Study on the Adsorption Characteristics of Mo-Doped Graphene on the Decomposition Products of SF 6 Substitute Gas Based on First-Principle Calculations

C 4 F 7 N, C 5 F 10 O, etc., as new environmental-friendly alternative gases decompose under partial discharge and produce a series of products such as CO, CF 4 , C 2 F 6 , C 3 F 8 , CF 3 CN, C 2 F 5 CN, and COF 2 . Based on the ﬁrst-principles calculation method of density functional theory (DFT), the adsorption characteristics of intrinsic state graphene and Mo-doped graphene adsorbing SF 6 and its substitute gas decomposition products are calculated and analyzed. By comparing the adsorption energy, adsorption distance, density of state, Mulliken charge population, charge transfer amount, and molecular orbital energy for adsorbing diﬀerent decomposition gases, it can be seen that the system structure is the most stable when Mo is doped at the T site of the graphene surface. The adsorption of Mo-doped graphene on gas molecules is signiﬁcantly stronger than that of intrinsic graphene, and the order of adsorption is: SO 2 F 2 > H 2 S > SO 2 > CF 4 . The adsorption of H 2 S gas molecules by intrinsic state and Mo-doped graphene is n-type adsorption, while the adsorption of SO 2 F 2 , CF 4 , and SO 2 gas molecules is p-type adsorption. Mo-doped graphene can be used as a detection device for SO 2 F 2 gas resistance sensors.


Introduction
Sulfur hexafluoride (SF 6 ) gas is widely used in direct current breakers and gas insulated switchgear (GIS) due to its excellent insulation performance and arc extinguishing performance [1][2][3][4]. However, SF 6 gas insulation device will inevitably leak in the process of installation, use, and commissioning. Because of its great greenhouse effect and extremely difficult decomposition in the atmosphere, scientists and scholars from all over the world began to study new SF 6 alternative gas. e research results show that C 4 F 7 N, C 5 F 10 O, CF 3 I, and other new environmental-friendly gases can replace SF 6 gas to a certain extent [5][6][7][8]. However, during long-term use, certain defects, and aging phenomena will inevitably occur in electrical equipment, resulting in a decline in its insulation performance, resulting in spark discharge or thermal breakdown under the action of the arc, which will decompose SF 6 and its new alternative gas. e decomposed gas will react with a small amount of carbon dioxide and water in electrical equipment and other trace substances, and then generate a variety of harmful gases to the environment, such as CF 4 , SO 2 F 2 , SO 2 , and H 2 S [9][10][11] (Figure 1). ese decomposition gases are not only harmful to the environment but also threaten the safety of workers and the normal and stable operation of electrical equipment. With the national "carbon peak and carbon neutral" goal proposed, environmental-friendly operation and management of electrical equipment has become a general trend [12]. erefore, it is essential to monitor and analyze the decomposition products of SF 6 gas.
In recent years, with the continuous in-depth research on semiconductor sensor materials, various types of gas sensors have attracted extensive attention. Zhang et al. [13] show that Ni-doped In 2 O 3 /WS 2 nanohybrid materials provide a new way for formaldehyde sensors. Another study Zhang et al. [14] found that Ni-doped MoS 2 films can be used as SO 2 gas sensors. Also, another study by Zhang et al. [15] show that ZnO/WS 2 nanoheterostructures can detect trace benzene gas at room temperature. Based on the firstprinciples calculations of density functional theory, it is believed that Au-doped MoSe 2 composites are more beneficial as NH 3 gas sensors [16]. Graphene is a new type of carbon nanofunctional material constructed by layers of carbon atoms. Similar to other inorganic oxide semiconductor metal oxides (CuO, Co 3 O 4 , SnO 2 , In 2 O 3 , and LaFeO 3 ) gas sensors, graphene not only has low cost [17], large surface area [18], high stability, good electrical conductivity [19], high mechanical strength [20], high carrier mobility [21], and a series of advantages but also in gas adsorption has a wider application. Pearce et al. [22] studied the adsorption of individual gas molecules by intrinsic graphene and found that intrinsic graphene only has good gas sensitivity to a small part of gas molecules such as NH 3 and NO 2 . It can be seen that intrinsic graphene because of its very stable structure and no dangling bonds, there are obvious limitations in the adsorption of gas molecules. Relevant studies have shown [23] that doping can improve the adsorption characteristics of intrinsic graphene for specific gases. Sun et al. [24] calculated the adsorption of O, O 2 , OH, and OOH by B-P atom single doping and codoping graphene and found that codoping has a synergistic effect and the adsorption effect is the best. Song et al. [25] comparatively analyzed the adsorption of C 2 H 4 gas on graphene doped with Fe, Co, and Ni based on density functional theory and found that Ni-doped graphene has the best adsorption capacity. Wang et al. [26] used B, P, Pd, and Pt single-doped graphene to adsorb H 2 and CO gases, respectively, and the results showed that the single-doped Pd and Pt graphene had a better adsorption effect on these two gases. Zhang et al. [27] found that the surface of Au-doped graphene has good gas sensitivity to SO 2 F 2 gas molecules based on simulations and experiments.
At present, although there have been some studies on the adsorption of gas molecules on the surface of element-doped graphene, most of the adsorbed gas molecules are conventional gases such as NH 3 and CO 2 , and the research used to detect the decomposition products of SF 6 and its substitute gas is still very rare. erefore, this paper is based on density functional theory (DFT) [28], using first-principle calculation methods to explore the four gas molecules of H 2 S, SO 2 , CF 4 , and SO 2 F 2 in Mo-doped graphite. e adsorption parameters, the density of states, the Mulliken charge population, the amount of charge transfer, and the energy of the molecular orbital on the surface of the alkene provide a theoretical basis for the study of high-performance gas sensors to monitor the decomposition products of SF 6 and its substitute gases.

