1Tâ²-MoS2, A Promising Candidate for Sensing NOx - The Journal of

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1T'-MoS, a Promising Candidate for Sensing NO Yaoyao Linghu, and Chao Wu

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00051 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 7, 2019

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The Journal of Physical Chemistry

1T'-MoS2, a Promising Candidate for Sensing NOx Yaoyao Linghu and Chao Wu＊ Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710054, China.

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ABSTRACT Recently, ultra-thin MoS2 sheets in the semi-conducting 2H phase have been fabricated into high performance gas sensors. Here, we investigated the gas sensing behavior of MoS2 in the metallic 1T' phase via first-principles calculations. We found that monolayered and few-layered 1T'-MoS2 exhibit high selectivity and sensitivity toward NO molecules. Moreover, the adsorption of NO and NO2 molecules on monolayered and few-layered 1T'-MoS2 can be significantly strengthened by tensile strain whereas the adsorption of other common gas molecules (e.g. CO, CO2, NH3, SO2) is strain insensitive. This distinct behavior suggests high selectivity of NOx over other gases on 1T'-MoS2 can be achieved through strain engineering. In addition, the 50% Spassivated Mo-edges of 1T'-MoS2 are found to be more active than the basal plane. Our results suggest that monolayered and few-layered 1T'-MoS2 sheets (both basal plane and edges) are promising gas sensing materials.

INTRODUCTION Toxic gas detection is important for industrial, environmental, and military purposes. It is highly desirable to have gas sensors with low concentration limit of detection, rapid response, and high selectivity. Over the past decades, gas sensors are usually fabricated with metal oxide semiconductors.1-3 In the past decade, twodimensional (2D) layered nanomaterials (i.e., graphene and layered transition metal dichalcogenides, TMDs) have been used to make gas sensors with superior performance, primarily due to their large surface-to-volume ratio and unique electronic structure. For example, in 2007, Schedin et al. demonstrated that graphene can be made into efficient gas sensors for detecting CO, NH3, H2O and NO2 with detection limit on the order of 1 ppb and even has response to single gas molecules.4 The success of graphene-based sensors has stimulated the research of using other 2D materials for gas sensing. Mechanically exfoliated few-layered MoS2 nanosheets (2H-MoS2) were proved to be highly sensitive to NOx and NH3 molecules, whose detection limits for NO, NO2, and NH3 are as low as 0.8 ppm, 20 ppb, and 300 ppb, correspondingly.5-7 The flexible MoS2 thin-film transistor (TFT) sensor even exhibit superior sensitivity


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toward NO2 than the reduced graphene oxide field-effect transistor (FET) sensor.8 The sensitivity largely relies on the adsorption strength of gas molecule over sensor's surface. Theoretical calculations indicate that most gas molecules weakly physisorb over the basal plane of 2H-MoS2 and adsorb more strongly at the edge sites.914

For example, the adsorptions of NO and NO2 are stronger by approximately 3-4 eV

at the edge than on the basal plane. As a result, the NO2 sensitivity of vertically aligned MoS2 films (edge exposed) is 5-fold higher than horizontally aligned MoS2 films (basal plane exposed).15-16 Furthermore, due to the large number of edge-like sites, sensors made of MoS2 nanowire can detect the concentration of NO2 down to 4.6 ppb.17 Additionally, the edge sites of 2H-MoS2 are excellent for catalytic reactions like hydrogen evolution reaction (HER), whereas the basal plane is catalytically inert.18-19 Again, the reactants and key reaction intermediates interact more strongly with the edge than with the basal plane (e.g. the H adsorption energy at the edge sites is stronger by 0.45 eV than over the basal plane).20 Fortunately, the electronic structure of the basal plane of 2H-MoS2 and its related properties can be altered by strain. For example, 10% biaxial tensile strain can effectively reduce the band gap of semiconducting monolayer 2H-MoS2 (by 1.7 eV) and enhance its performance as double-gate EFTs.21 A combination of defect and strain can improve the HER activity of 2H-MoS2.22-23 Recent first-principles calculations have discovered that gas adsorption can be greatly strengthened over strained 2D materials, such as monolayer 2H-MoS2, stanine, GeS, SnS2 and so on.24-27 For instance, the NO2 adsorption on the basal plane of monolayer GeS sheets is enhanced by 0.3 eV under 3% biaxial compressive strain. In contrast to the 2H phase, the metallic 1T' phase of MoS2 is much more catalytic active for HER, because the H adsorption strength on the basal plane of 1T'-MoS2 is about 1.5 eV higher than that of 2H-MoS2.28-30 Theoretically, Chen and Wang have demonstrated that tensile strain can enhance the catalytic activity of 1T-MoS2.31 Similarly, we anticipate that the 1T'-MoS2-based sensors should be better than the ones based on the basal plane of 2H-MoS2 and their properties may be further tuned by strain. However, no investigation has been performed with respect to the gas adsorption



