The Adsorption and Sensing Performances of Ir-modified MoS2 Monolayer toward SF6 Decomposition Products: A DFT Study

In this paper, the Ir-modified MoS2 monolayer is suggested as a novel gas sensor alternative for detecting the characteristic decomposition products of SF6, including H2S, SO2, and SOF2. The corresponding adsorption properties and sensing behaviors were systematically studied using the density functional theory (DFT) method. The theoretical calculation indicates that Ir modification can enhance the surface activity and improve the conductivity of the intrinsic MoS2. The physical structure formation, the density of states (DOS), deformation charge density (DCD), molecular orbital theory analysis, and work function (WF) were used to reveal the gas adsorption and sensing mechanism. These analyses demonstrated that the Ir-modified MoS2 monolayer used as sensing material displays high sensitivity to the target gases, especially for H2S gas. The gas sensitivity order and the recovery time of the sensing material to decomposition products were reasonably predicted. This contribution indicates the theoretical possibility of developing Ir-modified MoS2 as a gas sensor to detect characteristic decomposition gases of SF6.


Introduction
Due to the excellent insulation and arc-extinguishing ability of SF 6 gas, it has been widely used in gas-insulated switchgear (GIS) [1,2]. During long-term operations, SF 6 in GIS decomposes into different characteristic gases, such as H 2 S, SO 2 , SOF 2 , etc., when discharge faults occur [3,4]. Research showed that detecting these characteristic decomposition gases can reflect the operating status and potential faults of the gas-insulated equipment [5][6][7][8]. Therefore, the accurate detection of these gases is of important for the online monitoring of GIS and the safe operation of equipment.
Among various detection methods, the metal-oxides-based gas sensor was considered a convenient and effective approach [9]. However, these traditional gas sensors have defects, such as low sensitivity and instability, limiting their further development [10,11]. Twodimensional (2D) materials are widely used in gas sensing, catalysis, energy storage, etc., due to their large specific surface areas and distinctive physical and chemical properties [12][13][14]. The MoS 2 monolayer was studied and reported as a potential gas-sensing material [15]. Rathi et al. studied the gas-sensing performance of La-MoS 2 hybrid-heterostructure-based sensor of NO 2 gas. They found that the sensing response of the La-decorated MoS 2 gas sensor was six times more than the pristine MoS 2 , indicating that the fabricated sensor was suitable for NO 2 detection [16]. Urs et al. reported the sensitivity of MoS 2 modified with alloy nanoparticles to H 2 at temperatures in the range of 30 to 100 • C. The results showed that the composite nanomaterials can enhance the response to H 2 , and this phenomenon was explained using the density functional theory (DFT) simulation [17]. Based on these studies, we deduced that the gas sensitivity of intrinsic MoS 2 can be further effectively improved by introducing appropriate dopants, such as transition metal atoms or metal oxides [18]. In particular, iridium (Ir), which has excellent physicochemical stability, has been proven to effectively improve the Nanomaterials 2021, 11, 100 2 of 12 gas sensitivity of 2D materials [19,20]. Thus, we speculated that the reasonable combination of Ir and MoS 2 has potential for gas sensing. Considering the huge challenges faced by current sensor development and the importance of detecting SF 6 decomposition components, the adsorption and sensing performance of Ir-modified MoS 2 for these gases should be studied based on DFT for guiding the experiments.
In this paper, we predict the adsorption and sensing properties of Ir-modified MoS 2 monolayer to SF 6 decomposition components (H 2 S, SO 2, and SOF 2 ) based on the DFT calculation. The most stable adsorption structure of Ir-modified MoS 2 for different gases is proposed, and its electronic properties are also systematically explored. The gas-sensing response of the Ir-modified MoS 2 nanomaterial-based sensor to the target gases was reasonably predicted. These results indicated that the Ir-modified MoS 2 -based gas sensor is a promising alternative for detecting the decomposition components of SF 6 .

