Article pubs.acs.org/JPCA
Reversible Hydrogen Uptake by BN and BC3 Monolayers Functionalized with Small Fe Clusters: A Route to Effective Energy Storage Tanveer Hussain,† Debra J. Searles,†,‡ and Keisuke Takahashi*,§ †
Centre for Theoretical and Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, and School of Chemical and Molecular Biosciences, The University of Queensland, Brisbane, Qld 4072, Australia § Graduate School of Engineering, Hokkaido University, N-13, W-8, Sapporo 060-8628, Japan ‡
ABSTRACT: In an effort to design new functionalized nanostructures for clean energy storage, DFT calculations of Fen (n = 1−3) clusters on BC3 and BN monolayers are performed. The stability of the systems was considered by calculating the binding energies of the monolayers with Fen clusters on one or both sides. All the clusters bound strongly to both the monolayers and transferred electron density to the sheets. The cationic Fe clusters were then able to adsorb multiple H2 molecules through electrostatic and van der Waals interactions. The average adsorption energies per H2 in the case of maximum coverage were calculated to be −0.389 and −0.358 eV for systems with one Fe on both sides of BC3 and BN monolayers, respectively. In these cases four H2 molecules were adsorbed to the Fe atoms on both sides of the monolayer. These adsorption energies are such that there is potential for adsorption/ desorption at ambient conditions. The results provide insights into an efficient and reversible storage of H2 by using Fenfunctionalized BC3 and BN monolayers.
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INTRODUCTION
higher hydrogen storage capacity demands the use of large surface area nanostructures. Metal clusters on carbon-based nanostructures have been explored as promising energy storage materials where the metal clusters considered are alkali, alkaline earth, and transition metals (TMs).14−16 Binding energies of H2 to pure metals can be very high, in which case desorption can be problematic. However, when the metals are adsorbed on two-dimensional substrates, their binding energies with H2 can be reduced, and in some cases the new material will then fall into the ideal range for H2 storage materials. The binding of the metal clusters to the substrate needs to be high enough to minimize metal clustering and guarantee reversibility, and this binding is insufficient in the case of pristine graphene. Thus, graphene-like monolayers such as BN and BC3 sheets are interesting alternatives as they mimic graphene in various properties and yet can provide higher bindings for metal clusters. There are several studies on pure,16 lithium-doped,17 calcium-doped,18 and polylithiated molecule-doped19 BC3 as reversible H2 storage materials. Similarly, BN sheets have been considered as an efficient energy storage medium when doped with various TMs,20 oxygen,5 platinum (palladium),21 and lithium.22
The diminishing reserves of fossil fuels will eventually be incapable of dealing with the accelerating needs for energy and CO2 emissions associated with them are threatening to the environment. This situation results in demands for renewable and clean alternative fuels capable of replacing the current energy sources efficiently. Features including abundance, higher energy content, and zero emission have made hydrogen (H2) an ideal choice, though the proficient storage of this ideal energy carrier has been problematic for years as hydrogen is a low density gas.1−4 Because of their large surface area that enhances storage capacity, two-dimensional nanostructures can provide a compact and viable H2 storage option.5,6 However, for an ideal storage material the binding of molecular H2 with the host material should be within the energy range of 0.2−0.70 eV,7−9 and the interaction of H2 with several monolayers in pristine form lies in the weak physisorption range. Therefore, it cannot be considered for ambient condition H2 storage.9 However, the creation of defects, introduction of dopants, or adsorption of other materials on the monolayers can enhance the material− H2 interaction significantly which has been investigated extensively in recent past.9−11 Here are also some studies exploring the hydrogen storage properties of materials in bulk phases, like in BC3,12,13 where the adsorption of hydrogen takes place in a dissociative manner. However, the goal of achieving © XXXX American Chemical Society
Received: December 30, 2015 Revised: March 4, 2016
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DOI: 10.1021/acs.jpca.5b12739 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A Motivated by the great interest of research community into these fascinating monolayers, the interaction of small iron clusters (Fe, Fe2, and Fe3) with both BC3 and BN is investigated in this work, and their H2 storage properties are explored. Here, the novel interaction and storage of H2 (molecular form) with small iron (Fe) clusters on both sides of the two-dimensional materials are described. By means of van der Waals corrected first-principles calculations, the following questions are addressed: (1) Do the BC3 and BN sheets remain stable upon the exposure of small Fe clusters on either side of the sheets? (2) What is the binding location and strength of Fe clusters on BC3 and BN sheets, and how does it vary as the cluster size increases? (3) What is the charge transfer mechanism between the clusters and monolayers? (4) What is the type of interaction between Fe and H2 molecules?
