First-Principles Study of χ3-Borophene as a Candidate for Gas Sensing and the Removal of Harmful Gases

The potential application of borophene as a sensing material for gas-sensing devices is investigated in this work. We utilize density functional theory (DFT) to systematically study the adsorption mechanism and sensing performance of χ3-borophene to search for high-sensitivity sensors for minor pollutant gases. We compare the results to those for two Pmmn borophenes. The first-principles calculations are used to analyze the sensing performance of the three different borophenes (2 Pmmn borophene, 8 Pmmn borophene, and χ3-borophene) on five leading harmful gases (CO, NH3, SO2, H2S, and NO2). The adsorption configuration, adsorption energy, and electronic properties of χ3-borophene are investigated. Our results indicate that the mechanism of adsorption on χ3-borophene is chemisorption for NO2 and physisorption for SO2 and H2S. The mode of adsorption of CO and NH3 on χ3-borophene can be both physisorption and chemisorption, depending on the initially selected sites. Analyses of the charge transfer and density of states show that χ3-borophene is selective toward the adsorption of harmful gases and that N and O atoms form covalent bonds when chemisorbed on the surface of χ3-borophene. An interesting phenomenon is that when 8 Pmmn borophene adsorbs SO2, the gas molecules are dismembered and strongly adsorb on the surface of 8 Pmmn borophene, which provides a way of generating O2 while adsorbing harmful substances. Overall, the results of this work demonstrate the potential applications of borophene as a sensing material for harmful gas sensing or removal.


Introduction
The extraction of fossil fuels, such as oil, gas, and coal, has contributed considerably to meet the increasing global energy demand. However, these fuels generate large quantities of harmful gases [1]. Industrial exhaust gases can be categorized into particulates, gas, and radioactive pollutants. These combustion products have caused numerous global problems, such as the greenhouse effect, the hole in the ozone layer, acid rain, and widespread environmental pollution [2,3]. Hence, it is vital to remove these harmful gases from the environment.
Two-dimensional (2D) materials are considered to be nanomaterials with a sheet-like morphology featuring a large lateral size ranging from hundreds of nanometers to tens of micrometers or, even more significantly, a thickness of a single or a few atomic layers [4,5]. Two-dimensional materials with unusual properties are some of the most promising candidates for numerous applications, including electronics, optoelectronics, catalysis, energy storage, solar cells, biomedicine, sensors, environments, etc. [6]. Due to the larger available surface volume ratio of 2D materials, the adsorption of gas molecules can arouse significant signals in the sensor's materials, leading to higher sensing performance [7,8].
In 2004, the discovery of graphene ushered in a new era in the study of 2D materials [9]. The large surface area and high conductivity of graphene have resulted in extensive applications [10], such as electrode materials, molecular and other types of sensors, hydrogen storage materials, and data storage [11]. Graphene is the material used

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on thickness for the vacuum layer in the z direction to avoid the interaction between occasional images. The Brillouin zone (BZ) of the three borophenes is sampled using 1 × 2 × 1, 3 × 2 × 1, and 3 × 4 × 1 mesh points in the k-space base on the Monkhorst-Pack scheme [44]. Structural optimization is performed to relax the structure until the change in the energy and Hellmann-Feynman forces acting on the structure is less than 1.0 × 10 −8 eV/atom and 0.02 eV/ Nanomaterials 2023, 13, x FOR PEER REVIEW

Analysis of the Overall Trend of Gas Adsorption for Five Ga
The bond lengths of CO, NH3, NO2, SO2, and H2S are known t Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophen which different harmful gases were adsorbed. The distance from was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The are simple compounds composed of two elements. Therefore, eac adsorption methods. The configuration in which the atom corresp ment in the molecular gas formula of the gas was closer to χ3-bo atom was represented by −1. The configuration in which the atom second element in the molecular formula of the gas was closer to other atom was represented by −2. Therefore, a total of ten adsorpti investigated. Table 2 shows the adsorption energy, number of tran tance to the bottom borophene for the harmful gases adsorbed borophene. An appropriate sensor requires both sensitivity and structure of the adsorbed gas has sufficient charge transfer and a energy, it proves that χ3-borophene can be used as an applicatio harmful gases. Table 2 shows that the vertical adsorption of the CO molecul and D (D1 and D2) sites shifted the χ3-borophene layer up. The Bfrom 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an force exists between χ3-borophene and the C atom. When the O borophene, it always deflected the structure at this time. The dis molecule and the bottom borophene increased to approximately sponding adsorption energy was approximately −0.1 eV. When th sorbed on χ3-borophene with the C atom closer to the χ3-borophe atom, the distance between the CO and the bottom borophene decr 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 CO vertically on the H site did not lead to considerable rotation of ever, the distance between the gas molecules and the bottom boro proximately 3.1 Å, and the adsorption energy was relatively low. energy (−0.685 eV) was obtained for adsorption on B2 with the C phene than the O atom, which was, therefore, the most stable co adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was c (−0.538~−0.764 eV), the adsorption effect was remarkable, and the borophene (about 1.64 Å). During chemisorption, the B atom mov B bond increased in length (from 1.71 Å to 1.79 Å). However, whe borophene at the H site, the height of the gas molecule above th was slightly different, and the adsorption energy was approxima sorption on the D1 and D2 sites was more unstable than adsorption during which the gas molecules rotated slightly and shifted. The the NH3 was consistent with the adsorption process of the CO. T energy for NH3 (−0.764 eV) was obtained for the B2 site. When χ3-borophene adsorbed the NO2, the adsorption on all s As the N atom in the initial NO2 configuration was closer to the χ atom, the final optimized structures for adsorption at the H, B2, D1 of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As sorption energy (−2.067~−2.073 eV), the distance between the gas tom borophene decreased to approximately 1.55 Å. When the NO2 , respectively. The adsorption systems consist of a 2 × 4 × 1 χ 3 -borophene supercell, a 5 × 4 × 1 2 Pmmn borophene supercell, and a 2 × 2 × 1 8 Pmmn borophene supercell. To evaluate the stability of adsorption and bonding, we calculate the adsorption energy (E ad ) using the following equation: E X+Borophene , E Borophene , and E X are the total energy of borophene with the adsorbed gas molecule, the original borophene, and the isolated gas molecules, respectively. In addition, we calculate the charge transferred from borophene to a gas molecule using the Bader charge analysis code [45]. The transferred charge reflects the change in the electron density between a gas molecule and the borophene surface during the interaction, which is calculated using the following equation: where ρ X+Borophene , ρ Borophene , and ρ X represent the total charge density of borophene with the adsorbed gas molecule, the original borophene, and an isolated gas molecule, respectively.

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was establishe which different harmful gases were adsorbed. The distance from the bottom boro was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated are simple compounds composed of two elements. Therefore, each position selecte adsorption methods. The configuration in which the atom corresponding to the fir ment in the molecular gas formula of the gas was closer to χ3-borophene than the atom was represented by −1. The configuration in which the atom corresponding second element in the molecular formula of the gas was closer to χ3-borophene th other atom was represented by −2. Therefore, a total of ten adsorption configuration investigated. Table 2 shows the adsorption energy, number of transfer electrons, an tance to the bottom borophene for the harmful gases adsorbed at different sites borophene. An appropriate sensor requires both sensitivity and selectivity. Wh structure of the adsorbed gas has sufficient charge transfer and appropriate adso energy, it proves that χ3-borophene can be used as an application sensor for det harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 a and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length ch from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous inter force exists between χ3-borophene and the C atom. When the O atom was closer borophene, it always deflected the structure at this time. The distance between t molecule and the bottom borophene increased to approximately 3.3 Å, and the sponding adsorption energy was approximately −0.1 eV. When the CO molecule w sorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than atom, the distance between the CO and the bottom borophene decreased to approxim 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorp CO vertically on the H site did not lead to considerable rotation of the gas molecule. ever, the distance between the gas molecules and the bottom borophene increased proximately 3.1 Å, and the adsorption energy was relatively low. The highest adso energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3 phene than the O atom, which was, therefore, the most stable configuration for t

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was es which different harmful gases were adsorbed. The distance from the botto was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five inve are simple compounds composed of two elements. Therefore, each positio adsorption methods. The configuration in which the atom corresponding ment in the molecular gas formula of the gas was closer to χ3-borophene atom was represented by −1. The configuration in which the atom corresp second element in the molecular formula of the gas was closer to χ3-borop other atom was represented by −2. Therefore, a total of ten adsorption config investigated. Table 2 shows the adsorption energy, number of transfer elec tance to the bottom borophene for the harmful gases adsorbed at differe borophene. An appropriate sensor requires both sensitivity and selectiv structure of the adsorbed gas has sufficient charge transfer and appropri energy, it proves that χ3-borophene can be used as an application senso harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond le from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormo force exists between χ3-borophene and the C atom. When the O atom wa borophene, it always deflected the structure at this time. The distance be molecule and the bottom borophene increased to approximately 3.3 Å, sponding adsorption energy was approximately −0.1 eV. When the CO mo sorbed on χ3-borophene with the C atom closer to the χ3-borophene struct atom, the distance between the CO and the bottom borophene decreased to 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The CO vertically on the H site did not lead to considerable rotation of the gas m ever, the distance between the gas molecules and the bottom borophene in proximately 3.1 Å, and the adsorption energy was relatively low. The high energy (−0.685 eV) was obtained for adsorption on B2 with the C atom clo phene than the O atom, which was, therefore, the most stable configurati

