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Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages
international journal of hydrogen energy xxx (xxxx) xxx
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Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages M. Ghorbanzadeh Ahangari a, A. Hamed Mashhadzadeh b,* a
Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Mazandaran, Babolsar, Iran b Department of Mechanical Engineering, Azadshahr Branch,Islamic Azad Universty, Azadshahr, Iran
highlights H2 storage capacity and adsorption properties of nanocages studied using DFT method. Mg12O12 and Zn12O12 have highest capacity to storage hydrogen molecules. Zn12O12 is strongest adsorbent for H2 molecule.
article info
abstract
Article history:
In the current study, the density functional theory calculations (DFT) were employed to
Received 14 April 2019
determine the hydrogen storage properties of some nanoclusters including C24, B12N12, Al12
Received in revised form
N12, Be12O12, Mg12O12, and Zn12O12. After full geometrical optimization of all nanocages
14 December 2019
under the DFT framework, we found that C24 and B12N12 were unstable structures even in
Accepted 16 December 2019
case of incorporating only one hydrogen molecule to them due to positive obtained for-
Available online xxx
mation energy magnitudes while Al12N12 and Be12O12 were able to adsorb one hydrogen molecules and became thermodynamically unstable for more than one hydrogen mole-
Keywords:
cule. Also, Mg12O12 and Zn12O12 were capable of storing up to 4 hydrogen molecules ac-
DFT
cording to negative achieved formation energies. Also, calculated bulk modulus revealed
Nanocage
that when all studied structures stored H2 molecules the bulk modulus decreased
Hydrogen storage
compared to pristine nanoclusters. The highest reduction in bulk modulus was 10% which
Adsorption
occurred in C24 while storing 5H2. Furthermore, the adsorption properties of these nanocages were considered using DFT and the results showed that Zn12O12 was a stronger adsorbent for H2 in comparison to the rest of the studied nanocages. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Hamed Mashhadzadeh). https://doi.org/10.1016/j.ijhydene.2019.12.106 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Introduction Hydrogen has been introduced as a potential replacement for fossil fuels in sustainable energy systems due to its high energy density, great efficiency, low weight, and being environmentally friendly [1]. The development of appropriate systems for safe storage and movement of hydrogen has become a major challenge among researchers over the past few decades [2e4]. According to the technical targets that have been evolved by the U.S Department of Energy (DOE), hydrogen storage systems must meet some requirements regarding delivery temperature, kinetics, volumetric capacity and gravimetric capacity [2]. Since the conventional methods of storing hydrogen such as storage after being compressed or storing in liquid form showed some limitations in terms of safety and cost, focuses were turned to employ nano architectures for this purpose [5,6]. Nanomaterials including nanotubes, nanocapsules and nanocages have been reported to be suitable storage systems for hydrogen due to their superior electronic properties as well as their extended surface area [7e9] so a great deal of research works have focused on exploring the capability of such materials in adsorbing or storing hydrogen [9]. Takeo Oku et al., in their experimental study, investigated the hydrogen storage in Boron-Nitride (BN) nanomaterials including nanocapsules, nanotubes and nanocages that were synthesized from a mixture of LaB6, Pd and boron powder using arc melting method. Their results demonstrated that these nanostructures are capable of storage of up to 3 wt% hydrogen [10]. In another research work, Ye-Ji Han et al. considered the effect of adding nickel nanoparticles to MWCNT on its hydrogen storage properties and reported considerable improvement in this property in comparison to pristine MWCNT [11]. Besides, nanocages that are the frame-like hollow structures which include opening pores in their shells have been extensively employed in a variety of electronic and chemical industries applications especially storage and adsorption of hydrogen over the past few years due to their considerable advantages [12e15]. These nanostructures are able to adsorb hydrogen in reversible process. The low enthalpy of the adsorption process decreases the thermal management concerns and therefore the materials stability during the cycle would not be a problem [16,17]. Due to such capabilities, nanoclusters like fullerenes, Boron-Nitride, Aluminum-Nitride, and Metal-oxide nanocages have been introduced as appropriate hydrogen storage media [18e21]. In this regard, it has been reported that, up to eight hydrogen molecules could be incorporated to C120 nanocapsule forming a stable complex [22]. Up to 38 hydrogen molecules were stored in B60N60 nanocage as reported by Koi et al. [8]. Qi et al. employed density functional theory study (DFT) to investigate the capacity of Licoated B36N24 nanocluster as a hydrogen storage structure and found that each Li atom in this structure can adsorb two hydrogen molecules [4]. Wang et al. considered the hydrogen storage capacity of Li decorated (AlN)n nanocages under DFT frame work and reported the gravity density of storage equal to 4.7 wt% [23]. Sayhan and Kinal selected DFT calculations to make a comparison between the storage capacity of B24N24 and
Al24N24 nanocages. Their results demonstrated a tighter binding between H2 and Al in Al24N24 than B24N24 [24]. The main aim of the current study was to make a direct comparison between the hydrogen storage and adsorption properties of some potential nanoclusters through a DFT- MD based framework. With this purpose, the DFT calculations have been employed to consider the hydrogen storage and adsorption capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages. At first, these structures were geometrically optimized under the DFT framework and Density of States (DOS) was calculated for all of these nanocages to make a comprehensive understanding of the electronic properties of studied nanoclusters. Then, the stability of each structure was considered by incorporating different numbers of hydrogen molecules in them and comparisons were made among them according to the obtained formation energies. After the storage consideration, we investigated the effect of storage on, bulk modulus of all studied nanocages and finally, the adsorption properties of these nano architectures toward hydrogen molecule were investigated using density functional theory calculations.
Computational methods Density functional theory calculations have been vastly employed in solid-state physics over the past few decades. The results that have been obtained from this technique are usually in good agreement with those of experimental research [25,26]. Our calculations based on the adsorption and storage of the hydrogen molecules on and in the nanocages were performed within the framework of first-principles DFT implemented using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code [27]. We adopted the generalized gradient approximation (GGA) to treat the electronic exchange and correlation effects, as described by Perdew-Burke-Ernzerhof [28]. All calculations were performed with a double-z basis set of localized numerical atomic orbitals, including polarization functions (DZP), with an energy shift of 50 meV and a split norm of 0.15. Structural relaxations were performed using the conjugate gradient algorithm until the residual forces on each atom were lower than 0.02 eV A1. The mesh cutoff, an energy that corresponds to the grid spacing, was selected as 120 Ry. We have also performed DFT-based MD simulations for the system under study. The systems were anneals to specified pressure and temperature. All MD simulations were performed in a NPT ensemble (constant Number of atoms, Pressure and Temperature) with a specified temperature of 300 K at the constant pressure of 1e70 bar using the anneal thermostat. The simulation time duration of the molecular dynamics simulation was 5 ps, and the time step was 1 fs.
Results and discussions Optimization of pristine nanocages Geometrical optimization of pure C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 was investigated using DFT calculations.
