Gas Sensing Analysis of Ag-Decorated Graphene for Sulfur Hexafluoride Decomposition Products Based on the Density Functional Theory

Detection of decomposition products of sulfur hexafluoride (SF6) is one of the best ways to diagnose early latent insulation faults in gas-insulated equipment, and the occurrence of sudden accidents can be avoided effectively by finding early latent faults. Recently, functionalized graphene, a kind of gas sensing material, has been reported to show good application prospects in the gas sensor field. Therefore, calculations were performed to analyze the gas sensing properties of intrinsic graphene (Int-graphene) and functionalized graphene-based material, Ag-decorated graphene (Ag-graphene), for decomposition products of SF6, including SO2F2, SOF2, and SO2, based on density functional theory (DFT). We thoroughly investigated a series of parameters presenting gas-sensing properties of adsorbing process about gas molecule (SO2F2, SOF2, SO2) and double gas molecules (2SO2F2, 2SOF2, 2SO2) on Ag-graphene, including adsorption energy, net charge transfer, electronic state density, and the highest and lowest unoccupied molecular orbital. The results showed that the Ag atom significantly enhances the electrochemical reactivity of graphene, reflected in the change of conductivity during the adsorption process. SO2F2 and SO2 gas molecules on Ag-graphene presented chemisorption, and the adsorption strength was SO2F2 > SO2, while SOF2 absorption on Ag-graphene was physical adsorption. Thus, we concluded that Ag-graphene showed good selectivity and high sensitivity to SO2F2. The results can provide a helpful guide in exploring Ag-graphene material in experiments for monitoring the insulation status of SF6-insulated equipment based on detecting decomposition products of SF6.


Introduction
Sulfur hexafluoride (SF 6 ) has been widely applied in gas-insulated switchgears (GIS) because of its excellent insulation and arc-quenching performance [1]. When the insulation performance of electrical equipment begins to decline, the energy released by the partial discharge phenomena will cause SF 6 to decompose into multiple low fluorine sulfides [2]. These low fluorine sulfides react with trace water and oxygen to generate SO 2 F 2 , SOF 2 , SO 2 , and other characteristic gas products in GIS [3]. Based on previous research, decomposition and concentration of gas products are closely associated with the type, severity, and generation factor of partial discharge [4]. Therefore, detection of SF 6 decomposition products is an effective way to monitor and diagnose insulation faults in gas-insulated electrical equipment.
The broad application prospect of graphene is due to its unique structure, electronic properties, and excellent electrical performance [5][6][7][8]. Moreover, graphene possesses many advantages, such as an extremely large specific surface area [9], great carrier concentration [10], ultrahigh carrier mobility [10], low Johnson noise, fewer internal defects [11], and zero-gap materials (with metallicity) [12]. In 2007, Schedin first researched adsorption and desorption behavior of single gas molecules on intrinsic graphene surfaces and expanded the research of graphene as a gas sensor [13]. Researchers confirmed that graphene with certain defects or decorations had stronger response characteristics to specific gases, and the selectivity and sensitivity of graphene to a particular gas can be effectively improved through conducting specific doping or functional modification [14]. At present, the research on functionalized graphene is limited to a common detection field, such as NO, H 2 , and other common gases [15,16]. Meanwhile, a few research reports have been written about its application in detection of SF 6 decomposition products. According to the preliminary study in our study group and its high hydrogen uptake in a previous paper, Ag-graphene gas sensor has good gas-sensing properties for SF 6 decomposition products [17][18][19]. In addition, feasible fabrication of Ag-graphene will lead to large-scale industrial production [20]. To further guide experiments to enhance the detection sensitivity and selectivity of Ag-graphene gas sensors for SF 6 decomposition products, the gas-sensing properties and gas-response mechanism of Ag-graphene sensors for different SF 6 decomposition products must be further analyzed.
Based on density functional theory (DFT), adsorption processes of single (SO 2 F 2 , SOF 2 and SO 2 ) and double gas molecules (2SO 2 F 2 , 2SOF 2 , 2SO 2 ) on Ag-graphene were analyzed, respectively. The theory suggested that decorated Ag could effectively improve the electron density and charge transfer characteristics of graphene, which provided a new way of preparing sensing materials for functionalized graphene gas and developing a high-performance gas sensor for detection of SF 6 decomposition products.

