First-principles study of Pd-MoSe₂ as sensing material for characteristic SF₆ decomposition components Open Access

Sulfur hexafluoride (SF₆) is a nontoxic, colorless, odorless, and nonflammable inert gas at room temperature and atmospheric pressure, which possesses strong chemical stability, outstanding insulation, and strong arc extinction properties.^1,2 Currently, SF₆ has been the most used insulation medium filling in high-voltage insulation equipment, such as gas-insulated switchgear, gas-insulated transmission line, and gas circuit breaker.^3–5 However, electric discharge faults inevitably occur in SF₆-insulated equipment, including partial discharge, spark discharge, and flashover discharge.^6–8 The electric discharge faults accelerate the charged particle to bombard the SF₆ molecule, resulting in the decomposition of SF₆ gas. Under the combined action of trace moisture and trace oxygen in SF₆-insulated equipment, SF₆ decomposes to H₂S, SO₂, SOF₂, SO₂F₂, and other trace gases.⁹ As the insulation strength of the decomposition components is far less than that of SF₆, the discharge process may become more vigorous with time, which may even lead to serious equipment damage or regional blackout.

Therefore, it is urgent to detect the discharge faults in a timely manner and accurately. Currently, the most used detection methods are the ultrahigh frequency (UHF) method, ultrasonic method, optical measurement method, and gas sensing method.^10–12 Among them, in common gases detection, the gas sensing method based on specific gas sensors gives high detection sensitivity and less volume, and is of low cost, which makes it an ideal method when used with online monitoring.¹³ However, SOF₂ and SO₂F₂ are two unusual gases with extremely low concentration (parts per million level), and the sensitivity of current gas sensors is not high enough for practical applications.¹⁴ Carbon nanotubes, titanium dioxide, graphene, and molybdenum disulfide have been used to detect SOF₂ and SO₂F₂, which still shows limited gas sensing property to the gases.¹⁵ It is necessary to find a new gas sensing material for SOF₂ and SO₂F₂ detection.

Two-dimensional molybdenum selenide (MoSe₂) is a transition metal dichalcogenide (TMD) material, which shows better gas sensing property than traditional metal dioxides.¹⁶ Baek et al. reported a highly sensitive MoSe₂ based gas sensor for NO₂.¹⁷ Late et al. reported a MoSe₂ monolayer based sensor for NH₃ gas sensing applications with sensitivity of parts per million level.¹⁸ In addition, transition metal decoration on MoSe₂ has been adopted to enhance its gas sensitivity. Choi et al. reported that the gas sensing response of Nb doped MoSe₂ to NO₂ increased to 8%.¹⁹ Zhang et al. reported the gas sensing property of Pd doped MoSe₂ to NH₃ based on density functional theory (DFT) calculations.²⁰ Cui et al. and Zhang et al. analyzed the doping mechanism of Rh on MoSe₂ and its gas sensing property to H₂ and C₂H₂ in transformer oil, and other toxic gases.^21,22

Pd is one of the most used transition metals used in surface decoration of nanotubes and two dimensional and three dimensional porous nanomaterials. However, intrinsic MoSe₂ and Pd doped MoSe₂ (Pd-MoSe₂) have not been studied for SOF₂ and SO₂F₂ detection. In this study, the gas sensing property of intrinsic MoSe₂ and Pd-MoSe₂ to SOF₂ and SO₂F₂ has been analyzed based on DFT calculations. It is found that Pd doping has dramatically enhanced the gas sensitivity of MoSe₂ material. The study results prove that Pd-MoSe₂ can be a promising gas sensing material for SOF₂ and SO₂F₂ detection, which provides a key foundation for detecting and diagnosing the discharge fault in SF₆-insulated equipment online.

