Gas-Sensing Properties of B/N-Modified SnS2 Monolayer to Greenhouse Gases (NH3, Cl2, and C2H2)

The adsorption capacity of intrinsic SnS2 to NH3, Cl2 and C2H2 is very weak. However, non-metallic elements B and N have strong chemical activity, which can significantly improve the conductivity and gas sensitivity of SnS2. Based on density functional theory, SnS2 was modified with B and N atoms to analyze its adsorption mechanism and gas sensitivity for NH3, Cl2 and C2H2 gases. The optimal structure, adsorption energy, state density and frontier molecular orbital theory are analyzed, and the results are in good agreement with the experimental results. The results show that the adsorption of gas molecules is exothermic and spontaneous. Only the adsorption of NH3 and Cl2 on B-SnS2 belongs to chemical adsorption, whereas other gas adsorption systems belong to physical adsorption. Moderate adsorption distance, large adsorption energy, charge transfer and frontier molecular orbital analysis show that gas adsorption leads to the change of the conductivity of the modified SnS2 system. The adsorption capacity of B-SnS2 to these gases is Cl2 > NH3 > C2H2. The adsorption capacity of N-SnS2 is NH3 > C2H2 > Cl2. Therefore, according to different conductivity changes, B-SnS2 and N-SnS2 materials can be developed for greenhouse gas detection of gas sensors.


Introduction
With the continuous progress of society, the traditional agricultural production mode has been unable to meet the needs of modern civilization, which prompts the development of greenhouse planting [1][2][3]. It can be applied in the plateau, deep mountains, deserts, and other unique environments for agricultural production [4,5]. The illuminance, temperature, humidity, and gas composition are the critical environmental parameters that affect planting growth [6]. In the actual cultivation process, the illuminance, temperature, and humidity can be easily regulated by changing the ceiling coverage and ventilation rate [7,8]. It is urgent to accurately monitor the greenhouse's characteristic gas composition in the greenhouse online. Due to the half-open structure of the greenhouse, the accumulated gases are mainly NH 3 , Cl 2 , and C 2 H 2 [9,10].
Two-dimensional SnS 2 is widely used in the gas sensor industry because of its large specific surface area and pore structure [11]. Compared with carbon nanotubes, SnS 2 is more resistant to oxidation and more stable at high temperatures, making SnS 2 more suitable for gas-sensing detection than carbon nanotubes [12]. It has become one of the most promising materials used in high-temperature and high-pressure environments [13]. However, pristine SnS 2 has a limited reaction to gases, such as C 2 H 4 , C 2 H 2 , and NH 3 [14]. Studies showed that non-metals modification could improve the gas detection accuracy and adsorption capacity of gas-sensing materials by regulating their energy gap and conductivity upon gas adsorption [15]. B and N are the most widely used modified non-

Computational Details
All calculations were carried out based on DFT [23,25,26]. The SnS 2 crystal plane is modeled with a 4 × 4 × 1 supercell [27][28][29][30]. To prevent the interaction from repeating planes along the z-axis direction, a vacuum layer of 25 Å was set between the planes [31,32]. The electron exchange and correlation energy were treated with the generalized gradient approximation (GGA) and the Perdew-Burke-Ernzerhof (PBE) basis [30]. A double numerical plus polarization (DNP) basis set was used [27]. The ionic convergence criterion for the total energy and maximum force were set as 1 × 10 −5 Ha, and 2 × 10 −3 Ha/Å, respectively [33,34], and the electronic self-consistent field tolerance was 1 × 10 −6 Ha [30]. The Brillouin zone was sampled with a 5×5×1 Monkhorst-Pack mesh of k-points [33]. All calculations are performed under 0 K; the adsorption performance under room temperature is directly related to the results under 0 K. The adsorption energy (E ads ) of the molecule adsorbed on the SnS 2 surface was calculated by E ads = E slab/gas − E slab − E gas . E slab/gas is the total energy of the adsorption system; E slab and E gas are the energy of the SnS 2 surface and gas molecules of greenhouse gases, respectively [34]. A negative value of E ads means the adsorption process is exothermic and happens spontaneously [35]. The electron density distribution was calculated by Mulliken population analysis [36]. The charge transfer Q in the adsorption process was obtained by Q = Q ads − Q iso . Q iso and Q ads are the total charges of isolated gas and adsorbed gas molecules, respectively [37]. Q > 0 means electrons transfer from the gas molecules to the surface of SnS 2 . According to frontier molecular orbital theory, the energy gap represents the difference between the highest occupied orbital (HOMO) and the lowest occupied orbital (LUMO) [38]. The energy gap between HOMO and LUMO was defined by E g = |E HOMO − E LUMO |. The smaller the energy gap is, the more efficiently the reaction is excited.