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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In a structural view, these clusters are formed from six 4membered ring (4-MR) and eight 6-membered ring (6-MR) with Th symmetries. After full structural optimization of C24 nanocage, the C]C bond length in 4-MR and 6-MR were obtained equal to 1.49 A and 1.38 A respectively. These obtained results are in best agreement with those that Tahmasebi and her co-workers reported for the bond length in 4-MR and 6-MR A and 1.37 A, for C24 cage which were reported equal to 1.49 respectively [29]. Furthermore, we calculated the electronic density of states for C24 and found the HOMOeLUMO band gap energy (Eg) of 1.81 eV for this nanocage These results are comparable with the previous B3LYP/6-31G* computational level that was investigated by Chang et al. [30]. They reported A and about the average CeC bond length and Eg equal to 1.45 1.8 eV successively. Fig. 1 indicates the optimized structure and corresponding DOS of C24 cage. In this section the structures of all mentioned clusters, were fully optimized and then the DOSs were calculated. The optimized structures and the obtained DOS results are illustrated in Figs. 1 and 2, respectively. Two topologically nonequivalent BeN and AleN bonds are presented in B12N12 and Al12N12 clusters; one is shared by two 6-MRs rings (d6MR), and the other is shared by a 4-MR and a 6-MR (d4MR). The d6MR and d4MR in B12N12, and Al12N12 nanocages are equal to 1.44 A and 1.49 A, and 1.81 A and 1.88 A, respectively. Tahmasebi et al. investigated electro-optical features of the group IIInitrides and group IV carbides nanocluster encapsulated with alkali metals using DFT calculation and they reported that d6MR and d4MR in B12N12, and Al12N12 nanocages were equal to 1.44 A and 1.49 A, 1.79 A and 1.86 A, respectively, which are in best agreement with our obtained results [29]. Also, according to Fig. 3, the calculated Eg from the DOS results for the B12N12, and Al12N12 nanocages are about 5.6 eV and 3.23 eV. These results are in good agreement with the results of previous studies that have been obtained under DFT calculations [31,32]. In another work, Fallahi and her coworkers clearly indicated that the band gap energy for B12N12, and Al12N12 nanocages were about 6.84 eV and 3.96 eV, respectively, which is in acceptable agreements with our results [15].
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As mentioned above, we also optimized the structures of Be12O12, Mg12O12, and Zn12O12 nanocages which are illustrated in Fig. 2. The BeeO, MgeO and ZneO bond length in square ring for Be12O12, Mg12O12, and Zn12O12 clusters are calculated equal to 1.66 A, 1.95 A and 2.02 A, respectively. Moreover, six-member ring bond length for Be12O12, Mg12O12, and Zn12O12 are equal to 1.59 A, 1.89 A and 1.94 A, respectively. The results obtained from optimized structure of Be12O12 are so close to results reported in our previous article. Mashhadzadeh and his co-workers optimized geometrical parameters of Be12O12 using DFT calculation. That article reported that the bond length between Be and O elements in square and six-member ring are about 1.662 A and 1.587 A, respectively, which is in best accordance with the current article’s results [33]. In another work Beheshtian et al. reported that the Be12O12 in optimized structure had a bond length about 1.52 A between two hexagons and 1.58 A between a hexagon and square which is in acceptable agreement with our results [34]. By comparing the results obtained from Mg12O12 with previous studies we found that bond length between square and six-member ring were about 1.95 A and 1.89 A which is in best agreements with our results [35]. The HOMOeLUMO band gap energies for Be12O12, Mg12O12, and Zn12O12 were obtained equal to 7.41 eV, 4.83 eV and 3.99 eV, respectively. These results are in good agreement with the results which were presented by other investigators who employed the B3LYP method in the GAMESS program [36e38]. Omidi et al. investigated the DOS of different conformers of B12O12 and Mg12O12 nanocages using DFT method with the B3LYP/6-31 þ G(d) basis set [39]. They found that the HOMOeLUMO band gaps for B12O12 and Mg12O12 nanocages are equal to 7.57 and 4.80 eV, respectively. Oliveira et al. has showed that the HOMOeLUMO band gap of Zn12O12 nanocage is equal to 4.04 eV at the DFT/B3LYP hybrid functional with the 6311G(d,p) basis set [40]. Kakemam et al. investigated electronic, energetic, and structural properties of Mg12O12 nano-cages and they calculated Eg of mentioned nano-cage from DOS plot about 4.86 eV [35]. Fallahi et al. investigated adsorption of Tabun molecule onto surface of some nanocages using DFT method and they reported that the HOMOeLUMO band gap energies for mg12O12 and Be12O12 were about 4.87 eV and 8.29 eV, respectively, which is in acceptable agreement with our results [15].
Fig. 1 e (a) The optimized structure and geometrical parameters of C24, and (b) corresponding calculated density of sates (DOS) (the Fermi level is set to zero). Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 2 e The optimized structure and geometrical parameters of (a) B12N12 and (b) Al12N12, (c) Be12O12, (d) Mg12O12, and (e) Zn12O12 clusters.
By comparing between DOS of studied cages it can be found that C24 has a lowest HOMOeLUMO band gap energy which it means that it has a highest conductivity behavior while among non-carbonic nanocages, Be12O12 has a highest Eg that puts it in the category of non-conductive nanocages. The rest of nanocages show semiconductor behavior.
Encapsulation of hydrogen molecule in nanocages To investigate the stability of the nH2/nanocage complexes we first examined the formation energies of H2 molecules inserted into nanocages. The formation energy and/or adsorption energy are calculated from following equation:
Erelax ¼ EnH2 =cage Ecage nEH2
(1)
where EnH2 =cage is the total energy of the nanocage with n confined hydrogen molecules where n represents the number of hydrogen molecules that were encapsulated i the nanoscale, Ecage is the total energy of the pure nanocage and EH2 is the total energy of the isolated hydrogen molecule. The calculated formation energies of nH2/nanocage complexes are displayed in Table 1. As it can be seen, the formation energies of C24 and B12N12 complexes are positive even with encapsulating just one hydrogen molecule, which shows that these complexes are not thermodynamically stable. Also, we can encapsulate one hydrogen molecule inside the Be12O12
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 3 e Calculated density of sates (DOS) of (a) B12N12, (b) Al12N12, (c) Be12O12, (d) Mg12O12 and (e) Zn12O12 clusters (the Fermi level is set to zero).
