Adsorption of Greenhouse Decomposition Products on Ag2O-SnS2 and CuO-SnS2 Surfaces.

In this paper, based on density functional theory, the adsorption mechanism and gas sensitivity of Ag2O/CuO-modified SnS2 were analyzed. The results were analyzed according to the adsorption energy, total density of states, partial density of states, and frontier molecular orbital theory. The results show that the adsorption of all gas molecules is exothermic. NH3, Cl2, and C2H2 gases are chemisorbed on the modified SnS2 surfaces. After gas adsorption, the energy gap of the base changes by more than 10%, which fully shows that the conductivity changes greatly after gas adsorption, which can be reflected in the macroscopic resistance change. Ag2O-SnS2 is suitable as a gas sensor for NH3 gas sensors in terms of moderate adsorption distance, large adsorption energy, charge transfer, and frontier molecular orbital theory, while CuO-SnS2 is more suitable as a C2H2 gas sensor.

Introduction

Greenhouse cultivation is favored by more and more farmers for advancing or delaying the planting season, increasing planting density, and bringing greater economic benefits.[1,2] With the continuous improvement of cultivation technology, the scale of greenhouse cultivation is becoming larger and larger.[3,4] However, because of the relative isolation of the internal and external environment of the greenhouse, temperature, humidity, moisture, and other factors are relatively stable in the hut, it is easy for gases to accumulate in the hut, which may endanger crop life at a certain concentration.[5] Therefore, it is necessary to provide gas sensors that can detect the gas content in the greenhouse to achieve the effect of adjusting the amount and type of the gas content in the greenhouse in time. Besides, gas sensors are easy to popularize because of their low cost, which facilitates the achievement of certain economic benefits.[6,7] In this paper, NH3, Cl2, and C2H2 produced in the greenhouse are the main representative gases of the greenhouse through the analysis of these three gases carried out in this research.[8,9]

In recent years, SnS2-based materials have been widely used as gas sensors in the industry due to their large specific surface area and abundant pore structure.[10] Compared with carbon nanotubes, SnS2 has stronger oxidation resistance, high-temperature stability, and smaller dependence, so SnS2 is more suitable for gas detection than carbon nanotubes.[11,12] Therefore, it has become one of the most promising materials in high-temperature and high-pressure environments.[13,14] However, the reaction of intrinsic SnS2 to some inert gases is limited, such as CH4, C2H2, and H2. Fortunately, metal doping modification can adjust the energy gap of the gas-sensing material, thus changing the conductivity of the sensor upon gas adsorption, resulting in improved detection accuracy and adsorption capacity of these gases.[15] Pd, Pt, Au, Ag, Ni, and Co are often used to improve the sensitivity, measurement accuracy, selectivity, and reaction recovery time of gas response. However, the strong surface activity of metal atoms makes the gas sensor easily susceptible to oxidation during a long time operation, which significantly reduces the sensor’s stability. Metal oxides, such as Ag2O and CuO, are the most widely used surface modifications, which show good chemical stability. Salvatore found that SnS2/SnO2– mixed phases showed outstanding gas-sensing performance to NH3.[16] Hao fabricated SnS2/SnO2 sensors that exhibited ultrahigh response toward 1 ppm NO2 at 100 °C, roughly 10.2 times higher than that of pure SnS2 nanoflowers.[17]

