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Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst
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Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst Youngho Kang PII: DOI: Reference:
S0039-6028(20)30723-8 https://doi.org/10.1016/j.susc.2020.121759 SUSC 121759
To appear in:
Surface Science
Received date: Revised date: Accepted date:
25 August 2020 2 November 2020 3 November 2020
Please cite this article as: Youngho Kang , Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst, Surface Science (2020), doi: https://doi.org/10.1016/j.susc.2020.121759
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Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst
Youngho Kang Department of Materials Science and Engineering, Incheon National University, Incheon 22012, Korea e-mail address: [email protected]
Graphical Abstract
Highlights
Transition-metal doped MoS2 has remarkable HER activity comparable to the wellknown Pt catalyst. Transition-metal dopants in MoS2 form defect complexes with sulfur vacancies. Sulfur vacancy that forms defect complexes with transition-metal dopants serve as the critical catalytic sites in MoS2.
Abstract We investigate the effect of single-atom doping on the hydrogen evolution reaction (HER) activity of MoS2 using first-principles calculations. In Ru-doped MoS2 that was recently suggested as a promising HER catalyst, we show that sulfur vacancy (VS) prefers to form defect complexes with Ru substituting for Mo (RuMo). We examine hydrogen adsorption free energy at various potential active sites, which underlines that VS in the defect complex can serve as important active centers for HER. We also examine various transition-metal (TM)
dopants for MoS2. The results show that several TMs such as Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt can be used as dopants to promote the HER process on MoS2. By uncovering the active sites for HER in single-atom doped MoS2 as well as suggesting several promising dopants, this work will open the way to developing novel 2D catalysts for HER. Keywords Hydrogen evolution reaction, MoS2, transition metal doping, sulfur vacancy, density functional theory
1. Introduction Electrochemical hydrogen production from water electrolysis is a promising technology for alleviating the two current world-wide challenges, namely global warming and rapidly increasing energy demand.[1] For efficient hydrogen production using this technology, an efficient electrocatalyst for hydrogen evolution reaction (HER) is a vital component, and Pt is known to be the best catalyst for HER until now. However, Pt is one of the most expensive elements on earth, so that extensive efforts have been made to find alternatives to Pt over the last decade.[2] Recently, two-dimensional transition metal dichalcogenides (TMDs) have emerged as promising catalysts for HER. In particular, MoS2, an archetypal TMD, is attracting great attention due to various advantages such as good chemical stability, earth abundance, low cost and high catalytic activity.[2–6][7] According to earlier studies that examined HER on MoS2 electrodes, the basal plane of 2H-MoS2 is catalytically inert while its edge sites can serve as active centers for HER.[8] As a result, many researches have been conducted to prepare nanostructures that can maximize the exposure of active edge sites.[9,10] However, it
is still challenging to obtain 2H-MoS2 with extremely high concentrations of active edge sites due to large surface energy of the edges that include coordinatively unsaturated atoms.[11,12] Metallic 1T-MoS2 that is a metastable form of MoS2 was reported to promote hydrogen production because of the favorable energetics of hydrogen adsorption on its basal plane.[13,14] Nonetheless, due to the intrinsic metastable nature, 1T-MoS2 suffers from the lack of an effective synthesis method as well as the degradation of the catalytic performance over time.[15,16] Single-atom metal doping has been considered as a feasible route to activate the basal plane of stable 2H-MoS2. For instance, previous experiments showed that doping 2H-MoS2 with several transition metals (TMs) such as Ru, Pt and Sn generates active centers in the basal plane, significantly promoting the HER activity of the MoS2 electrode.[2,17–23] Despite the great potential of TM-doped MoS2, however, an understanding of the role of the dopants in the HER activity, which would help design new 2D catalysts, is yet to be established. For example, it was reported that Ru and Re doping leads to an increase of the concentration of sulfur vacancies (VSs) in MoS2 and many VSs occur near the defect sites where the dopants substitute for Mo (TMMo), but the origin for these experimental results and a function of the VS around TMMo have not been elucidated.[19,20][24] Moreover, S-top sites around TMMo in TM-doped MoS2 were often pointed out as critical active centers for HER, but this argument has not been justified thoroughly.[22] In this work, we investigate the HER activity of TM-doped MoS2 using first-principles calculations. We first focus on Ru-doped MoS2 for which various defect configurations were identified in experiments. By calculating the binding energy between RuMo and VS, we reveal that RuMo and VS strongly attract each other so that they are likely to make defect complexes (RuMo-VS), as suggested in the recent experiments. (Note that VS, a constituent of the defect
complex, is the major native defect that can be present in MoS2 more than 1013 cm−2.)[25] We evaluate hydrogen adsorption free energies at potential active centers including S-top and vacancy sites in Ru-doped MoS2. This turns out that the most critical catalytic sites for HER are VS sites in RuMo-VS defect complexes rather than S-top sites around isolated RuMo defects. We also explore 14 additional TM dopants whose ionic radius is similar to that of Mo, suggesting many dopants (8 out of 14) such as Os, Re, and Sn to promote the HER process of 2H-MoS2 by forming defect complexes with VS.
