Hydrogen Adsorption on Carbon-Doped Boron Nitride Nanotube
Rogerio 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, Institutode Fısica, UniVersidade Federal de Uberlaˆndia, Caixa Postal 593, 38400-902, Uberlaˆndia, MG, Brazil, andInstituto 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 investigatedwithin the ab initio density functional theory. The binding energy of adsorbed hydrogen on carbon-dopedboron nitride nanotube is substantially increased when compared with hydrogen on nondoped nanotube. Theseresults are in agreement with experimental results for boron nitride nanotubes (BNNT) where dangling bondsare present. The atomic hydrogen makes a chemical covalent bond with carbon substitution, while aphysisorption occurs for the molecular hydrogen. For the H2 molecule adsorbed on the top of a carbon atomin a boron site (BNNT+ CB-H2), a donor defect level is present, while for the H2 molecule adsorbed on thetop of a carbon atom in a nitrogen site (BNNT+ CN-H2), an acceptor defect level is present. The bindingenergies of H2 molecules absorbed on carbon-doped boron nitride nanotubes are in the optimal range to workas a hydrogen storage medium.
I. Introduction
In the pursuing of a clean energy alternative, hydrogen as afuel has received special attention of the scientific community.One challenge is to find new materials that present highhydrogen storage capacity in a stable configuration. A class ofmaterials that presents a great potential for hydrogen storage isthe nanostructured systems, which have an advantage over theconventional materials owing to their high surface-to-bulk ratio.Promising nanostructures such as carbon nanotubes (CNTs) andboron 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 4.2wt % for collapsed BNNTs.10 BNNTs are, in some aspects,similar to CNTs, but the differences between them seem to favorthe use of BNNTs for hydrogen storage. BNNTs are alwayssemiconductors, with a band gap around 5.5 eV, nearlyindependent of the tube diameter and helicity; they presentpartially ionic B-N bonds and these bonds form a slightbuckling on the tube surface. Further, the interaction betweenhydrogen molecules with material surfaces can be enhanced byheteropolar bonds at the surfaces, a feature that is present inBNNTs but absent in CNTs. In fact, the binding energy ofhydrogen on boron nitride (BN) materials is dependent on thestructure. In bulk BN powder, the hydrogen adsorption is just0.2 wt %, while in a more defective bamboolike nanotubestructure the capacity increases to 2.6 wt %.7 Recent theoreticalstudies have found that the binding energies of molecularhydrogen on BNNTs are greater (around 40%) than on CNTs.11
It was suggested that moderate substitutional doping in materialswith ionic-like bonding could enhance the binding energies ofH2 to values suitable for hydrogen storage.11
In this work, we explore the viability of using carbon-dopedBNNTs as a possible hydrogen storage medium through arigorous study of atomic and molecular hydrogen adsorption
on BNNTs and on carbon-doped BNNTs. Our results show thatthe molecular hydrogen is weakly bound on BNNTs (tenths ofmeV). On the other hand, the introduction of carbon substitutionin the BNNTs increases the binding energy substantially, reach-ing an optimized energy range to hydrogen storage. Althoughsome properties of boron carbonitride nanotubes (BCN) aredependent on their composition,12-14 we believe that our resultsfor carbon-doped BNNTs could be valuable for BCN nanotubes.
II. Calculations ProcedureThe calculations are based on spin-polarized density func-
tional theory (DFT) as implemented in the SIESTA-code,15
which performs fully self-consistent calculations by solving thestandard Kohn-Shan (KS) equations. The KS orbitals areexpanded using a linear combination of numerical pseudoatomicorbitals, similar to those proposed by Sankey and Niklewski.16
In all calculations, a split-valence double-ú quality basis setenhanced with polarization functions has been used. Toguarantee a good description of the charge density, a cutoff of150 Ry for the grid integration is employed to project the chargedensity in the real space and to calculate the self-consistentHamiltonian matrix elements. The ion-electron interactions aremodulated by norm-conserving Troullier-Martins17 pseudo-potentials in the fully separable Kleinman-Bylander form.18
To sample the Brillouin zone, a set of three Monkhorst-Packspecialk-points,19 along the tube axis, has been used.
