J. Phys. Chem. C 2008, 112, 16231–16235
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Ab Initio Calculations on the Magnetic Properties of Hydrogenated Boron Nitride Nanotubes Feng Li,†,‡ Zhonghua Zhu,*,‡ Mingwen Zhao,§ and Yueyuan Xia§ Department of Physics, Taishan UniVersity, Taian, Shandong, 271021, China, DiVision of Chemical Engineering, School of Engineering, UniVersity of Queensland, Brisbane 4072, Australia, and School of Physics and Microelectronics, Shandong UniVersity, Jinan, Shandong, 250100, China ReceiVed: March 23, 2008; ReVised Manuscript ReceiVed: July 27, 2008
Ab initio local spin density approximation calculations were performed to study the magnetic properties of hydrogenated boron nitride nanotubes (H-BNNTs). It is found that the adsorption of a single H atom on the external surface of BNNT can induce spontaneous magnetization in the H-BNNTs, whereas no magnetism is observed when two H atoms are adsorbed on two neighboring N atoms or on two neighboring B and N atoms. However, spontaneous magnetization is also found in the H-BNNTs with two H atoms on two B atoms not next to each other, the further the better. This may be experimentally accessible when the coverage of H atoms adsorbed on the external surface of BNNTs is low. Defects produce spontaneous magnetization on BNNTs. When one or three H atoms are adsorbed on vacancy defects of BNNTs, the H-BNNTs are nonmagnetic, while magnetic H-BNNT can be obtained by two H atoms adsorbed on vacancy defects, which may be difficult to control experimentally. The key issue for magnetism is the existence of unpaired electrons, which can be experimentally realized by either low coverage of hydrogen atoms or making defects on perfect BNNTs. This indicates that it is possible to tune the magnetic properties of BNNTs by hydrogenation or defects, thus providing a new synthetic route toward metal-free magnetic materials. I. Introduction The discoveries of magnetism in polymerized C601 and proton-irradiated graphite2 have stimulated wide interest in investigating the origin of magnetism in metal-free materials as well as searching for metal-free materials with similar properties due to their potential applications in spintronic devices. Lehtinen et al.3,4 performed ab initio calculations to study the properties of a carbon adatom on a graphite sheet and a carbon nanotube and found that the adatom is spin polarized in both cases. Fujita et al.5 performed tight-binding band structure calculations on graphite ribbons with armchair and zigzag edges, respectively. Ribbons with armchair edges show a sharp peak in the density of states at Fermi level, indicating the possibility of spontaneous magnetization. Ma et al.,6 on the basis of ab initio calculations, predicted that the vacancy defects can induce magnetism in graphite sheets and carbon nanotubes (CNTs). For graphitic BN sheets and BN nanotubes (BNNTs),7,8 the nitrogen vacancy or the boron vacancy can also induce spontaneous magnetization. The theoretical prediction of spontaneous magnetization in BNNTs induced by substitutional doping with carbon or silicon atoms (for either boron or nitrogen atoms) has also been reported by several groups.9,10 The magnetic properties of fluorinated BN nanotubes, which originate from the chemisorption of F atoms on the B atoms, were reported in our previous work.11 Recently, some investigations have shown that the hydrogenation treatment of carbon materials can also induce magnetism. For example, Kusakabe and Maruyama12 performed tightbinding band structure calculations on the hydrogenated graphite * To whom correspondence should be addressed. E-mail:
[email protected]. † Taishan University. ‡ University of Queensland. § Shandong University.
