DFT Study of Zn-Modified SnP3: A H2S Gas Sensor with Superior Sensitivity, Selectivity, and Fast Recovery Time

The adsorption properties of Cu, Ag, Zn, and Cd-modified SnP3 monolayers for H2S have been studied using density functional theory (DFT). Based on phonon spectrum calculations, a structurally stable intrinsic SnP3 monolayer was obtained, based on which four metal-modified SnP3 monolayers were constructed, and the band gaps of the modified SnP3 monolayers were significantly reduced. The adsorption capacity of Cu, Zn-modified SnP3 was better than that of Ag, Cd-modified SnP3. The adsorption energies of Cu-modified SnP3 and Zn-modified SnP3 for H2S were −0.749 eV and −0.639 eV, respectively. In addition, Cu-modified SnP3 exhibited chemisorption for H2S, while Zn-modified SnP3 exhibited strong physisorption, indicating that it can be used as a sensor substrate. Co-adsorption studies showed that ambient gases such as N2, O2, and H2O had little effect on H2S. The band gap change rate of Zn-modified SnP3 after adsorption of H2S was as high as −28.52%. Recovery time studies based on Zn-modified SnP3 showed that the desorption time of H2S was 0.064 s at 298 K. Therefore, Zn-modified SnP3 can be used as a promising sensor substrate for H2S due to its good selectivity, sensitivity, and fast recovery time.


Introduction
With the increasing advancement of urbanization and industrialization, the domestic sewage, industrial sewage and runoff sewage gathered by pipelines produce irritant gases such as hydrogen sulfide (H 2 S) [1].In addition, H 2 S as an industrial waste gas, its emission can pollute the atmosphere [2].H 2 S is a highly toxic and corrosive gas, and as a kind of flammable hazardous chemical, the leakage of H 2 S may result in significant economic and property losses or even casualties.Therefore, it is critical to investigate novel solutions for detecting H 2 S gas.
With its successful discovery in 2004, graphene has demonstrated considerable promise in a variety of technical areas.Due to their substantial theoretical specific surface area, two-dimensional (2D) materials such as graphene offer excellent options for extremely sensitive sensing and detection devices [3].However, after years of research, the sensing adsorption behavior of substrates such as transition metal dichalcogenides (TMDCs) and graphene has been systematically studied [4][5][6][7].
SnP 3 , a layered material made of Sn and P, has been studied and it was pointed out in the 1970s that it can be obtained by slow cooling to room temperature after heat treatment for 2 days at 575 • C [8].Based on theoretical calculations, it has been found that double and single-layer SnP 3 have relatively low cleavage energies of 0.45 J/m 2 and 0.71 J/m 2 , separately, approached by phosphorene (0.36 J/m 2 ) and graphene (0.32 J/m 2 ) [9].Therefore, SnP 3 is a novel 2D substance that is easy to peel off.A 2D SnP 3 not only has high thermodynamic stability [10], but also has excellent carrier mobility [11,12].For potassiumion, lithium-ion, and sodium-ion batteries, SnP 3 has been used as an anode material [13][14][15].Application studies in gas sensing have shown that intrinsic SnP 3 has a strong adsorption effect for NO 2 and NO [16][17][18].
In the study of gas sensing in 2D materials, computational works are frequently conducted by using density functional theory (DFT) [19][20][21].Moreover, doped 2D materials can enhance the adsorption capacity of gases, according to DFT computational studies.Besides the impurity doping, the doping could be realized by metallic gating, and high doping carrier densities of the order of 10 14 cm −2 have been achieved in 2D materials by electrolyte gating [22].Most of the time, impurity doping could offer better selectivity, in comparison with metallic gating.For example, the interaction of O 2 and CO is significantly enhanced by Au-doped graphene [23], As-doped WSe 2 shows a significant increase in NO 2 adsorption [24], and the adsorption ability of N 2 by InN decorated with Ni atom is significantly improved [25].However, only a few works on the doping adsorption of SnP 3 monolayers, for example, indium-doped SnP 3 monolayer is used for CO 2 adsorption [26], chromium-doped SnP 3 monolayer is used for SO 2 adsorption [27], and palladium-doped SnP 3 monolayer is used for H 2 adsorption research [28].
In this work, four transition metals were selected as doping elements.Cu and Ag have similar properties because they belong to group IB.Zn and Cd have similar properties as they belong to group IIB.In addition, Cu (1.35 Å) and Zn (1.35 Å) have the same atomic radius, and Ag (1.60 Å) and Cd (1.55 Å) have similar atomic radius.By studying Cu, Ag, Zn, and Cd, the doping effects of elements in the same and adjacent groups can be compared.Therefore, the adsorption properties of H 2 S on SnP 3 monolayers modified with Cu, Ag, Zn, and Cd atoms were studied for the first time in this paper using DFT calculations, providing theoretical support for the application of SnP 3 monolayer material in gas sensing.