Analysis of the Overall Trend of Gas Adsorption for Five
The bond lengths of CO, NH3, NO2, SO2, and H2S are know Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borop which different harmful gases were adsorbed. The distance fr was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. are simple compounds composed of two elements. Therefore, adsorption methods. The configuration in which the atom cor ment in the molecular gas formula of the gas was closer to χ atom was represented by −1. The configuration in which the second element in the molecular formula of the gas was close other atom was represented by −2. Therefore, a total of ten adso investigated. Table 2 shows the adsorption energy, number of tance to the bottom borophene for the harmful gases adsorb borophene. An appropriate sensor requires both sensitivity structure of the adsorbed gas has sufficient charge transfer a energy, it proves that χ3-borophene can be used as an appli harmful gases. Table 2 shows that the vertical adsorption of the CO mole and D (D1 and D2) sites shifted the χ3-borophene layer up. Th from 1.71 Å to 1.79 Å, forming a C-B bond. These results mea force exists between χ3-borophene and the C atom. When th borophene, it always deflected the structure at this time. The molecule and the bottom borophene increased to approxima sponding adsorption energy was approximately −0.1 eV. Whe sorbed on χ3-borophene with the C atom closer to the χ3-boro atom, the distance between the CO and the bottom borophene d 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0. CO vertically on the H site did not lead to considerable rotation ever, the distance between the gas molecules and the bottom b proximately 3.1 Å, and the adsorption energy was relatively lo energy (−0.685 eV) was obtained for adsorption on B2 with th phene than the O atom, which was, therefore, the most stabl , respectively, in agreement with the previously reported results [46]. The coordination numbers of B1 and B2 were 5 and 4, respectively. The Nanomaterials 2023, 13, 2117 4 of 16 optimized χ 3 -borophene was flat, without ripples along the A and B directions, and spliced by a triangular and hexagonal lattice. Several sites for adsorbing harmful gases were selected to determine the optimal adsorption configuration. These sites are shown in Figure 1a, where B, D, and H represent the top, bridge, and middle vacancy points, respectively. Nanomaterials 2023, 13, x FOR PEER REVIEW 4 of 16 lengths were 1.64 Å, 1.62 Å, and 1.71 Å, respectively, in agreement with the previously reported results [46]. The coordination numbers of B1 and B2 were 5 and 4, respectively. The optimized χ3-borophene was flat, without ripples along the A and B directions, and spliced by a triangular and hexagonal lattice. Several sites for adsorbing harmful gases were selected to determine the optimal adsorption configuration. These sites are shown in Figure 1a, where B, D, and H represent the top, bridge, and middle vacancy points, respectively.   Figure 2a shows the calculated band structure of χ3-borophene, where the high symmetry points follow a G-X-S-Y-G route in the reciprocal space of the BZ. From the electronic band structure and TDOS, one can find the typical metallic behavior and apparent anisotropy for χ3-borophene. At the Fermi level, χ3-borophene has a density of state (DOS) of 2.417 per eV. In the X-S and Y-G directions, multiple electron energy bands pass through the Fermi level, where the main contributions are from the py and pz orbitals of the B atom. However, in the S-Y direction, the electron bands near the Fermi level are relatively flat, and the electronic states are more localized, indicating that the electronic properties vary with the directions. Figure 2b shows the phonon spectrum along several highly symmetric paths, where no imaginary frequency arises in the BZ. Hence, the χ3borophene structure is stable. The optical phonon branch has a high eigenvalue of 38.89 THz, whereas the eigenvalue of graphene is 47.98 THz [48], showing that χ3-borophene is dynamically stable. The bond strength between boron atoms is comparable to that of the C-C bond.  a/ lengths were 1.64 Å, 1.62 Å, and 1.71 Å, respectively, in agreement with the previously reported results [46]. The coordination numbers of B1 and B2 were 5 and 4, respectively. The optimized χ3-borophene was flat, without ripples along the A and B directions, and spliced by a triangular and hexagonal lattice. Several sites for adsorbing harmful gases were selected to determine the optimal adsorption configuration. These sites are shown in Figure 1a, where B, D, and H represent the top, bridge, and middle vacancy points, respectively.   Figure 2a shows the calculated band structure of χ3-borophene, where the high symmetry points follow a G-X-S-Y-G route in the reciprocal space of the BZ. From the electronic band structure and TDOS, one can find the typical metallic behavior and apparent anisotropy for χ3-borophene. At the Fermi level, χ3-borophene has a density of state (DOS) of 2.417 per eV. In the X-S and Y-G directions, multiple electron energy bands pass through the Fermi level, where the main contributions are from the py and pz orbitals of the B atom. However, in the S-Y direction, the electron bands near the Fermi level are relatively flat, and the electronic states are more localized, indicating that the electronic properties vary with the directions. Figure 2b shows the phonon spectrum along several highly symmetric paths, where no imaginary frequency arises in the BZ. Hence, the χ3borophene structure is stable. The optical phonon branch has a high eigenvalue of 38.89 THz, whereas the eigenvalue of graphene is 47.98 THz [48], showing that χ3-borophene is dynamically stable. The bond strength between boron atoms is comparable to that of the C-C bond. b/ lengths were 1.64 Å, 1.62 Å, and 1.71 Å, respectively, in agreement reported results [46]. The coordination numbers of B1 and B2 were 5 The optimized χ3-borophene was flat, without ripples along the A a spliced by a triangular and hexagonal lattice. Several sites for adso were selected to determine the optimal adsorption configuration. T in Figure 1a, where B, D, and H represent the top, bridge, and mid respectively.   [47] 8.42 Figure 2a shows the calculated band structure of χ3-borophene, metry points follow a G-X-S-Y-G route in the reciprocal space of th tronic band structure and TDOS, one can find the typical metallic be anisotropy for χ3-borophene.
At the Fermi level, χ3-borophene has a d of 2.417 per eV. In the X-S and Y-G directions, multiple electron through the Fermi level, where the main contributions are from the the B atom. However, in the S-Y direction, the electron bands near relatively flat, and the electronic states are more localized, indicatin properties vary with the directions. Figure 2b shows the phonon spe highly symmetric paths, where no imaginary frequency arises in th borophene structure is stable. The optical phonon branch has a high THz, whereas the eigenvalue of graphene is 47.98 THz [48], showing dynamically stable. The bond strength between boron atoms is comp C-C bond.  Figure 2a shows the calculated band structure of χ 3 -borophene, where the high symmetry points follow a G-X-S-Y-G route in the reciprocal space of the BZ. From the electronic band structure and TDOS, one can find the typical metallic behavior and apparent anisotropy for χ 3 -borophene. At the Fermi level, χ 3 -borophene has a density of state (DOS) of 2.417 per eV. In the X-S and Y-G directions, multiple electron energy bands pass through the Fermi level, where the main contributions are from the p y and p z orbitals of the B atom. However, in the S-Y direction, the electron bands near the Fermi level are relatively flat, and the electronic states are more localized, indicating that the electronic properties vary with the directions. Figure 2b shows the phonon spectrum along several highly symmetric paths, where no imaginary frequency arises in the BZ. Hence, the χ 3 -borophene structure is stable. The optical phonon branch has a high eigenvalue of 38.89 THz, whereas the eigenvalue of graphene is 47.98 THz [48], showing that χ 3 -borophene is dynamically stable. The bond strength between boron atoms is comparable to that of the C-C bond. Nanomaterials 2023, 13, x FOR PEER REVIEW 4 of 16 lengths were 1.64 Å, 1.62 Å, and 1.71 Å, respectively, in agreement with the previously reported results [46]. The coordination numbers of B1 and B2 were 5 and 4, respectively. The optimized χ3-borophene was flat, without ripples along the A and B directions, and spliced by a triangular and hexagonal lattice. Several sites for adsorbing harmful gases were selected to determine the optimal adsorption configuration. These sites are shown in Figure 1a, where B, D, and H represent the top, bridge, and middle vacancy points, respectively.   Figure 2a shows the calculated band structure of χ3-borophene, where the high symmetry points follow a G-X-S-Y-G route in the reciprocal space of the BZ. From the electronic band structure and TDOS, one can find the typical metallic behavior and apparent anisotropy for χ3-borophene. At the Fermi level, χ3-borophene has a density of state (DOS) of 2.417 per eV. In the X-S and Y-G directions, multiple electron energy bands pass through the Fermi level, where the main contributions are from the py and pz orbitals of the B atom. However, in the S-Y direction, the electron bands near the Fermi level are relatively flat, and the electronic states are more localized, indicating that the electronic properties vary with the directions. Figure 2b shows the phonon spectrum along several highly symmetric paths, where no imaginary frequency arises in the BZ. Hence, the χ3borophene structure is stable. The optical phonon branch has a high eigenvalue of 38.89 THz, whereas the eigenvalue of graphene is 47.98 THz [48], showing that χ3-borophene is dynamically stable. The bond strength between boron atoms is comparable to that of the C-C bond. The bond lengths of CO, NH 3 , NO 2 , SO 2 , and H 2 S are known to be 1.13 harmful gases. energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site. When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on , 1.448 energy, it proves that χ3-borophene can be used as an application sensor for detectin harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length chang from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interacti force exists between χ3-borophene and the C atom. When the O atom was closer to χ borophene, it always deflected the structure at this time. The distance between the C molecule and the bottom borophene increased to approximately 3.3 Å, and the corr sponding adsorption energy was approximately −0.1 eV. When the CO molecule was a sorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the atom, the distance between the CO and the bottom borophene decreased to approximate 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption CO vertically on the H site did not lead to considerable rotation of the gas molecule. How ever, the distance between the gas molecules and the bottom borophene increased to a proximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorpti energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-bor phene than the O atom, which was, therefore, the most stable configuration for the C adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophe (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ borophene at the H site, the height of the gas molecule above the χ3-borophene surfa was slightly different, and the adsorption energy was approximately −0.134 eV. The a sorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 site during which the gas molecules rotated slightly and shifted. The adsorption process the NH3 was consistent with the adsorption process of the CO. The highest adsorpti energy for NH3 (−0.764 eV) was obtained for the B2 site. When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorptio As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consist of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high a sorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bo tom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed , and 1.543 energy, it proves that χ3-borophene can be used as an application sens harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on t and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enorm force exists between χ3-borophene and the C atom. When the O atom w borophene, it always deflected the structure at this time. The distance molecule and the bottom borophene increased to approximately 3.3 Å sponding adsorption energy was approximately −0.1 eV. When the CO m sorbed on χ3-borophene with the C atom closer to the χ3-borophene stru atom, the distance between the CO and the bottom borophene decreased t 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). Th CO vertically on the H site did not lead to considerable rotation of the gas ever, the distance between the gas molecules and the bottom borophene proximately 3.1 Å, and the adsorption energy was relatively low. The hig energy (−0.685 eV) was obtained for adsorption on B2 with the C atom c phene than the O atom, which was, therefore, the most stable configur adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height borophene (about 1.64 Å). During chemisorption, the B atom moved upw B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 borophene at the H site, the height of the gas molecule above the χ3-bo was slightly different, and the adsorption energy was approximately −0 sorption on the D1 and D2 sites was more unstable than adsorption on the during which the gas molecules rotated slightly and shifted. The adsor the NH3 was consistent with the adsorption process of the CO. The hig energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites wa As the N atom in the initial NO2 configuration was closer to the χ3-borop atom, the final optimized structures for adsorption at the H, B2, D1, and D of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a resul sorption energy (−2.067~−2.073 eV), the distance between the gas molecu tom borophene decreased to approximately 1.55 Å. When the NO2 molecu , respectively. A monolayer of χ 3 -borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 borophene. An appropriate sensor requires both sensitivity and selectivity structure of the adsorbed gas has sufficient charge transfer and appropriate energy, it proves that χ3-borophene can be used as an application sensor fo harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond leng from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous force exists between χ3-borophene and the C atom. When the O atom was c borophene, it always deflected the structure at this time. The distance betw molecule and the bottom borophene increased to approximately 3.3 Å, and sponding adsorption energy was approximately −0.1 eV. When the CO molec sorbed on χ3-borophene with the C atom closer to the χ3-borophene structure atom, the distance between the CO and the bottom borophene decreased to app 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The ad CO vertically on the H site did not lead to considerable rotation of the gas mole ever, the distance between the gas molecules and the bottom borophene incre proximately 3.1 Å, and the adsorption energy was relatively low. The highest energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer phene than the O atom, which was, therefore, the most stable configuration adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3 (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was c borophene (about 1.64 Å). During chemisorption, the B atom moved upward, B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adso borophene at the H site, the height of the gas molecule above the χ3-boroph was slightly different, and the adsorption energy was approximately −0.134 sorption on the D1 and D2 sites was more unstable than adsorption on the B1 a during which the gas molecules rotated slightly and shifted. The adsorption the NH3 was consistent with the adsorption process of the CO. The highest energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was che As the N atom in the initial NO2 configuration was closer to the χ3-borophene atom, the final optimized structures for adsorption at the H, B2, D1, and D2 site of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of t sorption energy (−2.067~−2.073 eV), the distance between the gas molecules a tom borophene decreased to approximately 1.55 Å. When the NO2 molecules a to control the variable. We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ 3 -borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ 3 -borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ 3 -borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ 3 -borophene can be used as an application sensor for detecting harmful gases. Table 2. The adsorption energy, transfer electron number, height, and structural changes of the adsorption of harmful gases at different sites on χ 3 -borophene.