In this paper, Ag2O and CuO-modified SnS2 (TMO-SnS2) are proposed as promising greenhouse gas-sensing materials, which can be used to monitor the change in indoor gas concentrations.[18] First, the structure of TMO-SnS2 is optimized to obtain highly stable structures. Then, the most stable structure is used to analyze its adsorption properties to NH3, Cl2, and C2H2.[19,20] The modification and gas-sensing mechanism of TMO-SnS2 are also described. By analyzing the structure optimization, adsorption energy, energy band structure, density of states, and charge transfer, it is found that the modified SnS2 sensor shows a high gas-sensing response.[21] The optimal site of gas adsorption can be found through structural optimization analysis, and the model can be used as the model of gas adsorption to analyze the gas-sensing response results. Based on this model, the reaction types and reaction intensities of gas adsorption are analyzed by the adsorption distance, adsorption energy, and charge transfer again through the density of states analysis and frontier molecular orbital theory analysis to further explore the mechanism of gas adsorption afterward, in which the density of states analysis shows that the gas adsorption type and reaction intensity can be determined through the analysis of the frontier molecular orbital theory, which shows that the energy gap of the gas changes and it changes to reflect the conductivity of the gas charge transfer. This study provides a new method for monitoring the concentration of indoor greenhouse gases.[22]

Computational Details and Methods

All calculations were carried out based on density functional theory (DFT).[23,24] A periodic boundary model was adopted to avoid the interaction between adjacent cells. The vacuum thickness of the (1 0 1) lattice plane was set to 18.0 Å.[25,26] The Brillouin zone k-point sampling was sampled by 3 × 3 × 1. The electron exchange and correlation energy were calculated by the GGA, and the interaction effect of electrons was treated by the PBE function, whereas a double numerical basis with polarization (DNP) was used.[27,28] The convergence criterion for energy and force was set as 10–5 Ha and 2 × 10–3 Ha/Å, respectively, and the charge density convergence accuracy of the self-consistent field was 1 × 10–6 Ha.[29,30]

The adsorption energy Eads shows the change in total energy in the process of adsorption, which can be calculated by eq .[31,32] In this formula, Esuf/gas, Esuf, and Egas represent the total energy of the system after the gas molecule’s adsorption on TMO-SnS2, the total energy of the TMO-SnS2 surface, and the total energy of individual gas molecules before adsorption, respectively.[33,34]

The electron density distribution was calculated by Mulliken population analysis. The charge transfer Q in the adsorption process was obtained by eq , where Qiso and Qads are the total charges of isolated gas and adsorbed gas molecules.[35,36]Q > 0 means that electrons transfer from gas molecules to the surface of TMO-SnS2.[37,38]

The energy gap of the molecular orbital can be calculated by the energy levels of the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbital (LUMO), defined as eq .[39−42] It determines the probability of charge movement in the adsorption system composed of gas molecules and the TMO-SnS2 surface.[43−46] The narrower the gap, the lower the energy required for electrons to transfer from the valence band to the conduction band.[47,48]

Results and Discussion

Structure of SnS2 and TMO-SnS2 Surfaces

As shown in Figure , the gas molecular structure and intrinsic SnS2 structure are optimized. C2H2 and Cl2 are linear gas molecules, while NH3 has a three-dimensional tetrahedral structure. The length of the Cl–Cl bond is 2.024 Å, and the lengths of the C–H bond and C–C bond in the C2H2 molecule are 1.071 and 1.211 Å, respectively. The N–H bond angle of the NH3 molecule is 105.35°. The symmetrical N–H bonds of the NH3 molecule ensure its chemical stability. SnS2 with a perfect crystal structure is shown in Figure d. The two-dimensional structure is beneficial in improving the adsorption performance.[49,50] The S–Sn bond length of 2.611 Å is appropriate for structural stability, and the axial and circumferential distances are close to this value. In general, SnS2 is a suitable material for gas adsorption.

Figure 1

(a) NH3 molecule, (b) Cl2 molecule, (c) C2H2 molecule, and (d) SnS2 molecule. The distance is in Å.

