Publication and dataset for "First-Principles Investigation of Gas Adsorption on Bilayer Transition Metal Dichalcogenides for Sensing Toxic Gases"

Results in Physics 70 (2025) 108183 Available online 28 February 2025 2211-3797/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). First-principles investigation of gas adsorption on bilayer transition metal dichalcogenides for sensing toxic gases Jemal Yimer Damte * , Hassan Ataalite Department of Physics and NTIS - European Centre of Excellence, University of West Bohemia in Pilsen, Univerzitni 8, 30100 Plzen, Czech Republic ARTICLE INFO Keywords: Transition metal dichalcogenides Bilayer Sensor Toxic gases ABSTRACT Transition metal dichalcogenides (TMDs) have shown significant promise in gas sensing applications due to their high catalytic activity and unique electronic properties, which facilitate effective interactions with various gas molecules. This makes them ideal candidates for high-performance gas sensors. In this study, we investigated the sensing properties of nitrogen-containing gases (NCGs) on several heterostructures—namely, MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 —using density functional theory calculations. The results indicate that NH 3 and NO X exhibit weak electronic interactions with MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, while strong electronic interactions are observed with MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Electron transport properties were further assessed using Non-Equilibrium Green’s Function calculations, revealing promising gas sensing characteristics for NH 3 detection across all heterostructures and particularly effective NO X detection with MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures. These findings highlight the potential of MoS 2 /WTe 2 and MoTe 2 / WS 2 as sensitive and selective gas sensors for both NH 3 and NO X , providing valuable insights for developing advanced gas-sensing technologies with diverse practical applications. Introduction Today, the world faces critical challenges from air pollution and the greenhouse effect, which lead to significant public health issues and environmental degradation [1–3]. According to the World Health Organization, millions of people die each year due to exposure to toxic gases in the air, which damages both the circulatory and respiratory systems [4–7]. Among the primary pollutants in the environment are nitrogen-containing gases, such as nitrogen oxides (NO X ) and ammonia (NH 3 ), which are primarily emitted by the petrochemical industry and vehicles. Nitrogen oxides, including nitric oxide and nitrogen dioxide (NO X ), are highly toxic and have severe impacts on human and animal metabolic processes. Even at low concentrations, NO X can damage the human respiratory system, potentially leading to various diseases. Additionally, NH 3 reacts rapidly with other gases, posing further risks by irritating the respiratory system, depositing in the lungs, and, in extreme cases, leading to loss of life [8–13]. Given these dangers, detecting and capturing harmful gases is essential for safeguarding human health and the environment. This urgent need has driven our efforts to design new sensing materials capable of detecting toxic gases. Gas sensors, which are indispensable for monitoring hazardous air pollutants, have recently gained attention for their applications in human health monitoring, environmental pollution assessment, military and public safety, wearable devices, and smart farming [14–19]. Developing high-performance gas sensors requires devices that maintain high stability, sensitivity, and selectivity. As a result, recent studies have focused on novel sensing materials, with metal oxide sensors receiving significant attention due to their low cost, small particle size, and ease of production. However, metal oxide sensors are limited by specific working conditions and stability, which restricts their sensitivity [20–26]. Consequently, it is essential to explore gas sensing materials that operate at room temperature, with high sensitivity, a large surface-to-volume ratio, and a strong binding force for gas adsorption. In this regard, two-dimensional materials have become an area of intense study, given their large surface-to-volume ratios, high mobility of surface charge carriers, and atomic-thin layered structures that make them ideal for high-performance sensors [21,27–29]. Among these materials, transition metal dichalcogenides (TMDs) offer excellent physical, chemical, and mechanical properties suitable for nanoelectronic devices. Due to their tunable structures, numerous reactive sites, and high surface-to-volume ratios, TMDs are promising candidates for sensing devices [30–35]. * Corresponding author. E-mail address: [email protected] (J.Y. Damte). Contents lists available at ScienceDirect Results in Physics journal homepage: www.elsevier.com/locate/rinp https://doi.org/10.1016/j.rinp.2025.108183 