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The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study
Accepted Manuscript The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study Nahong Song, Yusheng Wang, Songyang Ding, Yuye Yang, Jing Zhang, Bin Xu, Lin Yi, Yu Jia PII:
S0042-207X(15)00134-7
DOI:
10.1016/j.vacuum.2015.03.034
Reference:
VAC 6616
To appear in:
Vacuum
Received Date: 29 January 2015 Revised Date:
25 March 2015
Accepted Date: 26 March 2015
Please cite this article as: Song N, Wang Y, Ding S, Yang Y, Zhang J, Xu B, Yi L, Jia Y, The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study, Vaccum (2015), doi: 10.1016/ j.vacuum.2015.03.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study Nahong Song1, Yusheng Wang2,3*, Songyang Ding1, Yuye Yang1, Jing Zhang2,4, Bin Xu2,4, Lin Yi4, Yu
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Jia3* 1) College of Computer and Information Engineering, Henan University of Economics and Law, Zhengzhou, Henan, 450000, China
2) College of Mathematics and Information Science, North China University of Water Resources and
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Electric Power, Zhengzhou, Henan, 450011, China
3) International Joint Research Laboratory for Quantum Functional Materials of Henan, and School
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of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China 4) Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, China * Corresponding author, Tel.: 86-371-67739336; E-mail: [email protected] (Y. Wang); [email protected] (Y. Jia) Abstract
The hydrogen storage properties of Li-decorated graphene-like monolayer WS2 have been
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systematically studied by using first principles calculations based on density functional theory. The present results bring to light that the pristine WS2 is not suitable for storing hydrogen due to the very weak interaction between the pristine WS2 sheet and the H2 molecules. The Li atoms can be adsorbed
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strongly on both sides of WS2 without clustering owing to the larger binding energy of Li to WS2 sheet and the Coulomb repulsion between the Li adatoms. The results reveal that the adsorption of H2
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molecules comes from not only electrostatic force between the Li atom and H2 molecules, but also the hybridization mechanism between the σ orbital of H2 molecules and the p orbital of S atom. This is quite different from the other materials where there is no orbital hybridizations between the H2 molecules and the host materials. For the 2Li/WS2 system, the average adsorption energy is 0.13 eV/H2, indicating that Li decorated WS2 can be an optimal choice for the reversible hydrogen adsorption/desorption at room temperature. PACS number: 68.43.Bc, 73.20.At, 84.60.Ve Keywords: hydrogen storage; decoration; WS2 sheet 1
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I. Introduction In modern society, the problems of the energy shortage and environment pollution lead to the increasing need for the green energy carrier, which has aroused growing research interest in hydrogen
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due to its abundance, pollution-free and packing more energy unit mass than any other traditional fuels [1-6]. However, compressed gas and liquefied hydrogen as the traditional storing models, fail to be put into a large-scale application due to heavy energy consumption and safety problems. Moreover, metal hydride can’t perform hydrogen reversibly under ambient thermodynamic conditions for motor
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vehicles and other mechanical systems [7]. Therefore it is an urgent task to find a way to store hydrogen with high gravimetric density and moderate adsorption energy.
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Since the discovery of decorating metal atom to carbon-based materials enhancing interaction between hydrogen molecules and the host material, it opens a new strategy for hydrogen storage research recently[8-18]. Molecules adsorption on nanostructured surfaces and the hydrogenation of the carbon based materials are also studied systematically [19, 20]. Although transition metals (TMs) decorated system can yield high storage weight through the Kubas interaction, TMs tend to cluster on
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the surface of the substrate due to their lager cohesive energy [9, 21-24]. Up to now, there are no effective methods to prevent TMs clustering. In this view, none of TM decorated-material has yet been practically or commercially successful. Clustering is less severe for the system decorated with alkaline metal (AM) and alkaline-earth metal (AEM) because of their relatively small bulk cohesive energies
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[25, 26]. The nanomaterials decorated by Li atoms, the first AM with the light weight, have also widely attracted attention as potential hydrogen storage medium. In the Li decorated systems, Li atom
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can transfer charges to the substrate, and become the positive ion. Each positively charged Li ion can adsorb multi-hydrogen molecules by polarizing mechanism [27, 28]. For instance, in the Li-decorated graphene system, Li ion can absorb up to four H2 molecules through the polarization mechanism. However, the binding energy of Li to pure graphene is only 0.86 eV/Li, which is less than the cohesive energy of bulk Li (1.63 eV)[29]. Thus, the Li decorated graphene may be unstable, thereby significantly lowering hydrogen storage capacity. In the Li-decorated planar boron sheets, the maximal hydrogen storage capacity can reach up to 9.22 wt% with an average binding energy of H2 0.35 eV/H2[30], which is acceptable for reversible H2 adsorption/desorption near the room temperature. 2
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Due to the tantalizing two-dimensional characterization of the graphene, which scientists have created great interest in the last few years, it would seem natural to explore the synthesis of graphene analogues of layered materials. Among all kinds of the layered nanostructure materials, there exists a
