ARTICLE pubs.acs.org/JPCC
Anti-Kubas Type Interaction in Hydrogen Storage on a Li Decorated BHNH Sheet: A First-Principles Based Study Authors: S. Bhattacharya,*,† A. Bhattacharya, and G. P. Das Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India
bS Supporting Information ABSTRACT: We have performed first-principles DFT calculations to explore the possibility of using a metal-functionalized hydrogenated BN sheet for storage of molecular hydrogen. The chair BHNH conformer is ideally suited for adsorption of metal adatoms on the surface of the sheet. The Li metal, in particular, binds to the sheet with a binding energy (∼0.88 eV/Li atom) and becomes cationic, which thereby attracts hydrogen molecules. However, the interaction of the BHNH sheet and the absorbed H2 molecules with Li+ is different from the conventionally known Dewar coordination or Kubas-type interaction for hydrogen storage. Each Li+ can adsorb up to four H2 molecules, and the hydrogen binding energy is in the desired energy window for effective storage of molecular hydrogen. The fully Li-functionalized BHNH sheet yields a reasonably high gravimetric density, which is more than 7 wt %.
1. INTRODUCTION The safe and efficient storage of molecular hydrogen is a crucial target in the transition to a hydrogen-based energy economy.1 There are two main criteria for good hydrogenation. First, the system must adsorb a good weight percentage (DOE target is ∼6 wt %) of molecular hydrogen. Second, the binding energy (BE) of the adsorbed H2 molecules should ideally lie in the range of 0.2 0.7 eV/H2, i.e., in between the range of physisorption and atomic chemisorption.2 To achieve this target, various metal functionalized carbon and BN-based nanostructures, including fullerenes,3 nanotubes,4 planar graphene,5 and BN sheet,6 have been studied extensively in the past few years as possible storage materials. A hydrogenated graphene sheet known as graphane has been theoretically predicted7 9 and has also been recently synthesized in the laboratory by Andre Geim’s group.10 Subsequently, various interesting properties and applications of this material were revealed.11,12 Another analogous two-dimensional nanostructure, viz., hexagonal boron nitride sheet (h-BN sheet), has been cited as a possible substitute of graphene in various ways because of its similar lattice parameters13 and complementary properties. The 2-dimensional BN sheet has recently been synthesized in single and multiple layers.14,15 The system is insulating with a wide band gap of ∼7 eV. The apparent similarity of graphene and the BN sheet has prompted researchers to explore the functional aspect of a singlelayer hydrogenated BN sheet. Hydrogenation of the BN sheet takes place in such a way that one H atom gets bonded to each B and N atoms of the sheet in specific periodic fashion, giving rise to various conformers (with formula unit BHNH) of the sheet.16 20 In the chair and boat BHNH conformers, H atoms of each hexagon r 2011 American Chemical Society
alternate singly and in pair on both sides of the sheet.16 20 However, in the third BHNH conformer known as stirrup, three consecutive H atoms of each hexagon alternate on either side of the sheet.16 Unlike the case of graphene to graphane (where the band gap increases on hydrogenation), here the band gap decreases on hydrogenation, and all BHNH conformers are found to have direct band gaps at the Γ-point, having magnitudes lying between 3.5 and 4.2 eV.16,19 The relative stabilities of various conformers (viz., chair, boat, and stirrup) are different but close in magnitude (4.65 ( 0.15 eV/atom),16 and hence it might be possible to synthesize any of these conformers in the laboratory via nonequilibrium roots. In fact, most of the recent calculations have been carried out on the chair BHNH conformer with the aim of different kinds of applications, including ferromagnetism on partial dehydrogenation.17 19 In this article, we explore how a metal-functionalized planar BHNH sheet can be used for storage of molecular hydrogen. In our recently published letter,16 we have reported that in the chair BHNH conformer hydrogenation separates the H atoms of opposite polarities to either side of the sheet. This fact can be utilized to adsorb different metal adatoms on one surface of the hydrogenated BN sheet. We have performed first-principles DFT calculations to find the binding energies of different metal adatoms (viz., Li, Na, Mg, Al, K, Ca, etc.), and estimated binding energies have been provided in the Supporting Information. However, since Li is the lightest metal, the effective gravimetric efficiency will be more in the case of Li decoration as compared to Received: October 27, 2011 Published: December 19, 2011 3840
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Figure 1. Ball and stick representation of optimized structures (a) Li decorated chair BHNH unit and (b) each Li atom of the Li-decorated BHNH unit adsorbing up to four H2 molecules. (c) A 4 4 supercell storing up to 7 wt % of molecular hydrogen.