and E[H2] is the energy of a H2 molecule. Here y is an integer, which represents the number of H2 and y = 1−12 in this work.
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RESULTS Fe Clusters on BN and BC3. The binding mechanism of Fe clusters on BC3 and BN sheets and the adsorption of H2 molecules on these Fe functionalized systems were investigated. The structural details of BC3 and BN monolayers are discussed. The calculated B−C and C−C bond lengths in the BC3 sheet are found to be 1.57 and 1.42 Å, respectively, whereas the B−N bond length in the BN sheet is calculated to be 1.44 Å. These values are in good agreement with the previously reported work.20,22,28 There are several binding sites available in both the monolayers for accommodation of the Fe, Fe2, and Fe3 clusters. To obtain the most stable configurations, all the likely adsorption sites of Fe clusters are considered in the calculations and compared based on binding energies. Adsorption energies of Fe clusters on one or both sides of the BC3 and BN sheets are presented in Figure 1. As an overall trend, it can be seen
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COMPUTATIONAL METHOD The first-principles calculations were performed based on spinpolarized density functional theory using both the VASP and GPAW packages.23,24 The results from both the computational packages complemented each other. For the exchange and correlation effects, the generalized gradient approximation (GGA) using the Perdew−Burke−Ernzerhof scheme was employed, and the electron−ion interaction was dealt with the projector augmented wave method (PAW).25 However, the GGA functional does not treat the long-range forces correctly, which play a very important role in weakly interacting systems and therefore results in underestimation of the adsorption energies. To include the van der Waals correction, the DFT-D2 method as proposed by Grimme is employed.26 For the structural optimization and the calculations of adsorption energy, an energy cutoff of 500 eV was considered for the plane wave and a linear combination of atomic orbitals basis set was used. The sampling of the Brillouin zone (BZ) was performed by Monkhorst−Pack scheme with a 5 × 5 × 1 k-point mesh.27 All the structures were fully optimized until the forces on each ion became less than 0.01 eV/Å. A vacuum space of 15 Å was inserted along the z-direction to avoid the interaction between the periodically repeating units. For both the BC3 and BN sheet, a supercell of dimension 4 × 4 × 1 was used, which resulted in 32 atoms in the supercell of each monolayer. The binding energies of Fe on these sheets, per Fe atom, was calculated as Eb =
Figure 1. Adsorption energies per Fe for Fe, Fe2, and Fe3 clusters on BC3 and BN sheets (circles) and Fe, Fe2, and Fe3 clusters on both sides of BC3 and BN sheets (diamonds).