Table 1 e Calculated formation energies of nH2/nanocage complexes. Formation energy of nH2/nanocage (eV/H2 molecule)
nanocage
C24 B12N12 Al12N12 Be12O12 Mg12O12 Zn12O12
1H2
2H2
3H2
4H2
5H2
6H2
7H2
þ0.93 þ0.91 0.93 0.47 1.22 1.36
þ2.15 þ3.29 þ0.16 þ0.82 0.58 0.52
þ2.83 þ4.03 þ0.64 þ1.46 0.26 0.20
þ3.16 þ4.41 þ0.88 þ1.77 0.12 0.05
þ3.79 e e e þ0.11 þ0.19
e e e e e þ0.37
e e e e e e
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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and Al12N12 nanocages achieving negative formation energies while, these structures are not thermodynamically stable for n > 1 similar to what had already been obtained for C24 and B12N12. Our results also reveal that, up to four hydrogen molecules can be encapsulated inside the Mg12O12 and Zn12O12 cages without destroying the stability of these structures (negative formation energies regarding to Table 1). It is also observed that, encapsulating the fifth hydrogen molecule or more makes those structures metastable due to the positive formation energies. Therefore, among all considered nanocages, Mg12O12 and Zn12O12 nanocages which can form a stable complex with up to four hydrogen molecules are expected to be more promising materials for hydrogen storage than other mentioned nanocages. Fig. 4 represents the optimized geometrical structures of nH2/C24 for n ¼ 1, 2, 5 and 6. From this figure it can be seen that, when less than five hydrogen molecules were incorporated into this cage, only one hydrogen molecule exist in molecular form, and a new bond between hydrogen and inner
surface of the capsule was formed. In other words, except one molecule of hydrogen that highlighted with yellow color in Fig. 4, the HeH bond in H2 molecule was broken and each hydrogen atom has created a new bond with the inner surface of the cage. Hence, the length of the C]C double bond length increased to 1.58 A in 6-MR and to 1.63 A in 4-MR. These changes in the bond length indicate that the C]C double bond was transformed into a CeC single bond. When six hydrogen molecules were encapsulated inside the cage, the capsule side wall broke and some hydrogen molecules escaped from the cage. Thus, this complex seems to be highly improbable. The obtained results show that for the other considered complexes (other than C24), the hydrogen molecules preserved their molecular form after being inserted into the cage, and no new bonds were formed between hydrogen molecules and the side-wall of the nanocapsule. Our calculations also show that, when nH2 molecules were incorporated into the cage, the bond length of HeH is almost the same as an isolated H2 molecule (0.76 A). For instance, the optimized structures of
Fig. 4 e The optimized structure and geometrical parameters of nH2/C24 for n¼ (a) 1, (b) 2, (c) 5 and (d) 6. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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6H2/Zn12O12 and 5H2/Mg12O12 complexes are presented in Fig. 5. Furthermore, it is found that, the distance between the atom H of the hydrogen molecule and the nearest atoms of the nanocage (dX-H) obviously decreased. For example, the XeH A while distance for 1H2/Zn12O12 complex is equal to 2.61 this distance falls to 2.1 A with increasing in the number of encapsulated H2 molecules to six in this cage (6H2/Zn12O12 complex). This reduction of the average XeH distance is due to this point that, hydrogen density and pressure are higher in the complexes with more number of hydrogen atoms than those with fewer n. Our calculations, eventually depicted that, we can encapsulate maximum five hydrogen molecules in the B12N12, Al12N12 and Be12O12 cages, while Mg12O12 and Zn12O12 nanocages are capable of storing six and seven hydrogen molecules inside them, respectively. According to above results, it can be concluded that, Mg12O12 and Zn12O12 could be the best nanocages for hydrogen storage. In order to study the sustainability and mechanism of Mg12O12 and Zn12O12 nanocages opening with increase of the confined H2 molecules, we have carried out DFT-MD simulations on the optimized structures of 6H2/Mg12O12 and 7H2/ Zn12O12, which are put in contact with a thermostat at room temperature. Our results on 7H2/Zn12O12 nanocages, show that this complex is stable at the room temperature, 1 bar pressure and also without any break in the wall of nanocage after 5000 fs However, when the pressure is increased to 70 bar, it can be seen that after 1073 fs, one hydrogen atom of one of the molecules is connected to oxygen atom and the other one approaches the Zn atom. This results to breakage of the Zn12O12 nanocage (Fig. 6(a)). The calculated distance between the hydrogen molecule and the nearest atoms in the A. Therefore, Zn12O12 nanocage is in the range of 2.17e2.74 after the wall breakage, the internal pressure that exists among hydrogen molecules pushes them to escape from the capsule. The optimized structure of the broken 7H2/Zn12O12 nanocages after 1800 fs is illustrated in Fig. 6(b). As a result, Zn12O12 nanocage can accommodate seven hydrogen molecules at room temperature and atmosphere pressure, while in pressure 70 bar this nanocage releases six of the stored hydrogen molecules from itself. Moreover, our DFT-MD calculations results on 6H2/Mg12O12 nanocages at room temperature and in pressure of 1 bar show that after 359 fs one
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hydrogen molecule forms covalent bonds with the Mg and O atoms of the cage which leads to wall breakage. Afterward, up to 1500 fs the rest of the hydrogen molecules escape from the cage. The snapshots of DFT-MD simulations of 6H2/Mg12O12 structure after 359 and 1500 fs are illustrated in Fig. 7. As a consequence, the best of the above nanocages for hydrogen storage is Zn12O12 and the hydrogen storage capacities of this nanocapsules is estimated to be 1.4 wt%.
Mechanical properties of nanocages In this section, we calculated the bulk modulus of (C24, B12N12, Al12N12, Be12O12, Mg12O12 and Zn12O12) to evaluate their hardness. Similar calculation procedures also have been performed for the complexes with nH to estimate the effect of hydrogen storage on the mechanical properties of the mentioned nanocages. Because the nanocages have a spherical structure, the bulk modulus of these systems can be calculated by the conventional definition, which involves the second derivative of the total energy, Etot, of the cage with respect to its volume, V, according to the following equation: Ym ¼
1 v2 E V v2 V
(2)
The results that were obtained from investigating the bulk modulus of the different types of nanocages are listed in Table 2. With reference to these results, the orders of bulk modulus values in both pristine and full hydrogen encapsulated form of nanocages are as follows: C24 > B12N12 > Zn12O12 > Be12O12 > Mg12O12 > Al12N12. As it is seen, the number of encapsulated hydrogen molecules of B12N12 and Zn12O12 is much more than other studied nanocages as well as their bulk modulus. Therefore, according to the above results, Zn12O12 is superior to B12N12, due to higher number of stored hydrogen molecules.
Adsorption of hydrogen molecule on nanocages Here, we considered the adsorption of hydrogen molecule on the surface of each semiconductor nanocage. For this purpose, we selected different positions on the surface of nanocages to determine the favored position with a minimum
Fig. 5 e The optimized structure and geometrical parameters (a) 6H2/Zn12O12 and (b) 5H2/Mg12O12 complexes. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 6 e The structures of the of 7H2/Zn12O12 at the room temperature and 70 bar pressure after (a) 1073 and (b) 1800 femtosecond.
Fig. 7 e The schematic of the of 6H2/Mg12O12 at room temperature and 1 bar pressure after (a) 359 and (b) 1500 femtosecond.
Table 2 e Calculated bulk modulus of pure nanocages and H2/nanocage complexes. nanocage C24 B12N12 Al12N12 Be12O12 Mg12O12 Zn12O12
Bulk modulus (GPa)
H2/ nanocage
Bulk modulus (GPa)
774.14 710.31 327.92 455.17 416.07 686.10
5H2/C24 4H2/B12N12 4H2/Al12N12 4H2/Be12O12 5H2/Mg12O12 6H2/Zn12O12
691.80 665.07 323.93 434.76 400.33 631.27
total energy for a hydrogen molecule adsorption. After full structural optimization we found that, the hydrogen molecule tends to be adsorbed on the surface of 4-membered ring of nanocages. The calculated results show that, the adsorption of hydrogen molecule on the surface of nanocages follows the following trend: B12N12 (0.97 eV) < C24 (1.03 eV) < Be12O12 (1.23 eV) < A12N12 (1.36 eV) < Mg12O12 (1.41 eV) < Zn12O12 (1.54 eV). All these binding energies are indicative of chemisorption of hydrogen molecule. Our results also reveal that, the adsorption of hydrogen molecule on Zn12O12 is stronger
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 8 e The optimized structure and geometrical parameters of adsorbed hydrogen molecule on the surface of the (a) C24, (b) Al12N12, (c) Mg12O12, and (d) Zn12O12 nanocages.
than the other considered nanocages. Furthermore, the equilibrium distance between hydrogen and the nearest atom of the surface of C24, B12N12, A12N12, Be12O12, Mg12O12 and Zn12O12 are equal to 3.30 A, 3.35 A, 2.94 A, 3.08 A, 2.88 A, and
2.70 A, respectively. Thus, the order of binding energies for other semiconductors totally agrees with the order of equilibrium distances. The optimized structure and geometry parameters of adsorbed hydrogen molecule on the surface of the
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 9 e Calculated density of sates (DOS) of (a) H2eC24, (b) H2-A12N12, (c) H2eMg12O12 and (d) H2e Zn12O12 (the Fermi level is set to zero).