Received 20 November 2024; Received in revised form 20 February 2025; Accepted 27 February 2025 Results in Physics 70 (2025) 108183 2 Both theoretical and experimental studies have demonstrated that TMDs are excellent sensors for toxic gases, [36–38]. The sensing mechanism of TMDs is based on charge transfer during gas adsorption. Recent research has focused on monolayer MoS 2 -based sensors, which can detect NO 2 at sub-ppb levels (20 ppb) and NH 3 at 1 ppm [39–42]. Investigations by Babar et al. on monolayer and few-layer MoS 2 and WS 2 have revealed these materials’ sensitivity and stability in detecting nitrogen-containing gases and carbon monoxide. Their studies show that MoS 2 and WS 2 bilayers and heterobilayers exhibit excellent gas-sensing performance [20,22]. In our previous studies, we investigated various heterostructures and developed a single device incorporating a tribopiezoelectric nanogenerator [43]. This nanogenerator not only enhanced power output but also provided a stable power source, which is valuable for sensor applications requiring consistent power. In this study, we used selected heterostructures from previous work to investigate their toxic gas sensing capabilities. We examined adsorption energies for all gases and electronic properties, including electron density differences, Bader charge analysis, and density of states. Additionally, we evaluated the sensitivity of the selected structures by analyzing their current–voltage characteristics before and after gas adsorption. Computational details In our study, we examined various heterostructures of MX 2 materials, where M represents elements such as Mo and W, and X includes S, Te, and O. Specifically, we investigated the heterostructures MoS 2 / WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2. All density functional theory calculations were performed using the SIESTA 4.1.5 package [44,45]. We utilized Troullier–Martins-type norm-conserving pseudopotentials in our calculations [46]. The core radii for each atom were used as specified in the pseudopotential parameters without modification. Ir (s3.14, p-4.40, d-2.30, f-3.51 a 0 ), Mo (s1.06, p-1.35, d-2.17, f2.62 a 0 ), O (s–0.77, p-1.77 a 0 ), S (s1.35, p-2.04 a 0 ), Ti (s–0.91, p-0.95, d-0.89, f-2.99 a 0 ), Te (s–2.96, p-2.75, d-3.66 a 0 ), W (s3.39, p-2.64, d2.58, f-2.49 a 0 ). Based on previous simulations of similar heterostructures [47,48], we selected the van der Waals energy functional parameterized by Lee et al. [49]. As 2D heterostuctures are primarily held by dispersion forces, the van der Waals density functional(vdWDF2) exchange functional has been considered into the calculations [50,51]. We employed a double-zeta basis set with polarization functions (DZP) for our calculations. Following benchmark calculations, the electronic properties and geometry optimizations were investigated using the Monkhorst–Pack grid method, with a k-point sampling of 11 x 11 x 1 and the kinetic energy cutoff was set to 700 eV to ensure computational accuracy and efficiency. A 3 ×3 supercell was used for adsorption calculations and atomic positions were relaxed using a conjugate gradient algorithm until force and energy tolerances were below 0.01 eV/Å and 1x10 −5 eV, respectively. A vacuum-slab was utilized along the c-axis to prevent interactions among periodic images. The lattice mismatch between distinct TMD layers in the heterostructures is addressed by introducing a small amount of strain to align the lattice parameters of the layers. This approach ensures that the two layers match in periodicity, enabling the construction of a commensurate supercell suitable for computational simulations. The applied strain is carefully controlled to remain minimal, typically within a few percent, to prevent significant distortions or artifacts in the structural and electronic properties of the heterostructure. Structural relaxation is then performed to allow the system to adjust naturally and achieve a stable, low-energy configuration, ensuring the accuracy of subsequent calculations. The adsorption energy (E ads ) of gas molecules on TMD bilayers was calculated using the following formula: Eads =ETMD/gas −ETMD −Egas (1) Here, E TMD/gas represents the total energy of the system with gas adsorption, E TMD is the energy of TMD bilayers, and Egas is the energy of gas molecules. For investigating the electronic transport properties of the sensing systems, we calculated the current–voltage (I-V) characteristics using the TranSIESTA code, which employs the Non-Equilibrium Green’s Function approach [52,53]. Gold was used as the electrode material, with the scattering region and semi-infinite left and right electrodes forming part of the device. Both electrodes were connected to the scattering region. The transport electric current was calculated using the Landauer-Büttiker formula which is described as follows [54]. 