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on )  The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 Å. When the NO2 molecules adsorbed on  The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. As the N atom in the initial NO2 configuration was closer to the χ3-borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bot-  Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorption. The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1. Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established on which different harmful gases were adsorbed. The distance from the bottom borophen was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gas are simple compounds composed of two elements. Therefore, each position selected tw adsorption methods. The configuration in which the atom corresponding to the first el ment in the molecular gas formula of the gas was closer to χ3-borophene than the oth atom was represented by −1. The configuration in which the atom corresponding to th second element in the molecular formula of the gas was closer to χ3-borophene than th other atom was represented by −2. Therefore, a total of ten adsorption configurations we investigated. Table 2 shows the adsorption energy, number of transfer electrons, and d tance to the bottom borophene for the harmful gases adsorbed at different sites on χ borophene. An appropriate sensor requires both sensitivity and selectivity. When t structure of the adsorbed gas has sufficient charge transfer and appropriate adsorptio energy, it proves that χ3-borophene can be used as an application sensor for detectin harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length change from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interactio force exists between χ3-borophene and the C atom. When the O atom was closer to χ borophene, it always deflected the structure at this time. The distance between the C molecule and the bottom borophene increased to approximately 3.3 Å, and the corr sponding adsorption energy was approximately −0.1 eV. When the CO molecule was a sorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the atom, the distance between the CO and the bottom borophene decreased to approximate 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption CO vertically on the H site did not lead to considerable rotation of the gas molecule. How ever, the distance between the gas molecules and the bottom borophene increased to a proximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorptio energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-bor phene than the O atom, which was, therefore, the most stable configuration for the C adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophen (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ borophene at the H site, the height of the gas molecule above the χ3-borophene surfa was slightly different, and the adsorption energy was approximately −0.134 eV. The a sorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 site during which the gas molecules rotated slightly and shifted. The adsorption process the NH3 was consistent with the adsorption process of the CO. The highest adsorptio energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites was chemisorptio , forming a C-B bond. These results mean an enormous interaction force exists between χ 3 -borophene and the C atom. When the O atom was closer to χ 3 -borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3

Analysis of the Overall Trend of Gas Adso
The bond lengths of CO, NH3, NO2, SO2, and Å, 1.448 Å, and 1.543 Å, respectively. A monolay which different harmful gases were adsorbed. T was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D are simple compounds composed of two elemen adsorption methods. The configuration in which ment in the molecular gas formula of the gas w atom was represented by −1. The configuration second element in the molecular formula of the other atom was represented by −2. Therefore, a to investigated. Table 2 shows the adsorption energ tance to the bottom borophene for the harmful borophene. An appropriate sensor requires bo structure of the adsorbed gas has sufficient cha energy, it proves that χ3-borophene can be use harmful gases. Table 2 shows that the vertical adsorption o and D (D1 and D2) sites shifted the χ3-borophen from 1.71 Å to 1.79 Å, forming a C-B bond. The force exists between χ3-borophene and the C at borophene, it always deflected the structure at molecule and the bottom borophene increased sponding adsorption energy was approximately sorbed on χ3-borophene with the C atom closer atom, the distance between the CO and the bottom 1.5 Å, resulting in the higher adsorption efficien CO vertically on the H site did not lead to consid ever, the distance between the gas molecules an proximately 3.1 Å, and the adsorption energy w energy (−0.685 eV) was obtained for adsorption phene than the O atom, which was, therefore, t adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when th (−0.538~−0.764 eV), the adsorption effect was rem borophene (about 1.64 Å). During chemisorption B bond increased in length (from 1.71 Å to 1.79 Å borophene at the H site, the height of the gas m was slightly different, and the adsorption energ sorption on the D1 and D2 sites was more unstab during which the gas molecules rotated slightly , and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ 3 -borophene with the C atom closer to the χ 3 -borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.