C24, Al12N12, Mg12O12 and Zn12O12 nanocages are depicted in Fig. 8. Finally, The Mulliken charge analysis estimated that 0.007, 0.031, 0.047 and 0.066 electrons were transferred from the hydrogen molecule to the C24, A12N12, Mg12O12 and Zn12O12 nanocages, respectively. Our results show that the amount of adsorption energy of hydrogen molecule has a significant effect on the electronic charge transfer. To understand the electronic properties of H2/nanocages complex better, we used DOS calculations. The DOS plot of the mentioned relaxed complexes that are shown in Fig. 9. Fig. 9a indicates the DOS calculations for H2/C24 complex. From this figure it can be simply seen that H2 affects the electronic properties of the C24 slightly and the HOMOeLUMO band gap energy does not change. So the electrical conductivity will not vary and the baer C24 cannot detect the H2 molecules. Low adsorption energy (1.03 eV) and Mulliken charge transfer (0.007 e) that reported in previous section confirm this. Fig. 9b, 9c indicate the DOS calculations for the H2/Al12N12 and H2/
Mg12O12 complexes. It can be seen that the DOS of the H2/ A12N12 and H2/Mg12O12 complexes are shifted around 0.18 eV and 1.10 eV to the higher energy levels in comparison to the related nanocages. Fig. 9d shows the DOS plot for the H2/ Zn12O12 complex. The nature of the H2/Zn12O12 complex is completely changed to a zero band gap metallic system. The high adsorption energy (1.54 eV) and Mulliken charge transfer (0.066 e) between them confirm this variation.
Conclusion In this paper we used density functional theory calculations to investigate the hydrogen storage capacity as well as adsorption properties of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages. After full geometrical optimization and DOS calculation of all nanocages using DFT calculations which were done to determine the bond length and band gap energies of these nanoarchitectures, formation energies of
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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nH2/nanocage complexes were calculated to consider the hydrogen storage capacity of them and we found that two nanostructures including C24 and B12N12 were unstable even with incorporating only one H2 molecule due to positive formation energies (þ0.93 and þ 0.91 eV/H2 molecule), the formation energies of Be12O12 and Al12N12 were obtained 0.47 and 0.93 eV/H2 molecule with adding one H2 molecule and for n > 1 these complexes became unstable, while Mg12O12 and Zn12O12 could store up to 4 hydrogen molecules due to negative calculated formation energies and these nanostructures become unstable with incorporating the fifth H2 molecule. Then, the bulk modulus of nH2/nanocage were determined using DFT and the results showed that this property decreased for all mentioned complexes after encapsulating different numbers of hydrogen molecules inside them. Although the B12N12, Al12N12 and Be12O12 could store up to maximum five H2 molecules inside them, C24, Mg12O12 and Zn12O12 were found to store six, six and seven hydrogen molecules respectively without becoming unstable. Furthermore, the adsorption properties of these nanocages were considered using DFT and the results showed that Zn12O12 was a stronger adsorbent for H2 in comparison to all other considered nanocages.
references
[1] Garland NL, Papageorgopoulos DC, Stanford JM. Hydrogen and fuel cell technology: progress, challenges, and future directions. Energy Procedia 2012;28:2e11. [2] Yang J, et al. High capacity hydrogen storage materials: attributes for automotive applications and techniques for materials discovery. Chem Soc Rev 2010;39(2):656e75. [3] Jena P. Materials for hydrogen storage: past, present, and future. J Phys Chem Lett 2011;2(3):206e11. [4] Qi P, Zhang Y, Chen H. Li-coated B36N24 as potential hydrogen storage material. Comput. Theor. Chem. 2014;1047:30e7. [5] Mandal TK, Gregory DH. Hydrogen storage materials: present scenarios and future directions. Annu. Rep. Sect. (Inorg. Chem) 2009;105:21e54. 0. n CO. Materials for hydrogen [6] van den Berg AWC, Area storage: current research trends and perspectives. Chem Commun 2008;(6):668e81. [7] Tang C, et al. Catalyzed collapse and enhanced hydrogen storage of BN nanotubes. J Am Chem Soc 2002;124(49):14550e1. [8] Koi N, Oku T. Hydrogen storage in boron nitride and carbon clusters studied by molecular orbital calculations. Solid State Commun 2004;131(2):121e4. [9] Wang Q, et al. Potential of AlN nanostructures as hydrogen storage materials. ACS Nano 2009;3(3):621e6. [10] Oku T, Kuno M, Narita I. Hydrogen storage in boron nitride nanomaterials studied by TG/DTA and cluster calculation. J Phys Chem Solids 2004;65(2):549e52. [11] Han Y-J, Park S-J. Influence of nickel nanoparticles on hydrogen storage behaviors of MWCNTs. Appl Surf Sci 2017;415:85e9. [12] Mandal S, et al. Controlling the band gap energy of clusterassembled materials. Acc Chem Res 2013;46(11):2385e95. [13] Yong Y, Song B, He P. Growth pattern and electronic properties of cluster-assembled material based on Zn12O12: a density-functional study. J Phys Chem C 2011;115(14):6455e61.
