Ab Initio Study of SOF2 and SO2F2 Adsorption on Co-MoS2.

The detection of partial discharge by analyzing the decomposition components of SF6 gas in gas-insulated switchgears plays an important role in the diagnosis and assessment of the operational state of power equipment. Recently, the application of transition metal-modified MoS2 monolayer dioxide in gas detection has received wide attention. In this paper, first-principle density functional theory calculations were adopted to study the gas-sensitive response of Co-MoS2 monolayer to SOF2 and SO2F2. It is found that the conductivity of the Co-MoS2 monolayer has been effectively enhanced after Co atom doping on the MoS2 monolayer. After gas adsorption, electrons transfer from the Co-MoS2 monolayer to the gas molecules, resulting in significant reduction of conductivity of the adsorption system. The calculation results reveal that the Co-MoS2 monolayer is sensitive and selective to SOF2 and SO2F2 gases. This study provides the theoretical possibility of using Co-MoS2 as a gas sensor for SOF2 and SO2F2 gas detection.

Introduction

It is well known that SF6 has excellent insulation and arc extinguishing capability, which makes it a widely used insulating medium using in gas-insulated switchgear (GIS).[1,2] Because of the critical role of SF6-insulated GIS in power systems, its running stability is crucial for the entire power system. However, partial discharge inevitably occurs when potential insulation faults exist in GIS during the design, production, and long-term operation process.[3−5] Research showed that SF6 will decompose into fluoride (SF, x = 1–5) under strong energy of partial discharge. Then, the unstable SF will quickly react with trace of H2O and O2 present in GIS to various decomposition products, including the main components: SOF2 and SO2F2.[6−8] These decomposition products can significantly accelerate the corrosion of the insulation medium and further reduce the electrical insulation strength of GIS.[7−9] In addition, continuous partial discharge may turn into flashover or destructive discharge and even lead to sudden breakdown of GIS without timely handling.[10,11] Therefore, detecting and analyzing the type and concentration of SF6 decomposition products is of great significance for diagnosing the type and damage degree of partial discharge. Thanks to high sensitivity and fast response of gas sensors, it can be an effective method to detect the SF6 decomposition products.[12−14]

In recent years, two dimensional-layered MoS2 nanomaterials have attracted much attention because of its large specific surface area, high surface activity, good thermal conductivity, and strong chemical stability.[15−17] The semiconductor properties and high specific surface area of the MoS2 monolayer, makes it a promising material for gas detection.[18−20] Recently, studies showed that MoS2 monolayer sensors can effectively detect NH3, NO2, H2, and other gases.[21,22] However, because of the limited gas sensing properties of intrinsic MoS2, surface modification is usually adopted,[23] such as metallic and nonmetallic nanoparticles doping: Au, Pt, Ni, and Si.[24−26] Previous study results showed that surface modification significantly improves the sensitivity and selectivity of MoS2-based materials to gas molecules.[27−29] Liu et al. found that Si-doped MoS2 has obvious sensing properties for H2 at room temperature even at a low gas concentration of 0.5%.[30] According to previous studies, Co is one of the most used doping atoms among transition metals, which shows outstanding performance in enhancing the adsorption, gas sensitivity, and catalytic ability of MoS2.[31,32] In addition, Co doping on MoS2 is easy to be prepared, which makes it to be widely used as an adsorbent and gas sensor.[33,34] However, as far as we know, the Co-MoS2 monolayer has not been reported for its application in SF6 decomposition product detection.

In this study, the first-principles density functional theory (DFT) was used to study the gas sensing properties of Co-MoS2 to SOF2 and SO2F2 by analyzing the adsorption structure, adsorption energy, charge transfer, density of states (DOS), charge density difference (CDD), and molecular orbital. The theoretical analysis explains the effect of Co doping on the response performance of the MoS2 monolayer to SOF2 and SO2F2. This study not only puts forward an application of Co-MoS2 material in the SF6 decomposition components detection but also provides a theoretical foundation for the gas sensor design.

Results and Discussion

Simulation Structure of the Co-MoS2 Monolayer

The structure of Co-MoS2 is built based on the perfect MoS2 monolayer. The most stable configuration for the Co-MoS2 monolayer has been thoroughly studied. Figure and Table show the optimized geometry and corresponding structural parameters of the most stable configuration of the Co-MoS2 monolayer. The Co atom locates at the top of the MoS2 monolayer protruding out of the MoS2 surface, which forms stable chemical bonds with three adjacent S atoms labeled as S1, S2, and S3 with bond lengths of 2.114, 2.089, and 2.114 Å, respectively. The structure of MoS2 away from the Co doping site is almost unchanged after Co atom doping. Eads of Co-MoS2 is −0.325 eV, and 0.144 e of electrons transfers from the Co atom to MoS2.