Computational Detail
In this work, the DMol 3 package based on the DFT method was used to study the electronic and adsorption characteristics [21]. The 4 × 4 MoS 2 monolayer with the lattice constants of 12.664 × 12.664 Å was established, and its size proved to be large enough for gas adsorption [22]. To avoid influence between the adjacent layers of MoS 2 , we set the vacuum thickness to 20 Å. The electron exchange and correlation energy were treated by the generalized gradient approximation (GGA), Perdew-Burke-Ernzerhof (PBE), and the DFT-D method. The double numerical plus polarization (DNP) basis set was applied to deal with the relativistic effect of the molecular structure [23]. The k-point was set as 5 × 5 × 1 Monkhorst. To ensure the reasonableness and accuracy of the calculation, the energy convergence accuracy, maximum force, and maximum displacement in this study were selected as 10 −5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively [24]. The adsorption energy (E ad ), charge transfer (Q t ), energy gap (E g ), and recovery time (T r ) for each system were analyzed using the following equations: (1) where E gas/Ir-modified MoS2 , E gas, and E Ir-modified MoS2 denote the energy of the Ir-modified MoS 2 structure adsorbed gas, the bare Ir-modified MoS 2 substrate, and the isolated gas, respectively; Q a and Q b in represent the charge of gas molecules after and before adsorption, respectively. To study the conductivity change of various adsorption structures, we defined the energy gap (E g ) depicted in Equation (3). HOMO and LUMO represent the highest occupied molecular orbital and the lowest occupied molecular orbital, respectively; the lower the E g , the higher the conductivity [25]. T r , F 0 (10 12 s −1 ), K B (8.62 × 10 −5 eV K −1 ), and T w represent recovery time, attempt frequency, Boltzmann constant, and working temperature, respectively [26].

