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Improved catalytic performance of monolayer nano-triangles WS2 and MoS2 on HER by 3d metals doping
Computational Materials Science 159 (2019) 333–340
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Improved catalytic performance of monolayer nano-triangles WS2 and MoS2 on HER by 3d metals doping
T
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Yurong An, Xiaoli Fan , Hanjie Liu, Zhifen Luo State Key Laboratory of Solidification Processing, Center for Advanced Lubrication and Seal Materials, School of Material Science and Engineering, Northwestern Polytechnical University, 127 YouYi Western Road, Xi’an, Shaanxi 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D materials Transition-metaldichalcogenids Hydrogen evolution reaction Catalytic performance Density functional theory
Recently, monolayer transition metal dichalcogenides (TMDCs) such as MoS2 and WS2, exhibit distinguished catalytic performance on hydrogen evolution reaction (HER). Triangular shaped monolayers are the most popular morphology for the chemically synthesized monolayer-TMDCs. In this study, we find a way to further improve the catalytic performance of monolayer nano-triangles (mNT) MS2 (M = W, Mo) on HER. By adopting the first-principles calculation methods based on the density functional theory, we studied the catalytic activity of the mNT MS2 with the substitutional doping of Me (Me = Cr, Mn, Co) atoms on the edges. The detailed electronic structures were also investigated to better understand the catalytic activity. Our calculations show that the Me atoms doping on the edge changes the electronic states near the Fermi level. Additionally, the enhancement in the number of active sties and conductivity result in the improvement of the catalytic performance of mNT MS2 on HER. More importantly, we demonstrate that the electronic structures and edge configurations, particularly the structures on the vertexes of the triangular mNT MS2 are closely related to the edge composition.
1. Introduction Hydrogen is considered to be an ideal energy carrier [1] not only because it possesses the ultrahigh energy density but also because there is no green-house gas such as carbon dioxide emitting during combustion process. Efficient hydrogen production holds tremendous promise for the clean and renewable energy development [2]. Pt-group metals are the most active catalyst for electrochemistry hydrogen evolution reaction (HER) [3,4]. But these metals are too rare and expensive to be used for the large-scale production of hydrogen. Recent study shows that the catalytic performance of monolayer MoS2 on HER is comparable to that of the Pt-group metals [5]. Hence, extensive studies are devoted to investigate and improve the catalytic performance of transition-metal dichalcogenides (TMDCs) of MS2 (M = Mo, W, V) catalyst for HER [6–17]. Single-layer MS2 (M = Mo, W) is the sandwiched layer of S-M-S, which has several possible polymorphs, namely, 1H, 1T, ZT or 1T′, as well as the DT lattice [6,7,14,15,18–23]. The basal plane of the 1H phase is HER-inert, while the 1T and ZT phases are active toward HER. Atomic-thin nano-sheets of MoS2 and WS2 generated by the liquid exploiting method both showed advanced catalytic performance on HER [6,7], and their catalytic activity were attributed to the metallic 1T and
⁎
ZT phases. On the other hand, monolayer TMDCs were synthesized successfully by the thin-film deposition methods and studied by both experimental and computational tools [5,24–27]. The deposited monolayer TMDCs were reported as the advanced catalysts for HER [5,9,10]. The shapes of the deposited monolayer TMDCs are unexpected regular [5,24–27], including hexagonal, stars, bowties and triangles. The nano-triangles (mNT) are the most popular ones [5,24–37] among all the reported nanopolygons of monolayer TMDCs. As for the mNT MoS2, the most studied monolayer TMDCs, its edges are either Mo terminated or S terminated depending on the experimental conditions [28–37]. According to the previous study [5], the catalytic activity of the as grown mMT MoS2 is derived from the edge sites, particular the S atoms. Thus, various edge engineering and defect engineering techniques were adopted to create more active sites on the inert basal plane to further improve the catalytic performance of monolayer TMDCs on HER [8,11,13]. Actually, the catalytic activity of mNT MS2 on HER are attribute to their active edge sites with proper electronic structures to facilitate HER [5,17]. Additionally, it is well known that edges and corners of nanoclusters materials in nanoscale or sub-nanoscale dominantly affect the material properties. In this context, there is a big chance we can
Corresponding author. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.commatsci.2018.12.032 Received 22 June 2018; Received in revised form 7 December 2018; Accepted 17 December 2018 0927-0256/ © 2018 Published by Elsevier B.V.