Figure shows the most stable doping structure of Ag2O and CuO-modified SnS2. The distribution of SnS2 atoms shows centrosymmetry. The Ag–S bond length (2.406 Å) is slightly shorter than the O–S bond length (3.513 Å), which is consistent with the bond length between heavy metals. This phenomenon illustrates a strong interaction between the Ag and S atoms. The formation energy is −1.626 eV, which confirms the good stability of the interacting structure. The O–S bond length of -doped species is 1.593 Å, which is slightly shorter than the Cu–S bond length of 3.633 Å. It signifies the strong interaction between the O and S atoms. The adsorption energy reaches −1.713 eV, which proves that the structure of CuO–SnS2 is extremely stable. Based on the Mulliken population, 0.063 e electrons and 0.054 e electrons transfer to SnS2 from Ag2O and CuO as electron donors, respectively. This redistribution of charge results in a change in the conductivity of the doping system.

Figure 2

Most stable structure of the (a) Ag2O–SnS2 surface and (b) CuO–SnS2 surface. The distance is in Å.

The density of states was analyzed to further study the interaction mechanism of Ag2O and CuO-modified SnS2, as shown in Figure . After Ag2O and CuO doping on the SnS2 surface, the total density of states (TDOS) changes significantly in the range of −2 to −5 eV and −1 to 1 eV. The TDOS of TMO-SnS2 consists mainly of 3p orbitals of the S atom. According to the analysis of the partial density of states (PDOS), the 3p orbital of S and the 4d orbital of Ag overlap significantly around −4 eV. The orbital hybridization occurs when the Ag atom interacts with the S atom, indicating that the interaction of SnS2 with Ag2O mainly comes from the S atom of the SnS2. The 3p orbital of S and the 2p orbital of O shapes overlap significantly around −5 eV. The orbital hybridization occurs when the O atom interacts with the S atom, indicating that the interaction between SnS2 and CuO atom mainly comes from the S atom and the O atom.

Figure 3

TDOS and PDOS of before and after gas molecule adsorption on (a) Ag2O–SnS2 and (b) CuO–SnS2.

NH3, Cl2, and C2H2 Adsorption on TMO-SnS2 Surfaces

To study the adsorption characteristics of NH3, Cl2, and C2H2 on TMO-SnS2, the gas molecules were adsorbed on the TMO-SnS2 surface from different directions and angles. The most stable gas adsorption structures on Ag2O and CuO–SnS2 surfaces are shown in Figures and 5, respectively. The density of states, band structure, and molecular orbital of the adsorption process were analyzed by taking the most stable adsorption structure that has the largest adsorption energy.

Figure 5

TDOS and PDOS of before and after gas molecule adsorption on Ag2O–SnS2.

Gas Adsorption on the Ag2O–SnS2 Surface

The adsorption distances of NH3, Cl2, and C2H2 on Ag2O–SnS2 are 1.754, 1.724, and 1.905 Å, respectively. The structures of NH3 and C2H2 gas molecules stay intact in the adsorption process, but the Cl–Cl bond breaks in the adsorption process of Cl2. It can be seen that the adsorption distance of NH3, Cl2, and C2H2 by Ag–SnS2 is relatively moderate, which is conducive to the subsequent desorption, making the recycling of sensing material and improving the sensitivity of the gas sensor. In the adsorption process of NH3 gas, the H atom is absorbed on the O atom; it can be speculated that there is a chemical bond between the H atom and the O atom because when N atoms and H atoms combine to form NH3, N atoms have a larger radius and are less likely to lose electrons and gain electrons, while H atoms have a smaller radius and possess more ability to lose electrons. As can be seen from the previous paper, in the doping process of Ag2O on SnS2, the O atom gains electrons, thus making the O atom show more electrons to bond with the H atom that lost electrons. This is the mechanism in which the H atom approaches Ag2O–SnS2, while the N atom is far away from the Ag2O–SnS2. It can be seen that the adsorption of NH3, Cl2, and C2H2 by Ag2O–SnS2 belongs to chemical adsorption. In addition, when Ag2O–SnS2 adsorbed Cl2 gas, the chemical bond between Cl–Cl was broken due to excessive adsorption energy and charge transfer, so the adsorption was not reversible. After adsorption, the modified SnS2 substrate was in a state that could not be reused. The adsorption of the other two gases can be desorbed for recycling applications.