Structure and Properties of the Ir-modified MoS 2 Monolayer
The optimized structures of Ir-modified MoS 2 and target gases are shown in Figure  1. Figure 1a illustrates that the Ir atom forms an optimal configuration with MoS 2 at a distance of 2.742 Å directly above the Mo atom. This doping mode (the S vacancy) proves to be the most stable based on the formation energy analysis [27]. The three optimized Ir-S bond lengths of the Ir-modified MoS 2 structure are 2.250, 2.262, and 2.247 Å. Figure 1b depicts the optimized configurations of three characteristic decomposition components of SF 6 . The bond length and bond angle of each gas molecule are marked in the figure, which is consistent with the literature [28,29]. Nanomaterials 2021, 11, 100 3 of 12 Nanomaterials 2020, 10, x FOR PEER REVIEW 3 of 12 characteristic decomposition components of SF6. The bond length and bond angle of each gas molecule are marked in the figure, which is consistent with the literature [28,29]. To further analyze the electronic properties of Ir-modified MoS2, the density of states (DOS) distributions and band structure are depicted in Figure 2. Figure 2a compares the total density of states (TDOS) of Ir-modified MoS2 shifted to the left with the intrinsic MoS2. This phenomenon is mainly attributed to the hybridization of the orbitals of introduced Ir atom with the orbitals of Mo and S atoms in MoS2. Specifically, the Ir-5d orbital overlaps with Mo-4d and S-3p orbitals in the range of −7.0 to 2.0 eV as observed in the distribution of partial density of states (PDOS), which indicated that the structure formed by the introduced Ir atom and MoS2 is stable. The Ir-5d peak appears near the Fermi level, indicating that electrons are more easily transferred to the conduction band after Ir atom was introduced, and the conductivity of the gas-sensing material increases. Figure 2b depicts the band structures of intrinsic and Ir-modified MoS2. We noticed that the bandgap of the composite system reduced from 2.088 to 0.398 eV after introducing an Ir atom due to the introduction of impurity levels near the Fermi level after doping with Ir atom. The electrons are more easily excited from the valence band to the conduction band. This phenomenon is mainly attributed to the hybridization of the orbitals of introduced Ir atom with the orbitals of Mo and S atoms in MoS 2 . Specifically, the Ir-5d orbital overlaps with Mo-4d and S-3p orbitals in the range of −7.0 to 2.0 eV as observed in the distribution of partial density of states (PDOS), which indicated that the structure formed by the introduced Ir atom and MoS 2 is stable. The Ir-5d peak appears near the Fermi level, indicating that electrons are more easily transferred to the conduction band after Ir atom was introduced, and the conductivity of the gas-sensing material increases. Figure 2b depicts the band structures of intrinsic and Ir-modified MoS 2 . We noticed that the bandgap of the composite system reduced from 2.088 to 0.398 eV after introducing an Ir atom due to the introduction of impurity levels near the Fermi level after doping with Ir atom. The electrons are more easily excited from the valence band to the conduction band.   [22]. The electrons are mainly concentrated around the Ir atom, which illustrates that the bonds formed by Ir atom and surrounding S atoms have a strong binding force [30]. The Ir atom acts as an electron acceptor and obtains 0.274 e from MoS2 monolayer, and the S mainly behaves as an electron donator. The HOMO and LUMO distributions of the Ir-modified MoS2 system are depicted in Figure 3b. We found that a large part of HOMO is distributed near the Ir atom, which indicates that the addition of an Ir atom provides more active sites on the surface of MoS2 and enhances the sensitivity of the material.    [22]. The electrons are mainly concentrated around the Ir atom, which illustrates that the bonds formed by Ir atom and surrounding S atoms have a strong binding force [30]. The Ir atom acts as an electron acceptor and obtains 0.274 e from MoS 2 monolayer, and the S mainly behaves as an electron donator. The HOMO and LUMO distributions of the Ir-modified MoS 2 system are depicted in Figure 3b. We found that a large part of HOMO is distributed near the Ir atom, which indicates that the addition of an Ir atom provides more active sites on the surface of MoS 2 and enhances the sensitivity of the material.   [22]. The electrons are mainly concentrated around the Ir atom, which illustrates that the bonds formed by Ir atom and surrounding S atoms have a strong binding force [30]. The Ir atom acts as an electron acceptor and obtains 0.274 e from MoS2 monolayer, and the S mainly behaves as an electron donator. The HOMO and LUMO distributions of the Ir-modified MoS2 system are depicted in Figure 3b. We found that a large part of HOMO is distributed near the Ir atom, which indicates that the addition of an Ir atom provides more active sites on the surface of MoS2 and enhances the sensitivity of the material.

Structures of Different Adsorption Systems
To obtain the most stable adsorption structure for each gas, we established various adsorption models in which the gases were close to the Ir-modified MoS 2 surface at different positions. The optimized H 2 S-Ir-MoS 2 , SO 2 -Ir-dopd-MoS 2, and SOF 2 -Ir-dopd-MoS 2 structures in various positions are depicted in Figures 4-6, respectively. Based on the figures, we observed that all gas molecules were adsorbed on the surface of the Ir-modified MoS 2 surface in different spatial positions. In particular, in the H 2 S adsorption systems of P2 and P3 depicted in Figure 4b,c, the H-S bond in the H 2 S gas broke, and the H atom formed a new chemical bond with the Ir atom, indicating that the adsorption of H 2 S gas is a chemical adsorption process. The adsorption parameters of Ir-modified MoS 2 for various gases in different positions are depicted in Table 1.We observed from Table 1 that all the adsorption energies were negative, indicating that the adsorption of the target gases is an exothermic process. The absolute value of E ad in all systems was the largest for P3 structure, indicating that the adsorption structure is the most stable compared to the other two configurations, and the gas is most likely to be adsorbed on the surface of the material in this state during the adsorption reaction. The gas in each system was stably adsorbed on the Ir-modified MoS 2 surface at different distances, and the specific adsorption distance values are listed in Table 1. For Q t , we found that the amount of charge transfer between H 2 S and the sensing material was the largest compared with SO 2 and SOF 2 gas adsorption systems, which illustrates that the H 2 S gas causes a greater change in the conductivity of the material during the adsorption process. Nanomaterials 2020, 10, x FOR PEER REVIEW 5 of 12