2e ℏ∫+∞ −∞ T(E,Vb)[fL(E− μ L) − fR(E− μ R)]dE=I(Vb)(2) Here, f L and f R represent the Fermi–Dirac distribution functions of the left and right electrodes, respectively; e is the electron charge; I(V b ) denotes the electric current under the applied bias voltage V b ; μ L and μ R are the chemical potentials of the left and right electrodes, respectively; and ℏ is the reduced Planck’s constant. The chemical potentials of the two electrodes shift up and down relative to the Fermi energy E F due to the applied bias voltage. μ L(R)=EF±eVb 2(3) T(E,V b ) is the transmission coefficient, describing the probability of electron transport at a given energy E under the applied bias V b . T(E,Vb) = Tr[ΓL(E,Vb)G(E,Vb)ΓR(E,Vb)G*(E,Vb) ] (4) The retarded and advanced Green’s functions are represented by G(E,Vb)and G*(E,Vb), respectively. Γ L and Γ R denote the contact broadening functions associated with the left and right electrodes, respectively, describing the coupling between the transport region and the electrodes [55]. During the transport calculations, the k-point sampling was set to 13 ×1 ×1 along the transport direction to ensure accurate results. We have finally calculated the current flowing at constant bias condition using the post-processing tool TBTrans which is included in the TranSIESTA package [52]. In our transport property simulations, we did not include a buffer layer. However, the setup and parameters were carefully chosen to ensure accurate results. The system was designed to minimize artificial interactions between the transport region and the leads, providing reliable insights into the electronic transport properties. Results and discussion Adsorption of NCGs on different heterostructures Various heterostructures of MX 2 materials have been explored in our previous work [43]. The M elements include Mo, W, Cr, Ir, Ni, Pt, Ru, and Ti, while the X elements are Se, S, Te, and O. We considered twelve heterostructures: MoTe 2 /WTe 2 , MoS 2 /WS 2 , MoS 2 /NiS 2 , MoS 2 /RuS 2 , MoS 2 /PtS 2 , WS 2 /NiS 2 , MoTe 2 /WSe 2 , WS 2 /CrS 2 , MoTe 2 /WS 2 , MoS 2 / TiO 2 , MoS 2 /IrO 2 , and MoS 2 /WTe 2 . To perform an initial selection of the most promising systems as TENG (Triboelectric Nanogenerator) elements, we focused on models exhibiting the largest average charge differences between the two TMD layers. Notably, significant charge differences were observed in the systems MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /IrO 2 , and MoS 2 /TiO 2 . Since a large charge separation enhances triboelectric activity, we prioritized these systems for further analysis. These selected heterostructures were subsequently utilized in gas sensing applications, where their distinct properties were evaluated for potential performance in detecting various gas molecules. In this study, nitrogen-containing gases were optimized on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, leveraging the favorable adsorption sites identified in prior studies [56,57]. We have considered the top layer of a bilayer for adsorption of gas molecules. However, we J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 3 explored different surface terminations, including sulfur, tellurium, and oxygen, to interact with various gas molecules. These gas molecules adsorbed on all surface terminations, contributing to the gas sensing and adsorption properties. The optimized structures for nitrogen-containing gases are shown in Figs. 1-4, and the adsorption energies and bond distances are detailed in Table 1. For NH 3 , physisorption is observed in both the MoTe 2 /WS 2 and MoS 2 /WTe 2 heterostructures. However, in the MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, NH 3 adsorption energies are −0.68 eV and −0.54 eV, respectively. The adsorption behaviour of gas molecules on the heterostructures varies with the surface terminations. When the surface terminations are sulphur or tellurium, the adsorption energy of gas molecules is lower compared to oxygenterminated surfaces. This is because oxygen is more electronegative than sulphur and tellurium, leading to stronger interactions with the surface. This strong electron-withdrawing capability enhances the interaction between the gas molecules and the oxygen-terminated surface, leading to higher adsorption energy. On the other hand, sulphur and tellurium, being less electronegative, form weaker interactions with the gas molecules, resulting in lower adsorption energies. These variations in adsorption behaviour are critical for applications like gas sensing, as the strength of the interaction influences the sensitivity and selectivity of the heterostructures for detecting specific gas molecules. The calculated adsorption distances between NH 3 and the MoS 2 / WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures are 3.04 Å, 2.85 Å, 2.41 Å, and 3.11 Å, respectively. Additionally, the adsorption energy of NO 2 was calculated, showing that NO 2 adsorption on the MoS 2 /TiO 2 heterostructure has