Geometry Optimization
To obtain the adsorption characteristics of B-SnS 2 and N-SnS 2 to the greenhouse gases, the structures of the gas molecules and SnS 2 surface were initially optimized. The structures of NH 3 , Cl 2 , and C 2 H 2 gas molecules are established as shown in Figure 1a-c. The bond length of Sn-S in SnS 2 is 2.611 Å. C 2 H 2 gas molecule is a two-dimensional planar structure with only 1.211 Å C-C bond length and 1.071 Å C-H bond length. NH 3 gas molecule is a regular tetrahedral structure: all of the N-H bond lengths are 1.023 Å, and the bond angles are 105.350 • . Cl 2 has a bond length of 2.024 Å. The two most stable modification structures of B and N modification on the SnS 2 surface are obtained, respectively, as shown in Figure 1e,f. Based on the Mulliken population, B and N atoms as electron acceptors, 0.176 e electrons and 0.65 e electrons are obtained from SnS 2 . This redistribution of charge leads to a change in the system's conductivity. It can be seen that the modification distance is 1.843 Å and 1.527 Å, respectively. From the bonding distance and charge transfer, both B and N atoms have built a stable structure on the SnS 2 surface, which provides a foundation for further gas adsorption.  As shown in Figure 2, the total density of states (TDOS) and partial density of states (PDOS) are analyzed to further analyze the modification mechanism of the B and N atoms on SnS2. Both B and N atom modifications make the TDOS move to the left. Therefore, after the modification of SnS2 by B and N, the electrons in the conduction band are reduced, resulting in a decrease in the conductivity of SnS2. According to the PDOS, the peaks of S-3p and B-2p overlap range from −6 eV to −4 eV, and at the 1 eV for the B-SnS2 system. On the other hand, the peaks of S-3p and N-2p hybridize around −5 eV, −4 eV, −2.5 eV, 0 eV, and 2 eV. In general, the conductivity of the modified SnS2 systems decreases due to the strong electronegativity of the modified atoms.  As shown in Figure 2, the total density of states (TDOS) and partial density of states (PDOS) are analyzed to further analyze the modification mechanism of the B and N atoms on SnS 2 . Both B and N atom modifications make the TDOS move to the left. Therefore, after the modification of SnS 2 by B and N, the electrons in the conduction band are reduced, resulting in a decrease in the conductivity of SnS 2 . According to the PDOS, the peaks of S-3p and B-2p overlap range from −6 eV to −4 eV, and at the 1 eV for the B-SnS 2 system. On the other hand, the peaks of S-3p and N-2p hybridize around −5 eV, −4 eV, −2.5 eV, 0 eV, and 2 eV. In general, the conductivity of the modified SnS 2 systems decreases due to the strong electronegativity of the modified atoms.  As shown in Figure 2, the total density of states (TDOS) and partial density of states (PDOS) are analyzed to further analyze the modification mechanism of the B and N atoms on SnS2. Both B and N atom modifications make the TDOS move to the left. Therefore, after the modification of SnS2 by B and N, the electrons in the conduction band are reduced, resulting in a decrease in the conductivity of SnS2. According to the PDOS, the peaks of S-3p and B-2p overlap range from −6 eV to −4 eV, and at the 1 eV for the B-SnS2 system. On the other hand, the peaks of S-3p and N-2p hybridize around −5 eV, −4 eV, −2.5 eV, 0 eV, and 2 eV. In general, the conductivity of the modified SnS2 systems decreases due to the strong electronegativity of the modified atoms.   