3.1. Establishment of the Geometric Model
To find the steadiest state, gas molecule models of SF
6 characteristic decomposition (SO
2F
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].
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:
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].
Figure 3a and
Figure 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].
3.3. Adsorption of a Single Gas Molecule on Ag-Graphene
To obtain the most stable adsorption system, single gas molecules (SO
2F
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
2F
2 have a strong interaction. Two F atoms separated from SO
2F
2 and approached decorated Ag with two S-F bonds of SO
2F
2, extending to 1.948 Å and 3.942 Å, respectively, in the adsorption process, which indicates SO
2F
2 decomposition. Meanwhile, the value of Q upon SO
2F
2 adsorption on Ag-graphene was −0.947 e, which meant that SO
2F
2 acted as an electron acceptor during electron transfer, because electron-rich F atoms contributed to pulling electrons from Ag-graphene to SO
2F
2. Another essential parameter ∆E
ads verified that the strong interaction belonged to chemisorption, because ∆E
ads of SO
2F
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 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
2F
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
2F
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
2F
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
2F
2 in a certain filling rate degree, which bring a strong interaction between SO
2F
2 and decorated Ag.
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 SF
6 characteristic gas molecules (SO
2F
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
2F
2 was adsorbed on Ag-graphene, the energy gap of Ag-graphene/SO
2F
2 increased to 0.761 eV, which indicated that the SO
2F
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.
3.4. 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
2F
2, 2SOF
2, and 2SO
2) adsorbed in favorable positions on Ag-graphene. Optimized systems (Ag-graphene/2SO
2F
2, Ag-graphene/2SOF
2, and Ag-graphene/2SO
2) are shown in
Figure 9, and their related parameters are listed in
Table 5.
As shown in
Figure 9a, two F atoms break away from one SO
2F
2 and attached to the Ag atom, while another SO
2F
2 held its local position. In terms of bond length, only the S-F bond lengths of that SO
2F
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
2F
2 gas molecules interacted with Ag-graphene. Based on
Figure 5a and
Figure 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
2F
2 and Ag-Graphene/2SO
2F
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
Table 3 and
Table 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 SO2 and an O atom from another SO2 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 SO2 gas molecules. In addition, the net charge transfer (Q) of Ag-Graphene/2SO2 (−0.701 eV) were twice as much as that of Ag-Graphene/SO2 (−0.350 e) with considerable adsorption energy (∆Eads) of Ag-Graphene/2SO2 (−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/SO2.
As shown in
Table 6, we calculated HOMO, LUMO, and energy gap after double gas molecules (2SO
2F
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
2F
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
2F
2 gas molecules made conductivity rise further. The HOMO gathered in SO
2F, 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
2F 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).