the highest adsorption energy of −1.67 eV. The adsorption energies of NO 2 on the MoS 2 /WTe 2 , MoTe 2 / WS 2 , and MoS 2 /IrO 2 heterostructures are −1.02 eV, −0.78 eV, and −1.13 eV, respectively. The calculated adsorption distances between NO 2 and the heterostructures are 2.80 Å, 2.70 Å, 2.33 Å, and 2.13 Å for MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 , respectively. In terms of NO adsorption, the molecule is physisorbed on the MoTe 2 /WS 2 heterostructure but chemisorbed on the MoS 2 /WTe 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. The adsorption energies are −0.34 eV, −2.27 eV, and −1.92 eV for MoS 2 /WTe 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 , respectively. The calculated adsorption distances between NO and the MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures are provided in Table 1. The shorter distances between NO and the MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures indicate stronger interactions. In summary, the adsorption energy of nitrogencontaining gases on the MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures is higher compared to that on the MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, suggesting stronger interactions with these materials. Electronic property analysis To investigate the sensing mechanism of nitrogen-containing gases (NCGs) on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, we analyzed electronic properties, including Bader charge analysis, electron density difference and projected density of states. Bader charge analysis The Bader analysis method was employed to obtain the charges of nitrogen containing gases adsorbed on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 / TiO 2 and MoS 2 /IrO 2 heterostructures. The charge of each atom in the heterostructures was determined using the Bader charge analysis method [58], which calculates the charge density within each Bader volume. Each Bader volume represents the charge associated with a specific atom. The total charge was obtained by summing over all atoms in the system, both before and after the adsorption of gas molecules. According to this definition, positive values of indicate a loss of electrons, while negative values imply a gain of electrons. The calculated Bader charge values of NH 3 in MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures are 0.02, 0.07, 0.19 and 0.26 electron, respectively. This confirms that NH 3 acts as an electron donor in all heterostructures, and the charge transfer to the sensing materials is consistent with the adsorption energy. We observed that NO 2 and NO act as electron acceptors in MoS 2 / WTe 2 and MoTe 2 /WS 2 heterostructures, while they serve as electron donors in MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Additionally, the results indicate an inconsistency between the adsorption energy of NO 2 and the charge transfer in MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Specifically, the charge transfer of NO 2 is minimal compared to that of NO in these heterostructures (see Table 1). The lower adsorption energy of NO 2 and the longer bond length between the adsorbate and the Fig. 1. Optimized adsorption structures of a) NH 3 , b) NO 2 and c) NO on MoS 2 /WTe 2 heterostructure. White (H), Deep blue (N), Red (O), Light blue (W), Orange (Te), Green (Mo) and Yellow (S). J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 4 surface observed on MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, compared to NO, indicate weak interactions on these surfaces. Additionally, the oxygen in NO 2 points towards the surface, creating repulsion with the oxygen atoms on the surface, which could inhibit charge transfer of NO 2 on the surface. Electron density difference and projected density of states analysis We have further studied electron density difference plots and projected density of states in order to study the orbital interactions of NCGs and the heterostructures. The electron density difference is calculated as follows: ρ diff = ρ Total − ρ heterostructure − ρ molecule (5) where, ρ Total is the electron density distribution of gas molecule adsorbed on the heterostructure, ρ heterostructure represents the electron density for the heterostructure and ρ molecule is the electron density of gas molecule. We have generated plots using a range of isosurface values to find a balance between clarity and detail. Lower isosurface values are used to highlight subtle density differences and capture delicate redistributions of electrons. This approach ensures transparency, allowing readers to accurately assess the sensitivity of the analysis and interpret the visualizations effectively. The interaction between the molecule and the MoS 2 /IrO 2 surface forms a rectangular configuration. For consistency, all heterostructures were analyzed using the same isosurface value to interpret the electron density associated with different gas molecules. However, when