the same with other metals adatoms. This is the main reason for our choice for Li to decorate the chair BHNH surface. Moreover Li, in particular, has reasonably high binding (∼0.88 eV) to the sheet as compared with the other metal adatoms [see Supporting Information], and after adsorption on the sheet, it becomes cationic. From first-principles DFT-based calculations, we show that each Li cation of the Li-decorated BHNH sheet can adsorb up to four H2 molecules in the desired energy window for hydrogen storage and yields a gravimetric density of ∼7 wt %.
2. COMPUTATIONAL DETAILS Our calculations have been carried out using first-principles density functional theory (DFT)21,22 based total energy calculations with generalized gradient approximation (GGA). We have used VASP23 code with projected augmented wave (PAW) potential24 for all elemental constituents, viz., H, B, and N. The GGA calculations have been performed using the exchange-correlation functional of Perdew et al.25,26 An energy cutoff of 600 eV was used. The K-mesh was generated by the Monkhorst Pack27
method, and all results were tested for convergence with respect to mesh size. In all our calculations, self-consistency has been achieved with 0.0001 eV convergences in total energy. For optimizing ground state geometry,28,29 atomic forces were converged to less than 0.1 meV/Å via conjugate gradient minimization. To perform an accurate Mulliken population analysis, we have used D-MOL3 codes30 where exchange and correlation terms are treated within the GGA functional by Perdew et al.31 Core electrons were treated in a nonrelativistic all-electron implementation of the potential. A double numerical quality basis set with a polarization function (DNP) was considered. The simulation cell was modeled by taking a 4 4 BHNH supercell comprised of 16 B, 16 N, and 32 H atoms placed in a 16 Å vacuum.
3. RESULTS AND DISCUSSIONS 3.1. Li Decoration on a Chair BHNH Sheet [Li@BHNH Sheet]. As mentioned above, in a chair BHNH sheet, the H
atoms of each hexagon alternate on both sides of the BN plane, giving rise to a structure where all the HB (H bonded to B) atoms 3841
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Table 1. Mulliken Charge Analysis of Various Systemsa
Table 2. Binding Energies (BEs) of H2 Molecules to the Sheet
average Mulliken charge state per atom system
HN
N
B
HB
BE of last
average BE of all H2 molecules (eV)
Li
system
H2 molecule adsorbed (eV)
chair BHNH
+0.257
0.620
+0.370
0.073
-
Li@BHNH + 1H2
E1 = 0.10
0.10
Li@ BHNH
+0.257
0.670
+0.370
0.073
0.20
Li@BHNH + 2H2
E2 = 0.38
0.24
Li@BHNH + 4H2*
+0.257
0.670
+0.370
0.067
0.30
Li@BHNH + 3H2
E3 = 0.22
0.22
Li@BHNH + 4H2
E4 = 0.21
0.22
a
Negative sign indicates charge gained by the system, while positive sign indicates charge lost by the system.
