Abstract
In the current scenario, nitrogen-based gases such as nitric oxide (NO), nitrogen dioxide (NO2), and ammonia (NH3) are bringing about serious threats to human health and the ecosystems, even at minimal concentrations, making the purification of these gases vital for environmental safety. In this investigation, density functional theory (DFT) calculations were carried out to examine the gas sensing properties and adsorption behavior of ScSTe, TiSTe, and ZrSTe nanosheets towards the NO, NO2, and NH3 gas molecules. All the nanosheets exhibit metallic behavior, with the dominant contribution arising from the p orbitals in ScSTe, and from the d orbitals in TiSTe and ZrSTe. To identify the most stable gas adsorption site, two sites (S site and Te site) on the nanosheets were considered. Our adsorption energy calculations indicate weak physisorption across all the nanosheets. The adsorption energy ranges for NO, NO2, and NH3 gas molecules are from − 0.080 eV to − 0.406 eV for ScSTe, − 0.144 eV to − 0.287 eV for TiSTe, and − 0.103 eV to − 0.354 eV for ZrSTe. Among all nanosheets, the S site exhibits better adsorption energies with NO and NH3, and the Te site for NO2 gas molecules. During the interaction, the gas molecules generally donate charge to the nanosheets, except for NO2, which tends to withdraw charge due to the presence of two electronegative oxygen atoms. A low recovery time was observed for all gas–nanosheet interactions, which is favorable for gas sensing applications. Among the materials, ScSTe shows stronger interaction with NO and NH3, while ZrSTe is more responsive to NO2.
Keywords: DFT, Nitrogen-based gas, TMDs, DOS, Recovery time
Subject terms: Chemistry, Environmental sciences, Materials science, Nanoscience and technology, Physics
Introduction
Air pollution is a major environmental threat in the modern era, impacting nearly every organ system and contributing to millions of premature deaths. Long-term exposure to polluted air is linked to asthma, bronchitis, cardiovascular disease, premature births, and weakened immunity1–3. Air pollutants such as nitrogen oxides (NO, NO2) and ammonia (NH3) also damage the environment. NO2 is one of the major contributors to acid rain, also NO and NO2 are precursors of tropospheric ozone and smog formation4,5. NH3 is another critical pollutant because it reacts with acidic species in the atmosphere to form fine particulate matter (PM 2.5), which reduces visibility and worsens air quality6,7. These gases are particularly concerning in urban and biomass-burning regions. In these areas, they accumulate and pose serious risks8. Since air pollutants are often colorless and odorless, their presence usually goes unnoticed without proper monitoring systems. Therefore, the detection and control of these hazardous gases have become a pressing concern9. Gas sensors are crucial in this context, providing the necessary sensitivity, response speed, selectivity, and low energy consumption required for accurate and timely monitoring.
Over the last few decades, two-dimensional (2D) materials have emerged as promising candidates for gas sensing applications. Graphene was the first 2D material to gain global attention, but many others including MoS210, WS211, and phosphorene have also shown great potential. Among these, transition metal dichalcogenides (TMDs) form a particularly fascinating group of materials12–14. More recently, researchers have focused on Janus TMDs, which have the general formula MXY, where M represents a transition metal atom and X, Y are different chalcogen atoms15,16. Unlike conventional TMDs, Janus TMDs are structurally asymmetric because the two sides of the metal layer are occupied by different chalcogen atoms17–19. This asymmetry gives them unique properties, including an intrinsic dipole moment, enhanced charge redistribution, and greater chemical reactivity20,21. These features significantly improve their interaction with gas molecules, making Janus TMDs especially suitable for advanced gas sensing applications22. Recently, Density functional theory (DFT) has been widely used to explore the surface interaction characteristics of these materials, consistently showing favorable results.
