Gas-Sensing Property of TM-MoTe2 Monolayer towards SO2, SOF2, and HF Gases

Detecting the characteristic decomposition products (SO2, SOF2, and HF) of SF6 is an effective way to diagnose the electric discharge in SF6-insulated equipment. Based on first-principles calculations, Au, Ag, and Cu were chosen as the surface modification transition metal to improve the adsorption and gas-sensing properties of MoTe2 monolayer towards SO2, SOF2, and HF gases. The results show that Au, Ag, and Cu atoms tend to be trapped by TH sites on the MoTe2 monolayer, and the binding strength increases in the order of Ag < Au < Cu. In gas adsorption, the moderate adsorption energy provides the basis that the TM-MoTe2 monolayer can be used as gas-sensing material for SO2, SOF2, and HF. The conductivity of the adsorption system changes significantly. The conductivity decreases upon gases adsorption on TM-MoTe2 monolayer, except the conductivity of Ag-MoTe2 monolayer increases after interacting with SOF2 gas.


Introduction
In the compact design of high-voltage equipment and gas insulation systems in the power industry [1], SF 6 gas has been widely used as an insulation medium due to its comprehensive advantages, such as high dielectric strength, strong electronegativity, thermal stability, chemical inertness, and non-toxicity [2,3]. However, SF 6 gas inevitably decomposes to toxic and corrosive by-products under electric discharge [4]. Corona, spark, and arc discharge are three typical types of electric discharge observed in SF 6 -insulated equipment [5]. Under the electric discharge, the low-sulfur fluorides produced by ionizing SF 6 gas will quickly react with trace moisture and impurities in the gas insulation system, forming some common stable decomposition products, including SO 2 , SOF 2 , and HF [6][7][8]. If these decomposition products are not handled properly, they will reduce the insulation strength of the filling gas, and be harmful to the environment and human health [9]. In addition, these acid gases will also corrode the original insulation device inside the gas insulation system, aggravating electric discharge and further affecting the safe and stable operation of the power system [10]. Online detection of the concentration of SO 2 , SOF 2 , and HF gases in the gas insulation system is crucial to ensure the safe running of equipment [11]. Therefore, it is urgent to explore suitable gas-sensitive materials for high-efficiency detection of the SF 6 decomposed gases.
Based on the catalytic performance and unique electrical structure, two-dimensional layered materials-based chemical sensors have been extensively explored and studied in broad application prospects, such as equipment testing, environmental diagnosis, and industrial manufacturing [12,13]. Graphene-like materials, such as transition metal dihalides (TMD), InN monolayers, hexagonal boron nitride (h-BN), and carbon nitride compounds, have a large specific surface area, and are resistant to strong acids and alkalis, and high temperatures [14][15][16]. Its adsorption and gas-sensitivity properties to gases have been extensively studied [17,18]. Compared with other TMD, MoTe 2 has lower binding energy and a larger bond length [19,20]. TMD-based gas sensors have attracted broad focus in recent years [21]. Wang et al. studied the gas-sensing potential of MoTe 2 monolayer to SF 6 decomposition products based on theoretical calculations [22]. Feng et al. developed a MoTe 2 -based gas sensor for NH 3 and NO 2 detection, with an excellent recovery rate and high sensitivity [23]. These previous studies have significantly enhanced the chemical interaction with specific gases, providing a promising candidate for SF 6 decomposition product detection.
The introduction of transition metal atom modification on the surface of two-dimensional layered materials effectively improves surface activity and gas-sensing performance [24,25]. In particular, nano-noble metals, such as Au, Ag, and Cu, may show better surface performance [26]. This is because transition metal atom modification increase the chemical activity and electron mobility of pristine materials, opening up a new perspective on exploring high-performance gas sensor [27]. Based on density functional theory (DFT) calculations, transition metals (Au, Ag, and Cu) were selected as modifying atoms on the MoTe 2 surface in this study. Then, SO 2 , SOF 2 , and HF gas molecules are adsorbed on three transition metals (Au, Ag, and Cu) modified MoTe 2 monolayer, abbreviated as TM-MoTe 2 . The surface modification and gas-sensing mechanism have been studied by analyzing the geometric structures and electronic properties.