Materials and Methods
All calculations were based on DMol 3 module of Materials Studio. First, we chose a 6 × 6 graphene supercell (72 carbon atoms) to build the graphene structure, and the periodic boundary condition of supercell was set to 20 Å to avoid the interaction between adjacent layers in modeling. Then, optimization calculation was conducted for the gas molecule structure and the supercell to adjust their structure parameters in the model to be as close to the experimental parameters as possible. To find the most stable Int-graphene and Ag-graphene structure after adsorbing gas molecules, the system of gas molecules on Int-graphene and Ag-graphene was calculated and optimized. Finally, obtained adsorption systems were analyzed by a series of physical and chemical characteristic parameters. Because the Van der Waals force shows obvious effects in physical adsorption, this work selected the generalized gradient approximation method, using Perdew-Burke-Ernzehof format and dispersion correction for DFT algorithm to process exchange correlation and iteration process, respectively [21]. Thus, the influence of the Van der Waals force on adsorption effect could be explored more accurately. In addition, a basis set of P-polarized double-number orbit was added; a k-point mesh of 6 × 6 × 1 was selected for the Brillouin zone integration to obtain accurate energies and structures; and the convergence tolerance of energy was 2.72 × 10 −5 eV in structural optimization [22]. Previously published research agreed with the model and its related parameters established in this work [23].
After presenting the optimization structure, a series of physical and chemical parameters were analyzed to understand the adsorption process of gas molecules on Int-graphene and Ag-graphene. Their definitions are as follows: Adsorption distance (d) means the closest distance from Int-graphene or Ag-graphene to gas molecules. Adsorption energy (∆E ads ), the acting force of gas molecules adsorbed on Int-graphene and Ag-graphene, gives the change of total energy in the adsorption process of gas molecules on Int-graphene and Ag-graphene to reach the most stable adsorption system using the equation: where E (system) , E (Int-graphene/Ag-graphene) , and E (molecule) are the total energy of adsorption system after the molecules adsorbed on graphene or Ag-graphene, the energy of graphene or Ag-graphene, and the energy of gas molecules, respectively. Net charge transfer (Q) characterizes the quantity of electric charge transferred from gas molecules to Int-graphene or Ag-graphene in the absorption process and reflects the change of electrical properties of Int-graphene and Ag-graphene. The energy gap of adsorption system offers the complexity of electron transfer between the whole highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and its formula is as follows:

Establishment of the Geometric Model
To find the steadiest state, gas molecule models of SF 6 characteristic decomposition (SO 2 F 2 , SOF 2 and SO 2 ) and Int-graphene were constructed in material visualizer. Then, every geometrical structure was optimized by finding its lowest energy, so bond length and angle were marked in each optimized gas molecule model (see Figure 1), whose parameters were consistent with the published data from other simulation results [24].
where E(system), E(Int-graphene/Ag-graphene), and E(molecule) are the total energy of adsorption system after the molecules adsorbed on graphene or Ag-graphene, the energy of graphene or Ag-graphene, and the energy of gas molecules, respectively. Net charge transfer (Q) characterizes the quantity of electric charge transferred from gas molecules to Int-graphene or Ag-graphene in the absorption process and reflects the change of electrical properties of Int-graphene and Ag-graphene. The energy gap of adsorption system offers the complexity of electron transfer between the whole highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), and its formula is as follows:

Establishment of the Geometric Model
To find the steadiest state, gas molecule models of SF6 characteristic decomposition (SO2F2, SOF2 and SO2) and Int-graphene were constructed in material visualizer. Then, every geometrical structure was optimized by finding its lowest energy, so bond length and angle were marked in each optimized gas molecule model (see Figure 1), whose parameters were consistent with the published data from other simulation results [24].
We explored the decoration mechanism of a single Ag atom on graphene. After optimizing the geometrical structure of graphene, one Ag atom was embedded on the graphene plane in each unit cell. To reach the most stable decorated graphene with a single atom, three possible positions were considered according to the characteristics of highly symmetrical structure of graphene and the previous literature. Figure 2 shows the top site directly above a C atom (T-site), the hollow site at the center of a hexagon (H-site), and the bridge site at the midpoint of two C atoms (B-site). The binding energy describes the natural interaction of the bonding between decorated Ag and graphene as: We explored the decoration mechanism of a single Ag atom on graphene. After optimizing the geometrical structure of graphene, one Ag atom was embedded on the graphene plane in each unit cell. To reach the most stable decorated graphene with a single atom, three possible positions were considered according to the characteristics of highly symmetrical structure of graphene and the previous literature. Figure 2 shows the top site directly above a C atom (T-site), the hollow site at the center of a hexagon (H-site), and the bridge site at the midpoint of two C atoms (B-site). where E (C72), E (Ag), and E (Ag/C72) represent the respective energy of intrinsic graphene, energy of a single Ag atom, and the total energy of optimized Ag-graphene.  Table 1 shows details of the binding energy in three possible positions, demonstrating that T-site holds the minimum energy. Moreover, the T-site model could be speculated to be the main decorated structure during experimental preparation according to the single metal atom decorated in the same method [25]. Therefore, we regarded the T-site model as the gas-sensing material to study its gas sensing properties to SF6 decomposition products, which was verified and used in previous similar studies [26].