a lower electron density is used for the MoS 2 /IrO 2 system, the yellow and cyan curves representing the electron density distributions are far apart, as illustrated in Fig. 5d. Fig. 5 shows Fig. 2. Optimized adsorption structures of a) NH 3 , b) NO 2 and c) NO on MoTe 2 /WS 2 heterostructure. White (H), Deep blue (N), Red (O), Light blue (W), Orange (Te), Green (Mo) and Yellow (S). Fig. 3. Optimized adsorption structures of a) NH 3 , b) NO 2 and c) NO on MoS 2 /TiO 2 heterostructure. White (H), Deep blue (N), Red (O), Gray (Ti), Green (Mo) and Yellow (S). J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 5 the electron density difference plots for nitrogen-containing gases on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Regions of charge depletion are represented in cyan, while regions of electron accumulation are shown in yellow. Partial electron density depletion and accumulation were observed around NH 3 in each heterostructure. This result aligns with the Bader charge calculations, where NH 3 acts as an electron donor upon adsorption in all heterostructures. Thicker electron density accumulation is observed following NO 2 adsorption on MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, compared to MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. This accumulation indicates that NO 2 accepts electrons from MoS 2 /WTe 2 and MoTe 2 /WS 2 , consistent with Bader charge calculations showing NO 2 as an electron acceptor in these heterostructures. After NO adsorption, thicker electron density accumulation is observed on the MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, while thicker electron density depletion is seen on MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. This finding is consistent with the Bader charge calculations. Figs. 6, 7, 8, and 9 illustrate the projected density of states plots for NH 3 , NO 2 , and NO before and after adsorption on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. These projected density of states diagrams provide insights into the electronic interactions between the gas molecules and the heterostructures. After adsorption, a notable reduction in the band gap is observed in all heterostructures. This band gap narrowing indicates the formation of new electronic states near the Fermi level, which results from strong interactions between the adsorbed gas molecules and the surface atoms of the heterostructures. The interaction can involve charge transfer, hybridization of electronic orbitals, or both, depending on the specific gas molecule and surface termination. The PDOS analysis of MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures shows that Mo, Ti, and W d orbitals and S and Te p-orbitals are the main contributors to the valence band. In the MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, the Te p d-orbitals are the main contributors to the conduction band, with smaller contributions from S p-orbitals. On the other hand, S p-orbitals are the main contributors to the conduction band in both MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures with minimal contributions observed from O p-orbitals. The degree of band gap reduction varies across the different heterostructures and gases, reflecting the influence of the structural and electronic properties of each heterostructure on gas adsorption behaviour. A lower band gap after adsorption suggests enhanced conductivity, which is particularly relevant for gas-sensing applications, as it can lead to measurable changes in the material’s electrical properties in the presence of specific gas molecules. These findings highlight the strong adsorption capabilities of the heterostructures, further emphasizing their potential for applications such as gas detection and sensing. The choice of heterostructure and gas molecule plays a critical role in determining the extent of interaction and the resulting electronic changes. Current (I)–Voltage (V) characteristics To assess the sensing capabilities of the heterostructures for nitrogencontaining gas (NCG) molecules, we investigated their transmission properties. The transport characteristics of MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, both with and without adsorbed NCGs, were studied using the TranSIESTA module [52]. The transport setup was modeled with identical electrode materials (left and right), both semi-infinite, while the scattering region, including the NCG molecules, was placed between them. Current was calculated using the Landauer-Büttiker formula [54] across a bias voltage range of 0.0 to 1.0 Fig. 4. Optimized adsorption structures of a) NH 3 , b) NO 2 and c) NO on MoS 2 /IrO 2 heterostructure. White (H), Deep blue (N), Red (O), Blue black (Ir), Green (Mo) and Yellow (S). Table 1 Adsorption energy (E ads ) of NCGs, bond distance (d) between gas molecules and heterostructures and bader charge results. After the adsorption of gas molecules on the heterostructure, the