First principles study of gas molecules adsorption on monolayered β-SnSe

: For the purpose of exploring the application of two-dimensional (2D) material in the ﬁeld of gas sensors, the adsorption properties of gas molecules, CO, CO 2 , CH 2 O, O 2 , NO 2 , and SO 2 on the surface of monolayered tin selenium in β phase ( β -SnSe) has been researched by ﬁrst principles calculation based on density functional theory (DFT). The results indicate that β -SnSe sheet presents weak physisorption for CO and CO 2 molecules with small adsorption energy and charge transfers, which show that a β -SnSe sheet is not suitable for sensing CO and CO 2 . The adsorption behavior of CH 2 O molecules adsorbed on a β -SnSe monolayer is stronger than that of CO and CO 2, revealing that the β -SnSe layer can be applied to detect CH 2 O as physical sensor. Additionally, O 2 , NO 2 , and SO 2 are chemically adsorbed on a β -SnSe monolayer with moderate adsorption energy and considerable charge transfers. All related calculations reveal that β -SnSe has a potential application in detecting and catalyzing O 2 , NO 2 , and SO 2 molecules.


Introduction
The detection of gases, especially the toxic gases, has aroused tremendous interest for its extensive application in the fields of agricultural production, industrial control, medical diagnosis, and environmental detection [1].The traditional gas sensors, transition metal oxides sensors, have the shortcoming of a high operating temperature (200-600 • C) [2,3] and low sensing response [4], which have motivated researchers to search for appropriate materials as reliable and high performance gas sensors [5].Fortunately, because 2D materials exhibit excellent physicochemical properties, such as the high ratio of surface area to volume [6], ultrahigh carrier mobility [7], excellent mechanical performance [8], and low electrical noise [9,10], it is possible for them to sense gas molecules at room temperature and normal pressure [11,12].Within 2D materials, monolayered graphene has been immensely researched due to its commendable properties [13].However, the lack of an energy band gap limits its applications in the fields of field-effect transistors, gas sensors, and computer chips [14,15].On the contrary, some 2D materials have a large band gap, but they are unstable in air, such as silicene [16] and phosphorenes [17], which hinders their commercial applications.It is noteworthy that some stable layered materials also show semiconductor characteristics with an appropriate band gap, which is of vital importance for the sensing performance [18,19].
In addition, due to the low cost, rich elements, and environmental friendliness, 2D IV-VI semiconductors have become a research hotspot in recent years.Monolayered SnSe is a fascinating 2D material due to its ideal electric and thermal properties in the field of nanoelectronics [20], which means it has promising applications in the photovoltaic industry, cut-off devices, and infrared lasers [21][22][23].It has been reported that the adsorption behaviors of some small gas molecules on a GeS sheet [11], a SnS sheet [24], and an α-SnSe sheet [25].Significatively, there are some reports pointing out that monolayered SnSe allotropes also show high thermoelectric performance, chemical stability, no toxicity, and earth abundance, which indicate that these monolayer SnSe allotropes can have a potential application in the field of next generation nano-photovoltaic devices and 2D optoelectronic material [15,26].However, reports about the adsorption behaviors of β-SnSe for small gas molecules is scarce.Therefore, in this paper, the sensing behaviors of small gas molecules (CO, CO 2 , CH 2 O, O 2 , NO 2 , and SO 2 ) on a β-SnSe sheet were investigated by first principles calculation.

Materials and Methods
In this work, all the simulation calculations are presented by applying the DMOL 3 module in the Materials Studio [27].Due to the fact that the conventional generalized gradient approximation (GGA) methods are inclined to underestimate the adsorption energy [14], the Perdew-Burke-Ernzerhof (PBE) of generalized gradient approximation (GGA) was applied as the exchange-correlation functional in the process of structural optimization [28], which is widely employed for its precision and economy [29].Due to the existence of the tiny van der Waals interaction, the Grimme custom method for DFT-D correction was applied [30].Moreover, the double numerical plus d-function (DND) basis set was carried out to achieve high computational quality of the density functional theory (DFT) calculation.It has been verified that the basic set superposition error (BSSE) effect is not considered when the numerical basis sets are applied in DMOL 3 [31][32][33].For both geometric optimization and electronic properties calculations, the 4 × 4 × 1 Monkhorst-Pack k-point mesh was chosen.Considering the interaction between β-SnSe sheets of adjacent supercells, a 4 × 4 single-layered β-SnSe supercell with a vacuum region of 15 Å in the Z direction was utilized in the simulated system.Non-spin polarization was proposed in the investigation of the adsorbing properties, but not the two types of molecules (NO 2 and O 2 ) as they are paramagnetic [14].In addition, all the correlation simulation calculations were completely performed until the displacement, energy, and force were converged to 0.005 Å, 0.002 Ha/Å (1 Ha = 27.21eV), and 1 × 10 −5 Ha, respectively.
Considering the different adsorption effects for the different initial configurations of SnSe toward the gas molecules, eight different adsorption initial locations for each molecule were calculated.On the Sn atoms side of β-SnSe layer, four adsorption sites were calculated, the T 1 point is located on top of the Sn atom, the T 2 point is located on top of the Se atom, the T 3 point is located in the middle of the Se-Sn bond, and the T 4 point is located in the center of the puckered hexagon.There are four equal configurations (T 5 , T 6 , T 7 , T 8 ) on the Se atoms side of β-SnSe sheet, as shown in Figure 1.The initial distance of 3 Å is applied as the height of the gas molecules on top of the β-SnSe monolayer.For the purpose of intuitively accessing the adsorbing performance of the gas molecules adsorbed on the β-SnSe layer, we calculated the adsorption energy (E a ), the Hirshfeld charge transfer (Q c ), and the equilibrium distance (d i ).The negative value of Q c expresses that gas molecules acquire charge from the β-SnSe layer, and the d i stands for the balanced minimum distance between gas molecules and the β-SnSe layer.E a is defined as follows: where E (molecules + SnSe) , E SnSe , and E molecules denote the total energy of gas molecules on the SnSe sheet, a SnSe sheet, and a single gas molecule, respectively.