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large family of transition metal dichalcogenides (TMDS) having the chemical formula MX2, where M is transition metals and X stands for S, Se or Te [31]. In the course of studying MX2, recently WS2 have received a great amount of attention in the pursuit of application. Under ambient conditions, experimentally, thin nanosheet WS2 can be prepared through many techniques, which lay a foundation
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for bringing novel material WS2 into operation and studying the characterization of the WS2. To the best of our knowledge, there is no report about hydrogen storage properties of WS2. It is very
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interesting to explore the way of improving the hydrogen gravimetric density and the interaction between hydrogen molecules and the WS2 sheet. In this paper, we used first-principles method to investigate hydrogen storage performance of Li-decorated WS2. As shown below, the binding energy of Li to WS2 is larger than the cohesive energy of bulk Li, which has solved the problem of using metal-decorated single layer material as a hydrogen storage material: the formation of the metal
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cluster. 2. Computational method
Our studies are performed by using first-principles method based on density functional theory (DFT) as implemented in the Cambridge Serial Total Energy Package (CASTEP) [32]. The
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exchange-correlation potential of local density approximation (LDA) is used. To evaluate our results, we also compare the LDA results with those from the generalized gradient approximation (GGA) in
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the Perdew-Burke-Ernzerh (PBE) scheme. Because a lot of previous reports indicated that the exchange and correlation effects depicted by the scheme of GGA-PBE functional can obtain the reasonable results[33-35]. The kinetic cut-off energy for the plane wave expansion is taken to be 450 eV for high precision calculation. Periodic boundary condition and vacuum space of ~24 Å along c-directions are applied in order to avoid interactions between two sheets. The Brillouin zone of supercell is sampled by 4 × 4 ×1 k-points within the Monkhorst-Pack scheme [36] for geometry optimization. In all our calculations, the energy convergence for the self-consistent calculation is set to be 10-5 eV. All atoms are allowed to relax and all the structures are fully optimized without any 3
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symmetry constraints until the atomic force is less than 0.02 eV/Å. Four W atoms and eight S atoms contain in a supercell to simulate an infinite sheet. After full relaxation, we get the stable structure of the monolayer WS2 with the lattice parameters of a=6.24 Å, the S-W bond length 2.38 Å and the W-W
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distance 3.12 Å, which are agreement with the previous results [37]. To describe the adsorption of Li atoms on WS2 sheet, we define the average binding energy of Li as
Eb = −[ E (nLi / WS2 ) − E (WS2 ) − nE( Li)] / n
(1)
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where n is the number of the adsorbed Li atoms, E(nLi/WS2) is the total energy of the fully relaxed Li-decorated WS2 sheet, E(Li) is the energy of a free Li atom, and E(WS2) is the energy of the pristine
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WS2. The hydrogen adsorption energy is defined as the following:
(mH2 /Li/WS2 ) E(Li/WS2 ) (H2 )/
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where m indicates the number of the H2 molecules. E(Li/WS2), E(H2) and E(mH2/ Li/WS2) are the energies of the Li-decorated WS2, hydrogen molecules, and Li-decorated WS2 with m adsorbed H2 molecules, respectively. It is well known that the LDA overestimates the binding energy, while the
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GGA underestimates the binding energy [9, 12, 38]. Thus the accurate binding energy should be reasonable in between the LDA and the GGA values. In this work, both LDA and GGA are used for exchange-correlation energy in all calculations.
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To analysis the electronic structure of Li/WS2, we calculated the charge density difference using the following formula:
(3)
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∆ /
where ρ(Li/WS2), ρ(WS2) and ρ(Li) are the charge density of the system of Li/WS2, pristine WS2 and the Li atom, respectively.
3. Results and discussion 3.1 Li-decorated WS2
WS2 is characterized as quasi-two-dimensional layered configurations with the three-atom-thick, instead of one-atom-thick. The W and S atoms in the layer are bond with strong covalent bonding, while the different layers are packed together with weakly Van der Waals' force like graphite. Within 4
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WS2 monolayer, each W atom is coordinated to six S atoms, and each S atom is coordinated to three W atoms, forming a trigonal prismatic configuration as shown in Fig. 1(b). According to Mulliken charge analysis, one can find that W transfers a small number of charges to S atom, which can be confirmed
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by the Figs. 1(c) and 1(d). We now consider an individual hydrogen molecule adsorption on the pristine WS2 sheet. According to the geometry configuration of WS2, there are three different adsorption sites as shown in Fig. 1(a), which are the top site of S atom (T1), the top site of W atom (T2), and the hollow center of hexagonal ring (H). After full relaxation, for three different sites, it is
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found that the distances between H2 molecule and WS2 sheet are all larger than 3.2 Å indicating that the interaction between H2 molecule and WS2 sheet is too weak. Consequently the pristine WS2 sheet,
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like as the pristine graphene, is not suitable for practical hydrogen storage at ambient temperatures. Previous studies have shown that Li atom decorating on nanomaterials can adsorb muti-hydrogen molecules[39-42], this makes us want to know whether it is possible to enhance the hydrogen storage capacity of WS2 by decorating it with Li atoms. Next, we turn our attention to considering the stable structure of Li-decorated WS2. After full relaxation, the average binding energy (Eb) of Li atom is 1.77