Computational Details
Herein, adsorption characteristics of the SnP 3 monolayer were investigated using the DMol 3 module based on DFT calculations [29,30].The exchange correlation in the module configuration was described by the PBE (Perdew-Burke-Ernzerhof) functional in the GGA (generalized gradient approximation) [31,32].We adjusted the dispersion interactions using the TS (Tkatchenko-Scheffler) technique (a method for DFT-D) [33,34], and checked the spin unrestricted (spin-polarization) [35].The energy had a convergence tolerance of 10 −5 Ha (1 Ha is approximately equal to 27.2114 eV), the maximum displacement convergence tolerance was set to 5 × 10 −3 Å, and the convergence tolerance for maximum force was set to 2 × 10 −3 Ha/Å [36].DSPP (DFT Semi-core Pseudopots) was used as the core treatment technique for introducing relativistic corrections [37].The atomic orbital used a dual numerical basis set having the orbital polarization function [38].Smearing was set to 0.005 Ha to accelerate convergence [17].After convergence tests, the geometric structure was optimized with a k-point set to 6 × 6 × 1, and the properties were calculated using a more intensive k-point grid of 12 × 12 × 1 [27,28].Intrinsic and doped SnP 3 supercell structures of 2 × 2 × 1 size with a vacuum area of 20 Å were created for adsorption studies.
After geometry optimization, the energy of the structure was obtained.The following equation was utilized to compute the adsorption energy (E a ) of gas molecules on SnP 3 substrates: where E gas/sub is the system's total energy, E gas and E sub are the energies of gas molecules and substrates, respectively.The substrates are more strongly adsorbed to the gas molecules, the greater the absolute values of E a .Adsorption can typically happen on its own if the adsorption energy E a is negative.The relevant properties were analyzed by DFT module calculations.To calculate the transfer charge (Q) of a gas molecule after adsorption by a substrate, Mulliken population analysis was utilized to determine the charge gain or loss of individual atoms in the gas molecule [30,39].When Q is positive, the whole gas molecule consumes electrons; when Q is negative, the whole gas molecule accumulates electrons [40].The shortest distance between the surface and the adsorbed gas molecule in the adsorption system was indicated by the adsorption distance (d).A closer adsorption distance indicates a stronger interaction, and conversely, a farther adsorption distance indicates a weaker interaction.The interactions between the substrates and the doped atoms, as well as between the substrates and the adsorbed gas molecules, were investigated through analyzing the density of states (DOS).The transfer charge between substrates and gas molecules was studied using the charge density difference (CDD).A description of whether a bond forms between the gas molecule and the substrate could be given by the electron localization function (ELF) [41,42].

Establishment and Analysis of SnP 3 Monolayer
First, based on previous research reports [27], a supercell of intrinsic SnP 3 monolayer with a size of 2 × 2 × 1 was constructed in this paper for adsorption studies, as shown in Figure 1.The lattice parameter a = b = 7.359 Å of its unit cell is comparable to a = b = 7.37 Å in the literature [9].The top view shows that the structure contains 8 Sn atoms and 24 P atoms.The side view shows that the structure is not flat, but a sandwich structure consisting of P atoms in the center and Sn atoms on both sides.Each P atom creates a P-Sn bond with an adjacent Sn atom and two P-P bonds with two adjacent P atoms, while three Sn-P bonds are formed between each Sn atom and the three nearby P atoms.With a thickness of 2.888 Å, the SnP 3 monolayer is similar to that of 2.76 Å in the literature [28].
the greater the absolute values of Ea.Adsorption can typically happen on its own if the adsorption energy Ea is negative.
The relevant properties were analyzed by DFT module calculations.To calculate the transfer charge (Q) of a gas molecule after adsorption by a substrate, Mulliken population analysis was utilized to determine the charge gain or loss of individual atoms in the gas molecule [30,39].When Q is positive, the whole gas molecule consumes electrons; when Q is negative, the whole gas molecule accumulates electrons [40].The shortest distance between the surface and the adsorbed gas molecule in the adsorption system was indicated by the adsorption distance (d).A closer adsorption distance indicates a stronger interaction, and conversely, a farther adsorption distance indicates a weaker interaction.The interactions between the substrates and the doped atoms, as well as between the substrates and the adsorbed gas molecules, were investigated through analyzing the density of states (DOS).The transfer charge between substrates and gas molecules was studied using the charge density difference (CDD).A description of whether a bond forms between the gas molecule and the substrate could be given by the electron localization function (ELF) [41,42].