distance between the closest atom of the gas molecule and the surface is measured to determine the gas molecule/bilayer distance. MoS 2 /WTe 2 E ads (eV) d (Å) Q (e) NH 3 0.02 3.04 0.02 NO 2 −1.02 2.80 −0.49 NO −0.34 2.78 −0.18 MoTe 2 /WS 2 E ads (eV) d (Å) Q (e) NH 3 0.05 2.85 0.07 NO 2 −0.78 2.70 −0.29 NO 0.03 2.60 −0.02 MoS 2 /TiO 2 E ads (eV) d (Å) Q (e) NH 3 −0.68 2.41 0.19 NO 2 −1.67 2.33 0.01 NO −2.27 2.18 0.23 MoS 2 /IrO 2 E ads (eV) d (Å) Q (e) NH 3 −0.54 3.11 0.26 NO 2 −1.13 2.13 0.02 NO −1.92 1.87 0.24 J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 6 Fig. 5. Calculated electron density difference on a) MoS 2 /WTe 2 , b) MoTe 2 /WS 2 , c) MoS 2 /TiO 2 , and d) MoS 2 /IrO 2 heterostructure. The iso surface level is 0.0005 e/Å 3 . Fig. 6. Calculated projected density of states (PDOS) of adsorption of NH 3 , NO 2 and NO on MoS 2 /WTe 2 heterostructure. The Fermi level has been set to 0 eV. Fig. 7. Calculated projected density of states (PDOS) of adsorption of NH 3 , NO 2 and NO on MoTe 2 /WS 2 heterostructure. The Fermi level has been set to 0 eV. J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 7 V. Fig. 10 shows the current–voltage (I-V) characteristics of NCGs before and after adsorption on the MoS 2 /WTe 2 heterostructure. Overall, the current responses remain small before and after NCG adsorption on all heterostructures and are generally within the same order of magnitude, except for NH 3 and NO adsorbed on the MoS 2 /IrO 2 heterostructure at a bias voltage of 1.0 V. After NH 3 , NO 2 , and NO adsorption on MoS 2 / WTe 2 , the current response rises with increasing voltage. However, for NO 2 adsorption on MoS 2 /WTe 2 , the current response stabilizes beyond 0.8 V and decreases for NO adsorption at 1.0 V. This confirms that sensitivity begins at a low bias voltage of 0.1 V, indicating low power consumption. The I-V curves for MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, with and without adsorbed gases, are presented in Fig. 10. Changes in current are minimal after NH 3 and NO adsorption on MoTe 2 / WS 2 but increase significantly with NO 2 . Additionally, current response rises with bias voltage, both with and without NCG adsorption on MoS 2 / TiO 2 , with a slight increase after 1.0 V for NH 3 and NO adsorption. Fig. 10 shows that NO 2 adsorption on MoS 2 /IrO 2 leads to minor current increases compared to NH 3 and NO adsorption. Calculated sensitivity for NH 3 is generally lower than for NO and NO 2 in MoS 2 /WTe 2 , MoTe 2 /WS 2 and MoS 2 /TiO 2 but is higher than for NO 2 in MoS 2 /IrO 2 . Consequently, our results indicate that MoS 2 /WTe 2 , MoTe 2 /WS 2 and MoS 2 /TiO 2 heterostructures are more sensitive to NO X gases. These calculations reveal that MoS 2 /WTe 2 shows greater sensitivity to NO than to NH 3 and NO 2 , indicating a high selectivity for NO under the given conditions. Similarly, NO 2 demonstrates significant sensitivity in MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, while NO shows higher sensitivity in MoS 2 / IrO 2 structure. Overall, the findings indicate that NOx molecules have greater sensitivity in MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 / IrO 2 at lower applied bias voltages, contributing to improved selectivity for NO and NO 2 in these heterostructures. Variations in the I-V curves are observed in the MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 / IrO 2 bilayers, which are perturbed by the presence of different gas molecules. Specifically, the presence of NH 3 decreases the current flow in all four bilayer systems. This reduction is attributed to the NH 3 molecule transferring a significant number of electrons to the MoS 2 / WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 bilayers surfaces, which inhibits free electron mobility by creating scattering centers, thereby decreasing the overall current flow in the device. In contrast, the presence of NO 2 and NO increases the current flow on the MoS 2 /WTe 2 and MoTe 2 /WS 2 surfaces. This increase is attributed to the NO 2 and NO molecules accepting electrons from the MoS 2 /WTe 2 and MoTe 2 /WS 2 surfaces, which promotes free electron mobility through the scattering centers, thereby enhancing the overall current flow in the device. These insights are valuable for developing gas sensors with enhanced selectivity and practical applications. Band gap and recovery time The relationship between band gap and electrical conductivity is an important factor in understanding the gas sensing properties of heterostructures. In our study, we investigated how the band gap influences the gas sensing properties of MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures for detecting nitrogen-containing gases (NCGs). The relationship between band gap and electrical conductivity is described as follows [59,60]: σ ∝exp(−Eg 2kT)(6) where σ , k, E g and T represents the electrical conductivity, the Boltzmann constant, the band gap, and