@B36N36 carries a high magnetic moment of 4.0 µB, indicating its potential application in magnetic applications37 and for information storage.38 (ii) The chemisorption energy of H on the B36N36 cage shell varies with the metal atom encapsulated in the cage, and our analyses showed that it is required for the endohedral metal atom to be charged to achieve a decrease in H chemisorption energy. (iii) Endohedral metal atoms can exert influences on H2 molecules residing inside the B36N36 cage. Total energy calculations showed that it is energetically favorable for five hydrogen molecules to enter the Ti@B36N36 and form a Kubas complex with the endohedral Ti atom. Meanwhile, the endohedral Ti atom becomes charged owing to the formation of the Kubas complex, and the charged endohedral Ti atom modifies the chemisorption energies of the H atoms on the B36N36 cage shell, resulting in a high hydrogen content structure with the Kubas complex in the inner cavity of the cage and a full coverage of H chemisorptions on the outer cage shell. In summary, the metal atom doped inside BN fulleneres will dramatically affect the hydrogen binding with BN fullerenes. The proposed Ti@B36N36 structure can store hydrogen up to 8 wt %. Acknowledgment. This work was supported by NKBRSF (Grant 2007CB815202). The authors thank the crew of the Shanghai Supercomputing Center for their continuous support of the supercomputing facility. Supporting Information Available: The optimized geometry of pristine B36N36 cage and B36N36 cage with a H atom chemisorbed on the top site of the B or N atom (Figure S-1) and the density of states (DOS) for different configurations of the B-N cluster (Figure S-2) and the steps for high hydrogen storage in the metal doped inside B36N36 cage and the HOMO-LUMO energy gaps of different stages of the compound (Figure S-3). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schlapbach, L.; Zuttel, A. Nature 2001, 414, 353. (2) http://www1.eere.energy.gov/hydrogenandfuelcells/storage/national_proj.html. (3) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Nature 1997, 386, 377. (4) Yildirim, T.; Ciraci, S. Phys. ReV. Lett. 2005, 94, 175501. (a) Durgun, E.; Ciraci, S.; Zhou, W.; Yildirim, T. Phys. ReV. Lett. 2006, 97, 226102. (b) Durgun, E.; Jang, Y. R.; Ciraci, S. Phys. Chem. B 2007, 76, 073413. (5) Deng, W. Q.; Xu, X.; Goddard, W. A. Phys. ReV. Lett. 2004, 92, 166103. (a) Shin, W. H.; Yang, S. H.; Kang, J. K.; Goddard, W. A. Appl. Phys. Lett. 2006, 88, 053111. (6) (a) Zhao, Y. F.; Kim, Y. H.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Phys. ReV. Lett. 2005, 94, 155504. (b) Zhao, Y. F.; Dillon, A. C.; Kim, Y. H.; Heben, M. J.; Zhang, S. B. Chem. Phys. Lett. 2006, 425, 273. (c) Zhao, Y. F.; Lusk, M. T.; Dillon, A. C.; Heben, M. J.; Zhang, S. B. Nano Lett. 2008, 8, 157.

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