Boosting the adsorption and sensing performance of MoS₂ for SF₆ decomposition gases by non-metal atom doping:a DFT study

Abstract

In electrical power systems, SF₆ in Gas Insulated Switchgear (GIS) decomposes under partial discharge, yielding toxic products such as H₂S, SO₂, SOF₂, SO₂F₂. Molybdenum disulfide (MoS₂), a promising two-dimensional material, exhibits potential in gas sensing but its pristine form suffers from weak adsorption capacity for gas molecules. Herein, we carry out a systematic exploration of the gas-sensing capabilities of eight non-metal (NM)-doped MoS₂ (NM@MoS₂) materials toward SF₆ decomposition gases by leveraging first-principles calculations. The results reveal that all NM@MoS₂ substrates exhibit thermodynamic stability with negative binding energies ranging from −0.84 to −7.11 eV. Pristine MoS₂ shows weak physisorption of target gases, accompanied by low adsorption energies (−0.21 to −0.33 eV), large adsorption distances (2.90 to 3.70 Å), minimal charge transfer and limited sensitivity. In contrast, the NM@MoS₂ substrates demonstrate distinct adsorption behaviors: O@MoS₂, Se@MoS₂, and Te@MoS₂ retain physical adsorption (adsorption energies: −0.14 to −0.37 eV; distances: 2.73 to 4.02 Å), whereas B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂, and Si@MoS₂ demonstrate enhanced adsorption (adsorption energies: −0.39 to −1.67 eV; distances: 1.60 to 3.24 Å), accompanied by significant charge transfer and enhanced sensing-response. Of these substrates, Si@MoS₂ demonstrates moderate recovery times at ambient temperature (2.82 s) and demonstrates significant sensing-response to SF₆ decomposition components, highlighting its potential for practical gas sensing applications. This study demonstrates that non-metal doping can effectively enhance the gas-detection efficacy of MoS₂ towards SF₆ decomposition products, providing theoretical support for developing high-efficiency gas sensors.

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1. Introduction

In the realm of power systems, Gas Insulated Switchgear (GIS) has become increasingly prevalent as power grids expand, and its safe and reliable operation is of utmost importance.^1,2 A key component of GIS is sulfur hexafluoride (SF₆), which serves as a vital gas-insulating medium, renowned for its remarkable electrical insulation and arc-quenching properties. However, during extended periods of operation, local discharge events can trigger the breakdown of SF₆ gas, producing byproducts such as hydrogen sulfide (H₂S), sulfur dioxide (SO₂), sulfuryl fluoride (SOF₂), and thionyl difluoride (SO₂F₂).^3,4 These byproducts have significantly poorer insulating properties than SF₆, which weakens equipment insulation, intensifies local discharge incidents and poses a severe threat to system stability.^5,6 In light of these challenges, it is of the utmost importance to develop effective technologies that can detect and remove SF₆ decomposition gases. Such advancements are essential for enhancing the safety and reliability of power systems, as well as for mitigating potential environmental and health risks.^7,8

In recent years, two-dimensional transition metal dichalcogenides (TMDs) have garnered significant attention due to their ultra-high specific surface area and exceptional electronic transport properties. Taking MoS₂ as an example, chemical vapor deposition (CVD) enables the fabrication of atomic-thickness monolayer films.⁹ When thickness is reduced to the atomic level, the specific surface area increases significantly, expanding the interface area for gas molecule contact. Simultaneously, the bandgap transitions from the bulk indirect bandgap (1.2 eV) to a direct bandgap (1.8 eV), substantially enhancing electron mobility and accelerating rapid charge transfer between gas molecules and the surface.¹⁰ These attributes have propelled their extensive utilization in gas sensing applications.^11–14 Additionally, atomically functionalized TMDs have been employed for detecting SF₆ decomposition gases. For instance, Li et al. demonstrated the potential of platinum-doped HfS₂ (Pt-HfS₂) for dual gas detection of H₂S and SO₂.¹⁵ Jiang et al. investigated the adsorption behavior of iridium-embedded hafnium disulfide (Ir-HfS₂) monolayers toward three sulfur hexafluoride decomposition products (H₂S, SOF₂, and SO₂F₂) and their sensing prospects.¹⁶ TMDs are characterized by a layered two-dimensional structure, where each layer is composed of alternating transition metal and chalcogen atoms. This unique structural arrangement imparts distinctive properties to TMDs, unlocking a wide range of potential applications.^17,18 Currently, extensive research efforts have been directed toward exploring two-dimensional materials, particularly compounds such as MoS₂, MoSe₂, and SnS₂.^19–22 Investigations have revealed that TMDs monolayer exhibits favorable characteristics, including narrow band gaps and high carrier mobility. These features render them highly suitable for applications in optoelectronics and nanoelectronics.^23–25