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detailed edge structures are similar with each other. Particularly, every three metal atoms cluster in a three atoms chain on the edges. As showing in Fig. 1, Me atoms doping (Me = Cr, Mn, Co) changes the edge configurations of mNT WS2 and MoS2. For Mn-doped mNT WS2 and MoS2 shown in Fig. 1(c) and (g), all the S monomers on the three vertexes tilt away from its original position and bond to the S monomers next to it. This kind of deformation on the vertex also happens for the Co-doped mNT WS2 and MoS2 as showing Fig. 1(d) and (h). But the Cr-doped mNT WS2 and MoS2 maintains their original structures on the vertex. Indeed, all the structures showing in Fig. 1 are those having the lowest energy. Our calculation shows that the deformed structures of Cr-doped WS2 and MoS2 on the vertexes are less stable relative to the structures shown in Fig. 1(b) and (f) by 0.4 and 0.5 eV. Additional, the clustering phenomenon of three metal atom on the edges disappear after Me atoms doping for both WS2 and MoS2. The formation energies for the Me-doped mNT WS2 and MoS2 were calculated via the following formula [57],
tune the electronic structure through modulating the edge structures to enhance the catalytic performance of MS2 on HER. Doping is one of the most feasible methods to modulate the electronic properties of materials [38–42], which also has proved to be an efficient way to improve the HER performance [43–51]. In the present study, by performing the first-principles calculations, we study the effect of 3d metals Me (Me = Cr, Mn, Co) doping on the edge of mNT MS2 on the catalytic activity. We examined the atomic structures and the catalytic activity for HER, as well as the electronic and magnetic properties of Me atoms doped mNT WS2 and MoS2. It turns out that the Me-doping on the edge of mNT WS2 and MoS2 increase the number of active sites through change their electronic structure near the Fermi level. 2. Calculation methods All the calculations were performed by the first-principles method based on the density functional theory (DFT) within the Vienna ab-initio simulation package (VASP) [52]. The ion–electron interactions were treated with the projector augmented wave (PAW) pseudopotentials [53]. The Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [54] was employed to calculate the electronic exchange-correlation interaction. A plane wave basis set with a cutoff energy of 400 eV was used to expand the wave functions. The first Brillouin-Zone was sampled by the Monkhorst-Pack method [55]. A 15 Å vacuum layer was added along the normal direction above the monolayer to avoid interactions between the adjacent images. The convergence criterion for the self-consistency process was set to 10−5 eV between two ionic steps, and the atoms were fully relaxed until the force on each atom was less than 0.02 eV/Å. The Gibbs free energy change for the adsorption of hydrogen is calculated via
Ef = E (doped - MS2) − E (MS2) + nμ (M ) − nμ (Me )
3.2. Catalytic activity of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) for HER
(1)
ΔGH = ΔEH + ΔEZPE − T ΔSH
As we know, the high active catalyst for HER should have two essential characteristics, high conductivity which accelerates the combination of electrons and protons and abundant active sites. According to previous study, the Gibbs free energy change (ΔGH ) for the adsorption of hydrogen calculated by the density functional theory based method is a good descriptor for the catalytic activity toward HER [5], it shows that the best catalytic performance on HER achieves at neutral ΔGH . In other words, the optimum value of the free energy changes for hydrogen adsorption should be neutral (ΔGH = 0 ). In this context, we set −0.25 eV ≤ ΔGH ≤ 0.25 eV as a criterion to estimate the catalytic activity of mNT MS2 and Me-doped mNT MS2 for hydrogen evolution. It has been confirmed that the catalytic activity of monolayer WS2 and MoS2 nanoparticles comes from the edge sites [5,17]. Hence, we evaluated the catalytic activity of mNT MS2 and Me-doped mNT MS2 for HER by calculating the ΔGH for H adsorption on the edge sites. As we know, the edge configurations of mNT MS2 and Me-doped mNT MS2 both have certain symmetry as showing Fig. 1, thus we have studied 7 edge sites representing the whole 21 edge sties. Our calculated ΔGH are summarized in Table 1. We also calculated the ratio of number of active sites over the total number of edge sites to evaluate the catalytic performance. Our calculations show that all the ΔGH on the edge sites of mNT WS2 with 7 W atoms on each edge are positive and ranging from 0.31 to 0.89 eV, which means that hydrogen bonds too weak to be adsorbed. In other words, all the edge sites of mNT WS2 with 7 W atoms on each edge are inert for HER. However, we can see from Table 1 that the Medoping reduces ΔGH by as large as 1.53 eV, and really improves the catalytic performance by increasing the number of active sites. More specifically, Cr doping increases the ratio of number of active sites over the total number of edge sites from 0 to 18/21, while Mn and Co doping increase the ratios to 15/21. Compared to mNT WS2, the catalytic activity of mNT MoS2 with 7 Mo atoms on each edge is better. There are 12 active sites over the total 21 edge sites. Again, ΔGH on the edges of mNT MoS2 are all positive, and Me-doping on the edges reduces the
where ΔEH is the adsorption energy of hydrogen which is defined as:
ΔEH = EMe − MS2+ H − EMe − MS2 −
1 EH 2 2
(3)
where E (doped - MS2) and E (MS2) are the total energy of Me-doped mNT MS2 and mNT MS2, respectively. μ (M ) and μ (Me ) are the chemical potentials of M atom and Me atom, which are taken from the energy of most stable bulk phase. n is the number of doped Me atoms. The resulted formation energy per atom for Cr-, Mn-, and Co-doped WS2 are 0.47, 0.77, and 1.38 eV, and the counterpart results for mNT MoS2 are 0.53, 0.82, and 1.45 eV. Our calculations show that these doped systems are achievable in experiments. Moreover, the Cr atoms are readily doped than Mn and Co.
(2)
and ΔEZPE is the difference in zero point energy between the adsorbed atomic hydrogen and hydrogen in gas phase. ΔSH is the entropy difference between adsorbed state and gas phase. The entropy of the adsorbed atomic hydrogen is regarded as ΔSH = −S H2/2, where SH 2 is the entropy of molecule hydrogen in the gas phase at standard conditions and ΔEZPE − T ΔSH is 0.24 eV [7,16,56]. So Eq. (1) is simplified to ΔGH = ΔEH + 0.24 . In Eq. (2), EMe − MS2+ H and EMe − MS2 are the energies of the Me-doped mNT MS2 with and without hydrogen adsorption, while E H2 is the energy of hydrogen molecule in gas phase. 3. Results and discussions 3.1. Atomic structures of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) Previously, we investigates the catalytic performance of monolayer nanopolygons MS2 (Mo = W, Mo) for HER [17], it shows that the active site density of monolayer nano-triangles (mNT) MS2, particular mNT WS2 with 7 metal atoms on each edge drops down suddenly relative to the triangles with 5 and 6 metal atoms on the edge. So we simply pick mNT WS2 and MoS2 with 7 metal atoms on each edge as the representive mNTs to study the effect of Me atoms doping on HER performance. Fig. 1 shows the atomic structures for the mNT WS2 and MoS2 with 7 metal atoms on each edge. As we can see, mNT MS2 arranges in the sandwiched S-M-S way where one atomic layer of M atoms is enclosed by two atomic layers of S atoms. The M atoms on the edges are 50% terminated by the S atoms which are called as S monomers. It indicates that the both mNT WS2 and MoS2 have the C3 symmetry, and their 334
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Fig. 1. Atomic structures for monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) doping. (a) mNT WS2 and (e) mNT MoS2, Cr-doped (b) mNT WS2 and (f) mNT MoS2, Mn-doped (c) mNT WS2 and (g) mNT MoS2, Codoped (d) mNT WS2 and (h) mNT MoS2. The probable adsorption sites on the edge are illustrated in (a).