The exchange and correlation potential has been treated usingboth the local density approximation (LDA) and the generalizedgradient approximation (GGA). The use of any of theseapproximations to describe weak interactions, as those involvingthe hydrogen molecules in the present study, is controversial.However, previous calculations on hexagonal BN show thatGGA underestimates the interactions among the hexagonalplanes, while LDA describes these interactions more correctly.Although the trends found for binding energies using LDA andGGA in this study are the same, the LDA results systematicallygive higher binding energies.20 Considering that the bindingenergies are very important to hydrogen storage and that LDAbetter describes weakly interacting systems, only the LDAresults are presented.
* Author to whom correspondence should be addressed. Tel: 55-55-32208859; fax: 55-55-32208032; e-mail: [email protected].
† Universidade Federal de Santa Maria.‡ Universidade Federal de Uberlaˆndia.§ Universidade de Sa˜o Paulo.
The use of localized basis sets largely reduces the amount ofcomputational work required when working with large vacuumregions in the unit cell. The finiteness of the localized basissets leads, however, to basis set superposition errors (BSSE),as well described in a study of benzene on CNTs.21 To avoidthis problem, we have used “ghost” molecules which correspondto additional basis functions centered at the atomic positions ofan X system that is interacting with the BNNT or at the positionof the BNNT itself but without any atomic potential. Thus, thebinding energy of the hydrogen atom or hydrogen molecule onthe BNNT is determined through the following equation:
where ∆BSSE is added to correct the errors because of the
unbalance between the basis set used to describe the interactingsystem and the basis set used to the isolated reactants.22 Negativevalues ofEb indicate that the adsorption is exothermic. In thiswork, 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 approximately8.0 Å. We use periodic boundary conditions with a tetragonalsupercell of 20 Å lattice parameter in directions perpendicularto the tube axis and 8.6 Å along the tube axis (with a total of80 atoms in the unit cell). This construction should eliminatepossible intertube interactions.
The forces are calculated using the Hellmann-Feymannprocedure, and the geometry is optimized using the conjugatedgradient scheme. The systems are relaxed until the root-mean-square criterion of 0.05 eV/Å on the atomic forces is reached.
III. Results and Discussion
The adsorption of a H atom on the top site of B (BNNT+H-B) introduces an energy level close to the valence bandmaximum (VBM), while a H atom on the top site of N (BNNT+ H-N) introduces an energy level close to the conductionband minimum (CBM), as shown in Figure 1b and c, respec-tively. The two electronic levels in the nanotube gap correspondto the two spin polarization directions (up and down). Theacceptor levels correspond to bonding orbitals (see Figure 2b)and come from the N atoms close to the H-B bond. The donorlevels correspond to antibonding orbitals (see Figure 2d) andcome from the B atoms close to the H-N bond. Similar resultswere obtained by Wu et al.8 studying H adsorption on BNNTand by Van de Walle and Neugebauer25 studying the influenceof H impurities in semiconductor bulk systems. The calculatedbinding energy of the H atom on the top site of B is-490meV and a covalent bond between the adsorbed H atom andthe 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 onthe top side of B is an exothermic process. The calculatedbinding energy of H adsorption on the top site of N
Figure 1. Band structures for (a) the clean BNNT, (b) a H atomadsorbed on the top site of B, (c) a H atom adsorbed on the top site ofN, and (d) a H2 molecule adsorbed on the center of a hexagon. Thedotted lines represent the calculated Fermi energy.
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 aB atom and for a H atom bound to a N atom, respectively.