Establishment and Analysis of SnP3 Monolayer
First, based on previous research reports [27], a supercell of intrinsic SnP3 monolayer with a size of 2 × 2 × 1 was constructed in this paper for adsorption studies, as shown in Figure 1.The lattice parameter a = b = 7.359 Å of its unit cell is comparable to a = b = 7.37 Å in the literature [9].The top view shows that the structure contains 8 Sn atoms and 24 P atoms.The side view shows that the structure is not flat, but a sandwich structure consisting of P atoms in the center and Sn atoms on both sides.Each P atom creates a P-Sn bond with an adjacent Sn atom and two P-P bonds with two adjacent P atoms, while three Sn-P bonds are formed between each Sn atom and the three nearby P atoms.With a thickness of 2.888 Å, the SnP3 monolayer is similar to that of 2.76 Å in the literature [28].The intrinsic SnP3 band structure is shown in Figure 2a, where the horizontal coordinates are the K points taken based on the symmetry of the structural model, and the vertical coordinates indicate the energy, with the position of the zero-point corresponding to the Fermi energy level.There is a band gap of 0.498 eV for the SnP3 monolayer structure, which is consistent with the findings of earlier research [31,43].To verify the stability of the constructed monolayer structure, we calculated the phonons using the finite displacement method in the CASTEP module.The same functional and DFT-D corrections as in The intrinsic SnP 3 band structure is shown in Figure 2a, where the horizontal coordinates are the K points taken based on the symmetry of the structural model, and the vertical coordinates indicate the energy, with the position of the zero-point corresponding to the Fermi energy level.There is a band gap of 0.498 eV for the SnP 3 monolayer structure, which is consistent with the findings of earlier research [31,43].To verify the stability of the constructed monolayer structure, we calculated the phonons using the finite displacement method in the CASTEP module.The same functional and DFT-D corrections as in the DMol 3 were used in the module setup, and additionally, an energy cutoff of 500 eV and ultrasoft pseudopotentials were set.The phonon spectrum can reflect whether the material exists stably or not, if the phonon spectrum of the material has no imaginary frequency, it can be proved that the material can exist stably.As shown in Figure 2b, there is no imaginary frequency in the phonon spectrum of the intrinsic SnP 3 structure, which can be a good indication that the structure is stable.On this basis, we will construct several metal doped structures.
the DMol 3 were used in the module setup, and additionally, an energy cutoff of 500 eV and ultrasoft pseudopotentials were set.The phonon spectrum can reflect whether the material exists stably or not, if the phonon spectrum of the material has no imaginary frequency, it can be proved that the material can exist stably.As shown in Figure 2b, there is no imaginary frequency in the phonon spectrum of the intrinsic SnP3 structure, which can be a good indication that the structure is stable.On this basis, we will construct several metal doped structures.Next, the modified doping model was constructed based on intrinsic SnP3.Four transition metal atoms, Cu, Ag, Zn, and Cd were selected as doping elements.As depicted in Figure 1, four distinct modification locations were taken into account in the construction of the metal atom doping structure, namely, the TP location above the P atom, the TH1 location above the hexagonal ring composed of P atoms, the TSn location above the Sn atom, and the TH2 location above the hexagonal ring made up of P and Sn atoms.Usually, the stability of the doped structure can be evaluated by calculating the formation energy [44], and the formation energy (Ef) can be calculated according to the following equation: where EX-SnP3, ESnP3 and EX are the energies of the X-modified SnP3 (X = Cu, Ag, Zn, Cd), SnP3, and a metal atom, respectively.The more negative the formation energy, the more stable the doped structure.It can be seen that the energy of the doped structure determines the formation energy, since the energy of SnP3 and metal atoms are certain.After comparison, the doped structure with the metal atom modified above the Sn atom has the maximum absolute value of energy, that is, the most stable structure, so the TSn site is the best modification site, which is consistent with the doping site in the literature [28].
The stable structures of the SnP3 monolayers modified by four metal atoms (Cu-SnP3, Ag-SnP3, Zn-SnP3, Cd-SnP3) are shown in Figure 3.The average lengths of the Cu-P, Ag-P, Zn-P, and Cd-P bonds formed by the modified metal atoms with three P atoms are 2.275 Å, 2.622 Å, 2.480 Å, and 2.812 Å, respectively.In addition, the distances of the modified metal atoms from the Sn atoms directly below them are 2.943 Å, 3.576 Å, 3.324 Å, and 3.761 Å, respectively.Compared with the intrinsic SnP3 monolayer, the thickness of doped SnP3 monolayers increased to 3.278 Å, 3.303 Å, 3.353 Å, and 3.278 Å, respectively.Next, the modified doping model was constructed based on intrinsic SnP 3 .Four transition metal atoms, Cu, Ag, Zn, and Cd were selected as doping elements.As depicted in Figure 1, four distinct modification locations were taken into account in the construction of the metal atom doping structure, namely, the T P location above the P atom, the T H1 location above the hexagonal ring composed of P atoms, the T Sn location above the Sn atom, and the T H2 location above the hexagonal ring made up of P and Sn atoms.Usually, the stability of the doped structure can be evaluated by calculating the formation energy [44], and the formation energy (E f ) can be calculated according to the following equation: where E X-SnP3 , E SnP3 and E X are the energies of the X-modified SnP 3 (X = Cu, Ag, Zn, Cd), SnP 3 , and a metal atom, respectively.The more negative the formation energy, the more stable the doped structure.It can be seen that the energy of the doped structure determines the formation energy, since the energy of SnP 3 and metal atoms are certain.
After comparison, the doped structure with the metal atom modified above the Sn atom has the maximum absolute value of energy, that is, the most stable structure, so the T Sn site is the best modification site, which is consistent with the doping site in the literature [28].
The stable structures of the SnP 3 monolayers modified by four metal atoms (Cu-SnP 3 , Ag-SnP 3 , Zn-SnP 3 , Cd-SnP 3 ) are shown in Figure 3.The average lengths of the Cu-P, Ag-P, Zn-P, and Cd-P bonds formed by the modified metal atoms with three P atoms are 2.275 Å, 2.622 Å, 2.480 Å, and 2.812 Å, respectively.In addition, the distances of the modified metal atoms from the Sn atoms directly below them are 2.943 Å, 3.576 Å, 3.324 Å, and 3.761 Å, respectively.Compared with the intrinsic SnP 3 monolayer, the thickness of doped       The DOS of intrinsic and four doped SnP 3 monolayers are depicted in Figures 5 and 6 structure.The PDOS of Ag-SnP3 is shown in Figure 5d.It is clear that the Ag-4d and P-3p orbits significantly overlap between −4.831 eV and −1.913 eV, and the Ag-5s and P-3p orbits overlap slightly at the peak of 2.082 eV.It shows that Ag-4d, Ag-5s, and P-3p orbitals have strong interactions.Orbital hybridization mostly happens between Ag-4d and P-3p, which will cause the electron distribution to alter, and indicates the formation of the Ag-P bond.It shows that the Ag atom modified on the SnP3 monolayer surface has a stable structure.The PDOS of Zn-SnP3 is depicted in Figure 6c.The Zn-3d and P-3s, P-3p orbits overlap between −8.374 eV and −4.913 eV, and the Zn-4s and P-3p orbital peaks overlap at