molecular orbital-lowest unoccupied molecular orbital) energy gaps of about 0.9 eV. Additionally, Me-doping reduces their energy gaps largely as showing in Fig. 2. Particularly, both Mn and Co doped mNT WS2 and mNT MoS2 show metallic character with some electronic states crossing the Fermi level. Although Cr doped mNT WS2 and mNT MoS2 are still semiconducting, their HOMO-LUMO energy gaps reduce largely to 0.4 eV from the original value of 0.9 eV. Fig. 3 shows the projected DOS for mNT MS2 and Me-doped mNT MS2 with 7 metal atoms on each edge. It indicates that the electronic states of the Me-doped mNT WS2 and MoS2 around the Fermi level mainly come from the doped Me atoms and the S monomers. We further calculate the partial charge density near the Fermi level. The frontier orbitals showing in Fig. 4 tell us that the p orbitals of the S monomers indeed are the dominate states close to the Fermi level. We believe that the decrease of ΔGH is attribute to the reduction of the energy gaps. When the energy gaps are reducing, the electronic states of the S monomers come near to the Fermi level where the 1s state of hydrogen atom locates. In this way, the 1s state of the hydrogen interacts with the p state of S monomers which makes the ΔGH reduce. Not only this, Medoping improves the electrical conductivity by bringing some electronic states near to the Fermi level. Consequently, the catalytic performance of Me-doped mNT WS2 and mNT MoS2 with 7 metal atoms on each edge have been improved through increasing the number of active sites and promoting the electron transfer. Figs. 2 and 3 also show that the majority and minority spin density are not same with each other for Mn and Co doped mNT WS2 and MoS2,
ΔGH by at least 0.07 eV to make the hydrogen adsorption happen. Additional, Cr and Mn doping increase the number of active edge sites to 21 and 18, respectively. As listed in Table 1, it is noted that the ΔGH on edge_I site of Mndoped mNT WS2 is still quite positive. The reason is that after the hydrogen adsorption, the S monomer on the vertex next to edge_I site turns back to the normal position from the deformed structure as showing in Fig. 1(c). And as we have mentioned in the above section, the normal configuration on the vertexes is energetically less favorable relative to the deformed configuration. Additional, our results show that the Cr-doping is most effective in improving the catalytic activity of mNT WS2 for HER compared to Mn and Co doping. This is new because Co is often used to promote WS2 and MoS2 for catalyzing the hydrogen evolution [58,59].
3.3. Electronic and magnetic properties of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) In order to find out how does the Me doping enhance the catalytic activity of mNT MS2 for HER, more specifically, how does the Medoping decrease the hydrogen adsorption free energy to facilitate the HER process, we have studied the electronic structures of mNT MS2 and Me-doped mNT MS2. Fig. 2 show the total density of states (DOS) for mNT MS2 and Me-doped mNT MS2 with 7 metal atoms on each edge. It clearly shows that both mNT WS2 and MoS2 with 7 metal atoms on each edge are semiconducting with the HOMO-LUMO (highest occupied
Table 1 Calculated Gibbs free energy changes for hydrogen adsorption on the edges of monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. ΔGH
Vertex
Edge_I
Edge_II
Edge_III
Edge_IV
Edge_V
Edge_VI
Active sites/total sites
mNT WS2 Cr-doped mNT WS2 Mn-doped mNT WS2 Co-doped mNT WS2 mNT MoS2 Cr-doped mNT MoS2 Mn-doped mNT MoS2 Co-doped mNT MoS2
0.31 −0.17 −0.25 −0.31 0.17 −0.18 0.01 −0.28
0.60 0.01 0.45 −0.20 0.31 0 −0.19 −0.28
0.89 0.02 −0.05 −0.64 0.55 0.07 −0.16 −0.53
0.49 −0.14 −0.38 −0.20 0.20 −0.11 −0.32 −0.16
0.47 −0.31 −0.19 −0.24 0.21 −0.22 −0.23 −0.19
0.62 −0.13 −0.06 −0.19 0.36 −0.06 0 −0.13
0.46 −0.06 0.12 0.16 0.22 −0.07 −0.14 0.15
0/21 18/21 15/21 15/21 12/21 21/21 18/21 12/21
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Fig. 2. Density of states for monolayer nano-triangle (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) atoms doping.
which means that Mn and Co doping introduce magnetism into mNT WS2 and MoS2. The magnetic moments are 10.33 and 11.07 μB for Mn doped mNT WS2 and MoS2, and 1.57 and 0.67 μB for Co doped mNT WS2 and MoS2, respectively. We further calculated the spin-resolved charge density. As showing in Fig. 5, it indicates that the magnetic moment mainly comes from the doped Me atoms and the S monomers. For the Mn-doped mNT WS2 and MoS2, Fig. 5(a) and (c) shows that the
6 Mn atoms are anti-ferromagnetic coupling with the other 15 Mn atoms. As for the Co-doped mNT WS2, the magnetic moment on the Co atoms on the three vertexes are anti-ferromagnetic coupling with the other Co atoms. While the magnetic moment of the Co-doped mNT MoS2 mainly comes from the Co atoms on the three vertexes. Additional, the local magnetic moments of the doped Mn atoms are much larger than those of the doped Co atoms. Correspondingly, the total
Fig. 3. Projected density of states for monolayer nano-triangles (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) doping. The Fermi level is set to zero. 336
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Fig. 4. Partial charge density of Me (Me = Cr, Mn, Co) atoms doped monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. Charge accumulated around the Fermi level within the respective energy range of (a) ± 0.25 eV for Cr-doped mNT WS2, (b) −0.07 to 0.05 eV for Mn-doped mNT WS2, (c) −0.02 to 0.03 eV for Co-doped mNT WS2, (d) ± 0.26 eV for Cr-doped mNT MoS2, (e) −0.06 to 0.06 eV for Mn-doped mNT MoS2, (f) −0.04 to 0.03 eV for Co-doped mNT MoS2. Isosurface value is 0.001 e/Å3. Green depicting electrons in conduction band and red depicting holes in valence band. Fig. 5. Calculated spin-resolved charge density for Me-doped monolayer nano-triangles (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. (a) Mn-doped mNT WS2, (b) Co-doped mNT WS2, (c) Mn-doped mNT MoS2, (d) Co-doped mNT MoS2. The blue and orange color represent spin-up and spin-down charge, respectively. Isosurface value is 5 × 10−3 e/Å3.