Calculation Parameter Design
e calculation of this paper mainly uses DMol3 module. First, select a single-layer graphene cell with lattice constant a � b � 0.246 nm, c � 0.680 nm, crystal plane angle α � β � 90°, c � 120°, a total of two carbon atoms, and then expand 6 times in the a and b directions to create a 6 × 6 supercell with 72 carbon atoms in total. Take a 20Å vacuum layer in the c direction of the unit cell to prevent mutual interference between adjacent layers of graphene during the optimization process, thereby affecting the accuracy of the calculation results. e bond length between carbon-carbon bonds in the graphene structure is 0.142 nm, and the theoretical thickness of a single layer of atoms is 0.34 nm.
In this paper, the PBE (Perdew, Burke, and Ernzerhof ) basis set of the generalized gradient approximation (GGA) is used to deal with the exchange correlation energy between atoms [29], and the intrinsic state graphene and Mo-doped graphene unit cells are analyzed respectively. Optimize the geometric structure to obtain a stable geometric structure, and then calculate the adsorption process of the four typical decomposition gases for the stable geometric structure to find the most stable adsorption structure, and finally analyze the calculated results to find the best adsorption capacity gas sensor. Set the grid space k points to 6 × 6 × 1. In the adsorption model, consider the obvious role of van der Waals force in the physical adsorption effect. To achieve more accurate calculation results, the DFT-D used to correct the dispersion force is added during the iteration process (Tkatchenk and Scheffler, TS) algorithm [30], set the orbital cut-off radius 3.5Å, the single-atom energy convergence accuracy during the iteration process, the maximum stress, the maximum displacement between atoms, the convergence standards are 1.0 × 10 −5 Ha, 0.004Ha/Å and 0.005Å, respectively, Consider spin polarization.

Binding Energy Analysis.
e graphene crystal structure has high periodicity and symmetry. erefore, there are three kinds of doping at different positions during the doping process, namely, the top position T (directly above the carbon atom), doping, and the bridge position B (two  doped with a carbon atom in the middle), and doped with an acupuncture point H (in the center of the carbon ring), as shown in Figure 2. e most stable adsorption structure can be determined by the size of the binding energy. e smaller the binding energy, the more stable the adsorption structure. e formula for the binding energy (E form ) of Mo atom-doped graphene surface is as follows: Among them, E binding is the binding energy of Mo atomdoped graphene structure, E Mo-Gra is the energy when the entire doping system is most stable after Mo atoms doping the graphene surface, E Gra is the total energy of intrinsic graphene, and E Mo is the total energy of a single Mo atom. By comparing the size of the binding energy, the most stable adsorption structure for gas molecule adsorption can be obtained. e binding energy of different positions of Mo atom-doped graphene is shown in Table 1. It can be seen from Table 1 that in the three different doping processes, the binding energy is the lowest when Mo atoms are doped to replace the carbon atoms in the T position of graphene. It can be seen that the structure model after T position doping is more stable. It is also easier to form. erefore, the adsorption calculations of SF6 and its substitute gas decomposition products later in this article are based on the structural model of B-Au doped T-site graphene. e side view and top view of the Mo atom-doped intrinsic graphene surface are shown in Figures 3(a) and 3(b). It can be seen from Figure 3 that after Mo atoms doped the intrinsic graphene surface, the intrinsic graphene surface structure did not change significantly, but because the radius of Mo atoms is much larger than that of C atoms and the local sp 3 formed near the Mo atoms after doping hybrid configuration, the Mo atom and the surrounding three adjacent C atoms form a covalent bond, and the Mo atom protrudes directly above, the protruding height is about 1.549Å, and the C-Mo bond length is 2.02Å, this is similar to the structure studied in the literature, so the model calculated in this article is correct and reliable and can be used to calculate other properties.