All calculations are performed based on density functional theory (DFT) calculations which has been widely used for analyzing the adsorption properties of small gas molecules on nanomaterial surfaces.²³ The generalized gradient approximation/Perdew-Burke-Ernzerhof (GGA/PBE) functional was adopted to calculate the exchange correlation potential, and the Grimme has been introduced for the van der Waals (vdW) correction.²⁴ The double numerical plus (DNP) polarization basis set and the DFT semicore pseudopotential (DSSP) method have been employed.²⁵ The k point of the Monkhorst–Pack grid was sampled to 7 × 7 × 1.²² The energy tolerance accuracy, the energy gradient, and the atomic offset were set to 5.442 × 10⁻⁴ eV, 0.054 eV/Å, and 0.005 Å, respectively. The smearing was set to 0.136 eV to speed up the convergence of the self-consistent field (SCF).

A 4 × 4 × 1 supercell of the MoSe₂ slab was built to analyze its surficial adsorption properties, which is consist of 16 Mo atoms and 32 Se atoms.²⁶ In addition, a vacuum slab of 20 Å was set between the periodic MoSe₂ slabs, which is large enough to avoid the interaction from the adjacent slabs. The adsorption energy (E_ads), charge transfer (Q_T), and energy gap (E_g) are defined in Eqs. (1)–(3), respectively,²⁷ where E_sur/gas, E_sur, and E_gas are the total energy of the gas adsorbed surface (including SOF₂ and SO₂F₂ adsorption on MoSe₂ and Pd-MoSe₂), the energy of the isolated surface (MoSe₂ and Pd-MoSe₂), and the energy of the isolated gases (SOF₂ and SO₂F₂).^28–30 Q_ads and Q_iso are the net charge of gas molecules after and before gas adsorption. E_HOMO and E_LUMO are the energy of the highest occupied molecular orbit (HOMO) and the energy of the lowest occupied molecular orbit (LUMO), respectively,

Figure 1 shows the optimized structure of MoSe₂ and Pd-MoSe₂ in top views and side views. The MoSe₂ slab shown in Figs. 1(a) and 1(d) is a multilayer structure, where Se layers are the outmost surface and Mo atoms are located at the middle layer. Therefore, the gas molecules mainly interact with Se atoms during the adsorption process. The distance between the Se atom and three bonded Mo atoms are 2.543 Å, 2.537 Å, and 2.537 Å. The small difference between these three bonds is caused by the periodic boundary condition. For the Pd-MoSe₂ slab shown in Figs. 1(b) and 1(e), the decorated Pd atom bonds with three adjacent Se atoms, which protrudes out of the surface of MoSe₂. The bond lengths of these Pd–Se bonds are 2.472 Å, 2.472 Å, and 2.469 Å, which are shorter than those of Mo–Se bonds. The results calculated are in agreement with the results reported by Zhang et al., which exactly verifies the correction of the calculation method used in this study.²⁰ The pyramid structure of the Pd doping structure ensures its structural stability and is also beneficial for gas molecule adsorption as an active site. Figures 1(c) and 1(f) show the structures of SOF₂ and SO₂F₂ molecules, respectively. The bond lengths of O–S and F–S in the SOF₂ molecule are 1.461 Å and 1.669 Å, respectively. The lengths of O–S and F–S in the SO₂F₂ molecule are 1.442 Å and 1.611 Å, respectively.

Figure 2 shows the total density of states (TDOS) and projected density of states (PDOS) of MoSe₂ and Pd-MoSe₂. For TDOS of MoSe₂ shown in Fig. 2(a), the TDOS is continuous at the Fermi level, but an obvious interruption exists from 0.3 eV to 1.1 eV, verifying the semiconductor properties of intrinsic MoSe₂. For PDOS of MoSe₂ shown in Fig. 2(b), it can be found that the chemical bond between the Mo atom and Se atom is mainly due to hybridization between the Mo-p orbit and Se-p orbit as the PDOS is highly coincident at all energy levels. For TDOS of Pd-MoSe₂ shown in Fig. 2(c), its distribution tendency is similar to that of MoSe₂, and the crests and troughs almost occur at the same energy level. However, the peak value of Pd-MoSe₂ is larger than that of MoSe₂ due to the metallic properties of the doped Pd atom. For PDOS of the Pd atom and its bonded Mo atom in the Pd-MoSe₂ system shown in Fig. 2(d), the PDOS mainly comes from the Pd-d orbit and Mo-p orbit. In addition, the PDOS of the Pd-d orbit and Mo-p orbit distinctly overlap around −4.2 eV and the Fermi level. In general, Pd doping on the MoSe₂ surface increases its conductivity, and the protruding Pd atom enhances the adsorption properties of MoSe₂ to small gas molecules.