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Fig. 6. Density of states for vertex-doped and edge-doped monolayer nano-triangle (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers.
Table 2 Calculated Gibbs free energy changes for hydrogen adsorption on the edges of vertex-doped* and edge-doped** monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers.
*
ΔGH
Vertex
Edge_I
Edge_II
Edge_III
Edge_IV
Edge_V
Edge_VI
Active sites/total sites
Vertex-doped mNT WS2 Edge-doped mNT WS2 Vertex-doped mNT MoS2 Edge-doped mNT MoS2
−0.44 −0.20 −0.24 0.05
0.13 −0.04 0.28 0.06
0.24 0.23 0.26 0.29
0.12 −0.29 0.17 −0.17
0.19 −0.31 0.19 −0.12
0.55 0.38 0.53 0.44
0.12 0.01 0.26 0.23
15/21 12/21 9/21 15/21
The M atoms at the three vertex positions are substituted with Cr atoms. The M atoms at the middle of three edges are substituted with Cr atoms.
**
Comparing with the Cr-dope mNT MS2, the formation energy of vertexdoped MS2 is smaller, while the formation energy of the edge-doped mNT MS2 is higher. The density of states (DOS) for vertex-doped and edge-doped mNT MS2 are plotted in Fig. 6(a) and (b). Clearly, the vertex-doped and edge-doped mNT MS2 with mixed edge are semiconductors. Compared with the pure mNT MS2, the band gaps of vertexdoped and edge-doped mNT MS2 all decrease. We evaluated the HER catalytic activity of the vertex-doped and edge-doped mNT MS2 by calculating the ΔGH for H adsorption on the edge sites. The calculated ΔGH and the ratio of number of active sites over the total number of edge sites are summarized in Table 2. The number of active sites for the vertex-doped and edge-doped mNT WS2 are 15 and 12, respectively. Compared with mNT WS2, the catalytic performance of the vertex-doped and edge-doped mNT WS2 improves obviously. Additionally, compared with mNT MoS2, the number of active sites for the edge-doped mNT MoS2 slightly increases from 12 to 15, and the number of active edge sites for the vertex-doped mNT MoS2 decreases from 12 to 9. However, the number of active site for the vertex-doped and edge-doped mNT MS2 all decrease compared with the Cr-doped mNT MS2. Thus, our calculations show that the mixed doping is less effective than the whole edges doping on improving the HER catalytic performance of mNT MS2.
magnetic moments of the Mn doped mNT MS2 are much larger than the counterpart of the Co doped mNT MS2. Moreover, the magnetic moments distributes symmetrical along the doped Me atoms and the S monomers, consistent with the symmetry of the edge configurations. It is well known that monolayer WX2 and MoX2, as well as CrX2 (X = S, Se, Te) are nonmagnetic semiconducting, but monolayer VX2, as well as MnX2 and CoX2 are metallic and magnetic [16,17,60–64]. Our present study shows that the Mn and Co doping not only change the edge structures of mNT WS2 and MoS2 but also change their electronic structures and magnetic properties. Interesting, the edge configurations of the Mn and Co doped mNT WS2 and MoS2 are similar with that of mNT VS2, in which the S monomers on the vertexes tilt away to one edge [17]. More importantly, the Mn and Co doped mNT WS2 and MoS2 are metallic and magnetic just like mNT VS2. On the other hand, Crdoping does not change the edge configurations of mNT WS2 and MoS2, and they maintain to be semiconducting and nonmagnetic. In this context, our study finds the relation between the edge composition and edge configurations, as well as the electronic structures of mNT MS2, which can be used to manipulate the properties and performance of the monolayer MS2 in the applications of catalysis and electronic devices. 3.4. Catalytic activity of mNT MS2 (M = W, Mo) with mixed edges
4. Conclusions
According to previous work [65], the mixed edge of monolayer MoS2 affects the catalytic activity. In this context, taking Cr-doping as example, we further investigate mNT MS2 with mixed edge since the Cr doping effectively improve the catalytic performance of mNT MS2. We substituted the M atoms at the three vertex positions and the middle positions of the three edges of mNT MS2 with Cr atoms, respectively, which are named as vertex-doped and edge-doped mNT MS2. The optimized structures of the vertex-doped and edge-doped mNT MS2 are almost same with the structures of mNT MS2. The formation energies per atom for vertex-doped and edge-doped mNT WS2 are 0.41 and 0.71 eV, and are 0.32 and 0.66 eV for the counterpart of mNT MoS2.
By performing the first-principles calculations, we have investigated the catalytic activity of Me (Me = Cr, Mn, Co) atoms doped mNT WS2 and MoS2 for hydrogen evolution. Our calculations show that the Medoping on the edges of mNT WS2 and MoS2 effectively improve their catalytic performance for HER via increasing the number of active sites and promoting electron transferring. Our study on the electronic structure indicates that the Me-doping largely reduce the energy gaps of mNT WS2 and mNT MoS2 with 7 metal atoms on each edge by bringing electronic states of doped Me atoms and S monomers near to the Fermi 338
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level. In this way, the p states of S monomers interact weakly with the 1s state of hydrogen, which makes the ideal HER catalyst. Additional, it finds that the atomic structures, especially the edge configurations of mNT MS2 and the electronic structures are closely correlated to the metal atoms on the edge, which can be used to improve their performance as HER catalyst and engineer their electronic structures.
