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Ti-decorated B38 fullerene: A high capacity hydrogen storage material
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 6 ) 1 e6
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Ti-decorated B38 fullerene: A high capacity hydrogen storage material Pingping Liu a, Hong Zhang a,b,*, Xinlu Cheng c, Yongjian Tang d a
College of Physical Science and Technology, Sichuan University, Chengdu 610065, PR China Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR China c Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, PR China d Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, PR China b
article info
abstract
Article history:
In the present study we investigate the stabilities, electronic properties and the hydrogen
Received 3 March 2016
storage capacity of Ti-decorated B38 fullerene. All calculations have been performed by
Received in revised form
using first-principles calculations based on density functional theory. Our results show Ti
12 July 2016
atoms can be attached on top of the center of hexagonal holes of B38 fullerene with large
Accepted 26 July 2016
binding energy (5.67 eV). Each Ti atom can bind up to six hydrogen molecules with an
Available online xxx
average adsorption energy of 0.22 eV/H2. While the B38 fullerene coated with 4 Ti atoms (4Ti/B38) can store 24 H2 molecules, the gravimetric density reaches up to 7.44 wt% with an
Keywords:
average adsorption energy of 0.23 eV/H2. Based on these results we infer that Ti-decorated
High hydrogen storage
B38 fullerene is a potential material for hydrogen storage with high capacity and might
B38 fullerene
motivate active experimental efforts in designing hydrogen storage media.
Ti-decorated
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
DFT calculations
Introduction Hydrogen is one of the most important energy carriers due to its high efficiency, abundance and environmental friendly nature. Finding reliable materials that can store hydrogen safely and transport efficiently is one of the main challenges [1e3]. According to the requirement of U. S. Department of Energy (DOE), such reliable materials should meet a target: gravimetric hydrogen capacity of more than 5.5% [4,5]. In fact, Boron fullerene is suitable for substrates because of it is composed of light elements and have the capability to bind with metal atoms [6,7]. After B80 fullerene [8], various types of boron fullerenes nanostructures have been proposed [9e17].
Metal decorated boron fullerenes have been widely suggested as candidate materials for hydrogen storage [6,18e23]. Li et al. [18] report the alkali-metal-doped B80 have a gravimetric density of 11.2 and 9.8 wt%. However, the average H2 adsorption energies are of only 0.07 and 0.09 eV/H2. Dong et al. [6] suggest the Ca atoms even cannot stably bind to B40. And they find that the hydrogen storage capacity of Ti decorated B40 reaches 8.7 wt% with an average adsorption energy of 0.37 eV/H2. Ti-decorated on other nanostructures also have been widely reported as potential hydrogen storage materials [24e33]. Tang et al. [31] publish the largest hydrogen gravimetric density of Ti atoms decorated C24B24 fullerene is 8.1 wt % with average adsorption energies of 0.24e0.55 eV/H2. Lebon
* Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.223 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
2
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et al. [32] find an effective hydrogen coverage on Ti-coated graphene nanoribbons with gravimetric density of 6% and average adsorption energy of 0.2e0.6 eV/H2. Most recently, B38 fullerene (hereafter referred to as B38) has been predicted by first-principles swarm structure searching calculations [34]. It is considerably smaller and lighter than B80. This structure is conducive to the hydrogen storage properties. There is little study on hydrogen storage properties of B38 except Lu et al. [7]. They have investigate the hydrogen storage of alkali and alkaline-earth metals (AM) decorated B38, AM ¼ Li, Na, K, Mg, Ca based on DFT. They suggest that, among all of their metarials, Ca-decorated B38 has the best hydrogen storage capacity with gravimetric density of 6.47 wt%. However, the average H2 adsorption energy (0.075 eV/H2) produced by the generalized gradient approximation (GGA) function is too small for room temperature applications. All of the above mentioned researches inspire us to consider whether the transition metal Ti atom decorated B38 is a efficient hydrogen storage media. And to the best of our knowledge, there is little information about hydrogen storage properties of Ti-decorated B38. Thus, the aim of this paper is to study the hydrogen storage capacity of Ti decorated B38 by density functional theory (DFT) with GGA function. We also investigate the structural stability and electronic properties of Ti-decorated B38. When 4Ti atoms decorate on B38, the maximum hydrogen gravimetric density reaches to 7.44 wt% with the average binding energy of 0.23 eV/H2, which is exceeding the criterion specified by the U.S. Department of Energy.
