A first principles study of RbSnCl3 perovskite toward NH3, SO2, and NO gas sensing

The sensitivity of a RbSnCl3 perovskite 2D layer toward NH3, SO2, and NO toxic gases has been studied via DFT analysis. The tri-atomic layer of RbSnCl3 possessed a tetragonal symmetry with a band gap of 1.433 eV. The adsorption energies of RbSnCl3 for NH3, SO2 and NO are −0.09, −0.43, and −0.56 eV respectively with a recovery time ranging from 3.4 × 10−8 to 3.5 ms. RbSnCl3 is highly sensitive toward SO2 and NO compared to NH3. The adsorption of SO2 and NO results in a significant structural deformation and a semiconductor-to-metal transition of RbSnCl3 perovskite. A high absorption coefficient (>103 cm−1), excessive optical conductivity (>1014 s−1), and a very low reflectivity (<3%) make RbSnCl3 a potential candidate for numerous optoelectronic applications. A significant shift in optical responses is observed through SO2 and NO adsorption, which can enable identification of the adsorbed gases. The studied characteristics signify that RbSnCl3 can be a potential candidate for SO2 and NO detection.


Introduction
2][3][4] Among various toxic gases, NH 3 , NO, and SO 2 are quite soluble in water, and hence they can easily be absorbed into the body through the lungs or skin contact.Once in the body, they can dissolve in the bloodstream and cause harm to organs and tissues.6][7] NH 3 is a highly toxic gas with a pungent smell that can be produced by agricultural activity as well as industrial wastes. 1,8SO 2 gas is mainly generated by the burning of various fossil fuels, which is seriously injurious to health and the environment. 2,8NO is a highly hazardous gas which can cause even death by asphyxia. 9Hence, monitoring these gases is a vital task in order to provide a better living environment, which motivates researchers to create cutting-edge methods of sensing these substances.
Since the historic discovery of graphene, scientists have become more interested in two-dimensional materials. 10,11lthough graphene is a revolutionary discovery, pure graphene is not suitable for gas detection due to its poor sensitivity. 2][24][25][26] Perovskites are multifunctional materials with great potential for numerous optoelectronic (OTE) applications. 279][30][31][32][33][34][35][36] Paper-based sensors of CH 3 NH 3 PbI 3 (MAPI) perovskite showed a high sensitivity toward NH 3 gas. 28NbWO 6based perovskite thin sheets showed strong interaction with H 2 S gas at low temperatures. 29ZnSnO 3 perovskite nanospheres were demonstrated to be potential candidates for n-propanol sensors. 30Zhuang et al. fabricated a SCN-doped MAPI thin lm, which showed remarkable sensitivity toward NO 2 gas. 31Balamurugan and Lee synthesized YMnO 3 nanopowder which demonstrated a ne sensing performance for H 2 S gas. 32Liu et al. studied the sensitivity of CsPbX 3 (X = I, Br, and Cl) via density functional theory (DFT) calculation, which revealed strong adsorption of CH 2 O gas on CsPbBr 3 . 33Aranthady et al. reported that a La 0.6 Ca 0.4 FeO 3 perovskite thin lm showed enhanced sensitivity for SO 2 gas detection. 34According to the experimental report of Marikutsa et al., BaSnO 3 nanocrystals showed high sensitivity for SO 2 gas. 35Formamidinium (FA) lead iodide-based sensors showed strong sensitivity and high selectivity for NH 3 gas. 36RbSnCl 3 perovskites are reported to be potential candidates for solar cells and thermoelectric and photocatalytic applications. 37,38The sensing performance of RbSnCl 3 for various gases is yet to be studied.
Here we designed a 2D RbSnCl 3 perovskite with three atomic layers and studied its structural, electronic and optical properties via DFT calculations.We also studied the adsorption phenomenon of NH 3 , SO 2 , and NO toxic gases on the RbSnCl 3 layer.The sensitivity of the RbSnCl 3 layer toward the selected gases is understood via the variation in distinct properties of the perovskite.

Computational details
In the current study, we have used the CASTEP code to nd the global minimum structure and understand the sensing behavior implemented in "Materials Studio".All of our calculations are performed based on density functional theory with the plane wave basis energy cut-off set to 650 eV.1][42] We have set the convergence energy at 1.0 × 10 −5 eV per atom for the geometry optimization, the ionic displacement at 0.001 Å, and the Gaussian smearing at 0.05 GPa for stress.The Hellmann-Feynman force for every atom has been set at a value of 0.03 eV Å −1 .We have designed a triatomic layer of RbSnCl 3 perovskite by building a 2 × 2 × 1 supercell.To avoid the interaction with surroundings we have used a vacuum slab of 35 Å.All complexes are optimized using the same optimization criteria.
The adsorption energy (E ad ) and recovery time (T R ) of the complex structures are calculated from the following equations.
where E Gas+RbSnCl 3 , E RbSnCl 3 , E Gas , n o , K b , and T represent the energy of the gas adsorbed perovskite layer, the energy of the perovskite layer, the energy of isolated gas molecules, incident frequency of UV radiation (n o = 10 12 Hz), Boltzmann constant, and operating temperature (298 K).