Results and Discussion
The lattice dimensions of the optimized 4 × 4 supercell structure are a = b = 15.125 Å, and the unit cell has a lattice constant of a = b = 3.187 Å.The parameters of structural optimizations are in accordance with the results in the previous work [26].Additionally, a SnSe monolayer in β-phase is a semiconductor with a 1.974 eV band gap in the PBE method, which has a larger band gap than that of SnSe in α-phase (the band gap values of α-SnSe are 0.90 eV indirect and 1.30 eV direct) [15,26].Judging from the result of the DFT calculation, the most stable sites of structural optimizations for gas molecules on the β-SnSe layer are presented in Figure 2. In addition, the adsorption parameters of the gas molecules adsorbed on the β-SnSe layer, such as E a , d i , and Q c , are shown in Table 1.The E a values of the CO, CO 2 , and CH 2 O gas molecules adsorbed on the β-SnSe monolayer are −0.202,−0.175, and −0.322 eV, respectively.Moreover, Table 1 shows the large equilibrium distances (more than 3.2 Å) and the small charge transfer (less than 0.1 e, absolute value) of these molecules.Clearly, the values of d i are outside the length of the covalent radii of the C atom and Sn atom (2.15 Å), the O atom and Sn atom (2.22 Å), or the H atom and Sn atom (1.72 Å) [34,35].It follows that these three types of gas molecules show the tendency for physisorption on the β-SnSe layer.However, for O 2 on the surface of the β-SnSe sheet, the optimized structure has a slight distortion.The E a and Q c are −1.596 and −0.445 eV, respectively, which are much larger than that of other gas molecules (CO, CO 2 , CH 2 O, NO 2 , and SO 2 ) adsorbed on the β-SnSe monolayer.As for the d i for O 2 , it is 2.058 Å, which is within the length of the covalent radii of the O atom and Sn atom (2.22 Å) [35].From this is can be concluded that the O 2 molecules are chemisorbed on the β-SnSe.When it comes to the adsorption properties of polluting gas molecules (e.g., NO 2 and SO 2 ) on the surface of β-SnSe, the E a values are −0.829 and −0.499 eV, respectively, which are larger than the E a of the molecules on the antimonene [14].For NO 2 on the surface of GeS and α-SnSe monolayers, the values of E a are −0.519[11] and −0.770 eV [25], respectively, meaning that the adsorption behaviors of NO 2 and SO 2 molecules on β-SnSe are stronger than the aforementioned adsorption system.In addition, the charge transfer values for NO 2 and SO 2 are −0.279 and −0.278 eV, respectively, indicating that the charge transfer clearly happens between the gas molecules and the β-SnSe layer.The adsorption distance values for NO 2 and SO 2 are 2.531 Å and 2.692 Å, respectively, which is approaching the range of the Sn-O bond lengths (2.22 Å to 2.66 Å) [35].It is of great significance for the β-SnSe sheet to detect NO 2 and SO 2 in the field of gas sensors.It is worth mentioning that all the most energetically stable adsorption sites are on the Sn atom's side of the β-SnSe sheet, showing that the adsorption properties of metal atoms are stronger than that of non-metal atoms [36].
Furthermore, the first principles molecular dynamics (MD) simulation lasted 5 ps with a step of 1 fs and the canonical ensemble (NVT) was applied at 300 K to examine the structural stability of a pristine β-SnSe sheet.Judging from Figure S1 (in the Supplementary Materials), results show that the total potential energy of a pristine β-SnSe sheet fluctuates a small amount before 2500 fs, and then keeps stable during the remaining 2500 fs.As for the structure of a pristine β-SnSe sheet, it has a slight deformation.This all shows that a pristine β-SnSe sheet is stable at 300 K.For the purpose of fully understanding the adsorption mechanism of the molecule-substrate systems, Figure 3 shows the calculation results of the density of states (DOSs) of the β-SnSe monolayer, molecule-SnSe systems, and the projected DOS for the adsorbate gas molecules.In addition, the band information of the molecule-substrate systems is plotted in Figure 4. Clearly, the summits of the DOSs of CO and CO 2 molecules illustrated in Figure 3a,b mainly localize below −3 eV in the valence band (VB), which are doubtless out of the Fermi level (E f ).In addition, judging from the band structure of the adsorption system, it is found that there is a slight influence on the band structure of the β-SnSe sheet.All of this indicates that CO and CO 2 are weakly adsorbed on the β-SnSe layer.As for the adsorption performances of CH 2 O adsorbed on the β-SnSe sheet, the influences of the electronic levels of CH 2 O molecules to the β-SnSe monolayer are mainly located in the interval ranging from −1.5 to −0.5 eV in the VB, which is not at the position of E f .Thus, the changes in the band gaps and band structures are slight.In regard to the O 2 and SO 2 adsorbed on the surface of the β-SnSe sheet, it is found that these two gas molecules contribute the electronic levels of the adsorption system from −2.0 to 2.0 eV, which are close to the E f .Accordingly, the band gap values of the molecule-substrate systems of O 2 and SO 2 are 1.496 and 1.473 eV, respectively, revealing that the O 2 and SO 2 molecules have a strong influence on the electronic properties of the β-SnSe sheet.In terms of the electronic density of the state of NO 2 on β-SnSe, the peaks of NO 2 , SnSe, and NO 2 -SnSe gather and overlap near the E f .In addition, the band structures of β-SnSe induce great changes with the appearance of NO 2 on the β-SnSe sheet.Thus, it is inferred that the existence of NO 2 has a dramatic influence on the electronic properties of the β-SnSe monolayer.Furthermore, the slices of charge densities are presented in Figure 5.Using the slice of charge densities for the molecule-substrate systems, the electric distribution of the systems were investigated.Judging from Figure 5a-c, there are clearly no charges gathering between the atom in the gas molecule and the Sn atom.As for the charge density slices of O 2 , NO 2 , and SO 2 , the high electronic densities are presented in the areas between the O atom and the Sn atom, see Figure 5d-f, indicating the emergence of covalent bonding in the molecule-substrate systems.In addition, the total charge carrier in the substrate can be changed via charge transfer, so the electronic properties of the substrate will be altered after the molecules are adsorbed on it [37,38].Thus, it is of vital significance for β-SnSe to be applied in the field of gas sensors.
In addition, taking the effect of humidity on the substance into consideration, the adsorption performances of H 2 O adsorbed on the β-SnSe sheet are calculated by first principles calculation.The most stable site of structural optimizations for H 2 O on the β-SnSe layer is presented (see Figure S2 in the Supplementary Materials) The E a , Q c , and d i of H 2 O adsorbed on β-SnSe monolayer are −0.4038eV, −0.0577 e, and 2.898 Å, respectively.The E a and Q c are small, the value of d i is outside of the length of the covalent radii of the O atom and Sn atom (2.22 Å).The band gap value of the H 2 O-substrate system is 1.962 eV, which is shown in Figure S3 (in the Supplementary Materials).The change in value of the band gap is small (0.012 eV).In addition, the peak of DOSs of the H 2 O molecule illustrated in Figure S4 (in the Supplementary Materials) mainly localize below −2.5 eV in the valence band (VB), which are doubtless out of the Fermi level (E f ).Judging from Figure S5 (in the Supplementary Materials), there are clearly no charge distributions between the atom in the gas molecule and the Sn atom.All results show that H 2 O has little effect on the β-SnSe monolayer.