Hydrogen Adsorption on Boron Nitride Nanotube J. Phys. Chem. B, Vol. 110, No. 42, 200621185
is positive, 9 meV. This positive binding energy can beunderstood taking into account the outward radial relaxation ofthe N atom to form a covalent N-H bond (1.06 Å), weakeningthe 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 inthe isolated H2 molecule (0.80 Å). The H2 molecule can beadsorbed on top of a B atom and on top of a N atom as well ason the center of a hexagon. The H2 adsorption is slightly favoredon top of a N atom, with a calculated binding energy of-60meV, while the binding energies for the H2 molecule on thecenter of a hexagon and on top of a B atom are-43 meV. Theoptimized 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 ofthe H2 molecule approximating to the center of a hexagon, theaverage distance between the atoms of the hexagon and theclosest H (of the H2 molecule) is 2.80 Å. The bond lengths andthe binding energies are summarized in Table 1. The BSSEcorrections for these weakly bound systems are around half ofthe LDA binding energy. Also, without the BSSE correction,the binding energies for the adsorbed H2 molecule for the threestudied systems are similar to each other, in agreement withJhi et al.11 The band structure of the BNNT+ H2 system ispractically unchanged when compared with that of the cleanBNNT (see Figure 1a and d).
Carbon substitutional impurities, CB and CN, present lowformation energies and introduce energy levels inside thenanotube band gap (see Figure 3a and d), as describedelsewhere.26,27For a H atom on top of C impurity sites, CB andCN, a chemical adsorption occurs, with binding energies of-1.726 eV and-2.308 eV, respectively. These binding energiesare significantly enhanced when compared with the adsorptionof an atomic hydrogen on the defect-free BNNT surface.
As a result of the adsorption of atomic hydrogen on the topof CB and on the top of CN, CB-H and CN-H bonds are formed,with the occupied and empty impurity levels (before the Hadsorption) 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 impuritydangling bonds, cleaning the band gap and lowering the totalenergy of the system. The formation of CB-H and CN-H, inthe presence of a source of H2 molecules, cannot be a barrierlessprocess since the minimum energy required to furnish a H atomis 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 thatthere is almost no preference for the H2 molecule to be adsorbedon the CB or on the CN defect, the H2 binding energies being-156 meV and-163 meV, respectively. These binding energies
are higher than those for H2 on C-doped BN graphite sheet.11
Although the binding energies have almost the same value forthe BNNT+ CB-H2 and BNNT+ CN-H2 systems, the C-Hand H-H bond distances are slightly different: 1.43 and 0.92Å for the H2 on the top of CN and 2.14 and 0.82 Å for the H2on the top of CB, respectively (see the binding energies and themost relevant distances in Table 1).
The difference in the C-H and H-H bond distances whenthe H2 molecule is adsorbed on CB and on CN is a clearindication that the H2 adsorption process must be different forthese two sites, and it must be related to the ionic character ofthe BNNT. To show this, we have performed rigorous calcula-tions for H2 on the top of CN and on the top of CB fixing theC-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 distanceswhere the binding energy remains almost unchanged. What ishappening is that there is a binding energy compensationbetween the C-H bond and the H-H bond.
In Figure 5, we can observe that when the H2 moleculeapproaches the tube surface, the C-H bond turns stronger and
TABLE 1: Calculated Binding Energies (in meV), Distances(in Å) between the BNNT and the H Atom Closest to theTube Surface, and H-H Distances (in Å) for the H2Molecule Adsorbed on the BNNT Surface
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) Hatom and (f) H2 molecule adsorbed on the top site of CN.
Figure 4. Binding energy (eV) versus C-N bond distance for H2adsorbed on the top of CN defect.
21186 J. Phys. Chem. B, Vol. 110, No. 42, 2006 Baierle et al.
the H-H bond starts to weaken. In fact, Figure 5c shows atransference of charge between the CN and the H2 molecule,leading the system in a limit between the chemisorption andthe physisorption processes. These two effects almost compen-sate each other, resulting in a smooth curve for the bindingenergy and an optimized small CN-H distance (1.43 Å).