that Fe clusters have stronger binding with the BC3 sheet than the BN sheet. This suggests that Fe clusters on the BN sheet could have stronger binding with H2 than Fe clusters on a BC3 since the monolayer is having less effect on the cluster behavior, and they will behave more like isolated metal clusters. It is observed that the Fe adatom prefers to stay on the hollow site (i.e., above the center of a six-membered ring) of both the BC3 and BN-sheet with binding energies of −3.28 and −1.65 eV, respectively. The binding distance for BC3−Fe is 2.09 Å, and for BN−Fe it is 3.10 Å. The values for the binding energies are higher than previous reports describing the interaction of Fe with a BN and graphene sheet; however, it must be noted that those reports did not consider the van der Waals interactions.29,30 Both the monolayers preserve their planar geometry after the Fe binding although there is distortion of the BN sheet upon the introduction of Fe clusters. Since the intention of the current study was to adsorb the H2 molecules on Fe clusters on BC3 and BN sheets, thus to increase the H2 storage capacity, double-sided metal coverage is also considered by adding another Fe adatom to the other side of the
E[BC3 /BN + mFen] − E[BC3 /BN)] − mE[Fen] mn
(1)
where E[BC3/BN+mFen] is the total energy of a monolayer (either BC3 or BN) loaded with Fe, E[BC3/BN)] is the energy of the pristine monolayer, and E[Fen] is the energy of the Fe cluster. The number of Fe clusters is given by m and the number of Fe atoms in a cluster by n where m = 1, 2 and n = 1, 2, 3 in this work. A negative value of Eb indicates that the atom(s) or cluster(s) bind to the monolayer. The adsorption energy of H2 on the Fe functionalized BC3/ BN systems was calculated using E b = E[BC3 /BN + mFen + y H2] − E[BC3 /BN + mFen + (y − 1)H2] − yE[H2] (2)
where E[BC3/BN+mFen+yH2] is total energy of the Fe functionalized system loaded with y H2 molecules, E[BC3/BN+mFen+(y−1)H2] is the total energy of the system loaded with (y − 1) H2 molecules, B
DOI: 10.1021/acs.jpca.5b12739 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A monolayers. Again all the possible binding sites are considered for a second Fe atom, and it resulted in binding on the hollow site (on the other side of both the monolayers) as well. The binding energies in this case on BC3 and BN sheet are found to be −2.48 and −1.24 eV, respectively. The minimum binding distance of Fe with the BC3 sheet is 2.13 Å, and it is 1.99 Å with the BN sheet; however, the BN sheet became more distorted, and the binding distance between the BN sheet and the other atom is 3.10 Å. The binding energy of isolated Fe atoms on both the sheets is reasonably high, and the adatoms are far apart (>4 Å) from each other due to the size of the supercell. However, unlike the light alkali or alkaline earth metal atoms, the Fe atoms tend to form clusters since the cohesive energy of Fe is higher than its binding energy with the substrates (4.3 eV, see ref 32). Considering this, the binding strength of small Fe clusters (Fe2 and Fe3) to the BC3 and BN sheets was also investigated. In Fe2, the Fe−Fe distance is calculated to be 1.99 Å, which elongates a little to 2.01 and 2.09 Å upon structural relaxation over the BC3 and BN sheets with binding energies (per Fe) of −1.73 and −1.42 eV, respectively. Similarly to isolated Fe, all the possible binding sites over the monolayers are considered, and it was found that the Fe2 dimer binds horizontally above the hollow sites of the both the sheets. The binding distances of Fe2 on the BC3 and BN sheets are 2.05 and 3.10 Å, respectively, which are similar to the binding distances of an isolated Fe atom. For two-sided Fe2 coverage, the binding energies per Fe changed to −1.47 and −0.52 eV for BC3 and BN, respectively. For the Fe3 cluster, we considered a triangular cluster with Fe− Fe distances of 2.249, 2.255, and 2.267 Å. Following the previous approach Fe3 has been introduced to the monolayers at all likely binding, sites horizontally (parallel to the sheets) as well as vertically (perpendicular to the sheets). The minimumenergy configurations are the ones with BC3−Fe3 distance of 2.00 Å and significantly high binding energy (per Fe) of −1.65 eV and BN−Fe3 distance of 