Computational Details
All calculations are performed based on DFT calculations [28]. The Perdew-Burke-Ernzerhof (PBE) function of the generalized gradient approximation method (GGA) was selected to approximate the exchange and correlation of electrons [29]. DFT Semi-core Pseudopot (DSPP) was selected to eliminate the relativistic effect of TM atoms in the core processing [30]. The p-orbital dual-value plus polarization function (DNP) was used as the atomic orbital basis set, which increases the calculation accuracy of hydrogen bonds [31]. The Grimme method performed DFT-D correction on Ag and Cu modified MoTe 2 models. While DFT-D2 method by TS was used to analyze the intermolecular forces and long-range interactions in the Au-modified MoTe 2 model. A 7 × 7 × 1 Monkhorst pack grid was wet for Brillouin sampling. A static calculation with a self-consistent field convergence accuracy of 10 −6 Ha, a global orbit cut-off radius of 4.9 Å, and a smearing of 0.005 Ha were used to ensure the smooth convergence of the entire system [32]. According to the principle of periodic boundary conditions to eliminate the boundary effect, a single layer of MoTe 2 was constructed in a 4 × 4 × 1 supercell containing 16 Mo atoms and 32 Te atoms. A vacuum interval of 20 Å is used to avoid mutual influence between adjacent layers [33]. Four possible modification sites of TM atoms on MoTe 2 were considered, including T H , T Mo , T Te , and T B . T H is the location above the center of the MoTe 2 hexagonal ring, T Mo is the location on the top of the Mo atom, T Te is the location on the top of the Te atom, and T B means the location on the bridge between two Te atoms. The binding energy (E b ) of TM atoms on the MoTe 2 monolayer is defined in Equation (1). Where E TM-MoTe2 , E TM , and E MOTe2 represent the energy of TM-MoTe 2 monolayer, TM atom, and pristine MoTe 2 monolayer, respectively.
After obtaining the most stable TM-MoTe 2 monolayer structure, various possible adsorption positions of the gas molecule on TM-MoTe 2 were considered to study the adsorption behavior. Gas molecules undergo significant displacement and move to the highest stability position after structural optimization. The most stable configuration for gas molecule adsorption is determined by the largest adsorption energy (E ads ) calculated as Equation (2): where E TM-MoTe2/gas is the total energy of the adsorption system, while E TM-MoTe2 and E gas are the total energy of separated TM-MoTe 2 monolayer and gas molecules, respectively. The Mulliken atomic charges method is used to analyze the charge transfer. A negative value of charge transfer indicates the electrons transfer from TM-MoTe 2 monolayer to gas molecules.

Structural Optimization of Gas Molecules and TM-MoTe 2 Monolayer
As shown in Figure 1, the pristine MoTe 2 monolayer, TM atoms, SO 2 , SOF 2 , and HF gas molecules were optimized. The pristine MoTe 2 monolayer structure is composed of a hexagonal pattern with Se atoms and Mo atoms intersecting. The Mo-Te bond length of the MoTe 2 monolayer structure before TM modification is 2.758 Å. SO 2 gas molecule has a broken-line spatial configuration, which structure keeps good agreement with the reported theoretical and experimental results. where ETM-MoTe2/gas is the total energy of the adsorption system, while ETM-MoTe2 and Egas are the total energy of separated TM-MoTe2 monolayer and gas molecules, respectively. The Mulliken atomic charges method is used to analyze the charge transfer. A negative value of charge transfer indicates the electrons transfer from TM-MoTe2 monolayer to gas molecules.

