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Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
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Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube Jessiel Siaron Gueriba a,*, Allan Abraham Bustria Padama d, Al Rey Villagracia a,c,e, Melanie David a,c, Nelson Arboleda Jr. a,b,c, Hideaki Kasai f,g a
Department of Physics, De La Salle University, 2401 Taft Avenue, Manila 0922, Philippines ~ an, Laguna, Philippines De La Salle University-Science and Technology Complex, Bin c Computational Materials Design Research Unit, CENSER, De La Salle University, 2401 Taft Avenue, Manila 0922, Philippines d ~ os, Laguna, Philippines Institute of Mathematical Sciences and Physics, University of the Philippines Los Ban e Centre for Advanced 2D Materials and Graphene Research Center, National University of Singapore, Block S16, Level 6, 6 Science Drive 2, 117546 Singapore f Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan g National Institute of Technology, Akashi, Japan b
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abstract
Article history:
Ab initio study on the viability of calcium decorated silicon carbide nanotube as a hydrogen
Received 16 December 2016
storage material was conducted. Calcium strongly adsorbs on silicon carbide nanotube
Received in revised form
(SiCNT) with a significant binding energy of 2.83 eV, thus calcium's low cohesive energy
8 March 2017
and strong binding with SiCNT may prevent Ca to form clusters with other adsorbates.
Accepted 9 March 2017
Bader charge analysis also revealed a charge transfer of 1.45e from Ca to SiCNT resulting to
Available online xxx
calcium's cationic state, which may induce charge polarization to a nearby molecule such as hydrogen. Hydrogen molecule was then allowed to interact with the calcium
Keywords:
adatom where it exhibited charge polarization, induced by the electric field from calcium's
Density functional theory
positive charge. This resulted to a significant binding energy of 0.22 eV for the first
Hydrogen storage
hydrogen molecule. Results reveal that Ca on SiCNT can hold up to 7 hydrogen molecules
Nanotube
and can be a promising candidate for a hydrogen storage material. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen storage is considered as the bottleneck in exhausting the full potential of hydrogen as an energy source for various applications. The reason for this is the challenge of meeting the criterion of a good storage medium-high capacity, fast kinetics, and favorable thermodynamics [1]. Researchers
thus acknowledge this need to investigate on possible storage materials for hydrogen, which led to several researches involving hydrogen interaction with numerous materials up until now [2e13]. Metal decorated nanostructures are of great interest to researchers recently because their properties can be modified by varying the type of metals to be used for a wide range of possible applications [14e25]. One of the most promising
* Corresponding author. E-mail address: [email protected] (J.S. Gueriba). http://dx.doi.org/10.1016/j.ijhydene.2017.03.057 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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applications for these metal decorated nanostructures is for hydrogen storage, where the metal decorations serve as the active sites for molecular hydrogen adsorption [26e38]. Transition metal decorations allow strong binding with H2 due to the interaction of their partially filled d orbital with the orbitals of hydrogen molecule. This interaction is reported as Kubas effect where the binding of hydrogen molecule with the metal decoration is enhanced via orbital interaction with 3d metals [37,38]. However, the clustering of these metal decorations due to their high cohesive energy and weaker binding with the nanostructure is an obstacle to maximize the potential of these materials for hydrogen storage application [39,40]. This reduces the storage capability of the system where dissociated hydrogen tends to chemically bond with the transition metal clusters which prevents other H2 to interact. To address this problem, the metal adsorbate should have a strong binding energy with the nanostructure and overcome its cohesive energy thus lowering the possibility of clustering. A study on lithium decorated charged CNT showed that a net charge on the nanostructure enhances the binding energy of the metal decoration as well as that of the hydrogen molecule attracted to it [27]. This implies that nanostructures with point charges on its surface may provide better properties for metal decorated systems. In this study we investigate on hydrogen interaction with Ca/SiCNT to determine its viability as a hydrogen storage material. Calcium was chosen as the metal decoration due to its lower cohesive energy than transition metals [66e68]. Computational studies suggest that calcium may be a superior coating for decorated nanostructures to enhance its storage capacity and avoid metal clustering [66,67]. Interestingly, it may also provide similar interaction as transition metals due to its empty 3d orbital which may hybridize with the molecular orbital of hydrogen and improve its binding energy with the metal decorated system. On the other hand, SiCNT was used as a platform for calcium decoration due to its reactive surface compared to other carbon based nanostructure [69]. This allows stronger binding with the calcium adatom as presented in this study. SiCNT was first synthesized in 2001 by Pham-Huu et al. using shape memory synthesis method and other researchers as well tried to produce the nanomaterial through different experimental techniques [41e49]. After its synthesis, computational studies of its physical and electronic properties emerged and the possibility of functionalization through metal decorations [38,50e54]. Molecular dynamics simulations suggest that the surface of silicon carbide nanotube may capture molecules, such as hydrogen, cholesterols and phospholipids, more effectively than carbon nanotube [55,56]. This reactivity is due to point charges on the surface of SiCNT. It was also proposed that SiCNT may be a suitable material for industrial devices for its thermal stability with melting temperatures ranging from 1600 K to 4265 K which varies with the radius of the nanotube [57,58]. Recently, an experimental study on hydrogen storage capability of both SiCNT and CNT were investigated and results suggest that SiCNT has superior hydrogen uptake and desorption capability than CNT [59]. However, it's low gravimetric capacity prevents its commercial application thus they recommend further studies about SiCNT and the possibility of improving its capacity through alkali metal decoration. These serve as
the foundation for the present study which may provide insights on how to improve current hydrogen storage systems and design possible materials, such as calcium decorated silicon carbide nanotube, for hydrogen storage application.