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eV, 1.76 eV, 1.44 eV (GGA: 1.50 eV, 1.26 eV, 1.36 eV) for adsorption on three different sites of WS2 (T2, T1, H), respectively. When the Li atom is put on top of S atom, it moves to the T2 site. Obviously, the optimal adsorption position is on top of the W atom (T2) as shown in Figs. 2(a) and 2(b). The Li atom stands above the triangular S atoms with the distance between Li and the three nearest S atom of
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2.29 Å. It is found that the average binding energy (Eb) of Li atom (1.77 eV) is larger than that in Li-decorated graphene (0.86 eV) and the cohesive energy of bulk Li (1.63 eV)[29], which ensures Li
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atoms dispersing evenly on the surface of the WS2. To understand the binding mechanism between Li and WS2 sheet, it is essential to analysis the
electronic structure of Li/WS2. Figs. 2(c) and 2(d) show the three-dimensional charge density difference of Li/WS2 system with the isosurface of 0.015 e/Å3. The red isosurface describes the charge accumulation while the blue depicts the charge depletion. In the case of the pristine WS2, it is found that some electronic charges are transferred from W to S as shown in Figs. 1(c) and 1(d), which makes the monolayer WS2 to be viewed as a positively charged W plane sandwiched between two negatively charged planes of S atoms. As shown later, this structure plays an important role in hydrogen storage 5
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due to the average adsorption energy of hydrogen molecules enhanced by the orbital hybridization of hydrogen molecules and S atoms. For the system of Li/WS2, from the Figs. 2(c) and 2(d), one can find that a small number of charges accumulate around S atoms and few charges exit on top of the Li atom,
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suggesting that some charges transfer from Li atom to WS2 substrate. This is confirmed by the Mulliken charge analysis that the Li atom transfers 0.97e charges to the WS2 and becomes a cation (see Table 1). Moreover, the positive charged Li atoms can adsorb H2 molecules through static Coulomb interaction. There exists the same phenomenon in other host materials such as B2C and
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BC2N [41, 43]. In order to further understand the interaction between Li atom and the WS2, the partial density of states (PDOS) of WS2 and Li/WS2 is given in Fig. 3. From this figure, one can see that there
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is hybridization between the s orbital of the Li and the p orbital of the S at the -3.1 eV, which enhanced the interaction between Li atom and substrate WS2. The PDOS of WS2 shift toward the lower energies after the Li atom adsorption on it. As a result of the adsorption of an Li atom, semiconductor WS2 becomes metallic due to the donation of ~0.97e charge from the Li atom into the WS2 conduction band.
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To increase the available surface area for hydrogen storage, we now turn to consider the double-sided adsorption structures. On the basis of the above results about the single Li atom decorated WS2, there are three possible adsorption sites (T1, T2, H) on the opposite side of the WS2 layer as shown in Fig. 1(a). According to our results, the second Li atom also prefers to be on the T2
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site of the opposite side of WS2. Thus the preferable configuration is shown in Figs. 5(a) and 5(b) with the two Li atoms located on both sides of WS2. The average binding energy of the Li atom is slightly
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enhanced to 2.03 eV (GGA: 1.72 eV) compared to the value of 1.77 eV (GGA: 1.50 eV) for the case of a single Li atom adsorption, suggesting that Li atom can disperse stably on the WS2 surface. 3.2 Hydrogen storage property of Li-decorated WS2 After establishing the preferable structure of the Li-decorated WS2, we next focus on the
hydrogen storage property of the Li-decorated WS2. When one H2 molecule is adsorbed, there are mainly three different adsorption sites: on top of S, on top of Li, on the hollow center of hexagonal ring. After testing the different positions, one can find that the first H2 molecule prefers to be aslant toward the Li atom and tends to be seating above the H site with the lowest energy configuration as 6
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seen in Figs. 4(a) and 4(d). One by one, we add more H2 molecules to the Li atom. At last, three H2 molecules can be adsorbed around one Li atom. Table 2 summarizes the average bond length of H-H, the average distance between Li atom and H2 molecule, and the average adsorption energy of the
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Li-decorated WS2 with different numbers of H2. It is found that the H-H bond lengths of hydrogen molecules are in the range of 0.782 to 0.785 Å (GGA: 0.757 to 0.766 Å). Comparing with the isolated H2 molecule, the H-H bond lengths are slightly stretched with the number of adsorbed H2 molecules increasing. The molecular feature of hydrogen is unchanged after adsorbed on the surface of
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Li-decorated WS2 sheet. The distances between H2 and Li atom (dLi-H) increase from 2.03 Å to 2.09 Å (GGA: from 2.24 Å to 2.65 Å), which is agreement with the fact that the average adsorption energies
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slightly reduce with the number of adsorbed H2 molecules increasing. These distances are larger than that in Li hydrides (1.59 Å)[44], therefore the interaction between H2 molecules is not chemisorption but physisorption. Table 1 lists the average charge of H2 molecules, Li atom, and WS2 for different systems. It is found that the Li atom donates charges not only to WS2 sheet but also to H2 molecules, indicating that Li as a “bridge” interacts with both H2 and WS2. In order to gain the maximum
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hydrogen gravimetric density and the corresponding adsorption energy of Li-decorated WS2, we also consider the double-sided adsorption, which is given in Figs. 5(c) and 5(d). At most, six H2 molecules can be adsorbed in the 2Li/WS2 system with each Li atom capturing three H2 molecules. The average adsorption energy is 0.13 eV/H2, which falls in the reasonable range (0.1~0.2 eV/H2) [41].