Structural Optimization of Gas Molecules and TM-MoTe2 Monolayer
As shown in Figure 1, the pristine MoTe2 monolayer, TM atoms, SO2, SOF2, and HF gas molecules were optimized. The pristine MoTe2 monolayer structure is composed of a hexagonal pattern with Se atoms and Mo atoms intersecting. The Mo-Te bond length of the MoTe2 monolayer structure before TM modification is 2.758 Å. SO2 gas molecule has a broken-line spatial configuration, which structure keeps good agreement with the re- First, the geometric structure and electronic properties of the TM-MoTe2 monolayer were studied. Liu et al. achieved geometric relaxation of TM modification on the pristine MoTe2 monolayer through four possible positions [34]. After complete optimization, it can be seen that the TM atoms prefer to be trapped at the TH site (located above the center of the MoTe2 hexagonal ring). Therefore, this paper has three different TM atoms in the TH position for geometric optimization. It is worth noting that the three TM atoms form three bonds with three adjacent Te atoms after optimization, resulting in a certain degree of structural distortion in each system. However, in the optimized configuration, the lengths of the three TM-Te bonds are equal. It can be seen from Figure 2 that the average length of the TM-Mo bond in the TM-MoTe2 model is 2.942 Å, 2.974 Å, and 2.618 Å, respectively.  [34]. After complete optimization, it can be seen that the TM atoms prefer to be trapped at the TH site (located above the center of the MoTe 2 hexagonal ring). Therefore, this paper has three different TM atoms in the TH position for geometric optimization. It is worth noting that the three TM atoms form three bonds with three adjacent Te atoms after optimization, resulting in a certain degree of structural distortion in each system. However, in the optimized configuration, the lengths of the three TM-Te bonds are equal. It can be seen from Figure Table 1 shows that the bonding strengths of TM atoms on MoTe2, which increase in the order of Ag < Au < Cu, and the binding energies are 1.61 eV, 1.47 eV, and 1.13 eV, respectively. It shows that Au, Ag, and Cu of the same group can interact with the adjacent undercoordinated Mo atoms. The electron transfer from the TM atoms to the MoTe2 monolayer is −0.238 e, 0.007 e, and −0.104 e, respectively, which is crucial to the chemical activity and sensitivity of the TM modified MoTe2 monolayer. As shown in Figure 3, the band gaps of the MoTe2 single layer under TS optimization and Grimme optimization are 1.273 eV and 1.275 eV, respectively, which is not much different overall. All of the bandgaps of the TM-MoTe2 monolayer reduce, especially the bandgap of Au-MoTe2 reduces to 0.728 eV. The band gaps of Ag-MoTe2 and Cu-MoTe2 are also respectively reduced to 0.794 eV and 0.838 eV, which is very important for the performance of the activated material.

Adsorption of Gas Molecules on TM-MoTe2 Monolayer
The adsorption structures of SO2, SOF2, and HF gas molecules on the most stable TM-MoTe2 monolayer structure were obtained, as shown in Figure 4. Table 2 lists the adsorption energy and charge transfer between the TM-MoTe2 adsorption system and SO2, SOF2,  Table 1 shows that the bonding strengths of TM atoms on MoTe 2 , which increase in the order of Ag < Au < Cu, and the binding energies are 1.61 eV, 1.47 eV, and 1.13 eV, respectively. It shows that Au, Ag, and Cu of the same group can interact with the adjacent undercoordinated Mo atoms. The electron transfer from the TM atoms to the MoTe 2 monolayer is −0.238 e, 0.007 e, and −0.104 e, respectively, which is crucial to the chemical activity and sensitivity of the TM modified MoTe 2 monolayer. As shown in Figure 3, the band gaps of the MoTe 2 single layer under TS optimization and Grimme optimization are 1.273 eV and 1.275 eV, respectively, which is not much different overall. All of the bandgaps of the TM-MoTe 2 monolayer reduce, especially the bandgap of Au-MoTe 2 reduces to 0.728 eV. The band gaps of Ag-MoTe 2 and Cu-MoTe 2 are also respectively reduced to 0.794 eV and 0.838 eV, which is very important for the performance of the activated material.   Table 1 shows that the bonding strengths of TM atoms on MoTe2, which increase in the order of Ag < Au < Cu, and the binding energies are 1.61 eV, 1.47 eV, and 1.13 eV, respectively. It shows that Au, Ag, and Cu of the same group can interact with the adjacent undercoordinated Mo atoms. The electron transfer from the TM atoms to the MoTe2 monolayer is −0.238 e, 0.007 e, and −0.104 e, respectively, which is crucial to the chemical activity and sensitivity of the TM modified MoTe2 monolayer. As shown in Figure 3, the band gaps of the MoTe2 single layer under TS optimization and Grimme optimization are 1.273 eV and 1.275 eV, respectively, which is not much different overall. All of the bandgaps of the TM-MoTe2 monolayer reduce, especially the bandgap of Au-MoTe2 reduces to 0.728 eV. The band gaps of Ag-MoTe2 and Cu-MoTe2 are also respectively reduced to 0.794 eV and 0.838 eV, which is very important for the performance of the activated material.