Computational details Spin Polarized DFT calculations were performed as implemented in the Vienna Ab initio Simulation Package (VASP) [62]. The electron exchange correlation functional was treated within generalized gradient approximation by PerdeweBurkeeErnzerhof [63]. Convergence tests for cut off energy and k-point sampling were implemented. Total energy of the system was calculated with increasing values of cut off energy from 200 eV to 600 eV with increments of 50 eV and kpoints along the z axis from 1 to 11. The total energy of the system converged for cut off energies higher than 500 eV, with an energy difference of less than 10 meV, thus the cut off energy for the basis set was chosen to be 550 eV. Varying the number of k-points along the z-axis showed a convergence in energy with difference of less than 1 meV from 7 to 11 kpoints, thus Monkhorst-Pack scheme with 1 1 8 special Kpoints was used for Brillouin zone sampling. The atomic relaxation was carried out until the Helmann-Feynman forces on the unconstrained atoms were less than 0.01 eV/ A. An 18 18 10.5 A3 supercell (Fig. 1) was used to model the (5,0) nanotube which encloses two unit cells of the system, consisting of 20 carbon and 20 silicon atoms, periodic along its axis of symmetry (z-axis). (5,0) chirality was chosen due to its surface curvature which was found to directly affect the reactivity of nanomaterials thus having a more reactive surface for atomic or molecular adsorption [36,60,61]. Van der Waals interaction was also taken into consideration to address the long range interaction of hydrogen with the metal decoration by implementing DFT-D2 method of Grimme in the calculation [64]. Binding energy was calculated as Eq. (1), where Etot, Esub and Ead are the total energy of the system, total energy of the substrate and total energy of the adsorbate, respectively.
3 supercell of (5,0) SiCNT, periodic Fig. 1 e 18 £ 18 £ 10 A along the z-axis enclosing 20 carbon (black) and 20 silicon (yellow) atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Eb ¼ Etot ðEsub þ Ead Þ
(1)
Frozen lattice calculation was implemented where the total energy of Ca/SiCNT is obtained as Ca approaches the nanotube at different high symmetry adsorption sites (Fig. 2). From an initial separation distance of 4 A away from the substrate, total energy calculation was performed for every increment of 0.5 A, moving towards the wall of SiCNT. A fully relaxed calculation was conducted after obtaining the preferred reaction coordinate for each site. The binding energy of calcium on SiCNT is determined using Eq. (1) where Esub is the energy of SiCNT, Ead is the calculated total energy of an isolated calcium atom and Etot is the energy of the Ca/SiCNT complex. Hydrogen molecule was placed on the surrounding sites of calcium. The center of mass of the hydrogen molecule was 2e2.5 A away from the center of calcium where relaxed calculations at each site were conducted. The average binding energy of H2 is calculated as Eq. (2), where EnH2 =Ca=SiCNT is the total energy of nH2 over Ca decorated SiCNT, ECa/SiCNT is the energy of isolated Ca decorated SiCNT, EH2 is the energy of the isolated hydrogen molecule and n is the number of adsorbed H2 molecules.
Fig. 2 e Adsorption sites of calcium on SiCNT. Hollow site: center of the hexagon formed by the SieC bonds. Si-top and C-top sites: directly above the silicon and carbon atoms respectively. Bridge site: above the midpoint of the SieC bond.
Eb ¼ EnH2 =Ca=SiCNT ECa=SiCNT þ nEH2 =n
(2)
It is assumed that the maximum number of adsorbed hydrogen molecule is attained if the added H2 no longer binds with the nH2/Ca complex as suggested by the optimized structure, where the binding energy of a single hydrogen molecule on the nH2/Ca complex can be calculated as Eq. (3). Eb ¼ EH2 =nH2 =Ca=SiCNT EnH2 =Ca=SiCNT þ EH2
(3)
Results and discussion Silicon carbide nanotube Structural optimization shows that (5,0) SiCNT is a narrowgap semiconductor with an SieC bond length of 1.79e1.80 A which is in good agreement with other studies [50e54]. Partial charge distribution (Fig. 3) was obtained by plotting the charge density of the electron states within the energy range of 0.2 eV to 0 eV and 0 eV to 1 eV, which represent the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the system, respectively. Results show that the lowest unoccupied molecular orbital is mainly contributed by the pz orbital of silicon, which makes SiCNT more reactive than carbon nanotube by having more electron acceptor states. This characteristic is due to the heteropolar nature of SiCNT where the occupied electron state distribution is localized at the carbon atoms having a higher electronegativity than silicon atoms. This unoccupied electron states may elicit more charge transfer from a metal atom resulting to a greater binding energy and a more positive residual charge from the metal decoration. This indicates that the system can easily facilitate sidewall decoration and SiCNT can be a good platform for metal adsorption.
Calcium adsorption Total energy calculations were implemented as Ca atom approaches the wall of the nanotube at four adsorption sites, with
Fig. 3 e Density of states of SiCNT with partial charge density profiles of the HOMO and LUMO (Isosurface ¡3). Electron acceptor states from 0 eV to 1 eV are visualized by the charge distribution (cyan) level ¼ 0.00197697eA surrounding the silicon atoms. Highest occupied states from ¡0.2 eV to 0 eV are visualized by the charge distribution (cyan) surrounding the carbon atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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increments of 0.5 A from an initial separation distance of 4 A. Potential energy curves (Fig. 4a) show that the hollow site provides a lower energy configuration at a separation distance of 2 A from the wall of SiCNT. Relaxed calculation, where atoms were allowed to move, was performed which revealed a strong binding energy of 2.83 eV at the hollow site of the nanotube. This is much greater than the cohesive energy of bulk calcium
of about 1.87 eV as obtained from other theoretical studies [65,66]. This means that calcium more likely prefers to adsorb on SiCNT than to cluster with other calcium atoms. The optimized structure of Ca/SiCNT (Fig. 4b) shows the depression of the structure on the nearest silicon atom after Ca adsorption. This depression of silicon atom is the result of its charge gain from calcium which mainly enhances its sp3 configuration.