2. Methods 2.1. DFT calculations We perform density functional theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP) with the projector-augmented wave (PAW) pseudopotentials.[26,27] The generalized gradient approximation (GGA) functional[28] is employed for the exchange-correlation energy. The cutoff energy for expanding plane-wave basis is set to 400 eV. In this work, we use monolayer MoS2 in 2H phase that is stable at room temperature. For the Brillouin zone sample, we use 8×8×1 and 2×2×1 Γ-centered kpoint grid for a unit cell and 4×4×1 supercells, respectively. The supercells include a vacuum with thickness of at least 12 Å to minimize interactions between repeated images along the vertical direction within the periodic boundary condition. Throughout this work, spinpolarized calculations are carried out. The atomic positions are relaxed until the force action on each atom becomes less than 0.02 eV/Å. 2.2. Hydrogen adsorption free energy Hydrogen adsorption energy (
) is calculated as[29–32]
(
where (
)
( )
(
) (1)
) and ( ) are the DFT energies of the supercells with and without hydrogen
adsorbed on an active site , respectively. (
) is the DFT energy of the hydrogen
molecule. Here, we employ 4×4×1 supercells for calculating ( be converted into hydrogen adsorption free energy (
) and ( ). The
can
) as follows:
(2)
where
and
are the change of zero-point energy (ZPE) and entropy upon hydrogen
adsorption, respectively. We consider
of −0.021 eV evaluated for VS in MoS2 using
GGA in a previous work[32] for every system because
is known to rarely depend on
materials and the type of active sites.[29-31] In line with previous calculations, we assume (
), where
(
) is the entropy of H2 gas at the standard state considering
the fact that the entropy change of TMDs upon hydrogen adsorption is negligible.[33] We take
(
) of 1.36 meV/K from the thermochemical table.[34] Hydrogen adsorption free
energy is a strong descriptor for the catalytic activity for HER and its optimal value was suggested to be around 0 eV.[33] However,
on a given catalyst can vary within 0.1-0.2
eV depending on exchange-correlation functionals.[32] Therefore, in this work, we discuss the HER activity based on
in reference to that on Pt (
) that is the best catalyst for
HER as of now, because the relative values among different materials are well maintained in
each functional.[32] For
of Pt, we take into account the (111) surface that is modeled as
a 2×2 supercell with three atomic layers and a vacuum longer than 12 Å. The calculated s at a S-top site and VS site in undoped MoS2 are 2.40 and 0.30 eV, respectively, which are in good agreement with 2.32 and 0.28 eV in previous calculations.[32]
2.3. Binding energy of a complex defect The binding energy (
( )
( )
) between two defects A and B are computed as[35]
* (
)
(
)+ (3)
where ( ) and ( ) are the total energies of the supercells including the defect A and B, respectively. (
) and (
) are the total energies of the supercells including the
complex defect AB and perfect MoS2, respectively. When the attractive interaction exists between A and B,
becomes positive. In the present study, every total energy to calculate
equation 3 is obtained using 4×4×1 supercells.