Among TMDs materials, MoS₂ has attracted the most research interest in gas detection, particularly for the sensing of toxic gases. This is due to its favourable semiconductor properties, which include a substantial band gap, a large surface-to-volume ratio, an abundance of sites for redox reactions, and high carrier mobility.^26,27 MoS₂-based gas sensing materials achieve significant gas sensitivity through charge transfer between gas molecules and the surface. For instance, Late et al. utilised ab initio calculations to ascertain that the observed reduction in resistance of monolayer and bilayer MoS₂ under an applied magnetic field is attributable to charge transfer.²⁸ Dong et al. fabricated an Au/MoS₂/Au photoelectrochemical gas sensor exhibiting sensitivity as high as S = 4.9%/ppb (4900%/ppm) upon exposure to ppb-level NO₂.²⁹ However, chemical adsorption impedes recovery processes, and MoS₂ still faces bottlenecks such as poor selectivity and insufficient sensitivity.³⁰ Therefore, new strategies are urgently needed to overcome performance limitations.

In recent decades, a vast body of research has concentrated on modifying the MoS₂ monolayer to improve its gas-sensing performance. These modification strategies primarily include doping with metal (both precious and non-precious metals) and non-metal (NM) atoms, as well as compositing with other materials.^27,31–39 Such modifications are credited with significantly improving the electronic properties and structural stability of MoS₂ monolayers, thereby optimizing their gas-sensing capabilities.^31,39Therefore, doping has a significant influence on the electronic structure and gas-detection capability of MoS₂, including the adsorption strength, interfacial charge displacement and adsorbate–substrate interaction. For example, Fan et al. studied how the doping of transition metals (including several precious and non-precious metals) affects the gas-detecting performance of MoS₂ monolayer when exposed to various gas molecules (CO, NO, O₂, NO₂ and NH₃).³⁷ The results demonstrated that doping significantly enhanced the sensing performance. Luo et al. selected Al, Si, and P atoms as dopants due to the close similarity of their covalent radii to that of the S atom.³⁸ Doping MoS₂ with these atoms has demonstrated promising potential for NO₂ sensing despite of hazardous gasses sensing, the MoS₂ doped with metal (both precious and non-precious metals) atom materials also used detection of SF₆ decomposition gases.^3,36,40,41

Similarly, extensive research has been conducted on utilizing NM atoms to modify MoS₂ for detecting sulfur-containing gases.^42,43 For instance, Szary et al. systematically investigated adsorption selectivity by adsorbing H₂S, N₂, and O₂ molecules on pristine MoS₂ films and MoS₂ films doped with P, Cl and Ge.⁴² Piosik et al. used doping strategies involving Si, P, Cl, Ge and Se to significantly improve the sensing performance of MoS₂ for SO₂.⁴³ It should be emphasized that MoS₂ substrates doped with NM atoms are renowned for the simplicity of their preparation. For instance, through density functional theory (DFT) calculations, Ma and his colleagues showed that common gases such CO, NO, NO₂, and O₂ are capable of occupying S vacancies in MoS₂ at room temperature. In this way, doping with C, N, and O can be realized.⁴⁴ This implies that MoS₂ doped with NM atoms can be realized under gentle conditions. Moreover, Song and his colleagues managed to synthesize P-doped MoS₂ through a simple pyrolysis procedure.⁴⁵ Xie and their team successfully fabricated MoS₂ materials with different concentrations of oxygen doping by controlling the preparation temperature.⁴⁶ Zhang and his colleagues fabricated Se-doped MoS₂ materials employing a hydrothermal process, and Song et al. produced O-doped MoS₂ materials by means of pyrolysis.^47,48 In a similar vein, multiple experimental studies have emphasized that B-doped MoS₂ materials obtained via the hydrothermal route display outstanding performance in electrocatalytic reactions.^49–51 Consequently, these research outcomes provide strong theoretical backing for the synthesizability and stability of NM@MoS₂ materials.

In this study, we performed a comprehensive analysis of the adsorption and detection characteristics of NM@MoS₂ materials (where NM = B, C, N, O, P, Si, Se and Te) using DFT calculations, with regard to four SF₆ decomposition products: H₂S, SO₂, SOF₂ and SO₂F₂. Firstly, we calculated the formation energies of NM@MoS₂ to evaluate the thermodynamic stability of the different substitution sites. Next, we computed and analyzed the adsorption characteristics of the four SF₆ decomposition gases on NM@MoS₂ substrates, making comparisons with intrinsic MoS₂. Finally, we explored their gas-sensing potential further via electronic structure calculations and sensitivity analysis.