As shown in Fig. 3 and Table I, the adsorption structures and the corresponding parameter of the SOF₂ molecule on MoSe₂ and Pd-MoSe₂ surfaces are analyzed to understand the gas sensing mechanism. For SOF₂ adsorption on MoSe₂ shown in Figs. 3(a) and 3(c), it shows the most stable adsorption structure by approaching the MoSe₂ with the S atom; this is because of the multivalence property of the S atom. The nearest interaction distance from the molecule to MoSe₂ reaches a length of 3.258 Å. Due to the long interaction distance, the adsorption E_ads and Q_T are only −0.361 eV and −0.043e, respectively. In addition, the adsorption process does not change drastically the initial structure of MoSe₂ and SOF₂. Therefore, the adsorption effect of MoSe₂ to SOF₂ belongs to physisorption. For SOF₂ adsorption shown in Figs. 3(b) and 3(d), the SOF₂ molecule adsorbs on Pd-MoSe₂ by the Pd–S bond. The adsorption distance has dramatically decreased to 2.188 Å, which is obviously shorter than that between SOF₂ and MoSe₂. The adsorption belongs to chemisorption because of the large E_ads with a value of −1.238 eV. However, there is still only small charge transfer (−0.042e) between the gas molecule and Pd-MoSe₂. By comparing the adsorption properties before and after Pd doping, it can be concluded that Pd doping significantly enhances the adsorption of MoSe₂ based material to SOF₂, which plays a key foundation for gas sensing.

Figure 4 shows the TDOS before and after SOF₂ adsorption on MoSe₂ and Pd-MoSe₂, respectively. For SOF₂ adsorption on MoSe₂, it can be seen that the gas adsorption brings no change to the distribution of TDOS because the adsorption between SOF₂ and MoSe₂ is too weak to change the TDOS during the adsorption process. Therefore, intrinsic MoSe₂ shows weak gas sensing property to SOF₂. After SOF₂ adsorption on Pd-MoSe₂, the TDOS obviously increases at most areas below the Fermi level and moves right over the Fermi level. As a result, the probability of charge transfer from the valence band to conduction band increases after SOF₂ adsorption, reflecting the increase in conductivity. In summary, Pd doping enhances the gas sensing property of MoSe₂ based material to SOF₂.

Similarly, the properties of SO₂F₂ on MoSe₂ and Pd-MoSe₂ were studied to analyze the adsorption mechanism. Figure 5 and Table II present the adsorption structure and the corresponding adsorption parameter. For SO₂F₂ adsorption on the intrinsic MoSe₂ surface as shown in Figs. 5(a) and 5(c) in top and side views, the gas molecule is far from the surface with a nearest distance of 3.440 Å between F1 and Mo1 atoms. Due to the long interaction distance, the E_ads and Q_T are only −0.327 eV and −0.020e, respectively. Consequently, the interaction between SO₂F₂ and MoSe₂ should be weak physisorption. After Pd doping on the MoSe₂ surface, there is strong chemisorption between SO₂F₂ and Pd-MoSe₂ as shown in Figs. 5(b) and 5(d) in top and side views. The bond between the Pd atom and Mo4 atom broke during the adsorption process, and the F4 atom has dissociated from the SO₂F₂ molecule and built a new bond with the Pd atom with a distance of 1.959 Å. Besides, the dissociated SOF₂ molecule interacts with the Pd atom by the S2-Pd bond with a length of 2.275 Å. Because of the strong interaction, the E_ads and Q_T reach −1.547 eV and −0.700e, respectively. Therefore, the adsorption process is hard to reverse spontaneously.