Doping Sites H-site B-site T-site
Eform (eV) 1.51 1.32 0.85 Figures 3a and 4b respectively offer the side and top views of the T-site model after geometry optimization, which illustrates that the Ag atom embedded on the graphene does not lead to the change of the whole two-dimensional plane structure of graphene, but rather protrudes out of the C atomic layer plane with distance (1.586 Å ) along the Z-direction and forms covalent bonds with its adjacent C atoms with 2.053 Å bond length. This further supports the idea that an Ag-decorated graphene structure has a local sp 3 configuration. Otherwise, the similar decorated structures of local sp 3 configuration were also found in other metal-element-decorated graphene (Pd, Pt, Mn, etc.), and it is generally believed that local sp 3 configuration is correct in metal-doped graphene theory research [27].

Adsorption of a Single Gas Molecule on Int-Graphene
To have a better understanding of adsorption effects of gas molecules on Ag-graphene, we first investigated single gas molecule (SO2F2, SOF2, and SO2) adsorption with different initial positions. The binding energy describes the natural interaction of the bonding between decorated Ag and graphene as: where E (C 72 ), E (Ag), and E (Ag/C 72 ) represent the respective energy of intrinsic graphene, energy of a single Ag atom, and the total energy of optimized Ag-graphene. Table 1 shows details of the binding energy in three possible positions, demonstrating that T-site holds the minimum energy. Moreover, the T-site model could be speculated to be the main decorated structure during experimental preparation according to the single metal atom decorated in the same method [25]. Therefore, we regarded the T-site model as the gas-sensing material to study its gas sensing properties to SF 6 decomposition products, which was verified and used in previous similar studies [26]. Figures 3a and 4b respectively offer the side and top views of the T-site model after geometry optimization, which illustrates that the Ag atom embedded on the graphene does not lead to the change of the whole two-dimensional plane structure of graphene, but rather protrudes out of the C atomic layer plane with distance (1.586 Å) along the Z-direction and forms covalent bonds with its adjacent C atoms with 2.053 Å bond length. This further supports the idea that an Ag-decorated graphene structure has a local sp 3 configuration. Otherwise, the similar decorated structures of local sp 3 configuration were also found in other metal-element-decorated graphene (Pd, Pt, Mn, etc.), and it is generally believed that local sp 3 configuration is correct in metal-doped graphene theory research [27]. where E (C72), E (Ag), and E (Ag/C72) represent the respective energy of intrinsic graphene, energy of a single Ag atom, and the total energy of optimized Ag-graphene.  Table 1 shows details of the binding energy in three possible positions, demonstrating that T-site holds the minimum energy. Moreover, the T-site model could be speculated to be the main decorated structure during experimental preparation according to the single metal atom decorated in the same method [25]. Therefore, we regarded the T-site model as the gas-sensing material to study its gas sensing properties to SF6 decomposition products, which was verified and used in previous similar studies [26].

Doping Sites H-site B-site T-site
Eform (eV) 1.51 1.32 0.85 Figures 3a and 4b respectively offer the side and top views of the T-site model after geometry optimization, which illustrates that the Ag atom embedded on the graphene does not lead to the change of the whole two-dimensional plane structure of graphene, but rather protrudes out of the C atomic layer plane with distance (1.586 Å ) along the Z-direction and forms covalent bonds with its adjacent C atoms with 2.053 Å bond length. This further supports the idea that an Ag-decorated graphene structure has a local sp 3 configuration. Otherwise, the similar decorated structures of local sp 3 configuration were also found in other metal-element-decorated graphene (Pd, Pt, Mn, etc.), and it is generally believed that local sp 3 configuration is correct in metal-doped graphene theory research [27].

Adsorption of a Single Gas Molecule on Int-Graphene
To have a better understanding of adsorption effects of gas molecules on Ag-graphene, we first investigated single gas molecule (SO2F2, SOF2, and SO2) adsorption with different initial positions.