2. Computational methods

We performed spin-polarized DFT calculations using the Vienna Ab Initio Simulation Package (VASP).⁵² The electronic exchange and correlation terms were characterised using the generalized gradient approximation (GGA) and the Perdew–Burke–Ernzerhof (PBE) functional.⁵³ The GGA-PBE functional is one of the most widely used exchange-correlation approximations for evaluating electronic properties. Compared to the Local Density Approximation (LDA), meta-GGA, and hybrid functions, it strikes a favourable balance between computational cost and accuracy, and it has also been extensively studied for calculations involving two-dimensional materials.^3,41,44 Ion–electron interactions were characterized using the Projector Augmented Wave (PAW) approach, and the energy cutoff was fixed at 450 eV.⁵⁴ Complete relaxation was achieved for every atom, with standard convergence parameters of 1 × 10⁻⁴ eV and 0.02 eV Å⁻¹ established for energies and forces, respectively. The DFT-D3 approach was then employed to strengthen the weak van der Waals forces between the adsorbates and surfaces.⁵⁵ The limited capability of traditional functions in describing dispersion effects makes the introduction of empirical dispersion correction terms the most viable solution currently available.⁵⁶ Compared to methods such as DFT-D2 and optPBE, the DFT-D3 correction technique offers greater accuracy, broader elemental coverage (H–Pu) and improved computational efficiency.⁵⁷ It is particularly well-suited to large-scale or rapidly iterative research systems, and has been widely adopted in chemistry, materials science and related fields.^58,59 During the geometry optimization process, a 4 × 4 × 1 Monkhorst–Pack k-point grid was adopted to sample the Brillouin zone.⁶⁰ A single-layer supercell with dimensions of 4 × 4 × 1 was built to serve as a model for the substrate surface. To eradicate the possible effect of periodicity on computational results, a 15 Å-thick vacuum layer was established in the lattice's Z-direction. To examine the possibility of doping MoS₂ with NM atom, the formation energies (E_f) of NM@MoS₂ substrates were computed in line with eqn (1):

E_f = E_NM@MoS2 − E_MoS2 − µ_NM + µ_{substitute-atom} (1)

where, E_MoS2 and E_NM@MoS2 represent the total energy of a single-layer 4 × 4 × 1 MoS₂ supercell, and the total energy of the system after one S or Mo atom in the supercell is substituted with an NM atom, respectively. µ_NM represents the chemical potential of the NM atom. With regard to N and O atoms, their respective values are derived from the per-atom energies of their gaseous forms (N₂ and O₂). For all other types of atoms, their chemical potentials are obtained from the per-atom energies of their bulk or solid states: specifically, β-rhombohedral boron for B, graphite for C, black phosphorus for P, diamond-cubic silicon for Si, trigonal selenium for Se, hexagonal tellurium for Te. µ_{substitute-atom} represents the chemical potential of the replacement atom. NM atoms can replace either the S sites or Mo sites in MoS₂. The chemical potentials of S and Mo correspond to the energy per atom in the 8-membered rings and body-centered cubic (bcc) structures.

The adsorption energies (E_ad) of adsorbed gases are calculated according to eqn (2):

E_ad = E_{NM@MoS2+adsorbed gas} − E_NM@MoS2 − E_{gas molecule} (2)

where, E_{NM@MoS2+adsorbed gas} is the total energy of NM@MoS₂ adsorption system, E_NM@MoS2 is the energy of NM@MoS₂ substrate, and E_{gas molecule} is the energy of an isolated gas molecule. All these energies were computed from optimized atomic structures.