ribbons and predicted that spontaneous magnetization could appear in the graphene ribbon with one edge composed of monohydrogenated carbon atoms and another edge made of dihydrogenated ones. Pei et al.13 showed that the hydrogen adsorption on the two types of sites (A and B) of CNTs behaves differently in tuning the magnetic properties of CNTs. The AA structure formed by hydrogen adsorption on two A sites favors the appearance of spontaneous magnetization, but the groundstate of AB structure composed of two hydrogen atoms on A and B sites is nonmagnetic. Ma et al.14 found that hydrogen trapped at a vacancy can trigger delocalized π electron spin polarization on semiconducting zigzag CNTs. The hydrogenated BNNTs (H-BNNTs) have been studied as important materials in hydrogen storage by many groups,15-17 but the magnetic properties of H-BNNTs have not attracted considerable attention both experimentally and theoretically. In this paper, we report our study on the magnetic properties of hydrogenated (10,0) BNNTs using ab initio local spin density approximation calculations. Our results show that the stable spontaneous magnetization can be obtained in H-BNNTs by the adsorption of H atoms on the external surface or at vacancies of BNNTs under certain conditions. This provides vital information for understanding the origin of magnetism in metal-free materials. II. Thoeretical Methods Our calculations are based on the density functional theory and the local spin density functional formalism. The calculation is carried out using the SIESTA18-20 package. The pseudopotentials generated using the Trouiller and Martins scheme21 were used to describe the interaction of valence electron with the atomic core, and their nonlocal components were expressed in the fully separable form of Kleiman and Bylander.22,23 The
10.1021/jp802524m CCC: $40.75 2008 American Chemical Society Published on Web 09/25/2008
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TABLE 1: Adsorption Energies and Magnetic Moments of Various Hydrogenated BN (10,0) Nanotubesa H+B H+N 2H+B 2H+BN 2H+Bfar 2H+Nfar H+VB 2H+VB 3H+VB H+VN 2H+VN 3H+VN
E (eV)
magnetic moments (µB)
-0.389 0.385 -0.435 -1.788 -0.394 0.367 -4.892 -4.342 -4.585 -4.086 -2.731 -3.163
1.00 0.99 2.00 0.00 2.00 2.00 0.00 1.00 0.00 0.00 1.00 0.00
a The adsorption energy is defined as Eads ) [E(BNNT+nH)-E(BNNT)-nEH]/n, where E(BNNT+nH), E(BNNT), and EH stand for the total energies of the hydrogenated nanotubes, the pure nanotube (the vacancy-containing nanotube for substitution case), and the H atom, respectively, and n stands for the number of the H atoms.
generalized gradient approximation correction in the form of Perdew at al.24 was adopted for the exchange correlation potential. The atomic orbital basis set employed throughout was a double-ζ plus polarization (DZP) orbital, which has been used successfully to study magnetic properties of doped BNNTs.9,10 The charge density was calculated in a regular real space grid with a cutoff energy of 120 Ry. We also tested the convergence behavior by using a cutoff energy of 200 Ry. It indicated that the differences between the results (such as the adsorption energies, electronic properties, and magnetic properties) obtained using these cutoff values are negligible. Structural optimizations were performed for all configurations using the conjugate gradient (CG) algorithm until the maximum residual forces were smaller than 0.02 eV/Å. Periodic boundary condition along the tube axis was employed with a vacuum region (at least 18.0 Å) between BNNTs to make sure that there are no interaction between tubes. The length of the supercell was 12.84 Å for the (10,0) BNNT consisting of 120 atoms. The Monkhorst-Pack special k-points scheme25 was used to sample the Brillouin zone with 8 k points along the tube axis for the structure optimization. Convergence testes with 40 k points along the tube axis for the structure optimization were also performed. The difference between the results obtained using these k points is also negligible. III. Results and Discussion We first considered the chemisorption of a single H atom on a B (H+B) or N (H+N) atom of (10,0) BNNT. It was found that the H atom prefers to adsorb on a B atom and that the H+B adsorption is exothermic, whereas the H+N adsorption is endothermic (as shown in Table 1), which agrees well with the reported theoretical results.17 The electronic structures of (10,0) BNNT adsorbed with a single H atom on a B or N atom are plotted in Figure 1. The band structure of pristine (10,0) BNNT was also calculated for the purpose of comparison. No spontaneous magnetization is found, suggesting that the pristine (10,0) BNNT is nonmagnetic. The band gap of (10,0) BNNT is 3.93 eV, which is in agreement with the result of ab initio calculations by Xiang et al.26 However, the H+B adsorption gives rise to an impurity band near the valance-band edge, which is split into spin-up and spin-down branches (see Figure 1a). The spin-up branch is occupied, while the spin-down branch is empty, leading to a strong spontaneous magnetization in the
Figure 1. The band structures and the spin density isosurfaces of H-BNNTs with a single H atom adsorbed on (a and c) B (H+B) and (b and d) N (H+N) atom. Red lines and blue lines represent spin up and spin down, respectively. The Fermi level is indicated by dashed line. The isovalue is 0.02 e/Å3.