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To further understand the interaction mechanism between H2 molecule and the Li/WS2, as a prototype system, we depict the PDOS of the 2H2/Li/WS2 in Fig. 6. The PDOS of 2H2/Li/WS2 certifies
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that there are no orbital obvious hybridizations between Li atom and H2 molecules, which is in accord with the Li-decorated other nanostructure materials [45]. In addition, we pay attention to the phenomenon that the σ orbital of H2 split into several peaks in the range from -9.5 eV to -8.0 eV, the p orbital of S and the σ orbital of H2 molecule are overlapped at the -8.5 eV and -9.3 eV, indicating that there exist the orbital hybridizations between S atom and H2 molecules, which is quite different from the other materials where there is no orbital hybridizations between the H2 molecules and the host materials. The distance between the S atom and the nearest hydrogen atom is about 2.95 Å, indicating that there is no covalent bond between H and S atom. This is different from the chemical compound of 7
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H2S, in which the S-H bond is 1.33 Å. In order to deeply find out the interaction mechanism, the Mulliken charge analysis is employed to get the charge minute distribution. It is found that the adsorbed H2 molecules carry about 2.16e contrast to 2e in the isolated H2 molecules indicating that H2
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molecules get the charges from the Li/WS2. It is also noted that the net charge of Li atom is increasing from 1.13e to 1.40e with enhancing the number of H2 molecules from one to three. This suggests Li atom further transfers charges to the substrate. Consequently the positive charged Li ion can adsorb the negative charged H2 molecules by electrostatic force. Therefore, not only the Coulomb interaction
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between the Li and H2 molecule, but also the orbital hybridizations between S atom and H2 molecules are beneficial for storing molecular hydrogen.
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4. Conclusion
In conclusion, the behavior of hydrogen molecules adsorbed on Li/WS2 complex is deeply investigated by using density functional theory. It is found that there is very weak interaction between H2 molecule and the pristine WS2. Hence it is impossible to store molecular hydrogen with the pristine WS2. In the case of Li-decorated on one side of the WS2 sheet, the results announce that at most three
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H2 molecules can be bound to one Li atom. In the case of Li decorated on double sides of the WS2 sheet, Six H2 molecules can be held with each Li atom capturing three H2 molecules. The average adsorption energy is 0.13 eV/H2, promising for potential hydrogen storage application at ambient
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temperature. It is noted that the binding mechanism of hydrogen molecules on Li/WS2 system originates from two interactions: 1) the electrostatic Coulomb attraction between Li atoms and H2
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molecules. 2) the orbital hybridizations between the σ orbital of H2 molecule and the p orbital of S atom.
Acknowledgements
The work was support by the NSF of China (Grant Nos. 11404112, 11104072) and Research in Cutting-edge Technologies of Zhengzhou (Grant No. 141PRKXF622).
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Figure Captions: Figure 1. The optimized atomic structure to WS2 (a) Top view, (b) Side view. (c) Top view, (d) Side
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view of charge differences for WS2 with an isovalue of 0.055 e/Å3. The red and blue iso-surface indicates space charge accumulation and depletion. The dark brown and green balls in this figure and figures below are W and S atoms, respectively. The possible
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adsorption sites are T1, T2, H.
Figure 2. The preferable configuration of the Li-decorated WS2 (a) Side view, (b) Top view of Li/WS2.
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(c) Side view, (d) Top view of charge density differences for Li/WS2 with an isovalue of 0.015 e/Å3.
Figure 3. PDOS of WS2 (a) S atoms, (b) W atoms. PDOS of Li/ WS2 (c) S atoms, (d) W atoms, (e) Li atom. The Fermi level is set as zero.
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Figure 4. The optimized configurations of Li/WS2 with different numbers of H2 molecules. (a) Top view of 1H2/ Li/WS2, (b) Top view of 2H2/ Li/WS2, (c)Top view of 3H2/ Li/WS2, (d) Side view of 1H2/ Li/WS2 (e) Side view of 2H2/ Li/WS2 (f) Side view of 3H2/ Li/WS2. The silvery
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grey and yellow balls in this figure and figures below are Li and hydrogen atoms, respectively.
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Figure 5. The optimized configurations of 2Li/WS2 and 6H2/2Li/WS2. (a)Top view of 2Li/WS2, (b) Side view of 2Li/WS2, (c) Top view of 6H2/2Li/WS2, (h) Side view of 6H2/2Li/WS2.
Figure 6. PDOS of 2H2/ Li/WS2. The Fermi level is set as zero.
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Table 1. The average charges of Li atom(c-Li) (e), H2 molecule(c-H2) (e), the WS2 (c-WS2) (e), and the
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corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA.
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Table 2. Average bond length of H-H (dH-H) (Å), distance between Li and H2 molecule (dLi-H2) (Å), and
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in LDA and GGA, respectively.
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the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2
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c-Li
c-H2
c-WS2
Ead
Li/WS2 1H2/Li/WS2 2H2/Li/WS2 3H2/Li/WS2
0.97 1.13 1.31 1.40
--0.16 -0.17 -0.16
-0.97 -0.97 -0.97 -0.92
1.77 0.20 0.04 0.10
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Table 1. The average charges of Li atom(c-Li) (e), H2 molecule(c-H2) (e), the WS2 (c-WS2) (e), and the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA.
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Table 2. Average bond length of H-H (dH-H) (Å), distance between Li and H2 molecule (dLi-H2) (Å), and the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA and GGA, respectively.
GGA dH-H dLi-H 0.766 2.24 0.761 2.40 0.757 2.65
Ead 0.17 0.08 0.09
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1H2/Li/WS2 2H2/Li/WS2 3H2/Li/WS2
dH-H 0.782 0.782 0.785
LDA dLi-H Ead 2.03 0.20 2.06 0.04 2.09 0.09
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nH2/Li/WS2
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Li decorated WS2 sheet as hydrogen storage media. Li atom can disperse on the WS2 surface without clustering. H2 adsorption/desorption can operate under the room temperature. The adsorption of H2 is electrostatic force and small S-H hybridization.