Adsorption Performance Analysis.
First, optimize the structure of SF 6 and its substitute gas decomposition products, and then perform adsorption calculations on the optimized gas molecules and Mo-doped intrinsic graphene surface. Set the initial adsorption distance to 2Å. Figure 4 is the structural model of the optimized gas molecules, and Figures 5(a) and 5(b) are the most stable structural models of the gas molecules adsorbed on the eigenstate and Mo atomdoped intrinsic graphene surface, respectively.
In order to analyze the adsorption characteristics of SF 6 and its substitute gas decomposition products on the surface of different atom-doped graphene, the energy change value of the adsorption process between the different atom-doped T-site graphene surface and the SF 6 decomposition gas is introduced, which is the adsorption energy (△E ads ) [31]: Among them, E gas-Mo-Gra is the total energy of Mo atomdoped graphene surface adsorbed gas molecules, E Mo-Gra is the total energy of Mo atom-doped graphene surface, and E gas is the total energy of gas molecules. e positive or negative of the adsorption energy indicates whether the reaction is exothermic or endothermic. e absolute value of the adsorption energy indicates the strength of the adsorption capacity. e larger the absolute value, the stronger the adsorption capacity. Table 2 shows the adsorption energy (△E ads ), adsorption distance (d), and bond length (Å) of the four gas molecules decomposed by SF 6 and its substitute gas. As can be seen from Table 2, as for the adsorption energy, intrinsic graphene has weak adsorption capacity for four kinds of gas molecules but strong adsorption capacity for H 2 S gas, while Mo-doped graphene surface has increased adsorption capacity for four kinds of gas molecules, and has the strongest adsorption capacity for SO 2 F 2 gas. According to Zhou et al.  Advances in Condensed Matter Physics 3 [32], when the adsorption energy is greater than 0.8 eV, the adsorption effect is chemical adsorption, and vice versa, it is physical adsorption. It can be seen from the data in the table that the adsorption capacity of intrinsic graphene for these four gas molecules is relatively weak, all of which are physical adsorption; while the adsorption of SO 2 and CF 4 on the surface of Mo-doped graphene is still physical adsorption., the effect is not obvious, but the adsorption of H 2 S and SO 2 F 2 gas molecules is chemical adsorption, and the interaction is relatively strong. As for the adsorption distance, the eigenstate graphene adsorbs H 2 S gas molecules the shortest distance, and the Mo-doped graphene surface adsorbs SO 2 F 2 gas molecules the shortest distance. e shorter the adsorption distance, the stronger the interaction, which also corresponds to the adsorption energy. Regarding the bond length, after the intrinsic graphene adsorbs the four gas molecules, the bond length between the atoms in the gas molecules hardly changes, and the intrinsic graphene and the four gas molecules are also reacted from the side. e interaction between them is relatively weak, and after the Mo-doped graphene surface adsorbs these four gas molecules, the bond length between the atoms in the gas molecules has changed significantly, especially the SO 2 and SO 2 F 2 gas molecules, which adsorb SO 2 gas. After the molecule, the bond lengths of the two S-O bonds were extended to 3.639Å and 1.529Å, respectively. After the SO 2 F 2 gas was adsorbed, the bond lengths between the two S-F bonds were 3.431Å and 2.969Å, respectively, indicating that the S-F bond has been broken and the SO 2 F 2 gas molecule has cracked, indicating that a strong chemical reaction occurred between the SO 2 F 2 gas and the Mo-doped graphene surface. To more clearly show the changes in the surface structure parameters of the intrinsic graphene and Mo-doped graphene before and after adsorption. Figures 6(a) and 6(b) shows the adsorption energy and adsorption distance of the four gases in different graphene structures. It can be seen from Figure 6 that the absolute value of the adsorption energy of the intrinsic state graphene for the four gas molecules is H 2 S > SO 2 F 2 > CF 4 > SO 2 and the adsorption distance is SO 2 > CF 4 > SO 2 F 2 > H 2 S, Mo doped the adsorption energy of intrinsic graphene for the four decomposition gases is SO 2 F 2 > H 2 S > SO 2 > CF 4 , and the absolute value of charge transfer is CF 4 > SO 2 > H 2 S > SO 2 F 2 .
From the calculation and analysis of the above adsorption parameters, it can be seen that the Mo-doped intrinsic graphene has the strongest ability to adsorb SO 2 F 2 gas molecules. erefore, this article mainly takes the adsorption of SO 2 F 2 gas as an example to further explore the adsorption characteristics of SF 6 decomposition gas on the surface of Mo-doped graphene.