Structures of Different Adsorption Systems
To obtain the most stable adsorption structure for each gas, we established various adsorption models in which the gases were close to the Ir-modified MoS2 surface at different positions. The optimized H2S-Ir-MoS2, SO2-Ir-dopd-MoS2, and SOF2-Ir-dopd-MoS2 structures in various positions are depicted in Figures 4-6, respectively. Based on the figures, we observed that all gas molecules were adsorbed on the surface of the Ir-modified MoS2 surface in different spatial positions. In particular, in the H2S adsorption systems of P2 and P3 depicted in Figure 4b,c, the H-S bond in the H2S gas broke, and the H atom formed a new chemical bond with the Ir atom, indicating that the adsorption of H2S gas is a chemical adsorption process. The adsorption parameters of Ir-modified MoS2 for various gases in different positions are depicted in Table 1.We observed from Table 1 that all the adsorption energies were negative, indicating that the adsorption of the target gases is an exothermic process. The absolute value of Ead in all systems was the largest for P3 structure, indicating that the adsorption structure is the most stable compared to the other two configurations, and the gas is most likely to be adsorbed on the surface of the material in this state during the adsorption reaction. The gas in each system was stably adsorbed on the Ir-modified MoS2 surface at different distances, and the specific adsorption distance values are listed in Table 1. For Qt, we found that the amount of charge transfer between H2S and the sensing material was the largest compared with SO2 and SOF2 gas adsorption systems, which illustrates that the H2S gas causes a greater change in the conductivity of the material during the adsorption process.        To further compare and analyze the different characteristics of Ir-modified MoS2 sensing material for different SF6 decomposition components, we selected the most stable structures (Position 3) for each gas adsorption system. The optimized configurations and adsorption parameters of target gases adsorbed on the Ir-modified MoS2 monolayer are depicted in Figure 7 and Table 2, respectively. We suggest, based on Figure 7a, that the S-H bond in H2S gas was broken due to the strong metallicity of Ir atom during the adsorption process of H2S gas, which led to the formation of a new H2-Ir bond between H2 atom and Ir atom with a bond length of 1.584 Å. The H1-S1 bond length is 1.357 Å, which is slightly elongated compared to the bond length of the gas before adsorption (1.350 Å). For the SO2-Ir-MoS2 and SOF2-Ir-MoS2 adsorption systems depicted in Figure 7b,c, respectively, we observed that To further compare and analyze the different characteristics of Ir-modified MoS 2 sensing material for different SF 6 decomposition components, we selected the most stable structures (Position 3) for each gas adsorption system. The optimized configurations and adsorption parameters of target gases adsorbed on the Ir-modified MoS 2 monolayer are depicted in Figure 7 and Table 2, respectively. We suggest, based on Figure 7a, that the S-H bond in H 2 S gas was broken due to the strong metallicity of Ir atom during the adsorption process of H 2 S gas, which led to the formation of a new H2-Ir bond between H2 atom and Ir atom with a bond length of 1.584 Å. The H1-S1 bond length is 1.357 Å, which is slightly elongated compared to the bond length of the gas before adsorption (1.350 Å). For the SO 2 -Ir-MoS 2 and SOF 2 -Ir-MoS 2 adsorption systems depicted in Figure  7b,c, respectively, we observed that the S atom in the gas molecules is close to the Ir atom to form the stable structures. The bond lengths of S2-O1 and S2-O2 in SO 2 gas are 1.467 and 1.468 Å, respectively. The bond lengths of S3-O3, S3-F2, and S3-F1 in SOF 2 gas are 1.449, 1.642, and 1.643 Å, respectively. All of the bond lengths of SO 2 and SOF 2 gases undergo small changes during the adsorption progress, suggesting that gases have interactions with the sensing material.
Nanomaterials 2020, 10, x FOR PEER REVIEW 7 of 12 H2S gas interacts strongly with the Ir-modified MoS2 system, which is consistent with the calculated results of adsorption energy.