3. Results and discussion

3.1. Structural stability of NM@MoS₂

Starting from the 2H–MoS₂ unit cell, the geometric structure was first optimized to obtain lattice parameters. The optimized lattice constant was a = 3.16 Å, the S–Mo bond length was 2.41 Å, and the S–Mo–S bond angle was 80.61°, which are highly consistent with literature reports.^61,62 Given that MoS₂ monolayer features two substitutional sites (i.e., S and Mo), this study systematically investigated the substitution of B, C, N, O, P, Si, Se, and Te at both sites. The corresponding configurations are presented in Fig. 1a and S1a. To quantitatively evaluate the synthetic feasibility of these NM@MoS₂ materials, we initially computed their formation energies (E_f). As illustrated in Fig. 1b and S1b, the E_f values of NM atoms substituting S sites in MoS₂ monolayer range from −3.86 to 2.32 eV, whereas those substituting Mo sites span from −0.22 to 9.52 eV. The substantially lower energy barrier for S-site substitution highlights a distinct preference of NM atoms for occupying the S positions. It is noteworthy that although the E_f value for N atom replacing the S site is relatively large, this result stems from setting the chemical potential of the N atom to half that of nitrogen gas in this work. Nevertheless, the E_f value for N atom replacing S site remains significantly lower than the corresponding value for replacing the Mo site, indicating that the N atom preferentially substitutes the S site. Experimental synthesis of N@MoS₂ via S-site substitution has already been achieved,^63,64 thereby confirming its experimental feasibility and stability. Besides, the majority of other NM@MoS₂ materials have been successfully synthesized experimentally, which further validating their stability.^44–48,51 Huang et al. successfully synthesized ultrathin P-doped MoS₂ nanosheets via pyrolysis, achieving a surface phosphorus content of 4.7 at%.⁴⁵ Xie et al. used hydrothermal synthesis to produce oxygen-doped MoS₂ ultrathin nanosheets (O–MoS₂), with oxygen atom content ranging from 1.92 to 4.18 at%.⁴⁶ Huang et al. thermally decomposed MoS₂ ultrathin nanosheets synthesized using ammonium molybdate, thiourea, and layered g-C₃N₄ as templates. Following simple H₂O₂ treatment, they achieved controlled oxygen atom introduction at defect sites (including edges), yielding an oxygen content of 6.2 at%.⁴⁷ Yuan et al. synthesized N-doped MoS₂ photocatalysts via a two-step hydrothermal calcination process, achieving a nitrogen content of approximately 18.39 at%.⁶⁵

Fig. 1 (a) Structure of NM atoms replacing S sites in a monolayer MoS₂; (b) formation energies of NM atom substitutions at S sites in a monolayer MoS₂.

On the basis of the analysis mentioned above, we have gone for the S-site substitution of NM@MoS₂.

3.2. Adsorption performance analysis

3.2.1 Adsorption on pristine MoS₂. The adsorption characteristics of four SF₆ decomposed gases on the pristine MoS₂ structure were comprehensively examined. The optimized configurations of H₂S, SO₂, SOF₂, and SO₂F₂ on pristine MoS₂ are shown in Fig. 2, and the corresponding adsorption energy (E_ad), adsorption distance (d), as well as the transferred charges (Q), are summarized in Table 1.

Fig. 2 Optimized structure of adsorbed gases on pristine MoS₂. Green directional lines show the number of transferred charges, and dashed lines represent the adsorption distance.

Table 1 E_ad, d, and Q of adsorbed gases on pristine MoS₂

Adsorbed gases	Ead (eV)	d (Å)	Q (e)
H2S	−0.21	2.90	0.022
SO2	−0.33	3.05	0.070
SOF2	−0.29	3.31	0.022
SO2F2	−0.33	3.70	0.037

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Since adsorption configurations vary with surface structure, “d” must be explicitly defined for each system. In the case of SO₂F₂ bonding to the sulfur site on the pristine MoS₂ surface via its sulfur atom, and the corresponding d is defined as the nearest interatomic distance from the SO₂F₂–S bond to the surface S atom. In NM@MoS₂ system, SO₂F₂ couples to the NM site via oxygen atom. Here d is defined as the shortest distance from SO₂F₂–O bond to the NM atom. H₂S adsorbs onto the surface chalcogen site via the hydrogen atom, with d being defined as the distance from H₂S–H to the corresponding chalcogen atom. SO₂ and SOF₂ both adsorb onto the surface sulfur site or dopant site via the sulfur atom, with d defined as the shortest distance from adsorbate–S to the sulfur or NM atom.

Based on Fig. 2 and Table 1, the adsorption efficiency of pristine MoS₂ towards the four gases is comparatively low (E_ad ≈ −0.30 eV), with the corresponding d-values range from 2.9 to 3.7 Å, suggesting that the adsorption process is of a physisorptive in nature. In this scenario, charges consistently transfer from the MoS₂ substrate to the gas molecules, with corresponding Q values of merely 0.022, 0.070, 0.022, and 0.037 e. van der Waals forces are weak intermolecular interactions originating from transient charge fluctuations, with strengths significantly lower than those of covalent or ionic bonds.⁶⁶ Following adsorption onto the pristine MoS₂ surface, the d values of the four gases are significantly greater than the sum of the S–S covalent radii. Consequently, no chemical bonds are formed or broken. The interaction between these four gas molecules and the pristine MoS₂ surface is primarily dominated by van der Waals forces. Given the relatively low E_ad, greater adsorption distances, and smaller quantities of charge transfer, it can be inferred that pristine MoS₂ exhibits limited adsorption capability toward H₂S, SO₂, SOF₂, and SO₂F₂.