Results and discussion Ti dispersion on B38 We first examine the structure of a single Ti atom on B38. Fig. 1 shows the atomic structures of a Ti atom attached on B38. We can see that B38 is highly symmetric and connected with triangle and hexagon holes. And the average diameter of B38 is 5.85 A. A, B and C in Fig. 1 represent three adsorption sites for the Ti atom, including the centers of two kinds of hexagons (A, B) and the B-B bridges around the hexagons (C). The binding energy results shows that site “A” is the most stable position with the largest binding energy. The binding energy of Ti atom on B38 is 5.67 eV, which is larger than the cohesive energy of its bulk phases (4.85 eV [42]). The binding energy value is consistent with the results of the previous study earlier works, such as Ti-decorated B40 fullerene (5.58 eV) [6]. To understand the binding mechanism between the Ti atom and B38 (record as 1Ti/B38), the partial density of states (PDOS) of the 1Ti/B38 complex have been carried out, as shown in Fig. 2. From the PDOS, we find the significant contribution of
Computational details All geometry optimizations are conducted using the DMol3 package based on DFT calculations [35]. The double numerical polarized (DNP) basis set corresponds to a double-DFT semicore pseudo potentials (DSPP) treatment is adopted as the core treatment for relativistic effects. The exchange-correlation functional is treated using the generalized gradient approximation (GGA) [36] with the PerdeweBurkee Ernzerhof functional [37]. Dispersion-corrected DFT (DFT-D) [38e40] scheme put forward by Ortmann, Bechstedt, and Schmidt (OBS) [41] is used to describe the van der Waals (vdW) interaction. Selfconsistent field convergence tolerance is set to 1 106 Ha. A for The convergence criterion are specifed as of 4 103 Ha/ gradient, 5 103 Å for displacement, and 2 105 Hartree for energies. All calculations are spin-unrestricted, and smearing A is set is set at 5 103 Hartree. The global orbital cutoff at 5.5 in the spin-unrestricted calculations. The adsorption energy of transition metals on B38 is determined by the following function, Ead ðMÞ ¼ EðM BÞ EðMÞ EðBÞ
Fig. 1 e The structure of B38 fullerene. A, B and C represent the center of the two hexagon holes and the BeB bridge sites around the hexagon. Pink ball: B atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1)
where Ead(MB), E(M) and E(B) are total energies of transition metal on B38 after fully relaxation, an isolated transition metal atom and bare B38, respectively. The average adsorption energy of H2 on MB is defined by Ead ½H2 ¼ ½nH2=EðM BÞ EðM BÞ nEðH2Þ=n; where n is the number of H2 molecules adsorbed.
(2) Fig. 2 e The partial density of states (PDOS) of the 1Ti/B38.
Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
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 6 ) 1 e6
3
Fig. 3 e The optimized structures of the most stable configurations of adsorption 1e6 H2 molecules. Pink ball: B atom, grey ball: Ti atom, white ball: H atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4 e The partial density of density of states (PDOS) of 1Ti/B38/(H2)1e6.
Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
4
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the Ti-3d orbitals in binding of Ti to B38. The Ti-3d orbitals hybridize with the B orbitals in the range of 1 to 1 eV. There are also hybrid orbitals just far below the Fermi level, Ti-4s and Ti-2p located at 29 eV and 17 eV, which can help to increase the reactivity of the system. The attached Ti atom first donates the d electrons to the B38, leading to partially fill B-2s orbitals. Meanwhile, the Ti orbitals split due to the strong ligand field generated by the B38 graphene, and thus, the B38 back-donates some electrons to the Ti orbitals, resulting in strong hybridizations between the Ti atoms and the substrate. These hybridizations lead to a strong Ti-B bond, which plays a crucial role to ensure Ti atoms stably on the surface and aviod clustering. This is consistent with the results of charge population analysis. The Mulliken charge analysis shows that, there is 0.014 e positive charges on Ti atom in Ti/B38 system, as a result of the electron donation from B to the orbitals of Ti and the back-donation from Ti orbitals to B. So based on the PDOS, the Mulliken charge and the deformation electron density analysis, we can conclude that some charges have transferred from the Ti to B atoms and Ti remains in cationic state.
Adsorption of H2 molecules on Ti functionalized B38 We first investigate the interaction between B38 and different number of H2 molecules and the optimizing the geometry configurations. The most stable configurations of adsorption with one H2 molecules are plotted in Fig. 3A. As one H2 molecule is absorbed to the attached Ti atom, the A distance between the Ti atom and the H2 molecule is 2.206 and the adsorption energy is 0.31 eV. We plot the partial density of density of states (PDOS) of 1Ti/B38/(H2)1 in Fig. 4a. Through the PDOS analysis, the peak of Ti-2p, 4s orbitals in the range of 5 to 3.5eV hybridizes with the H-s orbitals, which indicate that a electron transfer between H-s orbitals and Ti-2p, 4s orbitals in the 1Ti/B38/(H2)1 system, which accords with the results of Mulliken charge calculation of the attached Ti atom in Table 1. Next, we add additional H2 molecules close to the Ti atom. The optimum configurations of adsorption with different numbers of H2 molecules are shown in Fig. 3B-F. We can find that additional five H2 molecules can be absorbed to 1Ti/B38/(H2)1 complexes, resulting in hydrogen capacities of 2.56 wt% with the average adsorption energy of 0.22 eV/H2. Fig. 4aef clearly presents the appearance of Ti2p orbitals owing to the 2p orbitals receiving more
electrons from B38. Simultaneously, the H orbitals in the 1Ti/ B38/(H2)2 system (Fig. 4b) has split and the peaks located at 4.2 and 3.5 eV overlap with the Ti-2p orbitals, respectively. A similar phenomenon can also be observed in the 1Ti/B38/(H2)3 (Fig. 4c) system, which has the weaker hybridization strength among the three systems. The orbital hybridizations between H-s and Ti-2p, 4s orbitals have become weaker as the number of adsorbed H2 molecules increases. The electron transfer from the H2 molecules to the Ti atom increases through the orbital hybridizations, as a result, the net charge of the Ti atom decrease as shown in Table 1. To gain insight into the nature of the Ti-H2 bonding, we have calculated the deformation electron density of these complexes Fig. 3aef. The deformation electron density exhibits the localized characteristics of H-H bonds and confirms the electron back donation between H2 and Ti atom. The Mulliken population analysis show that the charge of Ti atom decreased as from positive to negative as the increase of the number of adsorbed hydrogen molues, from positive to negative. When the fifth and sixth H2 adsorbed on the Ti atom, the Ti atom carried 1.012 and 1.007 electrons, respectively. When B38 are symmetrically coted by 4Ti atoms (record as 4Ti/B38), the 4Ti/B38 complex keeps stable and provide high average binding energy (0.52 eV). And 24 H2 molecules can be stored on 4Ti/B38, leading to a high hydrogen capacity up to 7.44 wt% with the average adsorption energy 0.23 eV/H2. The structure of 4Ti/B38/(H2)24 is shown in Fig. 5. The average H-H distance 0.761 A, which is slightly lengthened compared with free state H2. On the whole, we can conclude that the Ti/B38 system can adsorb a maximum of 24H2 molecules with high gravimetric density of 7.44 wt %. The average adsorption
Table 1 e The average adsorption energies (Eads) per H2 molecule on Ti-decorated B38 fullerene, the Mulliken charge of the attached Ti atom. Hydrogen number (n) 0 1 2 3 4 5 6
Ead [H2] (eV)
Charge of Ti
0.45 0.44 0.26 0.24 0.23 0.22
0.014 0.447 0.447 0.723 1.007 1.012 1.007
Fig. 5 e The optimized structure of 4Ti/B38 complex with 24 H2 molecules adsorbed. Pink ball: B atom, grey ball: Ti atom, white ball: H atom. (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: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
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 6 ) 1 e6
energy of 0.23 eV/H2 satisfies the requirements of DOE. Therefore, Ti-decorated B38 is a potential material to store hydrogen.