DOS Analysis of Different Adsorption Systems
The DOS distributions were used to study the electronic properties and gas sensitivity of SF6 decomposition components adsorbed on the Ir-modified MoS2 monolayer, and the results are depicted in Figure 8. Figure 8a depicts the overall TDOS of the H2S-Ir-MoS2 system moving to the left after adsorbing H2S gas, indicating that the electrons easily fill in the conduction band. The huge change in TDOS that happened near the Fermi level indicated that the Ir-modified MoS2 gas-sensing material has strong interactions with H2S gas, and the gas sensitivity is improved. The peaks of the PDOS spectrum overlap among Ir, S, and H atoms, suggesting that the adsorption structure has good stability. The hybridization of Ir-3p and S-3p orbitals should be responsible for the huge changes in TDOS near the Fermi level. In the SO2-Ir-MoS2 structure displayed in Figure 8b, the TDOS decreases around the Fermi level after SO2 gas adsorption. This phenomenon illustrates that the number of free electrons decreases after gas adsorption and the resistance of the material increases. We observed the PDOS distributions of Ir-5d orbital near the Fermi level, which may be the main reason for the decrease in TDOS. The hybridization of Ir-5d and S-3p orbitals increases the TDOS near the energy level of −7.0 eV after gas adsorption. For the SOF2-Ir-MoS2 system depicted in Figure 8c, the TDOS of the SOF2-Ir-MoS2 adsorption system slightly changes near the Fermi level compared with the DOS of Ir-MoS2, indicating that the material has weak gas-sensitivity to SOF2 gas. From the PDOS distributions, we found that the novel peaks appearing in DOS at the range of −9.0 to −8.0 eV are  Adsorption systems are displayed in Table 2. The E ad of H 2 S-Ir-MoS 2 was calculated to be −2.323 eV, which is smaller than the SO 2 -Ir-MoS 2 (−1.757 eV) and SOF 2 -Ir-MoS 2 (−1.492 eV) systems. This result suggested that all adsorption reactions can proceed spontaneously, the adsorption process of the H 2 S gas is the strongest, and the formed structure is the most stable among the three systems. According to the definition of adsorption distance, the D of H 2 S-Ir-MoS 2 , SO 2 -Ir-MoS 2, and SOF 2 -Ir-MoS 2 is 1.583, 2.175, and 2.171 Å, respectively. Based on the Mulliken population analysis, we calculated the Q t of the three adsorption systems as 0.286, 0.114, and 0.154 e for H 2 S-Ir-MoS 2 , SO 2 -Ir-MoS 2, and SOF 2 -Ir-MoS 2 configurations, respectively. The largest Q t of H 2 S-Ir-MoS 2 indicates that the H 2 S gas interacts strongly with the Ir-modified MoS 2 system, which is consistent with the calculated results of adsorption energy.

