21184
J. Phys. Chem. B 2006, 110, 21184-21188
Hydrogen Adsorption on Carbon-Doped Boron Nitride Nanotube Roge´ rio J. Baierle,*,† Paulo Piquini,† Tome´ M. Schmidt,‡ and Adalberto Fazzio§ Departamento de Fı´sica, UniVersidade Federal de Santa Maria, 97105-900, Santa Maria, RS, Brazil, Instituto de Fı´sica, UniVersidade Federal de Uberlaˆ ndia, Caixa Postal 593, 38400-902, Uberlaˆ ndia, MG, Brazil, and Instituto de Fı´sica, UniVersidade de Sa˜ o Paulo, Caixa Postal 66318, 05315-970, Sa˜ o Paulo, SP, Brazil ReceiVed: March 14, 2006; In Final Form: August 8, 2006
The adsorption of atomic and molecular hydrogen on carbon-doped boron nitride nanotubes is investigated within the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-doped boron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. These results are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bonds are present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while a physisorption occurs for the molecular hydrogen. For the H2 molecule adsorbed on the top of a carbon atom in a boron site (BNNT + CB-H2), a donor defect level is present, while for the H2 molecule adsorbed on the top of a carbon atom in a nitrogen site (BNNT + CN-H2), an acceptor defect level is present. The binding energies of H2 molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to work as a hydrogen storage medium.
I. Introduction In the pursuing of a clean energy alternative, hydrogen as a fuel has received special attention of the scientific community. One challenge is to find new materials that present high hydrogen storage capacity in a stable configuration. A class of materials that presents a great potential for hydrogen storage is the nanostructured systems, which have an advantage over the conventional materials owing to their high surface-to-bulk ratio. Promising nanostructures such as carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) have been closely studied.1-8 The hydrogen storage capacity in CNTs is about 0.2 wt %,6 while in BNNTs the capacity reaches up to 3 wt %8,9 and 4.2 wt % for collapsed BNNTs.10 BNNTs are, in some aspects, similar to CNTs, but the differences between them seem to favor the use of BNNTs for hydrogen storage. BNNTs are always semiconductors, with a band gap around 5.5 eV, nearly independent of the tube diameter and helicity; they present partially ionic B-N bonds and these bonds form a slight buckling on the tube surface. Further, the interaction between hydrogen molecules with material surfaces can be enhanced by heteropolar bonds at the surfaces, a feature that is present in BNNTs but absent in CNTs. In fact, the binding energy of hydrogen on boron nitride (BN) materials is dependent on the structure. In bulk BN powder, the hydrogen adsorption is just 0.2 wt %, while in a more defective bamboolike nanotube structure the capacity increases to 2.6 wt %.7 Recent theoretical studies have found that the binding energies of molecular hydrogen on BNNTs are greater (around 40%) than on CNTs.11 It was suggested that moderate substitutional doping in materials with ionic-like bonding could enhance the binding energies of H2 to values suitable for hydrogen storage.11 In this work, we explore the viability of using carbon-doped BNNTs as a possible hydrogen storage medium through a rigorous study of atomic and molecular hydrogen adsorption * Author to whom correspondence should be addressed. Tel: 55-5532208859; fax: 55-55-32208032; e-mail:
[email protected]. † Universidade Federal de Santa Maria. ‡ Universidade Federal de Uberla ˆ ndia. § Universidade de Sa ˜ o Paulo.
on BNNTs and on carbon-doped BNNTs. Our results show that the molecular hydrogen is weakly bound on BNNTs (tenths of meV). On the other hand, the introduction of carbon substitution in the BNNTs increases the binding energy substantially, reaching an optimized energy range to hydrogen storage. Although some properties of boron carbonitride nanotubes (BCN) are dependent on their composition,12-14 we believe that our results for carbon-doped BNNTs could be valuable for BCN nanotubes. II. Calculations Procedure The calculations are based on spin-polarized density functional theory (DFT) as implemented in the SIESTA-code,15 which performs fully self-consistent calculations by solving the standard Kohn-Shan (KS) equations. The KS orbitals are expanded using a linear combination of numerical pseudoatomic orbitals, similar to those proposed by Sankey and Niklewski.16 In all calculations, a split-valence double-ζ quality basis set enhanced with polarization functions has been used. To guarantee a good description of the charge density, a cutoff of 150 Ry for the grid integration is employed to project the charge density in the real space and to calculate the self-consistent Hamiltonian matrix elements. The ion-electron interactions are modulated by norm-conserving Troullier-Martins17 pseudopotentials in the fully separable Kleinman-Bylander form.18 To sample the Brillouin zone, a set of three Monkhorst-Pack special k-points,19 along the tube axis, has been used. The exchange and correlation potential has been treated using both the local density approximation (LDA) and the generalized gradient approximation (GGA). The use of any of these approximations to describe weak interactions, as those involving the hydrogen molecules in the present study, is controversial. However, previous calculations on hexagonal BN show that GGA underestimates the interactions among the hexagonal planes, while LDA describes these interactions more correctly. Although the trends found for binding energies using LDA and GGA in this study are the same, the LDA results systematically give higher binding energies.20 Considering that the binding energies are very important to hydrogen storage and that LDA better describes weakly interacting systems, only the LDA results are presented.