nanotube. In the case of the H+N adsorption, an impurity band also appears in the band gap, but it is close to the conductionband edge (see Figure 1b), which is different from the H+B adsorption because of the different contribution of B and N on the electronic structures of BNNTs. The impurity band is also split into an occupied spin-up branch and an unfilled spin-down branch, resulting in a strong spontaneous magnetization in the H-BNNTs. The net magnetic moment is about 1 µB in both cases. For clarification, the spin density isosurfaces (∆F£1/2Fv£-FV) of the H-BNNTs at a density value of 0.02 e/Å3 is given in parts c and d of Figure 1, which shows that the magnetization density of H+B adsorption localizes mainly on the H atom and the three nearest-neighboring N atoms (N1, N2, and N3) of the adsorption B sites (as shown in Figure 1c), while for the H+N adsorption the magnetization density localizes mainly on the H atom and B2, B3, and B4 (as shown in Figure 1d). Moreover, Mulliken population analysis shows that the total net spin (S ) 1/ ) in the H+B adsorption mainly comes from the s orbital of 2 the H atom and the 2p orbitals of the three N atoms, and the contributions of these atoms to the magnetic moment are 0.34, 0.14, 0.14, and 0.17 µB for H, N1, N2, and N3, respectively, while in the H-B adsorption it mainly comes from the s orbital of the H atom and the 2p orbitals of the three B atoms and the contributions of these atoms to the magnetic moment are 0.07, 0.29, 0.29, and 0.12 µB for H, B2, B3, and B4, respectively. The energies of paramagnetic (S ) 0) states are 0.17 and 0.14 eV higher than that of ferromagnetic states for H+B and H+N adsorption, respectively. To study the magnetic properties of H-BNNT containing two adsorbed H atoms per supercell, different adsorption configurations were considered. The electronic structure of H-BNNTs with two H atoms adsorbed on two B atoms within a B-N
Hydrogenated Boron Nitride Nanotubes
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Figure 2. (a and b) The band structure and the spin density isosurfaces of H-BNNTs with two H atoms adsorbed on two B atoms within a B-N hexagon (2H+B). Red lines and blue lines represent spin up and spin down, respectively. The Fermi level is indicated by dashed line. The isovalue is 0.02 e/Å3. The wave functions isosurfaces of (c) b1 and (d) b2 spin-down bands at the Γ point for 2H+B.
Figure 3. The spin density isosurfaces and the band structures of H-BNNTs with two H atoms placed as far away as possible on two symmetric (a) and (c) B (2H+Bfar) and (b) and (d) N (2H+Nfar) atoms. The isovalue is 0.02 e/Å3. Red lines and blue lines represent spin up and spin down, respectively. The Fermi level is indicated by dashed line.
hexagon (2H+B) is shown in Figure 2. From Figure 2a, we can see that all of the valence bands of spin-up branch are fully occupied, whereas two spin-down bands (b1 and b2) near the Fermi level are empty, resulting in the net magnetic moment of 2.00 µB. These two bands mainly arise from the s orbitals of the H atoms and the 2p orbitals of five N atoms around the two hydrogenated B atoms. The wave function of the state of b1 band at the Γ point mainly locates on the H atoms and the nitrogen (N5) atom connecting the two hydrogenated B atoms (Figure 2c), whereas that of b2 bands mainly distributes in the region near the other four nitrogen (N1, N2, N3, and N4) atoms (see Figure 2d). However, the configurations of H-BNNT with two H atoms adsorbed on two N atoms within a B-N hexagon (2H+N) or adjacent B and N atoms (2H+BN) are nonmagnetic. When the two H atoms are placed as far away as possible on two symmetric B (2H+Bfar) or N (2H+Nfar) atoms (as shown in parts a and b of Figure 3), spontaneous magnetizations induced by H atoms are found, and their band structures are similar to that of a single H atom adsorbed on a B or N atom, as shown in parts c and d of Figure 3. It is noteworthy that the nonmagnetic configure of 2H+BN is energetically the most favorable (as shown in Table 1), and H atoms prefer to adsorb on the top sites of adjacent B and N atoms to form an armchair chain along the tube axis in our calculations, which agrees well with the other theoretical results.17 Therefore, the realization of magnetic H-BNNTs is an experimental challenge. However, we think that it is not impossible. According to our results, the H+N adsorption is endothermic and the hydrogen adsorbed on a N atom cannot occur automatically. However, once the structures of H+N can be experimentally accessible, the H atom adsorbed on the N atom cannot easily move from the adsorption site of N atom to that of the nearest-neighboring B atom along the tube axis at room temperature due to an energy barrier of 0.56 eV of H