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S0042-207X(15)00134-7
DOI:
10.1016/j.vacuum.2015.03.034
Reference:
VAC 6616
To appear in:
Vacuum
Received Date: 29 January 2015 Revised Date:
25 March 2015
Accepted Date: 26 March 2015
Please cite this article as: Song N, Wang Y, Ding S, Yang Y, Zhang J, Xu B, Yi L, Jia Y, The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study, Vaccum (2015), doi: 10.1016/ j.vacuum.2015.03.034. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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The hydrogen storage behavior of Li-decorated monolayer WS2: A first-principles study Nahong Song1, Yusheng Wang2,3*, Songyang Ding1, Yuye Yang1, Jing Zhang2,4, Bin Xu2,4, Lin Yi4, Yu
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Jia3* 1) College of Computer and Information Engineering, Henan University of Economics and Law, Zhengzhou, Henan, 450000, China
2) College of Mathematics and Information Science, North China University of Water Resources and
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Electric Power, Zhengzhou, Henan, 450011, China
3) International Joint Research Laboratory for Quantum Functional Materials of Henan, and School
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of Physics and Engineering, Zhengzhou University, Zhengzhou 450001, China 4) Department of Physics, Huazhong University of Science and Technology, Wuhan 430074, China * Corresponding author, Tel.: 86-371-67739336; E-mail: [email protected] (Y. Wang); [email protected] (Y. Jia) Abstract
The hydrogen storage properties of Li-decorated graphene-like monolayer WS2 have been
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systematically studied by using first principles calculations based on density functional theory. The present results bring to light that the pristine WS2 is not suitable for storing hydrogen due to the very weak interaction between the pristine WS2 sheet and the H2 molecules. The Li atoms can be adsorbed
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strongly on both sides of WS2 without clustering owing to the larger binding energy of Li to WS2 sheet and the Coulomb repulsion between the Li adatoms. The results reveal that the adsorption of H2
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molecules comes from not only electrostatic force between the Li atom and H2 molecules, but also the hybridization mechanism between the σ orbital of H2 molecules and the p orbital of S atom. This is quite different from the other materials where there is no orbital hybridizations between the H2 molecules and the host materials. For the 2Li/WS2 system, the average adsorption energy is 0.13 eV/H2, indicating that Li decorated WS2 can be an optimal choice for the reversible hydrogen adsorption/desorption at room temperature. PACS number: 68.43.Bc, 73.20.At, 84.60.Ve Keywords: hydrogen storage; decoration; WS2 sheet 1
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I. Introduction In modern society, the problems of the energy shortage and environment pollution lead to the increasing need for the green energy carrier, which has aroused growing research interest in hydrogen
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due to its abundance, pollution-free and packing more energy unit mass than any other traditional fuels [1-6]. However, compressed gas and liquefied hydrogen as the traditional storing models, fail to be put into a large-scale application due to heavy energy consumption and safety problems. Moreover, metal hydride can’t perform hydrogen reversibly under ambient thermodynamic conditions for motor
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vehicles and other mechanical systems [7]. Therefore it is an urgent task to find a way to store hydrogen with high gravimetric density and moderate adsorption energy.
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Since the discovery of decorating metal atom to carbon-based materials enhancing interaction between hydrogen molecules and the host material, it opens a new strategy for hydrogen storage research recently[8-18]. Molecules adsorption on nanostructured surfaces and the hydrogenation of the carbon based materials are also studied systematically [19, 20]. Although transition metals (TMs) decorated system can yield high storage weight through the Kubas interaction, TMs tend to cluster on
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the surface of the substrate due to their lager cohesive energy [9, 21-24]. Up to now, there are no effective methods to prevent TMs clustering. In this view, none of TM decorated-material has yet been practically or commercially successful. Clustering is less severe for the system decorated with alkaline metal (AM) and alkaline-earth metal (AEM) because of their relatively small bulk cohesive energies
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[25, 26]. The nanomaterials decorated by Li atoms, the first AM with the light weight, have also widely attracted attention as potential hydrogen storage medium. In the Li decorated systems, Li atom
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can transfer charges to the substrate, and become the positive ion. Each positively charged Li ion can adsorb multi-hydrogen molecules by polarizing mechanism [27, 28]. For instance, in the Li-decorated graphene system, Li ion can absorb up to four H2 molecules through the polarization mechanism. However, the binding energy of Li to pure graphene is only 0.86 eV/Li, which is less than the cohesive energy of bulk Li (1.63 eV)[29]. Thus, the Li decorated graphene may be unstable, thereby significantly lowering hydrogen storage capacity. In the Li-decorated planar boron sheets, the maximal hydrogen storage capacity can reach up to 9.22 wt% with an average binding energy of H2 0.35 eV/H2[30], which is acceptable for reversible H2 adsorption/desorption near the room temperature. 2
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Due to the tantalizing two-dimensional characterization of the graphene, which scientists have created great interest in the last few years, it would seem natural to explore the synthesis of graphene analogues of layered materials. Among all kinds of the layered nanostructure materials, there exists a