For H2 on the top of CB, these compensation effects do notoccur and the smooth behavior of the binding energy curve atlong distances is not observed. The optimized CB-H distance(2.14 Å) is similar to previous calculations for H2 adsorptionon carbon-doped BN sheet.11
In Figure 4, we can observe the importance of correcting forBSSE, which reduces the binding energy. Also, the resultspresented in Figures 4 and 5 allow us to conclude that the H2
molecule should not dissociate when it approaches the carbon-doped BNNTs. This is confirmed by looking at the bindingenergies of BNNT+ CB-H (-1.726 eV) and BNNT+ CN-H(-2.308 eV) systems, which are always smaller than half ofthe calculated H2 binding energy (-2.345 eV). In this way, ifonly a source of H2 molecules is present, only H2 will beadsorbed on C-doped BNNTs, neither BNNT+ CB-H norBNNT + CN-H will be formed. Other defects in BNNT likevacancies and antisites, which have higher formation energies,can induce the H2 dissociation.28
Another observation that helps clarify the interaction betweenthe H2 molecule and the BNNT concerns the distances betweenthe H2 molecule and the BNNT matrix. For the undoped andthe 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 arereduced to 2.31 and 2.41 Å (Figure 6d). These results reinforcethe compensation mechanism between the CN-H and the H-H(bond distances and binding energies), which explains why thebinding 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-dopedBNNTs are in the range where it is predicted that the hydrogencan be stored in ambient temperature and pressure.11 Bycomparing Figure 1 and Figure 3, we observe that the electronicproperties of the BNNT+ CB-H2 and BNNT + CN-H2
systems are similar to those for the H atom on the clean BNNTsurface (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 calculationswere performed for a carbon concentration of 1.25% in theBNNT. By increasing the carbon concentration, more levels willappear inside the band gap. As the H2 levels are resonant at thevalence band, we do not expect significant changes on thebinding energies by increasing the carbon concentration. Onthe other hand, if the substitutional carbon atoms are dilutedon the BNNT surface, avoiding C-C interaction and theconsequent C-C dangling bonds reconstruction, more H2
molecules can be adsorbed, increasing the hydrogen storagecapacity.
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).
Figure 6. Schematic representation of (a) N-H distances of BNNT+ H2-B; (B) B-H distances of BNNT+ H2-N; (c) N-H distancesof BNNT + CB-H2; and (d) B-H distances of BNNT+ CN-H2.
Hydrogen Adsorption on Boron Nitride Nanotube J. Phys. Chem. B, Vol. 110, No. 42, 200621187
IV. Summary and Conclusions
Our ab initio calculations show that the H2 binding energieson carbon-doped BNNTs are enhanced when compared withclean BNNTs, in agreement with experimental results forBNNTs where dangling bonds are present.10 Different from theadsorption of atomic hydrogen, the adsorption of the H2
molecule is a physisorption process. The magnitude of thebinding energies for the H2 molecule on the BNNT+ CB issimilar to the H2 on the BNNT + CN, and they are in theoptimized range to work as a hydrogen storage medium. Forthe H2 molecule adsorbed on the top of a carbon atom in aboron site, a donor defect level is present, while for the H2
molecule adsorbed on the top of carbon in a nitrogen site, anacceptor defect level is present. Although the properties of theBCN nanotubes depend on their composition, these nanotubesare promising candidates for storing hydrogen.
Acknowledgment. This work was supported by Brazilianagencies CAPES, CNPq, and FAPERGS. The calculations havebeen performed using the facilities of the Centro Nacional deProcessamento at UNICAMP/CAMPINAS.
References and Notes
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(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.Nature2003, 423, 626.(26) Schmidt, T. M.; Baierle, R. J.; Piquini, P.; Fazzio, A.Phys. ReV B
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