Study on Adsorption of H2S by SnP3 Monolayer
In constructing the intrinsic SnP3 monolayer adsorption system for H2S, the initial separation between the H2S and the SnP3 monolayer was adjusted to 3 Å.As shown in Figure S1, various adsorption positions of the H2S molecule are considered, including H2S molecules placed parallel or vertically above the Sn atom, P atom, and the hexagonal ring.According to modular calculations, the intrinsic SnP3 monolayer adsorbed H2S has the most stable structure, as illustrated in Figure S1b.Similarly, the system of X-SnP3 (X = Cu, Ag, Zn, Cd) for H2S was constructed using the above method.Since the doped metal atoms Figures 5a,b and 6a,b illustrate the TDOS of four doped SnP 3 monolayers compared with the intrinsic SnP 3 , respectively.It is obvious that the TDOS of the doped SnP 3 monolayers is shifted to the left with respect to the intrinsic monolayer, and combined with the energy band structure in Figure 4, indicates that doping reduces the band gap, which results in enhanced metal characteristics and increased conductivity.Moreover, as the band gap narrows, the valence band electrons can move more readily into the conduction band [45].
The PDOS of Cu-SnP 3 is seen in Figure 5c.The P-3p and Cu-3d orbits significantly superimpose between −4.682 eV and −0.361 eV.It shows that the P-3p and Cu-3d orbitals have strong interactions and orbital hybridization mostly happens between them, which will cause the electron distribution to alter, and denotes the formation of the Cu-P bond.It demonstrates that the Cu atom modified on the SnP 3 monolayer surface has a stable structure.The PDOS of Ag-SnP 3 is shown in Figure 5d.It is clear that the Ag-4d and P-3p orbits significantly overlap between −4.831 eV and −1.913 eV, and the Ag-5s and P-3p orbits overlap slightly at the peak of 2.082 eV.It shows that Ag-4d, Ag-5s, and P-3p orbitals have strong interactions.Orbital hybridization mostly happens between Ag-4d and P-3p, which will cause the electron distribution to alter, and indicates the formation of the Ag-P bond.It shows that the Ag atom modified on the SnP 3 monolayer surface has a stable structure.
The PDOS of Zn-SnP 3 is depicted in Figure 6c.The Zn-3d and P-3s, P-3p orbits overlap between −8.374 eV and −4.913 eV, and the Zn-4s and P-3p orbital peaks overlap at −4.580 eV, −1.212 eV, and 1.151 eV.It indicates that all orbitals interact with each other.Orbital hybridization mostly happens between Zn-4s and P-3p, which will cause the electron distribution to alter, and indicates the formation of the Zn-P bond.It shows that the Zn atom modified on the SnP 3 monolayer surface has a stable structure.The PDOS of Cd-SnP 3 is seen in Figure 6d.The Cd-4d and P-3s, P-3p orbits overlap between −9.852 eV and −7.281 eV, and the peaks of the Cd-5s and P-3p orbits coincide at −4.276 eV, −1.456 eV, and 0.973 eV, which means there are interactions between the orbitals.Orbital hybridization mostly happens between P-3p and Cd-5s, which will cause the electron distribution to alter, and indicates the formation of the Cd-P bond.It shows that the Cd atom modified on the SnP 3 monolayer surface has a stable structure.