Density of State Analysis.
To further analyze the influence of Mo-doped graphene surface on the adsorption characteristics of SF 6 and its substitute gas decomposition products, this paper analyzes the total density of states and atomic partial wave states of intrinsic graphene and Mo-doped intrinsic graphene adsorbed SO 2 F 2 gas molecules on the surface. e density was calculated and analyzed. Since the region far away from the Fermi level has little effect on the properties of the system and can be ignored, this paper mainly selects the density of states in    Figure 7(a) shows the calculated total spin state density of the intrinsic state graphene and Mo-doped graphene adsorbed SO 2 F 2 gas molecules, and Figure 7(b) shows the partial waves of Mo, S, F, O, and C atoms. Spin state density: it can be seen from Figure 7(a) that the intrinsic graphene spin-up state density and spin-down state density coincide near the Fermi level, and the Fermi level passes through the Dirac point, indicating that the intrinsic graphene is graphene. It is a direct band gap semiconductor, which is consistent with the results obtained in [33]. After the surface of graphene is doped with Mo atoms, there is overlap between the bottom of the conduction band and the top of the valence band, which shows the characteristics of the metal, and the density of state has changed to a certain extent, which shows that the doped Mo atoms have a certain influence on the adsorption substrate. After adsorption of SO 2 F 2 gas molecules, the spin state density of intrinsic graphene basically coincides with that before adsorption, with little change, which reflects that the adsorption of intrinsic graphene to SO 2 F 2 gas molecules is not strong, while the spin state density of Mo-doped graphene shifts to the right as a whole, which indicates that its conductivity becomes stronger, and the state density changes greatly near its Fermi level (E f � 0 eV) and at the top of valence band and at the bottom of conduction band, which also reflects the adsorption of Mo-doped graphene to SO 2 F 2 gas molecules. Figure 7(b) shows the local spin state density. After Mo-doped graphene adsorbs SO 2 F 2 gas molecules, the state density near the Fermi level is mainly composed of Mo-4d and S-3p orbitals, and the density of states with energy of −8 eV ∼ −4 eV is mainly composed of Mo-4d, O-2p, F-2p, C-2p, and S-3p orbitals, and in this energy domain, there is overlapping hybridization between Mo-4d and S-3p orbitals, and Mo-4d and F-2p orbitals overlap to some extent at energy of −1 eV∼2 eV. It can be seen that it is precisely. Table 3 shows the orbital charge distribution of SO 2 F 2 gas molecules adsorbed by Mo atom-doped graphene. As can be seen from Table 3, the total charge numbers of Mo atoms and S atoms are 1.028e and 0.448e, respectively, which are positively charged and lose electrons, while O atoms and F atoms are negatively charged and gain electrons, which indicates that when the surface of Modoped intrinsic graphene adsorbs SO 2 F 2 gas molecules, the fracture of S-F is mainly caused by the hybridization between molecular orbitals, which is also consistent with the analysis results of the density of spin states.