As shown in Figure 3, after B and N modification on SnS 2 , HOMO is mainly distributed on B and N, indicating that B and N atoms provide electrons as electron donors and are active sites that can provide adsorption sites for NH 3 , Cl 2 , and C 2 H 2 gases. Moreover, the energy gap increases significantly after modification as listed in Table 1, making the system's conductivity significantly decrease; therefore, the measurement system's conductivity change is more pronounced. The results obtained by the frontier molecular orbital theory are consistent with those obtained by the density of state analysis. Figure 3, after B and N modification on SnS2, HOMO is mainly distributed on B and N, indicating that B and N atoms provide electrons as electron donors and are active sites that can provide adsorption sites for NH3, Cl2, and C2H2 gases. Moreover, the energy gap increases significantly after modification as listed in Table 1, making the system's conductivity significantly decrease; therefore, the measurement system's conductivity change is more pronounced. The results obtained by the frontier molecular orbital theory are consistent with those obtained by the density of state analysis. Table 1. Energy of HOMO, LUMO, and energy gap of B-SnS2 and N-SnS2.

Configuration
Structure

NH3, Cl2, and C2H2 Adsorption on B-SnS2 and N-SnS2 Surfaces
To study the adsorption properties of the three greenhouse gases on B-SnS2 and N-SnS2, NH3, Cl2, and C2H2 gases were made to approach the B-SnS2 and N-SnS2 surfaces from different positions to obtain the most stable adsorption structures. Figure 4 shows the most stable adsorption structures after gas molecules adsorption on B-SnS2. Figure 4 and Table 2 show the optimal adsorption structure of NH3, Cl2, and C2H2 gas molecules on B-SnS2. Since the B atom is in a prominent position on the SnS2 surface, it provides a better attachment point for gas adsorption, making the adsorption of SnS2 more stable. The H and N atoms in the NH3 molecule were used to approach the surface of SnS2. The results showed that the H atom approaching the B atom method acts as the most stable structure with the largest adsorption energy (−1.735 eV). When the N atom is closed to the B atom, the adsorption energy is only −0.712 eV. The greater the absolute value of the adsorption energy, the more intense the reaction is. In addition, the negative adsorption energy means that the reaction is exothermic and can be carried out spontaneously. From the microscopic point of the adsorption structure, the bending stress also causes the surface deformation of B-SnS2 to different degrees. The adsorption distance of B-SnS2 to NH3 gas is 2.055 Å. The small adsorption distance indicates that the reaction may be strong chemisorption. After gas adsorption, SnS2 has slight deformation, and the Sn-S bond is slightly elongated. After B-SnS2 adsorbs NH3 gas, 0.254 e electrons transfer from the H2S gas to B-SnS2, mainly provided by the H atom. After the Cl2 adsorption on B-SnS2, the adsorption energy is −2.204 eV, the charge transfer is −0.422 e, and the adsorption distance is 1.776 Å. The reaction is also chemical adsorption due to the large adsorption energy and shorter adsorption distance. Upon C2H2 adsorption on B-SnS2, the adsorption distance is 2.531 Å, the adsorption energy is −0.272 eV, and the charge transfer is 0.172 e. It can be deduced that the C2H2 adsorption on B-SnS2 belongs to physical adsorption.