Study on Adsorption of H 2 S by SnP 3 Monolayer
In constructing the intrinsic SnP 3 monolayer adsorption system for H 2 S, the initial separation between the H 2 S and the SnP 3 monolayer was adjusted to 3 Å.As shown in Figure S1, various adsorption positions of the H 2 S molecule are considered, including H 2 S molecules placed parallel or vertically above the Sn atom, P atom, and the hexagonal ring.According to modular calculations, the intrinsic SnP 3 monolayer adsorbed H 2 S has the most stable structure, as illustrated in Figure S1b.Similarly, the system of X-SnP 3 (X = Cu, Ag, Zn, Cd) for H 2 S was constructed using the above method.Since the doped metal atoms act as interacting active sites between the substrate and the adsorbed gases [46], the H 2 S molecule was placed parallel or vertically on the doped metal atoms in the construction of the adsorption systems of the four doped SnP 3 monolayers to H 2 S.
As shown in Figure 7, the most stable structures of the intrinsic as well as the four doped SnP 3 monolayers for H 2 S adsorption are displayed.Table 1 summarizes the results of H 2 S adsorbed on the intrinsic SnP 3 and the SnP 3 doped with four metal atoms.It includes the adsorption energy (E a ) of the SnP 3 monolayer to the H 2 S molecule, the transfer charge (Q) of the H 2 S molecule, and the shortest adsorption distance (d) between the SnP 3 monolayer and the H 2 S. According to Equation (1), the adsorption energy of intrinsic SnP 3 for H 2 S is −0.392 eV.The transfer charge and adsorption distance are 0.024 e and 2.574 Å, respectively.The X-SnP 3 (X = Cu, Ag, Zn, Cd) monolayer adsorption energies for H 2 S are −0.749eV, −0.595 eV, −0.639 eV, and −0.402 eV, correspondingly.The absolute values of adsorption energies are Cu-SnP 3 > Zn-SnP 3 > Ag-SnP 3 > Cd-SnP 3 .The transfer charges of H 2 S molecules amounted to 0.272 e, 0.211 e, 0.234 e, and 0.187 e, correspondingly.The transfer charges show numerically Cu-SnP 3 > Zn-SnP 3 > Ag-SnP 3 > Cd-SnP 3 .The adsorption distances, which are 2.336 Å, 2.601 Å, 2.520 Å, and 2.877 Å, showed numerically that Cu-SnP 3 < Zn-SnP 3 < Ag-SnP 3 < Cd-SnP 3 .
−0.749 eV, −0.595 eV, −0.639 eV, and −0.402 eV, correspondingly.The absolute values of adsorption energies are Cu-SnP3 > Zn-SnP3 > Ag-SnP3 > Cd-SnP3.The transfer charges of H2S molecules amounted to 0.272 e, 0.211 e, 0.234 e, and 0.187 e, correspondingly.The transfer charges show numerically Cu-SnP3 > Zn-SnP3 > Ag-SnP3 > Cd-SnP3.The adsorption distances, which are 2.336 Å, 2.601 Å, 2.520 Å, and 2.877 Å, showed numerically that Cu-SnP3 < Zn-SnP3 < Ag-SnP3 < Cd-SnP3.Compared with the intrinsic SnP3 monolayer, the transfer charge and adsorption energy values of the four doped SnP3 monolayers are increased, and the adsorption distances between Cu-SnP3, Zn-SnP3, and H2S molecule are smaller than those of the intrinsic SnP3 monolayer.The larger the value of the transfer charge and adsorption energy, and the closer the adsorption distance, the better the adsorption effect is considered.Therefore, based on the information in Table 1, the SnP3 monolayer doped with four kinds of metals can improve the adsorption of H2S, only Cu-SnP3 and Zn-SnP3 can significantly improve the adsorption of H2S, while Ag-SnP3 and Cd-SnP3 have a smaller enhancing effect on the adsorption of H2S.Next, to study the effects of adsorbed H2S on the electronic structure of the SnP3 monolayers, DOS, CDD, and ELF analyses were performed for the adsorption system.Since Cu-SnP3 and Zn-SnP3 have better adsorption capacities for H2S than Ag-SnP3 and Cd-SnP3, here we only took Cu-SnP3 and Zn-SnP3 as examples to study the adsorption of H2S.
As illustrated in Figure 8a,b, the TDOS of Cu-SnP3 and Zn-SnP3 monolayers is not significantly shifted after H2S adsorption, but there is an increase at some positions (marked by circles), and the increased positions correspond exactly to the S-3s, 3p orbitals of the PDOS in Figure 8c,d.From Figure 8c, the Cu-3d, 4s, and S-3p orbital peaks overlap at −6.362 eV, −4.708 eV, −4.197 eV, −2.762 eV, and 2.663 eV.The overlap of the same waveform of the density of states peak represents the hybridization between the adsorbed molecule and the substrate, thus indicating a strong interaction of the S atom in H2S with the substrate-modified Cu atom.In addition, as seen in Figure 8d, the Zn-3d, 4s, and S-3p orbital peaks overlap at −7.465 eV, −6.069 eV, −4.745 eV, −4.065 eV, and 2.528 eV, suggesting that the doped Zn atom interacts strongly with the S atom in the H2S molecule.Compared with the intrinsic SnP 3 monolayer, the transfer charge and adsorption energy values of the four doped SnP 3 monolayers are increased, and the adsorption distances between Cu-SnP 3, Zn-SnP 3 , and H 2 S molecule are smaller than those of the intrinsic SnP 3 monolayer.The larger the value of the transfer charge and adsorption energy, and the closer the adsorption distance, the better the adsorption effect is considered.Therefore, based on the information in Table 1, the SnP 3 monolayer doped with four kinds of metals can improve the adsorption of H 2 S, only Cu-SnP 3 and Zn-SnP 3 can significantly improve the adsorption of H 2 S, while Ag-SnP 3 and Cd-SnP 3 have a smaller enhancing effect on the adsorption of H 2 S.
Next, to study the effects of adsorbed H 2 S on the electronic structure of the SnP 3 monolayers, DOS, CDD, and ELF analyses were performed for the adsorption system.Since Cu-SnP 3 and Zn-SnP 3 have better adsorption capacities for H 2 S than Ag-SnP 3 and Cd-SnP 3 , here we only took Cu-SnP 3 and Zn-SnP 3 as examples to study the adsorption of H 2 S.