3.2.2 Adsorption on NM@MoS₂. Fig. 3 and 4 show the optimized configurations of H₂S, SO₂, SOF₂ and SO₂F₂ on NM@MoS₂, with the corresponding d and Q values labelled on the respective configurations. The E_ad of these four gases on NM@MoS₂ are summarized in Tables 2 and 3.

Fig. 3 Optimized structure of adsorbed gases on O@MoS₂, Se@MoS₂ and Te@MoS₂, respectively. Green directional lines show the number of transferred charges, and dashed lines represent the adsorption distance.

Fig. 4 Optimized structure of adsorbed gases on B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂, and Si@MoS₂, respectively. Green directional lines show the number of transferred charges, and dashed lines represent the adsorption distance.

Table 2 E_ad of adsorbed gases on NM@MoS₂(NM = O, Se, Te)

Substrates	Ead (eV)
	H2S	SO2	SOF2	SO2F2
O@MoS2	−0.32	−0.37	−0.33	−0.31
Se@MoS2	−0.15	−0.26	−0.23	−0.26
Te@MoS2	−0.14	−0.20	−0.19	−0.24

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Table 3 E_ad of adsorbed gases on NM@MoS₂(NM = B, C, N, P, Si)

Substrates	Ead (eV)
	H2S	SO2	SOF2	SO2F2
B@MoS2	−1.66	−1.22	−0.98	−0.49
C@MoS2	−1.62	−1.15	−1.00	−0.44
N@MoS2	−0.46	−0.68	−0.51	−0.39
P@MoS2	−0.55	−0.75	−0.44	−0.39
Si@MoS2	−1.67	−0.93	−0.64	−0.74

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This study categorises dopant atoms as either chalcogens (O, Se and Te) or non-chalcogens (B, C, N, P and Si). Non-chalcogen doping significantly enhances the gas adsorption capacity, as evidenced by the E_ad values of the four gases adsorbed on the NM@MoS₂ substrate. As shown in Table 2, the E_ad values of O@MoS₂, Se@MoS₂, and Te@MoS₂ exhibit range from −0.14 to −0.37 eV, indicating a lack of significant adsorption enhancement compared to pristine MoS₂ (−0.21 to −0.33 eV).

As shown in Table 3, the E_ad values of B@MoS₂, C@MoS₂, P@MoS₂, N@MoS₂ and Si@MoS₂ range from −0.39 to −1.67 eV, indicating enhanced adsorption. This adsorption enhancement is reflected by the significant reduction in the intermolecular distance between the substrate and the molecules. It can be concluded that a shorter adsorption distance generally implies a closer interaction between the gas molecule and the NM@MoS₂ substrate. The relatively short distance of adsorption implies a more intimate contact, aligns with its higher adsorption energy. In contrast, the longer distance of adsorption may be related to its relatively weaker interaction with NM@MoS₂ substrate. In terms of the transferred charge (Q), the amount of charge transfer reflects the degree of electron interaction during adsorption. The adsorption process shows a significant charge transfer, indicating a strong electron – donating or – accepting behaviour in the adsorption process, which associated with its relatively high adsorption energy as well.

We systematically compared the calculated adsorption energies with those of previously reported doping strategies (see Table S1), which further confirms the significant application potential of single-layer NM@MoS₂ (where NM = B, C, N, P or Si) in gas sensing. The study also reveals that MoS₂ substituted with transition metal atoms has a significantly higher adsorption capacity for SF₆ decomposition components.^3,40,67,68 However, the excessively strong adsorption strength makes the adsorption sites difficult to regenerate, which severely hinders sensor recovery. In contrast, H₂S, SO₂, SOF₂, and SO₂F₂ exhibit insufficient adsorption stability on the PtSe₂ surface and readily desorbing at room temperature due to thermal agitation.⁶⁹ Compared to these materials, SF₆ decomposition gases show moderately reduced adsorption strength on the NM@MoS₂ monolayer, thereby avoiding the regeneration hindrance caused by excessively strong chemisorption.^{15,16,70–72}

In summary, the adsorption of SF₆ decomposition gases onto eight NM@MoS₂ substrates can be categorized into two group: substitution systems involving chalcogen elements (O, Se, Te) retain the original surface characteristics of MoS₂ and have no significant impact on the adsorption behavior of SF₆ decomposition products. In contrast, substitution systems involving non-chalcogen elements (B, C, N, P and Si) exhibit distinct adsorption properties, indicating that surface modification significantly alters gas adsorption behaviour.