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behavior over 1T'-MoS2. In this work, we employed first-principles calculations to explore the potential of using 1T'-MoS2 as gas sensor materials. Firstly, we compared the gas adsorption behavior of a number of common gases (e.g. NOx, NH3, CO, CO2, SO2) over monolayer 2H- and 1T'-MoS2 sheets. Secondly, we examined the strain effects on the gas adsorption. We next investigate the effect of multilayer MoS2. Further, we also explored whether the edges of 1T'-MoS2 are suitable for sensing gas molecules. Finally, we summarized our results and conclude if 1T'-MoS2 nanosheets are potentially suitable for sensing NOx.

METHODS The first-principles calculations based on the density functional theory (DFT) were conducted using the Vienna ab initio simulation package (VASP).32 The exchange correlation term was treated by the Perdew−Burke−Ernzerhof (PBE) approach within the generalized gradient approximation (GGA).33 Spin-polarized calculations were carried out for NO, NO2 and the systems involving NO and NO2. In order to describe the long range van der Waals (vdW) interactions in the weakly bonding systems, a dispersion correction of total energy was added using the DFT-D3 method.34 The projector augmented wave (PAW) method35 and the energy cutoff of 450 eV were adopted for the plane-wave basis. The MoS2 monolayer was modeled by a 4  4 supercell and a Monkhorst-Pack mesh36 of 4 × 4 × 1 for the Brillouin zone integration were employed. In order to eliminate the interactions between image monolayers, a vacuum of 15 Å was added in the z-direction of the supercell. All the geometric configurations were fully relaxed until the energy and forces acting on each atom were less than 10-4 eV and 0.01 eV/Å, respectively. The adsorption energy of gas molecules on MoS2 was calculated according to Ead = Etotal - EMoS - Egas 2

(1)

where Etotal is the total energy of the gas molecule adsorbed on the MoS2 sheet, EMoS and 2

Egas are the energies of the clean MoS2 sheet and the gas molecule, respectively. To


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investigate the effect of strain on the gas adsorption, a biaxial strain was applied. The strain  was defined as

  a  a0  / a0  100%

(2)

where a and a0 are the lattice constants of strained and free MoS2, respectively. The calculated bond lengths and bond angles of gas molecules and Mo-Mo spacing of pristine 1T'-MoS2 sheets listed in Table S1 agree well with the experimental results 28, 37

The lattice constants of pristine 1T'-MoS2 were optimized to be 6.55 and 3.18 Å,

respectively, also consistent well with the previous theoretical reports.28 Ab initio molecular dynamics (AIMD) simulations using the Nose΄–Hoover heat bath scheme were employed to evaluate the thermal stability of NO adsorbed over 1T'MoS2 monolayers.

RESULTS AND DISCUSSION We first obtained the stable configurations of a series of common gas molecules (CO, CO2, NH3, SO2, NO and NO2) over monolayer 2H-MoS2 (Figure 1a) and 1T'MoS2 sheets (Figure 1b; Figure S1 and Table S2, in the Supporting Information). Most considered molecules weakly adsorb over both kinds of monolayers through vdW interactions (Table 1). The adsorption strengths of the gases (except CO2) on 1T'-MoS2 are stronger than those on 2H-MoS2. The calculations based on the 4×4 and 6×6 supercells show little lateral interactions (Table S3). The results suggest that the 4×4 supercell is large enough to ignore molecular interactions, so we kept using this supercell model in subsequent calculations. The geometric parameters (Figure 1) of the stable adsorption configurations indicate that the molecules are closer to the 1T'-MoS2 surface, allowing closer and stronger interaction. Since 2H-MoS2 sheets have been demonstrated as efficient sensors for these gases,5, 7-8, 15-16, 38-40 it is reasonable to believe that 1T'-MoS2 sheets are good candidate for making gas sensors.