Fig. 4 e a) Potential Energy Curve as Ca approaches SiCNT at different adsorption sites: bridge, carbon top (C-top), hollow, silicon top (Si-top). Plots represent the varying total energy of the system derived from static calculations. b) Optimized away from a depressed Si. Structure of Ca/SiCNT as a result of relaxed calculation with Ca 2.74 A
Table 1 e Change in Bader charge of Ca and the nearest neighbor atoms after Ca adsorption. Atom
D charge (e)
Si 10 Si 12 Si 19 C 22 C 30 C 39 Ca 41
0.38 0.12 0.12 0.10 0.10 0.05 þ1.45
Fig. 5 e PDOS of Ca/SiCNT projected on Si p, C p, Ca s and d orbitals with partial charge density profile of the system's highest ¡3). occupied molecular orbitals (Isosurface level ¼ 0.00197697eA Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Bader charge analysis confirmed that there was a significant electron transfer of 1.45e from calcium to SICNT. This significant charge transfer resulted to the strong binding energy of calcium over SiCNT. The transfer of electrons also provided Ca a residual charge of þ1.45e which may induce charge polarization to a nearby hydrogen molecule. The charge population after adsorption reveals that the electrons were mostly transferred to the silicon atoms which further confirms the existence of the previously discussed acceptor states of silicon. Table 1 also shows that Si 10 atom gained much of the electrons which may explain its depression leading to sp3 hybridization. The alphanumeric labels of the atoms in Table 1 refers to the atom type and ID number written in the structural input file used in the calculation. Orbital interactions were visualized by plotting the partial charge distribution represented by the peak near the Fermi level as shown in the density of states of Ca/SiCNT (Fig. 5). Projected density of states revealed an overlap of an occupied d state below the Fermi level with the electron state of Si 10 which received the most charge from calcium. This can be explained by a back-donation of electrons from the depressed Si to Ca d orbital. The peak within the energy range of 0.5 eV to 0 eV shows that there is also an interaction between the partially filled d orbital of Ca with the p orbital formed by Si and C, after Ca adsorption. This suggests that Ca on SiCNT
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Fig. 8 e Optimized Structure of Multiple H2 adsorption on Ca/SiCNT (Increasing number of H2 from left to right). Ca/ SiCNT can hold 6 hydrogen molecules using standard DFT calculation.
also manifests the existence of occupied d state which may behave similarly with transition metals as it interacts with hydrogen molecules.
Hydrogen interaction Relaxed calculation of the first hydrogen placed at the surrounding sites of calcium show that the first hydrogen molecule has a stronger binding energy of 0.22 eV near the depressed silicon site. This is due to a more charge-depleted region of the calcium adatom near the said site, where it donated the most electrons as shown by the Bader charge analysis on Table 1. This interaction is near the atomic orbital hybridization between Ca and Si 10 as shown in Fig. 5 thus resulting to a stronger binding energy. The presence of the
away Fig. 6 e a) Optimized structure of H2/Ca/SiCNT. H2 preferred an orientation at an angle of 55.20 from the z-axis 2.61 A ¡3). Charge loss from the center of Ca b) Charge density difference profile of H2/Ca/SiCNT (Isosurface level ¼ 0.00104107e A and charge gain profile of H2 shows an induced polarization after adsorption on Ca/SiCNT complex.
Fig. 7 e PDOS of H2/Ca/SiCNT projected on H2-s (black), Ca-d (red) orbitals. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Table 2 e Adsorption energy of calcium on silicon carbide nanotube and carbon based nanostructures; CNT, nitrogen/boron doped CNT, graphene, and fullerene (C60), and the corresponding average binding energy of hydrogen molecules. Ca/Nanomaterial
Ca (eV) 1H2 (eV/H2) 2H2 (eV/H2) 3H2 (eV/H2) 4H2 (eV/H2) 5H2 (eV/H2) 6H2 (eV/H2) 7H2 (eV/H2) a
Ca/SiCNT (GGA) this work Ca/SiCNT (GGA þ VdW) this worka Ca/CNT (GGA) [68] Ca/N-CNT (GGA) [68] Ca/B-CNT (GGA) [68] 2Ca/graphene (GGA) [73] Ca/graphene (GGA þ VdW) [31] Ca/C60 (GGA) [74] a
2.83 0.88 0.61 1.82 1.09 0.63 1.30
0.22 0.27 0.21 0.23 0.2 0.075 0.17 0.22
0.18 0.24 0.21 0.22 0.21 0.142 0.20 0.214
0.16 0.23 0.21 0.21 0.2 0.27 0.26 0.233
0.15 0.23 0.21 0.21 0.19 0.164 0.30 0.206
0.13 0.21 0.19 0.19 0.18
0.12 0.21 0.17
0.10 0.20
0.31 0.204
The positive values presented in this work are the magnitude of the binding energy of Ca on SiCNT and the average binding energy of nH2 where n is the number of H2 molecules.