In 2022, Gajjir et al. demonstrated through DFT analysis that Janus MoSSe monolayers exhibit strong gas–surface interaction dynamics with NH3 at 300 K, making them effective room-temperature sensors with site-specific detection capability21. Deobrat et al. explored the gas detection performance of Janus NbSeTe monolayers for NO and NO2, revealing significant charge transfers of − 0.12e and − 0.53e, respectively. Both gases exhibited ultra-fast recovery times, with 8.07 µs for NO and 75.74 µs for NO223. The superior gas interaction and detection performance of Te-defective Janus WSTe monolayers for detecting O2, NO, and CO were investigated by Dou et al. They found, O2 binds most strongly, followed by NO and then CO, with Te-defective WSTe, and shows remarkable electron transfer24. Moreover, Donarelli et al. (2018) proved that 2D materials like MoSe2, MoS2, WS2, and WSe2 possess high surface-to-volume ratios and tunable electronic properties, making them promising candidates for room-temperature gas detection25. Zhang et al. reported a significantly enhanced sensing response of approximately 200% for NO2 when mixed with other gases, compared to the performance of an unmodified intrinsic WSe2 device26. Using the DFT method, Kong et al. revealed that the NO2 molecule exhibits excellent sensing performance on 2D diboron dinitride, supported by a notable adsorption energy of − 2.06 eV27.
In this study, we investigated the adsorption and gas sensing properties of NO, NO2, and NH3 on Janus MSTe (M = Sc, Ti, Zr) nanosheets based on the DFT method. To find out the gas–surface interaction of gas molecules on nanosheets, we calculated the adsorption energy, adsorption distance, and charge transfer. For more details, we also calculated band structure, density of states (DOS), charge difference density (CDD), electron density, recovery time, and work function. However, this study focuses only on the adsorption behavior of gases in pristine nanosheet structures, considers temperature effects in a limited way, and lacks experimental validation to confirm the theoretical results.
Computational details
To examine the interaction between gas molecules (NO, NO2, NH3) and nanosheets (ScSTe, TiSTe, ZrSTe), DFT calculations were executed using the Dmol3 module in BIOVIA Materials Studio (version 2017) software available at (https://www.3ds.com/products/biovia). To optimize the nanosheets and the gas molecule complexes, the Generalized Gradient Approximation (GGA) was employed rather than the Local Density Approximation (LDA)28. The Perdew–Burke–Ernzerhof (PBE) method was designed to capture the exchange and correlation contributions in electron systems29. The Tkatchenko–Scheffler (TS) dispersion correction was used to improve the description of long-range Van der Waals interactions30,31. A 3 × 3 × 1 supercell consisting of 27 atoms and a vacuum layer of 20 Å was used in this investigation. To ensure accurate and efficient modeling of nanosheets, the global cut-off radius and k-point grid were set to 5.4 Å and 4 × 4 × 1, respectively, for all calculations. During the optimization process, the convergence thresholds were set to 1 × 10−5 Ha for energy, 0.002 Ha/Å for the maximum force, and 0.005 Å for the maximum atomic displacement. These values were chosen based on convergence tests, where the total energy was found to stabilize at a minimum, and further tightening of the parameters resulted in negligible changes. To examine the interaction between gas molecules and the nanosheet, the adsorption energy was determined by32,
 |
1 |
In this formula,
represents the combined energy of the nanosheet and the absorbed gas molecules,
and 
represent the individual energies of the nanosheet and the gas molecules before the adsorption process33. To analyze the charge exchange between the gas molecules and nanosheets, the net charge transfer (Qt) was obtained using Mulliken and Hirshfeld charge analysis. Qt was determined by34,
 |
2 |
where, the net charge of the gas molecule after adsorption is donated by 
and 
represents its charge in the isolated state.
To effectively analyze the charge transfer mechanism, charge density difference (CDD) maps were computed using the CASTEP code with ultrasoft pseudopotentials and the formula is given by35.
 |
3 |
ρtotal, ρnanosheet, and ρgas correspond to the charge densities of the gas-adsorbed nanosheet, the pristine nanosheet, and the isolated gas molecules, respectively.
Results and discussions
Geometry and structural stability
In this investigation, we designed a 3 × 3 × 1 supercell structure featuring three Janus TMDs, such as ScSTe, TiSTe, and ZrSTe, containing 27 atoms each, where 9 S atoms, 9 Te atoms, and 9 (Sc, Ti, and Zr) atoms, respectively. The ScSTe monolayer is composed of three distinct atom layers, Sc, S, and Te, where the Scandium layer remains at the middle of the Sulfur and Tellurium layers, displayed in Fig. 1a. TiSTe monolayer contains a three-layer structure where the Titanium layer remains at the middle of the S and Te layers, displayed in Fig. 1b. Similarly, ZrSTe also contains three layers where the Zirconium layer remains at the middle of the S and Te layers, as shown in Fig. 1c.