lie on one side and all the HN (H bonded to N) atoms lie on the other side.16 Mulliken population analysis shows that all B atoms lose an electron, which in turn is gained by the N atoms of the sheet. The HN atoms also lose an electron, which is gained by the HB atoms as well as the N atoms of the sheet. Thus, the two surfaces of the sheet become oppositely charged with anionic HB atoms (Mulliken charge state of 0.073) and cationic HN atoms (Mulliken charge state of +0.257) separated on either surface of the sheet16 [Figure 1a, b]. We have tried to functionalize the surface of the BHNH sheet with suitable light metal adatoms, such as Li, B, Na, Mg, Al, K, Ca, etc., to use it for the purpose of H-storage. When metal adatoms are placed on the sheet, they get adsorbed to the HB surface by losing an electron to the atoms of the sheet. Li metal, in particular, is found to have reasonably higher binding on the sheet as compared to other metal adatoms [see Supporting Information]. Out of the three probable sites for Li adsorption, viz., on-top, bridge, and hollow (i.e., hexagonal center top), the hollow site has been found to be energetically most preferred, with a binding of 0.88 eV/atom as shown in Figure 1(a). However, the binding energies are relatively smaller at the bridge (0.68 eV/atom) and top sites (0.72 eV/atom), respectively. In the optimized structure, only one Li atom can be decorated on the top position of the three HB atoms in one hexagon, and the average Li HB distance is calculated to be ∼1.86 Å [Figure 1(a)]. In our earlier letter,16 we had found the Mulliken charge states of B, N, HB, and HN, which were, respectively, +0.374, 0.619, 0.073, and +0.257. When we decorate this sheet with the Li atom, the overall charge distribution changes as 0.374, 0.670, 0.073, and +0.257, respectively, for B, N, HB, and HN along with a +0.20 charge state for Li [Table 1]. Therefore, Mulliken population analysis of a Li adsorbed BHNH system implies that an electronic charge of +0.20 is transferred from the Li atom to the N atoms of the sheet via HB and B, respectively. Therefore, each N atom of the sheet becomes more anionic with its charge state changing from 0.62 to 0.67 after a single Li adsorption. Next, this Li+ and N generates an electrostatic attraction, which is responsible for the Li binding with the sheet. The binding energy of Li is rather weak (∼0.88 eV/Li) but sufficient to avoid Li clustering on the surface as the nearest Li Li distance is ∼4.92 Å, and they suffer a repulsive Coulomb interaction between them [Figure 1c]. Therefore, this kind of weak interaction is not similar to the conventional Kubas-type backdonation effect32 where the binding energy of the Li atom should be quite high (∼2 3 eV/Li).33 Moreover, this is exactly opposite of the conventional Kubas-type interaction, which will be discussed in more detail in the next section. Hence, we name this interaction as the “Anti-Kubas interaction”. The situation here is similar to that of the Li-decorated graphene sheet where also the adsorbed Li atom donates an electron to the graphene
sheet and becomes cationic, but the overall charge transfer mechanism does not follow the conventional Dewar Kubas interaction.34 3.2. Hydrogen Adsorption in the Li@BHNH Sheet. We now explore the possibility of using a Li-decorated BHNH sheet for the purpose of storage of molecular hydrogen. When H2 molecules are introduced one by one to the sheet, they get adsorbed to the Li+ ions with binding energies (BE) that are listed in Table 2. The BE of the first H2 molecule to the sheet is found to be ∼0.10 eV/H2, and on subsequent H2 adsorption, the average BE values of the adsorbed H2 molecules increases. The BE is highest (∼0.33 eV) for the second H2 adsorption, after which it decreases again. The BEs of the third and fourth H2 molecules are ∼0.22 eV (Table 2), while the average distance between Li and H2 has been estimated to be ∼1.8 Å. The average BE of adsorbed H2 molecules is ∼0.20 eV/H2, which exactly falls in the desirable energy window for effective hydrogen storage as set by the Department of Energy, USA [DOE]. Thus, each Li+ ion can adsorb up to four H2 molecules at the most [Figure 1(b)]. Mulliken population analysis of the H2 adsorbed Li@BHNH system shows that after hydrogen adsorption the Li atom and the HB atoms of the sheet become more cationic. The charge state of Li and HB changes to +0.30 and 0.067, respectively, after hydrogen adsorption. Thus, the H2 adsorption is accompanied by transfer of electrons from Li and HB atoms to the H2 molecules, which is similar to the charge transfer trend in hydrogen adsorption in the Li-decorated graphene sheet.34 Therefore, H2 molecules