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Improved catalytic performance of monolayer nano-triangles WS2 and MoS2 on HER by 3d metals doping
T
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Yurong An, Xiaoli Fan , Hanjie Liu, Zhifen Luo State Key Laboratory of Solidification Processing, Center for Advanced Lubrication and Seal Materials, School of Material Science and Engineering, Northwestern Polytechnical University, 127 YouYi Western Road, Xi’an, Shaanxi 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: 2D materials Transition-metaldichalcogenids Hydrogen evolution reaction Catalytic performance Density functional theory
Recently, monolayer transition metal dichalcogenides (TMDCs) such as MoS2 and WS2, exhibit distinguished catalytic performance on hydrogen evolution reaction (HER). Triangular shaped monolayers are the most popular morphology for the chemically synthesized monolayer-TMDCs. In this study, we find a way to further improve the catalytic performance of monolayer nano-triangles (mNT) MS2 (M = W, Mo) on HER. By adopting the first-principles calculation methods based on the density functional theory, we studied the catalytic activity of the mNT MS2 with the substitutional doping of Me (Me = Cr, Mn, Co) atoms on the edges. The detailed electronic structures were also investigated to better understand the catalytic activity. Our calculations show that the Me atoms doping on the edge changes the electronic states near the Fermi level. Additionally, the enhancement in the number of active sties and conductivity result in the improvement of the catalytic performance of mNT MS2 on HER. More importantly, we demonstrate that the electronic structures and edge configurations, particularly the structures on the vertexes of the triangular mNT MS2 are closely related to the edge composition.
1. Introduction Hydrogen is considered to be an ideal energy carrier [1] not only because it possesses the ultrahigh energy density but also because there is no green-house gas such as carbon dioxide emitting during combustion process. Efficient hydrogen production holds tremendous promise for the clean and renewable energy development [2]. Pt-group metals are the most active catalyst for electrochemistry hydrogen evolution reaction (HER) [3,4]. But these metals are too rare and expensive to be used for the large-scale production of hydrogen. Recent study shows that the catalytic performance of monolayer MoS2 on HER is comparable to that of the Pt-group metals [5]. Hence, extensive studies are devoted to investigate and improve the catalytic performance of transition-metal dichalcogenides (TMDCs) of MS2 (M = Mo, W, V) catalyst for HER [6–17]. Single-layer MS2 (M = Mo, W) is the sandwiched layer of S-M-S, which has several possible polymorphs, namely, 1H, 1T, ZT or 1T′, as well as the DT lattice [6,7,14,15,18–23]. The basal plane of the 1H phase is HER-inert, while the 1T and ZT phases are active toward HER. Atomic-thin nano-sheets of MoS2 and WS2 generated by the liquid exploiting method both showed advanced catalytic performance on HER [6,7], and their catalytic activity were attributed to the metallic 1T and
⁎
ZT phases. On the other hand, monolayer TMDCs were synthesized successfully by the thin-film deposition methods and studied by both experimental and computational tools [5,24–27]. The deposited monolayer TMDCs were reported as the advanced catalysts for HER [5,9,10]. The shapes of the deposited monolayer TMDCs are unexpected regular [5,24–27], including hexagonal, stars, bowties and triangles. The nano-triangles (mNT) are the most popular ones [5,24–37] among all the reported nanopolygons of monolayer TMDCs. As for the mNT MoS2, the most studied monolayer TMDCs, its edges are either Mo terminated or S terminated depending on the experimental conditions [28–37]. According to the previous study [5], the catalytic activity of the as grown mMT MoS2 is derived from the edge sites, particular the S atoms. Thus, various edge engineering and defect engineering techniques were adopted to create more active sites on the inert basal plane to further improve the catalytic performance of monolayer TMDCs on HER [8,11,13]. Actually, the catalytic activity of mNT MS2 on HER are attribute to their active edge sites with proper electronic structures to facilitate HER [5,17]. Additionally, it is well known that edges and corners of nanoclusters materials in nanoscale or sub-nanoscale dominantly affect the material properties. In this context, there is a big chance we can
Corresponding author. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.commatsci.2018.12.032 Received 22 June 2018; Received in revised form 7 December 2018; Accepted 17 December 2018 0927-0256/ © 2018 Published by Elsevier B.V.
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detailed edge structures are similar with each other. Particularly, every three metal atoms cluster in a three atoms chain on the edges. As showing in Fig. 1, Me atoms doping (Me = Cr, Mn, Co) changes the edge configurations of mNT WS2 and MoS2. For Mn-doped mNT WS2 and MoS2 shown in Fig. 1(c) and (g), all the S monomers on the three vertexes tilt away from its original position and bond to the S monomers next to it. This kind of deformation on the vertex also happens for the Co-doped mNT WS2 and MoS2 as showing Fig. 1(d) and (h). But the Cr-doped mNT WS2 and MoS2 maintains their original structures on the vertex. Indeed, all the structures showing in Fig. 1 are those having the lowest energy. Our calculation shows that the deformed structures of Cr-doped WS2 and MoS2 on the vertexes are less stable relative to the structures shown in Fig. 1(b) and (f) by 0.4 and 0.5 eV. Additional, the clustering phenomenon of three metal atom on the edges disappear after Me atoms doping for both WS2 and MoS2. The formation energies for the Me-doped mNT WS2 and MoS2 were calculated via the following formula [57],