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Figure 1. Top and side view of the most stable configurations of gas molecules adsorbed on (a) 2H-MoS2 and (b) 1T'-MoS2. Yellow: S, cyan: Mo, gray: C, red: O, blue: N, white: H. The distance (Å) between the molecule (the lowest atom) and the MoS2 sheets (the plane of the uppermost S atoms) are labeled.

Table 1. Adsorption energies (eV) of gas molecules on both 2H- and 1T'-MoS2 monolayers. MoS2

CO

CO2

NH3

SO2

NO

NO2

2H

-0.14

-0.31

-0.18

-0.30

-0.16

-0.21


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1T'

-0.15

-0.22

-0.22

-0.35

-0.47

-0.25

As a consequence of weak adsorption, the bond lengths of CO, CO2 and NH3 remain nearly unchanged after adsorption, while the bond lengths of NO2 and SO2 marginally increase by 0.001Å and 0.004 Å, respectively. Only the NO molecule has a less than 2 Å adsorption distance to the 1T'-MoS2 monolayer, which corresponds to its much stronger binding (-0.47 eV) than all other adsorptions. Notably, the bond length of NO decreases by 0.007Å. The relatively strong adsorption of NO over monolayer 1T'-MoS2 sheets has also been verified using AIMD (Figure S2). All the results suggest that monolayer 1T'-MoS2 sheets should be a better sensor (especially for NO) than monolayer 2H-MoS2 sheets. Then, we try to explain the selectivity of MoS2 towards NO from the electronic structure perspective. We calculated the density of states (DOS) and band structure of MoS2 monolayer adsorbed with CO, CO2, NH3, SO2, NO and NO2 molecules, respectively (Figure S3 and Figure S4). The number of states of a gas molecule near the Fermi level involved in hybridization largely determines the strength of the interaction between the gas molecule and the adsorbent. In most cases (CO, CO2, NH3, SO2 and NO2), the overlap between the DOS of the gas molecule and the MoS2 sheet around the Fermi level is small. This is also echoed in their band structures. For the NO2-adsorbed systems, the slight change in the electronic band structure is mainly due to the magnetism induced by the NO2 molecule (Figure S4f). Notably, the significant orbital hybridizations between NO and MoS2 are localized near the Fermi level (Figure S3e). In the band structure of the NO case (Figure S4e), an almost flat band corresponding to the NO spin up states around the Fermi level confirms its high sensitivity to NO adsorption. If the adsorption free energy is considered, the adsorptions are weak due to the reduction of entropy (Table S4). However, the entropy change and zero-point energy correction do not affect the order of the gas adsorption strength, we will use the adsorption energy rather than the adsorption free energy to characterize the adsorption



strength, as the former is more frequently used in literature (for easy and direct comparison) and the latter is much more time consuming. Fortunately, strain can change the adsorption strength of gas molecules on 2D materials. For example, Chen and Wang have shown that the hydrogen evolution activity of 1T-MoS2 sheets can be tuned by applying -6% (compressive) to 10% (tensile) strain.31 Here, we applied three values of strain respectively (−7%, 3% and 7%) to 1TMoS2 monolayers to check the response of gas adsorption (Figure 2). Only NO is sensitive to 3% tensile strain and its adsorption is strengthened by 0.2 eV. CO, CO2, NH3 and SO2 show no appreciable response (adsorption energy change < 0.15 eV) even for 7% tensile strain. The adsorptions of NO and NO2 are evidently strengthened under 7% tensile strain (adsorption strengthened by over 0.85 eV for both). The biaxial -7% (compressive) strain significantly weakens the adsorption strength of NO and has little effect on other gas molecules. Differently, the adsorption strengths of NO and NO2 are always weakened on strained (either tensile or compressive) 2H-MoS2.26 The response of gas adsorption over other 2D materials to strained can be quite different. Ma et al. found that tensile strain has little effect on the adsorption strength of NO2 on GeS monolayers, whereas compressive strain greatly improves the binding of NO2 molecules.25

Figure 2. Adsorption energies of gas molecules on 1T'-MoS2 monolayers under selected strain values.