Adsorption of Gas Molecules on TM-MoTe2 Monolayer
The adsorption structures of SO2, SOF2, and HF gas molecules on the most stable TM-MoTe2 monolayer structure were obtained, as shown in Figure 4. Table 2 lists the adsorption energy and charge transfer between the TM-MoTe2 adsorption system and SO2, SOF2,

Adsorption of Gas Molecules on TM-MoTe 2 Monolayer
The adsorption structures of SO 2 , SOF 2 , and HF gas molecules on the most stable TM-MoTe 2 monolayer structure were obtained, as shown in Figure 4. Table 2 lists the adsorption energy and charge transfer between the TM-MoTe 2 adsorption system and SO 2 , SOF 2 , and HF gas molecules. The calculated E ads ranges from −0.23 eV to −1.18 eV, determining whether gas adsorption processes belong to chemical or physical adsorption. Q T ranges from +0.031 e to −0.341 e, indicating the redistribution of the electrons for all systems. It can be seen that SOF 2 prefers to interact with the TM-MoTe 2 monolayer by the S atom of SO 2 F 2 approaching the TM atom. This is because the unique molecular configuration of SOF 2 makes the S atom more multivalent. and HF gas molecules. The calculated Eads ranges from −0.23 eV to −1.18 eV, determining whether gas adsorption processes belong to chemical or physical adsorption. QT ranges from +0.031 e to −0.341 e, indicating the redistribution of the electrons for all systems. It can be seen that SOF2 prefers to interact with the TM-MoTe2 monolayer by the S atom of SO2F2 approaching the TM atom. This is because the unique molecular configuration of SOF2 makes the S atom more multivalent.  It can be seen from Figure 4 that the structure of the TM-MoTe2 monolayer has undergone significant deformation after the adsorption of SO2, SOF2, and HF gas molecules. In particular, the Au atom undergoes a significant movement from the initial modification position, the position above the center of the MoTe2 hexagonal ring to a position partial to the top of the Te atom, and two of the original three Au-Te bonds are broken. The length of the remained Au-Te bond changes to 2.857 Å, 2.937 Å, and 2.947 Å when SO2, SOF2, and HF gases adsorb on the Au-MoTe2 monolayer, respectively. On the other hand, the Ag and Cu modified MoTe2 monolayer have not undergone significant geometric deformation during the adsorption of SO2, SOF2, and HF gases, though the Ag-Te bond and It can be seen from Figure 4 that the structure of the TM-MoTe 2 monolayer has undergone significant deformation after the adsorption of SO 2 , SOF 2 , and HF gas molecules. In particular, the Au atom undergoes a significant movement from the initial modification position, the position above the center of the MoTe 2 hexagonal ring to a position partial to the top of the Te atom, and two of the original three Au-Te bonds are broken. The length of the remained Au-Te bond changes to 2.857 Å, 2.937 Å, and 2.947 Å when SO 2 , SOF 2 , and HF gases adsorb on the Au-MoTe 2 monolayer, respectively. On the other hand, the Ag and Cu modified MoTe 2 monolayer have not undergone significant geometric deformation during the adsorption of SO 2 , SOF 2 , and HF gases, though the Ag-Te bond and Cu-Te bond are elongated to some extent. It can be determined that the Ag-MoTe 2 adsorption system is similar to the Cu-MoTe 2 adsorption system. The O atoms in the SO 2 gas molecule and the S atoms in the SOF 2 gas molecule are stably captured by the Ag atom and Cu atom by  Figure 5 shows the TDOS distribution of TM-MoTe 2 monolayers before and after SO 2 , SOF 2 , and HF gas adsorption. It can be seen that the TDOS distribution of the Au-MoTe 2 monolayer shifts to the right as a whole after the adsorption of SO 2 , SOF 2 , and HF gases, proving that the modification of Au atoms improves the chemical activity and conductivity of the adsorption system. In addition, due to the activated state of the adsorbed SOF 2 gas molecule, the TDOS distribution of the SOF 2 adsorbed TM-MoTe 2 system showed a continuous new peak