10.1021/jp061587s CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
Hydrogen Adsorption on Boron Nitride Nanotube
J. Phys. Chem. B, Vol. 110, No. 42, 2006 21185 unbalance between the basis set used to describe the interacting system and the basis set used to the isolated reactants.22 Negative values of Eb indicate that the adsorption is exothermic. In this work, we use the LDA23 as parametrized by Perdew-Zunger24 plus the BSSE correction. The hydrogen (atomic and molecular) was adsorbed on a (10,0) semiconducting BNNT with a diameter of approximately 8.0 Å. We use periodic boundary conditions with a tetragonal supercell of 20 Å lattice parameter in directions perpendicular to the tube axis and 8.6 Å along the tube axis (with a total of 80 atoms in the unit cell). This construction should eliminate possible intertube interactions. The forces are calculated using the Hellmann-Feymann procedure, and the geometry is optimized using the conjugated gradient scheme. The systems are relaxed until the root-meansquare criterion of 0.05 eV/Å on the atomic forces is reached. III. Results and Discussion
Figure 1. Band structures for (a) the clean BNNT, (b) a H atom adsorbed on the top site of B, (c) a H atom adsorbed on the top site of N, and (d) a H2 molecule adsorbed on the center of a hexagon. The dotted lines represent the calculated Fermi energy.
The use of localized basis sets largely reduces the amount of computational work required when working with large vacuum regions in the unit cell. The finiteness of the localized basis sets leads, however, to basis set superposition errors (BSSE), as well described in a study of benzene on CNTs.21 To avoid this problem, we have used “ghost” molecules which correspond to additional basis functions centered at the atomic positions of an X system that is interacting with the BNNT or at the position of the BNNT itself but without any atomic potential. Thus, the binding energy of the hydrogen atom or hydrogen molecule on the BNNT is determined through the following equation:
Eb[BNNT + X] ) ET(BNNT + X) [ET(BNNT) + ET(X)] + ∆BSSE (1) where ∆BSSE is added to correct the errors because of the
The adsorption of a H atom on the top site of B (BNNT + H-B) introduces an energy level close to the valence band maximum (VBM), while a H atom on the top site of N (BNNT + H-N) introduces an energy level close to the conduction band minimum (CBM), as shown in Figure 1b and c, respectively. The two electronic levels in the nanotube gap correspond to the two spin polarization directions (up and down). The acceptor levels correspond to bonding orbitals (see Figure 2b) and come from the N atoms close to the H-B bond. The donor levels correspond to antibonding orbitals (see Figure 2d) and come from the B atoms close to the H-N bond. Similar results were obtained by Wu et al.8 studying H adsorption on BNNT and by Van de Walle and Neugebauer25 studying the influence of H impurities in semiconductor bulk systems. The calculated binding energy of the H atom on the top site of B is -490 meV and a covalent bond between the adsorbed H atom and the BNNT is formed (Figure 2a), with a bond length of 1.36 Å. Thus, if a source of H atoms is present, H atom adsorption on the top side of B is an exothermic process. The calculated binding energy of H adsorption on the top site of N
Figure 2. Spatial localization of the total charge density (a and c) and the highest occupied molecular orbital (b and d) for a H atom bound to a B atom and for a H atom bound to a N atom, respectively.
21186 J. Phys. Chem. B, Vol. 110, No. 42, 2006
Baierle et al.