3. Results and discussion 3.1. Interplay between RuMo and VS for HER Defects in materials can be presented as isolated centers but also form defect complexes at high defect concentrations or at low temperatures. A key parameter that determines the concentration of defect complexes is the binding energy between the constituent defects. If
two defects have a large, positive binding energy, they are likely to form complexes. For instance, most of hydrogen in Mg-doped GaN form defect complexes with Mg at room temperature because of the large binding energy (
~0.7 eV).[35] To examine a possibility to
form defect complexes between RuMo and VS, we calculate the binding energy between them considering the atomic configurations in Figure 1. We find the significant binding energy of 1.56 eV, indicating the strong attractive interaction between RuMo and VS. Thus, many VSs in Ru-doped MoS2 are expected to exist in a form of the defect complexes. The large binding energy also means that the VS concentration can increase with Ru doping because VS is exclusively stable around RuMo. For obtaining insights into the attraction between RuMo and VS, we analyze the electronic structures of defective MoS2. In isolated VS (Figure 2a), three defect states are created between the band gap by the hybridization of the three dangling states on the surrounding Mo atoms, mainly consisting of Mo 4d orbitals. Due to trigonal symmetry of VS, these defect states can be categorized as a singlet a1 state and two doublet e states. The a1 state is lower in energy than the e states and two excess electrons that occur due to one missing S atom occupy the a1 state while the e states remain empty. On the other hand, in RuMo (Figure 2b), a defect state that is localized to the RuMo site develops inside the band gap. This state is mainly derived by the 4d states of the Ru atom and those of the neighboring Mo atoms. Because Ru has two more valence electrons than Mo, there are two excess electrons that occupy the defect state. When RuMo-VS is formed, namely VS occurs near the RuMo site, three defect states are generated (Figure 2c), as in VS. However, in this case, the degeneracy of the e states is lifted because the trigonal symmetry of VS is broken. Thus, the three defect states are different in energy and the two lower states are fully occupied by four excess electrons. It is noticed that the lowest and second lowest defect states in RuMo-VS lie below the occupied
defect levels in VS and RuMo, respectively. In other words, the occupied defect states of VS and RuMo are stabilized when forming defect complexes. Consequently, forming defect complexes can be energetically favorable, leading to the positive value of Eb. Table 1 shows
s at several active sites in pure and Ru-doped MoS2. As reported in
previous studies, the basal plane of pure MoS2 is inert for HER, resulting in very high at S-top sites.[32] When Ru is incorporated into a substitutional Mo site,
at the S-top
site near RuMo is predicted to be 0.79 eV that is advantageous over that in pure MoS2. However, this
is not small enough to explain the Pt-like catalytic performance of Ru-
doped MoS2 in experiments.[19,20] Meanwhile, VS of MoS2 is known to be catalytically active,[3][36] and indeed leads to favorable
, 0.30 eV.
at VS is reduced to the
more optimal value, 0.14 eV, when forming defect complexes with RuMo. Due to this benefit in energetics of hydrogen adsorption, along with the large Eb between RuMo and VS, sulfur vacancies in the defect complexes can constitute the most critical active centers for HER in Ru-doped MoS2, enabling dramatic improvement of the catalytic performance. We want to point out that smaller
(0.19 eV) at S-top sites around RuMo in a previous
study results from the consideration of the metastable ferromagnetic ordering for RuMo.[19] However, we find that the non-magnetic electronic configuration is more stable than the ferromagnetic one by 0.27 eV and
at the S-top site near RuMo taking into account the
stable non-magnetic ordering increases to 0.45 eV.
3.2. Identification of transition metal dopants for HER With the insights obtained from the study of Ru doped MoS2, we search for TM dopants for MoS2 that can enhance the electrocatalytic HER performance. Herein, for the feasibility of
doping, we focus on TMs whose ionic radius in the 4+ charge state matches that of Mo within a 10% difference. Such dopants are Ti, Zr, Nb, Tc, Ru, Rh, Pd, Sn, Hf, Ta, W, Re, Os, Ir, and Pt. First, we examine
at the S-top sites around isolated TMMo. Note that there
are two possible atomic configurations for TMMo: one is six-fold coordinated (Figure S1a) and the other is four-fold coordinated by the surrounding S atoms (Figure S1b). We test the total energy of these two configurations and select the more stable one for investigating the catalytic activity of the S-top sites (Table 2). Note that there are two distinct S-top sites around 4-fold coordinated TM dopants, but they lead to the identical free energy change upon hydrogen adsorption. The calculated
is plotted in Figure 3a for the energy range
between −0.4 and 0.4 eV, along with the volcano relation for transition metal catalysts like Pt, Ni, Cu, and Au[33] (the full list of data is provided in Table 2). Within a given energy range, a few dopants such as Pt, Pd, Rh, and Ir appear, indicating they can substantially activate the basal plane of MoS2 for HER without forming defect complexes with VS. Next, we examine the possibility for TMMo to form defect complexes with VS by computing the binding energy (Figure 4). It is seen that the Eb varies between −0.03 and 2.09 eV over the different dopants. The large, positive binding energy (Eb>0.6 eV) between VS and several TMMo such as RhMo and PtMo can be explained by the fact that the TMMo-VS produces the more stable defect states compared to the constituent defects (e.g., the band structure of RhMo in Figure S2a and b), as shown for RuMo-VS. In contrast, in TMMo like WMo and TaMo, occupied defect states that can be stabilized when forming defect complexes are not generated (e.g., the band structure of WMo in Figure S2c), leading to smaller binding energy (Eb<0.6 eV). For the dopants that yield Eb larger than 0.6 eV (Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt), we further evaluate
at VS in TMMo-Vs (Figure 3b). Interestingly, every defect
complex leads to favorable
at the VS site (<0.3 eV). As a result, significant
improvement in the HER performance of MoS2 is expected when these TMs are doped. The reason for the modifications of
in TM-doped MoS2 compared to the basal plane
of pure MoS2 is associated to the presence of defect states in the band gap that may strongly interact with the hydrogen state. As pointed out by Li et al.[3], VSs in MoS2, for example, generate localized gap states inside the band gap, resulting in lower
at the vacancy sites.