Configuration
Structure

NH 3 , Cl 2 , and C 2 H 2 Adsorption on B-SnS 2 and N-SnS 2 Surfaces
To study the adsorption properties of the three greenhouse gases on B-SnS 2 and N-SnS 2 , NH 3 , Cl 2 , and C 2 H 2 gases were made to approach the B-SnS 2 and N-SnS 2 surfaces from different positions to obtain the most stable adsorption structures. Figure 4 shows the most stable adsorption structures after gas molecules adsorption on B-SnS 2 .    Figure 5 shows the DOS analysis diagram of B-SnS2 after adsorption of NH3, Cl2, and C2H2; the TDOS after gas adsorption moves to the right, where the dotted line represents the Fermi energy level. From Figure 5(a1,a2), it can be figured out that the TDOS has a distinct increase above the Fermi level after NH3 adsorption. It facilitates the transition of electrons from the valence band to the conduction band, resulting in an overall increase in conductivity. After NH3 adsorption, the TDOS increases range from −5 eV to −2.5 eV, −10 eV to −12.5 eV, and 2.5 eV to 3 eV, respectively, which are caused by the hybridization of H-1s and B-2p orbitals. The strong orbital hybridization and the considerable TDOS increase indicate that this reaction is chemisorption. Additionally, the adsorption structure is very stable. When the Cl2 molecule is adsorbed, the TDOS of B-SnS2/Cl2 shifts to the right as a whole, and the TDOS will increase at the energy level of 0 eV, whereas the TDOS  Table 2 show the optimal adsorption structure of NH 3 , Cl 2 , and C 2 H 2 gas molecules on B-SnS 2 . Since the B atom is in a prominent position on the SnS 2 surface, it provides a better attachment point for gas adsorption, making the adsorption of SnS 2 more stable. The H and N atoms in the NH 3 molecule were used to approach the surface of SnS 2 . The results showed that the H atom approaching the B atom method acts as the most stable structure with the largest adsorption energy (−1.735 eV). When the N atom is closed to the B atom, the adsorption energy is only −0.712 eV. The greater the absolute value of the adsorption energy, the more intense the reaction is. In addition, the negative adsorption energy means that the reaction is exothermic and can be carried out spontaneously. From the microscopic point of the adsorption structure, the bending stress also causes the surface deformation of B-SnS 2 to different degrees. The adsorption distance of B-SnS 2 to NH 3 gas is 2.055 Å. The small adsorption distance indicates that the reaction may be strong chemisorption. After gas adsorption, SnS 2 has slight deformation, and the Sn-S bond is slightly elongated. After B-SnS 2 adsorbs NH 3 gas, 0.254 e electrons transfer from the H 2 S gas to B-SnS 2 , mainly provided by the H atom. After the Cl 2 adsorption on B-SnS 2 , the adsorption energy is −2.204 eV, the charge transfer is −0.422 e, and the adsorption distance is 1.776 Å. The reaction is also chemical adsorption due to the large adsorption energy and shorter adsorption distance. Upon C 2 H 2 adsorption on B-SnS 2 , the adsorption distance is 2.531 Å, the adsorption energy is −0.272 eV, and the charge transfer is 0.172 e. It can be deduced that the C 2 H 2 adsorption on B-SnS 2 belongs to physical adsorption.   Figure 5 shows the DOS analysis diagram of B-SnS 2 after adsorption of NH 3 , Cl 2 , and C 2 H 2 ; the TDOS after gas adsorption moves to the right, where the dotted line represents the Fermi energy level. From Figure 5(a1,a2), it can be figured out that the TDOS has a distinct increase above the Fermi level after NH 3 adsorption. It facilitates the transition of electrons from the valence band to the conduction band, resulting in an overall increase in conductivity. After NH 3 adsorption, the TDOS increases range from −5 eV to −2.5 eV, −10 eV to −12.5 eV, and 2.5 eV to 3 eV, respectively, which are caused by the hybridization of H-1s and B-2p orbitals. The strong orbital hybridization and the considerable TDOS increase indicate that this reaction is chemisorption. Additionally, the adsorption structure is very stable. When the Cl 2 molecule is adsorbed, the TDOS of B-SnS 2 /Cl 2 shifts to the right as a whole, and the TDOS will increase at the energy level of 0 eV, whereas the TDOS will decrease at the energy level range of −15 eV to −12 eV and −5 eV to −2 eV. The conductivity of the surface system enhances as the DOS at the Fermi level increases. The hybridization of the B-2p orbital and Cl-3p orbital shows that the reaction is very violent. According to the analysis of the DOS diagram shown in Figure 5(c1,c2), the distribution of TDOS nearly does not change before and after C 2 H 2 adsorption.   