Geometry analysis
Fig. 1 shows the optimized geometries of the pristine RbSnCl 3 layer along with their gas adsorbed complexes.The lattice parameters of the optimized structure are shown in Table 1.The RbSnCl 3 layer possessed an orthorhombic phase with lattice parameters analogous to those in a previous study. 43A slight deformation of the structure is observed due to the adsorption of NH 3 gas, which suggests a poor interaction of NH 3 with the adsorbent layer.However, for SO 2 and NO adsorption, a significant structural deformation is observed, signifying a strong adsorbate-adsorbent interaction.The volume of the RbSnCl 3 unit cell increased due to the interaction with SO 2 and NO gas, whereas in the presence of NH 3 , the volume is slightly decreased.
The average bond lengths between atoms are shown in Table 2.The values in the parenthesis represent the bond lengths before adsorption.It is observed that the gas molecules suffer a slight deformation through the adsorption process.The average Sn-Cl bond length of the SnCl 6 octahedra is about 2.77 Å, which agrees with a previous study. 44The average Sn-Cl bond lengths vary slightly due to the interaction of NH 3 with RbSnCl 3 , whereas a signicant structural deformation of the SnCl 6 octahedra is a result of the interaction with SO 2 and NO gas molecules.

Adsorption of NH 3 , SO 2 , and NO gases
The adsorption energy, recovery time, and adsorption length (L ad ) are displayed in Table 3.The NH 3 gas is very weakly adsorbed on the RbSnCl 3 surface resulting in a very small recovery time of 3.4 × 10 −8 ms, which is not suitable for practical applications at room temperature.SO 2 gas is adsorbed signicantly by the perovskite layer, whose recovery time makes RbSnCl 3 suitable for SO 2 sensing.Among the three selected gases, NO shows the strongest adsorption on the RbSnCl 3 surface, with a recovery time of 3.5 ms.The strong interaction of NO and SO 2 with adsorbent can result from the presence of high electronegative elements N, O and S, which can offer strong interaction via partial charge transfer between the adsorbent and adsorbate.Among them, NO possesses an unpaired electron in the p* antibonding orbital, which makes it more susceptible to bonding with the adsorbent surface, resulting in the strongest interaction.On the other hand, no unpaired electron is available in NH 3 .Though N is signicantly electronegative, the presence of highly electropositive H atoms can generate an opposite force N atom on the adsorbent resulting in weakening of interaction strength.RbSnCl 3 perovskite showed comparatively poor sensitivity toward NH 3 gas compared to BC 3 , 45 graphene, 46 phosphorene, 47 silicene, BNNS, 48 and MoSe 2 (ref.49) 2D layers.On the other hand, SO 2 adsorption energy has a comparatively higher value than that of graphene, 50 MoS 2 , 51 MoSe 2 , 52 and BNNSs. 2 The NO adsorption on RbSnCl 3 perovskite is comparatively stronger than on silicene, 23 BN nanotubes, 53 MoS 2 , 26 and MoSe 2 . 54

Electronic properties
The interactions between the atoms in pristine RbSnCl 3 and gas-adsorbed RbSnCl 3 can be well understood with the help of the electron density difference map and Mulliken (or Hirshfeld) population of the corresponding atoms.Tables 4 and 5 show the Mulliken and Hirshfeld charge distribution of the adsorbent and the complex structures, respectively.In Fig. 2 the red region indicates an electron-enriched region, while the green region shows an electron depletion region.The average Mulliken charge on each Rb, Sn and Cl is +0.87jej +0.67jej and −0.6jej respectively.The A-site element, Rb, shows a partially positive charge due to high electropositivity, and hence Rb atoms act as electron donors.On the other hand, the Cl atoms of the SnCl 6 octahedra show a partially negative charge due to Cl's higher electronegativity than Sn suggesting the displacement of bonding electrons of Sn-Cl bonds. 55The signicant amount of