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large family of transition metal dichalcogenides (TMDS) having the chemical formula MX2, where M is transition metals and X stands for S, Se or Te [31]. In the course of studying MX2, recently WS2 have received a great amount of attention in the pursuit of application. Under ambient conditions, experimentally, thin nanosheet WS2 can be prepared through many techniques, which lay a foundation
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for bringing novel material WS2 into operation and studying the characterization of the WS2. To the best of our knowledge, there is no report about hydrogen storage properties of WS2. It is very
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interesting to explore the way of improving the hydrogen gravimetric density and the interaction between hydrogen molecules and the WS2 sheet. In this paper, we used first-principles method to investigate hydrogen storage performance of Li-decorated WS2. As shown below, the binding energy of Li to WS2 is larger than the cohesive energy of bulk Li, which has solved the problem of using metal-decorated single layer material as a hydrogen storage material: the formation of the metal
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cluster. 2. Computational method
Our studies are performed by using first-principles method based on density functional theory (DFT) as implemented in the Cambridge Serial Total Energy Package (CASTEP) [32]. The
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exchange-correlation potential of local density approximation (LDA) is used. To evaluate our results, we also compare the LDA results with those from the generalized gradient approximation (GGA) in
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the Perdew-Burke-Ernzerh (PBE) scheme. Because a lot of previous reports indicated that the exchange and correlation effects depicted by the scheme of GGA-PBE functional can obtain the reasonable results[33-35]. The kinetic cut-off energy for the plane wave expansion is taken to be 450 eV for high precision calculation. Periodic boundary condition and vacuum space of ~24 Å along c-directions are applied in order to avoid interactions between two sheets. The Brillouin zone of supercell is sampled by 4 × 4 ×1 k-points within the Monkhorst-Pack scheme [36] for geometry optimization. In all our calculations, the energy convergence for the self-consistent calculation is set to be 10-5 eV. All atoms are allowed to relax and all the structures are fully optimized without any 3
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symmetry constraints until the atomic force is less than 0.02 eV/Å. Four W atoms and eight S atoms contain in a supercell to simulate an infinite sheet. After full relaxation, we get the stable structure of the monolayer WS2 with the lattice parameters of a=6.24 Å, the S-W bond length 2.38 Å and the W-W
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distance 3.12 Å, which are agreement with the previous results [37]. To describe the adsorption of Li atoms on WS2 sheet, we define the average binding energy of Li as
Eb = −[ E (nLi / WS2 ) − E (WS2 ) − nE( Li)] / n
(1)
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where n is the number of the adsorbed Li atoms, E(nLi/WS2) is the total energy of the fully relaxed Li-decorated WS2 sheet, E(Li) is the energy of a free Li atom, and E(WS2) is the energy of the pristine
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WS2. The hydrogen adsorption energy is defined as the following:
(mH2 /Li/WS2 ) E(Li/WS2 ) (H2 )/
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where m indicates the number of the H2 molecules. E(Li/WS2), E(H2) and E(mH2/ Li/WS2) are the energies of the Li-decorated WS2, hydrogen molecules, and Li-decorated WS2 with m adsorbed H2 molecules, respectively. It is well known that the LDA overestimates the binding energy, while the
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GGA underestimates the binding energy [9, 12, 38]. Thus the accurate binding energy should be reasonable in between the LDA and the GGA values. In this work, both LDA and GGA are used for exchange-correlation energy in all calculations.
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To analysis the electronic structure of Li/WS2, we calculated the charge density difference using the following formula:
(3)
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∆ /
where ρ(Li/WS2), ρ(WS2) and ρ(Li) are the charge density of the system of Li/WS2, pristine WS2 and the Li atom, respectively.
3. Results and discussion 3.1 Li-decorated WS2
WS2 is characterized as quasi-two-dimensional layered configurations with the three-atom-thick, instead of one-atom-thick. The W and S atoms in the layer are bond with strong covalent bonding, while the different layers are packed together with weakly Van der Waals' force like graphite. Within 4
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WS2 monolayer, each W atom is coordinated to six S atoms, and each S atom is coordinated to three W atoms, forming a trigonal prismatic configuration as shown in Fig. 1(b). According to Mulliken charge analysis, one can find that W transfers a small number of charges to S atom, which can be confirmed
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by the Figs. 1(c) and 1(d). We now consider an individual hydrogen molecule adsorption on the pristine WS2 sheet. According to the geometry configuration of WS2, there are three different adsorption sites as shown in Fig. 1(a), which are the top site of S atom (T1), the top site of W atom (T2), and the hollow center of hexagonal ring (H). After full relaxation, for three different sites, it is
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found that the distances between H2 molecule and WS2 sheet are all larger than 3.2 Å indicating that the interaction between H2 molecule and WS2 sheet is too weak. Consequently the pristine WS2 sheet,
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like as the pristine graphene, is not suitable for practical hydrogen storage at ambient temperatures. Previous studies have shown that Li atom decorating on nanomaterials can adsorb muti-hydrogen molecules[39-42], this makes us want to know whether it is possible to enhance the hydrogen storage capacity of WS2 by decorating it with Li atoms. Next, we turn our attention to considering the stable structure of Li-decorated WS2. After full relaxation, the average binding energy (Eb) of Li atom is 1.77