Figure shows the TDOS and PDOS before and after adsorption of NH3, Cl2, and C2H2 by Ag2O–SnS2, where dotted lines represent the Fermi levels. It can be seen in Figure a1–c1 that the peak value of the TDOS shifts significantly to the left after adsorption, making it continuous at the Fermi level. In Figure a1,b1, it can be seen that Ag2O–SnS2 generates a new peak value at −5.0 to −7.5 eV due to the hybridization of Ag 4d, N 2p, O 2p, and H 1s in the adsorption process of NH3. In Figure a2,b2, it can be seen that during the adsorption of Cl2, Ag2O–SnS2 produced a new peak due to the hybridization of Ag 4d, O 2p, and Cl 3p at −0 to −5.0 eV. In Figure a3–b3, Ag2O–SnS2 generates a new peak due to the hybridization of Ag 4d, C 2p, O 2p, and H 1s at −5.0 to −10.0 eV during the adsorption of C2H2. The generation of the new peak value makes the chemisorption more stable, and the larger the overlapping peak value, the stronger the chemical interaction. In Figure b1, it can be seen that the peak value of the overlap is very large when Ag2O–SnS2 adsorbs Cl2, which also confirms that the adsorption chemistry of Cl2 is very strong, leading to the rupture of the Cl–Cl chemical bond. However, Ag2O–SnS2 has no new peak value for C2H2, which also confirms that the chemical adsorption of C2H2 is too weak, making C2H2 desorb easily.

Figure 4

Most stable structures of gas molecules on Ag2O–SnS2: (a) Ag2O–SnS2/NH3, (b) Ag2O–SnS2/Cl2, and (c) Ag2O–SnS2/C2H2. The distance is in Å.

The adsorption parameters of Ag2O–SnS2 for the three gases are shown in Table , including the adsorption distance, adsorption energy, and charge transfer. The adsorption energies of the NH3, Cl2, and C2H2 adsorption structures are −0.463, −0.742, and −0.354 eV, in which the negative adsorption energy means that the reaction can be spontaneous. Therefore, Ag2O–SnS2 has moderate adsorption energy for NH3, Cl2, and C2H2, which will not be too large for hard desorption nor too small to detect these three gases due to low adsorption energy. The charge transfer of the three adsorption structures during the adsorption process is 0.138 e, −0.176 e, and 0.013 e, in which the negative charge transfer represents the transfer of electrons from gas to Ag2O–SnS2, while the positive charge transfer represents the transfer of electrons from Ag2O–SnS2 to gas molecules.

Table 1

Adsorption Parameters of Gas Molecules on Ag2O–SnS2

configuration	structure	d (Å)	Eads (eV)	QT (e)
Ag2O–SnS2/NH3	Figure 4a	1.754	–0.463	0.138
Ag2O–SnS2/Cl2	Figure 4b	1.724	–0.742	–0.176
Ag2O–SnS2/C2H2	Figure 4c	1.905	–0.354	–0.013

Gas Adsorption on the CuO–SnS2 Surface

As the adsorption structures shown in Figure , the distances of NH3, Cl2, and C2H2 on the CuO–SnS2 substrate are 2.266, 2.168, and 2.277 Å, respectively. It can be seen that the structures of the three gas molecules and CuO–SnS2 are kept intact when gas molecules are adsorbed on the surface of CuO–SnS2. The adsorption distance, adsorption energy, and charge transfer of the three gas adsorption structures are listed in Table . In the adsorption process of CuO–SnS2 toward NH3 gas, H atoms are close to the substrate, while N atoms stay far away. The large adsorption distance of CuO–SnS2 to NH3 can be inferred as physical adsorption. In the adsorption process of Cl2 gas, the Cl atom closes to the Cu atom because of the abundant electronegativity of the Cl atom and the abundant positive charge of the Cu atom. In the adsorption process of C2H2 gas, the H atom is absorbed on the O atom of CuO. This is because when C and H combine to form C2H2, C has a larger radius and is less likely to lose electrons and gain electrons, while H has a smaller radius and is more likely to lose electrons. The adsorption energies of NH3, Cl2, and C2H2 gases are −0.327, −0.925, and −0.299 eV, respectively. Charge transfer is 0.046 e, −0.115 e, and −0.072 e, respectively.