Similarly, as shown in the band structure of Ru-doped MoS2 in Figures 2b (without VS) and 2c (with VS), the defect levels appear inside the band gap, giving rise to the smaller
s
(0.79 eV at the S-top site without VS and 0.14 eV at the vacancy site in the defect complex), compared to that of the basal plane of perfect MoS2 (2.40 eV). As discussed above, defect states evidently affects
, but the effect of defect states on
can be different among
distinct dopants because the interaction between the defect and hydrogen states are influenced by various factors like the spatial extent and energetic position of the defect states and the size of active sites. On the other hand, we check the stability of TM-doped MoS2 by calculating the formation energy (Table S1). It turns out that every material exhibits negative formation energies which are close to that of pure MoS2, indicating that they are stable and synthesizable. Indeed, several dopants such as Sn[23], Re[2,17], Pd[18], and Pt[21,22], in addition to Ru, were already examined in experiments and found to substantially enhance the HER activity of MoS2. Considering our calculation results, we believe in such experiments that VS in TMMoVS serves as important active sites for HER. On the other hand, we also suggest other candidate dopants like Tc, Ir, Rh, and Os to promote the catalytic performance of MoS2 for HER by forming defect complexes with VS. Developing strategies to increase both VS and dopants would help to further optimize the HER activity for these dopants.
4. Conclusion In conclusion, we reported the DFT results about the effect of TM doping on the HER activity of MoS2. We showed that VS in Ru-doped MoS2 prefers to form defect complexes with RuMo, leading to the large, positive binding energy. The calculations of hydrogen adsorption free energy highlighted that VS in RuMo-VS exhibits Pt-like energetics for hydrogen adsorption, and thus, it can serve as the important active centers for HER in Ru-doped MoS2. We also explored the binding energy and hydrogen adsorption free energy in MoS2 doped with other TMs to discover optimal dopants for HER. The results showed that various TMs such as Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt can be used as dopants to promote the HER process on MoS2. By enlightening the role of single-atom doping in the HER activity of MoS2 as well as suggesting several promising dopants, this work will pave the way to developing noble 2D catalysts for HER.
mmc1.docx
Declaration of Competing Interest None.
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Figure 1. Atomic structures of the sulfur vacancy (VS), substitutional Ru defect (RuMo), and defect complex (RuMo-VS).
Figure 2. Band structures of the sulfur vacancy (VS), substitutional Ru defect (RuMo), and defect complex (RuMo-VS). The defect levels are colored in red. The black dashed line denotes the Fermi level. The valence band maximum is set to 0.
Figure 3. Hydrogen adsorption free energies (
) of (a) basal planes (S-top sites) around
substitutional transition metal defects (TMMo) and at (b) sulfur vacancies (VS) in defect complexes (TMMo-VS). The black straight line indicates the volcano plot for metal catalysts that shows the relation between the experimental exchange current that is a measure of the rate of HER at the equilibrium potential and calculated
. For comparison purpose, the
data for pure Pt is marked in blue.
Figure 4. Binding energies between substitutional transition metal defects and the sulfur vacancy.
Table 1.
(in eV) of the basal plane (S-top site) and at sulfur vacancies (VS) in pure and
Ru-doped MoS2. In Ru-doped MoS2, VS refers to the sulfur vacancy around the RuMo-VS defect complex.
pure MoS2
Ru-doped MoS2
Basal plane
2.40
0.79
VS
0.30
0.14
Table 2. Coordination numbers (CN) of substitutional transition metal defects (TMMo) and (in eV) of basal planes (S-top sites) and at sulfur vacancies (VS) in TMMo-VS defect complexes.