tune the electronic structure through modulating the edge structures to enhance the catalytic performance of MS2 on HER. Doping is one of the most feasible methods to modulate the electronic properties of materials [38–42], which also has proved to be an efficient way to improve the HER performance [43–51]. In the present study, by performing the first-principles calculations, we study the effect of 3d metals Me (Me = Cr, Mn, Co) doping on the edge of mNT MS2 on the catalytic activity. We examined the atomic structures and the catalytic activity for HER, as well as the electronic and magnetic properties of Me atoms doped mNT WS2 and MoS2. It turns out that the Me-doping on the edge of mNT WS2 and MoS2 increase the number of active sites through change their electronic structure near the Fermi level. 2. Calculation methods All the calculations were performed by the first-principles method based on the density functional theory (DFT) within the Vienna ab-initio simulation package (VASP) [52]. The ion–electron interactions were treated with the projector augmented wave (PAW) pseudopotentials [53]. The Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [54] was employed to calculate the electronic exchange-correlation interaction. A plane wave basis set with a cutoff energy of 400 eV was used to expand the wave functions. The first Brillouin-Zone was sampled by the Monkhorst-Pack method [55]. A 15 Å vacuum layer was added along the normal direction above the monolayer to avoid interactions between the adjacent images. The convergence criterion for the self-consistency process was set to 10−5 eV between two ionic steps, and the atoms were fully relaxed until the force on each atom was less than 0.02 eV/Å. The Gibbs free energy change for the adsorption of hydrogen is calculated via
Ef = E (doped - MS2) − E (MS2) + nμ (M ) − nμ (Me )
3.2. Catalytic activity of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) for HER
(1)
ΔGH = ΔEH + ΔEZPE − T ΔSH
As we know, the high active catalyst for HER should have two essential characteristics, high conductivity which accelerates the combination of electrons and protons and abundant active sites. According to previous study, the Gibbs free energy change (ΔGH ) for the adsorption of hydrogen calculated by the density functional theory based method is a good descriptor for the catalytic activity toward HER [5], it shows that the best catalytic performance on HER achieves at neutral ΔGH . In other words, the optimum value of the free energy changes for hydrogen adsorption should be neutral (ΔGH = 0 ). In this context, we set −0.25 eV ≤ ΔGH ≤ 0.25 eV as a criterion to estimate the catalytic activity of mNT MS2 and Me-doped mNT MS2 for hydrogen evolution. It has been confirmed that the catalytic activity of monolayer WS2 and MoS2 nanoparticles comes from the edge sites [5,17]. Hence, we evaluated the catalytic activity of mNT MS2 and Me-doped mNT MS2 for HER by calculating the ΔGH for H adsorption on the edge sites. As we know, the edge configurations of mNT MS2 and Me-doped mNT MS2 both have certain symmetry as showing Fig. 1, thus we have studied 7 edge sites representing the whole 21 edge sties. Our calculated ΔGH are summarized in Table 1. We also calculated the ratio of number of active sites over the total number of edge sites to evaluate the catalytic performance. Our calculations show that all the ΔGH on the edge sites of mNT WS2 with 7 W atoms on each edge are positive and ranging from 0.31 to 0.89 eV, which means that hydrogen bonds too weak to be adsorbed. In other words, all the edge sites of mNT WS2 with 7 W atoms on each edge are inert for HER. However, we can see from Table 1 that the Medoping reduces ΔGH by as large as 1.53 eV, and really improves the catalytic performance by increasing the number of active sites. More specifically, Cr doping increases the ratio of number of active sites over the total number of edge sites from 0 to 18/21, while Mn and Co doping increase the ratios to 15/21. Compared to mNT WS2, the catalytic activity of mNT MoS2 with 7 Mo atoms on each edge is better. There are 12 active sites over the total 21 edge sites. Again, ΔGH on the edges of mNT MoS2 are all positive, and Me-doping on the edges reduces the
where ΔEH is the adsorption energy of hydrogen which is defined as:
ΔEH = EMe − MS2+ H − EMe − MS2 −
1 EH 2 2
(3)
where E (doped - MS2) and E (MS2) are the total energy of Me-doped mNT MS2 and mNT MS2, respectively. μ (M ) and μ (Me ) are the chemical potentials of M atom and Me atom, which are taken from the energy of most stable bulk phase. n is the number of doped Me atoms. The resulted formation energy per atom for Cr-, Mn-, and Co-doped WS2 are 0.47, 0.77, and 1.38 eV, and the counterpart results for mNT MoS2 are 0.53, 0.82, and 1.45 eV. Our calculations show that these doped systems are achievable in experiments. Moreover, the Cr atoms are readily doped than Mn and Co.
(2)
and ΔEZPE is the difference in zero point energy between the adsorbed atomic hydrogen and hydrogen in gas phase. ΔSH is the entropy difference between adsorbed state and gas phase. The entropy of the adsorbed atomic hydrogen is regarded as ΔSH = −S H2/2, where SH 2 is the entropy of molecule hydrogen in the gas phase at standard conditions and ΔEZPE − T ΔSH is 0.24 eV [7,16,56]. So Eq. (1) is simplified to ΔGH = ΔEH + 0.24 . In Eq. (2), EMe − MS2+ H and EMe − MS2 are the energies of the Me-doped mNT MS2 with and without hydrogen adsorption, while E H2 is the energy of hydrogen molecule in gas phase. 3. Results and discussions 3.1. Atomic structures of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) Previously, we investigates the catalytic performance of monolayer nanopolygons MS2 (Mo = W, Mo) for HER [17], it shows that the active site density of monolayer nano-triangles (mNT) MS2, particular mNT WS2 with 7 metal atoms on each edge drops down suddenly relative to the triangles with 5 and 6 metal atoms on the edge. So we simply pick mNT WS2 and MoS2 with 7 metal atoms on each edge as the representive mNTs to study the effect of Me atoms doping on HER performance. Fig. 1 shows the atomic structures for the mNT WS2 and MoS2 with 7 metal atoms on each edge. As we can see, mNT MS2 arranges in the sandwiched S-M-S way where one atomic layer of M atoms is enclosed by two atomic layers of S atoms. The M atoms on the edges are 50% terminated by the S atoms which are called as S monomers. It indicates that the both mNT WS2 and MoS2 have the C3 symmetry, and their 334
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Fig. 1. Atomic structures for monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) doping. (a) mNT WS2 and (e) mNT MoS2, Cr-doped (b) mNT WS2 and (f) mNT MoS2, Mn-doped (c) mNT WS2 and (g) mNT MoS2, Codoped (d) mNT WS2 and (h) mNT MoS2. The probable adsorption sites on the edge are illustrated in (a).