TABLE 1: Calculated Binding Energies (in meV), Distances (in Å) between the BNNT and the H Atom Closest to the Tube Surface, and H-H Distances (in Å) for the H2 Molecule Adsorbed on the BNNT Surface BNNT + H-B BNNT + H-N BNNT + H2-B BNNT + H2-N BNNT + H2-hex BNNT + CB-H BNNT + CN-H BNNT + CB-H2 BNNT + CN-H2
Eb
dBNNT-X
-490 9 -43 -60 -43 -1726 -2308 -156 -163
1.36 1.06 2.30 2.40 ∼2.80 1.13 1.13 2.14 1.43
dH-H
0.80 0.80 0.80 0.82 0.92
is positive, 9 meV. This positive binding energy can be understood taking into account the outward radial relaxation of the N atom to form a covalent N-H bond (1.06 Å), weakening the bonds between the N atom and its first neighbor B atoms (see Figure 2c and d). When a hydrogen molecule is brought closer to the BNNT, physisorption occurs, as already reported.11 After the H2 adsorption, the H-H distance remains almost the same as in the isolated H2 molecule (0.80 Å). The H2 molecule can be adsorbed on top of a B atom and on top of a N atom as well as on the center of a hexagon. The H2 adsorption is slightly favored on top of a N atom, with a calculated binding energy of -60 meV, while the binding energies for the H2 molecule on the center of a hexagon and on top of a B atom are -43 meV. The optimized distance between the closest H (of the H2 molecule) and the B (N) atom is 2.30 Å (2.40 Å), while for the case of the H2 molecule approximating to the center of a hexagon, the average distance between the atoms of the hexagon and the closest H (of the H2 molecule) is 2.80 Å. The bond lengths and the binding energies are summarized in Table 1. The BSSE corrections for these weakly bound systems are around half of the LDA binding energy. Also, without the BSSE correction, the binding energies for the adsorbed H2 molecule for the three studied systems are similar to each other, in agreement with Jhi et al.11 The band structure of the BNNT + H2 system is practically unchanged when compared with that of the clean BNNT (see Figure 1a and d). Carbon substitutional impurities, CB and CN, present low formation energies and introduce energy levels inside the nanotube band gap (see Figure 3a and d), as described elsewhere.26,27 For a H atom on top of C impurity sites, CB and CN, a chemical adsorption occurs, with binding energies of -1.726 eV and -2.308 eV, respectively. These binding energies are significantly enhanced when compared with the adsorption of an atomic hydrogen on the defect-free BNNT surface. As a result of the adsorption of atomic hydrogen on the top of CB and on the top of CN, CB-H and CN-H bonds are formed, with the occupied and empty impurity levels (before the H adsorption) shifted down to the VBM and up to the CBM, respectively (compare Figure 3a and d with Figure 3b and e, respectively). The atomic hydrogen saturates the C impurity dangling bonds, cleaning the band gap and lowering the total energy of the system. The formation of CB-H and CN-H, in the presence of a source of H2 molecules, cannot be a barrierless process since the minimum energy required to furnish a H atom is half the energy to break the H2 molecule, which is 2.345 eV. For the H2 molecule on the surface of a C-doped BNNT, physisorption occurs. The binding energy calculations show that there is almost no preference for the H2 molecule to be adsorbed on the CB or on the CN defect, the H2 binding energies being -156 meV and -163 meV, respectively. These binding energies
Figure 3. Band structures for (a) CB and (d) CN defects in BNNT, (b) H atom and (c) H2 molecule adsorbed on the top site of CB, and (e) H atom and (f) H2 molecule adsorbed on the top site of CN.
Figure 4. Binding energy (eV) versus C-N bond distance for H2 adsorbed on the top of CN defect.
are higher than those for H2 on C-doped BN graphite sheet.11 Although the binding energies have almost the same value for the BNNT + CB-H2 and BNNT + CN-H2 systems, the C-H and H-H bond distances are slightly different: 1.43 and 0.92 Å for the H2 on the top of CN and 2.14 and 0.82 Å for the H2 on the top of CB, respectively (see the binding energies and the most relevant distances in Table 1). The difference in the C-H and H-H bond distances when the H2 molecule is adsorbed on CB and on CN is a clear indication that the H2 adsorption process must be different for these two sites, and it must be related to the ionic character of the BNNT. To show this, we have performed rigorous calculations for H2 on the top of CN and on the top of CB fixing the C-H bond distance. As can be see in Figure 4 for the BNNT + CN-H2 system, there is a vast range of C-H bond distances where the binding energy remains almost unchanged. What is happening is that there is a binding energy compensation between the C-H bond and the H-H bond. In Figure 5, we can observe that when the H2 molecule approaches the tube surface, the C-H bond turns stronger and
Hydrogen Adsorption on Boron Nitride Nanotube
J. Phys. Chem. B, Vol. 110, No. 42, 2006 21187
Figure 5. Total charge densities for H2 molecule bound to CN in different C-N distances. All distances shown in the figure are in angstroms (Å).