atom diffusing between these two adsorption sites. In addition, the H+B adsorption is exothermic and the hydrogen adsorbed on a B atom is favorable. So the magnetic H-BNNTs, such as 2H+Bfar, may be experimentally accessible when the coverage of H atoms adsorbed on the external surface of BNNTs is not high. We also studied the magnetic properties of BNNTs with H atoms adsorbed on vacant defects. When a B or N atom is removed from BNNTs (denoted by VB and VN, respectively), the neighboring atoms partially reconstruct around the defect (see parts c and d of Figure 4), forming new homoelemental bonds with the distance of 1.47 and 1.81 Å for VB and VN, respectively, which agrees well with the reported theoretical results.8 The band structures are plotted in parts a and b of Figure 4 for VB and VN, respectively. From these figures, we can see that there are three local states near the Fermi level in the gap region for VB. Two of them are spin-up state and occupied; the other is spin-down and unfilled, leading to a strong spontaneous magnetization. The net magnetic moment is 1.00 µB, which comes from the 2p electron of the dangling bond N atom (see Figure 4c). However, in the case of VN defect, there are only two spin splitting defective states in the gap region. The spinup state is occupied, while the spin-down state is unfilled, resulting in a strong spontaneous magnetization of 1.00 µB, which comes from the dangling bond and the new homoelemental bond (see Figure 4d). The dangling bonds of undercoordinated B or N atoms can easily be saturated by introducing H atoms, and three configurations are obtained for VB (as shown in Figure parts a- of Figure 5) and VN, respectively. These configures are denoted by nH+Vx (n ) 1, 2, or 3 and x ) B or N). Our calculations show that the H+Vx and 3H+Vx configurations are nonmagnetic, whereas every 2H+Vx configuration has a net magnetic moment of about 1.00 µB. This is understandable since no unpaired electron exists
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Li et al. net spin are mainly contributed by the unpaired 2p electron of the dangling bond of N3 in 2H+VB, while that of 2H+VN mainly comes from the σ bond between H1 and B1 and the dangling bond of B3. The s electrons also contribute to 2H+VN gap states, indicating the different hybrid features of atoms in the defect region between 2H+VN and 2H+VN. In these configurations of hydrogenated BNNTs, the configuration H+Vx is the most stable, and the 2H+Vx is the most unstable (see Table 1), meaning that hydrogenated BNNTs containing vacancies in our calculations prefer the nonmagnetic ground states. This provides an interesting way to modify the magnetic properties of BNNTs containing vacancies. Apparently it may not be easy to experimentally control the number and distribution of hydrogen atoms adsorbed on the defected BNNTs to maintain or induce spontaneous magnetism, but the defected BNNTs are spontaneously magnetic already; hydrogenation is not required. IV. Conclusion
Figure 4. The band structures and the spin density isosurfaces of BNNTs with (a and c) B vacancy (VB) and (b and d) N vacancy (VN). Red lines and blue lines represent spin up and spin down, respectively. The Fermi energy is indicated by a dashed line. The isovalue is 0.02 e/Å3.
In summary, we have performed ab initio local spin density approximation calculations to study the magnetic properties of H-BNNTs. Our calculated results show that the adsorption of a single H atom on the external surface of BNNT can induce spontaneous magnetization in the H-BNNTs, whereas no magnetism was found when two H atoms are adsorbed on the neighboring N atoms or two adjacent B and N atoms. However, spontaneous magnetization is also found in the H-BNNTs with H atoms adsorbed on two B atoms not next to each other, the further the better. This may be experimentally accessible at a low coverage of chemisorbed H atoms on BNNTs. The defected BNNTs are spontaneously magnetic already; hydrogenation is not needed. Acknowledgment. This work was supported by the National Natural Science Foundation of China under Grant Nos. 10547131, 10674099, 50402017, and 10374059 and the National Basic Research 973 Program of China (Grant No. 2005CB623602). Financial support from Australian Research Council Linkage International Fellowship Grant and the University of Queensland Early Career funding are also appreciated. Feng Li also thanks Taishan University for the financial support. References and Notes
Figure 5. The optimized structures of (a) H+VB and (b) 3H+VB. The spin density isosurfaces of (c) 2H+VB and (d and e) 2H+VN (from the front view (d) and the side view (e)). The isovalue is 0.02 e/Å3.
in H+Vx and 3H+Vx configuration, due to the formation of B-B (N-N) homoelemental bond and hydrogenation. But 2H+Vx, which is not fully hydrogenated, has one unpaired electron at a B (N) atom and contributes a 1.00 µB magnetic moment. To clarify it, the spin density isosurfaces of the H-BNNTs at a density value of 0.02 e/Å3 are shown in parts c-e of Figure 5 for 2H+VB and 2H+VN, from which the total
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