DOS Analysis of Different Adsorption Systems
The DOS distributions were used to study the electronic properties and gas sensitivity of SF 6 decomposition components adsorbed on the Ir-modified MoS 2 monolayer, and the results are depicted in Figure 8. Figure 8a depicts the overall TDOS of the H 2 S-Ir-MoS 2 system moving to the left after adsorbing H 2 S gas, indicating that the electrons easily fill in the conduction band. The huge change in TDOS that happened near the Fermi level indicated that the Ir-modified MoS 2 gas-sensing material has strong interactions with H 2 S gas, and the gas sensitivity is improved. The peaks of the PDOS spectrum overlap among Ir, S, and H atoms, suggesting that the adsorption structure has good stability. The hybridization of Ir-3p and S-3p orbitals should be responsible for the huge changes in TDOS near the Fermi level. In the SO 2 -Ir-MoS 2 structure displayed in Figure 8b, the TDOS decreases around the Fermi level after SO 2 gas adsorption. This phenomenon illustrates that the number of free electrons decreases after gas adsorption and the resistance of the material increases. We observed the PDOS distributions of Ir-5d orbital near the Fermi level, which may be the main reason for the decrease in TDOS. The hybridization of Ir-5d and S-3p orbitals increases the TDOS near the energy level of −7.0 eV after gas adsorption. For the SOF 2 -Ir-MoS 2 system depicted in Figure 8c, the TDOS of the SOF 2 -Ir-MoS 2 adsorption system slightly changes near the Fermi level compared with the DOS of Ir-MoS 2 , indicating that the material has weak gas-sensitivity to SOF 2 gas. From the PDOS distributions, we found that the novel peaks appearing in DOS at the range of −9.0 to −8.0 eV are mainly caused by the hybridization of the S-3p and F-2p orbitals.

DCD Analysis of Various Adsorption Systems
The interactions between SF6 decomposition components and Ir-modified sensitive materials were studied via the DCD analysis. Figure 9 depicts the corresponding DCD results of various adsorption systems. The charge accumulation in all structures is mainly concentrated on the gas molecules, and charge depletion is distributed around the Ir atom. This distribution suggests that the gases have interactions with the Ir-modified MoS2 material, which is also consistent with the DOS analysis results. In these adsorption systems, the SF6 decomposition components act as electron donors, while the Ir-modified MoS2 material acts as an electron acceptor. Compared with SO2-Ir-MoS2 and SOF2-Ir-MoS2 adsorption configurations, there is a large amount of charge accumulation and dissipation between the H2S gas and the Ir-modified MoS2 layer, which is caused by the strong reaction in the adsorption process [31].

DCD Analysis of Various Adsorption Systems
The interactions between SF 6 decomposition components and Ir-modified sensitive materials were studied via the DCD analysis. Figure 9 depicts the corresponding DCD results of various adsorption systems. The charge accumulation in all structures is mainly concentrated on the gas molecules, and charge depletion is distributed around the Ir atom. This distribution suggests that the gases have interactions with the Ir-modified MoS 2 material, which is also consistent with the DOS analysis results. In these adsorption systems, the SF 6 decomposition components act as electron donors, while the Ir-modified MoS 2 material acts as an electron acceptor. Compared with SO 2 -Ir-MoS 2 and SOF 2 -Ir-MoS 2 adsorption configurations, there is a large amount of charge accumulation and dissipation between the H 2 S gas and the Ir-modified MoS 2 layer, which is caused by the strong reaction in the adsorption process [31]. molecules, and charge depletion is distributed around the Ir atom. This distribution suggests that the gases have interactions with the Ir-modified MoS2 material, which is also consistent with the DOS analysis results. In these adsorption systems, the SF6 decomposition components act as electron donors, while the Ir-modified MoS2 material acts as an electron acceptor. Compared with SO2-Ir-MoS2 and SOF2-Ir-MoS2 adsorption configurations, there is a large amount of charge accumulation and dissipation between the H2S gas and the Ir-modified MoS2 layer, which is caused by the strong reaction in the adsorption process [31].