Fig. 1.
Top and side perspectives of the optimized TMDs structures: (a) ScSTe, (b) TiSTe, (c) ZrSTe.
Transition metal atoms like Sc, Ti, and Zr are linked by covalent bonds with three adjoining S atoms and three adjoining Te atoms. The structure showed the bonding of Sc atoms with S and Te atoms at a distance of 2.438 Å and 3.052 Å, and S―Sc―Te bond angle is 97.2˚, which aligns with previously reported data of Yang et al. for ScSTe nanosheets36 For TiSTe nanosheet, the calculated Ti―S and Ti―Te bond lengths are 2.407 Å and 2.820 Å, and also S―Ti―Te bond angle is 93.3˚. Besides in ZrSTe nanosheet, Zr―S and Zr―Te bond lengths are 2.522Å and 2.925Å and the S―Zr―Te bond angle is 97.9˚closely similar to the values reported by Dasadis et al. who found, Zr―S and Zr―Te bond lengths are 3.02Å and 2.52Å and the S―Zr―Te bond angle is 100˚37. The chemical environment of each atom results in slight variations in bond lengths. To evaluate the structural stability of the nanosheets, we calculated their cohesive energies, which were − 4.44 eV for ScSTe, − 4.61 eV for TiSTe, and − 5.02 eV for ZrSTe. The determination of cohesive energy provides critical insight into the intrinsic bond strength and thermodynamic stability of crystalline materials. This parameter serves as a fundamental indicator of the robustness of interatomic interactions within the crystal lattice. Naseri et al. studied the cohesive energy of nanostructures and their values are − 4.23 eV, − 3.70 eV, and − 4.84 eV for SiS2, SiSe2, and Be2C nanosheets, which exhibit notable similarity to our work38. Cohesive energy calculations indicate ZrSTe stands out as the most stable compound, succeeded by TiSTe and then ScSTe. We examined the optimized structures of NO, NO2, and NH3 gas molecules as represented in Fig. 2. NO has a linear structure with the N–O bond length is 1.66 Å. For NO2 and NH3 molecules, the NO2 molecule has an N–O bond length of 0.75 Å and the NH3 molecule has N―H bond length of 1.02 Å, respectively. Besides, the computed covalent bond angle of O―N―O is 133.2˚ from NO2 and covalent bond angle of H―N―H is 106.1˚ from NH3. Structurally, NO2 is V-shaped and NH3 is trigonal pyramidal.
Fig. 2.
Optimized structure of (a) NO, (b) NO2, and (c) NH3 gas molecules.
The electronic properties of ScSTe, TiSTe, and ZrSTe nanosheets were analyzed through partial density of states (PDOS) and band structure calculations, as visualized in Fig. 3. The DOS spectrum revealed that the p-orbitals primarily from Te and S atoms dominated near the Fermi level. This indicates that these chalcogen elements play a central role in governing the electronic behavior of the nanosheets. This p-orbital dominance suggests strong covalent interactions and hybridization with the transition metal d-states, particularly in the valence band region. The band structure analysis revealed that the valence and conduction bands overlapped with the Fermi level, indicating that there was no band gap and the material behaved like a metal. In ScSTe, the energy bands cross the Fermi level near the F and K points, which meant electrons could move freely through the material. TiSTe showed stronger band overlap, particularly between the Γ and K points, and beyond the K point toward higher energy levels, where the bands were more widely dispersed. This suggests better electron mobility and stronger metallic conductivity. ZrSTe also showed bands crossing the Fermi level near the F and K points, but with slightly flatter shapes, indicating a mix of mobile and more localized electrons. Overall, the lack of a band gap and the consistent band crossings in all three materials confirm their metallic behavior.
Fig. 3.
Partial DOS (up row) and band structure (down row) of (a) ScSTe, (b) TiSTe, and (c) ZrSTe nanosheets.