The DOS and electronic band structure of MoS2 under 7% tensile strain adsorbed


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by gas molecules are presented in Figure S5 and Figure S6. Similar to free states (0% strain), the electronic structure of MoS2 monolayer is not significantly influenced by the adsorption of CO, CO2, NH3 and SO2 molecules. However, the influences of NO and NO2 adsorption on the electronic structure of MoS2 are much more pronounced. Compared to free states, the introduced additional new states of adsorbed NO and NO2 molecules around the Fermi level strongly hybridize with the states of MoS2 monolayer, which are responsible for the stronger adsorption of NO and NO2 molecules on MoS2 monolayers at 7% strain. Meanwhile, the band structures of MoS2 monolayer at 7% strain are significantly changed by the adsorption of NO and NO2 molecules. We also obtained the electronic structure of MoS2 under -7% strain adsorbed with these gas molecules (Figure S7 and Figure S8). There exists little hybridization around the Fermi level between most gas molecules (CO, CO2, NH3, SO2 and NO2) and MoS2 monolayers. In the NO case, the number of hybridized states of the system near the Fermi level under -7% strain is also smaller than in the free state (0% strain). Correspondingly, the original flat band near the Fermi level in the free state also vanishes. Next, we focus on the adsorption behavior of NO and NO2 on 1T'-MoS2 monolayers under a series of tensile strain (from 0% to 12% with an increment of 1%, Figure 3). Beyond 12% tensile strain, 1T'-MoS2 monolayers transform into 1T-like phase and then the structures break down (Figure S9).

Figure 3. Adsorption energies of NO and NO2 on 1T'-MoS2 monolayers under 0-12% tensile strain. Both the adsorptions are highly strain-sensitive and become stronger with the



increasing tensile strain. The NO adsorption on 1T'-MoS2 monolayers can be naturally divided into three sections by two kinks: (1) At around 6% strain, the partial fracture of Mo-Mo bonds (the bond lengths of Mo-Mo at site ‘1’ and site ‘2’ are elongated by 0.74 Å and 0.55 Å, respectively) appears in MoS2 monolayers (highlighted in Figure 4a). (2) At around 10% strain, 1T'-MoS2 monolayers begin to break down due to the adsorbed NO molecule (Figure 4). In comparison, the NO adsorption energy on 2H-MoS2 monolayers varies in a similar fashion in response to strain.27 However, the sharp drop of the adsorption energy is due to the metallization of MoS2 rather than the structure breakdown.

Figure 4. (a) NO and (b) NO2 adsorbed on 1T'-MoS2 monolayers under various strain. The change of the NO2 adsorption energy over strained 1T'-MoS2 monolayers is similar to that of NO (Figure 3). A closer inspection of the configurations reveals that under small strain (< 3%), NO2 molecules assume the "^"-shape configuration with respect to the MoS2 surface (Figure 4b). As strain increases, NO2 molecules begin to tilt. When strain goes beyond 5%, the adsorption energy drops more rapidly and the Mo-Mo bonds around the adsorbed NO2 molecule start to break (the bond lengths of Mo-Mo at site ‘3’ and site ‘4’ are elongated by 0.77 Å and 0.49 Å, respectively,


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highlighted in Figure 4b). This trend continues as strain further increases. There is evident adsorption-accelerated structure failure by NO and NO2 molecules on strained 1T'-MoS2 monolayers. It is well-known that the number of layers affect the electronic properties of 2D materials including 2H-MoS2,41-42 SnP3 and C3N,43 2D polyaniline.44-45 Furthermore, Li et al. pointed out that monolayer 2H-MoS2 devices were not as stable for sensing applications as their multilayer counterparts, which exhibited better sensing properties.5 Recently, Late et al. also proved that transistors of few MoS2 layers exhibit sensitivity comparable to their monolayer counterpart38. More importantly, the exfoliated MoS2 sheets tend to form bilayers due to the interlayer vdW interaction.46 Understanding the thickness effect of multilayer 1T'-MoS2 sensors is important to harvest the true potential of this material. Here, we investigated the adsorption behavior of both tensile strain-insensitive molecules (SO2 and NH3) and tensile strain-sensitive molecules (NO and NO2) molecules on bilayered and trilayered 1T'-MoS2 sheets (Figure 5), which were built by stacking monolayers in the same sequence. The interlayer spacings were optimized to be 2.471 Å for the bilayer model, 2.462 Å and 2.480 Å for the trilayer model, respectively (Figure S10 and Figure S11, for stable adsorption configurations, see Figure S12 and S13).