2.99 Å with binding energy of −1.13 eV. The Fe atoms elongate their bond length to 2.23 and 2.35 Å in Fe3 cluster on BC3 and BN respectively. In case when Fe3 clusters were placed on both the sides of the monolayers, the corresponding binding energies per Fe are −1.44 eV and −0.44 eV for BC3 and BN, respectively. In general, the BN sheet binds the Fe adatom and clusters less strongly than the BC3 sheet. This would suggest that the Fe cluster on the BC3 would behave more differently than an isolated cluster. All results obtained for the binding energies of Fe, Fe2, and Fe3 clusters on BC3 and BN sheets are shown in Figure 1. Hydrogenation. In all the systems studied, a considerable amount of charge is transferred from the Fe to the monolayers due to the difference in electronegativities. This causes the Fe to become cationic which we see can be of advantage for anchoring the H2 molecules. It is of interest to consider the hydrogen adsorption mechanism on Fe clusters on BC3 and BN monolayers. All results obtained for the adsorption energies of H2 on Fe, Fe2 and Fe3 on BC3, and BN systems are shown in Figure 2. The structures corresponding to the fully hydrogenated double-sided Fe, Fe2, and Fe3 clusters on BN and BC3 are shown in Figure 3. Although the hydrogen can adsorb to the Fe adatoms or clusters in its atomic or molecular form, it is known that atomic adsorption involves dissociation of the hydrogen molecule and strong bonding that requires higher desorption temperatures and limits the reversibility of the system. Thus, to attain a high storage capacity and (reversible) desorption with feasible conditions, molecular adsorption must
Figure 2. van der Waals corrected adsorption energies of H2 molecules on (a) single and (b) double-sided Fe, Fe2, and Fe3 functionalized BC3 and BN monolayers. The H2 adsorption over pristine graphene, BN, and BC3 is also shown for comparison.
be able to be obtained. The calculation of the interactions of H2 with both the Fe on BC3 and Fe on BN systems follow the same approach; therefore, we focus on the 2Fe−BC3 systems here. In this case one Fe atom is attached to either on or both sides of the BC3 sheet, and each Fe transfers a significant amount of its electronic charge to the sheet. Most of this transferred charged goes to those atoms that constitute the hexagonal ring binding the Fe. The Fe can then anchor multiple H2 molecules through polarization interactions. The introduction of one H2 on each Fe on both sides of the BC3 sheet is performed, and the system is allowed to relax to its minimum energy state in the calculation. Once H2 is in the vicinity of Fe, it becomes polarized due to the cation and held to it through an electrostatic interaction. This kind of Fe−H2 binding where the partially empty d orbital of the Fe binds with hydrogen is referred to as the Kubas interaction.31 Each H2 molecule adsorbs to Fe on BC3 in the 2Fe−BC3 systems at a short distance of 1.66 Å and with a reasonably high adsorption energy of −0.74 eV/H2. The corresponding adsorption energy per H2 for the 2Fe−BN systems was found to be −0.83 eV, which is a slightly stronger binding than for the 2Fe−BC3 systems. For adsorption of a single H2, the C
DOI: 10.1021/acs.jpca.5b12739 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry A
Figure 3. Structures of the fully hydrogenated systems with (a) Fe, (b) Fe2, and (c) Fe3 clusters on both sides of BN and (d) Fe, (e) Fe2, and (f) Fe3 clusters on both sides of BC3. Atomic color code: Fe: orange; B: pink; C: gray; N: blue; H: light white.
room temperature and pressure, and a temperature of approximately 370 K would be required to remove molecules bound by 0.5 eV, at 1 atm. Therefore, adsorption and desorption of H2 from the BC3 systems could be achieved close to ambient conditions. Desorption will be more difficult from the BN systems, except if single atom Fe is present. However, both systems will adsorb significant amounts of H2 at room temperature. This is in agreement with experimental studies of hydrogenation of small Fe clusters which indicate that the hydrogenation process occurs at room temperature.16 It is in contrast to the pure graphene, BN sheet and BC3 sheet that all require very low temperatures in order to adsorb hydrogen.