NO, NO2 and NH3 adsorption on ScSTe
To assess the adsorption characteristics of NO, NO2, and NH3 on the ScSTe nanosheet, key parameters such as adsorption energy (Eads.), closest adsorption distance (dmin.), and charge transfer (Qt) were computed and tabulated in Table 1. In examining the interaction of these gas molecules with the optimized ScSTe nanosheet, two prominent adsorption sites parallel to the nanosheet were considered. These were the S site and the Te site shown in Fig. 4, where two adsorption configurations were also examined on ScSTe for each gas. Also, the stability of the structure after adsorption was calculated. The negative adsorption values indicate the attractive interaction between the gas molecules and the nanosheets also signify the exothermic reaction for the adsorption process. And when the adsorption energy is positive, it reflects an endothermic adsorption reaction39. Negative values of Eads. are preferable for our research. When Eads. is less than 0.6 eV, it indicates physisorption, and above 0.8 eV, it suggests chemisorption40.
Table 1.
The adsorption energy (
, minimum adsorption distance (
) and charge transfer (Qt) between gases and ScSTe at S and Te sites.
| Gas | Adsorption site |
|
 (Å) |
 (Mulliken) |
 (Hirshfeld) |
|---|---|---|---|---|---|
| NO | S | − 0.405 | 2.548 | 0.287 | 0.254 |
| Te | − 0.228 | 2.766 | 0.076 | 0.070 | |
| NO2 | S | − 0.080 | 3.118 | 0.075 | 0.057 |
| Te | − 0.132 | 3.22 | − 0.124 | − 0.137 | |
| NH3 | S | − 0.248 | 2.811 | 0.153 | 0.038 |
| Te | − 0.201 | 3.784 | 0.041 | 0.015 |
Fig. 4.
Top and side views of NO, NO2, and NH3 adsorbed on ScSTe where NO on (a) S-site, (b) Te-site; NO2 on (c) S-site, (d) Te-site; NH2 on (e) S-site, and (f) Te-site.
The calculated amounts of Eads. values were − 0.405, -0.080, and − 0.248 eV for NO, NO2, and NH3 gas molecules adsorption on ScSTe nanosheet (S site) at a distance of 2.548, 3.118, and 2.811 Å, respectively. On the other side, the Eads. values of ScSTe (Te site) respectively were − 0.228, -0.132, and − 0.201 eV for NO, NO2, and NH3 gas molecules at a distance 2.766, 3.220, and 3.784 Å. Raya. et al. examined NO and NH3 adsorption properties and sensing capabilities of ZrTe2 and HfSe2, and their adsorption energy value were − 0.94 eV and − 0.39 eV for ZrTe2; on the other hand, − 0.20 eV and − 0.19 eV for HfSe2 nanosheets41. Hu et al. observed an adsorption energy of − 0.562, − 0.403, and − 0.292 eV for NO, NO2, and NH3, respectively on PtSTe42. Both studies are nearly similar to our calculated values. The adsorption values indicate that S sites on ScSTe are more preferable than Te sites. All the values are less than 0.6 eV, so gas molecules show a physisorption mechanism on S sites. Qt was calculated using Eq. (2), with all results summarized in Table 1. Before adsorption, gas molecules usually carry no net charge; however, their net charge becomes non-zero after adsorption on nanosheets. A positive net charge value indicates that electron transfer has occurred from the gas molecules to the surface, confirming that the nanosheet exhibits electron-accepting behavior. If Qt is negative, the adsorbed surface donates the electron to gas molecules, and gas molecules become electron acceptors.
Mulliken and Hirshfeld charge analysis revealed a minor electron transfer of 0.124 e and 0.137 e, respectively, from the Te site to NO2, whereas the remaining Qt values indicated the nanosheet’s consistent role as an electron acceptor. To gain insight into the electronic interactions between the gas molecules and the surfaces, we evaluated the CDD of the proposed materials. By analyzing CDD, we can track where electrons concentrate or vacate due to adsorption events. Figure 5 shows the CDD map, where red highlights denote increased electron density and blue marks signify electron loss. Such interaction highlights the substrate’s role in transferring electrons to the adsorbed species, resulting in notable charge exchange. Electron depletion from the substrate, as reflected by negative charge transfer values, signifies its positive charge consistent with findings from the CDD maps.
Fig. 5.
CCD maps of (a) NH3 adsorbed on ScSTe (S site), (b) NO2 adsorbed on TiSTe (Te site), and (c) NO2 adsorbed on ZrSTe (Te site).