Dopant
CN
Basal plane
VS
Ti
6
0.65
−0.30
Zr
6
0.69
−0.59
Nb
6
0.64
−0.56
Tc
6
1.00
0.08
Ru
6
0.79
0.14
Rh
4
0.01
0.11
Pd
4
−0.02
0.25
Sn
6
0.48
0.02
Hf
6
0.72
−0.57
Ta
6
0.69
−0.60
W
6
2.35
0.28
Re
6
1.17
0.18
Os
6
0.70
0.18
Ir
6
0.038
0.09
Pt
4
−0.13
0.30
Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst Youngho Kang PII: DOI: Reference:
S0039-6028(20)30723-8 https://doi.org/10.1016/j.susc.2020.121759 SUSC 121759
To appear in:
Surface Science
Received date: Revised date: Accepted date:
25 August 2020 2 November 2020 3 November 2020
Please cite this article as: Youngho Kang , Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst, Surface Science (2020), doi: https://doi.org/10.1016/j.susc.2020.121759
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Interplay between transition-metal dopants and sulfur vacancies in MoS2 electrocatalyst
Youngho Kang Department of Materials Science and Engineering, Incheon National University, Incheon 22012, Korea e-mail address: [email protected]
Graphical Abstract
Highlights
Transition-metal doped MoS2 has remarkable HER activity comparable to the wellknown Pt catalyst. Transition-metal dopants in MoS2 form defect complexes with sulfur vacancies. Sulfur vacancy that forms defect complexes with transition-metal dopants serve as the critical catalytic sites in MoS2.
Abstract We investigate the effect of single-atom doping on the hydrogen evolution reaction (HER) activity of MoS2 using first-principles calculations. In Ru-doped MoS2 that was recently suggested as a promising HER catalyst, we show that sulfur vacancy (VS) prefers to form defect complexes with Ru substituting for Mo (RuMo). We examine hydrogen adsorption free energy at various potential active sites, which underlines that VS in the defect complex can serve as important active centers for HER. We also examine various transition-metal (TM)
dopants for MoS2. The results show that several TMs such as Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt can be used as dopants to promote the HER process on MoS2. By uncovering the active sites for HER in single-atom doped MoS2 as well as suggesting several promising dopants, this work will open the way to developing novel 2D catalysts for HER. Keywords Hydrogen evolution reaction, MoS2, transition metal doping, sulfur vacancy, density functional theory
1. Introduction Electrochemical hydrogen production from water electrolysis is a promising technology for alleviating the two current world-wide challenges, namely global warming and rapidly increasing energy demand.[1] For efficient hydrogen production using this technology, an efficient electrocatalyst for hydrogen evolution reaction (HER) is a vital component, and Pt is known to be the best catalyst for HER until now. However, Pt is one of the most expensive elements on earth, so that extensive efforts have been made to find alternatives to Pt over the last decade.[2] Recently, two-dimensional transition metal dichalcogenides (TMDs) have emerged as promising catalysts for HER. In particular, MoS2, an archetypal TMD, is attracting great attention due to various advantages such as good chemical stability, earth abundance, low cost and high catalytic activity.[2–6][7] According to earlier studies that examined HER on MoS2 electrodes, the basal plane of 2H-MoS2 is catalytically inert while its edge sites can serve as active centers for HER.[8] As a result, many researches have been conducted to prepare nanostructures that can maximize the exposure of active edge sites.[9,10] However, it
is still challenging to obtain 2H-MoS2 with extremely high concentrations of active edge sites due to large surface energy of the edges that include coordinatively unsaturated atoms.[11,12] Metallic 1T-MoS2 that is a metastable form of MoS2 was reported to promote hydrogen production because of the favorable energetics of hydrogen adsorption on its basal plane.[13,14] Nonetheless, due to the intrinsic metastable nature, 1T-MoS2 suffers from the lack of an effective synthesis method as well as the degradation of the catalytic performance over time.[15,16] Single-atom metal doping has been considered as a feasible route to activate the basal plane of stable 2H-MoS2. For instance, previous experiments showed that doping 2H-MoS2 with several transition metals (TMs) such as Ru, Pt and Sn generates active centers in the basal plane, significantly promoting the HER activity of the MoS2 electrode.[2,17–23] Despite the great potential of TM-doped MoS2, however, an understanding of the role of the dopants in the HER activity, which would help design new 2D catalysts, is yet to be established. For example, it was reported that Ru and Re doping leads to an increase of the concentration of sulfur vacancies (VSs) in MoS2 and many VSs occur near the defect sites where the dopants substitute for Mo (TMMo), but the origin for these experimental results and a function of the VS around TMMo have not been elucidated.[19,20][24] Moreover, S-top sites around TMMo in TM-doped MoS2 were often pointed out as critical active centers for HER, but this argument has not been justified thoroughly.[22] In this work, we investigate the HER activity of TM-doped MoS2 using first-principles calculations. We first focus on Ru-doped MoS2 for which various defect configurations were identified in experiments. By calculating the binding energy between RuMo and VS, we reveal that RuMo and VS strongly attract each other so that they are likely to make defect complexes (RuMo-VS), as suggested in the recent experiments. (Note that VS, a constituent of the defect
complex, is the major native defect that can be present in MoS2 more than 1013 cm−2.)[25] We evaluate hydrogen adsorption free energies at potential active centers including S-top and vacancy sites in Ru-doped MoS2. This turns out that the most critical catalytic sites for HER are VS sites in RuMo-VS defect complexes rather than S-top sites around isolated RuMo defects. We also explore 14 additional TM dopants whose ionic radius is similar to that of Mo, suggesting many dopants (8 out of 14) such as Os, Re, and Sn to promote the HER process of 2H-MoS2 by forming defect complexes with VS.