Analysis of the Overall Trend o
The bond lengths of CO, NH3, N Å, 1.448 Å, and 1.543 Å, respectively which different harmful gases were was set to 2 Å to control the variable We selected five adsorption site are simple compounds composed of adsorption methods. The configurat ment in the molecular gas formula o atom was represented by −1. The co second element in the molecular for other atom was represented by −2. Th investigated. Table 2 shows the adso tance to the bottom borophene for borophene. An appropriate sensor structure of the adsorbed gas has su energy, it proves that χ3-borophene harmful gases. Table 2 shows that the vertical a and D (D1 and D2) sites shifted the χ from 1.71 Å to 1.79 Å, forming a C-B force exists between χ3-borophene a borophene, it always deflected the s molecule and the bottom borophen sponding adsorption energy was app sorbed on χ3-borophene with the C a atom, the distance between the CO an 1.5 Å, resulting in the higher adsorp CO vertically on the H site did not lea ever, the distance between the gas m proximately 3.1 Å, and the adsorptio energy (−0.685 eV) was obtained for phene than the O atom, which was, adsorption on the χ3-borophene laye At the B1, B2, D1, and D2 sit (−0.538~−0.764 eV), the adsorption eff borophene (about 1.64 Å). During ch B bond increased in length (from 1.7 borophene at the H site, the height , resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1

Analysis of the Overall Tren
The bond lengths of CO, NH Å, 1.448 Å, and 1.543 Å, respectiv which different harmful gases we was set to 2 Å to control the varia We selected five adsorption s are simple compounds composed adsorption methods. The configu ment in the molecular gas formu atom was represented by −1. The second element in the molecular other atom was represented by −2 investigated. Table 2 shows the ad tance to the bottom borophene f borophene. An appropriate sens structure of the adsorbed gas has energy, it proves that χ3-borophe harmful gases. Table 2 shows that the vertic and D (D1 and D2) sites shifted th from 1.71 Å to 1.79 Å, forming a force exists between χ3-borophen borophene, it always deflected th molecule and the bottom boroph sponding adsorption energy was sorbed on χ3-borophene with the atom, the distance between the CO 1.5 Å, resulting in the higher ads CO vertically on the H site did not ever, the distance between the ga proximately 3.1 Å, and the adsorp energy (−0.685 eV) was obtained phene than the O atom, which w adsorption on the χ3-borophene la At the B1, B2, D1, and D2 , and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ 3 -borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ 3 -borophene layer. At the B1, B2, D1, and D2 sites, when the N atom was closer to χ 3 -borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ 3 -borophene (about 1.64 harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on t and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enor force exists between χ3-borophene and the C atom. When the O atom borophene, it always deflected the structure at this time. The distance molecule and the bottom borophene increased to approximately 3.3 Å sponding adsorption energy was approximately −0.1 eV. When the CO m sorbed on χ3-borophene with the C atom closer to the χ3-borophene stru atom, the distance between the CO and the bottom borophene decreased 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). T CO vertically on the H site did not lead to considerable rotation of the gas ever, the distance between the gas molecules and the bottom borophene proximately 3.1 Å, and the adsorption energy was relatively low. The hi energy (−0.685 eV) was obtained for adsorption on B2 with the C atom phene than the O atom, which was, therefore, the most stable configur adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height borophene (about 1.64 Å). During chemisorption, the B atom moved up B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH borophene at the H site, the height of the gas molecule above the χ3-bo was slightly different, and the adsorption energy was approximately −0 sorption on the D1 and D2 sites was more unstable than adsorption on th during which the gas molecules rotated slightly and shifted. The adsor the NH3 was consistent with the adsorption process of the CO. The hi energy for NH3 (−0.764 eV) was obtained for the B2 site.
When χ3-borophene adsorbed the NO2, the adsorption on all sites wa As the N atom in the initial NO2 configuration was closer to the χ3-borop atom, the final optimized structures for adsorption at the H, B2, D1, and D of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a resu sorption energy (−2.067~−2.073 eV), the distance between the gas molec tom borophene decreased to approximately 1.55 Å. When the NO2 molec ). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 energy, it proves that χ3-borophene can be used as an harmful gases. Table 2 shows that the vertical adsorption of the CO and D (D1 and D2) sites shifted the χ3-borophene layer u from 1.71 Å to 1.79 Å, forming a C-B bond. These result force exists between χ3-borophene and the C atom. Wh borophene, it always deflected the structure at this tim molecule and the bottom borophene increased to appr sponding adsorption energy was approximately −0.1 eV. sorbed on χ3-borophene with the C atom closer to the χ3 atom, the distance between the CO and the bottom boroph 1.5 Å, resulting in the higher adsorption efficiency (−0.4 CO vertically on the H site did not lead to considerable ro ever, the distance between the gas molecules and the bo proximately 3.1 Å, and the adsorption energy was relativ energy (−0.685 eV) was obtained for adsorption on B2 w phene than the O atom, which was, therefore, the most adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N ato (−0.538~−0.764 eV), the adsorption effect was remarkable borophene (about 1.64 Å). During chemisorption, the B a B bond increased in length (from 1.71 Å to 1.79 Å). How borophene at the H site, the height of the gas molecule was slightly different, and the adsorption energy was ap sorption on the D1 and D2 sites was more unstable than a during which the gas molecules rotated slightly and sh the NH3 was consistent with the adsorption process of energy for NH3 (−0.764 eV) was obtained for the B2 site. When χ3-borophene adsorbed the NO2, the adsorptio As the N atom in the initial NO2 configuration was close atom, the final optimized structures for adsorption at the of N-O bonds parallel to the D2, N-B bonds, and O-B bo sorption energy (−2.067~−2.073 eV), the distance between tom borophene decreased to approximately 1.55 Å. When to 1.79 energy, it proves that χ3-borophene can be us harmful gases. Table 2 shows that the vertical adsorption and D (D1 and D2) sites shifted the χ3-borophe from 1.71 Å to 1.79 Å, forming a C-B bond. Th force exists between χ3-borophene and the C a borophene, it always deflected the structure a molecule and the bottom borophene increased sponding adsorption energy was approximatel sorbed on χ3-borophene with the C atom closer atom, the distance between the CO and the botto 1.5 Å, resulting in the higher adsorption efficie CO vertically on the H site did not lead to consid ever, the distance between the gas molecules an proximately 3.1 Å, and the adsorption energy w energy (−0.685 eV) was obtained for adsorption phene than the O atom, which was, therefore, adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when (−0.538~−0.764 eV), the adsorption effect was re borophene (about 1.64 Å). During chemisorptio B bond increased in length (from 1.71 Å to 1.79 borophene at the H site, the height of the gas was slightly different, and the adsorption ener sorption on the D1 and D2 sites was more unsta during which the gas molecules rotated slight the NH3 was consistent with the adsorption p energy for NH3 (−0.764 eV) was obtained for th When χ3-borophene adsorbed the NO2, the As the N atom in the initial NO2 configuration atom, the final optimized structures for adsorpt of N-O bonds parallel to the D2, N-B bonds, a sorption energy (−2.067~−2.073 eV), the distanc tom borophene decreased to approximately 1.55 ). However, when NH 3 adsorbed on χ 3 -borophene at the H site, the height of the gas molecule above the χ 3 -borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH 3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH 3 (−0.764 eV) was obtained for the B2 site.
When χ 3 -borophene adsorbed the NO 2 , the adsorption on all sites was chemisorption. As the N atom in the initial NO 2 configuration was closer to the χ 3 -borophene than the O atom, the final optimized structures for adsorption at the H, B2, D1, and D2 sites consisted of N-O bonds parallel to the D2, N-B bonds, and O-B bonds. As a result of the high adsorption energy (−2.067~−2.073 eV), the distance between the gas molecules and the bottom borophene decreased to approximately 1.55 was set to 2 Å to control the variable.
We selected five adsorption sites: B1 are simple compounds composed of two adsorption methods. The configuration i ment in the molecular gas formula of th atom was represented by −1. The config second element in the molecular formul other atom was represented by −2. Theref investigated. Table 2 shows the adsorptio tance to the bottom borophene for the h borophene. An appropriate sensor requ structure of the adsorbed gas has suffici energy, it proves that χ3-borophene can harmful gases. Table 2 shows that the vertical adso and D (D1 and D2) sites shifted the χ3-bo from 1.71 Å to 1.79 Å, forming a C-B bon force exists between χ3-borophene and t borophene, it always deflected the struc molecule and the bottom borophene in sponding adsorption energy was approx sorbed on χ3-borophene with the C atom atom, the distance between the CO and th 1.5 Å, resulting in the higher adsorption CO vertically on the H site did not lead to ever, the distance between the gas molec proximately 3.1 Å, and the adsorption en energy (−0.685 eV) was obtained for adso phene than the O atom, which was, ther adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, w (−0.538~−0.764 eV), the adsorption effect borophene (about 1.64 Å). During chemi B bond increased in length (from 1.71 Å borophene at the H site, the height of th was slightly different, and the adsorptio sorption on the D1 and D2 sites was more during which the gas molecules rotated the NH3 was consistent with the adsorp energy for NH3 (−0.764 eV) was obtained When χ3-borophene adsorbed the N As the N atom in the initial NO2 configu atom, the final optimized structures for a of N-O bonds parallel to the D2, N-B bo sorption energy (−2.067~−2.073 eV), the d tom borophene decreased to approximat . When the NO 2 molecules adsorbed on the B1 site, with the N atom closer to the χ 3 -borophene than the O atoms, the optimized structure remained unchanged, but the adsorption was strong (−1.245 eV). Adsorption of the five selected sites with the O atoms closer to χ 3 -borophene than the N atom resulted in the formation of O-B bonds. Compared to the results for adsorption with the N atom closer to χ 3 -borophene than the O atoms, the NO 2 adsorption shifted from B1 to D1, from B2 to D2, and from H to the left. This result showed that the NO 2 adsorption on χ 3 -borophene was stronger when the N atoms were closer to χ 3 -borophene than the O atoms. High energy is required for NO 2 to adsorb on the χ 3 -borophene surface relative to that required for adsorption on graphene [39] and blue-black phosphorene [49].
The adsorption of SO 2 on χ 3 -borophene occurred via physisorption at all sites. For the direct adsorption at the H site, the bond angle of the small gas molecule did not change, but the gas molecules were far from the bottom of χ 3 -borophene (approximately 3.0