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[14] Jouypazadeh H, Farrokhpour H. DFT and TD-DFT study of the adsorption and detection of sulfur mustard chemical warfare agent by the C24, C12Si12, Al12N12, Al12P12, Be12O12, B12N12 and Mg12O12 nanocages. J Mol Struct 2018;1164:227e38. [15] Fallahi P, Jouypazadeh H, Farrokhpour H. Theoretical studies on the potentials of some nanocages (Al12N12, Al12P12, B12N12, Be12O12, C12Si12, Mg12O12 and C24) on the detection and adsorption of Tabun molecule: DFT and TDDFT study, vol. 260; 2018. [16] Orimo S-i, et al. Complex hydrides for hydrogen storage. Chem Rev 2007;107(10):4111e32. [17] Song B, et al. Design of cluster structure units with large surface areas for high-capacity hydrogen storage: in the case of Si12C12H24. Int J Hydrogen Energy 2017;42(31):20003e15. [18] Lu QL, et al. Alkali and alkaline-earth atom-decorated B38 fullerenes and their potential for hydrogen storage. Int J Hydrogen Energy 2015;40(38):13022e8. [19] Wang G, et al. High-capacity hydrogen storage in Lidecorated (AlN)n (n ¼ 12, 24, 36) nanocages 2014;39:3780e9. [20] Ren H, et al. A DFT study of the hydrogen storage potentials and properties of Na- and Li-doped fullerenes. Int J Hydrogen Energy 2017;42(1):312e21. [21] Dong H, et al. New Ti-decorated B40 fullerene as a promising hydrogen storage material. Sci Rep 2015;5(1):9952. [22] Ganji MD. First-principles simulation of the encapsulation of molecular hydrogen in C120 nanocapsules. Phys E Lowdimens Syst Nanostruct 2009;41(8):1433e8. [23] Wang G, et al. High-capacity hydrogen storage in Lidecorated (AlN)n (n ¼ 12, 24, 36) nanocages. Int J Hydrogen Energy 2014;39(8):3780e9. [24] Sayhan S, Kinal A. Computational investigation and comparison of hydrogen storage properties of B24N24 and Al24N24 nanocages. Int J Hydrogen Energy 2017;42(20):14166e80. [25] Ye L-H, et al. Ab initio phonon dispersions of single-wall carbon nanotubes. Phys Rev B 2004;69(23):235409. [26] Lazar P, Redinger J, Podloucky R. Density functional theory applied to VN∕TiN multilayers, vol. 76; 2013. M Soler EA, Gale Julian D, Garcı´a Alberto, [27] Jose n Pablo, Sa nchez-Portal Daniel. The Junquera Javier, Ordejo SIESTA method for ab initio order-N materials simulation. J Phys Condens Matter 2002;14(11):2745. [28] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77(18):3865e8. [29] Tahmasebi E, Shakerzadeh E, Biglari Z. Theoretical assessment of the electro-optical features of the group III nitrides (B12N12, Al12N12 and Ga12N12) and group IV carbides (C24, Si12C12 and Ge12C12) nanoclusters encapsulated with alkali metals (Li, Na and K). Appl Surf Sci 2016;363:197e208. [30] Chang Y, et al. In: Geometry and stability of fullerene cages: C24 to C70, vol. 105; 2005. p. 142e7. [31] Esrafili MD, Nurazar R. A density functional theory study on the adsorption and decomposition of methanol on B12N12 fullerene-like nanocage. Superlattice Microstruct 2014;67:54e60. [32] Rad AS, Ayub K. Coordination of nickel atoms with Al12X12 (X ¼ N, P) nanocages enhances H2 adsorption: a surface study by DFT. Vacuum 2016;133:70e80. [33] Hamed Mashhadzadeh A, et al. Theoretical studies on the mechanical and electronic properties of 2D and 3D structures of Beryllium-Oxide graphene and graphene nanobud. Appl Surf Sci 2019;476:36e48. [34] Beheshtian J, Ravaei I. Hydrogen storage by BeO nano-cage: a DFT study. Appl Surf Sci 2016;368:76e81.
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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[35] Kakemam J, Peyghan AA. Electronic, energetic, and structural properties of C- and Si-doped Mg12O12 nanocages. Comput Mater Sci 2013;79:352e5. [36] Salimi H, Ahmadi Peyghan A, Noei M. Adsorption of formic acid and formate anion on ZnO nanocage. A DFT Study 2014;26. [37] Beheshtian J, Ravaei I. Hydrogen storage by BeO nano-cage: a DFT study, vol. 368; 2016. [38] Shamlouei HR, et al. Influence of transition metal atoms doping on structural, electronic and nonlinear optical
properties of Mg12O12 nanoclusters: a DFT study, vol. 77; 2015. [39] Omidi M, Shamlouei HR, Noormohammadbeigi M. The influence of Sc doping on structural, electronic and optical properties of Be12O12, Mg12O12 and Ca12O12 nanocages: a DFT study, vol. 23; 2017. [40] de Oliveira OV, et al. Computational studies of the Ca12O12, Ti12O12, Fe12O12 and Zn12O12 nanocage clusters. Chem Phys Lett 2015;634:25e8.
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages M. Ghorbanzadeh Ahangari a, A. Hamed Mashhadzadeh b,* a
Department of Mechanical Engineering, Faculty of Engineering and Technology, University of Mazandaran, Babolsar, Iran b Department of Mechanical Engineering, Azadshahr Branch,Islamic Azad Universty, Azadshahr, Iran
highlights H2 storage capacity and adsorption properties of nanocages studied using DFT method. Mg12O12 and Zn12O12 have highest capacity to storage hydrogen molecules. Zn12O12 is strongest adsorbent for H2 molecule.
article info
abstract
Article history:
In the current study, the density functional theory calculations (DFT) were employed to
Received 14 April 2019
determine the hydrogen storage properties of some nanoclusters including C24, B12N12, Al12
Received in revised form
N12, Be12O12, Mg12O12, and Zn12O12. After full geometrical optimization of all nanocages
14 December 2019
under the DFT framework, we found that C24 and B12N12 were unstable structures even in
Accepted 16 December 2019
case of incorporating only one hydrogen molecule to them due to positive obtained for-
Available online xxx
mation energy magnitudes while Al12N12 and Be12O12 were able to adsorb one hydrogen molecules and became thermodynamically unstable for more than one hydrogen mole-
Keywords:
cule. Also, Mg12O12 and Zn12O12 were capable of storing up to 4 hydrogen molecules ac-
DFT
cording to negative achieved formation energies. Also, calculated bulk modulus revealed
Nanocage
that when all studied structures stored H2 molecules the bulk modulus decreased
Hydrogen storage
compared to pristine nanoclusters. The highest reduction in bulk modulus was 10% which
Adsorption
occurred in C24 while storing 5H2. Furthermore, the adsorption properties of these nanocages were considered using DFT and the results showed that Zn12O12 was a stronger adsorbent for H2 in comparison to the rest of the studied nanocages. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail addresses: [email protected], [email protected] (A. Hamed Mashhadzadeh). https://doi.org/10.1016/j.ijhydene.2019.12.106 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Introduction Hydrogen has been introduced as a potential replacement for fossil fuels in sustainable energy systems due to its high energy density, great efficiency, low weight, and being environmentally friendly [1]. The development of appropriate systems for safe storage and movement of hydrogen has become a major challenge among researchers over the past few decades [2e4]. According to the technical targets that have been evolved by the U.S Department of Energy (DOE), hydrogen storage systems must meet some requirements regarding delivery temperature, kinetics, volumetric capacity and gravimetric capacity [2]. Since the conventional methods of storing hydrogen such as storage after being compressed or storing in liquid form showed some limitations in terms of safety and cost, focuses were turned to employ nano architectures for this purpose [5,6]. Nanomaterials including nanotubes, nanocapsules and nanocages have been reported to be suitable storage systems for hydrogen due to their superior electronic properties as well as their extended surface area [7e9] so a great deal of research works have focused on exploring the capability of such materials in adsorbing or storing hydrogen [9]. Takeo Oku et al., in their experimental study, investigated the hydrogen storage in Boron-Nitride (BN) nanomaterials including nanocapsules, nanotubes and nanocages that were synthesized from a mixture of LaB6, Pd and boron powder using arc melting method. Their results demonstrated that these nanostructures are capable of storage of up to 3 wt% hydrogen [10]. In another research work, Ye-Ji Han et al. considered the effect of adding nickel nanoparticles to MWCNT on its hydrogen storage properties and reported considerable improvement in this property in comparison to pristine MWCNT [11]. Besides, nanocages that are the frame-like hollow structures which include opening pores in their shells have been extensively employed in a variety of electronic and chemical industries applications especially storage and adsorption of hydrogen over the past few years due to their considerable advantages [12e15]. These nanostructures are able to adsorb hydrogen in reversible process. The low enthalpy of the adsorption process decreases the thermal management concerns and therefore the materials stability during the cycle would not be a problem [16,17]. Due to such capabilities, nanoclusters like fullerenes, Boron-Nitride, Aluminum-Nitride, and Metal-oxide nanocages have been introduced as appropriate hydrogen storage media [18e21]. In this regard, it has been reported that, up to eight hydrogen molecules could be incorporated to C120 nanocapsule forming a stable complex [22]. Up to 38 hydrogen molecules were stored in B60N60 nanocage as reported by Koi et al. [8]. Qi et al. employed density functional theory study (DFT) to investigate the capacity of Licoated B36N24 nanocluster as a hydrogen storage structure and found that each Li atom in this structure can adsorb two hydrogen molecules [4]. Wang et al. considered the hydrogen storage capacity of Li decorated (AlN)n nanocages under DFT frame work and reported the gravity density of storage equal to 4.7 wt% [23]. Sayhan and Kinal selected DFT calculations to make a comparison between the storage capacity of B24N24 and
Al24N24 nanocages. Their results demonstrated a tighter binding between H2 and Al in Al24N24 than B24N24 [24]. The main aim of the current study was to make a direct comparison between the hydrogen storage and adsorption properties of some potential nanoclusters through a DFT- MD based framework. With this purpose, the DFT calculations have been employed to consider the hydrogen storage and adsorption capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages. At first, these structures were geometrically optimized under the DFT framework and Density of States (DOS) was calculated for all of these nanocages to make a comprehensive understanding of the electronic properties of studied nanoclusters. Then, the stability of each structure was considered by incorporating different numbers of hydrogen molecules in them and comparisons were made among them according to the obtained formation energies. After the storage consideration, we investigated the effect of storage on, bulk modulus of all studied nanocages and finally, the adsorption properties of these nano architectures toward hydrogen molecule were investigated using density functional theory calculations.