2. Methods 2.1. DFT calculations We perform density functional theory (DFT) calculations using the Vienna Ab-initio Simulation Package (VASP) with the projector-augmented wave (PAW) pseudopotentials.[26,27] The generalized gradient approximation (GGA) functional[28] is employed for the exchange-correlation energy. The cutoff energy for expanding plane-wave basis is set to 400 eV. In this work, we use monolayer MoS2 in 2H phase that is stable at room temperature. For the Brillouin zone sample, we use 8×8×1 and 2×2×1 Γ-centered kpoint grid for a unit cell and 4×4×1 supercells, respectively. The supercells include a vacuum with thickness of at least 12 Å to minimize interactions between repeated images along the vertical direction within the periodic boundary condition. Throughout this work, spinpolarized calculations are carried out. The atomic positions are relaxed until the force action on each atom becomes less than 0.02 eV/Å. 2.2. Hydrogen adsorption free energy Hydrogen adsorption energy (
) is calculated as[29–32]
(
where (
)
( )
(
) (1)
) and ( ) are the DFT energies of the supercells with and without hydrogen
adsorbed on an active site , respectively. (
) is the DFT energy of the hydrogen
molecule. Here, we employ 4×4×1 supercells for calculating ( be converted into hydrogen adsorption free energy (
) and ( ). The
can
) as follows:
(2)
where
and
are the change of zero-point energy (ZPE) and entropy upon hydrogen
adsorption, respectively. We consider
of −0.021 eV evaluated for VS in MoS2 using
GGA in a previous work[32] for every system because
is known to rarely depend on
materials and the type of active sites.[29-31] In line with previous calculations, we assume (
), where
(
) is the entropy of H2 gas at the standard state considering
the fact that the entropy change of TMDs upon hydrogen adsorption is negligible.[33] We take
(
) of 1.36 meV/K from the thermochemical table.[34] Hydrogen adsorption free
energy is a strong descriptor for the catalytic activity for HER and its optimal value was suggested to be around 0 eV.[33] However,
on a given catalyst can vary within 0.1-0.2
eV depending on exchange-correlation functionals.[32] Therefore, in this work, we discuss the HER activity based on
in reference to that on Pt (
) that is the best catalyst for
HER as of now, because the relative values among different materials are well maintained in
each functional.[32] For
of Pt, we take into account the (111) surface that is modeled as
a 2×2 supercell with three atomic layers and a vacuum longer than 12 Å. The calculated s at a S-top site and VS site in undoped MoS2 are 2.40 and 0.30 eV, respectively, which are in good agreement with 2.32 and 0.28 eV in previous calculations.[32]
2.3. Binding energy of a complex defect The binding energy (
( )
( )
) between two defects A and B are computed as[35]
* (
)
(
)+ (3)
where ( ) and ( ) are the total energies of the supercells including the defect A and B, respectively. (
) and (
) are the total energies of the supercells including the
complex defect AB and perfect MoS2, respectively. When the attractive interaction exists between A and B,
becomes positive. In the present study, every total energy to calculate
equation 3 is obtained using 4×4×1 supercells.