Analysis of th
The bond lengt Å, 1.448 Å, and 1.54 which different har was set to 2 Å to con We selected fiv are simple compoun adsorption method ment in the molecu atom was represent second element in t other atom was repr investigated. Table  tance to the bottom borophene. An app structure of the ads energy, it proves th harmful gases. Table 2 shows and D (D1 and D2) from 1.71 Å to 1.79 force exists between borophene, it alway molecule and the b sponding adsorptio sorbed on χ3-borop atom, the distance b 1.5 Å, resulting in t CO vertically on the ever, the distance b proximately 3.1 Å, a energy (−0.685 eV) w phene than the O a adsorption on the χ At the B1, B2, (−0.538~−0.764 eV), borophene (about 1 B bond increased in borophene at the H was slightly differe sorption on the D1 a during which the g the NH3 was consis energy for NH3 (−0. When χ3-borop As the N atom in th atom, the final optim of N-O bonds paral sorption energy (−2 tom borophene decr ), and the adsorption energy was minimal. The adsorption was strongest when the S atom was closer to the χ 3 -borophene than the O atoms, indicating that the interaction force between the χ 3 -borophene and the S atom was higher than that between the χ 3 -borophene and the O atom. The angle between the S and O atoms deflected when the gas molecules adsorbed on the bridge sites (D1 and D2). On the other hand, when the SO 2 adsorbed on the B site (B1 or B2), the angle deflection and movement of the gas molecule along the a-and b-axis occurred, and the gas molecule moved above the bridge site, suggesting that the adsorption on the B site (B1, B2) was unstable.
Similarly, the adsorption of H 2 S on χ 3 -borophene occurred via physisorption at all sites. The optimization of the adsorption at the aforementioned ten sites resulted in almost no movement of the gas molecule along the ab plane and an indiscernible deflection of the H 2 S bond angle. When adsorption occurred with the H atom closer to the χ 3 -borophene than the S atom, the distance between the H atom and the borophene bottom ranged between 2.40

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, during which the gas molecules rotated slightly and shifted. The adsorption process of the NH3 was consistent with the adsorption process of the CO. The highest adsorption energy for NH3 (−0.764 eV) was obtained for the B2 site.

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established o which different harmful gases were adsorbed. The distance from the bottom boroph was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated g are simple compounds composed of two elements. Therefore, each position selected adsorption methods. The configuration in which the atom corresponding to the first ment in the molecular gas formula of the gas was closer to χ3-borophene than the o atom was represented by −1. The configuration in which the atom corresponding to second element in the molecular formula of the gas was closer to χ3-borophene than other atom was represented by −2. Therefore, a total of ten adsorption configurations w investigated. Table 2 shows the adsorption energy, number of transfer electrons, and tance to the bottom borophene for the harmful gases adsorbed at different sites on borophene. An appropriate sensor requires both sensitivity and selectivity. When structure of the adsorbed gas has sufficient charge transfer and appropriate adsorp energy, it proves that χ3-borophene can be used as an application sensor for detec harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length chan from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interac force exists between χ3-borophene and the C atom. When the O atom was closer to borophene, it always deflected the structure at this time. The distance between the molecule and the bottom borophene increased to approximately 3.3 Å, and the co sponding adsorption energy was approximately −0.1 eV. When the CO molecule was sorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than th atom, the distance between the CO and the bottom borophene decreased to approxima 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorptio CO vertically on the H site did not lead to considerable rotation of the gas molecule. H ever, the distance between the gas molecules and the bottom borophene increased to proximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorp energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-b phene than the O atom, which was, therefore, the most stable configuration for the adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-boroph (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and th B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on borophene at the H site, the height of the gas molecule above the χ3-borophene sur was slightly different, and the adsorption energy was approximately −0.134 eV. The sorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 s during which the gas molecules rotated slightly and shifted. The adsorption proces the NH3 was consistent with the adsorption process of the CO. The highest adsorp energy for NH3 (−0.764 eV) was obtained for the B2 site.
, and the adsorption energy ranged between −0.074 eV and −0.081 eV. By comparison, when adsorption occurred with the S atom closer to the borophene surface than the H atom, the distance between the H atom and the bottom borophene was shorter

Analysis of the Overall Trend of Gas Adsorption for Five Gases
The bond lengths of CO, NH3, NO2, SO2, and H2S are known to be 1.13 Å, 1.01 Å, 1.20 Å, 1.448 Å, and 1.543 Å, respectively. A monolayer of χ3-borophene was established onto which different harmful gases were adsorbed. The distance from the bottom borophene was set to 2 Å to control the variable.
We selected five adsorption sites: B1, B2, D1, D2, and H. The five investigated gases are simple compounds composed of two elements. Therefore, each position selected two adsorption methods. The configuration in which the atom corresponding to the first element in the molecular gas formula of the gas was closer to χ3-borophene than the other atom was represented by −1. The configuration in which the atom corresponding to the second element in the molecular formula of the gas was closer to χ3-borophene than the other atom was represented by −2. Therefore, a total of ten adsorption configurations were investigated. Table 2 shows the adsorption energy, number of transfer electrons, and distance to the bottom borophene for the harmful gases adsorbed at different sites on χ3borophene. An appropriate sensor requires both sensitivity and selectivity. When the structure of the adsorbed gas has sufficient charge transfer and appropriate adsorption energy, it proves that χ3-borophene can be used as an application sensor for detecting harmful gases. Table 2 shows that the vertical adsorption of the CO molecules on the B (B1 and B2) and D (D1 and D2) sites shifted the χ3-borophene layer up. The B-B bond length changed from 1.71 Å to 1.79 Å, forming a C-B bond. These results mean an enormous interaction force exists between χ3-borophene and the C atom. When the O atom was closer to χ3borophene, it always deflected the structure at this time. The distance between the CO molecule and the bottom borophene increased to approximately 3.3 Å, and the corresponding adsorption energy was approximately −0.1 eV. When the CO molecule was adsorbed on χ3-borophene with the C atom closer to the χ3-borophene structure than the O atom, the distance between the CO and the bottom borophene decreased to approximately 1.5 Å, resulting in the higher adsorption efficiency (−0.44~−0.685 eV). The adsorption of CO vertically on the H site did not lead to considerable rotation of the gas molecule. However, the distance between the gas molecules and the bottom borophene increased to approximately 3.1 Å, and the adsorption energy was relatively low. The highest adsorption energy (−0.685 eV) was obtained for adsorption on B2 with the C atom closer to χ3-borophene than the O atom, which was, therefore, the most stable configuration for the CO adsorption on the χ3-borophene layer.
At the B1, B2, D1, and D2 sites, when the N atom was closer to χ3-borophene (−0.538~−0.764 eV), the adsorption effect was remarkable, and the height was closer to χ3borophene (about 1.64 Å). During chemisorption, the B atom moved upward, and the B-B bond increased in length (from 1.71 Å to 1.79 Å). However, when NH3 adsorbed on χ3borophene at the H site, the height of the gas molecule above the χ3-borophene surface was slightly different, and the adsorption energy was approximately −0.134 eV. The adsorption on the D1 and D2 sites was more unstable than adsorption on the B1 and B2 sites, ), and the adsorption energy was higher (−0.032~−0.040 eV). In summary, CO and NH 3 adsorbed onto χ 3 -borophene by both physisorption and chemisorption. When a CO molecule adsorbed at a B site (B1 or B2) or a bridge site D (D1 or D2), the adsorption was stronger when the C atom was closer to the bottom borophene than the O atom (C > O). Similarly, the adsorption of NH 3 was stronger when the N atom was closer to the bottom borophene than the H atom (N > H). Chemisorption occurred when the C and N atoms were closer to the bottom borophene than the O and H atoms, respectively, whereas physisorption occurred when the O and H atoms were closer to the bottom borophene than the C and N atoms, respectively. Finally, the adsorption was stronger at the B site than at the bridge site (B > D).
In addition, both the SO 2 and H 2 S adsorbed on the χ 3 -borophene by physisorption. The adsorption of SO 2 was stronger when the S atom was closer to the bottom borophene than the O atoms (S > O). Similarly, the adsorption of H 2 S was stronger when the H atom was closer to the bottom borophene than the S atom (H > S). Finally, the adsorption was stronger at the bridge site than at the top site (D > B). Last, NO 2 adsorbed on the χ 3 -borophene by chemisorption. In the optimized final structure, the N-O bonds oriented parallel to D2 to form N-B and O-B bonds. The adsorption was stronger at the bridge site than at the top site (D > B) and when the gas molecule was closer to the surface than farther away (2 > 1). Finally, when the absolute value of the adsorption energy was smaller, the adsorption was relatively weaker. The optimization of the gas molecule configuration increased the distance between the optimized gas molecule and the bottom χ 3 -borophene, indicating a minor interaction between the gas molecule and χ 3 -borophene.