between −10 eV and −5 eV. Moreover, the TDOS distribution of the SOF 2 gas adsorption system fluctuates the most in all distributions due to a certain amount of charge transfer during the adsorption process. On the right side of the Fermi level, the TDOS distribution decreases after SO 2 gas adsorption on the Ag-MoTe 2 monolayer and Cu-MoTe 2 monolayer, indicating that the filled electrons reduce, and a strong chemical effect occurs during the adsorption process. The TDOS distribution of the HF adsorbed Au-MoTe2 system shifts slightly to the right, but the TDOS distribution of the other two HF gas adsorption systems nearly does not change.  Figure 5 shows the TDOS distribution of TM-MoTe2 monolayers before and after SO2, SOF2, and HF gas adsorption. It can be seen that the TDOS distribution of the Au-MoTe2 monolayer shifts to the right as a whole after the adsorption of SO2, SOF2, and HF gases, proving that the modification of Au atoms improves the chemical activity and conductivity of the adsorption system. In addition, due to the activated state of the adsorbed SOF2 gas molecule, the TDOS distribution of the SOF2 adsorbed TM-MoTe2 system showed a continuous new peak between −10 eV and −5 eV. Moreover, the TDOS distribution of the SOF2 gas adsorption system fluctuates the most in all distributions due to a certain amount of charge transfer during the adsorption process. On the right side of the Fermi level, the TDOS distribution decreases after SO2 gas adsorption on the Ag-MoTe2 monolayer and Cu-MoTe2 monolayer, indicating that the filled electrons reduce, and a strong chemical effect occurs during the adsorption process. The TDOS distribution of the HF adsorbed Au-MoTe2 system shifts slightly to the right, but the TDOS distribution of the other two HF gas adsorption systems nearly does not change. PDOS analysis was performed to understand the electronic behavior of TM-MoTe2 monolayer when adsorbing SO2, SOF2, and HF gases, as shown in Figure 6. It can be found that the 5d, 4d and 3d orbits of Au, Ag, and Cu atoms have a significant influence on their respective TDOS distributions due to their mental activities. It can be seen from the PDOS PDOS analysis was performed to understand the electronic behavior of TM-MoTe 2 monolayer when adsorbing SO 2 , SOF 2 , and HF gases, as shown in Figure 6. It can be found that the 5d, 4d and 3d orbits of Au, Ag, and Cu atoms have a significant influence on their respective TDOS distributions due to their mental activities. It can be seen from the PDOS distribution of each adsorption system that the d orbits of TM atoms highly hybridize with the p orbits of S atoms or O atoms of the adsorbed SO 2 gas molecules and SOF 2 gas molecules. It confirms the stable formation of TM-S or TM-O bonds in the TM-MoTe 2 adsorption system. In the TDOS distribution of all TM-MoTe 2 /HF adsorption systems, a slight fluctuation between −12.5 eV and −7.5 eV appears as a new peak derived from the 2p and 1s orbits of HF gas molecule. These orbital interactions indicate that SO 2 , SOF 2 , and HF gases have an ideal adsorption effect on the TM-MoTe 2 monolayer, which leads to the redistribution of electrons on the substrate and a change of conductivity to a large extent. s 2022, 27, x FOR PEER REVIEW 7 of 11 distribution of each adsorption system that the d orbits of TM atoms highly hybridize with the p orbits of S atoms or O atoms of the adsorbed SO2 gas molecules and SOF2 gas molecules. It confirms the stable formation of TM-S or TM-O bonds in the TM-MoTe2 adsorption system. In the TDOS distribution of all TM-MoTe2/HF adsorption systems, a slight fluctuation between −12.5 eV and −7.5 eV appears as a new peak derived from the 2p and 1s orbits of HF gas molecule. These orbital interactions indicate that SO2, SOF2, and HF gases have an ideal adsorption effect on the TM-MoTe2 monolayer, which leads to the redistribution of electrons on the substrate and a change of conductivity to a large extent.