Figure 1 .
Figure 1.(a) Structure and initial adsorption sites of the Sn atom's side of the β-SnSe monolayer are presented in the picture; (b) structure and initial adsorption sites of the Se atom's side of the β-SnSe monolayer are presented in the picture, and (c) the band structure of the β-SnSe monolayer is shown in the picture.

Figure 2 .
Figure 2. The most stable sites of optimized configurations of the adsorbate molecules: (a) CO, (b) CO 2 , (c) CH 2 O, (d) O 2 , (e) NO 2 , and (f) SO 2 adsorbed on a β-SnSe monolayer.The most stable sites are exhibited.

Figure 3 .
Figure 3.Total DOSs of the (a) CO, (b) CO 2 , (c) CH 2 O, (d) O 2 , (e) NO 2 and (f) SO 2 on SnSe (black curve), the projected DOS of SnSe (red curve), and the adsorbate molecules (blue curve) for CO, CO 2 , CH 2 O, O 2 , NO 2 , SO 2 on SnSe monolayer.The E f is set to zero, as illustrated by the black dotted line.

Figure 4 .
Figure 4. Band structures of the monolayered β-SnSe with the adsorbate molecules: (a) CO, (b) CO 2 , (c) CH 2 O, (d) O 2 , (e) NO 2 , and (f) SO 2 , respectively.The E f is set to zero, as presented via the blue dotted line.