Adsorption of a Single Gas Molecule on Int-Graphene
To have a better understanding of adsorption effects of gas molecules on Ag-graphene, we first investigated single gas molecule (SO 2 F 2 , SOF 2 , and SO 2 ) adsorption with different initial positions. We focused on a series of parameters presenting gas-sensing properties of adsorption process, such as d, ∆E ads , and Q. Figure 4 shows the optimized stable adsorption systems holding the lowest energy among similar configurations, and Table 2 shows their parameters. We focused on a series of parameters presenting gas-sensing properties of adsorption process, such as d, ∆Eads, and Q. Figure 4 shows the optimized stable adsorption systems holding the lowest energy among similar configurations, and Table 2 shows their parameters. As shown in Figure 4 and Table 2, graphene interacted weakly with gas molecules, and the structure of these molecules almost remains the same (see in Figure 1). The Eads and d were −0.463 eV and 3.486 Å for SO2F2, respectively. ∆Eads and d of graphene/SO2 were −0.459 eV and 3.531 Å . The value of ∆Eads and d upon SO2 adsorption on graphene were −0.298 eV and 3.344 Å , respectively. In addition, net charge transfers (Q) were −0.004 e, −0.002 e and −0.003 e in graphene/SO2F2, graphene/SOF2 and graphene/SO2, respectively. Consequently, the interaction for gas molecule adsorption on graphene is just van der Waals interactions, which are too weak to influence the change of conductivity in the gas adsorption process. Therefore, intrinsic graphene material is not suitable for decomposition component detection in SF6-insulated equipment. Thus, we do not further discuss the adsorption details for gas molecules adsorption on intrinsic graphene.

Adsorption of a Single Gas Molecule on Ag-Graphene
To obtain the most stable adsorption system, single gas molecules (SO2F2, SOF2 and SO2) were made closer to Ag-graphene with different initial positions. We computed same parameters to analyze the gas-sensing properties of the adsorbing process. These optimized stable adsorption systems are exhibited in Figure 5, and all their parameters are shown in Table 3.
As shown in Figure 5 and Table 3, Ag-graphene and SO2F2 have a strong interaction. Two F atoms separated from SO2F2 and approached decorated Ag with two S-F bonds of SO2F2, extending to 1.948 Å and 3.942 Å , respectively, in the adsorption process, which indicates SO2F2 decomposition. Meanwhile, the value of Q upon SO2F2 adsorption on Ag-graphene was −0.947 e, which meant that SO2F2 acted as an electron acceptor during electron transfer, because electron-rich F atoms contributed to pulling electrons from Ag-graphene to SO2F2. Another essential parameter ∆Eads verified that the strong interaction belonged to chemisorption, because ∆Eads of SO2F2 on Ag-graphene (−1.448 eV) had far exceeded the critical value of chemical adsorption (0.8 eV) [26].   As shown in Figure 4 and Table 2, graphene interacted weakly with gas molecules, and the structure of these molecules almost remains the same (see in Figure 1). The E ads and d were −0.463 eV and 3.486 Å for SO 2 F 2 , respectively. ∆E ads and d of graphene/SO 2 were −0.459 eV and 3.531 Å. The value of ∆E ads and d upon SO 2 adsorption on graphene were −0.298 eV and 3.344 Å, respectively. In addition, net charge transfers (Q) were −0.004 e, −0.002 e and −0.003 e in graphene/SO 2 F 2 , graphene/SOF 2 and graphene/SO 2 , respectively. Consequently, the interaction for gas molecule adsorption on graphene is just van der Waals interactions, which are too weak to influence the change of conductivity in the gas adsorption process. Therefore, intrinsic graphene material is not suitable for decomposition component detection in SF 6 -insulated equipment. Thus, we do not further discuss the adsorption details for gas molecules adsorption on intrinsic graphene.