partially filled d orbital of Ca contributed to the binding energy of the first hydrogen molecule. After energy optimization hydrogen settled at a separation distance of 2.61 A from the Ca atom. It conforms in an orientation where the bond axis is nearly perpendicular to the radial distance from Ca (see Fig. 6a). Charge density difference was obtained to see how calcium affects the charge distribution of the hydrogen molecule. Charge density difference profile shows that the calcium atom was able to induce charge polarization on the hydrogen molecule (see Fig. 6b). This induced charge polarization is due to the electric field created by calcium's cationic state after adsorption on SiCNT. This reveals that the binding nature of H2 on Ca/SiCNT mainly involves van der Waals interaction. Density of states projected on hydrogen and calcium reveals an in-phase overlap of states of H2-s and Ca-d orbitals as a confirmation of orbital interaction between H2 and Ca (see Fig. 7). This reveals a similar interaction as for transition metal decorations, where partially filled d orbital interacts with hydrogen and bind significantly to the metal decoration [37,38]. To determine the number of hydrogen molecules that the calcium adatom can hold, hydrogen molecules were placed on the remaining adsorption sites where relaxed calculations were implemented (see Fig. 8). In the standard DFT-PBE calculation, the seventh H2 molecule preferred not to bind with the H2/Ca complex due to a weak adsorption energy of 0.02 eV where optimization suggests its stable configuration 5 A away from calcium. Knowing that the binding nature of H2 mainly involves van der Waals interaction, van der Waals correction was implemented in the calculation by using DFT D2 method of Grimme [64]. Table 2 shows the average binding energies of H2 with and without van der Waals correction. Both calculations have a trend of decreasing average binding energy as the number of H2 increases. It can also be said that the van der Waals correction enhances the binding energy of the system and fall between the desired range of (0.15e0.4 eV) [70e72]. The implementation of van der Waals correction allowed 7 hydrogen molecules to be attached on one calcium adatom. Given that 1/2 monolayer of Ca can hold 7 hydrogen molecules per calcium atom, the gravimetric percentage can ideally reach ~10 weight %. However, due to steric effect the number of hydrogen molecules per Ca decoration may lessen but a minimum of 4 hydrogen molecules per Ca decoration
may still meet the standard provided by US Department of Energy. Nevertheless, we recommend that increasing the amount or concentration of Ca on SiCNT be conducted in the future to further clarify the potential use of Ca-decorated SiCNT in hydrogen storage applications. The results of the study are comparable with other researches on the potential use of calcium-decorated nanostructures for hydrogen storage application (see Table 2). Ca/ SiCNT can theoretically hold 7 hydrogen molecules per isolated calcium decoration. It can also be inferred from these studies that carbon based nanostructures with atoms having lower electronegativity than carbon, such as silicon and boron, enhances the binding of calcium on the nanostructures thus allowing much stable configurations. The results of the studies presented in Table 2 were calculated using the same exchange correlation functional (GGA) to calculate for the total energy of the metal decorated systems and the isolated calcium atom as a basis for determining its binding energy. Results show that calcium adsorbs strongly to pristine SiCNT as compared to carbon nanotube, graphene and fullerene. This strong binding is caused by a significant amount of charge transfer to SiCNT which improved the binding of hydrogen molecules as it interacts with the cationic calcium decoration. The amount of hydrogen molecule adsorbed on Ca/SiCNT complex is significantly improved by using SiCNT as the platform for Ca decoration. This capacity of Ca/SiCNT may indicate a promising material for hydrogen storage application.
Summary and conclusion Hydrogen interaction with calcium decorated silicon carbide nanotube was investigated through density functional theory based calculations. Physical and electronic properties of SiCNT and Ca/SiCNT as well as the interaction of the latter to hydrogen molecule were obtained to determine the viability of the system as a hydrogen storage material. Electronic property of SiCNT presented a reactive surface due to the acceptor states of the silicon atoms which may easily facilitate sidewall decorations. Calcium preferred to be adsorbed on the hollow site of SiCNT with a binding energy of 2.83 eV. This strong binding energy of calcium was caused by a large charge transfer of 1.43e from Ca to SiCNT as well as the interaction of the partially filled d orbital of Ca and p orbital formed by the nearest Si and C
Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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atoms. This large charge transfer from calcium created a charge-poor region on its surface which indicates the cationic state of Ca. This induced charge polarization to H2 and thus bonded with Ca through van der Waals interaction. This interaction caused a significant binding energy of 0.22 eV for the first H2 molecule adsorbed on Ca/SiCNT. The average binding energy of H2 decreases with increasing number of adsorbed hydrogen molecule. The same trend was observed when van der Waals correction was added in the calculation but with a stronger binding energy per hydrogen molecule. Results showed that Ca/SiCNT could hold 7 hydrogen molecules for one calcium decoration. The data obtained from this study suggest that Ca/SiCNT can be a good candidate for a possible hydrogen storage material. This theoretical design may be a basis for future experimental work and validation.
Acknowledgement We would like to acknowledge JASSO for the financial support provided for the Quantum Engineering Design Course Short Term Program at Osaka University where majority of the research was conducted. We also express our deepest gratitude to Kasai Laboratory for the permission and assistance in using their facilities for computational research. Also, this study will not be possible without the support of the Department of Science and Technology - Accelerated Science and Technology Human Resource Development Program, Department of Physics and the Center for Natural Sciences and Environmental Research at De La Salle University through the Computational Materials Design Group and Unit.