the H-H bond starts to weaken. In fact, Figure 5c shows a transference of charge between the CN and the H2 molecule, leading the system in a limit between the chemisorption and the physisorption processes. These two effects almost compensate each other, resulting in a smooth curve for the binding energy and an optimized small CN-H distance (1.43 Å). For H2 on the top of CB, these compensation effects do not occur and the smooth behavior of the binding energy curve at long distances is not observed. The optimized CB-H distance (2.14 Å) is similar to previous calculations for H2 adsorption on carbon-doped BN sheet.11 In Figure 4, we can observe the importance of correcting for BSSE, which reduces the binding energy. Also, the results presented in Figures 4 and 5 allow us to conclude that the H2 molecule should not dissociate when it approaches the carbondoped BNNTs. This is confirmed by looking at the binding energies of BNNT + CB-H (-1.726 eV) and BNNT + CN-H (-2.308 eV) systems, which are always smaller than half of the calculated H2 binding energy (-2.345 eV). In this way, if only a source of H2 molecules is present, only H2 will be adsorbed on C-doped BNNTs, neither BNNT + CB-H nor BNNT + CN-H will be formed. Other defects in BNNT like vacancies and antisites, which have higher formation energies, can induce the H2 dissociation.28 Another observation that helps clarify the interaction between the H2 molecule and the BNNT concerns the distances between the H2 molecule and the BNNT matrix. For the undoped and the CB doped BNNT, the N-H (Figure 6a and c) and the B-H (Figure 6b) bond lengths are in the range from 2.73 to 2.96 Å. However, for the CN doped BNNT, the B-H distances are reduced to 2.31 and 2.41 Å (Figure 6d). These results reinforce the compensation mechanism between the CN-H and the H-H (bond distances and binding energies), which explains why the binding energies of the H2 on the CB and on the CN are similar, as described before. The binding energies for the H2 adsorption on carbon-doped BNNTs are in the range where it is predicted that the hydrogen can be stored in ambient temperature and pressure.11 By comparing Figure 1 and Figure 3, we observe that the electronic properties of the BNNT + CB-H2 and BNNT + CN-H2
Figure 6. Schematic representation of (a) N-H distances of BNNT + H2-B; (B) B-H distances of BNNT + H2-N; (c) N-H distances of BNNT + CB-H2; and (d) B-H distances of BNNT + CN-H2.
systems are similar to those for the H atom on the clean BNNT surface (BNNT-HN and BNNT-HB, respectively). The BNNT + CB-H2 introduces a donor defect level, while the BNNT + CN-H2 introduces an acceptor defect level. The calculations were performed for a carbon concentration of 1.25% in the BNNT. By increasing the carbon concentration, more levels will appear inside the band gap. As the H2 levels are resonant at the valence band, we do not expect significant changes on the binding energies by increasing the carbon concentration. On the other hand, if the substitutional carbon atoms are diluted on the BNNT surface, avoiding C-C interaction and the consequent C-C dangling bonds reconstruction, more H2 molecules can be adsorbed, increasing the hydrogen storage capacity.