NO, NO2, and NH3 adsorption on TiSTe
Just as in previous observations, the adsorption energies for the gas molecules on TiSTe nanosheets were also assessed, as shown in Fig. 6, and the values are mentioned in Table 2. Upon studying the nanosheet, it was clearly observed that TiSTe also showed nearly the same adsorption as ScSTe. To understand the interaction more precisely, the gas molecules were.
Fig. 6.
Top and side views of NO, NO2, and NH3 adsorbed on TiSTe where NO on (a) S-site, (b) Te-site; NO2 on (c) S-site, (d) Te-site; NH3 on (e) S-site, and (f) Te-site.
Table 2.
The adsorption energy (
, minimum adsorption distance (
) and charge transfer (Qt) between gases and TiSTe at S and Te sites.
| Gas | Adsorption site |
|
 (Å) |
(Mulliken) |
(Hirshfeld) |
|---|---|---|---|---|---|
| NO | S | − 0.169 | 3.363 | 0.176 | 0.152 |
| Te | − 0.167 | 3.392 | 0.042 | 0.026 | |
| NO2 | S | − 0.144 | 3.026 | − 0.091 | − 0.1 |
| Te | − 0.286 | 3.195 | − 0.225 | − 0.227 | |
| NH3 | S | − 0.248 | 3.08 | 0.076 | − 0.021 |
| Te | − 0.184 | 3.294 | 0.041 | − 0.052 |
analyzed at two distinct sites on the TiSTe surface, one near S and the other near Te. Adsorption energy values indicate the strength of their interaction. At the S site, NO, NO2, and NH3 exhibited energies of − 0.169, − 0.144, and − 0.248 eV, while at the Te site, the values were stronger at − 0.167, − 0.286, and − 0.184 eV, indicating that the gases bind more strongly to the Te site. The minimum distance between each gas molecule and the TiSTe surface was different for each case. At the S site, the distances were 3.363, 3.026, and 3.08 Å for NO, NO2, and NH3. At the Te site, the gases are 3.392, 3.195, and 3.294 Å away, showing that the closeness depends on where the gas attaches. These variations in both energy and distance highlight the importance of site-specific interactions and gas type in determining adsorption behavior. TiSTe exhibits comparable adsorption energies for all three gas molecules, reflecting the trends Eads. (NO2) > Eads. (NH3) > Eads. (NO). Zhou et al. reported the adsorption process and evaluated the sensing efficiency of NO, NO2, and NH3 on Janus WS2, and their adsorption energy values are − 0.206 eV, − 0.41 eV, and − 0.216 eV, respectively43. Similarly, Quan et al. reported that TaS2 absorbs both NH3 (− 0.22 eV) and NO2 (− 0.39 eV) gas molecules44. These studies are consistent with our calculated values. The results of this study showed that the Eads. values for gas adsorption on TiSTe were consistent with physisorption.
From the viewpoint of Mulliken and Hirshfeld charge analysis, it was observed that the charge transfer for NO was higher than that for NO2 and NH3 molecules. Hence, a strong interaction was observed between TiSTe and NO molecules relative to other gases, with charge transfers of 0.176 e (S site) and 0.042 e (Te site) from NO to TiSTe, suggesting that TiSTe behaves as an electron acceptor. For NO2 gas molecules, both Mulliken and Hirshfeld charge analyses gave a negative value of charge transfer, indicating gas molecules are electron acceptors. And for NH3 gas molecule, Mulliken charge analysis showed a positive value of charge transfer, which means TiSTe is an electron acceptor, and a negative value was shown by Hirshfeld charge analysis, which means gas molecules are electron acceptor and TiSTe is an electron donor.