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Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube Jessiel Siaron Gueriba a,*, Allan Abraham Bustria Padama d, Al Rey Villagracia a,c,e, Melanie David a,c, Nelson Arboleda Jr. a,b,c, Hideaki Kasai f,g a
Department of Physics, De La Salle University, 2401 Taft Avenue, Manila 0922, Philippines ~ an, Laguna, Philippines De La Salle University-Science and Technology Complex, Bin c Computational Materials Design Research Unit, CENSER, De La Salle University, 2401 Taft Avenue, Manila 0922, Philippines d ~ os, Laguna, Philippines Institute of Mathematical Sciences and Physics, University of the Philippines Los Ban e Centre for Advanced 2D Materials and Graphene Research Center, National University of Singapore, Block S16, Level 6, 6 Science Drive 2, 117546 Singapore f Institute of Industrial Science, The University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan g National Institute of Technology, Akashi, Japan b
article info
abstract
Article history:
Ab initio study on the viability of calcium decorated silicon carbide nanotube as a hydrogen
Received 16 December 2016
storage material was conducted. Calcium strongly adsorbs on silicon carbide nanotube
Received in revised form
(SiCNT) with a significant binding energy of 2.83 eV, thus calcium's low cohesive energy
8 March 2017
and strong binding with SiCNT may prevent Ca to form clusters with other adsorbates.
Accepted 9 March 2017
Bader charge analysis also revealed a charge transfer of 1.45e from Ca to SiCNT resulting to
Available online xxx
calcium's cationic state, which may induce charge polarization to a nearby molecule such as hydrogen. Hydrogen molecule was then allowed to interact with the calcium
Keywords:
adatom where it exhibited charge polarization, induced by the electric field from calcium's
Density functional theory
positive charge. This resulted to a significant binding energy of 0.22 eV for the first
Hydrogen storage
hydrogen molecule. Results reveal that Ca on SiCNT can hold up to 7 hydrogen molecules
Nanotube
and can be a promising candidate for a hydrogen storage material. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen storage is considered as the bottleneck in exhausting the full potential of hydrogen as an energy source for various applications. The reason for this is the challenge of meeting the criterion of a good storage medium-high capacity, fast kinetics, and favorable thermodynamics [1]. Researchers
thus acknowledge this need to investigate on possible storage materials for hydrogen, which led to several researches involving hydrogen interaction with numerous materials up until now [2e13]. Metal decorated nanostructures are of great interest to researchers recently because their properties can be modified by varying the type of metals to be used for a wide range of possible applications [14e25]. One of the most promising
* Corresponding author. E-mail address: [email protected] (J.S. Gueriba). http://dx.doi.org/10.1016/j.ijhydene.2017.03.057 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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applications for these metal decorated nanostructures is for hydrogen storage, where the metal decorations serve as the active sites for molecular hydrogen adsorption [26e38]. Transition metal decorations allow strong binding with H2 due to the interaction of their partially filled d orbital with the orbitals of hydrogen molecule. This interaction is reported as Kubas effect where the binding of hydrogen molecule with the metal decoration is enhanced via orbital interaction with 3d metals [37,38]. However, the clustering of these metal decorations due to their high cohesive energy and weaker binding with the nanostructure is an obstacle to maximize the potential of these materials for hydrogen storage application [39,40]. This reduces the storage capability of the system where dissociated hydrogen tends to chemically bond with the transition metal clusters which prevents other H2 to interact. To address this problem, the metal adsorbate should have a strong binding energy with the nanostructure and overcome its cohesive energy thus lowering the possibility of clustering. A study on lithium decorated charged CNT showed that a net charge on the nanostructure enhances the binding energy of the metal decoration as well as that of the hydrogen molecule attracted to it [27]. This implies that nanostructures with point charges on its surface may provide better properties for metal decorated systems. In this study we investigate on hydrogen interaction with Ca/SiCNT to determine its viability as a hydrogen storage material. Calcium was chosen as the metal decoration due to its lower cohesive energy than transition metals [66e68]. Computational studies suggest that calcium may be a superior coating for decorated nanostructures to enhance its storage capacity and avoid metal clustering [66,67]. Interestingly, it may also provide similar interaction as transition metals due to its empty 3d orbital which may hybridize with the molecular orbital of hydrogen and improve its binding energy with the metal decorated system. On the other hand, SiCNT was used as a platform for calcium decoration due to its reactive surface compared to other carbon based nanostructure [69]. This allows stronger binding with the calcium adatom as presented in this study. SiCNT was first synthesized in 2001 by Pham-Huu et al. using shape memory synthesis method and other researchers as well tried to produce the nanomaterial through different experimental techniques [41e49]. After its synthesis, computational studies of its physical and electronic properties emerged and the possibility of functionalization through metal decorations [38,50e54]. Molecular dynamics simulations suggest that the surface of silicon carbide nanotube may capture molecules, such as hydrogen, cholesterols and phospholipids, more effectively than carbon nanotube [55,56]. This reactivity is due to point charges on the surface of SiCNT. It was also proposed that SiCNT may be a suitable material for industrial devices for its thermal stability with melting temperatures ranging from 1600 K to 4265 K which varies with the radius of the nanotube [57,58]. Recently, an experimental study on hydrogen storage capability of both SiCNT and CNT were investigated and results suggest that SiCNT has superior hydrogen uptake and desorption capability than CNT [59]. However, it's low gravimetric capacity prevents its commercial application thus they recommend further studies about SiCNT and the possibility of improving its capacity through alkali metal decoration. These serve as
the foundation for the present study which may provide insights on how to improve current hydrogen storage systems and design possible materials, such as calcium decorated silicon carbide nanotube, for hydrogen storage application.