molecular orbital-lowest unoccupied molecular orbital) energy gaps of about 0.9 eV. Additionally, Me-doping reduces their energy gaps largely as showing in Fig. 2. Particularly, both Mn and Co doped mNT WS2 and mNT MoS2 show metallic character with some electronic states crossing the Fermi level. Although Cr doped mNT WS2 and mNT MoS2 are still semiconducting, their HOMO-LUMO energy gaps reduce largely to 0.4 eV from the original value of 0.9 eV. Fig. 3 shows the projected DOS for mNT MS2 and Me-doped mNT MS2 with 7 metal atoms on each edge. It indicates that the electronic states of the Me-doped mNT WS2 and MoS2 around the Fermi level mainly come from the doped Me atoms and the S monomers. We further calculate the partial charge density near the Fermi level. The frontier orbitals showing in Fig. 4 tell us that the p orbitals of the S monomers indeed are the dominate states close to the Fermi level. We believe that the decrease of ΔGH is attribute to the reduction of the energy gaps. When the energy gaps are reducing, the electronic states of the S monomers come near to the Fermi level where the 1s state of hydrogen atom locates. In this way, the 1s state of the hydrogen interacts with the p state of S monomers which makes the ΔGH reduce. Not only this, Medoping improves the electrical conductivity by bringing some electronic states near to the Fermi level. Consequently, the catalytic performance of Me-doped mNT WS2 and mNT MoS2 with 7 metal atoms on each edge have been improved through increasing the number of active sites and promoting the electron transfer. Figs. 2 and 3 also show that the majority and minority spin density are not same with each other for Mn and Co doped mNT WS2 and MoS2,
ΔGH by at least 0.07 eV to make the hydrogen adsorption happen. Additional, Cr and Mn doping increase the number of active edge sites to 21 and 18, respectively. As listed in Table 1, it is noted that the ΔGH on edge_I site of Mndoped mNT WS2 is still quite positive. The reason is that after the hydrogen adsorption, the S monomer on the vertex next to edge_I site turns back to the normal position from the deformed structure as showing in Fig. 1(c). And as we have mentioned in the above section, the normal configuration on the vertexes is energetically less favorable relative to the deformed configuration. Additional, our results show that the Cr-doping is most effective in improving the catalytic activity of mNT WS2 for HER compared to Mn and Co doping. This is new because Co is often used to promote WS2 and MoS2 for catalyzing the hydrogen evolution [58,59].
3.3. Electronic and magnetic properties of Me-doped (Me = Cr, Mn, Co) mNT MS2 (M = W, Mo) In order to find out how does the Me doping enhance the catalytic activity of mNT MS2 for HER, more specifically, how does the Medoping decrease the hydrogen adsorption free energy to facilitate the HER process, we have studied the electronic structures of mNT MS2 and Me-doped mNT MS2. Fig. 2 show the total density of states (DOS) for mNT MS2 and Me-doped mNT MS2 with 7 metal atoms on each edge. It clearly shows that both mNT WS2 and MoS2 with 7 metal atoms on each edge are semiconducting with the HOMO-LUMO (highest occupied
Table 1 Calculated Gibbs free energy changes for hydrogen adsorption on the edges of monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. ΔGH
Vertex
Edge_I
Edge_II
Edge_III
Edge_IV
Edge_V
Edge_VI
Active sites/total sites
mNT WS2 Cr-doped mNT WS2 Mn-doped mNT WS2 Co-doped mNT WS2 mNT MoS2 Cr-doped mNT MoS2 Mn-doped mNT MoS2 Co-doped mNT MoS2
0.31 −0.17 −0.25 −0.31 0.17 −0.18 0.01 −0.28
0.60 0.01 0.45 −0.20 0.31 0 −0.19 −0.28
0.89 0.02 −0.05 −0.64 0.55 0.07 −0.16 −0.53
0.49 −0.14 −0.38 −0.20 0.20 −0.11 −0.32 −0.16
0.47 −0.31 −0.19 −0.24 0.21 −0.22 −0.23 −0.19
0.62 −0.13 −0.06 −0.19 0.36 −0.06 0 −0.13
0.46 −0.06 0.12 0.16 0.22 −0.07 −0.14 0.15
0/21 18/21 15/21 15/21 12/21 21/21 18/21 12/21
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Fig. 2. Density of states for monolayer nano-triangle (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) atoms doping.
which means that Mn and Co doping introduce magnetism into mNT WS2 and MoS2. The magnetic moments are 10.33 and 11.07 μB for Mn doped mNT WS2 and MoS2, and 1.57 and 0.67 μB for Co doped mNT WS2 and MoS2, respectively. We further calculated the spin-resolved charge density. As showing in Fig. 5, it indicates that the magnetic moment mainly comes from the doped Me atoms and the S monomers. For the Mn-doped mNT WS2 and MoS2, Fig. 5(a) and (c) shows that the
6 Mn atoms are anti-ferromagnetic coupling with the other 15 Mn atoms. As for the Co-doped mNT WS2, the magnetic moment on the Co atoms on the three vertexes are anti-ferromagnetic coupling with the other Co atoms. While the magnetic moment of the Co-doped mNT MoS2 mainly comes from the Co atoms on the three vertexes. Additional, the local magnetic moments of the doped Mn atoms are much larger than those of the doped Co atoms. Correspondingly, the total
Fig. 3. Projected density of states for monolayer nano-triangles (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers without and with Me (Me = Cr, Mn, Co) doping. The Fermi level is set to zero. 336
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Fig. 4. Partial charge density of Me (Me = Cr, Mn, Co) atoms doped monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. Charge accumulated around the Fermi level within the respective energy range of (a) ± 0.25 eV for Cr-doped mNT WS2, (b) −0.07 to 0.05 eV for Mn-doped mNT WS2, (c) −0.02 to 0.03 eV for Co-doped mNT WS2, (d) ± 0.26 eV for Cr-doped mNT MoS2, (e) −0.06 to 0.06 eV for Mn-doped mNT MoS2, (f) −0.04 to 0.03 eV for Co-doped mNT MoS2. Isosurface value is 0.001 e/Å3. Green depicting electrons in conduction band and red depicting holes in valence band. Fig. 5. Calculated spin-resolved charge density for Me-doped monolayer nano-triangles (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers. (a) Mn-doped mNT WS2, (b) Co-doped mNT WS2, (c) Mn-doped mNT MoS2, (d) Co-doped mNT MoS2. The blue and orange color represent spin-up and spin-down charge, respectively. Isosurface value is 5 × 10−3 e/Å3.