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eV, 1.76 eV, 1.44 eV (GGA: 1.50 eV, 1.26 eV, 1.36 eV) for adsorption on three different sites of WS2 (T2, T1, H), respectively. When the Li atom is put on top of S atom, it moves to the T2 site. Obviously, the optimal adsorption position is on top of the W atom (T2) as shown in Figs. 2(a) and 2(b). The Li atom stands above the triangular S atoms with the distance between Li and the three nearest S atom of
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2.29 Å. It is found that the average binding energy (Eb) of Li atom (1.77 eV) is larger than that in Li-decorated graphene (0.86 eV) and the cohesive energy of bulk Li (1.63 eV)[29], which ensures Li
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atoms dispersing evenly on the surface of the WS2. To understand the binding mechanism between Li and WS2 sheet, it is essential to analysis the
electronic structure of Li/WS2. Figs. 2(c) and 2(d) show the three-dimensional charge density difference of Li/WS2 system with the isosurface of 0.015 e/Å3. The red isosurface describes the charge accumulation while the blue depicts the charge depletion. In the case of the pristine WS2, it is found that some electronic charges are transferred from W to S as shown in Figs. 1(c) and 1(d), which makes the monolayer WS2 to be viewed as a positively charged W plane sandwiched between two negatively charged planes of S atoms. As shown later, this structure plays an important role in hydrogen storage 5
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due to the average adsorption energy of hydrogen molecules enhanced by the orbital hybridization of hydrogen molecules and S atoms. For the system of Li/WS2, from the Figs. 2(c) and 2(d), one can find that a small number of charges accumulate around S atoms and few charges exit on top of the Li atom,
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suggesting that some charges transfer from Li atom to WS2 substrate. This is confirmed by the Mulliken charge analysis that the Li atom transfers 0.97e charges to the WS2 and becomes a cation (see Table 1). Moreover, the positive charged Li atoms can adsorb H2 molecules through static Coulomb interaction. There exists the same phenomenon in other host materials such as B2C and
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BC2N [41, 43]. In order to further understand the interaction between Li atom and the WS2, the partial density of states (PDOS) of WS2 and Li/WS2 is given in Fig. 3. From this figure, one can see that there
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is hybridization between the s orbital of the Li and the p orbital of the S at the -3.1 eV, which enhanced the interaction between Li atom and substrate WS2. The PDOS of WS2 shift toward the lower energies after the Li atom adsorption on it. As a result of the adsorption of an Li atom, semiconductor WS2 becomes metallic due to the donation of ~0.97e charge from the Li atom into the WS2 conduction band.
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To increase the available surface area for hydrogen storage, we now turn to consider the double-sided adsorption structures. On the basis of the above results about the single Li atom decorated WS2, there are three possible adsorption sites (T1, T2, H) on the opposite side of the WS2 layer as shown in Fig. 1(a). According to our results, the second Li atom also prefers to be on the T2
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site of the opposite side of WS2. Thus the preferable configuration is shown in Figs. 5(a) and 5(b) with the two Li atoms located on both sides of WS2. The average binding energy of the Li atom is slightly
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enhanced to 2.03 eV (GGA: 1.72 eV) compared to the value of 1.77 eV (GGA: 1.50 eV) for the case of a single Li atom adsorption, suggesting that Li atom can disperse stably on the WS2 surface. 3.2 Hydrogen storage property of Li-decorated WS2 After establishing the preferable structure of the Li-decorated WS2, we next focus on the
hydrogen storage property of the Li-decorated WS2. When one H2 molecule is adsorbed, there are mainly three different adsorption sites: on top of S, on top of Li, on the hollow center of hexagonal ring. After testing the different positions, one can find that the first H2 molecule prefers to be aslant toward the Li atom and tends to be seating above the H site with the lowest energy configuration as 6
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seen in Figs. 4(a) and 4(d). One by one, we add more H2 molecules to the Li atom. At last, three H2 molecules can be adsorbed around one Li atom. Table 2 summarizes the average bond length of H-H, the average distance between Li atom and H2 molecule, and the average adsorption energy of the
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Li-decorated WS2 with different numbers of H2. It is found that the H-H bond lengths of hydrogen molecules are in the range of 0.782 to 0.785 Å (GGA: 0.757 to 0.766 Å). Comparing with the isolated H2 molecule, the H-H bond lengths are slightly stretched with the number of adsorbed H2 molecules increasing. The molecular feature of hydrogen is unchanged after adsorbed on the surface of
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Li-decorated WS2 sheet. The distances between H2 and Li atom (dLi-H) increase from 2.03 Å to 2.09 Å (GGA: from 2.24 Å to 2.65 Å), which is agreement with the fact that the average adsorption energies
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slightly reduce with the number of adsorbed H2 molecules increasing. These distances are larger than that in Li hydrides (1.59 Å)[44], therefore the interaction between H2 molecules is not chemisorption but physisorption. Table 1 lists the average charge of H2 molecules, Li atom, and WS2 for different systems. It is found that the Li atom donates charges not only to WS2 sheet but also to H2 molecules, indicating that Li as a “bridge” interacts with both H2 and WS2. In order to gain the maximum
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hydrogen gravimetric density and the corresponding adsorption energy of Li-decorated WS2, we also consider the double-sided adsorption, which is given in Figs. 5(c) and 5(d). At most, six H2 molecules can be adsorbed in the 2Li/WS2 system with each Li atom capturing three H2 molecules. The average adsorption energy is 0.13 eV/H2, which falls in the reasonable range (0.1~0.2 eV/H2) [41].