Frontier Molecular Orbital Analysis of Different Systems
The HOMO and LUMO distributions were used to study the electronic behavior of various adsorption systems. According to the frontier molecular orbital analysis, we analyzed the possible change trends of material conductivity to predict the gas-sensing performance of materials [32]. After

Frontier Molecular Orbital Analysis of Different Systems
The HOMO and LUMO distributions were used to study the electronic behavior of various adsorption systems. According to the frontier molecular orbital analysis, we analyzed the possible change trends of material conductivity to predict the gas-sensing performance of materials [32]. After the target gas is adsorbed, the electron clouds of Ir-modified MoS 2 material is redistributed, causing changes in the energy values of HOMO and LUMO, as depicted in Figure 10. A large amount of HOMO is distributed on and around the gas, especially in the H 2 S-Ir-MoS 2 system, indicating that these electrons are not bound and can undergo charge transfer during the adsorption reaction. The Ir-modified MoS 2 sensitive material has obvious electron transfer behavior for the adsorption of these three target gases; thus, we speculate that Ir-modified MoS 2 gas-sensing material can be used as a resistive gas sensor to detect H 2 S, SO 2, and SOF 2 gases. Nanomaterials 2020, 10, x FOR PEER REVIEW 9 of 12 the target gas is adsorbed, the electron clouds of Ir-modified MoS2 material is redistributed, causing changes in the energy values of HOMO and LUMO, as depicted in Figure 10. A large amount of HOMO is distributed on and around the gas, especially in the H2S-Ir-MoS2 system, indicating that these electrons are not bound and can undergo charge transfer during the adsorption reaction. The Ir-modified MoS2 sensitive material has obvious electron transfer behavior for the adsorption of these three target gases; thus, we speculate that Ir-modified MoS2 gas-sensing material can be used as a resistive gas sensor to detect H2S, SO2, and SOF2 gases.

Gas-sensing Prediction of Ir-modified MoS2 to SF6 Decomposition Products
After gas adsorption, a large change in Eg indicates that the conductivity of the sensing material increases based on the definition of resistive gas sensor sensitivity [33]. The comparative analysis of Eg for different optimized systems are proposed and displayed in Figure 11a. The Eg increased to varying degrees after gas adsorption due to the different changes in the energy levels of HOMO and LUMO (marked in Figure 10). Compared with the Eg of the Ir-modified MoS2, the degree of Eg change in these systems is as follows: H2S-Ir-MoS2 > SO2-Ir-MoS2 > SOF2-Ir-MoS2. Combining the above results, we suggest that the gas sensitivity order of the Ir-modified MoS2 to these SF6 decomposition products is H2S > SO2 > SOF2. Work function (WF) refers to the minimum energy required for electrons to release from the surface. It represents the contact barrier between the target gas and the

Gas-Sensing Prediction of Ir-modified MoS 2 to SF 6 Decomposition Products
After gas adsorption, a large change in E g indicates that the conductivity of the sensing material increases based on the definition of resistive gas sensor sensitivity [33]. The comparative analysis of E g for different optimized systems are proposed and displayed in Figure 11a. The E g increased to varying degrees after gas adsorption due to the different changes in the energy levels of HOMO and LUMO (marked in Figure 10). Compared with the E g of the Ir-modified MoS 2 , the degree of E g change in these systems is as follows: H 2 S-Ir-MoS 2 > SO 2 -Ir-MoS 2 > SOF 2 -Ir-MoS 2 . Combining the above results, we suggest that the gas sensitivity order of the Ir-modified MoS 2 to these SF 6 decomposition products is H 2 S > SO 2 > SOF 2 . Work function (WF) refers to the minimum energy required for electrons to release from the surface. It represents the contact barrier between the target gas and the material during the gas adsorption process [31]. Figure 11b shows the calculated values of WF for various optimized structures. We observed that WF decreases to 5.252 eV after the adsorption of H 2 S gas, but increases to 5.905 and 5.878 eV for SO 2 and SOF 2 , respectively, compared with the Ir-modified MoS 2 system (5.469 eV). In other words, the smaller work function value of H 2 S-Ir-MoS 2 indicates that H 2 S more easily adsorbs on the Ir-modified MoS 2 material compared with the other two systems.