3. Results and discussion 3.1. Interplay between RuMo and VS for HER Defects in materials can be presented as isolated centers but also form defect complexes at high defect concentrations or at low temperatures. A key parameter that determines the concentration of defect complexes is the binding energy between the constituent defects. If
two defects have a large, positive binding energy, they are likely to form complexes. For instance, most of hydrogen in Mg-doped GaN form defect complexes with Mg at room temperature because of the large binding energy (
~0.7 eV).[35] To examine a possibility to
form defect complexes between RuMo and VS, we calculate the binding energy between them considering the atomic configurations in Figure 1. We find the significant binding energy of 1.56 eV, indicating the strong attractive interaction between RuMo and VS. Thus, many VSs in Ru-doped MoS2 are expected to exist in a form of the defect complexes. The large binding energy also means that the VS concentration can increase with Ru doping because VS is exclusively stable around RuMo. For obtaining insights into the attraction between RuMo and VS, we analyze the electronic structures of defective MoS2. In isolated VS (Figure 2a), three defect states are created between the band gap by the hybridization of the three dangling states on the surrounding Mo atoms, mainly consisting of Mo 4d orbitals. Due to trigonal symmetry of VS, these defect states can be categorized as a singlet a1 state and two doublet e states. The a1 state is lower in energy than the e states and two excess electrons that occur due to one missing S atom occupy the a1 state while the e states remain empty. On the other hand, in RuMo (Figure 2b), a defect state that is localized to the RuMo site develops inside the band gap. This state is mainly derived by the 4d states of the Ru atom and those of the neighboring Mo atoms. Because Ru has two more valence electrons than Mo, there are two excess electrons that occupy the defect state. When RuMo-VS is formed, namely VS occurs near the RuMo site, three defect states are generated (Figure 2c), as in VS. However, in this case, the degeneracy of the e states is lifted because the trigonal symmetry of VS is broken. Thus, the three defect states are different in energy and the two lower states are fully occupied by four excess electrons. It is noticed that the lowest and second lowest defect states in RuMo-VS lie below the occupied
defect levels in VS and RuMo, respectively. In other words, the occupied defect states of VS and RuMo are stabilized when forming defect complexes. Consequently, forming defect complexes can be energetically favorable, leading to the positive value of Eb. Table 1 shows
s at several active sites in pure and Ru-doped MoS2. As reported in
previous studies, the basal plane of pure MoS2 is inert for HER, resulting in very high at S-top sites.[32] When Ru is incorporated into a substitutional Mo site,
at the S-top
site near RuMo is predicted to be 0.79 eV that is advantageous over that in pure MoS2. However, this
is not small enough to explain the Pt-like catalytic performance of Ru-
doped MoS2 in experiments.[19,20] Meanwhile, VS of MoS2 is known to be catalytically active,[3][36] and indeed leads to favorable
, 0.30 eV.
at VS is reduced to the
more optimal value, 0.14 eV, when forming defect complexes with RuMo. Due to this benefit in energetics of hydrogen adsorption, along with the large Eb between RuMo and VS, sulfur vacancies in the defect complexes can constitute the most critical active centers for HER in Ru-doped MoS2, enabling dramatic improvement of the catalytic performance. We want to point out that smaller
(0.19 eV) at S-top sites around RuMo in a previous
study results from the consideration of the metastable ferromagnetic ordering for RuMo.[19] However, we find that the non-magnetic electronic configuration is more stable than the ferromagnetic one by 0.27 eV and
at the S-top site near RuMo taking into account the
stable non-magnetic ordering increases to 0.45 eV.
3.2. Identification of transition metal dopants for HER With the insights obtained from the study of Ru doped MoS2, we search for TM dopants for MoS2 that can enhance the electrocatalytic HER performance. Herein, for the feasibility of
doping, we focus on TMs whose ionic radius in the 4+ charge state matches that of Mo within a 10% difference. Such dopants are Ti, Zr, Nb, Tc, Ru, Rh, Pd, Sn, Hf, Ta, W, Re, Os, Ir, and Pt. First, we examine
at the S-top sites around isolated TMMo. Note that there
are two possible atomic configurations for TMMo: one is six-fold coordinated (Figure S1a) and the other is four-fold coordinated by the surrounding S atoms (Figure S1b). We test the total energy of these two configurations and select the more stable one for investigating the catalytic activity of the S-top sites (Table 2). Note that there are two distinct S-top sites around 4-fold coordinated TM dopants, but they lead to the identical free energy change upon hydrogen adsorption. The calculated
is plotted in Figure 3a for the energy range
between −0.4 and 0.4 eV, along with the volcano relation for transition metal catalysts like Pt, Ni, Cu, and Au[33] (the full list of data is provided in Table 2). Within a given energy range, a few dopants such as Pt, Pd, Rh, and Ir appear, indicating they can substantially activate the basal plane of MoS2 for HER without forming defect complexes with VS. Next, we examine the possibility for TMMo to form defect complexes with VS by computing the binding energy (Figure 4). It is seen that the Eb varies between −0.03 and 2.09 eV over the different dopants. The large, positive binding energy (Eb>0.6 eV) between VS and several TMMo such as RhMo and PtMo can be explained by the fact that the TMMo-VS produces the more stable defect states compared to the constituent defects (e.g., the band structure of RhMo in Figure S2a and b), as shown for RuMo-VS. In contrast, in TMMo like WMo and TaMo, occupied defect states that can be stabilized when forming defect complexes are not generated (e.g., the band structure of WMo in Figure S2c), leading to smaller binding energy (Eb<0.6 eV). For the dopants that yield Eb larger than 0.6 eV (Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt), we further evaluate
at VS in TMMo-Vs (Figure 3b). Interestingly, every defect
complex leads to favorable
at the VS site (<0.3 eV). As a result, significant
improvement in the HER performance of MoS2 is expected when these TMs are doped. The reason for the modifications of
in TM-doped MoS2 compared to the basal plane
of pure MoS2 is associated to the presence of defect states in the band gap that may strongly interact with the hydrogen state. As pointed out by Li et al.[3], VSs in MoS2, for example, generate localized gap states inside the band gap, resulting in lower
at the vacancy sites.