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To further understand the interaction mechanism between H2 molecule and the Li/WS2, as a prototype system, we depict the PDOS of the 2H2/Li/WS2 in Fig. 6. The PDOS of 2H2/Li/WS2 certifies
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that there are no orbital obvious hybridizations between Li atom and H2 molecules, which is in accord with the Li-decorated other nanostructure materials [45]. In addition, we pay attention to the phenomenon that the σ orbital of H2 split into several peaks in the range from -9.5 eV to -8.0 eV, the p orbital of S and the σ orbital of H2 molecule are overlapped at the -8.5 eV and -9.3 eV, indicating that there exist the orbital hybridizations between S atom and H2 molecules, which is quite different from the other materials where there is no orbital hybridizations between the H2 molecules and the host materials. The distance between the S atom and the nearest hydrogen atom is about 2.95 Å, indicating that there is no covalent bond between H and S atom. This is different from the chemical compound of 7
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H2S, in which the S-H bond is 1.33 Å. In order to deeply find out the interaction mechanism, the Mulliken charge analysis is employed to get the charge minute distribution. It is found that the adsorbed H2 molecules carry about 2.16e contrast to 2e in the isolated H2 molecules indicating that H2
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molecules get the charges from the Li/WS2. It is also noted that the net charge of Li atom is increasing from 1.13e to 1.40e with enhancing the number of H2 molecules from one to three. This suggests Li atom further transfers charges to the substrate. Consequently the positive charged Li ion can adsorb the negative charged H2 molecules by electrostatic force. Therefore, not only the Coulomb interaction
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between the Li and H2 molecule, but also the orbital hybridizations between S atom and H2 molecules are beneficial for storing molecular hydrogen.
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4. Conclusion
In conclusion, the behavior of hydrogen molecules adsorbed on Li/WS2 complex is deeply investigated by using density functional theory. It is found that there is very weak interaction between H2 molecule and the pristine WS2. Hence it is impossible to store molecular hydrogen with the pristine WS2. In the case of Li-decorated on one side of the WS2 sheet, the results announce that at most three
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H2 molecules can be bound to one Li atom. In the case of Li decorated on double sides of the WS2 sheet, Six H2 molecules can be held with each Li atom capturing three H2 molecules. The average adsorption energy is 0.13 eV/H2, promising for potential hydrogen storage application at ambient
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temperature. It is noted that the binding mechanism of hydrogen molecules on Li/WS2 system originates from two interactions: 1) the electrostatic Coulomb attraction between Li atoms and H2
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molecules. 2) the orbital hybridizations between the σ orbital of H2 molecule and the p orbital of S atom.
Acknowledgements
The work was support by the NSF of China (Grant Nos. 11404112, 11104072) and Research in Cutting-edge Technologies of Zhengzhou (Grant No. 141PRKXF622).
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Figure Captions: Figure 1. The optimized atomic structure to WS2 (a) Top view, (b) Side view. (c) Top view, (d) Side
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view of charge differences for WS2 with an isovalue of 0.055 e/Å3. The red and blue iso-surface indicates space charge accumulation and depletion. The dark brown and green balls in this figure and figures below are W and S atoms, respectively. The possible
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adsorption sites are T1, T2, H.
Figure 2. The preferable configuration of the Li-decorated WS2 (a) Side view, (b) Top view of Li/WS2.
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(c) Side view, (d) Top view of charge density differences for Li/WS2 with an isovalue of 0.015 e/Å3.
Figure 3. PDOS of WS2 (a) S atoms, (b) W atoms. PDOS of Li/ WS2 (c) S atoms, (d) W atoms, (e) Li atom. The Fermi level is set as zero.
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Figure 4. The optimized configurations of Li/WS2 with different numbers of H2 molecules. (a) Top view of 1H2/ Li/WS2, (b) Top view of 2H2/ Li/WS2, (c)Top view of 3H2/ Li/WS2, (d) Side view of 1H2/ Li/WS2 (e) Side view of 2H2/ Li/WS2 (f) Side view of 3H2/ Li/WS2. The silvery
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grey and yellow balls in this figure and figures below are Li and hydrogen atoms, respectively.
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Figure 5. The optimized configurations of 2Li/WS2 and 6H2/2Li/WS2. (a)Top view of 2Li/WS2, (b) Side view of 2Li/WS2, (c) Top view of 6H2/2Li/WS2, (h) Side view of 6H2/2Li/WS2.
Figure 6. PDOS of 2H2/ Li/WS2. The Fermi level is set as zero.
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Table 1. The average charges of Li atom(c-Li) (e), H2 molecule(c-H2) (e), the WS2 (c-WS2) (e), and the
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corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA.
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Table 2. Average bond length of H-H (dH-H) (Å), distance between Li and H2 molecule (dLi-H2) (Å), and
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in LDA and GGA, respectively.
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the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2
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c-Li
c-H2
c-WS2
Ead
Li/WS2 1H2/Li/WS2 2H2/Li/WS2 3H2/Li/WS2
0.97 1.13 1.31 1.40
--0.16 -0.17 -0.16
-0.97 -0.97 -0.97 -0.92
1.77 0.20 0.04 0.10
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Table 1. The average charges of Li atom(c-Li) (e), H2 molecule(c-H2) (e), the WS2 (c-WS2) (e), and the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA.
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Table 2. Average bond length of H-H (dH-H) (Å), distance between Li and H2 molecule (dLi-H2) (Å), and the corresponding adsorption energy of H2 (Ead) (eV/H2) for a single Li atom decorated WS2 in LDA and GGA, respectively.
GGA dH-H dLi-H 0.766 2.24 0.761 2.40 0.757 2.65
Ead 0.17 0.08 0.09
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1H2/Li/WS2 2H2/Li/WS2 3H2/Li/WS2
dH-H 0.782 0.782 0.785
LDA dLi-H Ead 2.03 0.20 2.06 0.04 2.09 0.09
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nH2/Li/WS2
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Li decorated WS2 sheet as hydrogen storage media. Li atom can disperse on the WS2 surface without clustering. H2 adsorption/desorption can operate under the room temperature. The adsorption of H2 is electrostatic force and small S-H hybridization.
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