Analysis of Mulliken Population and Transfer Charge.
According to Bader's charge analysis [34], the charge amount of a single atom in different systems can be calculated, and the charge transfer amount of the whole system can be obtained by subtracting the charge amount of the system after doping or adsorption of atoms (molecules) from the charge amount of the system before doping or adsorption.
where △Q is the charge transfer amount of the system before and after doping or adsorption, △Q 1 is the charge amount of the system after doping or adsorption, and △Q 2 is the charge amount of the system before doping or adsorption. If the charge transfer amount is greater than 0, then, the system loses electrons in the doping or adsorption process, and the doping or adsorption substrate is the acceptor, otherwise, it means that the system gets electrons in the doping or adsorption process, and the doping or adsorption substrate is the donor. Table 4 shows the transfer charge amounts of four kinds of gas molecules on the surface of intrinsic graphene and Mo-doped intrinsic graphene. According to the charge transfer amount in Table 4, only the surface of intrinsic graphene and Mo-doped intrinsic graphene has a positive transfer charge when adsorbing H 2 S gas molecules, while the others are all negative, which indicates that when the surface of intrinsic graphene and Modoped intrinsic graphene adsorbs H 2 S gas molecules, H 2 S gas transfers electrons to the substrate, while the surface of intrinsic graphene and Mo-doped intrinsic graphene acts as a collector and H 2 S gas molecules act as a donor. On the contrary, the surface of intrinsic graphene and Mo-doped intrinsic graphene acts as a donor when adsorbing other gases and transfers electrons to other gases.

Molecular Orbital Energy Analysis.
According to the molecular orbital theory [35], the charge transfer is directly related to the direction of molecular energy level change. To further study the principle of charge transfer during adsorption, in this paper, the highest orbital (HOMO) occupied the energy and the lowest orbital (LUMO) occupied the energy of these four gas molecules, the energy of intrinsic graphene Fermi level (E f � −4.338 eV), and the energy of Mo-doped intrinsic graphene Fermi level (E f-Mo � −3.993 eV) are calculated, respectively. E f-L or E f-Mo-L , respectively, represents the difference between LUMO molecular orbital energy of four gas molecules and Fermi energy level of intrinsic graphene or Mo-doped graphene.
Similarly, E f-H or E f-Mo-H , respectively, represents the difference between HOMO molecular orbital energy of four gas molecules and Fermi energy level of intrinsic graphene or Mo-doped graphene. e electron cloud distribution of four gas molecules is shown in Figure 8.
Among them, E 0 � E LUMO-EHOMO is the orbital energy gap of gas molecules, which mainly reflects the energy needed when gas molecules react with other substances. It can be seen from Figure 8 that the orbital gaps of four kinds of gas molecules from small to large are CF 4 > SO 2 F 2 > H 2 S > SO 2 , the maximum orbital gap of CF 4 gas molecules is 12.495 eV, and the minimum orbital gap of SO 2 gas molecules is 5.642 eV. It can be calculated that the energy differences E f-L between the HOMO-LUMO energy levels of H 2 S, SO 2 F 2 , CF 4 , and SO 2 [36], which indicates that in the process of adsorbing H 2 S gas, the surfaces of intrinsic graphene and Mo-doped graphene are current collectors, and H 2 S gas is donor. However, E F-H of SO 2 F 2 , CF 4 , and SO 2 is greater than E F-L , and the adsorption process with intrinsic graphene and Mo-doped graphene is all P-type adsorption, and the surfaces of intrinsic graphene and Mo-doped graphene are donors and gas molecules are acceptors, which is also consistent with the analysis results of transfer charge.

Conclusion
In this work, we discuss the interaction between SF 6 decomposition products (H 2 S, SO 2 , CF 4 , and SO 2 F 2 ) with intrinsic graphene and Mo-doped graphene, considering whether there is the effect of Mo atom doping. It was found that the doped architecture was the most stable when Mo atoms were doped at the T sites of graphene. Intrinsic graphene does not have strong adsorption of H 2 S, SO 2 , CF 4 , and SO 2 F 2 , these four SF 6 decomposition gases, all of which are physical adsorption, while Mo-doped graphene surface has enhanced adsorption capacity for the four gases. Also, the adsorption strength is SO 2 F 2 > H 2 S > SO 2 > CF 4 . After Mo-doped graphene surface, the overall density of states of the system shifts, the conductivity becomes stronger, and orbital hybridization occurs between F, S atoms, and Mo atoms in SO 2 F 2 gas, which leads to the breaking of SF bonds and also makes adsorption the most obvious effect.
According to the direction of electron transfer and molecular orbital energy, the adsorption of H 2 S gas molecules on the surface of Mo-doped graphene is n-type adsorption, while the adsorption of SO 2 F 2 , CF 4 , and SO 2 gas molecules is p-type adsorption. is study theoretically demonstrates that the Mo-doped graphene surface can be used as a detection device for H 2 S and SO 2 F 2 gas scavengers and SO 2 F 2 gas resistance sensors, which is of great significance for maintaining the safe and stable operation of power systems.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors have no conflicts of interest.