3.2.3 Analysis of electronic properties. In view of the preceding analysis of the adsorption of H₂S, SO₂, SOF₂ and SO₂F₂ gases onto B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂ and Si@MoS₂ substrates, which was classified as chemical adsorption, we further investigated the interaction mechanism between dopant atoms and adsorbed molecules by calculating the partial density of states (PDOS). As presented in Fig. 5, significant orbital hybridization occurs between the dopant (B, C, N, P and Si) atoms and the atoms of the adsorbed molecules. Fig. 5, for example, shows that the B-2p orbital in B@MoS₂ overlaps substantially with the S-3p, O-2p, and F-2p orbitals of the adsorbed gases. The same phenomenon also occurs for the C@MoS₂, N@MoS₂, P@MoS₂ and Si@MoS₂ substrates (see Fig. 5). These results indicate that the obvious orbital hybridization between the dopant atoms (B, C, N, P and Si) and the atoms of the adsorbed molecules further explains the strong E_ad and short d between the adsorbed molecules and the substrates.

Fig. 5 PDOS of H₂S, SO₂, SOF₂ and SO₂F₂ adsorbed on B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂ and Si@MoS₂ substrates, respectively.

3.3. Recovery time

A high-performance gas sensor demands strong sensitivity toward the sensing gas, as well as a moderate recovery (desorption) time after adsorption. As previously confirmed, B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂ and Si@MoS₂ substrates exhibit excellent adsorption efficiency for H₂S, SO₂, SOF₂ and SO₂F₂ molecules. This section examines the scientific feasibility of using these materials in practice by exploring the specifics of recovery time during the desorption process. Recovery time is can be calculated using eqn (3):⁴where, the experimental frequency (ω) corresponds to the vibration frequency of surface atoms and is set to 10¹² s⁻¹.^73–75 T is temperature (K), and K_B is the Boltzmann constant, which has a value of 8.62 × 10⁻⁵ eV K⁻¹.

A moderate recovery period is essential for gas-sensing materials and is correlated with adsorption strength.^4,6 Stronger adsorption of gases enhances the influence of individual molecule–substrate interactions, leading to a prompt and clear response. However, excessively strong adsorption can also slow down the recovery process. Therefore, the optimal adsorption strength must strike a balance between recovery behaviour and response capability. Intrinsic MoS₂ exhibits weak adsorption for SF₆ decomposition gas, resulting in an extremely short recovery time and limited response capability. However, doping with non-halogen elements (B, C, N, P and Si) enhances adsorption strength to a moderate level, thereby optimizing the recovery time. Recovery times of H₂S, SO₂, SOF₂ and SO₂F₂ gases at 298 K, 398 K and 498 K were summarized in Table 4.

Table 4 Recovery times (in s) at 298 K, 398 K, and 498 K

System	Gas	298 K	398 K	498 K
B@MoS2	H2S	1.27 × 10^16	1.11 × 10^9	6.85 × 10^4
	SO2	4.04× 10^8	2.68 × 10^3	2.14
	SOF2	3.11 × 10^4	2.23	7.40 × 10^−3
	SO2F2	1.97 × 10^−4	1.62 × 10^−6	9.18 × 10^−8
C@MoS2	H2S	2.99 × 10^15	3.73 × 10^8	2.79 × 10^4
	SO2	3.56 × 10^7	4.35 × 10^2	5.0 × 10^−1
	SOF2	9.45 × 10^4	5.13	1.44 × 10^−2
	SO2F2	3.38 × 10^−5	4.33 × 10^−7	3.20 × 10^−8
N@MoS2	H2S	6.61 × 10^−5	7.17 × 10^−7	4.78 × 10^−8
	SO2	3.28 × 10^−1	4.18 × 10^−4	7.78 × 10^−6
	SOF2	5.50 × 10^−4	3.50 × 10^−6	1.70 × 10^−7
	SO2F2	4.22 × 10^−6	9.13 × 10^−8	9.21 × 10^−9
P@MoS2	H2S	1.73 × 10^−3	8.27 × 10^−6	3.38 × 10^−7
	SO2	3.39	3.41 × 10^−3	4.15 × 10^−5
	SOF2	2.67 × 10^−5	3.64 × 10^−7	2.78 × 10^−8
	SO2F2	1.03 × 10^−6	3.19 × 10^−8	3.79 × 10^−9
Si@MoS2	H2S	1.73 × 10^16	1.39 × 10^9	4.51 × 10^4
	SO2	5.62 × 10^3	5.92 × 10^−1	2.55 × 10^−3
	SOF2	6.11 × 10^−2	1.19 × 10^−4	2.84 × 10^−6
	SO2F2	2.82	2.10 × 10^−3	2.81 × 10^−5