[12] [13]
Conclusion [14]
In summary, we propose a high-capacity hydrogen storage media, Ti-decorated B38 fullerene, by using first-principles density functional calculations. The Ti atoms can be attached to the centers of hexagon in the B38 fullerene because of the charge transfer from the Ti atoms to the B38 fullerene. Our results indicate that the strong binding between the Ti atoms and B38 fullerene can prevent aggregation of Ti atoms on B38 fullerene efficiently. The results indicate that, in Ti-decorated B38 fullerene system, six H2 molecules can be adsorbed to per Ti atom with an ideal adsorption energy of 0.23 eV/H2. And the hydrogen storage capacities reach to 7.44 wt%. Our results present here might motivate active experimental efforts in designing high-efficiency hydrogen storage media.
Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 11474207 and 11374217).
references
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Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
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[34] Lv J, Wang Y, Zhu L, Ma Y. B38: an all-boron fullerene analogue. Nanoscale 2014;6(20):11692e6. [35] Delley B. From molecules to solids with the DMol3 approach. J Chem Phys 2000;113:7756e64. [36] Perdew JP, Wang Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys Rev B 1992;45:13244e9. [37] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865e8. [38] Wu Q, Yang W. Empirical correction to density functional theory for van der Waals interactions. J Chem Phys 2002;116:515e24.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Ti-decorated B38 fullerene: A high capacity hydrogen storage material Pingping Liu a, Hong Zhang a,b,*, Xinlu Cheng c, Yongjian Tang d a
College of Physical Science and Technology, Sichuan University, Chengdu 610065, PR China Key Laboratory of High Energy Density Physics and Technology of Ministry of Education, Sichuan University, Chengdu 610064, PR China c Institute of Atomic and Molecular Physics, Sichuan University, Chengdu 610065, PR China d Science and Technology on Plasma Physics Laboratory, Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, PR China b
article info
abstract
Article history:
In the present study we investigate the stabilities, electronic properties and the hydrogen
Received 3 March 2016
storage capacity of Ti-decorated B38 fullerene. All calculations have been performed by
Received in revised form
using first-principles calculations based on density functional theory. Our results show Ti
12 July 2016
atoms can be attached on top of the center of hexagonal holes of B38 fullerene with large
Accepted 26 July 2016
binding energy (5.67 eV). Each Ti atom can bind up to six hydrogen molecules with an
Available online xxx
average adsorption energy of 0.22 eV/H2. While the B38 fullerene coated with 4 Ti atoms (4Ti/B38) can store 24 H2 molecules, the gravimetric density reaches up to 7.44 wt% with an
Keywords:
average adsorption energy of 0.23 eV/H2. Based on these results we infer that Ti-decorated
High hydrogen storage
B38 fullerene is a potential material for hydrogen storage with high capacity and might
B38 fullerene
motivate active experimental efforts in designing hydrogen storage media.
Ti-decorated
© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
DFT calculations
Introduction Hydrogen is one of the most important energy carriers due to its high efficiency, abundance and environmental friendly nature. Finding reliable materials that can store hydrogen safely and transport efficiently is one of the main challenges [1e3]. According to the requirement of U. S. Department of Energy (DOE), such reliable materials should meet a target: gravimetric hydrogen capacity of more than 5.5% [4,5]. In fact, Boron fullerene is suitable for substrates because of it is composed of light elements and have the capability to bind with metal atoms [6,7]. After B80 fullerene [8], various types of boron fullerenes nanostructures have been proposed [9e17].