Molecular Orbital Analysis of Gas Adsorbed TM-MoTe2 Monolayer
The adsorption of SO2, SOF2, and HF gases on the TM-MoTe2 monolayer was analyzed by molecular orbital analysis. According to the molecular orbital theory, the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) distributions of the adsorption system were calculated as shown in Figure 7. The yellow and blue areas in Figure 7 represent the positive and negative phases of the wave function. Table 3 shows the energy gap (Eg) between HOMO and LUMO, which helps evaluate the change in conductivity.

Molecular Orbital Analysis of Gas Adsorbed TM-MoTe 2 Monolayer
The adsorption of SO 2 , SOF 2 , and HF gases on the TM-MoTe 2 monolayer was analyzed by molecular orbital analysis. According to the molecular orbital theory, the highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) distributions of the adsorption system were calculated as shown in Figure 7. The yellow and blue areas in Figure 7 represent the positive and negative phases of the wave function. Table 3 shows the energy gap (E g ) between HOMO and LUMO, which helps evaluate the change in conductivity.
Molecular orbit is an effective method to evaluate the probability of electron transfer between characteristic molecules and the surface of TM-MoTe 2 . This confirms the previous DOS analysis that SO 2 , SOF 2 , and HF gas molecules are non-magnetic molecules during adsorption. At the same time, the E g of the TM-MoTe 2 monolayer changes after the gas molecules' adsorption. For the Ag-MoTe 2 system, the E g of the adsorption system decreases after SOF 2 gas molecules adsorption. While for other TM-MoTe 2 systems, the E g of the adsorption system increases after the gas adsorption. As the LUMO distribution obviously increases around the gas adsorption site, and HOMO distribution on Ag-MoTe 2 slightly increases when SOF 2 adsorption on the Ag-MoTe 2 system. As a result, the electron transition from HOMO to LUMO becomes easier, and E g decreases simultaneously. Therefore, the conductivity of the Ag-MoTe 2 monolayer increases after interacting with SOF 2 gas, while the conductivity of other adsorption systems decreases. As shown in Figure 7, the adsorption of gas molecules causes the redistribution of electrons in TM-MoTe 2 , which changes the energy of HOMO and LUMO accordingly, which matches the changing trend of E g .

Conclusions
Based on first-principles calculations, this paper studies the stable structure of TM (Au, Ag, and Cu) modification on the MoTe 2 monolayer. Then the adsorption structures of the characteristic decomposition products of SF 6 (SO 2 , SOF 2 , and HF) on the TM-MoTe 2 monolayer were calculated. By analyzing the adsorption structure, adsorption energy, charge transfer, adsorption distance, TDOS, PDOS, and molecular orbit, the adsorption performance and electronic behavior of TM-MoTe 2 monolayer towards SO 2 , SOF 2 , and HF gases were explored. TM atoms tend to be trapped by T H sites on the MoTe 2 monolayer with a binding strength of Ag <Au < Cu. The adsorption energy of the TM-MoTe 2 monolayer to SO 2 , SOF 2 , and HF gas is moderate, indicating that they are suitable gas-sensing materials for detecting SO 2 , SOF 2 , and HF gases. The adsorption of SO 2 , SOF 2 , and HF gases on the TM-MoTe 2 monolayer leads to the redistribution of electrons in the TM-MoTe 2 systems, affecting its conductivity to a large extent. After SOF 2 adsorption on the Ag-MoTe 2 monolayer, the conductivity increases along with the decreased E g , while the other gas adsorption on the TM-MoTe 2 monolayer leads to an increase of E g and a decrease in conductivity. Based on the different change rules of conductivity of the systems, it realizes the selective detection of the mixed decomposition products.