As illustrated in Figure 8a,b, the TDOS of Cu-SnP 3 and Zn-SnP 3 monolayers is not significantly shifted after H 2 S adsorption, but there is an increase at some positions (marked by circles), and the increased positions correspond exactly to the S-3s, 3p orbitals of the PDOS in Figure 8c,d.From Figure 8c, the Cu-3d, 4s, and S-3p orbital peaks overlap at −6.362 eV, −4.708 eV, −4.197 eV, −2.762 eV, and 2.663 eV.The overlap of the same waveform of the density of states peak represents the hybridization between the adsorbed molecule and the substrate, thus indicating a strong interaction of the S atom in H 2 S with the substrate-modified Cu atom.In addition, as seen in Figure 8d, the Zn-3d, 4s, and S-3p orbital peaks overlap at −7.465 eV, −6.069 eV, −4.745 eV, −4.065 eV, and 2.528 eV, suggesting that the doped Zn atom interacts strongly with the S atom in the H 2 S molecule.Figure 9a-d shows the top and side perspectives of Cu-SnP3 and Zn-SnP3 structures that are most stable to H2S adsorption, respectively.The CDD of H2S adsorbed by Cu-SnP3 and Zn-SnP3 is seen in Figure 9e,f.In the study of adsorption, CDD can be used to observe the direction of charge transfer in space after molecular adsorption.The isosurface can be obtained by connecting the points with the same charge gain and loss probability.Here, we set the isosurface value to 0.015 e/Å 3 , which can obtain a better graphical effect.The yellow and blue parts of the CDD illustration indicate charge consumption and accumulation, respectively.The charge of H atoms inside the H2S molecule is consumed, while the charge of the S atom is accumulated.However, the charge density of the H2S molecule is reduced from the overall view of the adsorption system, indicating that the substrate receives the charge from H2S.As a comparison, the CDD of the intrinsic SnP3 adsorbed H2S is displayed in Figure S2c, showing little transfer charge between the substrate and H2S molecule.From Table 1, the charge transfer between H2S and Cu-SnP3, and Zn-SnP3 was calculated by Mulliken population analysis to be 0.272 e, and 0.234 e, separately.The positive transfer charge value shows that the H2S molecule is positively charged, which implies that the H2S molecule loses electrons and transfers to the substrate, so the population analysis value is consistent with the CDD result.
Electron localization function (ELF) is one of the means to study electronic structure, which can characterize the localization degree of electrons.ELF is used to show the distribution of electrons outside the nucleus and analyze the properties of electrons near the nucleus and bonding regions.The range of the ELF value is 0 to 1: when ELF = 1, the electron is completely localized; when ELF = 0, it corresponds to complete delocalization of the electron, which also means that there is no electron at that place [47].Figure 9g,h formed along the lines in Figure 9a,b show the ELF section plots of Cu-SnP3 and Zn-SnP3 adsorbing H2S.In the ELF diagram, the closer the blue area is, the less and more dispersed the electrons are.The closer the red area is, the more concentrated the electrons are.From the dashed box in Figure 9g, the overlap of electron localization between the doped Cu atom and the S atom in H2S, indicates the existence of shared electrons and the formation Figure 9a-d shows the top and side perspectives of Cu-SnP 3 and Zn-SnP 3 structures that are most stable to H 2 S adsorption, respectively.The CDD of H 2 S adsorbed by Cu-SnP 3 and Zn-SnP 3 is seen in Figure 9e,f.In the study of adsorption, CDD can be used to observe the direction of charge transfer in space after molecular adsorption.The isosurface can be obtained by connecting the points with the same charge gain and loss probability.Here, we set the isosurface value to 0.015 e/Å 3 , which can obtain a better graphical effect.The yellow and blue parts of the CDD illustration indicate charge consumption and accumulation, respectively.The charge of H atoms inside the H 2 S molecule is consumed, while the charge of the S atom is accumulated.However, the charge density of the H 2 S molecule is reduced from the overall view of the adsorption system, indicating that the substrate receives the charge from H 2 S. As a comparison, the CDD of the intrinsic SnP 3 adsorbed H 2 S is displayed in Figure S2c, showing little transfer charge between the substrate and H 2 S molecule.From Table 1, the charge transfer between H 2 S and Cu-SnP 3 , and Zn-SnP 3 was calculated by Mulliken population analysis to be 0.272 e, and 0.234 e, separately.The positive transfer charge value shows that the H 2 S molecule is positively charged, which implies that the H 2 S molecule loses electrons and transfers to the substrate, so the population analysis value is consistent with the CDD result.
Electron localization function (ELF) is one of the means to study electronic structure, which can characterize the localization degree of electrons.ELF is used to show the distribution of electrons outside the nucleus and analyze the properties of electrons near the nucleus and bonding regions.The range of the ELF value is 0 to 1: when ELF = 1, the electron is completely localized; when ELF = 0, it corresponds to complete delocalization of the electron, which also means that there is no electron at that place [47].Figure 9g,h formed along the lines in Figure 9a,b show the ELF section plots of Cu-SnP 3 and Zn-SnP 3 of a chemical bond between them, suggesting that Cu-SnP3 is chemically adsorbed to H2S.As shown in Figure 9h, it is clear from the dotted box that the Zn atom and the S atom electron localization edges are very close to each other with strong interactions, indicating that Zn-SnP3 is a strong physisorption for H2S.As a comparison, the ELF of H2S adsorption by intrinsic SnP3 is shown in Figure S2d, which shows that the electron localization is not overlapped between the H2S molecule and the SnP3 and is far away, indicating that intrinsic SnP3 has only weak physical adsorption of H2S.