temperature, respectively. The Fig. 8. Calculated projected density of states (PDOS) of adsorption of NH 3 , NO 2 and NO on MoS 2 /TiO 2 heterostructure. The Fermi level has been set to 0 eV. Fig. 9. Calculated projected density of states (PDOS) of adsorption of NH 3 , NO 2 and NO on MoS 2 /IrO 2 heterostructure. The Fermi level has been set to 0 eV. Fig. 10. Current-Voltage curves for the adsorption of NCGs on bilayers (MoS 2 / WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 ). J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 8 calculated bandgap values for the pristine heterostructures MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 , and MoS 2 /IrO 2 are 1.81 eV, 1.93 eV, 2.62 eV, and 2.83 eV, respectively. Upon adsorption of gas molecules, the band gap decreases across all heterostructures, indicating a change in electrical conductivity due to the alteration of their electronic properties. Additionally, we calculated the recovery times of these heterostructures (MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 , and MoS 2 /IrO 2 ) at various temperatures. The results show that NH 3 exhibits the shortest recovery times for all heterostructures. Furthermore, NO 2 and NO molecules have shorter recovery times on MoS 2 /WTe 2 , MoTe 2 /WS 2 compared to their recovery times on MoS 2 /IrO 2 , and MoS 2 /TiO 2 . This trend highlights a relationship between adsorption energy and recovery time: lower adsorption energy corresponds to shorter recovery times, while higher adsorption energy leads to longer recovery times. Moreover, the recovery time increases with rising temperature, reflecting the thermally activated nature of the desorption process. The observed changes in band gap before and after the adsorption of nitrogen-containing gases (NCGs) on the various heterostructures (MoS 2 /WTe 2 and MoTe 2 /WS 2 ) are crucial indicators of their sensitivity as gas sensors (Fig. S1 and S2). As shown in Figs. S1 and S2, the band gap decreases after the adsorption of gas molecules on the heterostructures. However, the changes in the band gap between different gas molecules are relatively small, and their respective band gap curves appear to overlap. Despite this apparent overlap, there are still subtle differences between the band gaps corresponding to each gas molecule. These differences, although small, indicate variations in the interaction strength and the extent of charge transfer or orbital hybridization between the gas molecules and the heterostructure surfaces. The overlapping trends suggest that the electronic structures of the heterostructures are modified in a similar manner upon gas adsorption, but the specific electronic states introduced or shifted depend on the type of gas molecule. Such distinctions, even if minor, can play a critical role in applications like gas sensing, where sensitivity relies on detecting these subtle changes. A significant change in band gap implies a corresponding alteration in the conductivity of the material. In this context, the observed band gap variations suggest that the conductivity of the heterostructures can be enhanced upon exposure to NCGs. This is a critical factor in determining the suitability of these materials for gas sensing applications. Evaluating the recovery time is a crucial aspect of assessing the performance of gas sensors. It provides valuable information about the sensor’s ability to return to its initial state after being exposed to a specific gas. A shorter recovery time indicates a faster response and regeneration of the sensor, which is desirable for practical applications. The recovery time ( τ ) can be calculated as follows: τ = ν −1exp(−Eads kT )(7) where k, T and ν −1 , E ads is Boltzmann constant, temperature, the attempted frequency of the molecules and adsorption energy, respectively. It’s valuable to know that the recovery times are shorter at room temperature in MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures. Additionally, the observation of shorter recovery times for NH 3 in MoS 2 /IrO 2 and MoS 2 /TiO 2 heterostructures further highlights their potential for efficient desorption and sensor reusability. This means that NH 3 molecules have a tendency to desorb or detach from the surface of the heterostructures more quickly than NO 2 and NO molecules, indicating faster recovery times. It suggests that the gas sensors utilizing MoS 2 / WTe 2 and MoTe 2 /WS 2 bilayers have the potential to be efficiently recycled. As shown in Table 2, as the temperature increases the recovery times increases. This information is significant for practical applications, especially in gas sensing or catalytic processes. It’s worth noting that these findings highlight the potential for these heterostructures in environmental and industrial applications where the detection and removal of NH 3 , NO 2 , and NO gases are important. Conclusions We conducted first-principles calculations to investigate the sensing