Metal decorated boron fullerenes have been widely suggested as candidate materials for hydrogen storage [6,18e23]. Li et al. [18] report the alkali-metal-doped B80 have a gravimetric density of 11.2 and 9.8 wt%. However, the average H2 adsorption energies are of only 0.07 and 0.09 eV/H2. Dong et al. [6] suggest the Ca atoms even cannot stably bind to B40. And they find that the hydrogen storage capacity of Ti decorated B40 reaches 8.7 wt% with an average adsorption energy of 0.37 eV/H2. Ti-decorated on other nanostructures also have been widely reported as potential hydrogen storage materials [24e33]. Tang et al. [31] publish the largest hydrogen gravimetric density of Ti atoms decorated C24B24 fullerene is 8.1 wt % with average adsorption energies of 0.24e0.55 eV/H2. Lebon
* Corresponding author. E-mail address: [email protected] (H. Zhang). http://dx.doi.org/10.1016/j.ijhydene.2016.07.223 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
2
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 6 ) 1 e6
et al. [32] find an effective hydrogen coverage on Ti-coated graphene nanoribbons with gravimetric density of 6% and average adsorption energy of 0.2e0.6 eV/H2. Most recently, B38 fullerene (hereafter referred to as B38) has been predicted by first-principles swarm structure searching calculations [34]. It is considerably smaller and lighter than B80. This structure is conducive to the hydrogen storage properties. There is little study on hydrogen storage properties of B38 except Lu et al. [7]. They have investigate the hydrogen storage of alkali and alkaline-earth metals (AM) decorated B38, AM ¼ Li, Na, K, Mg, Ca based on DFT. They suggest that, among all of their metarials, Ca-decorated B38 has the best hydrogen storage capacity with gravimetric density of 6.47 wt%. However, the average H2 adsorption energy (0.075 eV/H2) produced by the generalized gradient approximation (GGA) function is too small for room temperature applications. All of the above mentioned researches inspire us to consider whether the transition metal Ti atom decorated B38 is a efficient hydrogen storage media. And to the best of our knowledge, there is little information about hydrogen storage properties of Ti-decorated B38. Thus, the aim of this paper is to study the hydrogen storage capacity of Ti decorated B38 by density functional theory (DFT) with GGA function. We also investigate the structural stability and electronic properties of Ti-decorated B38. When 4Ti atoms decorate on B38, the maximum hydrogen gravimetric density reaches to 7.44 wt% with the average binding energy of 0.23 eV/H2, which is exceeding the criterion specified by the U.S. Department of Energy.