21188 J. Phys. Chem. B, Vol. 110, No. 42, 2006 IV. Summary and Conclusions Our ab initio calculations show that the H2 binding energies on carbon-doped BNNTs are enhanced when compared with clean BNNTs, in agreement with experimental results for BNNTs where dangling bonds are present.10 Different from the adsorption of atomic hydrogen, the adsorption of the H2 molecule is a physisorption process. The magnitude of the binding energies for the H2 molecule on the BNNT + CB is similar to the H2 on the BNNT + CN, and they are in the optimized range to work as a hydrogen storage medium. For the H2 molecule adsorbed on the top of a carbon atom in a boron site, a donor defect level is present, while for the H2 molecule adsorbed on the top of carbon in a nitrogen site, an acceptor defect level is present. Although the properties of the BCN nanotubes depend on their composition, these nanotubes are promising candidates for storing hydrogen. Acknowledgment. This work was supported by Brazilian agencies CAPES, CNPq, and FAPERGS. The calculations have been performed using the facilities of the Centro Nacional de Processamento at UNICAMP/CAMPINAS. References and Notes (1) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethume, D. S.; Heben, M. J. Nature 1997, 386, 377. (2) Liu, C.; Fan, Y. Y.; Liu, M.; Cong, H. T.; Cheng, H. M.; Dresselhaus, M. S. Science 1999, 286, 1127. (3) Zandonella, C. Nature 2001, 410, 734. (4) Yang, R, T. Carbon 2000, 38, 623. (5) Hirscher, M.; Becher, M.; Haluska, M.; Dettlaff-Weglikowska, U.; Quintel, A.; Duesberg, G. S.; Choi, Y.-M.; Downes, P.; Hulman, M.; Roth, S.; Stepanek, I.; Bernier, P. Appl. Phys. A 2001, 72, 129.
Baierle et al. (6) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. J. Am. Chem. Soc. 2002, 124, 7672. (7) Oku, T.; Kuno, M.; Narita, I. J. Phys. Chem. Solids 2004, 65, 549. (8) Wu, X.; Yang, J.; Hou, J. G.; Zhu, Q. J. Chem. Phys. 2004, 121, 8481; Phys. ReV. B 2004, 69, 153411. (9) Lan, A.; Mukasyan, A. J. Phys. Chem. B 2005, 109, 16011. (10) Tang, C.; Bando, Y.; Ding, X.; Qi, S.; Golberg, D. J. Am. Chem. Soc. 2002, 124, 14550. (11) Jhi, S.-H.; Kwon, Y.-K. Phys. ReV. B 2004, 69, 245407. (12) Terrones, M.; Golberg, D.; Grobert, N.; Seeger, T.; Reyes-Reyes, M.; Mayne, M.; Kamalakaran, R.; Dorozhkin, P.; Dong, Z. C.; Terrones, H.; Ruhle, M.; Bando, Y. AdV. Mater. 2003, 15, 1899. (13) Golberg, D.; Bando, Y.; Bourgeois, L.; Kurashima, K.; Sata, T. Carbon 2000, 38, 2017. (14) Yin, L. W.; Bando, Y.; Golberg, D.; Gloter, A.; Li, M. S.; Yuan, X. L.; Sekiquchi, T. J. Am. Chem. Soc. 2005, 127, 16354. (15) Ordejo´n, P.; Artacho, E.; Soler, J. M. Phys. ReV. B 1996, 53, 10441. Sa´nchez-Portal, D.; Ordejo´n, P.; Artacho, E.; Soler, J. M. Int. J. Quantum Chem. 1997, 65, 453. (16) Sankey, O. F.; Nikleswsky, D. J. Phys. ReV. B 1989, 40, 3979. (17) Troullier, N.; Martins, J. L. Phys. ReV. B 1991, 43, 1993. (18) Kleinman, L.; Bylander, D. M. Phys. ReV. Lett. 1982, 48, 1425. (19) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (20) For example, the calculated binding energy of the CN-H is -2.02 eV, using the GGA approach, while the LDA + BSSE gives -2.31 eV. On the other hand, the weakly bound H2 molecule on the CN defect gives a binding energy of -0.06 eV using the GGA and -0.16 eV using the LDA + BSSE. (21) Tournus, F.; Charlier, J.-C. Phys. ReV. B 2005, 71, 165421. (22) Boys, S.; Bernardi, F. Mol. Phys. 1970, 19, 553. (23) Ceperley, D. M.; Alder, B. J. Phys. ReV. Lett. 1980, 45, 566. (24) Perdew, J. P.; Zunger, A. Phys. ReV. B 1981, 23, 5048. (25) Van de Walle, C. G.; Neugebauer, J. Nature 2003, 423, 626. (26) Schmidt, T. M.; Baierle, R. J.; Piquini, P.; Fazzio, A. Phys. ReV B 2003, 67, 113407. (27) Piquini, P.; Baierle, R. J.; Schmidt, T. M.; Fazzio, A. Nanotechnology 2005, 16, 827. (28) Wu, X.; Yang, J.; How, J. H.; Zhu, Q. J. Chem. Phys. 2006, 124, 54706.