Adsorption of a Single Gas Molecule on Ag-Graphene
To obtain the most stable adsorption system, single gas molecules (SO 2 F 2 , SOF 2 and SO 2 ) were made closer to Ag-graphene with different initial positions. We computed same parameters to analyze the gas-sensing properties of the adsorbing process. These optimized stable adsorption systems are exhibited in Figure 5, and all their parameters are shown in Table 3.
As shown in Figure 5 and Table 3, Ag-graphene and SO 2 F 2 have a strong interaction. Two F atoms separated from SO 2 F 2 and approached decorated Ag with two S-F bonds of SO 2 F 2 , extending to 1.948 Å and 3.942 Å, respectively, in the adsorption process, which indicates SO 2 F 2 decomposition. Meanwhile, the value of Q upon SO 2 F 2 adsorption on Ag-graphene was −0.947 e, which meant that Sensors 2016, 16, 1830 6 of 13 SO 2 F 2 acted as an electron acceptor during electron transfer, because electron-rich F atoms contributed to pulling electrons from Ag-graphene to SO 2 F 2 . Another essential parameter ∆E ads verified that the strong interaction belonged to chemisorption, because ∆E ads of SO 2 F 2 on Ag-graphene (−1.448 eV) had far exceeded the critical value of chemical adsorption (0.8 eV) [26].  However, a different case happened with SOF2. Figure 5b displays the most stable adsorption system, and its related data is shown in Table 3. When SOF2 came close to Ag-graphene, the F atom tried to separate from SOF2, so the S-F bond was stretched to 2.718 Å , which offered a chance to build a new bond between Ag interacted, and lone F and left S atoms, respectively. However, these interactions were some kind of physical adsorption because of low Van der Waals force with bonding energy (−0.678 eV). In addition, the value of Q upon SOF2 adsorption on Ag-graphene was only −0.351 e. This agrees with the conclusion about physical adsorption, which indicates that electron also transfers from Ag-graphene to gas molecule. SO2 adsorption did not show obvious changes on molecular gas structure (see Figure 5). While the bond length of S-O in SO2 molecule had extended from 1.480 Å to 1.572 Å , distances from O and S atoms of SO2 to Ag atom gradually shortened to 2.210 Å and 2.558 Å , respectively. SO2-Ag-graphene interaction formed new Ag-S and Ag-O covalent bonds, as shown in Figure 5, thus leading to S-O bond stretching in SO2 molecule. Meanwhile, ∆Eads of Ag-graphene/SO2 (−1.075 eV) was lower than that of Ag-graphene/SO2F2, but higher than that of Ag-graphene/SOF2. Similarly, the value of Q (−0.801 e) upon SO2 adsorption on Ag-graphene was the middle value among three cases. Therefore, SO2 exhibited chemisorption with Ag-graphene.
Net charge transfer has an effect on the density of states (DOS), which results in the change of conductance in system. Thus, the density of states (DOS) and its corresponding partial DOS (PDOS) of Ag-graphene before and after adsorption were calculated to further analyze the chemical adsorption mechanism of gas on Ag-graphene.
DOS change was evident before and after SO2F2 adsorption by comparison, as shown in Figure  6a, where DOS decreased near the 0.8 eV conduction band and then increased in valence band range from −3 eV to −6 eV, which mainly led to an increase in the conductance of Ag-graphene/SO2F2 system. Figure 7a shows that PDOS peak near the −6 eV valence band causes DOS increase; because the 4d orbital of Ag atom hybridized with the 3p orbital of S atom and 2p orbital of O atom of SO2F2 in a certain filling rate degree, which bring a strong interaction between SO2F2 and decorated Ag.
DOS change before and after SOF2 only occurred below the Fermi level with a slight increase (see Figure 6b), suggesting that the conductivity of the system did not change, which was consistent with the conclusion of low adsorption energy discussed previously. More information from PDOS in