Electronic Structure of a System of a Gas Molecule Adsorbed on χ 3 -Borophene for Five Different Gases
Based on the data for adsorption on χ 3 -borophene given in Table 2 for the pairs of gas configurations, we selected a representative set of structures for each gas (highlighted in bold in the table). Figure 3 shows the adsorption sites that optimized the system. Chemisorption of gas occurs via the formation of chemical bonds between the gas and χ 3 -borophene, which can cause the χ 3 -borophene to deform [50]. The deformation of χ 3 -borophene can enhance the interaction between the gas molecule and the χ 3 -borophene surface. By contrast, physisorption does not induce changes in the χ 3 -borophene structure. Nanomaterials 2023, 13, x FOR PEER REVIEW 7 o when the C and N atoms were closer to the bottom borophene than the O and H ato respectively, whereas physisorption occurred when the O and H atoms were closer to bottom borophene than the C and N atoms, respectively. Finally, the adsorption w stronger at the B site than at the bridge site (B > D). In addition, both the SO2 and H2S adsorbed on the χ3-borophene by physisorpti The adsorption of SO2 was stronger when the S atom was closer to the bottom boroph than the O atoms (S > O). Similarly, the adsorption of H2S was stronger when the H at was closer to the bottom borophene than the S atom (H > S). Finally, the adsorption w stronger at the bridge site than at the top site (D > B). Last, NO2 adsorbed on the χ3-bo phene by chemisorption. In the optimized final structure, the N-O bonds oriented para to D2 to form N-B and O-B bonds. The adsorption was stronger at the bridge site than the top site (D > B) and when the gas molecule was closer to the surface than farther aw (2 > 1). Finally, when the absolute value of the adsorption energy was smaller, the adso tion was relatively weaker. The optimization of the gas molecule configuration increa the distance between the optimized gas molecule and the bottom χ3-borophene, indicat a minor interaction between the gas molecule and χ3-borophene.

Electronic Structure of a System of a Gas Molecule Adsorbed on χ3-Borophene fo Five Different Gases
Based on the data for adsorption on χ3-borophene given in Table 2 for the pairs of configurations, we selected a representative set of structures for each gas (highlighted bold in the table). Figure 3 shows the adsorption sites that optimized the system. Che sorption of gas occurs via the formation of chemical bonds between the gas and χ3-bo phene, which can cause the χ3-borophene to deform [50]. The deformation of χ3-bo phene can enhance the interaction between the gas molecule and the χ3-borophene s face. By contrast, physisorption does not induce changes in the χ3-borophene structur  Next, we present the contributions of DFT and DFT-D3 to the adsorbed gases and plot the adsorption energy data in Table 3. The system we studied involved weak interaction, while the traditional DFT method has some shortcomings in describing the dispersion interaction. The DFT-D3 considers the geometric information of the structure to calculate the dispersion correction energy, which can reasonably predict the energy of the Van der Waals system. After using such two frameworks to calculate the harmful gases adsorbed by the χ 3 -borophene species, we found that the energy obtained by DFT-D3 was in better agreement with the charge transfer results. Therefore, we considered the DFT-D3 in the adsorption system. Next, we analyzed the charge transfer, electronic band structure, and TDOS of the optimal adsorption sites for the aforementioned five gases. First, we performed a Bader charge analysis to study the stability of the χ 3 -borophene system further. Figure 4 shows a differential charge density plot of χ 3 -borophene, where the yellow and blue regions correspond to the charge accumulation and depletion, respectively. The yellow region around the small gas molecules adsorbed on χ 3 -borophene indicated that charge accumulated near the gas molecules. Figure 3 and Table 2 show that the charge transfer of only 0.025e occurred from χ 3 -borophene to CO. This result suggests that the main interaction between χ 3 -borophene and CO is the Van der Waals interaction, which proves that physisorption was the adsorption mechanism. A qualitative analysis of the differential charge density map of CO molecules above B1 indicates that the charge depletion is likely to occur for adsorption at this site. By comparison, NH 3 and H 2 S are more likely to adsorb above H, whereas NO 2 and SO 2 are more likely to adsorb above D2. Different gases are likely to lose electrons at different positions because of the difference in the gas molecular structures and interactions between each atom in the gas and the B atom. For example, as N-B and O-B bonds form easily, the B atoms of χ 3 -borophene bonded with the N and O atoms of NO 2 at the D2 site. Next, we present the contributions of DFT and DFT-D3 to the adsorbed gases and plot the adsorption energy data in Table 3. The system we studied involved weak interaction, while the traditional DFT method has some shortcomings in describing the dispersion interaction. The DFT-D3 considers the geometric information of the structure to calculate the dispersion correction energy, which can reasonably predict the energy of the Van der Waals system. After using such two frameworks to calculate the harmful gases adsorbed by the χ3-borophene species, we found that the energy obtained by DFT-D3 was in better agreement with the charge transfer results. Therefore, we considered the DFT-D3 in the adsorption system. Next, we analyzed the charge transfer, electronic band structure, and TDOS of the optimal adsorption sites for the aforementioned five gases. First, we performed a Bader charge analysis to study the stability of the χ3-borophene system further. Figure 4 shows a differential charge density plot of χ3-borophene, where the yellow and blue regions correspond to the charge accumulation and depletion, respectively. The yellow region around the small gas molecules adsorbed on χ3-borophene indicated that charge accumulated near the gas molecules. Figure 3 and Table 2 show that the charge transfer of only 0.025e occurred from χ3-borophene to CO. This result suggests that the main interaction between χ3-borophene and CO is the Van der Waals interaction, which proves that physisorption was the adsorption mechanism. A qualitative analysis of the differential charge density map of CO molecules above B1 indicates that the charge depletion is likely to occur for adsorption at this site. By comparison, NH3 and H2S are more likely to adsorb above H, whereas NO2 and SO2 are more likely to adsorb above D2. Different gases are likely to lose electrons at different positions because of the difference in the gas molecular structures and interactions between each atom in the gas and the B atom. For example, as N-B and O-B bonds form easily, the B atoms of χ3-borophene bonded with the N and O atoms of NO2 at the D2 site.  Figure 5 shows an electronic band structure plot. For χ3-borophene with an adsorbed gas molecule, all the bands passed through the Fermi level, as in the case of χ3-borophene without adsorbed gas. Therefore, there was no gap in the band structure of χ3-borophene with an adsorbed gas molecule (for all five gases), indicating a metallic behavior. For NO2, NH3, or CO adsorption, the gas molecules interacted with the occupied and unoccupied electronic states of χ3-borophene far from the Fermi level. Thus, the molecules of these three gases had almost no effect on the electronic properties of χ3-borophene near the Fermi level.  Figure 5 shows an electronic band structure plot. For χ 3 -borophene with an adsorbed gas molecule, all the bands passed through the Fermi level, as in the case of χ 3 -borophene without adsorbed gas. Therefore, there was no gap in the band structure of χ 3 -borophene with an adsorbed gas molecule (for all five gases), indicating a metallic behavior. For NO 2 , NH 3 , or CO adsorption, the gas molecules interacted with the occupied and unoccupied electronic states of χ 3 -borophene far from the Fermi level. Thus, the molecules of these three gases had almost no effect on the electronic properties of χ 3 -borophene near the Fermi level. Figure 6 shows the TDOS diagrams of χ 3 -borophene after the adsorption of a gas molecule for the five gases. The TDOS crossed the Fermi energy level for all five adsorbed gases, proving that adsorption did not change the metallic properties of χ 3 -borophene. The DOS between the two peaks near the Fermi level was not zero for all five gases. This pseudo energy gap directly reflected the covalence of bonding in the χ 3 -borophene-adsorbed gas system. The strongest covalent bonding was observed for the adsorption of NO 2 . Nanomaterials 2023, 13, x FOR PEER REVIEW 9 of Figure 5. Electronic band structures after the adsorption of five gases by χ3-borophene. Figure 6 shows the TDOS diagrams of χ3-borophene after the adsorption of a g molecule for the five gases. The TDOS crossed the Fermi energy level for all five adsorb gases, proving that adsorption did not change the metallic properties of χ3-borophe The DOS between the two peaks near the Fermi level was not zero for all five gases. T pseudo energy gap directly reflected the covalence of bonding in the χ3-borophene-a sorbed gas system. The strongest covalent bonding was observed for the adsorption NO2.     Figure 6 shows the TDOS diagrams of χ3-borophene after the adsorption of a gas molecule for the five gases. The TDOS crossed the Fermi energy level for all five adsorbed gases, proving that adsorption did not change the metallic properties of χ3-borophene. The DOS between the two peaks near the Fermi level was not zero for all five gases. This pseudo energy gap directly reflected the covalence of bonding in the χ3-borophene-adsorbed gas system. The strongest covalent bonding was observed for the adsorption of NO2.     CO-χ3-borophene revealed that the DOS peaks of the p-orbitals of the B atom (B-p) overlapped with those of the C-p and O-p atoms in the −6~−8 eV range of the valence region. It can therefore be understood that χ3-borophene-p interacted with the O and C atoms. We then focused on the PDOS of the gas molecules near the Fermi level. It was found that the B, C, and O atoms all contributed to the TDOS near the Fermi level. In the PDOS diagram of NH3-χ3-borophene, the DOS peak of the B-p orbit overlapped with the N-p and H-s orbit in the −10~−12 eV range of the other valence region. This indicates that the p-orbital of χ3-borophene interacted weakly with the N and H atoms. In the PDOS diagram of NO2-χ3-borophene, the orbitals overlapped in the −8 to 3 eV range. The N-p and O-p orbits transitioned from the conduction band to the valence band near the Fermi level, which confirmed that charge transfer from the χ3-borophene surface to the NO2 molecules caused enhanced metallic properties of the NH3-χ3-borophene system. In the PDOS diagram of SO2-χ3-borophene, the broadening and shift of the peak in the range of −8~2 eV was caused by the electron transfer from χ3-borophene to the SO2 molecule, leading to the overlap of the B-p, S-p, and O-p orbits. Gas adsorption increased the TDOS at the Fermi energy level and enhanced the metallic properties of χ3-borophene for all configurations of the adsorbed gas molecule. In the PDOS diagram of H2S-χ3-borophene, there were several peaks in the range of −6 to −1 eV over which the S-p orbital interacted with the B-p We then focused on the PDOS of the gas molecules near the Fermi level. It was found that the B, C, and O atoms all contributed to the TDOS near the Fermi level. In the PDOS diagram of NH 3 -χ 3 -borophene, the DOS peak of the B-p orbit overlapped with the N-p and H-s orbit in the −10~−12 eV range of the other valence region. This indicates that the p-orbital of χ 3 -borophene interacted weakly with the N and H atoms. In the PDOS diagram of NO 2 -χ 3 -borophene, the orbitals overlapped in the −8 to 3 eV range. The N-p and O-p orbits transitioned from the conduction band to the valence band near the Fermi level, which confirmed that charge transfer from the χ 3 -borophene surface to the NO 2 molecules caused enhanced metallic properties of the NH 3 -χ 3 -borophene system. In the PDOS diagram of SO 2 -χ 3 -borophene, the broadening and shift of the peak in the range of −8~2 eV was caused by the electron transfer from χ 3 -borophene to the SO 2 molecule, leading to the overlap of the B-p, S-p, and O-p orbits. Gas adsorption increased the TDOS at the Fermi energy level and enhanced the metallic properties of χ 3 -borophene for all configurations of the adsorbed gas molecule. In the PDOS diagram of H 2 S-χ 3 -borophene, there were several peaks in the range of −6 to −1 eV over which the S-p orbital interacted with the B-p orbital that all occurred in the valence region. In addition, SO 2 contributed to the Fermi energy state, which may affect the conductivity of SO 2 -χ 3 -borophene.
The Fermi energy level was increased, and the metallic properties of χ 3 -borophene were enhanced by the adsorption of all five gases. The electronic structure analysis shows that χ 3 -borophene has broad prospects as a gas sensor.