Figure 5. The adsorption energies of (a) SO2, (b) NH3, (c) NO and (d) NO2 on 1T'MoS2 sheets with different number of layers and different strain.

On relaxed 1T'-MoS2 sheets with all the considered thicknesses, the change of the adsorption energies of SO2, NH3, NO and NO2 is only about 0.01 eV. Like 1T'-MoS2 monolayers, relaxed multilayered 1T'-MoS2 sheets also exhibit high selectivity toward NO molecules, as its adsorption is the strongest. No appreciable change in adsorption energy can be observed for any molecule over all 1T'-MoS2 models at 3% strain (for NH3, this observation remains even at 7% strain). At 7% strain, the SO2 adsorption energy decreases to -0.75 eV on multilayered 1T'-MoS2 sheets, agreeing with the study of Tang et al47, who found that the Fe-decorated bilayer graphene exhibited higher sensitivity toward SO2 molecules than monolayer graphene. Also, Chen et al. found that the adsorption strengths of NH3 and NO2 on g-ZnO increase with the number of layers. They attributed the enhancement of adsorption strength to van der Waals forces between molecules and sublayers.48 In their case, the adsorption enhancement of NH3 with the layer number is probably due to the N-metal (including sublayer Zn) interactions, while in MoS2, the weaker H(NH3)-S(MoS2) interactions are less likely to be enhanced by sublayers compared to the S(SO2)-S(MoS2) interactions. In contrast,


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the adsorptions of NO and NO2 molecules on 7% strained multilayered 1T'-MoS2 sheets are weaker than those over monolayers. The stable configurations of NO2 over those strained multilayer 1T'-MoS2 sheets remain almost unchanged regardless of strain (i.e., the "^"-shaped configuration, Figure S14), which is very different from the configurations over monolayers (breaking of Mo-Mo bonds under similar strain and NO2 adsorption, Figure S14). The results indicate the higher stability of the multilayered 1T'-MoS2 sensors and greater strain is needed to enhance the gas adsorption strength than in monolayer systems. To better understand the relationship between the response of different gases to strain and the number of layers, we calculated the total DOS and corresponding partial DOS of bilayer MoS2 at 7% strain (Figure S15). Compared to the monolayer systems, the new hybridized states in multi-layer systems around the Fermi level from both NO and NO2 molecules decrease (probably related to the structural intactness), while the occupied states of SO2 near the Fermi level increase. Therefore, the adsorption strength of SO2 molecule on multilayer MoS2 at 7% strain is stronger than on monolayer systems whereas NO and NO2 are weaker.

Figure 6. The configurations of NO and NO2 molecules adsorbed at the Mo-edges covered by 50% S for 1T'-MoS2 monolayers. The vacuum space extends along both the x- and z-direction.

It has been demonstrated that the edge sites of 2H-MoS2 exhibited higher activity



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than its basal plane.15,16,18 Recently, the calculations by Gao et al. suggest that the catalytic HER activity on (1010) Mo-edge of 1T′-MoS2 is superior to that of 2HMoS2.49 Therefore, it is highly anticipated that the gas adsorption behavior at the edge sites of 1T'-MoS2 is very different.

Table 2. Adsorption energies (eV) of NO and NO2 adsorbed at Mo-edge with 50% S covered for 1T'-MoS2 and 2H-MoS2. Model

1T'-NO

1T'-NO2

2H-NO

2H-NO2

Edge-I

-0.92

-0.78

-1.78

-1.08

Edge-II

-0.44

-0.45

We built a 4 × 4 stripe model for 2H- and 1T′-MoS2 with the edges uncovered. Both Mo and S atoms are considered as adsorption sites. All adsorptions at edge sites are much stronger than on their basal planes (Table S5).15 The adsorption over 1T'MoS2 is much stronger than over 2H-MoS2 in most cases. Unfortunately, the stripe of 1T′-MoS2 with exposed edges is highly unstable that the structure severely distorts upon adsorption (Figure S16). Therefore, the exposed edge sites of 1T'-MoS2 are not suitable for serving as sensors directly and need to be passivated. Previous studies have proved that the Mo-edge sites of 2H-MoS2 are stable if passivated with 50% S coverage.19, 50 Also, Du et al. demonstrated that this passivation also works for 1T′-MoS2 stripes, whose edge sites have comparable catalytic activity to 2H-MoS2.49 Therefore, we built one-dimensional (1D) semi-infinite stripes for both 2H- and 1T′-MoS2 with Mo-edge covered by 50% S. For 1T′-MoS2, there are two different Moedges as 1T′-MoS2 sheets feature alternating wide and narrow MoS2 stripes.49 Thus, for 1T′-phase, we built stripes whose width has odd number of Mo atoms (N) across the stripe to include the two different edges, which are labeled as edge-I (Figure 6a and 6c) and edge-II (Figure 6b and 6d), respectively. To study the dependence between the