Study on Co-Adsorption of H2S with Ambient Gases
To study the interference of ambient gases with H2S, we calculated isotherm adsorption curves of the SnP3 monolayer for H2S as well as for the three interfering gases (N2, O2, and H2O) using the Metropolis method in the Sorption module.Cu-SnP3 and Zn-SnP3 were chosen as the adsorption substrates because of their better adsorption capacity for H2S than Ag-SnP3 and Cd-SnP3.As shown in Figure 10, the horizontal axis represents the pressure, and the vertical axis represents the adsorption capacity.When the pressure is between 0 and 200 kPa and the temperature is 298 K, the adsorption capacity of the substrate for several gases improves with increasing pressure.Notably, the adsorption of H2S by the substrate was significantly greater than that of N2, O2, and H2O throughout the coadsorption process.At pressures ranging from 20 to 200 kPa, the adsorption capacity of Cu-SnP3 and Zn-SnP3 for H2S was approximately six to seven times that of N2, O2, and H2O.The results showed that Cu-SnP3 and Zn-SnP3 had good selectivity for H2S, which provided a strong theoretical support for the adsorption of H2S in the ambient environment.

Study on Co-Adsorption of H 2 S with Ambient Gases
To study the interference of ambient gases with H 2 S, we calculated isotherm adsorption curves of the SnP 3 monolayer for H 2 S as well as for the three interfering gases (N 2 , O 2 , and H 2 O) using the Metropolis method in the Sorption module.Cu-SnP 3 and Zn-SnP 3 were chosen as the adsorption substrates because of their better adsorption capacity for H 2 S than Ag-SnP 3 and Cd-SnP 3 .As shown in Figure 10, the horizontal axis represents the pressure, and the vertical axis represents the adsorption capacity.When the pressure is between 0 and 200 kPa and the temperature is 298 K, the adsorption capacity of the substrate for several gases improves with increasing pressure.Notably, the adsorption of H 2 S by the substrate was significantly greater than that of N 2 , O 2 , and H 2 O throughout the co-adsorption process.At pressures ranging from 20 to 200 kPa, the adsorption capacity of Cu-SnP 3 and Zn-SnP 3 for H 2 S was approximately six to seven times that of N 2 , O 2 , and H 2 O.The results showed that Cu-SnP 3 and Zn-SnP 3 had good selectivity for H 2 S, which provided a strong theoretical support for the adsorption of H 2 S in the ambient environment.

Study on Sensing of H 2 S by Zn-SnP 3
The above studies have shown that Zn-SnP 3 has strong physical adsorption on H 2 S, therefore Zn-SnP 3 is a potentially sensitive material for the detection of H 2 S. Sensor devices can be fabricated from Zn-SnP 3 materials and applied to the detection of the target gas H 2 S. Typically, when a sensitive material adsorbs a target substance, the electrical conductivity of that material changes at the macro level.The conductivity (σ) can be described by the following relation [48]:

Study on Sensing of H2S by Zn-SnP3
The above studies have shown that Zn-SnP3 has strong physical adsorption o therefore Zn-SnP3 is a potentially sensitive material for the detection of H2S.Sens vices can be fabricated from Zn-SnP3 materials and applied to the detection of the gas H2S.Typically, when a sensitive material adsorbs a target substance, the electrica ductivity of that material changes at the macro level.The conductivity (σ) can be des by the following relation [48]: where

Study on Sensing of H2S by Zn-SnP3
The above studies have shown that Zn-SnP3 has strong physical adsorption on H2S, therefore Zn-SnP3 is a potentially sensitive material for the detection of H2S.Sensor devices can be fabricated from Zn-SnP3 materials and applied to the detection of the target gas H2S.Typically, when a sensitive material adsorbs a target substance, the electrical conductivity of that material changes at the macro level.The conductivity (σ) can be described by the following relation [48]: where Eg in the relation represents the band gap, and T and KB are the temperature and Boltzmann constant, respectively.It is easy to notice from this relation that only the parameter Eg affects the conductivity of the adsorption system at a certain temperature.The change in band gap after gas adsorption directly affects the change in conductivity, and the more obvious the change in conductivity, the easier the signal of current or voltage in the device can be detected.The band gap of Zn-SnP3 is 0.291 eV, as shown in Figure 4.
The energy band structure of Zn-SnP3 after adsorption of H2S is shown in Figure 11, with the band gap of 0.208 eV.The adsorption energy, transfer charge, band gap, and band gap change rate of H 2 S adsorbed by different substrate materials were summarized, as shown in Table 2.It can be seen from the table that compared with the intrinsic substrate, the absolute values of adsorption energy and transfer charge of H 2 S on doped substrate have increased, indicating the enhancement of adsorption performance.However, it does not mean that these doped substrate materials can be used as sensitive materials for sensors.For example, after adsorption of H 2 S by Zn-MoSe 2 , the band gap change rate was −7.06%, indicating its low sensitivity to H 2 S, but due to its larger adsorption energy (−1.361 eV), it can be applied to gas elimination [49].In this work, the band gap change rate of Zn-SnP 3 after adsorption of H 2 S is as high as −28.52%, significantly higher than that of other references, which strongly suggests that Zn-SnP 3 can be used as a sensitive material for H 2 S. In addition, the recovery time is one of the important indicators of the gas sensor.Generally, a good gas sensor should have a short recovery time, and the recovery time (τ) can be estimated from Van't Hoff-Arrhenius expression for the rate constant, which can be calculated by the following definition [51]: where according to transition state theory the attempt frequency γ is commonly assumed to be 'a typical value' in the range of 10 12 -10 13 s −1 [52], which is taken here to be 10 12 s −1 .E a represents the adsorption energy with the values shown in Table 1.T is the temperature, in K. K B denotes the Boltzmann constant with a value of 8.62 × 10 −5 eV K −1 .The recovery time increases with the absolute value of the adsorption energy and decreases with the temperature.The recovery time of the H 2 S gas molecule at different temperatures is shown in Figure 12.The desorption time at 298 K is only 0.064 s, indicating that Zn-SnP 3 has a fast recovery rate for H 2 S at room temperature.As a comparison, the recovery time after H 2 S adsorption on Cu-SnP 3 was calculated, as shown in Figure S3, which shows that the desorption time of H 2 S adsorbed on Cu-SnP 3 is significantly longer than that of Zn-SnP 3 .
In addition, the attempt frequency γ can be associated with the vibrational frequency [51].Some studies have shown that light exposure is related to gas desorption [53,54], which may be due to the fact that different wavelengths of light change the vibrational frequency, which in turn has an effect on the attempt frequency and alters the recovery time.