Figure 5 shows the DOS analysis diagram of B-SnS2 after adsorption of NH3, Cl2, and C2H2; the TDOS after gas adsorption moves to the right, where the dotted line represents the Fermi energy level. From Figure 5(a1,a2), it can be figured out that the TDOS has a distinct increase above the Fermi level after NH3 adsorption. It facilitates the transition of electrons from the valence band to the conduction band, resulting in an overall increase in conductivity. After NH3 adsorption, the TDOS increases range from −5 eV to −2.5 eV, −10 eV to −12.5 eV, and 2.5 eV to 3 eV, respectively, which are caused by the hybridization of H-1s and B-2p orbitals. The strong orbital hybridization and the considerable TDOS increase indicate that this reaction is chemisorption. Additionally, the adsorption structure is very stable. When the Cl2 molecule is adsorbed, the TDOS of B-SnS2/Cl2 shifts to the right as a whole, and the TDOS will increase at the energy level of 0 eV, whereas the TDOS will decrease at the energy level range of −15 eV to −12 eV and −5 eV to −2 eV. The conductivity of the surface system enhances as the DOS at the Fermi level increases. The hybridization of the B-2p orbital and Cl-3p orbital shows that the reaction is very violent. According to the analysis of the DOS diagram shown in Figure 5(c1,c2), the distribution of TDOS nearly does not change before and after C2H2 adsorption.   Figure 6 shows the most stable structures of gas molecules on N-SnS 2 . For NH 3 adsorption in Figure 6a, the structure of NH 3 keeps intact in the adsorption process. The most stable structure for NH 3 adsorption is obtained by the H atom of NH 3 closing the N atom of N-SnS 2 , and the adsorption distance is 2.162 Å. The large adsorption distance indicates that the adsorption is physical adsorption. For Cl 2 adsorption in Figure 6b, Cl atoms approach the surface of N with a single Cl atom. The adsorption distance reaches 2.915 Å. In addition, the chemical bond in Cl 2 keeps intact in the adsorption process, only a slight elongation occurs in the Cl-Cl bond length, indicating that the adsorption is also weak physical adsorption. The adsorption structure of C 2 H 2 is shown in Figure 6c. Its adsorption characteristics are similar to NH 3 , and the adsorption distance is 2.361 Å. The structure of C 2 H 2 has not been damaged during the adsorption process.  Figure 6 shows the most stable structures of gas molecules on N-SnS2. For NH3 adsorption in Figure 6a, the structure of NH3 keeps intact in the adsorption process. The most stable structure for NH3 adsorption is obtained by the H atom of NH3 closing the N atom of N-SnS2, and the adsorption distance is 2.162 Å. The large adsorption distance indicates that the adsorption is physical adsorption. For Cl2 adsorption in Figure 6b, Cl atoms approach the surface of N with a single Cl atom. The adsorption distance reaches 2.915 Å. In addition, the chemical bond in Cl2 keeps intact in the adsorption process, only a slight elongation occurs in the Cl-Cl bond length, indicating that the adsorption is also weak physical adsorption. The adsorption structure of C2H2 is shown in Figure 6c. Its adsorption characteristics are similar to NH3, and the adsorption distance is 2.361 Å. The structure of C2H2 has not been damaged during the adsorption process. The adsorption parameters of gases adsorbed N-SnS2 systems are listed in Table 3, including adsorption distance, adsorption energy, and charge transfer. It can be seen from the table that the adsorption energy of NH3 is −0.408 eV, and negative adsorption energy means that the reaction is exothermic and spontaneous. The charge transfer is 0.147 e, indicating a 0.147 e electron transfer from NH3 to N-SnS2. The small adsorption energy, long adsorption distance, and charge transfer confirm that the adsorption is physical adsorption. The adsorption energy of Cl2 is −0.245 eV, which is the lowest among the three gas adsorption, and its charge transfer is −0.136 e. The adsorption energy of C2H2 is −0.272 eV, which is the most moderate among the three gases. In total, 0.197 e electrons have been transferred to C2H2 from N-SnS2. By comparing the adsorption of these three gases on B-SnS2 and N-SnS2, NH3 and C2H2 always give electrons to the substrate, whereas Cl2 always gains electrons. In addition, B-SnS2 has larger adsorption energy, larger charge transfer amount, and a shorter adsorption distance for the Cl2 adsorption system, indicating that the B-SnS2 monolayer has the most robust adsorption performance for Cl2 gas molecules. Based on the above analysis, it can be concluded that the modification of B enhances the adsorption activity of NH3, Cl2, and C2H2 to SnS2. The adsorption capacity of B-SnS2 to these gases is Cl2 > NH3 > C2H2.