Results and discussion Ti dispersion on B38 We first examine the structure of a single Ti atom on B38. Fig. 1 shows the atomic structures of a Ti atom attached on B38. We can see that B38 is highly symmetric and connected with triangle and hexagon holes. And the average diameter of B38 is 5.85 A. A, B and C in Fig. 1 represent three adsorption sites for the Ti atom, including the centers of two kinds of hexagons (A, B) and the B-B bridges around the hexagons (C). The binding energy results shows that site “A” is the most stable position with the largest binding energy. The binding energy of Ti atom on B38 is 5.67 eV, which is larger than the cohesive energy of its bulk phases (4.85 eV [42]). The binding energy value is consistent with the results of the previous study earlier works, such as Ti-decorated B40 fullerene (5.58 eV) [6]. To understand the binding mechanism between the Ti atom and B38 (record as 1Ti/B38), the partial density of states (PDOS) of the 1Ti/B38 complex have been carried out, as shown in Fig. 2. From the PDOS, we find the significant contribution of
Computational details All geometry optimizations are conducted using the DMol3 package based on DFT calculations [35]. The double numerical polarized (DNP) basis set corresponds to a double-DFT semicore pseudo potentials (DSPP) treatment is adopted as the core treatment for relativistic effects. The exchange-correlation functional is treated using the generalized gradient approximation (GGA) [36] with the PerdeweBurkee Ernzerhof functional [37]. Dispersion-corrected DFT (DFT-D) [38e40] scheme put forward by Ortmann, Bechstedt, and Schmidt (OBS) [41] is used to describe the van der Waals (vdW) interaction. Selfconsistent field convergence tolerance is set to 1 106 Ha. A for The convergence criterion are specifed as of 4 103 Ha/ gradient, 5 103 Å for displacement, and 2 105 Hartree for energies. All calculations are spin-unrestricted, and smearing A is set is set at 5 103 Hartree. The global orbital cutoff at 5.5 in the spin-unrestricted calculations. The adsorption energy of transition metals on B38 is determined by the following function, Ead ðMÞ ¼ EðM BÞ EðMÞ EðBÞ
Fig. 1 e The structure of B38 fullerene. A, B and C represent the center of the two hexagon holes and the BeB bridge sites around the hexagon. Pink ball: B atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1)
where Ead(MB), E(M) and E(B) are total energies of transition metal on B38 after fully relaxation, an isolated transition metal atom and bare B38, respectively. The average adsorption energy of H2 on MB is defined by Ead ½H2 ¼ ½nH2=EðM BÞ EðM BÞ nEðH2Þ=n; where n is the number of H2 molecules adsorbed.
(2) Fig. 2 e The partial density of states (PDOS) of the 1Ti/B38.
Please cite this article in press as: Liu P, et al., Ti-decorated B38 fullerene: A high capacity hydrogen storage material, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.07.223
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3
Fig. 3 e The optimized structures of the most stable configurations of adsorption 1e6 H2 molecules. Pink ball: B atom, grey ball: Ti atom, white ball: H atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 4 e The partial density of density of states (PDOS) of 1Ti/B38/(H2)1e6.
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the Ti-3d orbitals in binding of Ti to B38. The Ti-3d orbitals hybridize with the B orbitals in the range of 1 to 1 eV. There are also hybrid orbitals just far below the Fermi level, Ti-4s and Ti-2p located at 29 eV and 17 eV, which can help to increase the reactivity of the system. The attached Ti atom first donates the d electrons to the B38, leading to partially fill B-2s orbitals. Meanwhile, the Ti orbitals split due to the strong ligand field generated by the B38 graphene, and thus, the B38 back-donates some electrons to the Ti orbitals, resulting in strong hybridizations between the Ti atoms and the substrate. These hybridizations lead to a strong Ti-B bond, which plays a crucial role to ensure Ti atoms stably on the surface and aviod clustering. This is consistent with the results of charge population analysis. The Mulliken charge analysis shows that, there is 0.014 e positive charges on Ti atom in Ti/B38 system, as a result of the electron donation from B to the orbitals of Ti and the back-donation from Ti orbitals to B. So based on the PDOS, the Mulliken charge and the deformation electron density analysis, we can conclude that some charges have transferred from the Ti to B atoms and Ti remains in cationic state.