NO, NO2, and NH3 adsorption on ZrSTe
To investigate the adsorption capacity of NO. NO2 and NH3 gas molecules on another nanosheet ZrSTe, we calculated adsorption energy and net charge transfer by using Eqs. (1) and (2), respectively, and listed in Table 3. Following the adsorption of gas molecules, the structural configurations of the ZrSTe nanosheet (top and side views) are shown in Fig. 7. The different adsorption sites on the nanosheet and the different orientations for gas molecules were studied to get the most stable complex. We investigated the adsorption characteristics of ZrSTe had changed in comparison to those of the TiSTe nanosheet. The adsorption energies of NO, NO2, and NH3 gas molecules on the S site of the ZrSTe nanosheet are − 0.174, − 0.206, and − 0.262 eV, respectively. On the Te site, the corresponding adsorption energies were − 0.103, − 0.354, and − 0.185 eV. The gas molecules were positioned at distances of 2.601, 2.461, and 3.028 Å from the S site, and 2.864, 3.237, and 3.225 Å from the Te site, respectively. Lu et al. observed an adsorption energy of − 0.182 eV for NO2 and − 0.02 eV for NH3 on Graphene45, which closely matches our findings. Similarly, Cui et al. demonstrated that Rh- and Pd-dispersed WSTe can be used as effective materials for capturing the toxic NH3 gas46. In our findings, the adsorption of NO2 is greater than the value of the adsorption energy of NO and NH3.
Table 3.
The adsorption energy (
, minimum adsorption distance (
) and charge transfer (Qt) between gases and ZrSTe at S and Te sites.
| Gas | Adsorption site | Eads. (eV) | dmin. (Å) | Qt (e) (Mulliken) | Qt (e) (Hirshfeld) |
|---|---|---|---|---|---|
| NO | S | − 0.174 | 2.601 | 0.159 | 0.138 |
| Te | − 0.103 | 2.864 | 0.014 | 0.015 | |
| NO2 | S | − 0.206 | 2.461 | − 0.074 | − 0.089 |
| Te | − 0.354 | 3.237 | − 0.268 | − 0.263 | |
| NH3 | S | − 0.262 | 3.028 | 0.078 | − 0.026 |
| Te | − 0.185 | 3.225 | 0.047 | − 0.051 |
Fig. 7.
Top and side views of gas NO, NO2, and NH3 adsorbed on ZrSTe where NO on (a) S-site, (b) Te-site; NO2 on (c) S-site, (d) Te-site; NH3 on (e) S-site, and (f) Te-site.
According to Mulliken and Hirshfeld charge analysis, only a small amount of charge transfer occured between the NO, NO2, and NH3 gas molecules and the ZrSTe nanosheet, as shown in Table 3. According to Mulliken charge analysis, a minor charge transfer (− 0.074 e) occurred from the S site of the nanosheet to the NO2 molecule, whereas the remaining Qt values suggest that the nanosheet functions as an electron acceptor. For Hirshfeld charge analysis, we noticed that the charge transfer value was negative for NO2 and NH3 gas molecules to the absorbed surface, which indicates that gas molecules are electron acceptors and the nanosheets are donors. In case of NO gas to be absorbed surface, the charge transfer was positive, which means the adsorbed surface is an electron acceptor.
Recovery time
To allow the sensor to be used again, the device should exhibit a quick recovery time47,48. In this context, the recovery time (τ) is defined as the time taken for the desorption of gas molecules from the nanosheet surface. The recovery time was calculated using the following Eqs30,31. ,
 |
4 |
In the above expression, 
indicates the attempt frequency, kB is the Boltzmann constant, and T signifies temperature.
As shown by the equation, τ depends on both Eads and T, which means that lower adsorption energy results in a shorter recovery time, allowing the gas molecules to desorb more quickly. Experimental findings revealed that sensor recovery rates can be markedly enhanced through exposure to either an electric field or vacuum ultraviolet (UV) irradiation, under tested frequencies of 1012, 3 × 1014, and 1015 s−1, and temperatures of 300, 400, and 500 K49,50. Our results show that all the nanosheets have a very fast recovery time since the adsorption energy lies within the physisorption range. The longest recovery time (5.2 × 10− 6 s) was observed when NO was adsorbed on the S site of the ScSTe surface, due to its relatively high adsorption energy (− 0.405 eV). This indicates that NO has strong adsorption stability on ScSTe, while still allowing quick desorption.
Work function
The analysis of the work function (φ) is considered essential for sensing materials to use that material as a sensing device. The work function of Janus TMD material plays a significant role in gas sensing by affecting the adsorption energy and charge transfer process.
The work function change (Δφ) upon gas adsorption was evaluated by51,
 |
5 |
Here, 
refers to the nanosheet’s work function before adsorption, while 
indicates the value after adsorption.