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Fig. 6. Density of states for vertex-doped and edge-doped monolayer nano-triangle (mNT) WS2 (a) and MoS2 (b) with 7 metal atoms on each edge which are terminated by S monomers.
Table 2 Calculated Gibbs free energy changes for hydrogen adsorption on the edges of vertex-doped* and edge-doped** monolayer nano-triangle (mNT) MS2 (M = W, Mo) with 7 metal atoms on each edge which are terminated by S monomers.
*
ΔGH
Vertex
Edge_I
Edge_II
Edge_III
Edge_IV
Edge_V
Edge_VI
Active sites/total sites
Vertex-doped mNT WS2 Edge-doped mNT WS2 Vertex-doped mNT MoS2 Edge-doped mNT MoS2
−0.44 −0.20 −0.24 0.05
0.13 −0.04 0.28 0.06
0.24 0.23 0.26 0.29
0.12 −0.29 0.17 −0.17
0.19 −0.31 0.19 −0.12
0.55 0.38 0.53 0.44
0.12 0.01 0.26 0.23
15/21 12/21 9/21 15/21
The M atoms at the three vertex positions are substituted with Cr atoms. The M atoms at the middle of three edges are substituted with Cr atoms.
**
Comparing with the Cr-dope mNT MS2, the formation energy of vertexdoped MS2 is smaller, while the formation energy of the edge-doped mNT MS2 is higher. The density of states (DOS) for vertex-doped and edge-doped mNT MS2 are plotted in Fig. 6(a) and (b). Clearly, the vertex-doped and edge-doped mNT MS2 with mixed edge are semiconductors. Compared with the pure mNT MS2, the band gaps of vertexdoped and edge-doped mNT MS2 all decrease. We evaluated the HER catalytic activity of the vertex-doped and edge-doped mNT MS2 by calculating the ΔGH for H adsorption on the edge sites. The calculated ΔGH and the ratio of number of active sites over the total number of edge sites are summarized in Table 2. The number of active sites for the vertex-doped and edge-doped mNT WS2 are 15 and 12, respectively. Compared with mNT WS2, the catalytic performance of the vertex-doped and edge-doped mNT WS2 improves obviously. Additionally, compared with mNT MoS2, the number of active sites for the edge-doped mNT MoS2 slightly increases from 12 to 15, and the number of active edge sites for the vertex-doped mNT MoS2 decreases from 12 to 9. However, the number of active site for the vertex-doped and edge-doped mNT MS2 all decrease compared with the Cr-doped mNT MS2. Thus, our calculations show that the mixed doping is less effective than the whole edges doping on improving the HER catalytic performance of mNT MS2.
magnetic moments of the Mn doped mNT MS2 are much larger than the counterpart of the Co doped mNT MS2. Moreover, the magnetic moments distributes symmetrical along the doped Me atoms and the S monomers, consistent with the symmetry of the edge configurations. It is well known that monolayer WX2 and MoX2, as well as CrX2 (X = S, Se, Te) are nonmagnetic semiconducting, but monolayer VX2, as well as MnX2 and CoX2 are metallic and magnetic [16,17,60–64]. Our present study shows that the Mn and Co doping not only change the edge structures of mNT WS2 and MoS2 but also change their electronic structures and magnetic properties. Interesting, the edge configurations of the Mn and Co doped mNT WS2 and MoS2 are similar with that of mNT VS2, in which the S monomers on the vertexes tilt away to one edge [17]. More importantly, the Mn and Co doped mNT WS2 and MoS2 are metallic and magnetic just like mNT VS2. On the other hand, Crdoping does not change the edge configurations of mNT WS2 and MoS2, and they maintain to be semiconducting and nonmagnetic. In this context, our study finds the relation between the edge composition and edge configurations, as well as the electronic structures of mNT MS2, which can be used to manipulate the properties and performance of the monolayer MS2 in the applications of catalysis and electronic devices. 3.4. Catalytic activity of mNT MS2 (M = W, Mo) with mixed edges
4. Conclusions
According to previous work [65], the mixed edge of monolayer MoS2 affects the catalytic activity. In this context, taking Cr-doping as example, we further investigate mNT MS2 with mixed edge since the Cr doping effectively improve the catalytic performance of mNT MS2. We substituted the M atoms at the three vertex positions and the middle positions of the three edges of mNT MS2 with Cr atoms, respectively, which are named as vertex-doped and edge-doped mNT MS2. The optimized structures of the vertex-doped and edge-doped mNT MS2 are almost same with the structures of mNT MS2. The formation energies per atom for vertex-doped and edge-doped mNT WS2 are 0.41 and 0.71 eV, and are 0.32 and 0.66 eV for the counterpart of mNT MoS2.
By performing the first-principles calculations, we have investigated the catalytic activity of Me (Me = Cr, Mn, Co) atoms doped mNT WS2 and MoS2 for hydrogen evolution. Our calculations show that the Medoping on the edges of mNT WS2 and MoS2 effectively improve their catalytic performance for HER via increasing the number of active sites and promoting electron transferring. Our study on the electronic structure indicates that the Me-doping largely reduce the energy gaps of mNT WS2 and mNT MoS2 with 7 metal atoms on each edge by bringing electronic states of doped Me atoms and S monomers near to the Fermi 338
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level. In this way, the p states of S monomers interact weakly with the 1s state of hydrogen, which makes the ideal HER catalyst. Additional, it finds that the atomic structures, especially the edge configurations of mNT MS2 and the electronic structures are closely correlated to the metal atoms on the edge, which can be used to improve their performance as HER catalyst and engineer their electronic structures.
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