Computational details Spin Polarized DFT calculations were performed as implemented in the Vienna Ab initio Simulation Package (VASP) [62]. The electron exchange correlation functional was treated within generalized gradient approximation by PerdeweBurkeeErnzerhof [63]. Convergence tests for cut off energy and k-point sampling were implemented. Total energy of the system was calculated with increasing values of cut off energy from 200 eV to 600 eV with increments of 50 eV and kpoints along the z axis from 1 to 11. The total energy of the system converged for cut off energies higher than 500 eV, with an energy difference of less than 10 meV, thus the cut off energy for the basis set was chosen to be 550 eV. Varying the number of k-points along the z-axis showed a convergence in energy with difference of less than 1 meV from 7 to 11 kpoints, thus Monkhorst-Pack scheme with 1 1 8 special Kpoints was used for Brillouin zone sampling. The atomic relaxation was carried out until the Helmann-Feynman forces on the unconstrained atoms were less than 0.01 eV/ A. An 18 18 10.5 A3 supercell (Fig. 1) was used to model the (5,0) nanotube which encloses two unit cells of the system, consisting of 20 carbon and 20 silicon atoms, periodic along its axis of symmetry (z-axis). (5,0) chirality was chosen due to its surface curvature which was found to directly affect the reactivity of nanomaterials thus having a more reactive surface for atomic or molecular adsorption [36,60,61]. Van der Waals interaction was also taken into consideration to address the long range interaction of hydrogen with the metal decoration by implementing DFT-D2 method of Grimme in the calculation [64]. Binding energy was calculated as Eq. (1), where Etot, Esub and Ead are the total energy of the system, total energy of the substrate and total energy of the adsorbate, respectively.
3 supercell of (5,0) SiCNT, periodic Fig. 1 e 18 £ 18 £ 10 A along the z-axis enclosing 20 carbon (black) and 20 silicon (yellow) atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Eb ¼ Etot ðEsub þ Ead Þ
(1)
Frozen lattice calculation was implemented where the total energy of Ca/SiCNT is obtained as Ca approaches the nanotube at different high symmetry adsorption sites (Fig. 2). From an initial separation distance of 4 A away from the substrate, total energy calculation was performed for every increment of 0.5 A, moving towards the wall of SiCNT. A fully relaxed calculation was conducted after obtaining the preferred reaction coordinate for each site. The binding energy of calcium on SiCNT is determined using Eq. (1) where Esub is the energy of SiCNT, Ead is the calculated total energy of an isolated calcium atom and Etot is the energy of the Ca/SiCNT complex. Hydrogen molecule was placed on the surrounding sites of calcium. The center of mass of the hydrogen molecule was 2e2.5 A away from the center of calcium where relaxed calculations at each site were conducted. The average binding energy of H2 is calculated as Eq. (2), where EnH2 =Ca=SiCNT is the total energy of nH2 over Ca decorated SiCNT, ECa/SiCNT is the energy of isolated Ca decorated SiCNT, EH2 is the energy of the isolated hydrogen molecule and n is the number of adsorbed H2 molecules.
Fig. 2 e Adsorption sites of calcium on SiCNT. Hollow site: center of the hexagon formed by the SieC bonds. Si-top and C-top sites: directly above the silicon and carbon atoms respectively. Bridge site: above the midpoint of the SieC bond.
Eb ¼ EnH2 =Ca=SiCNT ECa=SiCNT þ nEH2 =n
(2)
It is assumed that the maximum number of adsorbed hydrogen molecule is attained if the added H2 no longer binds with the nH2/Ca complex as suggested by the optimized structure, where the binding energy of a single hydrogen molecule on the nH2/Ca complex can be calculated as Eq. (3). Eb ¼ EH2 =nH2 =Ca=SiCNT EnH2 =Ca=SiCNT þ EH2
(3)
Results and discussion Silicon carbide nanotube Structural optimization shows that (5,0) SiCNT is a narrowgap semiconductor with an SieC bond length of 1.79e1.80 A which is in good agreement with other studies [50e54]. Partial charge distribution (Fig. 3) was obtained by plotting the charge density of the electron states within the energy range of 0.2 eV to 0 eV and 0 eV to 1 eV, which represent the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the system, respectively. Results show that the lowest unoccupied molecular orbital is mainly contributed by the pz orbital of silicon, which makes SiCNT more reactive than carbon nanotube by having more electron acceptor states. This characteristic is due to the heteropolar nature of SiCNT where the occupied electron state distribution is localized at the carbon atoms having a higher electronegativity than silicon atoms. This unoccupied electron states may elicit more charge transfer from a metal atom resulting to a greater binding energy and a more positive residual charge from the metal decoration. This indicates that the system can easily facilitate sidewall decoration and SiCNT can be a good platform for metal adsorption.
Calcium adsorption Total energy calculations were implemented as Ca atom approaches the wall of the nanotube at four adsorption sites, with
Fig. 3 e Density of states of SiCNT with partial charge density profiles of the HOMO and LUMO (Isosurface ¡3). Electron acceptor states from 0 eV to 1 eV are visualized by the charge distribution (cyan) level ¼ 0.00197697eA surrounding the silicon atoms. Highest occupied states from ¡0.2 eV to 0 eV are visualized by the charge distribution (cyan) surrounding the carbon atoms. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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increments of 0.5 A from an initial separation distance of 4 A. Potential energy curves (Fig. 4a) show that the hollow site provides a lower energy configuration at a separation distance of 2 A from the wall of SiCNT. Relaxed calculation, where atoms were allowed to move, was performed which revealed a strong binding energy of 2.83 eV at the hollow site of the nanotube. This is much greater than the cohesive energy of bulk calcium
of about 1.87 eV as obtained from other theoretical studies [65,66]. This means that calcium more likely prefers to adsorb on SiCNT than to cluster with other calcium atoms. The optimized structure of Ca/SiCNT (Fig. 4b) shows the depression of the structure on the nearest silicon atom after Ca adsorption. This depression of silicon atom is the result of its charge gain from calcium which mainly enhances its sp3 configuration.