after receiving charge from Li and HB atoms become negatively charged, and the covalent H2 bond becomes polarized. On the other hand, a weak ionic bond is formed due to the attractive Coulomb interaction between positively charged Li and negatively charged H2 molecules. This kind of weak interaction is reminiscent of van der Waals interaction, which is responsible for the formation of weak bonding between H2 molecules and the Li + BHNH complex. Here the bonding interaction is very different from the conventional Dewar Kubas interaction33 as from our earlier papers in H-storage in transitional metal (TM) decorated nanostructures, where the H2 molecules are adsorbed by transfer of electrons from the H2 s-orbital to the cationic TM d-orbital.2,5,6 3.2.1. Electronic Structure. Evidence of the above-mentioned charge transfer can also be observed from the partial electronic density of state (p-DOS) analysis. We have plotted the site projected DOS (p-DOS) plots for all components of the Li@BHNH system before and after H2 intake. However, only the DOS peaks of Li and HB are found to undergo changes after H2 adsorption, while the p-DOS of HN and N remain unaltered. Before H2 adsorption, the Li atom of the Li@BHNH system shows a cationic charge state by losing its electron to the sheet, which is evident from unoccupied Li peaks above the Fermi level in the DOS plot [Figure 2(a)]. However, after H2 adsorption, the 3842
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Figure 2. Site projected density of states plots of the (a) Li atom of a Li-decorated BHNH sheet before H2 adsorption, (b) Li atom of a Li-decorated BHNH sheet after H2 adsorption, (c) sum of all HB atoms of a Li-decorated BHNH sheet before H2 adsorption (averaged by the no. of HB atoms), (d) sum of all HB atoms of a Li-decorated BHNH sheet after H2 adsorption (averaged by the no. of HB atoms).
binding]. This in a way validates the presence of weak interaction, which is reminiscent of van der Waals interaction. This type of interaction is responsible for the formation of weak bonding between H2 molecules and the Li + BHNH complex. Hence, we call it an “anti-Kubas type interaction” from the conventionally known Dewar Kubas interaction for most of the metal decorated nanostructures. We have found that a fully decorated, Li-adsorbed, 64 atom BHNH sheet with the supercell formula of Li4@B16N16H32 can adsorb up to 16 H2 molecules as shown in Figure 1(c). The gravimetric weight percentage of the adsorbed H2 molecules is calculated to be 7.02 wt %, which is higher than the DOE target of H-storage of 6 wt %.
Figure 3. Site projected density of states plot of H, HB, and Li of the Li@BHNH + 4H2 system, showing the H2 peaks to shift toward occupied after adsorption. Here H represents the externally adsorbed hydrogen in molecular form, while HB denotes the hydrogen atom chemisorbed to B.
unoccupied peaks of the Li atom increase further as shown in Figure 2(b). The HB atoms of the sheet also lose an electron to the H2 molecule, which is visible from the increase in unoccupied DOS above the Fermi level after H2 adsorption as shown in Figure 2(c) and 2(d), respectively. The p-DOS of adsorbed H2 molecules given in Figure 3 indicates the absence of strong hybridization of Li and H2 molecules [viz., strong Li H
4. CONCLUSION In summary, we find that the hydrogenated chair BHNH sheet can be functionalized by metal adatoms. Being the lightest among various light metal adatoms, Li has a reasonably good binding energy to the sheet, ∼0.88 eV/atom, compared to others. However, this type of binding of Li is unconventional and rather weak, but sufficient to avoid Li clustering on the surface as the nearest Li Li distance is ∼4.92 Å, and they suffer a repulsive Coulomb interaction between them. A Li-decorated BHNH sheet can be used to store molecular hydrogen. The charge transfer is different from Dewar Kubas interaction and is similar to charge transfer in Li-decorated graphene. The H2 molecules are adsorbed by gaining electrons from the Li and HB atoms of the sheet. Each Li atom can adsorb up to 4 H2 molecules there by yielding an estimated gravimetric density of 7.02 wt % of hydrogen. 3843
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’ ASSOCIATED CONTENT
bS
Supporting Information. Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
ARTICLE
(31) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (32) Kubas, G. J. Metal Dihydrogen and Bond Complexes Structure, Theory, and Reactivity; Kluwer: Dordrecht, The Netherlands, 2001. (33) An, H; Liu, C; Zeng, Z; Fan, C; Ju, X. Appl. Phys. Lett. 2011, 98, 173101. (34) Ataca, C.; Akt€urk, E.; Ciraci, S.; Ustunel, H. Appl. Phys. Lett. 2008, 93, 043123.