Computational methods Density functional theory calculations have been vastly employed in solid-state physics over the past few decades. The results that have been obtained from this technique are usually in good agreement with those of experimental research [25,26]. Our calculations based on the adsorption and storage of the hydrogen molecules on and in the nanocages were performed within the framework of first-principles DFT implemented using the Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) code [27]. We adopted the generalized gradient approximation (GGA) to treat the electronic exchange and correlation effects, as described by Perdew-Burke-Ernzerhof [28]. All calculations were performed with a double-z basis set of localized numerical atomic orbitals, including polarization functions (DZP), with an energy shift of 50 meV and a split norm of 0.15. Structural relaxations were performed using the conjugate gradient algorithm until the residual forces on each atom were lower than 0.02 eV A1. The mesh cutoff, an energy that corresponds to the grid spacing, was selected as 120 Ry. We have also performed DFT-based MD simulations for the system under study. The systems were anneals to specified pressure and temperature. All MD simulations were performed in a NPT ensemble (constant Number of atoms, Pressure and Temperature) with a specified temperature of 300 K at the constant pressure of 1e70 bar using the anneal thermostat. The simulation time duration of the molecular dynamics simulation was 5 ps, and the time step was 1 fs.
Results and discussions Optimization of pristine nanocages Geometrical optimization of pure C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 was investigated using DFT calculations.
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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In a structural view, these clusters are formed from six 4membered ring (4-MR) and eight 6-membered ring (6-MR) with Th symmetries. After full structural optimization of C24 nanocage, the C]C bond length in 4-MR and 6-MR were obtained equal to 1.49 A and 1.38 A respectively. These obtained results are in best agreement with those that Tahmasebi and her co-workers reported for the bond length in 4-MR and 6-MR A and 1.37 A, for C24 cage which were reported equal to 1.49 respectively [29]. Furthermore, we calculated the electronic density of states for C24 and found the HOMOeLUMO band gap energy (Eg) of 1.81 eV for this nanocage These results are comparable with the previous B3LYP/6-31G* computational level that was investigated by Chang et al. [30]. They reported A and about the average CeC bond length and Eg equal to 1.45 1.8 eV successively. Fig. 1 indicates the optimized structure and corresponding DOS of C24 cage. In this section the structures of all mentioned clusters, were fully optimized and then the DOSs were calculated. The optimized structures and the obtained DOS results are illustrated in Figs. 1 and 2, respectively. Two topologically nonequivalent BeN and AleN bonds are presented in B12N12 and Al12N12 clusters; one is shared by two 6-MRs rings (d6MR), and the other is shared by a 4-MR and a 6-MR (d4MR). The d6MR and d4MR in B12N12, and Al12N12 nanocages are equal to 1.44 A and 1.49 A, and 1.81 A and 1.88 A, respectively. Tahmasebi et al. investigated electro-optical features of the group IIInitrides and group IV carbides nanocluster encapsulated with alkali metals using DFT calculation and they reported that d6MR and d4MR in B12N12, and Al12N12 nanocages were equal to 1.44 A and 1.49 A, 1.79 A and 1.86 A, respectively, which are in best agreement with our obtained results [29]. Also, according to Fig. 3, the calculated Eg from the DOS results for the B12N12, and Al12N12 nanocages are about 5.6 eV and 3.23 eV. These results are in good agreement with the results of previous studies that have been obtained under DFT calculations [31,32]. In another work, Fallahi and her coworkers clearly indicated that the band gap energy for B12N12, and Al12N12 nanocages were about 6.84 eV and 3.96 eV, respectively, which is in acceptable agreements with our results [15].
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As mentioned above, we also optimized the structures of Be12O12, Mg12O12, and Zn12O12 nanocages which are illustrated in Fig. 2. The BeeO, MgeO and ZneO bond length in square ring for Be12O12, Mg12O12, and Zn12O12 clusters are calculated equal to 1.66 A, 1.95 A and 2.02 A, respectively. Moreover, six-member ring bond length for Be12O12, Mg12O12, and Zn12O12 are equal to 1.59 A, 1.89 A and 1.94 A, respectively. The results obtained from optimized structure of Be12O12 are so close to results reported in our previous article. Mashhadzadeh and his co-workers optimized geometrical parameters of Be12O12 using DFT calculation. That article reported that the bond length between Be and O elements in square and six-member ring are about 1.662 A and 1.587 A, respectively, which is in best accordance with the current article’s results [33]. In another work Beheshtian et al. reported that the Be12O12 in optimized structure had a bond length about 1.52 A between two hexagons and 1.58 A between a hexagon and square which is in acceptable agreement with our results [34]. By comparing the results obtained from Mg12O12 with previous studies we found that bond length between square and six-member ring were about 1.95 A and 1.89 A which is in best agreements with our results [35]. The HOMOeLUMO band gap energies for Be12O12, Mg12O12, and Zn12O12 were obtained equal to 7.41 eV, 4.83 eV and 3.99 eV, respectively. These results are in good agreement with the results which were presented by other investigators who employed the B3LYP method in the GAMESS program [36e38]. Omidi et al. investigated the DOS of different conformers of B12O12 and Mg12O12 nanocages using DFT method with the B3LYP/6-31 þ G(d) basis set [39]. They found that the HOMOeLUMO band gaps for B12O12 and Mg12O12 nanocages are equal to 7.57 and 4.80 eV, respectively. Oliveira et al. has showed that the HOMOeLUMO band gap of Zn12O12 nanocage is equal to 4.04 eV at the DFT/B3LYP hybrid functional with the 6311G(d,p) basis set [40]. Kakemam et al. investigated electronic, energetic, and structural properties of Mg12O12 nano-cages and they calculated Eg of mentioned nano-cage from DOS plot about 4.86 eV [35]. Fallahi et al. investigated adsorption of Tabun molecule onto surface of some nanocages using DFT method and they reported that the HOMOeLUMO band gap energies for mg12O12 and Be12O12 were about 4.87 eV and 8.29 eV, respectively, which is in acceptable agreement with our results [15].