Fig. 4 e a) Potential Energy Curve as Ca approaches SiCNT at different adsorption sites: bridge, carbon top (C-top), hollow, silicon top (Si-top). Plots represent the varying total energy of the system derived from static calculations. b) Optimized away from a depressed Si. Structure of Ca/SiCNT as a result of relaxed calculation with Ca 2.74 A
Table 1 e Change in Bader charge of Ca and the nearest neighbor atoms after Ca adsorption. Atom
D charge (e)
Si 10 Si 12 Si 19 C 22 C 30 C 39 Ca 41
0.38 0.12 0.12 0.10 0.10 0.05 þ1.45
Fig. 5 e PDOS of Ca/SiCNT projected on Si p, C p, Ca s and d orbitals with partial charge density profile of the system's highest ¡3). occupied molecular orbitals (Isosurface level ¼ 0.00197697eA Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Bader charge analysis confirmed that there was a significant electron transfer of 1.45e from calcium to SICNT. This significant charge transfer resulted to the strong binding energy of calcium over SiCNT. The transfer of electrons also provided Ca a residual charge of þ1.45e which may induce charge polarization to a nearby hydrogen molecule. The charge population after adsorption reveals that the electrons were mostly transferred to the silicon atoms which further confirms the existence of the previously discussed acceptor states of silicon. Table 1 also shows that Si 10 atom gained much of the electrons which may explain its depression leading to sp3 hybridization. The alphanumeric labels of the atoms in Table 1 refers to the atom type and ID number written in the structural input file used in the calculation. Orbital interactions were visualized by plotting the partial charge distribution represented by the peak near the Fermi level as shown in the density of states of Ca/SiCNT (Fig. 5). Projected density of states revealed an overlap of an occupied d state below the Fermi level with the electron state of Si 10 which received the most charge from calcium. This can be explained by a back-donation of electrons from the depressed Si to Ca d orbital. The peak within the energy range of 0.5 eV to 0 eV shows that there is also an interaction between the partially filled d orbital of Ca with the p orbital formed by Si and C, after Ca adsorption. This suggests that Ca on SiCNT
5
Fig. 8 e Optimized Structure of Multiple H2 adsorption on Ca/SiCNT (Increasing number of H2 from left to right). Ca/ SiCNT can hold 6 hydrogen molecules using standard DFT calculation.
also manifests the existence of occupied d state which may behave similarly with transition metals as it interacts with hydrogen molecules.
Hydrogen interaction Relaxed calculation of the first hydrogen placed at the surrounding sites of calcium show that the first hydrogen molecule has a stronger binding energy of 0.22 eV near the depressed silicon site. This is due to a more charge-depleted region of the calcium adatom near the said site, where it donated the most electrons as shown by the Bader charge analysis on Table 1. This interaction is near the atomic orbital hybridization between Ca and Si 10 as shown in Fig. 5 thus resulting to a stronger binding energy. The presence of the
away Fig. 6 e a) Optimized structure of H2/Ca/SiCNT. H2 preferred an orientation at an angle of 55.20 from the z-axis 2.61 A ¡3). Charge loss from the center of Ca b) Charge density difference profile of H2/Ca/SiCNT (Isosurface level ¼ 0.00104107e A and charge gain profile of H2 shows an induced polarization after adsorption on Ca/SiCNT complex.
Fig. 7 e PDOS of H2/Ca/SiCNT projected on H2-s (black), Ca-d (red) orbitals. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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Table 2 e Adsorption energy of calcium on silicon carbide nanotube and carbon based nanostructures; CNT, nitrogen/boron doped CNT, graphene, and fullerene (C60), and the corresponding average binding energy of hydrogen molecules. Ca/Nanomaterial
Ca (eV) 1H2 (eV/H2) 2H2 (eV/H2) 3H2 (eV/H2) 4H2 (eV/H2) 5H2 (eV/H2) 6H2 (eV/H2) 7H2 (eV/H2) a
Ca/SiCNT (GGA) this work Ca/SiCNT (GGA þ VdW) this worka Ca/CNT (GGA) [68] Ca/N-CNT (GGA) [68] Ca/B-CNT (GGA) [68] 2Ca/graphene (GGA) [73] Ca/graphene (GGA þ VdW) [31] Ca/C60 (GGA) [74] a
2.83 0.88 0.61 1.82 1.09 0.63 1.30
0.22 0.27 0.21 0.23 0.2 0.075 0.17 0.22
0.18 0.24 0.21 0.22 0.21 0.142 0.20 0.214
0.16 0.23 0.21 0.21 0.2 0.27 0.26 0.233
0.15 0.23 0.21 0.21 0.19 0.164 0.30 0.206
0.13 0.21 0.19 0.19 0.18
0.12 0.21 0.17
0.10 0.20
0.31 0.204
The positive values presented in this work are the magnitude of the binding energy of Ca on SiCNT and the average binding energy of nH2 where n is the number of H2 molecules.