Gas Adsorption on N-SnS2 Surface
As shown in Figure 7, TDOS and PDOS of all adsorbed gas systems were analyzed to further study the adsorption mechanism of the N-SnS2 system to the gas molecules, where dotted lines represent Fermi energy levels. TDOS and PDOS of N-SnS2 adsorption by NH3, Cl2, and C2H2 are shown in Figure 7(a1-c1) and Figure 7(a2-c2), respectively. For NH3 adsorption, the TDOS of the adsorption system increases a little near the Fermi level. The adsorption parameters of gases adsorbed N-SnS 2 systems are listed in Table 3, including adsorption distance, adsorption energy, and charge transfer. It can be seen from the table that the adsorption energy of NH 3 is −0.408 eV, and negative adsorption energy means that the reaction is exothermic and spontaneous. The charge transfer is 0.147 e, indicating a 0.147 e electron transfer from NH 3 to N-SnS 2 . The small adsorption energy, long adsorption distance, and charge transfer confirm that the adsorption is physical adsorption. The adsorption energy of Cl 2 is −0.245 eV, which is the lowest among the three gas adsorption, and its charge transfer is −0.136 e. The adsorption energy of C 2 H 2 is −0.272 eV, which is the most moderate among the three gases. In total, 0.197 e electrons have been transferred to C 2 H 2 from N-SnS 2 . Table 3. Adsorption parameters of gas molecules on N-SnS 2 . By comparing the adsorption of these three gases on B-SnS 2 and N-SnS 2 , NH 3 and C 2 H 2 always give electrons to the substrate, whereas Cl 2 always gains electrons. In addition, B-SnS 2 has larger adsorption energy, larger charge transfer amount, and a shorter adsorption distance for the Cl 2 adsorption system, indicating that the B-SnS 2 monolayer has the most robust adsorption performance for Cl 2 gas molecules. Based on the above analysis, it can be concluded that the modification of B enhances the adsorption activity of NH 3 , Cl 2 , and C 2 H 2 to SnS 2 . The adsorption capacity of B-SnS 2 to these gases is Cl 2 > NH 3 > C 2 H 2 .

System
As shown in Figure 7, TDOS and PDOS of all adsorbed gas systems were analyzed to further study the adsorption mechanism of the N-SnS 2 system to the gas molecules, where dotted lines represent Fermi energy levels. TDOS and PDOS of N-SnS 2 adsorption by NH 3 , Cl 2 , and C 2 H 2 are shown in Figure 7(a1-c1) and Figure 7(a2-c2), respectively. For NH 3 adsorption, the TDOS of the adsorption system increases a little near the Fermi level. It indicates that the conductivity of the adsorption system increases slightly. The interaction between N-2p and H-1s is fragile. After Cl 2 adsorption, the atomic orbitals are strongly hybridized between the peaks of Cl-3p and N-2p. The TDOS of C 2 H 2 nearly does not change after C 2 H 2 adsorption on N-SnS 2 , and the corresponding interatomic orbital hybridization is also faint. Only the C-2p and H-1s peaks of the adsorption system overlap with the N atomic orbitals between −5.0 eV and −10.0 eV.