The observed work function values are tabulated in Table 4, and the corresponding variations after gas adsorption are visually depicted in the bar diagram shown in Fig. 8. In this investigation, we determined work function for ScSTe, TiSTe, and ZrSTe nanosheets, 5.96, 5.32, and 5.19, respectively. After adsorption of gas molecules, the negative value of work function change indicates a decrease in work function; on the other hand, a positive change indicates an increase in work function. We found that NO adsorption created only negative change, whereas NO2 and NH3 produced both negative and positive change. The highest value of negative change was − 6.54% for NO adsorption on ScSTe. the greater value of positive change was 5.97% for NO2 adsorption on ZrSTe.
Table 4.
The calculated work function (φ) in eV and change in work function, were computed for the nanosheets before and after gas adsorption.
| Nanosheets | Adsorption site | Before adsorption φ | After adsorption | |||||
|---|---|---|---|---|---|---|---|---|
| NO | NO2 | NH3 | ||||||
| φ | Δ φ | φ | Δ φ | φ | Δ φ | |||
| ScSTe | S | 5.96 | 5.57 | -6.54 | 5.84 | -2.01 | 5.94 | -0.34 |
| ScSTe | Te | 5.96 | 5.80 | -2.68 | 6.08 | 2.01 | 5.74 | -3.69 |
| TiSTe | S | 5.32 | 5.12 | -3.75 | 5.44 | 2.26 | 5.43 | 2.07 |
| TiSTe | Te | 5.32 | 5.26 | -1.13 | 5.59 | 5.08 | 5.47 | 2.74 |
| ZrSTe | S | 5.19 | 5.01 | -3.46 | 5.34 | 2.89 | 5.30 | 2.12 |
| ZrSTe | Te | 5.19 | 5.16 | -0.57 | 5.50 | 5.97 | 5.34 | 2.89 |
Fig. 8.
Bar diagram showing work function shifts in nanosheets pre- and post-gas adsorption.
We observed only a negative change in work function for NO adsorption means that for all cases, it leads to a decrease in work function. In case of NO2, its work function sometimes increases, such as during adsorption on ScSTe nanosheets, while for the other two nanosheets, ZrSTe and TiSTe, the work function decreases. For NH3 gas molecules, we observed a negative change on ScSTe nanosheets and a positive change on TiSTe and ZrSTe nanosheets, respectively. Therefore in case of ScSTe nanosheet, the work function has been reduced after interaction with the gases which indicates the more sensitivity of the nanosheet.
Electronic properties
We calculated DOS, and band structure of the nanosheets to examine the changes in electronic properties of the nanosheets. Basically, band structure provides information about how electron behave in materials. It also helps us to see changes of nanosheets after adsorption, like when gas molecules are adsorbed on a nanosheet band gap may decrease or increase52. Band structure analysis makes it easier to understand how strongly a material interacts with gas molecules. In our study, our band structure analysis revealed that the conduction band intersects the Fermi level (EF), confirming the metallic nature of the nanosheets. The calculated band structures of NO, NO2, and NH3 gas adsorbed on nanosheets are displayed in Fig. 9. These calculations were performed along the high-symmetry path Γ–F–K–Γ in reciprocal space, corresponding to the coordinates Γ (0, 0, 0), F (0, 0.5, 0), K (− 0.333, 0.667, 0), and back to Γ (0, 0, 0). After the adsorption of the gas molecules, the conductivity increased, which means the systems became more metallic.
Fig. 9.
The band structures after interaction with (a) NO on ScSTe, (b) NO2 on TiSTe, and (c) NH3 on ZrSTe.
We investigated the DOS of ScSTe, TiSTe, and ZrSTe nanosheets before and after adsorption of NO, NO2, and NH3 gas molecules on both S and Te sites shown in Fig. 10. The DOS spectrum reveals that gas adsorption notably modifies the electronic properties of nanosheets. The dashed line at zero energy denotes the Fermi level (EF), which serves as a reference for evaluating electronic changes. Upon interaction with gas molecules, distinct changes occurred near the Fermi level. These included the emergence of new states and shifts in existing peaks, indicating strong coupling and charge redistribution. Among the three nanosheets, ScSTe showed the most pronounced response for NO and NO2, with a notable increase in DOS intensity around the Fermi level, highlighting its superior sensitivity. The red curves highlight the contribution of the adsorbed gas molecules. These spectral variations confirm that gas adsorption influences the nanosheets conductivity, making them promising candidates for sensing applications.