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adsorption energy and the stripe width, stripes with various widths (N = 4 - 8 for 2HMoS2 and N = 5 - 9 for 1T′-MoS2) were considered. The adsorption energy converges when the stripe widths are 5 and 7 for 2H- (Table S6) and 1T′-MoS2 (Table S7), respectively. The stripes are stable after adsorption (e.g. stripes of 1T′-MoS2 (N = 7) in Figure 6 and 2H-MoS2 (N = 5) in Figure S18). Moreover, the adsorptions of both NO and NO2 at edge-I are more favorable than at edge-II of 1T′-MoS2. Although the edge sites of 1T′ are not as active as those of 2H, they can still serve as gas adsorption sites (the adsorption energy of nearly 1 eV).

CONCLUSIONS Using first-principles calculations, we demonstrate that monolayered, multilayered 1T'-MoS2 sheets (both basal plane and edges) have great potential in detecting toxic gas molecules like NO and NO2.

This is based on the following observations.

1. Among the common gas molecules (CO, CO2, NH3, SO2, NO and NO2), NO adsorbs most strongly on relaxed 1T'-MoS2 monolayers with an adsorption energy of -0.47 eV, which ensures the material's high sensitivity and selectivity toward NO. 2. When moderate (e.g. 5%) biaxial tensile strain is applied, the adsorption energies of NO and NO2 decrease (become stronger) significantly from -0.47 eV and -0.25 eV to 0.85 eV and -0.80 eV, respectively, whereas the adsorption of other gas molecules exhibit weak strain dependence (< 0.15 eV variation in adsorption energy), which suggest the properly strained 1T'-MoS2 monolayers will be more sensitive to both NO and NO2. 3. The adsorption strengths of NO and NO2 molecules do not vary evidently with the increase of the number of layers, indicating the possibility of using multilayered 1T'MoS2 sheets as gas sensors. Although over 7% tensile strained bilayered and trilayered 1T'-MoS2 sheets, both the NO and NO2 adsorptions are weaker than over monolayers, the adsorption energies are still strong (-0.94 and -0.79 eV for NO and NO2, respectively). 4. The adsorption energies of NO and NO2 molecules at the Mo-edges of monolayer



1T'-MoS2 sheets are respectively 0.92 eV and 0.78 eV, stronger than on the basal plane of 1T'-MoS2. Therefore, the edges of 1T'-MoS2 can be used as gas adsorption for NO and NO2. Conflicts of interest The authors declare no conflicts of interest.

Acknowledgements This work is supported by National Natural Science Foundation of China (No. 21477096). We also thank the support from the Fundamental Research Funds for the Central Universities and the World-Class Universities (Disciplines) and the Characteristic Development Guidance Funds for the Central Universities. We acknowledge Xi'an Jiaotong University High Performance Computing Center, the Materials Physics Center of FIST and the support of H2 cluster in XJTU for providing the computational resources.

Supporting Information The optimized structures of monolayers of 2H- and 1T-related MoS2 (free and under various strain), monolayers of 2H- and 1T'-MoS2 with gas molecules adsorbed at the edges, 1T'-MoS2 multilayers with gas molecules adsorbed on basal plane (free and under % tensile strain); ab initio MD simulation of the 1T'-MoS2 monolayer adsorbed with NO; the electronic structure of MoS2 monolayer adsorbed with gas molecule at free and strained (7% and -7%) states; the table of calculated structural properties of free gas molecules and 1T-MoS2; the table of the adsorption energies of gas molecules on 1T'-MoS monolayer at different adsorption sites and supercell; the table of 2

adsorption energies of NO and NO2 adsorbed at edges of 1T'-MoS2 and 2H-MoS2 nanoribbons of different widths (edges naked and 50% S covered) and the free adsorption energy of gas molecules on 1T'-MoS2 monolayers.


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