Fig. 1 e (a) The optimized structure and geometrical parameters of C24, and (b) corresponding calculated density of sates (DOS) (the Fermi level is set to zero). Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 2 e The optimized structure and geometrical parameters of (a) B12N12 and (b) Al12N12, (c) Be12O12, (d) Mg12O12, and (e) Zn12O12 clusters.
By comparing between DOS of studied cages it can be found that C24 has a lowest HOMOeLUMO band gap energy which it means that it has a highest conductivity behavior while among non-carbonic nanocages, Be12O12 has a highest Eg that puts it in the category of non-conductive nanocages. The rest of nanocages show semiconductor behavior.
Encapsulation of hydrogen molecule in nanocages To investigate the stability of the nH2/nanocage complexes we first examined the formation energies of H2 molecules inserted into nanocages. The formation energy and/or adsorption energy are calculated from following equation:
Erelax ¼ EnH2 =cage Ecage nEH2
(1)
where EnH2 =cage is the total energy of the nanocage with n confined hydrogen molecules where n represents the number of hydrogen molecules that were encapsulated i the nanoscale, Ecage is the total energy of the pure nanocage and EH2 is the total energy of the isolated hydrogen molecule. The calculated formation energies of nH2/nanocage complexes are displayed in Table 1. As it can be seen, the formation energies of C24 and B12N12 complexes are positive even with encapsulating just one hydrogen molecule, which shows that these complexes are not thermodynamically stable. Also, we can encapsulate one hydrogen molecule inside the Be12O12
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 3 e Calculated density of sates (DOS) of (a) B12N12, (b) Al12N12, (c) Be12O12, (d) Mg12O12 and (e) Zn12O12 clusters (the Fermi level is set to zero).
Table 1 e Calculated formation energies of nH2/nanocage complexes. Formation energy of nH2/nanocage (eV/H2 molecule)
nanocage
C24 B12N12 Al12N12 Be12O12 Mg12O12 Zn12O12
1H2
2H2
3H2
4H2
5H2
6H2
7H2
þ0.93 þ0.91 0.93 0.47 1.22 1.36
þ2.15 þ3.29 þ0.16 þ0.82 0.58 0.52
þ2.83 þ4.03 þ0.64 þ1.46 0.26 0.20
þ3.16 þ4.41 þ0.88 þ1.77 0.12 0.05
þ3.79 e e e þ0.11 þ0.19
e e e e e þ0.37
e e e e e e
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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and Al12N12 nanocages achieving negative formation energies while, these structures are not thermodynamically stable for n > 1 similar to what had already been obtained for C24 and B12N12. Our results also reveal that, up to four hydrogen molecules can be encapsulated inside the Mg12O12 and Zn12O12 cages without destroying the stability of these structures (negative formation energies regarding to Table 1). It is also observed that, encapsulating the fifth hydrogen molecule or more makes those structures metastable due to the positive formation energies. Therefore, among all considered nanocages, Mg12O12 and Zn12O12 nanocages which can form a stable complex with up to four hydrogen molecules are expected to be more promising materials for hydrogen storage than other mentioned nanocages. Fig. 4 represents the optimized geometrical structures of nH2/C24 for n ¼ 1, 2, 5 and 6. From this figure it can be seen that, when less than five hydrogen molecules were incorporated into this cage, only one hydrogen molecule exist in molecular form, and a new bond between hydrogen and inner
surface of the capsule was formed. In other words, except one molecule of hydrogen that highlighted with yellow color in Fig. 4, the HeH bond in H2 molecule was broken and each hydrogen atom has created a new bond with the inner surface of the cage. Hence, the length of the C]C double bond length increased to 1.58 A in 6-MR and to 1.63 A in 4-MR. These changes in the bond length indicate that the C]C double bond was transformed into a CeC single bond. When six hydrogen molecules were encapsulated inside the cage, the capsule side wall broke and some hydrogen molecules escaped from the cage. Thus, this complex seems to be highly improbable. The obtained results show that for the other considered complexes (other than C24), the hydrogen molecules preserved their molecular form after being inserted into the cage, and no new bonds were formed between hydrogen molecules and the side-wall of the nanocapsule. Our calculations also show that, when nH2 molecules were incorporated into the cage, the bond length of HeH is almost the same as an isolated H2 molecule (0.76 A). For instance, the optimized structures of
Fig. 4 e The optimized structure and geometrical parameters of nH2/C24 for n¼ (a) 1, (b) 2, (c) 5 and (d) 6. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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6H2/Zn12O12 and 5H2/Mg12O12 complexes are presented in Fig. 5. Furthermore, it is found that, the distance between the atom H of the hydrogen molecule and the nearest atoms of the nanocage (dX-H) obviously decreased. For example, the XeH A while distance for 1H2/Zn12O12 complex is equal to 2.61 this distance falls to 2.1 A with increasing in the number of encapsulated H2 molecules to six in this cage (6H2/Zn12O12 complex). This reduction of the average XeH distance is due to this point that, hydrogen density and pressure are higher in the complexes with more number of hydrogen atoms than those with fewer n. Our calculations, eventually depicted that, we can encapsulate maximum five hydrogen molecules in the B12N12, Al12N12 and Be12O12 cages, while Mg12O12 and Zn12O12 nanocages are capable of storing six and seven hydrogen molecules inside them, respectively. According to above results, it can be concluded that, Mg12O12 and Zn12O12 could be the best nanocages for hydrogen storage. In order to study the sustainability and mechanism of Mg12O12 and Zn12O12 nanocages opening with increase of the confined H2 molecules, we have carried out DFT-MD simulations on the optimized structures of 6H2/Mg12O12 and 7H2/ Zn12O12, which are put in contact with a thermostat at room temperature. Our results on 7H2/Zn12O12 nanocages, show that this complex is stable at the room temperature, 1 bar pressure and also without any break in the wall of nanocage after 5000 fs However, when the pressure is increased to 70 bar, it can be seen that after 1073 fs, one hydrogen atom of one of the molecules is connected to oxygen atom and the other one approaches the Zn atom. This results to breakage of the Zn12O12 nanocage (Fig. 6(a)). The calculated distance between the hydrogen molecule and the nearest atoms in the A. Therefore, Zn12O12 nanocage is in the range of 2.17e2.74 after the wall breakage, the internal pressure that exists among hydrogen molecules pushes them to escape from the capsule. The optimized structure of the broken 7H2/Zn12O12 nanocages after 1800 fs is illustrated in Fig. 6(b). As a result, Zn12O12 nanocage can accommodate seven hydrogen molecules at room temperature and atmosphere pressure, while in pressure 70 bar this nanocage releases six of the stored hydrogen molecules from itself. Moreover, our DFT-MD calculations results on 6H2/Mg12O12 nanocages at room temperature and in pressure of 1 bar show that after 359 fs one
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hydrogen molecule forms covalent bonds with the Mg and O atoms of the cage which leads to wall breakage. Afterward, up to 1500 fs the rest of the hydrogen molecules escape from the cage. The snapshots of DFT-MD simulations of 6H2/Mg12O12 structure after 359 and 1500 fs are illustrated in Fig. 7. As a consequence, the best of the above nanocages for hydrogen storage is Zn12O12 and the hydrogen storage capacities of this nanocapsules is estimated to be 1.4 wt%.