*E-mail:
[email protected]. Present Addresses †
Theory Department, Fritz-Haber-Institut der Max-PlanckGesellschaft, Faradayweg 4-6, D-14195, Berlin-Dahlem, Germany.
’ REFERENCES (1) Schlappbach, L.; Zuttel, A. Nature 2001, 414, 353. (2) Bhattacharya, S.; Majumder, C.; Das, G. P. J. Phys. Chem. C 2008, 112, 17487. (3) Sun, Q.; Jena, P.; Wang, Q.; Marquez, M. J. Am. Chem. Soc. 2006, 128, 9741. (4) Yildirim, T.; Ciraci, S. Phys. Rev. Lett. 2005, 94, 175501. (5) Bhattacharya, A.; Bhattachrya, S.; Majumder, C.; Das, G. P. J. Phys. Chem. C 2010, 114, 10297. (6) Bhattacharya, S.; Majumder, C.; Das, G. P. J. Phys. Chem. C 2009, 113, 15783. (7) Sluiter, M. H. F.; Kawazoe, Y. Phys. Rev. B 2003, 68, 85410. (8) Sofo, J. O.; Chaudhari, A. S.; Barber, G. D. Phys. Rev. B 2007, 75, 153401. (9) Bhattacharya, A.; Bhattacharya, S.; Majumder, C.; Das, G. P. Phys. Rev. B 2011, 83, 033404. (10) Elias, D. C.; Nair, R. R.; Mohiuddin, T. M. G.; Morozov, S. V.; Blake, P.; M. P. Halsall, M. P.; Ferrari, A. C.; Boukhvalov, D. W.; Katsnelson, M. I.; Geim, A. K.; Novoselov, K. S. Science 2009, 323, 610. (11) Singh, A. K.; Penev, E. S.; Yakobson, B. I. ACS Nano 2010, 4, 2510. (12) Lu, Y. H.; Feng, Y. P. J. Phys. Chem. C 2009, 113, 20841. (13) Topsakal, M.; Akt€urk, E.; Ciraci, S. Phys. Rev. B 2009, 79, 115442. (14) Nag, A.; Raidongia, K.; Hembram, K.; Datta, R.; Wagmare, U. V.; Rao, C. N. R. ACS Nano 2010, 4, 1539. (15) Jin, C.; Lin, F.; Suenaga, K.; Iijima, S. Phys. Rev. Lett. 2009, 102, 195505. (16) Bhattacharya, A.; Bhattacharya, S.; Majumder, C.; Das, G. P. Phys. Status Solidi RRL 2010, 4, 368. (17) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Adv. Mater. 2009, 21, 2889. (18) Chen, W.; Li, Y.; Yu, G.; Li, C.; Zhang, S. B.; Zhou, Z.; Chen, Z. J. Am. Chem. Soc. 2010, 132, 1699. (19) Zhou, J.; Wang, Q.; Sun, Q.; Jena, P. Phys. Rev. B 2010, 81, 085442. Wang, Y. Phys. Status Solidi RRL 2009, 4, 34. (20) Averill, F. W.; Morris, J. R.; Cooper, V. R. Phys. Rev. B 2009, 80, 195411. Tang, S.; Cao, Z. Chem. Phys. Lett. 2010, 488, 67. (21) Hohenberg, P.; Kohn, W. Phys. Rev. B 1964, 136, 864. (22) Kohn, W.; Sham, L. Phys. Rev. A 1965, 140, 1133. (23) Kresse, G.; Hafner, J. Phys. Rev. B 1994, 49, 14251. (24) Kresse, G.; Furthm€uller, J. J. Comput. Mater. Sci. 1996, 6, 15. (25) Bl€ochl, P. E. Phys. Rev. B 1994, 50, 17953. (26) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (27) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. (28) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188. (29) Press, W. H.; Flannery, B. P.; Tenkolsky, S. A.; Vetterling, W. T. Numerical Recipes; Cambridge University Press: New York, 1986. Pulay, P. Chem. Phys. Lett. 1980, 73, 393. (30) Delley, B. J. Chem. Phys. 1990, 92, 508. Delley, B. J. Chem. Phys. 2000, 113, 7756. 3844
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