Figure 6 shows the TDOS before and after SO₂F₂ adsorption on MoSe₂ and Pd-MoSe₂. For SO₂F₂ adsorption on MoSe₂, the change of TDOS only occurs below the energy level of −3.5 eV with a slight increase. The increased electrons at this area are hard to transfer from the valence to conduction band, and the conductivity does not change in the adsorption process. This result can also be provided by the weak adsorption between SO₂F₂ and MoSe₂, as the adsorption energy is too weak to increase the redistribution of the electron cloud. For SO₂F₂ adsorption on Pd-MoSe₂, the TDOS slightly increases from 0.8 eV to 1.3 eV and above 1.9 eV, while it decreases obviously below the Fermi level. Therefore, more electrons can transfer from the valence band to conduction band, resulting in the increase in conductivity after gas molecule adsorption.

Based on molecular orbital theory, the electron property was studied to further analyze the gas sensing mechanism. Figure 7 and Table III show the HOMO and LUMO distribution and their corresponding values. The HOMO and LUMO only distribute around the Mo atom due to its more strong chemical activity compared with the Se atom. There is no energy gap between the HOMO and LUMO, as both of the energy levels are −3.868 eV. After Pd atom doping, the HOMO and LUMO distribute not only on Mo atoms but also on the doped Pd atom and the surrounded Se atoms. The energy level decreases to −3.791 eV and −3.689 eV with an energy gap of 0.102 eV. Although there is a little energy gap, the large increased molecular orbit distribution will lead to the increase in conductivity. The molecular orbit distributions of SOF₂ and SO₂F₂ adsorption on intrinsic MoSe₂ are very similar, the HOMO distribution does not change drastically, and a small part of LUMO exists on the gas molecule. The energy gaps are only 0.009 eV and 0.002 eV for SOF₂ and SO₂F₂ adsorption, respectively. Therefore, the small change of molecular orbit does not change the conductivity during the adsorption of gases. Upon SOF₂ on Pd-MoSe₂, a large area of HOMO mainly exists on the gas molecule, Pd atom, and Se atoms, while the distribution area of LUMO is much smaller. The energy gap has decreased to 0.087 compared with Pd-MoSe₂ of 0.102 eV. Upon SO₂F₂ on Pd-MoSe₂, a small part of HOMO locates on the Pd atom and the dissociated gas molecule and a large area of LUMO on the gas adsorption position. The energy gap dramatically increased to 0.552 eV.

SOF₂ and SO₂F₂ are two characteristic decomposition components of SF₆ under electric discharge. This study focuses on developing a new gas sensing material for detecting these two types of gases. Intrinsic MoSe₂ and Pd doped MoSe₂ were adopted as the gas sensing material to explore their gas adsorption properties to SOF₂ and SO₂F₂. In order to analyze the adsorption mechanism, the adsorption structure, adsorption energy, charge transfer, density of states, and molecular orbit were calculated. It is found that intrinsic MoSe₂ shows weak adsorption property to SO₂F₂ and SO₂F₂, signifying that it shows weak gas sensing property to these gas molecules. Pd doping on the MoSe₂ surface significantly increases its adsorption property, and the adsorption energy for SOF₂ and SO₂F₂ reach −1.238 eV and −1.547 eV, respectively. SOF₂ and SO₂F₂ adsorption increases the conductivity of the adsorption system to different extents during the adsorption process. Due to the different variation of conductivity, it can be used to diagnose the severity and type of electric discharge. This study provides important theoretical support for developing an ideal SOF₂ and SO₂F₂, which is critical for online monitoring the running status of SF₆-insulated equipment.

This study was supported by the National Key Research and Development Program of China (Grant No. 2017YFB0902701).