2-Pmmn Borophene and 8 Pmmn Borophene
We also studied the adsorption of harmful gases by the two other types of borophene, 2 Pmmn and 8 Pmmn. The adsorption by χ 3 -borophene of the toxic gases was good relative to the other two borophenes. The lattice constant of Pmmn borophene is well-matched with the (110) surface of certain metals or metal oxides. Thus, Pmmn borophene can be synthesized by depositing boron atoms on specific metal substrates. It is known that graphene has been experimentally prepared by this method [10]. Figure 8 shows the structures of 2 Pmmn and 8 Pmmn borophene. The 2 Pmmn borophene is the most studied type of borophene and has no ripples along the a-axis direction and a W-shaped ripple structure along the b-axis with a considerable buckling height. Calculation of the energy band and density of states shows that 2 Pmmn borophene exhibits strong anisotropic metallic properties, which can induce facile electron transfer and electrical conduction at room temperature. However, the absence of ripples along the a-axis direction limits the conductivity of 2 Pmmn borophene. orbital that all occurred in the valence region. In addition, SO2 contributed to the Fermi energy state, which may affect the conductivity of SO2-χ3-borophene. The Fermi energy level was increased, and the metallic properties of χ3-borophene were enhanced by the adsorption of all five gases. The electronic structure analysis shows that χ3-borophene has broad prospects as a gas sensor.