Figure 1

Structure of the Co-MoS2 monolayer from different views: (a) top view; (b) side view.

Table 1

Structural Parameters of the Co-MoS2 Monolayer

system	dCo-S1/Å	dCo-S2/Å	dCo-S3/Å	QCo/e	ECo/eV
Co-MoS2	2.114	2.089	2.114	0.144	–0.325

To understand the adsorption properties of the Co atom on the MoS2 monolayer, the total DOS (TDOS) was analyzed as shown in Figure . For the TDOS, it can be seen that the TDOS of the Co-MoS2 monolayer shifts left after Co doping. Because of the contribution of Co doping, the TDOS increases significantly around the Fermi level, signifying the conductivity of the MoS2 improves. For the partial DOS (PDOS), it is found that the change of TDOS is mainly contributed by strong hybridization of the 3p orbital of the S atom and the 3d orbital of the Co atom in the interval of −3 to −2 and 1 to 2 eV.

Figure 2

TDOS and PDOS of intrinsic MoS2 and Co-MoS2 monolayers: (a) TDOS; (b) PDOS.

The electron transfer behavior was analyzed based on the molecular orbital theory as shown in Figure . For the distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), the HOMO and LUMO are mainly located around the Co doping site, indicating that Co doping benefits the electron transfer. Eg of the intrinsic MoS2 monolayer is 2.058 eV, and it dramatically reduces to 0.387 eV for the Co-MoS2 monolayer. Therefore, the electrons in the valence band are more likely to jump and effectively improve the electrical conductivity of the intrinsic MoS2 monolayer.

Figure 3

HOMO and LUMO orbitals on Co-MoS2 from different views: (a) HOMO of Co-MoS2; (b) LUMO of Co-MoS2.

Adsorption of SOF2 on the Co-MoS2 Monolayer

To analyze the adsorption performance of the Co-MoS2 monolayer to the target SOF2 molecule, a large number of potential structures have been built and optimized to obtain the most stable adsorption structure. Two typical adsorption structures, labeled as P1 and P2, are obtained as shown in Figure . Its corresponding adsorption parameters are presented in Table .

Figure 4

Adsorption structures of SOF2-adsorbed Co-MoS2 monolayer: (a) P1 structure; (b) P2 structure.

Table 2

Adsorption Parameters of the SOF2-Adsorbed Co-MoS2 Monolayer by P1 and P2, Respectively

position	Eads/eV	Q/e	Eg/eV	d/Å
P1	–1.468	–0.026	0.330	Co–S 2.043
P2	–1.765	–0.491	0.800	Co–F 1.842
				Co–S 2.187

For the P1 structure given in Figure a, though the SOF2 molecule has not directly bonded to the surface of the Co-MoS2 monolayer, Eads still reaches −1.468 eV during the adsorption process, which means that the adsorption structure is relatively stable. The distance from the S atom of SOF2 to the Co atom is 2.043 Å, and −0.026 e transfers from Co-MoS2 to SOF2. Because of the strong adsorption energy between the SOF2 molecule and Co-MoS2, its interaction belongs to chemical adsorption. By comparing Eg before and after gas molecule adsorption, Eg has only decreased 0.057 eV, indicating that the conductivity of the Co-MoS2 system is almost unchanged before and after adsorption.

For the P2 structure given in Figure b, it is found that one S–F bond in SOF2 breaks and interacts with the Co atom of Co-MoS2 by forming Co–S and Co–F bonds upon SOF2 adsorption. The bond lengths of Co–S and Co–F are 2.187 and 1.842 Å, respectively. The charge transfer from Co-MoS2 to the SOF2 molecule is −0.491 e based on the Milliken population results. In addition, Co-MoS2 shows strong interaction with SOF2 with an Eads of −1.765 eV, which is significantly larger than that in the P1 structure. Comparing with Eg (0.387 eV) of Co-MoS2, it increases to 0.800 eV after SOF2 molecule adsorption, indicating that SOF2 adsorption leads to the conductivity of the Co-MoS2 system.

To understand the electronic behavior and analyze the gas sensitivity of Co-MoS2 to SOF2, the TDOS, PDOS, and CDD were further analyzed. Because of the main contribution of the outmost orbitals of the interacted atom in the adsorption process, only the F 2p, S 3p, and Co 3d orbitals were discussed in the PDOS, as seen in Figure .

Figure 5

TDOS and PDOS of the SOF2-adsorbed Co-MoS2 monolayer: (a) TDOS; (b) PDOS.