H−H bond length increases significantly from 0.74 to 0.87 Å on BC3; however, the H2 preserves it molecular nature. This elongation in bond length is thought to be the result of the fractional charge transfer between the Fe and the H2 molecule. An additional H2 is introduced to each Fe on the 2Fe−BC3 system (so four H2 molecules are adsorbed), and the system is then relaxed completely. A decrease in the adsorption energy was observed giving a value of −0.68 eV/H2, and the H2−2Fe− BC3 adsorption distance increased to 1.80 Å. For 2Fe−BN, the adsorption decreases more, to −0.56 eV/H2. The average H−H bond length of adsorbed H2 molecules is found to be 0.82 Å. To determine the maximum H2 storage capacity, the systems are exposed to more H2 molecules and allowed to relax. Each time a H2 is introduced to the systems, a reasonable distance from the other H2 molecules is maintained to avoid an unwanted interaction among the H2, which carry a fraction charge from the positively charged Fe clusters. It is found that a maximum of 4 H2 could be adsorbed on each side of the 2Fe− BC3 system (eight altogether) with adsorption energy of −0.389 eV/H2 that falls within the ideal adsorption/desorption range. After adsorbing 4 H2 on each Fe, the systems become saturated and start repelling the additional H2 molecules. For the BC3 system, similar behavior is observed when clusters of two or three Fe atoms are adsorbed on the monolayer; however, there is some degree of dissociation of the H2 molecule, with adsorption of the H atoms to the Fe. Comparing BN and BC3, Figure 2 shows that the H2 adsorption energy is generally higher on the BN sheet than on the BC3 when there is one Fe atom on each side. In fact, the H2 at least partly splits into H atoms which are bound to the Fe clusters on BN, whereas it mostly remains molecular when interacting with the Fe clusters on BC3. This means that the BN systems behave more like isolated Fe clusters than do the BC3 systems. Stronger binding of H2 to BN than BC3 is observed in all cases except when there is one Fe adsorbed to both sides of the BN monolayer. In that case one of the Fe atoms bound quite, as evident from a short bond distance and this might explain why this system behaved differently. This general trend of stronger binding on BN can be explained by the adsorption energy of Fe clusters on BN and BC3. Figure 1 indicates that Fe clusters are more strongly adsorbed on BC3 than on the BN sheet. The difference in adsorption behavior for H2 on the two substrates is shown in Figure 3 where it is clear that the H2 remains fully molecular on the BC3 sheet, but not on the BN sheet. The standard entropy of desorption of H2 is approximately 1.35 meV/K; therefore, H2 molecules with a binding energy of about −0.4 eV or stronger would expected to be adsorbed at
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CONCLUSION In pursuit of an efficient and reversible storage medium for the clean energy carrier H2, an extensive DFT study on the structural and hydrogen storage properties of BC3 and BN sheets is performed by means of DFT calculations. In their pristine form, neither of the monolayers can adsorb H2 sufficiently strongly; however, adsorption of small iron clusters, Fen (n = 1−3), on these monolayers resulted in stable functionalized monolayers which could adsorb H2 effectively. It is observed that the binding energies of Fen are higher on BC3 than on the BN sheet. Because of the difference of electronegativities, the monolayers there is charge transfer from the Fe clusters to the sheets to give the Fe clusters a partial positive charge. These cations polarize the H2 molecules that are brought into their vicinities and then bind them with electrostatic and van der Waals interactions. The well-known underestimation of H2 adsorption energies caused by the GGA functional has treated by the application of van der Waals corrections to obtain more reliable values. It is revealed that both functionalized systems adsorb H2 with average adsorption energies suitable for practical H2 storage applications although desorption from the BN systems is generally more difficult. Thus, Fen−BC3 and Fen−BN could be promising choices as reversible H2 storage materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (K.T.). Notes
The authors declare no competing financial interest. D
DOI: 10.1021/acs.jpca.5b12739 J. Phys. Chem. A XXXX, XXX, XXX−XXX
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ACKNOWLEDGMENTS This research was undertaken with the assistance of resources provided at the NCI National Facility systems at the Australian National University through the National Computational Merit Allocation Scheme supported by the Australian Government, support from the Queensland Cyber Infrastructure Foundation (QCIF) and the University of Queensland Research Computing Centre. CPU time is also funded by Japan Society for the Promotion of Science and part of calculations are performed at Hokkaido University’s academic cloud, Information Initiative Center.
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