Gas-sensing Prediction of Ir-modified MoS2 to SF6 Decomposition Products
After gas adsorption, a large change in Eg indicates that the conductivity of the sensing material increases based on the definition of resistive gas sensor sensitivity [33]. The comparative analysis of Eg for different optimized systems are proposed and displayed in Figure 11a. The Eg increased to varying degrees after gas adsorption due to the different changes in the energy levels of HOMO and LUMO (marked in Figure 10). Compared with the Eg of the Ir-modified MoS2, the degree of Eg change in these systems is as follows: H2S-Ir-MoS2 > SO2-Ir-MoS2 > SOF2-Ir-MoS2. Combining the above results, we suggest that the gas sensitivity order of the Ir-modified MoS2 to these SF6 decomposition products is H2S > SO2 > SOF2. Work function (WF) refers to the minimum energy required for electrons to release from the surface. It represents the contact barrier between the target gas and the material during the gas adsorption process [31]. Figure 11b shows the calculated values of WF for various optimized structures. We observed that WF decreases to 5.252 eV after the adsorption of H2S gas, but increases to 5.905 and 5.878 eV for SO2 and SOF2, respectively, compared with the Ir-modified MoS2 system (5.469 eV). In other words, the smaller work function value of H2S-Ir-MoS2 indicates that H2S more easily adsorbs on the Ir-modified MoS2 material compared with the other two systems.  The predicted recovery time for the Ir-modified MoS 2 based sensor is displayed in Figure 12. The recovery time decreases with the increase in temperature due to the rapid desorption of gas molecules at high temperatures. The sequence of the recovery time for these gases at the same temperature is as follows: SOF 2 < SO 2 < H 2 S. Although the recovery time of H 2 S is very long due to the strong adsorption capacity, the time can be less than 2 min by appropriately increasing the working temperature during the experimental test. This result can provide a theoretical basis for guiding the gas-sensing performance test experiment.
Nanomaterials 2020, 10, x FOR PEER REVIEW 10 of 12 The predicted recovery time for the Ir-modified MoS2 based sensor is displayed in Figure 12. The recovery time decreases with the increase in temperature due to the rapid desorption of gas molecules at high temperatures. The sequence of the recovery time for these gases at the same temperature is as follows: SOF2 < SO2 < H2S. Although the recovery time of H2S is very long due to the strong adsorption capacity, the time can be less than 2 min by appropriately increasing the working temperature during the experimental test. This result can provide a theoretical basis for guiding the gas-sensing performance test experiment.

Conclusions
In this work, we used the theoretical calculation based on the DFT method to investigate the adsorption characteristic and gas-sensing mechanism of Ir-modified MoS2 to decomposition components of SF6, including H2S, SO2, and SOF2. We optimized and analyzed the geometric parameters and electronic properties of the Ir-modified MoS2 system. The results indicated that the

Conclusions
In this work, we used the theoretical calculation based on the DFT method to investigate the adsorption characteristic and gas-sensing mechanism of Ir-modified MoS 2 to decomposition components of SF 6 , including H 2 S, SO 2 , and SOF 2 . We optimized and analyzed the geometric parameters and electronic properties of the Ir-modified MoS 2 system. The results indicated that the introduction of the Ir atom enhances the surface activity of the material and reduces the bandgap of intrinsic MoS 2 from 2.088 to 0.398 eV as well as increases the conductivity. The most stable adsorption structure of Ir-modified MoS 2 for different gases was proposed, and their electronic properties were systematically explored via analyzing the DOS, DCD, molecular orbital theory, and WF. The gas-sensing mechanism study demonstrated that Ir-modified MoS 2 monolayer can adsorb the target gases and cause microscopic electron behavior, especially for H 2 S gas. The gas sensitivity order of SF 6 decomposition products was predicted as follows: H 2 S > SO 2 > SOF 2 . The predicted recovery time of the sensor to all target gases can be less than 2 min by appropriately increasing the temperature. Based on these results, the Ir-modified MoS 2 is suggested as a potential candidate for detecting decomposition components of SF 6 .