Figure 6

Most stable structures of gas molecules on CuO–SnS2. (a) CuO–SnS2/NH3, (b) CuO–SnS2/Cl2, (c) CuO–SnS2/C2H2. The distance is in Å.

Table 2

Adsorption Parameters of Gas Molecules on CuO–SnS2

configuration	structure	d (Å)	Eads (eV)	QT (e)
CuO–SnS2/NH3	Figure 6a	2.266	–0.327	0.046
CuO–SnS2/Cl2	Figure 6b	2.168	–0.925	–0.115
CuO–SnS2/C2H2	Figure 6c	2.277	–0.299	0.072

Figure shows the TDOS and PDOS before and after CuO–SnS2 adsorbs NH3, Cl2, and C2H2, where dotted lines represent Fermi energy levels. It can be seen in Figure a1–c1 that the peak value of the total density of states shifts significantly to the left after adsorption, making it continuous at the Fermi level. As can be seen from Figure a1,b1, CuO–SnS2 generated a new peak value at −5.0 to −7.5 eV due to the hybridization of Cu 4s, N 2p, O 2p, and H 1s in the adsorption process of NH3. In Figure a2,b2, it can be seen that CuO–SnS2 generates a new peak value due to the hybridization of Cu 4s, O 2p, and Cl 3p at −2.5 to −5.0 eV during the adsorption of Cl2. In Figure a3–b3, it can be seen that CuO–SnS2 generates a new peak due to the hybridization of Cu 4s, C 2p, O 2p, and H 1s at −7.5 to −10.0 eV during C2H2 adsorption.

Figure 7

TDOS and PDOS of before and after gas molecule adsorption on CuO–SnS2.

The adsorption parameters of CuO–SnS2 for the three gases are shown in Table , including adsorption distance, adsorption energy, and charge transfer. The adsorption energies of the three adsorption structures are −0.327, −0.925, and −0.299 eV in order, in which the negative adsorption energy means that the reaction can be spontaneous. Therefore, it can be seen that CuO–SnS2 has moderate adsorption energy for NH3, Cl2, and C2H2 and does not generate large adsorption energy so that NH3, Cl2, and C2H2 gases cannot be desorbed nor can it detect these three gases due to low adsorption energy. The charge transfer of the three adsorption structures is 0.046 e, −0.115 e, and 0.072 e respectively.

Molecular Orbital Theory Analysis of Gas Adsorption on Ag2O–SnS2 and CuO–SnS2

The behavior of system electrons in the adsorption process was analyzed by frontier molecular orbital theory, and HOMO and LUMO after adsorption of NH3, Cl2, and C2H2 gases were obtained, as shown in Figures and 9. Since the HOMO is denser, the system is more likely to give away electrons, whereas since LUMO is less dense, the electron affinity is stronger, and the size of the energy gap can be an indicator of a system’s conductivity. The increase in metal oxide doping can reduce Eg and thus increase the conductivity.

Figure 8

HOMO and LUMO of Ag2O–SnS2 and adsorption systems.

Figure 9

HOMO and LUMO of CuO–SnS2 and adsorption systems.