partially filled d orbital of Ca contributed to the binding energy of the first hydrogen molecule. After energy optimization hydrogen settled at a separation distance of 2.61 A from the Ca atom. It conforms in an orientation where the bond axis is nearly perpendicular to the radial distance from Ca (see Fig. 6a). Charge density difference was obtained to see how calcium affects the charge distribution of the hydrogen molecule. Charge density difference profile shows that the calcium atom was able to induce charge polarization on the hydrogen molecule (see Fig. 6b). This induced charge polarization is due to the electric field created by calcium's cationic state after adsorption on SiCNT. This reveals that the binding nature of H2 on Ca/SiCNT mainly involves van der Waals interaction. Density of states projected on hydrogen and calcium reveals an in-phase overlap of states of H2-s and Ca-d orbitals as a confirmation of orbital interaction between H2 and Ca (see Fig. 7). This reveals a similar interaction as for transition metal decorations, where partially filled d orbital interacts with hydrogen and bind significantly to the metal decoration [37,38]. To determine the number of hydrogen molecules that the calcium adatom can hold, hydrogen molecules were placed on the remaining adsorption sites where relaxed calculations were implemented (see Fig. 8). In the standard DFT-PBE calculation, the seventh H2 molecule preferred not to bind with the H2/Ca complex due to a weak adsorption energy of 0.02 eV where optimization suggests its stable configuration 5 A away from calcium. Knowing that the binding nature of H2 mainly involves van der Waals interaction, van der Waals correction was implemented in the calculation by using DFT D2 method of Grimme [64]. Table 2 shows the average binding energies of H2 with and without van der Waals correction. Both calculations have a trend of decreasing average binding energy as the number of H2 increases. It can also be said that the van der Waals correction enhances the binding energy of the system and fall between the desired range of (0.15e0.4 eV) [70e72]. The implementation of van der Waals correction allowed 7 hydrogen molecules to be attached on one calcium adatom. Given that 1/2 monolayer of Ca can hold 7 hydrogen molecules per calcium atom, the gravimetric percentage can ideally reach ~10 weight %. However, due to steric effect the number of hydrogen molecules per Ca decoration may lessen but a minimum of 4 hydrogen molecules per Ca decoration
may still meet the standard provided by US Department of Energy. Nevertheless, we recommend that increasing the amount or concentration of Ca on SiCNT be conducted in the future to further clarify the potential use of Ca-decorated SiCNT in hydrogen storage applications. The results of the study are comparable with other researches on the potential use of calcium-decorated nanostructures for hydrogen storage application (see Table 2). Ca/ SiCNT can theoretically hold 7 hydrogen molecules per isolated calcium decoration. It can also be inferred from these studies that carbon based nanostructures with atoms having lower electronegativity than carbon, such as silicon and boron, enhances the binding of calcium on the nanostructures thus allowing much stable configurations. The results of the studies presented in Table 2 were calculated using the same exchange correlation functional (GGA) to calculate for the total energy of the metal decorated systems and the isolated calcium atom as a basis for determining its binding energy. Results show that calcium adsorbs strongly to pristine SiCNT as compared to carbon nanotube, graphene and fullerene. This strong binding is caused by a significant amount of charge transfer to SiCNT which improved the binding of hydrogen molecules as it interacts with the cationic calcium decoration. The amount of hydrogen molecule adsorbed on Ca/SiCNT complex is significantly improved by using SiCNT as the platform for Ca decoration. This capacity of Ca/SiCNT may indicate a promising material for hydrogen storage application.
Summary and conclusion Hydrogen interaction with calcium decorated silicon carbide nanotube was investigated through density functional theory based calculations. Physical and electronic properties of SiCNT and Ca/SiCNT as well as the interaction of the latter to hydrogen molecule were obtained to determine the viability of the system as a hydrogen storage material. Electronic property of SiCNT presented a reactive surface due to the acceptor states of the silicon atoms which may easily facilitate sidewall decorations. Calcium preferred to be adsorbed on the hollow site of SiCNT with a binding energy of 2.83 eV. This strong binding energy of calcium was caused by a large charge transfer of 1.43e from Ca to SiCNT as well as the interaction of the partially filled d orbital of Ca and p orbital formed by the nearest Si and C
Please cite this article in press as: Gueriba JS, et al., Ab initio study on hydrogen interaction with calcium decorated silicon carbide nanotube, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.03.057
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atoms. This large charge transfer from calcium created a charge-poor region on its surface which indicates the cationic state of Ca. This induced charge polarization to H2 and thus bonded with Ca through van der Waals interaction. This interaction caused a significant binding energy of 0.22 eV for the first H2 molecule adsorbed on Ca/SiCNT. The average binding energy of H2 decreases with increasing number of adsorbed hydrogen molecule. The same trend was observed when van der Waals correction was added in the calculation but with a stronger binding energy per hydrogen molecule. Results showed that Ca/SiCNT could hold 7 hydrogen molecules for one calcium decoration. The data obtained from this study suggest that Ca/SiCNT can be a good candidate for a possible hydrogen storage material. This theoretical design may be a basis for future experimental work and validation.
Acknowledgement We would like to acknowledge JASSO for the financial support provided for the Quantum Engineering Design Course Short Term Program at Osaka University where majority of the research was conducted. We also express our deepest gratitude to Kasai Laboratory for the permission and assistance in using their facilities for computational research. Also, this study will not be possible without the support of the Department of Science and Technology - Accelerated Science and Technology Human Resource Development Program, Department of Physics and the Center for Natural Sciences and Environmental Research at De La Salle University through the Computational Materials Design Group and Unit.
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