It indicates that the conductivity of the adsorption system increases slightly. The interaction between N-2p and H-1s is fragile. After Cl2 adsorption, the atomic orbitals are strongly hybridized between the peaks of Cl-3p and N-2p. The TDOS of C2H2 nearly does not change after C2H2 adsorption on N-SnS2, and the corresponding interatomic orbital hybridization is also faint. Only the C-2p and H-1s peaks of the adsorption system overlap with the N atomic orbitals between −5.0 eV and −10.0 eV.

Analysis of Gas-Sensing Response
The behavior of electrons in the adsorption process was analyzed by frontier molecular orbital theory. The HOMO and LUMO were obtained after NH3, Cl2, and C2H2 gas adsorption. It helps to explore gas sensors with selectivity and sensitivity. The HOMO and LUMO distributions before and after gas adsorption on B-SnS2 are shown in Figure 8, and the energy gap values are shown in Table 4. The adsorption charge transfer of Cl2 and NH3 molecules is significant. The HOMO and LUMO distributions are improved by gas adsorption, part of HOMO and LUMO transfer to Cl2 and NH3 molecules. The specific charge numbers corresponding to the analysis of the three gases are 0.254 e, −0.422 e, and 0.172 e, respectively. The charge transfer amount during the analysis of the adsorption process mainly comes from modified B atoms. Overall, the energy gap upon Cl2 and NH3 adsorption on the surface of B-SnS2 is bigger than that of C2H2. After adsorption, the energy gap value changes from 0.019 eV (B-SnS2) to 0.024 eV (B-SnS2/NH3), 0.014 eV (B-SnS2/Cl2), and 0.019 eV (B-SnS2/C2H2), respectively. A smaller energy gap indicates that the system's conductivity improves, which is consistent with the previous DOS analysis. Table 4. Energy of HOMO, LUMO, and energy gap of B-SnS2 and adsorption systems.

Configuration
Structure

Analysis of Gas-Sensing Response
The behavior of electrons in the adsorption process was analyzed by frontier molecular orbital theory. The HOMO and LUMO were obtained after NH 3 , Cl 2 , and C 2 H 2 gas adsorption. It helps to explore gas sensors with selectivity and sensitivity. The HOMO and LUMO distributions before and after gas adsorption on B-SnS 2 are shown in Figure 8, and the energy gap values are shown in Table 4. The adsorption charge transfer of Cl 2 and NH 3 molecules is significant. The HOMO and LUMO distributions are improved by gas adsorption, part of HOMO and LUMO transfer to Cl 2 and NH 3 molecules. The specific charge numbers corresponding to the analysis of the three gases are 0.254 e, −0.422 e, and 0.172 e, respectively. The charge transfer amount during the analysis of the adsorption process mainly comes from modified B atoms. Overall, the energy gap upon Cl 2 and NH 3 adsorption on the surface of B-SnS 2 is bigger than that of C 2 H 2 . After adsorption, the energy gap value changes from 0.019 eV (B-SnS 2 ) to 0.024 eV (B-SnS 2 /NH 3 ), 0.014 eV (B-SnS 2 /Cl 2 ), and 0.019 eV (B-SnS 2 /C 2 H 2 ), respectively. A smaller energy gap indicates that the system's conductivity improves, which is consistent with the previous DOS analysis. The HOMO and LUMO distributions before and after gas adsorption on N-SnS2 are shown in Figure 9, and the energy gap values are shown in Table 5. HOMO is mainly distributed on N before N-SnS2 adsorbs gas, indicating that the N atom provides electrons as electron donors and is also the active site that provides adsorption sites for NH3, Cl2, and C2H2 gas. After adsorbing NH3, Cl2, and C2H2 gases, the HOMO becomes more con-   The HOMO and LUMO distributions before and after gas adsorption on N-SnS 2 are shown in Figure 9, and the energy gap values are shown in Table 5. HOMO is mainly distributed on N before N-SnS 2 adsorbs gas, indicating that the N atom provides electrons as electron donors and is also the active