System d (Å) ∆E ads (eV) Bond Length (Å) Q t (e)
Ag-graphene/SO 2 F 2 2.013 −1. However, a different case happened with SOF 2 . Figure 5b displays the most stable adsorption system, and its related data is shown in Table 3. When SOF 2 came close to Ag-graphene, the F atom tried to separate from SOF 2 , so the S-F bond was stretched to 2.718 Å, which offered a chance to build a new bond between Ag interacted, and lone F and left S atoms, respectively. However, these interactions were some kind of physical adsorption because of low Van der Waals force with bonding energy (−0.678 eV). In addition, the value of Q upon SOF 2 adsorption on Ag-graphene was only −0.351 e. This agrees with the conclusion about physical adsorption, which indicates that electron also transfers from Ag-graphene to gas molecule. SO 2 adsorption did not show obvious changes on molecular gas structure (see Figure 5). While the bond length of S-O in SO 2 molecule had extended from 1.480 Å to 1.572 Å, distances from O and S atoms of SO 2 to Ag atom gradually shortened to 2.210 Å and 2.558 Å, respectively. SO 2 -Ag-graphene interaction formed new Ag-S and Ag-O covalent bonds, as shown in Figure 5, thus leading to S-O bond stretching in SO 2 molecule. Meanwhile, ∆E ads of Ag-graphene/SO 2 (−1.075 eV) was lower than that of Ag-graphene/SO 2 F 2 , but higher than that of Ag-graphene/SOF 2 . Similarly, the value of Q (−0.801 e) upon SO 2 adsorption on Ag-graphene was the middle value among three cases. Therefore, SO 2 exhibited chemisorption with Ag-graphene.
Net charge transfer has an effect on the density of states (DOS), which results in the change of conductance in system. Thus, the density of states (DOS) and its corresponding partial DOS (PDOS) of Ag-graphene before and after adsorption were calculated to further analyze the chemical adsorption mechanism of gas on Ag-graphene. DOS change was evident before and after SO 2 F 2 adsorption by comparison, as shown in Figure 6a, where DOS decreased near the 0.8 eV conduction band and then increased in valence band range from −3 eV to −6 eV, which mainly led to an increase in the conductance of Ag-graphene/SO 2 F 2 system. Figure 7a shows that PDOS peak near the −6 eV valence band causes DOS increase; because the 4d orbital of Ag atom hybridized with the 3p orbital of S atom and 2p orbital of O atom of SO 2 F 2 in a certain filling rate degree, which bring a strong interaction between SO 2 F 2 and decorated Ag.
Sensors 2016, 16,1830 7 of 13 Figure 6c shows DOS change during SO2 adsorption. Obvious change was not found before and after SO2 adsorption. However, the weak increase near valence bands of −3 eV and −6 eV was due to the net charge transfer at the effective overlapping part between the 4d orbital of the Ag atom, 2p orbital of the S atom, and 2p orbital of the O atom in PDOS (see Figure 7c). As a result, the effective charge number increased on a macroscopic scale after SO2 molecule adsorption, as did the conductance of Ag-graphene/SO2.   DOS change before and after SOF 2 only occurred below the Fermi level with a slight increase (see Figure 6b), suggesting that the conductivity of the system did not change, which was consistent with the conclusion of low adsorption energy discussed previously. More information from PDOS in Figure 7b reveals that a little energy overlap existed among atomic orbits. This indicated that the interaction between atoms was weak, which further verified that SOF 2 adsorption on Ag-graphene belonged to physical adsorption. Figure 6c shows DOS change during SO 2 adsorption. Obvious change was not found before and after SO 2 adsorption. However, the weak increase near valence bands of −3 eV and −6 eV was due to the net charge transfer at the effective overlapping part between the 4d orbital of the Ag atom, 2p orbital of the S atom, and 2p orbital of the O atom in PDOS (see Figure 7c). As a result, the effective charge number increased on a macroscopic scale after SO 2 molecule adsorption, as did the conductance of Ag-graphene/SO 2 .   We investigated the transition ability of electrons from the top of valence band to the bottom of conduction band and electronic structure through HOMO and LUMO. The calculation results of HOMO and LUMO of Ag-graphene structure before and after absorbing SF6 characteristic gas molecules (SO2F2, SO2 and SOF2) are shown in Table 4 and Figure 8. Before absorbing gas molecules, the HOMO and LUMO of Ag-graphene were mainly distributed at the Ag-decorated site and its opposite site in Figure 8a1,a2, and the energy gap of Ag-graphene was 0.272 eV (see Table 4). The energy decline of HOMO and LUMO was due to the adsorption process, but its HOMO-LUMO energy gap increased in the system. When SO2F2 was adsorbed on Ag-graphene, the energy gap of Ag-graphene/SO2F2 increased to 0.761 eV, which indicated that the SO2F2 adsorption greatly increased system conductivity. After SO2 was adsorbed, the energy gap of the system slightly increased to 0.359 eV, which supported that conductivity of the Ag-graphene/SO2 increased to a certain extent. However, SOF2 adsorption almost had no effect on changing the energy gap of the system. All cases were consistent with the conclusion obtained through DOS analysis above. We investigated the transition ability of electrons from the top of valence band to the bottom of conduction band and electronic structure through HOMO and LUMO. The calculation results of HOMO and LUMO of Ag-graphene structure before and after absorbing SF 6 characteristic gas molecules (SO 2 F 2 , SO 2 and SOF 2 ) are shown in Table 4 and Figure 8. Before absorbing gas molecules, the HOMO and LUMO of Ag-graphene were mainly distributed at the Ag-decorated site and its opposite site in Figure 8a1,a2, and the energy gap of Ag-graphene was 0.272 eV (see Table 4). The energy decline of HOMO and LUMO was due to the adsorption process, but its HOMO-LUMO energy gap increased in the system. When SO 2 F 2 was adsorbed on Ag-graphene, the energy gap of Ag-graphene/SO 2 F 2 increased to 0.761 eV, which indicated that the SO 2 F 2 adsorption greatly increased system conductivity. After SO 2 was adsorbed, the energy gap of the system slightly increased to 0.359 eV, which supported that conductivity of the Ag-graphene/SO 2 increased to a certain extent. However, SOF 2 adsorption almost had no effect on changing the energy gap of the system. All cases were consistent with the conclusion obtained through DOS analysis above.
adsorbed on Ag-graphene, the energy gap of Ag-graphene/SO2F2 increased to 0.761 eV, which indicated that the SO2F2 adsorption greatly increased system conductivity. After SO2 was adsorbed, the energy gap of the system slightly increased to 0.359 eV, which supported that conductivity of the Ag-graphene/SO2 increased to a certain extent. However, SOF2 adsorption almost had no effect on changing the energy gap of the system. All cases were consistent with the conclusion obtained through DOS analysis above.