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As shown in Table 4, a rise in temperature leads to an enhancement in the intensity of molecular Brownian motion, thereby shortening the recovery time. Therefore, controlling temperature is an effective way to adjust the system's recovery time. However, the B@MoS₂ and C@MoS₂ substrates have prolonged recovery times because they have high adsorption energies for H₂S, SO₂ and SOF₂ molecules. Meanwhile, the Si@MoS₂ substrate exhibits a prolonged recovery time for H₂S and SO₂ molecules at room temperature. Even at an elevated temperature of 498 K, the recovery times for the H₂S/B@MoS₂, H₂S/C@MoS₂ and H₂S/Si@MoS₂ systems are still extremely long at 6.85 × 10⁴ s, 2.79 × 10⁴ s and 4.51 × 10⁴ s, respectively. Therefore, for systems with high adsorption energies, integrating with heating or UV irradiation is essential for accelerating the gas desorption process and improving the recycling of the B@MoS₂, C@MoS₂ and Si@MoS₂ substrates. Nevertheless, it is feasible to reduce the recovery time for the SOF₂/B@MoS₂, SO₂/C@MoS₂, SOF₂/C@MoS₂ and SO₂/C@MoS₂ adsorption systems by increasing the temperature. For example, at 398 K, the desorption times for the SOF₂/B@MoS₂ and SOF₂/C@MoS₂ systems are reduced to 2.23 and 5.13 seconds, respectively. Notably, the recovery times for the P@MoS₂ and Si@MoS₂ substrates are relatively short: at 298 K, the recovery times are 3.39 and 2.82 s for SO₂ and SO₂F₂ gases, respectively. This suggests that both the P@MoS₂ and Si@MoS₂ substrates can rapidly desorb SO₂ and SO₂F₂ at ambient temperature, indicating excellent recyclability.

3.4. Response capability

The work function is defined as the minimum energy required to move an electron from the surface of a material into a vacuum.⁷⁶ As a key parameter characterizing the surface electronic structure of sensing materials, its variation can quantitatively reflect the degree of charge transfer between gas molecules and the material surface, thereby providing an important indication of the potential response capability of the sensing material. Thus,we systematically calculated the work functions (Φ) and the percentage change in work function (ΔΦ%) of four gases before and after adsorption on pristine MoS₂ and NM@MoS₂ substrates. Φ and ΔΦ% can be calculated using eqn (4) and (5):

Φ = V_Vacuum − V_Fermi (4)

ΔΦ% = (|Φ_Unadsorbed − Φ_adsorbed|/Φ_Unadsorbed) × 100% (5)

where V_Vacuum, V_Fermi, Φ_Unadsorbed, and Φ_adsorbed represent the vacuum energy level, Fermi energy level, the work function of the substrate, and the work function for molecular adsorption on the substrate, respectively. The results are shown in Fig. 6 and 7.

Fig. 6 Work function of four gases before and after adsorption on MoS₂, O@MoS₂, Se@MoS₂, Te@MoS₂, B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂, and Si@MoS₂ substrates, respectively.

Fig. 7 The percentage change in work function of four gases after adsorption on MoS₂, O@MoS₂, Se@MoS₂, Te@MoS₂, B@MoS₂, C@MoS₂, N@MoS₂, P@MoS₂, and Si@MoS₂ substrates, respectively.