Mechanical properties of nanocages In this section, we calculated the bulk modulus of (C24, B12N12, Al12N12, Be12O12, Mg12O12 and Zn12O12) to evaluate their hardness. Similar calculation procedures also have been performed for the complexes with nH to estimate the effect of hydrogen storage on the mechanical properties of the mentioned nanocages. Because the nanocages have a spherical structure, the bulk modulus of these systems can be calculated by the conventional definition, which involves the second derivative of the total energy, Etot, of the cage with respect to its volume, V, according to the following equation: Ym ¼
1 v2 E V v2 V
(2)
The results that were obtained from investigating the bulk modulus of the different types of nanocages are listed in Table 2. With reference to these results, the orders of bulk modulus values in both pristine and full hydrogen encapsulated form of nanocages are as follows: C24 > B12N12 > Zn12O12 > Be12O12 > Mg12O12 > Al12N12. As it is seen, the number of encapsulated hydrogen molecules of B12N12 and Zn12O12 is much more than other studied nanocages as well as their bulk modulus. Therefore, according to the above results, Zn12O12 is superior to B12N12, due to higher number of stored hydrogen molecules.
Adsorption of hydrogen molecule on nanocages Here, we considered the adsorption of hydrogen molecule on the surface of each semiconductor nanocage. For this purpose, we selected different positions on the surface of nanocages to determine the favored position with a minimum
Fig. 5 e The optimized structure and geometrical parameters (a) 6H2/Zn12O12 and (b) 5H2/Mg12O12 complexes. Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 6 e The structures of the of 7H2/Zn12O12 at the room temperature and 70 bar pressure after (a) 1073 and (b) 1800 femtosecond.
Fig. 7 e The schematic of the of 6H2/Mg12O12 at room temperature and 1 bar pressure after (a) 359 and (b) 1500 femtosecond.
Table 2 e Calculated bulk modulus of pure nanocages and H2/nanocage complexes. nanocage C24 B12N12 Al12N12 Be12O12 Mg12O12 Zn12O12
Bulk modulus (GPa)
H2/ nanocage
Bulk modulus (GPa)
774.14 710.31 327.92 455.17 416.07 686.10
5H2/C24 4H2/B12N12 4H2/Al12N12 4H2/Be12O12 5H2/Mg12O12 6H2/Zn12O12
691.80 665.07 323.93 434.76 400.33 631.27
total energy for a hydrogen molecule adsorption. After full structural optimization we found that, the hydrogen molecule tends to be adsorbed on the surface of 4-membered ring of nanocages. The calculated results show that, the adsorption of hydrogen molecule on the surface of nanocages follows the following trend: B12N12 (0.97 eV) < C24 (1.03 eV) < Be12O12 (1.23 eV) < A12N12 (1.36 eV) < Mg12O12 (1.41 eV) < Zn12O12 (1.54 eV). All these binding energies are indicative of chemisorption of hydrogen molecule. Our results also reveal that, the adsorption of hydrogen molecule on Zn12O12 is stronger
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 8 e The optimized structure and geometrical parameters of adsorbed hydrogen molecule on the surface of the (a) C24, (b) Al12N12, (c) Mg12O12, and (d) Zn12O12 nanocages.
than the other considered nanocages. Furthermore, the equilibrium distance between hydrogen and the nearest atom of the surface of C24, B12N12, A12N12, Be12O12, Mg12O12 and Zn12O12 are equal to 3.30 A, 3.35 A, 2.94 A, 3.08 A, 2.88 A, and
2.70 A, respectively. Thus, the order of binding energies for other semiconductors totally agrees with the order of equilibrium distances. The optimized structure and geometry parameters of adsorbed hydrogen molecule on the surface of the
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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Fig. 9 e Calculated density of sates (DOS) of (a) H2eC24, (b) H2-A12N12, (c) H2eMg12O12 and (d) H2e Zn12O12 (the Fermi level is set to zero).
C24, Al12N12, Mg12O12 and Zn12O12 nanocages are depicted in Fig. 8. Finally, The Mulliken charge analysis estimated that 0.007, 0.031, 0.047 and 0.066 electrons were transferred from the hydrogen molecule to the C24, A12N12, Mg12O12 and Zn12O12 nanocages, respectively. Our results show that the amount of adsorption energy of hydrogen molecule has a significant effect on the electronic charge transfer. To understand the electronic properties of H2/nanocages complex better, we used DOS calculations. The DOS plot of the mentioned relaxed complexes that are shown in Fig. 9. Fig. 9a indicates the DOS calculations for H2/C24 complex. From this figure it can be simply seen that H2 affects the electronic properties of the C24 slightly and the HOMOeLUMO band gap energy does not change. So the electrical conductivity will not vary and the baer C24 cannot detect the H2 molecules. Low adsorption energy (1.03 eV) and Mulliken charge transfer (0.007 e) that reported in previous section confirm this. Fig. 9b, 9c indicate the DOS calculations for the H2/Al12N12 and H2/
Mg12O12 complexes. It can be seen that the DOS of the H2/ A12N12 and H2/Mg12O12 complexes are shifted around 0.18 eV and 1.10 eV to the higher energy levels in comparison to the related nanocages. Fig. 9d shows the DOS plot for the H2/ Zn12O12 complex. The nature of the H2/Zn12O12 complex is completely changed to a zero band gap metallic system. The high adsorption energy (1.54 eV) and Mulliken charge transfer (0.066 e) between them confirm this variation.
Conclusion In this paper we used density functional theory calculations to investigate the hydrogen storage capacity as well as adsorption properties of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages. After full geometrical optimization and DOS calculation of all nanocages using DFT calculations which were done to determine the bond length and band gap energies of these nanoarchitectures, formation energies of
Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106
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nH2/nanocage complexes were calculated to consider the hydrogen storage capacity of them and we found that two nanostructures including C24 and B12N12 were unstable even with incorporating only one H2 molecule due to positive formation energies (þ0.93 and þ 0.91 eV/H2 molecule), the formation energies of Be12O12 and Al12N12 were obtained 0.47 and 0.93 eV/H2 molecule with adding one H2 molecule and for n > 1 these complexes became unstable, while Mg12O12 and Zn12O12 could store up to 4 hydrogen molecules due to negative calculated formation energies and these nanostructures become unstable with incorporating the fifth H2 molecule. Then, the bulk modulus of nH2/nanocage were determined using DFT and the results showed that this property decreased for all mentioned complexes after encapsulating different numbers of hydrogen molecules inside them. Although the B12N12, Al12N12 and Be12O12 could store up to maximum five H2 molecules inside them, C24, Mg12O12 and Zn12O12 were found to store six, six and seven hydrogen molecules respectively without becoming unstable. Furthermore, the adsorption properties of these nanocages were considered using DFT and the results showed that Zn12O12 was a stronger adsorbent for H2 in comparison to all other considered nanocages.
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Please cite this article as: Ghorbanzadeh Ahangari M, Hamed Mashhadzadeh A, Density functional theory based molecular dynamics study on hydrogen storage capacity of C24, B12N12, Al12 N12, Be12O12, Mg12O12, and Zn12O12 nanocages, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.106