Adsorption of Double Gas Molecules on Ag-Graphene
Before understanding the mechanism of gas molecules interaction with Ag-graphene, it was necessary to explore whether more gas molecules could be adsorbed onto the Ag-graphene surface. Thus, we constructed all kinds of configurations with two gas molecules (2SO2F2, 2SOF2, and 2SO2) adsorbed in favorable positions on Ag-graphene. Optimized systems (Ag-graphene/2SO2F2, Ag-graphene/2SOF2, and Ag-graphene/2SO2) are shown in Figure 9, and their related parameters are listed in Table 5.

Adsorption of Double Gas Molecules on Ag-Graphene
Before understanding the mechanism of gas molecules interaction with Ag-graphene, it was necessary to explore whether more gas molecules could be adsorbed onto the Ag-graphene surface. Thus, we constructed all kinds of configurations with two gas molecules (2SO 2 F 2 , 2SOF 2 , and 2SO 2 ) adsorbed in favorable positions on Ag-graphene. Optimized systems (Ag-graphene/2SO 2 F 2 , Ag-graphene/2SOF 2 , and Ag-graphene/2SO 2 ) are shown in Figure 9, and their related parameters are listed in Table 5.

Adsorption of Double Gas Molecules on Ag-Graphene
Before understanding the mechanism of gas molecules interaction with Ag-graphene, it was necessary to explore whether more gas molecules could be adsorbed onto the Ag-graphene surface. Thus, we constructed all kinds of configurations with two gas molecules (2SO2F2, 2SOF2, and 2SO2) adsorbed in favorable positions on Ag-graphene. Optimized systems (Ag-graphene/2SO2F2, Ag-graphene/2SOF2, and Ag-graphene/2SO2) are shown in Figure 9, and their related parameters are listed in Table 5.