Fig. 10.
The total and partial DOS spectrum for ScSTe, TiSTe, and ZrSTe nanosheets for before and after adsorption of NO, NO2, and NH3 on S sites (a), (c), (e); and on Te sites (b), (d), (f), respectively.
We also investigated the electron density to better understand the electronic properties of nanosheets. The electron density reveals charge transfer, which is a key factor in gas detection. By comparing electron density maps before and after adsorption, we can observe where electrons are gained or lost. Also, the formation of chemical bonds between nanosheets and gas molecules can be visualized through electron density maps. If electron density overlaps between the gas and nanosheet, it indicates strong chemisorption; if there is no overlap, that suggests weak density interactions. In this study, electron density maps showed the overlap between the gas molecules and the nanosheets, as displayed in Fig. 11. This indicates ScSTe, TiSTe, and ZrSTe nanosheets are suitable for gas sensor material.
Fig. 11.
Electron density maps of the complexes.
Conclusions
In this study, we investigated the adsorption behavior of three nitrogen-containing gas molecules on Janus ScSTe, TiSTe, and ZrSTe nanosheets by using DFT calculations. All the nanosheets exhibit metallic behavior, with the dominant contribution arising from the p orbitals in ScSTe, and from the d orbitals in TiSTe and ZrSTe. Negative cohesive energies have been found for the three nanosheets, which confirm the good thermodynamic stability. Among the nanosheets, ZrSTe shows higher stability than ScSTe and TiSTe. To identify the most stable gas adsorption site, two sites (S site and Te site) on the nanosheets were considered. Our adsorption energy calculations indicate physisorption across all the nanosheets. Among all nanosheets, the S site exhibits better adsorption energies with NO and NH3, and the Te site for NO2 gas molecules. The adsorption energy ranges for NO, NO2, and NH3 gas molecules are from − 0.080 eV to − 0.406 eV for ScSTe, − 0.144 eV to − 0.287 eV for TiSTe, and − 0.103 eV to − 0.354 eV for ZrSTe. During the interaction, the gas molecules generally donate charge to the nanosheets, except for NO2, which tends to withdraw charge due to the presence of two electronegative oxygen atoms. After the adsorption of gas molecules on ScSTe, a prominent peak emerges at the Fermi level in the DOS spectra, accompanied by a reduction in the work function for all three gases. Additionally, all gas–nanosheet interactions exhibit short recovery times, which is advantageous for gas-sensing applications. Based on adsorption energy calculations, ScSTe demonstrates stronger interactions with NO and NH3 at the S site, whereas ZrSTe shows higher responsiveness to NO2 at the Te site. Therefore, ScSTe and ZrSTe can be considered highly sensitive gas sensor for these three gases.
Acknowledgements
We thankfully acknowledge the Bangladesh Research and Education Network (BdREN) for their computational access.
Author contributions
Md.J.I.S.: Investigation, formal analysis, data curation, writing-original draft. B.K.D.: Validation, writing-review and editing. A.R.: Validation, review and editing. A.A.P.: Methodology, writing-review and editing. S.U.D.S.: Supervision, conceptualization, software, writing-review and editing.
Funding
The authors declare that no funds, grants, or other support were received for the research.
Data availability
All data supporting the findings of this study, including optimized structures, adsorption energy values, charge transfer, work function, etc., are available within the article. All the calculations have been implemented by the DMol³ module in BIOVIA Materials Studio 2017. Additional data can be provided by the corresponding author upon reasonable request.
Declarations
Competing interests
The authors declare no competing interests.
Statement of usage of artificial intelligence
Artificial intelligence (AI) tools, specifically ChatGPT developed by OpenAI, were used solely for improving the grammar, language, and clarity of the manuscript. The authors ensured that the scientific content, interpretations, and conclusions were entirely their own and not influenced by the AI tool.
Footnotes
Publisher’s note
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Associated Data
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Data Availability Statement
All data supporting the findings of this study, including optimized structures, adsorption energy values, charge transfer, work function, etc., are available within the article. All the calculations have been implemented by the DMol³ module in BIOVIA Materials Studio 2017. Additional data can be provided by the corresponding author upon reasonable request.