As shown in Figure a, the TDOS slightly moves to the right side, and the TDOS near the Fermi level drops between 0 and 1 eV after SOF2 adsorption, indicating a reduction in the conductivity of Co-MoS2-based material. According to the distribution of PDOS in Figure b, this change of TDOS is mainly because of the obvious hybridization of the S 3p orbital of SOF2 and the Co 3d orbital in the range of −6 to −4 eV and the hybridization of the F 2p orbital with Co 3d from −2 to −1 eV. The analysis of TDOS and PDOS confirms that it is a strong interaction between SOF2 and Co-MoS2 monolayers.

To further analyze the change of the electron distribution, the CDD of the P2 structure was studied as shown in Figure , where the acquisition and loss of electrons are represented by red and blue, respectively. It can be seen that two F atoms of SOF2 get a large number of electrons. According to the results of Mulliken population analysis, the obtained quantity of electrons by two F atoms reaches −0.780 e. On the contrary, Co and Mo atoms lose during gas adsorption. Therefore, it further verifies that the conductivity of the Co-MoS2 monolayer decreases after SOF2 adsorption.

Figure 6

CDD of the SOF2-adsorbed Co-MoS2 monolayer by the P2 structure: (a) side view 1; (b) side view 2.

Adsorption of SO2F2 on the Co-MoS2 Monolayer

In order to obtain the most stable adsorption structure of SO2F2-adsorbed Co-MoS2, single SO2F2 was set to approach the surface of the Co-MoS2 monolayer from different initial approaches sites. After geometry optimization, two typical adsorption structures were obtained as shown in Figure , labeled as P1 and P2. Table shows Eads, Q, and Eg of SO2F2-adsorbed Co-MoS2 by P1 and P2.

Figure 7

Adsorption structures of the SO2F2-adsorbed Co-MoS2 monolayer by P1 and P2: (a) P1 side view; (b) P1 top view; (c) P2 side view; and (d) P2 top view.

Table 3

Adsorption Parameters of the SO2F2-Adsorbed Co-MoS2 Monolayer by P1 and P2, Respectively

structure	Eads/eV	Q/e	Eg/eV	d/Å
P1	–0.881	–0.636	0.540	Co–F1 1.841
P2	–2.390	–1.062	1.040	Co–F1 1.798
				Co–F2 1.864

For the P1 structure shown in Figure a, it can be seen that one S–F bond of SO2F2 breaks during the adsorption process. The dissociated F atom builds a bond with the Co atom with a bond length of 1.841 Å, and the dissociated SO2F gas keeps away from the surface of Co-MoS2. SO2F2 gas gets −0.636 e from Co-MoS2, and the adsorption process releases 0.881 eV. Because of the obvious charge transfer and strong adsorption energy, Co-MoS2 adsorbs on the surface of Co-MoS2 by a stable structure. Eg of SO2F2-adsorbed Co-MoS2 is 0.540 eV, which is distinctly larger than that of Co-MoS2 (0.387 eV). As a result, SO2F2 adsorption leads to the decrease of conductivity.

For the P2 structure shown in Figure b, it is found that both of the F atoms dissociate from SO2F2 and form two stable Co–F bonds by lengths of 1.798, 1.864 Å, respectively. The dissociated SO2 was away from the surface of Co-MoS2. In the process, a large amount of electrons transfer from the Co-MoS2 monolayer to SO2F2. Meanwhile, Eads of the P2 structure is up to −2.390 eV, exhibiting it as a strong interaction. Moreover, Eg increases sharply from 0.387 to 1.040 eV after adsorption. We conclude that P2 is the most stable adsorption structure by comparing the adsorption property of Co-MoS2 to SO2F2 in P1 and P2 structures.

To further analyze the adsorption behavior of SO2F2 gas on the Co-MoS2 monolayer, the TDOS, PDOS were also discussed as shown in Figure . After SO2F2 adsorption, the TDOS is apparently shifted to the right side, signifying that the energy level of the electron filling increases, which is favorable for the free movement of electrons. The TDOS increases between −1 and 0 eV, but decreases to zero in the range of 0.3–1 eV. According to the distribution of PDOS, its can be found that the change of TDOS is mainly owing to the orbital hybridization of the F 2p orbital and the S 3p orbital of Co-MoS2 in the range of −4.8 to −3.5 and −2 to −1.5 eV. In addition, the Co 3d orbital hybrids with F 2p range from −1.5 to 0.8 eV.

Figure 8

TDOS and PDOS of SO2F2 adsorbed on the Co-MoS2 monolayer: (a) TDOS; (b) PDOS.