System d (Å) ∆E ads (eV) Bond Length(Å) Q t (e)
Ag-graphene/SO 2 F 2 2.004 −1. As shown in Figure 9a, two F atoms break away from one SO 2 F 2 and attached to the Ag atom, while another SO 2 F 2 held its local position. In terms of bond length, only the S-F bond lengths of that SO 2 F 2 were respectively extended to 2.241 Å and 3.612 Å with other bond lengths unchanged, indicating that these two S-F bonds were broken in the adsorption process. These results reveal that only one of the double SO 2 F 2 gas molecules interacted with Ag-graphene. Based on Figures 5a and 9a and Table 3, adsorption distances (d) are 2.013 Å and 2.004 Å, absorption energies (∆E ads ) are −1.448 eV and −1.365 eV, and net charge transfers (Q) are −1.084 e and −1.113 e in Ag-Graphene/SO 2 F 2 and Ag-Graphene/2SO 2 F 2 , respectively. By comparison, the adsorption process of double gas molecules is basically similar to that of the corresponding single gas molecule.
Similarly, when F and S atoms from the first SOF 2 came close to Ag-Graphene with the S-F bond extended to 1.702 Å, the second SOF 2 kept its molecule structure unchanged. As the adsorption distance (d) for single gas molecule of SOF 2 was 0.051 Å shorter than that of a double gas molecule (see Tables 3 and 5), ∆E ads slightly decreased to −0.503 eV, and a total of 0.348 e electrons transferred from the gas molecule to the Ag-graphene. These demonstrated that only one of the double SOF 2 gas molecules was adsorbed on the Ag-graphene.
However, an S atom from one SO 2 and an O atom from another SO 2 approached the Ag atom, adsorbing in opposite directions. The bond lengths of S-O bonds, 1.546, 1.503, 1.498, and 1.493 Å, were received from the initial 1.480 Å during the adsorption of double SO 2 gas molecules. In addition, the net charge transfer (Q) of Ag-Graphene/2SO 2 (−0.701 eV) were twice as much as that of Ag-Graphene/SO 2 (−0.350 e) with considerable adsorption energy (∆E ads ) of Ag-Graphene/2SO 2 (−1.465 eV). Thus, new Ag-S and Ag-O bonds were formed, which led to electron enrichment of the Ag-graphene. These results reveal that more gas molecules were involved in interaction than Ag-Graphene/SO 2 .
As shown in Table 6, we calculated HOMO, LUMO, and energy gap after double gas molecules (2SO 2 F 2 , 2SOF 2 , and 2SO 2 ) adsorbed on Ag-graphene. The adsorption results of double gas molecules are similar to that of single gas molecule adsorption; HOMO and LUMO energies declined after adsorbing gas molecules. When 2SO 2 F 2 gas molecules were adsorbed on Ag-graphene, energy gap width increased to 1.050 eV, as shown in Table 6. The energy gap showed obvious changes compared to single gas molecule (−0.761), denoting that 2SO 2 F 2 gas molecules made conductivity rise further. The HOMO gathered in SO 2 F, and all of LUMOs were located at Ag-graphene (see Figure 10b1,b2); and the HOMO-LUMO energy gap of Ag-graphene/2SOF 2 was 0.282 eV, which was slightly higher compared with the intrinsic Ag-graphene that limits the performance of Ag-graphene to detect SO 2 F in SF 6 decomposition components. For 2SO 2 gas molecules, HOMO was mainly concentrated around Ag atom and gas molecules, while LUMO mainly distributed at the C atoms of Ag-graphene, as shown in Figure 10b1,b2. The width of energy gap increased to 0.542 eV, as shown in Table 6, indicating that the adsorption energy of 2SO 2 gas molecules increased system conductivity to a certain extent. Nevertheless, 2SOF 2 adsorption did not have an impact on the energy gap of the system. All adsorption results of double gas molecules are consistent with the conclusion of the corresponding adsorption energy, DOS, and other parameters above. (see Table 5 and Figure 11).

Conclusions
In summary, we mainly investigated the adsorption properties and mechanism for decomposition into SF 6 product gases (SO 2 F 2 , SOF 2 , and SO 2 ) on Ag-graphene based on density functional theory. A series of parameters presenting the adsorption characteristics in the adsorption process were calculated, and the main conclusions of the adsorption process were obtained as follows: (1) After calculating and comparing the binding energy of three different Ag atom adsorption positions on graphene, the T-site was the best decorated location for Ag atom. (2) Comparing with weak gas molecule interaction with Int-graphene, the Ag atom significantly enhances the electrochemical reactivity of graphene, reflected in the change of conductivity during the adsorption process. (3) For single gas molecule adsorption, SO 2 and SO 2 F 2 gas molecules were adsorbed on Ag-graphene with strong chemical adsorption effect, and the interaction strength decreased in following order: SO 2 F 2 > SO 2 . Also, Ag-graphene physically interacted with SOF 2 . The information from DOS, HOMO, and LUMO supported the conclusion that the electrical resistivity of the system was reduced during the adsorption process. (4) The adsorption results of double gas molecules were similar to those of single gas molecules, and the increase of the number of molecules did not change the type of interaction. The decorating Ag atom only provided one adsorption site, so that only one gas molecule could be adsorbed on Ag-graphene when 2SO 2 F 2 or 2SOF 2 were near Ag-graphene. However, the adsorption energy and net charge transfer were significantly improved compared with that of single gas molecule adsorption. Based on the system conductivity change, the electrical conductivity of the system would further increase with the increase of adsorbed molecules (2SO 2 F 2 and 2SOF 2 ).
Thus, Ag-graphene showed distinct interaction with SO 2 F 2 , a kind of decomposition product of SF 6 . Results of theoretical study played a guiding role in preparing a gas sensor with strong selectivity and high sensitivity in the experiment. Meanwhile, the conclusions are beneficial to realizing online monitoring and diagnosis of SF 6 -insulated equipment in engineering.