properties of toxic gases, namely NH 3 , NO 2 , and NO, on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Our findings indicate that NH 3 physisorbs onto MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, with adsorption energies of −0.68 eV and −0.54 eV for MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures, respectively. The adsorption energies of NO 2 and NO are higher in MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Bader charge analysis reveals that the charge transfer between nitrogen-containing gases (NCGs) and the heterostructures is less than 0.5 electrons. Specifically, NO 2 and NO act as electron acceptors in MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures, while they act as electron donors in MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures. Further electronic interactions were explored using electron density difference and projected density of states calculations, which showed substantial electron donation from MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures to NO and NO 2 . Additionally, electron transport calculations demonstrated minimal changes in current response before and after NCG adsorption on the heterostructures. The simulated current–voltage (I-V) curves of the NO X molecules on MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures revealed that NH 3 induced only small changes in current response, while NO and NO 2 led to significant current response changes within a specific bias range. This suggests that the heterostructures exhibit varying sensitivities to different gases, which is valuable for potential gas sensing applications. Our study identifies specific heterostructures (MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 ) with excellent potential for NH 3 detection, considering factors like adsorption energy, electronic property changes, and recovery times. Moreover, MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures are also promising candidates for NO 2 and NO detection. Generally, all results demonstrate that the recovery time for NOx sensing in MoS 2 /WTe 2 and MoTe 2 /WS 2 heterostructures is sufficient for recycling gas sensors at room temperature. However, we observed that the recovery time for NOx molecules in MoS 2 /TiO 2 and MoS 2 /IrO 2 is longer compared to MoS 2 /WTe 2 and MoTe 2 /WS 2 . Our results show that MoS 2 /WTe 2 and MoTe 2 /WS 2 exhibit lower recovery times for NOx molecules. Furthermore, the calculation of the I/V characteristics reveals that MoS 2 /TiO 2 and MoS 2 /IrO 2 have higher current flow with increasing voltage, making them unsuitable for achieving stable charge separation. These findings indicate that the most promising heterostructures for building sensor devices are MoS 2 /WTe 2 and MoTe 2 /WS 2 . The insights gained from this study offer valuable information for the development of sensitive and reliable gas sensors. The specificity and sensitivity of these heterostructures to different toxic gases could have significant implications for real-world applications. Table 2 Recovery time ( τ in second) of NCGs in MoS 2 /WTe 2 , MoTe 2 /WS 2 , MoS 2 /TiO 2 and MoS 2 /IrO 2 heterostructures at 300 K, 400 K and 500 K. MoS 2 /WTe 2 τ (300) τ (400) τ (500) band gap NH 3 1.06 ×10 −12 1.05 ×10 −12 1.04 ×10 −12 0.76 NO 2 1.47 ×10 5 7.53 2.00 ×10 −2 0.82 NO 7.14 ×10 −7 2.45 ×10 −8 3.26 ×10 −9 0.71 MoTe 2 /WS 2 τ (300) τ (400) τ (500) band gap NH 3 2.34 ×10 −12 1.89 ×10 −12 1.66 ×10 −12 1.53 NO 2 13.4 7.00 ×10 −3 7.50 ×10 −5 1.40 NO 1.06 ×10 −12 1.04 ×10 −12 1.03 ×10 −12 1.47 MoS 2 /TiO 2 τ (300) τ (400) τ (500) band gap NH 3 0.27 3.81 ×10 −4 7.34 ×10 −6 1.51 NO 2 1.28 ×10 16 1.20 ×10 9 7.30 ×10 4 1.43 NO 1.61 ×10 26 4.51 ×10 16 8.44 ×10 12 1.31 MoS 2 /IrO 2 τ (300) τ (400) τ (500) band gap NH 3 1.23 ×10 −3 6.51 ×10 −6 2.84 ×10 −7 1.82 NO 2 1.05 ×10 7 182 0.256 2.11 NO 2.06 ×10 20 1.73 ×10 12 2.46 ×10 7 1.91 J.Y. Damte and H. Ataalite Results in Physics 70 (2025) 108183 9 CRediT authorship contribution statement Jemal Yimer Damte: Writing – review & editing, Writing – original draft, Supervision, Methodology, Investigation, Formal analysis. Hassan Ataalite: Writing – review & editing, Validation. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgment This work was supported by the project Quantum materials for applications in sustainable technologies (QM4ST), funded as project No. CZ.02.01.01/00/22_008/0004572 by Programme Johannes Amos Comenius, call Excellent Research. Computational resources were provided by the e-INFRA CZ project (ID:90254), supported by the Ministry of Education, Youth and Sports of the Czech Republic. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.rinp.2025.108183. Data availability Data will be made available on request. References [1] Tang X, Du A, Kou L. Gas sensing and capturing based on two-dimensional layered materials: Overview from theoretical perspective. 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