The CDD of the P2 structure is shown in Figure , where red and blue represent acquisition and loss of electrons, respectively. During the strong chemical interaction between SO2F2 and Co-MoS2, the O atoms and the F atoms of the SO2F2 molecule receive a large amount of electrons, and the electron density surrounding the S atom of SO2F2 decreases. Milliken population analysis shows that the F atom and O atom receive 0.947 and 0.608 e of electrons, respectively, while the S atom loses 0.493 e of electrons. In general, SO2F2 acts as an electron acceptor during the adsorption process.

Figure 9

Electron density difference of the SO2F2-adsorbed Co-MoS2 monolayer: (a) side view 1; (b) side view 2.

Comparison of SOF2 and SO2F2 Adsorbed on the Co-MoS2 Monolayer

The adsorption properties of the Co-MoS2 monolayer to SOF2 and SO2F2 gases are compared below. For adsorption structures, SOF2 was completely bonded onto the surface of Co-MoS2 by a stable chemical bond while two F atoms of SO2F2 break away from the gas molecule and builds a new bond with the Co atom, and the dissociated SO2 gas adsorbs on the surface of MoS2 by weak interaction. Because of the strong electronegativity of the F atom, SO2F2 receives more electrons from the Co-MoS2 during the adsorption process. In addition, the redistribution of electrons leads to change of the DOS. The DOS shows different change rules for different gas molecules adsorption, indicating that the adsorption of these two types of gases causes a difference change in the conductivity of Co-MoS2. It builds up the theoretical basis for gas sensors to sensitively detect different gases. Generally, Co-MoS2 shows a better gas sensing property to SO2F2 than SOF2.

Conclusions

In this study, Co-MoS2 has been proposed as the gas-sensing material to adsorb the characteristic decomposition components of SF6 under partial electric discharge: SOF2 and SO2F2. The adsorption energy, charge transfer, energy gap, and CDD of all potential adsorption structures were calculated to analyze the adsorption mechanism based on the density functional theory. Following are the conclusions:

The doped Co atom builds a stable adsorption structure with MoS2 by interacting with three adjacent S atoms. Because of the strong chemical activity of the Co atom, the DOS near the Fermi level become continuous after Co doping, and the conductivity of the Co-MoS2 monolayer is greatly improved comparing with intrinsic MoS2.

SOF2 and SO2F2 adsorb on the Co doping site of Co-MoS2 by chemical adsorption. The gas molecules act as electron acceptors, and the Co-MoS2 monolayer plays electron donors role in the adsorption process. Co-MoS2 possesses different degrees of change in the electrical conductivity to different gas molecules adsorption. As a result, Co-MoS2-based material is sensitive to SOF2 and SO2F2, which builds up the theoretical basis for gas sensors to sensitively detect different gases.

Computational Details and Methods

All calculations were calculated based on the DFT with Tkatchenko–Scheffler dispersion correction. An MoS2 monolayer was built with a 4 × 4 × 1 supercell, including 32 S atoms and 16 Mo atoms, which is large enough to explore its adsorption to monomolecular gases.[35,36] In addition, a vacuum thickness of 15 Å was set to avoid the interaction between different layers.[37] The exchange and correlation energy were calculated by the general gradient approximate with the Perdew–Burke–Ernzerhof, which has been widely used in DFT calculations.[38−41] The self-consistent field (SCF) approximation was calculated by double numerical basis set plus polarization functions (DNP).[42,43] The Brillouin zone k-point sampling was sampled by 5 × 5 × 1.[44] All of the optimized structures have an energy accuracy of 2 × 10–5 Ha, and a maximum force of 4 × 10–3 Ha/Å.[45] The SCF convergence accuracy was set to 1 × 10–6 Ha. In addition, the DIIS size was set to 6 to speed up the convergence of SCF.

The adsorption energy of Co on the MoS2 monolayer is defined by eq , where ECo-MoS, EMoS, and Eco are the total energy of Co-MoS2, MoS2, and Co, respectively. The adsorption energy of the Co-MoS2 monolayer to gas molecules is defined by eq .[46]ECo-MoS, ECo-MoS, and Egas represent the total energy of the Co-MoS2 monolayer after gas molecules adsorption, the Co-MoS2 monolayer, and the free gas molecules before adsorption, respectively.

The charge transfer Q during gas adsorption was calculated using the Mulliken population analysis obtained by eq , where Qiso and Qads are the carried charge of gas molecules (or Co atom) before and after adsorption. Q > 0 means that electrons transfer from gas molecules to the surface of the Co-MoS2 monolayer.

The energy gap of the molecular orbital was calculated through the energy levels of the HOMO and the LUMO, as defined in eq . It determines the probability of charge movement in the adsorption system composed of the gas molecules and the Co-MoS2 monolayer. The wider the gap is, the more energy is required for electrons transferring from the valence band to the conduction band.