Although the work function of the material increases slightly with the substitution of B, C, N, P and Si atoms, it remains lower than that of h-BN (5.986 eV) and Ti₃C₂O₂ MXene (5.97 eV).^77,78 It is noteworthy that when the four gases adsorb onto the pristine MoS₂ surface, ΔΦ% ranges from 0.17% to 0.64%, indicating that monolayer MoS₂ exhibits low response to these gases. Similar observations were made in the case of the SO₂F₂-chalcogen substituted systems, where ΔΦ% ranged from 0.03% to 0.29%. The response of the other three adsorbed gases in the chalcogen substitution system increased slightly, with ΔΦ% ranging from 1.16% to 4.53%. In non-chalcogen-substituted systems, particularly in the cases of Si@MoS₂ and B@MoS₂, the ΔΦ% range of was found to be 4.74% to 10.85% and 2.91% to 5.71%, respectively. This demonstrates that the substrate exhibits a high response to all four of these gases. While the C@MoS₂, N@MoS₂ and P@MoS₂ substrates cannot maintain high response for all four gases simultaneously, they do significantly enhance the response of one or tow of these four gases. If it were possible, combining N@MoS₂ and C@MoS₂ materials achieves concurrent high-response of all four gases.

In summary, the theoretical analysis of work function variation and gas–solid adsorption interactions suggests that B@MoS₂ and Si@MoS₂ substrates both have high potential for responding to SF₆ decomposition gases. Therefore, they can be considered promising candidate sensing materials for all four target gases. Furthermore, combining N@MoS₂ and C@MoS₂ substrates enhances the theoretical response potential towards the four gases even further, suggesting a feasible strategy for optimising the sensing performance of MoS₂-based materials.

However, the boundaries between theoretical predictions and practical sensing performance must be clarified, as must the limitations of the current computational model. This is necessary in order to contextualize the significance of these results. Although work function modulation indicates the potential sensing response, it differs from actual sensitivity, which depends on various factors, such as surface coverage, doping concentrations and humidity. The current model makes idealized assumptions (uniform doping, single-gas saturated adsorption and ignoring humidity), which deviate from reality. Future experimental work in this field should therefore consider controllable synthesis, testing in humid conditions, and optimizing the model with realistic parameters.

4. Conclusion

In the present study, DFT calculations were utilized to systematically explore the adsorption characteristics of NM@MoS₂ in relation to four SF₆ decomposition gases (H₂S, SO₂, SOF₂, SO₂F₂). The aim was to enhance the gas-detection efficacy of MoS₂. The key findings are summarized below:

(1) According to the formation energy calculations for NM@MoS₂, the results suggest that substitution at the S site is more feasible than substitution at the Mo site, as the former exhibits a lower (more negative) formation energy.

(2) Pristine MoS₂ shows weak physisorption for the target gases, with low adsorption energies (−0.21 to −0.33 eV), large adsorption distances (2.90 to 3.70 Å) and minimal charge transfer, which limits its sensing efficiency.

(3) The substitution systems involving chalcogen elements (O, Se and Te) retain the original surface characteristics of MoS₂ and have no significant impact on the adsorption behavior of SF₆ decomposition products. In contrast, substitution systems involving non-chalcogen elements (B, C, N, P and Si) exhibit distinct adsorption properties, indicating that surface modification significantly alters gas adsorption behaviour.

(4) Recovery time analysis indicates that temperature control can optimize reusability. In particular, P@MoS₂ and Si@MoS₂ exhibit moderate recovery times for SO₂ and SO₂F₂ at ambient temperature, demonstrating their potential for repeated use in gas sensing.

(5) Work functional analysis confirms that NM atomic substitution significantly enhances the response capability of the intrinsic MoS₂ surface's response to SF₆ decomposition gas. The B@MoS₂ and Si@MoS₂ surfaces demonstrate a potential gas-sensing response to these gases.

Overall, non-metal doping is a viable strategy for enhancing the gas-sensing response of MoS₂ towards SF₆ decomposition products. This study provides valuable theoretical insights for designing high-sensitivity, reusable gas sensors for monitoring the safety of GIS operation.

Author contributions

Mamutjan Tursun: conceptualization, data curation, writing – original draft, Funding acquisition; Yifan Liu: data curation, formal analysis; Abulimiti Yumaier: data curation, formal analysis.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and as its supplementary information (SI). Supplementary information: structure of NM atoms replacing Mo sites in monolayer MoS₂; formation energies of NM atoms substitution at Mo sites in monolayer MoS₂; comparison of adsorption energies for SF₆ decomposition gases; references. See DOI: https://doi.org/10.1039/d5ra08522e.

Acknowledgements

We thank the financial support from the “Tianchi Talented young Doctors Program of Xinjiang Uygur Autonomous Region”, “Fundamental Research Grants for Universities in the Autonomous Region (Grant No. XJEDU2024P114)”, “Tianshan Innovation Team Plan of Xinjiang Uygur Autonomous Region (2023D14002)”, and the “Research Initiation Fund for High-level Talents at Kashi University (Grant No. GCC2023ZK-008)”.

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