2-Pmmn Borophene and 8 Pmmn Borophene
We also studied the adsorption of harmful gases by the two other types of borophene, 2 Pmmn and 8 Pmmn. The adsorption by χ3-borophene of the toxic gases was good relative to the other two borophenes. The lattice constant of Pmmn borophene is well-matched with the (110) surface of certain metals or metal oxides. Thus, Pmmn borophene can be synthesized by depositing boron atoms on specific metal substrates. It is known that graphene has been experimentally prepared by this method [10]. Figure 8 shows the structures of 2 Pmmn and 8 Pmmn borophene. The 2 Pmmn borophene is the most studied type of borophene and has no ripples along the a-axis direction and a W-shaped ripple structure along the b-axis with a considerable buckling height. Calculation of the energy band and density of states shows that 2 Pmmn borophene exhibits strong anisotropic metallic properties, which can induce facile electron transfer and electrical conduction at room temperature. However, the absence of ripples along the a-axis direction limits the conductivity of 2 Pmmn borophene. Moreover, 8 Pmmn borophene is a zero-gap semiconductor. The density of states at the Fermi level is zero. In the band structure, there is a Dirac cone, and the valence band and conduction band meet at the junction point (0, 0.3, 0) at the Fermi level. We investigated five adsorption sites on 2 Pmmn and 8 Pmmn borophene: B1, B2, D1, and D2. The same nomenclature was used for χ3-borophene; that is, the configuration in which the atom corresponding to the first (second) element in the molecular formula of the gas was closer to borophene than the other atom in the gas molecule was represented by −1 (−2). Eight adsorption mechanisms were considered for each type of borophene. The adsorption energy and charge transfer number of 2 Pmmn and 8 Pmmn borophene for the absorption of harmful gases at different sites are shown in Tables 4 and 5, respectively. Moreover, 8 Pmmn borophene is a zero-gap semiconductor. The density of states at the Fermi level is zero. In the band structure, there is a Dirac cone, and the valence band and conduction band meet at the junction point (0, 0.3, 0) at the Fermi level. We investigated five adsorption sites on 2 Pmmn and 8 Pmmn borophene: B1, B2, D1, and D2. The same nomenclature was used for χ 3 -borophene; that is, the configuration in which the atom corresponding to the first (second) element in the molecular formula of the gas was closer to borophene than the other atom in the gas molecule was represented by −1 (−2). Eight adsorption mechanisms were considered for each type of borophene. The adsorption energy and charge transfer number of 2 Pmmn and 8 Pmmn borophene for the absorption of harmful gases at different sites are shown in Tables 4 and 5, respectively.
The results in the two tables presented above clearly demonstrate that these two borophenes can adsorb harmful gases, and the adsorption site determined the magnitude of the adsorption energy. Also, the physisorption results from relatively weak interactions between a gas molecule and the absorbent surface. Physisorption is nonspecific and involves relatively weak van der Waals forces and low adsorption energies. In addition, physically adsorbed molecules can diffuse along the surface of an adsorbent and are usually not bound to specific locations on the surface. Because the gas molecules are only weakly bound to the adsorbent surface, physisorption can be rapidly reversed. The chemical bond can be created by the sharing of electrons between the adsorbate and the adsorbent and can be regarded as the formation of a surface compound. Chemisorption is difficult to reverse, because of the strong adhesion between the adsorbate and adsorbent [51]. However, as chemisorbed gas molecules cannot be easily desorbed into the gas phase, boron cannot be reused after the gas has been adsorbed onto borophene. Raw materials are thus wasted. Figure 9 shows the results of the differential charge density analysis of the two Pmmn borophenes for the adsorption of (from left to right) CO, NH 3 , NO 2 , SO 2 , and H 2 S. In Figure 9, the sizable blue area below the gas molecule after adsorption by 2 Pmmn borophene indicates that borophene lost electrons. The small gas molecule was surrounded by yellow regions, indicating that the gas received electrons. The quantity of charge transferred can be used to preliminarily determine the type of adsorption involved. After a gas molecule was adsorbed on the 8 Pmmn borophene, a "#"-shaped blue area appeared high above the borophene surface. By contrast, a "#"-shaped yellow area appeared below the B atom at the bottom of borophene. An intriguing result is that the absolute value of the adsorption energy of the 8 Pmmn borophene for SO 2 was considerably higher than those of the other two borophenes, indicating chemisorption. The optimized result after adsorption was S-O bond breakage, meaning that 8 Pmmn borophene may be able to dismember toxic gas molecules. It is speculated that this behavior results from the migration of half of the electrons in 8 Pmmn borophene from the interior to the bridge B atom, transforming 8 Pmmn borophene into a covalent single-element 2D material with ionic properties. Compared to planar borophene, 8 Pmmn borophene is more stable and therefore less prone to deformation. This property may enable 8 Pmmn borophene to dismember and thereby adsorb SO 2 more effectively than planar borophene. The fracture of the chemical bond can occur via a free radical reaction, which can be realized by ionization or electron transfer. The cleavage of SO 2 molecular bonds could occur via charge transfer, resulting in a more robust and stable O-B bond for 8 Pmmn borophene than for planar borophene.
atom, transforming 8 Pmmn borophene into a covalent single-element 2D material with ionic properties. Compared to planar borophene, 8 Pmmn borophene is more stable and therefore less prone to deformation. This property may enable 8 Pmmn borophene to dismember and thereby adsorb SO2 more effectively than planar borophene. The fracture of the chemical bond can occur via a free radical reaction, which can be realized by ionization or electron transfer. The cleavage of SO2 molecular bonds could occur via charge transfer, resulting in a more robust and stable O-B bond for 8 Pmmn borophene than for planar borophene. After a comparative analysis of the three types of borophene, it was found that the adsorption capacity decreased in the order of 2 Pmmn borophene > χ3-borophene > 8 Pmmn borophene. This result suggests that borophene with metallic properties has better adsorption performance than borophene with semiconductor properties. The site most prone to electron loss was the H for the χ3-borophene and the B1 for the 2 Pmmn and 8 Pmmn borophene. The adsorption mechanism determines the adsorption energy for small gas molecules. Vertical adsorption was most efficient for CO because the C atoms were located closer to the borophene surface than the O atoms. The most efficient NH3 adsorption occurred when the N atoms were closer to the borophene surface than the H atoms. The most efficient NO2 adsorption occurred when the O atoms were closer to the borophene surface than the N atoms. The most efficient SO2 adsorption occurred when the O atoms were closer to the borophene surface than the S atoms for 2 Pmmn and 8 Pmmn borophene) but when the S atoms were closer to the borophene surface than the O atoms for χ3-borophene. The adsorption of H2S was most efficient when the H atoms were closer to the borophene surface than the S atoms for the χ3-and 8-Pmmn borophene but when the S atoms were closer to the borophene surface than the H atoms for 2 Pmmn borophene.
As shown in Figure 10, if the adsorption energy is too high, it will cause small molecules of gas and borophene to tightly adsorb in the form of chemical adsorption, leading to the waste of raw materials and turning borophene into a disposable sensor device. If the adsorption energy is too low, it will lead to very unstable adsorption, and once there is a slight change in the external environment, it will lead to the desorption of gas on the adsorption. In addition, especially when 8 Pmmn borophene adsorbs gas, the difference in the adsorption energy at each site is too large, so it can be inferred that its adsorption of harmful gases is very unstable. Thus, compared to the other two Pmmn borophenes, After a comparative analysis of the three types of borophene, it was found that the adsorption capacity decreased in the order of 2 Pmmn borophene > χ 3 -borophene > 8 Pmmn borophene. This result suggests that borophene with metallic properties has better adsorption performance than borophene with semiconductor properties. The site most prone to electron loss was the H for the χ 3 -borophene and the B1 for the 2 Pmmn and 8 Pmmn borophene. The adsorption mechanism determines the adsorption energy for small gas molecules. Vertical adsorption was most efficient for CO because the C atoms were located closer to the borophene surface than the O atoms. The most efficient NH 3 adsorption occurred when the N atoms were closer to the borophene surface than the H atoms. The most efficient NO 2 adsorption occurred when the O atoms were closer to the borophene surface than the N atoms. The most efficient SO 2 adsorption occurred when the O atoms were closer to the borophene surface than the S atoms for 2 Pmmn and 8 Pmmn borophene) but when the S atoms were closer to the borophene surface than the O atoms for χ 3 -borophene. The adsorption of H 2 S was most efficient when the H atoms were closer to the borophene surface than the S atoms for the χ 3 -and 8-Pmmn borophene but when the S atoms were closer to the borophene surface than the H atoms for 2 Pmmn borophene.
As shown in Figure 10, if the adsorption energy is too high, it will cause small molecules of gas and borophene to tightly adsorb in the form of chemical adsorption, leading to the waste of raw materials and turning borophene into a disposable sensor device. If the adsorption energy is too low, it will lead to very unstable adsorption, and once there is a slight change in the external environment, it will lead to the desorption of gas on the adsorption. In addition, especially when 8 Pmmn borophene adsorbs gas, the difference in the adsorption energy at each site is too large, so it can be inferred that its adsorption of harmful gases is very unstable. Thus, compared to the other two Pmmn borophenes, without the excessive waste of raw materials, χ 3 -borophene has a better ability to adsorb harmful gases, and the gas structure after the adsorption by χ 3 -borophene is relatively stable.
without the excessive waste of raw materials, χ3-borophene has a better ability to ad harmful gases, and the gas structure after the adsorption by χ3-borophene is rela stable.

Summary
In summary, we performed a first-principles calculation to investigate the adsor potentials and effects of harmful gas molecules (CO, NH3, NO2, SO2, and H2S) on types of borophenes and then determined the most efficient adsorption site and m nism. Our used lattice constant of χ3-borophene was almost consistent with the pre experimental report, which ensured the correctness of our structure and provided a basis for the following experiments. Compared to the two Pmmn borophenes, χ3phene was found to have a better ability to adsorb harmful gases and can be used wi excessive waste of raw materials. The adsorption capacity of χ3-borophene was diff for the five gases and was strongest for NO2 because a covalent bond formed betwee NO2 and χ3-borophene. The high energy and large charge transfer of χ3-borophene fo adsorption makes χ3-borophene a candidate material for gas sensor applications. H ever, chemisorption results in the waste of raw materials, because χ3-borophene cann reused and becomes a disposable sensor device. The adsorption mechanism for H2 SO2 on χ3-borophene was pure physisorption, which requires low adsorption energ a high transfer charge. This result indicates that χ3-borophene-adsorbed gas struc are relatively stable after adsorption. Therefore, χ3-borophene is a good adsorbent. A and NH3 can be both physisorbed and chemisorbed on χ3-borophene, χ3-borophen high selectivity and is, therefore, a good choice for adsorbing these gases. In additi has been found that the adsorption of SO2 by 8 Pmmn borophene occurs by the deco sition of the gas molecules followed by the strong adsorption of the atoms on the su of 8 Pmmn borophene, which could be exploited to generate O2 during the adsorpti

Summary
In summary, we performed a first-principles calculation to investigate the adsorption potentials and effects of harmful gas molecules (CO, NH 3 , NO 2 , SO 2 , and H 2 S) on three types of borophenes and then determined the most efficient adsorption site and mechanism. Our used lattice constant of χ 3 -borophene was almost consistent with the previous experimental report, which ensured the correctness of our structure and provided a good basis for the following experiments. Compared to the two Pmmn borophenes, χ 3 -borophene was found to have a better ability to adsorb harmful gases and can be used without excessive waste of raw materials. The adsorption capacity of χ 3 -borophene was different for the five gases and was strongest for NO 2 because a covalent bond formed between the NO 2 and χ 3 -borophene. The high energy and large charge transfer of χ 3 -borophene for gas adsorption makes χ 3 -borophene a candidate material for gas sensor applications. However, chemisorption results in the waste of raw materials, because χ 3 -borophene cannot be reused and becomes a disposable sensor device. The adsorption mechanism for H 2 S and SO 2 on χ 3 -borophene was pure physisorption, which requires low adsorption energy but a high transfer charge. This result indicates that χ 3 -borophene-adsorbed gas structures are relatively stable after adsorption. Therefore, χ 3 -borophene is a good adsorbent. As CO and NH 3 can be both physisorbed and chemisorbed on χ 3 -borophene, χ 3 -borophene has high selectivity and is, therefore, a good choice for adsorbing these gases. In addition, it has been found that the adsorption of SO 2 by 8 Pmmn borophene occurs by the decomposition of the gas molecules followed by the strong adsorption of the atoms on the surface of 8 Pmmn borophene, which could be exploited to generate O 2 during the adsorption of harmful substances. All the results obtained in this work demonstrate that χ 3 -borophene has broad prospects as a gas sensor for adsorbing toxic gases.