site that provides adsorption sites for NH 3 , Cl 2 , and C 2 H 2 gas. After adsorbing NH 3 , Cl 2 , and C 2 H 2 gases, the HOMO becomes more concentrated on N, and the LUMO is nearly not located on gas molecules. From the HOMO and LUMO distribution of NH 3 molecule adsorption, the E g of N-SnS 2 /NH 3 decreases to 0.023 eV. In addition, HOMO electrons are mainly located at H and N atoms, whereas LUMO electrons do not change significantly, which is consistent with the result obtained from TDOS and PDOS analysis. In contrast, the energy gap of N-SnS 2 /Cl 2 is reduced to 0.025 eV, because HOMO electrons are mainly concentrated around N atoms, indicating that the adsorption of Cl 2 molecules dramatically improves the conductivity and has better reactivity on N-SnS 2 surfaces. In the C 2 H 2 system, LUMO mainly concentrates around N atoms with long N-H bonds, reaching −0.196 eV. The increase in LUMO also reduces the energy gap of the system to 0.026 eV, resulting in a decrease in the conductivity of the system. The HOMO and LUMO distributions before and after gas adsorption on N-SnS2 are shown in Figure 9, and the energy gap values are shown in Table 5. HOMO is mainly distributed on N before N-SnS2 adsorbs gas, indicating that the N atom provides electrons as electron donors and is also the active site that provides adsorption sites for NH3, Cl2, and C2H2 gas. After adsorbing NH3, Cl2, and C2H2 gases, the HOMO becomes more concentrated on N, and the LUMO is nearly not located on gas molecules. From the HOMO and LUMO distribution of NH3 molecule adsorption, the Eg of N-SnS2/NH3 decreases to 0.023 eV. In addition, HOMO electrons are mainly located at H and N atoms, whereas LUMO electrons do not change significantly, which is consistent with the result obtained from TDOS and PDOS analysis. In contrast, the energy gap of N-SnS2/Cl2 is reduced to 0.025 eV, because HOMO electrons are mainly concentrated around N atoms, indicating that the adsorption of Cl2 molecules dramatically improves the conductivity and has better reactivity on N-SnS2 surfaces. In the C2H2 system, LUMO mainly concentrates around N atoms with long N-H bonds, reaching −0.196 eV. The increase in LUMO also reduces the energy gap of the system to 0.026 eV, resulting in a decrease in the conductivity of the system. Table 5. Energy of HOMO, LUMO, and energy gap of N-SnS2 and adsorption systems.

Conclusions
The adsorption of NH 3 , Cl 2 , and C 2 H 2 molecules on B-SnS 2 and N-SnS 2 surfaces has been studied based on DFT calculation. The adsorption structure, charge transfer, DOS, and molecular orbital were analyzed to study the influence of B and N modification on the gas sensitivity of SnS 2 monolayer to the gas molecules. Pristine SnS 2 has low adsorption energy and a long adsorption distance for NH 3 , Cl 2 , and C 2 H 2 gas molecules. Compared with the pristine SnS 2 , the adsorption capacity of the three gases on B-SnS 2 and N-SnS 2 is improved. The adsorption capacity of B-SnS 2 to these gases is Cl 2 > NH 3 > C 2 H 2 . The adsorption capacity of N-SnS 2 is NH 3 > C 2 H 2 > Cl 2 . TDOS and PDOS analysis results show that B-SnS 2 has the strongest interaction with Cl 2 and the weakest interaction with C 2 H 2 . Frontier molecular orbital analysis shows that the influence of gas molecules on the conductivity of the B-SnS 2 adsorption system is NH 3 > C 2 H 2 > Cl 2 . The influence order of gas molecules on the conductivity of the N-SnS 2 adsorption system is C 2 H 2 > Cl 2 > NH 3 . The results lay a theoretical foundation for developing B-SnS 2 and N-SnS 2 gas sensors for greenhouse gas detection.
Author Contributions: Conceptualization, A.Z. and A.D.; methodology, Y.G.; investigation, Y.G.; writing-review and editing, Y.G. and A.D. All authors have read and agreed to the published version of the manuscript.