Before Ag2O–SnS2 adsorbs gas, HOMO is mainly distributed on Ag, indicating that the Ag atom provides electrons as electron donors and is also the active site that can provide adsorption sites for NH3, Cl2, and C2H2 gases. After adsorption of NH3, Cl2, and C2H2 gases, HOMO becomes more concentrated on Ag, while LUMO becomes more uniform. The band gaps of Ag2O–SnS2 adsorbed by the three gases are 0.016, 0.025, and 0.019 eV, as shown in Table , respectively. The HOMO and LUMO distribution on the Ag atom after Ag2O–SnS2 is adsorbed by NH3 is very small, indicating that the electron distribution of the system is more uniform. The moderate band gap also indicates that Ag2O–SnS2 is more suitable for the gas sensor of NH3.

Table 3

Energy of HOMO, LUMO, and the Energy Gap of Ag2O–SnS2 and Adsorption Systems

configuration	structure	EHOMO (eV)	ELUMO (eV)	Eg (eV)
Ag2O–SnS2/NH3	Figure 8a	–0.212	–0.196	0.016
Ag2O–SnS2/Cl2	Figure 8b	–0.226	–0.201	0.025
Ag2O–SnS2/C2H2	Figure 8c	–0.220	–0.201	0.019

HOMO is mainly distributed on Cu before CuO–SnS2 adsorbs gas, indicating that the Cu atom provides electrons as electron donors and is also the active site to provide adsorption sites for NH3, Cl2, and C2H2 gases. After adsorption of NH3, Cl2, and C2H2 gases, HOMO changes become more concentrated on Cu, while LUMO becomes more uniform. Moreover, the change is most obvious after the adsorption of Cl2 gas. It can be seen from the above that this is due to the strong interaction of molecular orbitals after CuO–SnS2 adsorption on Cl2, which makes HOMO and LUMO more homogeneous. In addition, there is almost no LUMO on CuO, which is also due to the strong chemical interaction between CuO and Cl2 gas, making CuO relatively stable. However, when CuO–SnS2 adsorbs C2H2, LUMO still exists in large quantities on CuO, which is caused by the weak interaction between C2H2 and CuO. As shown in Table , the band gaps of CuO–SnS2 adsorbed by the three gases are 0.005, 0.007, and 0.007 eV, respectively. The HOMO and LUMO distribution on Cu atoms after CuO–SnS2 is adsorbed by C2H2 is very small, indicating that the electron distribution of the system is more uniform. The moderate band gap also indicates that CuO–SnS2 is more suitable for the C2H2 gas sensor.

Table 4

Energy of HOMO, LUMO, and the Energy Gap of CuO–SnS2 and Adsorption Systems

configuration	structure	EHOMO (eV)	ELUMO (eV)	Eg (eV)
CuO–SnS2/NH3	Figure 9a	–0.196	–0.191	0.005
CuO–SnS2/Cl2	Figure 9b	–0.206	–0.199	0.007
CuO–SnS2/C2H2	Figure 9c	–0.189	–0.182	0.007

Conclusions

In this paper, the adsorption properties of Ag2O–SnS2 and CuO–SnS2 for NH3, Cl2, and C2H2 gases were calculated based on first principles. First, the most stable structures of the Ag2O and CuO modification on SnS2 were analyzed with various doping sites. The introduction of Ag2O and CuO metal oxide doping increases the conductivity of SnS2-based materials. Then, the interaction between Ag2O and CuO-modified SnS2 and gas molecules was comprehensively understood by the adsorption structure, the density of states, and frontier molecular orbital theory. Ag2O–SnS2 and CuO–SnS2 structures are chemisorbed for NH3, Cl2, and C2H2 gases. From a moderate adsorption distance, large adsorption energy, charge transfer, and frontier molecular orbital theory analysis, different gas molecules’ adsorption will induce changes in conductivity to different degrees. As a result, Ag2O–SnS2 is suitable for the NH3 gas sensor, while CuO–SnS2 is suitable for the C2H2 gas sensor. Under the doping of Ag2O and CuO, the adsorption process mechanism and adsorption capacity are slightly different, but the adsorption of NH3, Cl2, and C2H2 is consistent. Ag2O and CuO-modified SnS2 is a suitable gas sensor and has a promising application in greenhouse cultivation.