Conclusions
Overall, this work has used the density functional theory (DFT) to study in detail the adsorption of intrinsic SnP3 and metal atom modified SnP3 to irritant H2S gases.The monolayers of X-SnP3 (X = Cu, Ag, Zn, Cd) with stable structures were constructed, and all four atoms were modified at the same position, showing good consistency.The intrinsic SnP3 monolayer showed only physical adsorption and weak interaction for H2S, while the four doped monolayers showed enhanced adsorption capacity for H2S.Because the ad-

Conclusions
Overall, this work has used the density functional theory (DFT) to study in detail the adsorption of intrinsic SnP 3 and metal atom modified SnP 3 to irritant H 2 S gases.The

Figure 1 .
Figure 1.SnP3 monolayer structure, top and side views, with solid circles representing the modification sites.

Figure 1 .
Figure 1.SnP 3 monolayer structure, top and side views, with solid circles representing the modification sites.

Figure 2 .
Figure 2. The (a) band structure and (b) phonon spectrum of the intrinsic SnP3 monolayer.

Figure 2 .
Figure 2. The (a) band structure and (b) phonon spectrum of the intrinsic SnP 3 monolayer.

Figure 3 .
Figure 3. Optimized structures of (a) Cu-SnP 3 , (b) Ag-SnP 3 , (c) Zn-SnP 3 , and (d) Cd-SnP 3 .The modified SnP 3 monolayer energy band structures are shown in Figure 4.The band gap of intrinsic SnP 3 is 0.498 eV.The band gap of the atom-doped SnP 3 monolayer is reduced, and the energy bands of Cu-SnP 3 and Ag-SnP 3 cross the Fermi energy level and exhibit metallic properties, and the band gaps of Zn-SnP 3 and Cd-SnP 3 are 0.291 eV and 0.327 eV, respectively.
. The zero point of the energy coordinate indicates the Fermi energy level.Below zero is the valence band, and all positions are filled with electrons.The material properties have a strong correlation with the energy band structure close to the Fermi energy level.By studying the TDOS (total DOS) and the PDOS (projected DOS), we analyzed the effect of Cu, Ag, Zn, and Cd doping on the SnP 3 monolayer, and the interactions between the SnP 3 monolayer and the doped transition metal atoms.
Eg in the relation represents the band gap, and T and KB are the temperatur Boltzmann constant, respectively.It is easy to notice from this relation that only th rameter Eg affects the conductivity of the adsorption system at a certain temperatur change in band gap after gas adsorption directly affects the change in conductivit the more obvious the change in conductivity, the easier the signal of current or volt the device can be detected.The band gap of Zn-SnP3 is 0.291 eV, as shown in Fig The energy band structure of Zn-SnP3 after adsorption of H2S is shown in Figure 11 the band gap of 0.208 eV.

Figure 10 .
Figure 10.Adsorption isotherms of H 2 S, N 2 , O , and H 2 O on (a) Cu-SnP 3 and (b) Zn-SnP 3 at pressures ranging from 0 to 200 kPa and at temperatures of 298 K.

Figure 11 .
Figure 11.Band structure of Zn-SnP3 after adsorption of H2S.Figure 11.Band structure of Zn-SnP 3 after adsorption of H 2 S.

Figure 11 .
Figure 11.Band structure of Zn-SnP3 after adsorption of H2S.Figure 11.Band structure of Zn-SnP 3 after adsorption of H 2 S.

Figure 12 .
Figure 12.The recovery time of H 2 S at 298 K, 398 K, and 498 K, respectively.

Table 1 .
Parameters of H2S molecule adsorption by five SnP3 substrates.Ea, Q, and d stand for the adsorption energy, transfer charge, and shortest adsorption distance, respectively.

Table 1 .
Parameters of H 2 S molecule adsorption by five SnP 3 substrates.E a , Q, and d stand for the adsorption energy, transfer charge, and shortest adsorption distance, respectively.

Table 2 .
Summary of adsorption energy (E a ), transfer charge (Q), band gap of the substrate before and after adsorption of the gases (E g1 and E g2 ), change in band gap (∆E g = E g2 − E g1 ), and rate of change in band gap (∆E g /E g1 ) for H 2 S adsorption on different substrates.