Adsorption of H2 molecules on Ti functionalized B38 We first investigate the interaction between B38 and different number of H2 molecules and the optimizing the geometry configurations. The most stable configurations of adsorption with one H2 molecules are plotted in Fig. 3A. As one H2 molecule is absorbed to the attached Ti atom, the A distance between the Ti atom and the H2 molecule is 2.206 and the adsorption energy is 0.31 eV. We plot the partial density of density of states (PDOS) of 1Ti/B38/(H2)1 in Fig. 4a. Through the PDOS analysis, the peak of Ti-2p, 4s orbitals in the range of 5 to 3.5eV hybridizes with the H-s orbitals, which indicate that a electron transfer between H-s orbitals and Ti-2p, 4s orbitals in the 1Ti/B38/(H2)1 system, which accords with the results of Mulliken charge calculation of the attached Ti atom in Table 1. Next, we add additional H2 molecules close to the Ti atom. The optimum configurations of adsorption with different numbers of H2 molecules are shown in Fig. 3B-F. We can find that additional five H2 molecules can be absorbed to 1Ti/B38/(H2)1 complexes, resulting in hydrogen capacities of 2.56 wt% with the average adsorption energy of 0.22 eV/H2. Fig. 4aef clearly presents the appearance of Ti2p orbitals owing to the 2p orbitals receiving more
electrons from B38. Simultaneously, the H orbitals in the 1Ti/ B38/(H2)2 system (Fig. 4b) has split and the peaks located at 4.2 and 3.5 eV overlap with the Ti-2p orbitals, respectively. A similar phenomenon can also be observed in the 1Ti/B38/(H2)3 (Fig. 4c) system, which has the weaker hybridization strength among the three systems. The orbital hybridizations between H-s and Ti-2p, 4s orbitals have become weaker as the number of adsorbed H2 molecules increases. The electron transfer from the H2 molecules to the Ti atom increases through the orbital hybridizations, as a result, the net charge of the Ti atom decrease as shown in Table 1. To gain insight into the nature of the Ti-H2 bonding, we have calculated the deformation electron density of these complexes Fig. 3aef. The deformation electron density exhibits the localized characteristics of H-H bonds and confirms the electron back donation between H2 and Ti atom. The Mulliken population analysis show that the charge of Ti atom decreased as from positive to negative as the increase of the number of adsorbed hydrogen molues, from positive to negative. When the fifth and sixth H2 adsorbed on the Ti atom, the Ti atom carried 1.012 and 1.007 electrons, respectively. When B38 are symmetrically coted by 4Ti atoms (record as 4Ti/B38), the 4Ti/B38 complex keeps stable and provide high average binding energy (0.52 eV). And 24 H2 molecules can be stored on 4Ti/B38, leading to a high hydrogen capacity up to 7.44 wt% with the average adsorption energy 0.23 eV/H2. The structure of 4Ti/B38/(H2)24 is shown in Fig. 5. The average H-H distance 0.761 A, which is slightly lengthened compared with free state H2. On the whole, we can conclude that the Ti/B38 system can adsorb a maximum of 24H2 molecules with high gravimetric density of 7.44 wt %. The average adsorption
Table 1 e The average adsorption energies (Eads) per H2 molecule on Ti-decorated B38 fullerene, the Mulliken charge of the attached Ti atom. Hydrogen number (n) 0 1 2 3 4 5 6
Ead [H2] (eV)
Charge of Ti
0.45 0.44 0.26 0.24 0.23 0.22
0.014 0.447 0.447 0.723 1.007 1.012 1.007
Fig. 5 e The optimized structure of 4Ti/B38 complex with 24 H2 molecules adsorbed. Pink ball: B atom, grey ball: Ti atom, white ball: H atom. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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energy of 0.23 eV/H2 satisfies the requirements of DOE. Therefore, Ti-decorated B38 is a potential material to store hydrogen.
[12] [13]
Conclusion [14]
In summary, we propose a high-capacity hydrogen storage media, Ti-decorated B38 fullerene, by using first-principles density functional calculations. The Ti atoms can be attached to the centers of hexagon in the B38 fullerene because of the charge transfer from the Ti atoms to the B38 fullerene. Our results indicate that the strong binding between the Ti atoms and B38 fullerene can prevent aggregation of Ti atoms on B38 fullerene efficiently. The results indicate that, in Ti-decorated B38 fullerene system, six H2 molecules can be adsorbed to per Ti atom with an ideal adsorption energy of 0.23 eV/H2. And the hydrogen storage capacities reach to 7.44 wt%. Our results present here might motivate active experimental efforts in designing high-efficiency hydrogen storage media.
Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 11474207 and 11374217).
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