Similarly, as shown in the band structure of Ru-doped MoS2 in Figures 2b (without VS) and 2c (with VS), the defect levels appear inside the band gap, giving rise to the smaller
s
(0.79 eV at the S-top site without VS and 0.14 eV at the vacancy site in the defect complex), compared to that of the basal plane of perfect MoS2 (2.40 eV). As discussed above, defect states evidently affects
, but the effect of defect states on
can be different among
distinct dopants because the interaction between the defect and hydrogen states are influenced by various factors like the spatial extent and energetic position of the defect states and the size of active sites. On the other hand, we check the stability of TM-doped MoS2 by calculating the formation energy (Table S1). It turns out that every material exhibits negative formation energies which are close to that of pure MoS2, indicating that they are stable and synthesizable. Indeed, several dopants such as Sn[23], Re[2,17], Pd[18], and Pt[21,22], in addition to Ru, were already examined in experiments and found to substantially enhance the HER activity of MoS2. Considering our calculation results, we believe in such experiments that VS in TMMoVS serves as important active sites for HER. On the other hand, we also suggest other candidate dopants like Tc, Ir, Rh, and Os to promote the catalytic performance of MoS2 for HER by forming defect complexes with VS. Developing strategies to increase both VS and dopants would help to further optimize the HER activity for these dopants.
4. Conclusion In conclusion, we reported the DFT results about the effect of TM doping on the HER activity of MoS2. We showed that VS in Ru-doped MoS2 prefers to form defect complexes with RuMo, leading to the large, positive binding energy. The calculations of hydrogen adsorption free energy highlighted that VS in RuMo-VS exhibits Pt-like energetics for hydrogen adsorption, and thus, it can serve as the important active centers for HER in Ru-doped MoS2. We also explored the binding energy and hydrogen adsorption free energy in MoS2 doped with other TMs to discover optimal dopants for HER. The results showed that various TMs such as Sn, Tc, Ir, Rh, Ru, Re, Os, Pd, and Pt can be used as dopants to promote the HER process on MoS2. By enlightening the role of single-atom doping in the HER activity of MoS2 as well as suggesting several promising dopants, this work will pave the way to developing noble 2D catalysts for HER.
mmc1.docx
Declaration of Competing Interest None.
Acknowledgement This work was supported by Incheon National University (International Cooperative) Research Grant in 2020. References [1]
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Figure 1. Atomic structures of the sulfur vacancy (VS), substitutional Ru defect (RuMo), and defect complex (RuMo-VS).
Figure 2. Band structures of the sulfur vacancy (VS), substitutional Ru defect (RuMo), and defect complex (RuMo-VS). The defect levels are colored in red. The black dashed line denotes the Fermi level. The valence band maximum is set to 0.
Figure 3. Hydrogen adsorption free energies (
) of (a) basal planes (S-top sites) around
substitutional transition metal defects (TMMo) and at (b) sulfur vacancies (VS) in defect complexes (TMMo-VS). The black straight line indicates the volcano plot for metal catalysts that shows the relation between the experimental exchange current that is a measure of the rate of HER at the equilibrium potential and calculated
. For comparison purpose, the
data for pure Pt is marked in blue.
Figure 4. Binding energies between substitutional transition metal defects and the sulfur vacancy.
Table 1.
(in eV) of the basal plane (S-top site) and at sulfur vacancies (VS) in pure and
Ru-doped MoS2. In Ru-doped MoS2, VS refers to the sulfur vacancy around the RuMo-VS defect complex.
pure MoS2
Ru-doped MoS2
Basal plane
2.40
0.79
VS
0.30
0.14
Table 2. Coordination numbers (CN) of substitutional transition metal defects (TMMo) and (in eV) of basal planes (S-top sites) and at sulfur vacancies (VS) in TMMo-VS defect complexes.
Dopant
CN
Basal plane
VS
Ti
6
0.65
−0.30
Zr
6
0.69
−0.59
Nb
6
0.64
−0.56
Tc
6
1.00
0.08
Ru
6
0.79
0.14
Rh
4
0.01
0.11
Pd
4
−0.02
0.25
Sn
6
0.48
0.02
Hf
6
0.72
−0.57
Ta
6
0.69
−0.60
W
6
2.35
0.28
Re
6
1.17
0.18
Os
6
0.70
0.18
Ir
6
0.038
0.09
Pt
4
−0.13
0.30