Nanoscale Advances Paper
the charge transfer indicates a strong covalent bonding between Sn and Cl atoms. 56er the adsorption of NH 3 , a very slight variation in charge distribution is observed due to the interaction with NH 3 .About −0.08jej charge is transferred from the adsorbent to the NH 3 molecule suggesting a weak interaction.The average charge on Rb, Sn, and Cl varies signicantly due to SO 2 and NO adsorption.About −0.1jej Mulliken charge is transferred between the adsorbent and SO 2 /NO molecule.The extensive charge transfer between the absorbent and adsorbate through SO 2 and NO adsorption results in a strong interaction.Hirshfeld charge analysis also veries the charge transfer.Hence, the variation of Mulliken or Hirshfeld charge is commensurate with the adsorption energy of the three gas molecules under study.
Fig. 3 shows the band structures of the perovskite layer and its gas-adsorbed complexes.The RbSnCl 3 perovskite possesses a direct band gap of about 1.43 eV.Although a similar functional is used in the present study to that previously described, the obtained band gap is signicantly higher. 43The increase of the band gap can be the result of the quantum connement effect since in the present study a 2D layer of RbSnCl 3 perovskite is designed.The band structures are determined along the G / Hence, the conductivity signicantly increases aer gas adsorption.The absolute measurements of conductivity can be calibrated in order to identify the type of the present toxic gases.Fig. 4 shows the partial density of states (PDOS) of the RbSnCl 3 layer and its gas-adsorbed complexes.In RbSnCl 3 , the electronic conguration of Sn and Cl is 5s 2 5p 2 and 3s 2 3p 5 respectively.Hence the contribution of the VM comes from the p-orbital Cl, and the Sn-p orbital contributes to the CM, which is analogous to a previous study. 43The A-site element Rb has no signicant contribution to the VM or CM, and hence it does not directly affect the crystal band edge, which agrees with previous studies. 57,58No signicant variation in electronic contribution to the VM and CM is observed due to NH 3 adsorption, which is consistent with the nominal variation in the band gap.Aer SO 2 adsorption, band overlapping was observed due to the contribution of p orbitals of S and O atoms near the Fermi level, which as a result reduced the band gap.A similar phenomenon is observed with NO adsorption.The p-orbitals of both N and O contribute signicantly to the Fermi level resulting in a zeroband gap.

Optical properties
The optical responses of the RbSnCl 3 perovskite layer in the visible ranges satisfy previous ndings. 43The RbSnCl 3 perovskite shows an absorption coefficient (AC) over 10 3 cm −1 order (Fig. 5) in the visible region, which makes it a potential material for various optoelectronic applications.The high AC along with The shi in the absorption peak can be used in determining the type of adsorbed gas.An extensive redshi of the reectivity peak is observed through NO adsorption, which suggests that there would be a change in the color of the RbSnCl 3 material in the presence of NO gas.This idea can also be applied in sensing and detecting the nature of the toxic gas present in the environment.All the structures showed very poor reectivity (<3%) in the visible region, suggesting a small portion of energy loss via reection.
RbSnCl 3 showed high optical conductivity (OC) in the lower wavelength region of the visible spectrum.The OC peak is observed in the near UV region of value over 10 14 s.The high OC is preferable for various optoelectronic applications, i.e., photodiodes, detectors, etc.No observable variation in the OC occurred in the presence of NH 3 gas; however, a redshi of the OC peak in the presence of NO and SO 2 suggests a signicant optical response in the visible wavelength.Conductivity gradually decreases with increasing wavelength.
The RbSnCl 3 perovskite shows a low refractive index (h = 1.24-1.33)for visible wavelength.A low h is always preferable for optoelectronic research and the materials are suitable as antireection coatings. 59A low h signies a low energy loss via reection.NH 3 adsorption shows no signicant variation in the refractive index; however, a drastic change is observed via SO 2 and NO adsorption.The h increased up to 1.36 due to NO adsorption.The imaginary part of the refractive index (k) is also known as the extinction coefficient.A non-zero value of k signies optical absorption, which increases with increasing the value of k.Hence, the k-spectra are analogous to the The signicant variation in optical responses, i.e., shiing of absorption, reection, and conductivity maxima, can be calibrated in order to identify the nature of adsorbed gases.Since both RbSnCl 3 + SO 2 and RbSnCl 3 + NO are zero band gap complexes, the variation in optical characteristics (color) can be a way to distinguish between SO 2 and NO adsorption.

Conclusion
A triatomic layer of RbSnCl 3 perovskite is designed and optimized to ground state geometry successfully through DFT calculations.The toxic gases NH 3 , SO 2 , and NO are adsorbed on the RbSnCl 3 layer with negative adsorption energies with a strong interaction between RbSnCl 3 -SO 2 and RbSnCl 3 -NO.The RbSnCl 3 -SO 2 and RbSnCl 3 -NO interactions result in comparatively more structural deformation of RbSnCl 3 crystals than RbSnCl 3 -NH 3 interaction.The charge transfer between adsorbent and adsorbate also veries the strong interactions.The recovery times in the ms range also verify the applicability of the RbSnCl 3 gas sensor in a practical environment.The band gap of the pure RbSnCl 3 layer is about 1.43 eV which decreased to 1.412 in the presence of NH 3 ; however, a complete semiconductor-to-metal transition is observed due to SO 2 and NO adsorption.The RbSnCl 3 layer showed a high AC, OC and very low reectivity endowing it with potential for numerous optoelectronic research studies.A signicant red shi in the AC and OC is observed due to SO 2 and NO adsorption, which can be used to detect the type of adsorbed gas.The strong adsorption energies and variation in electronic and optical properties suggest that RbSnCl 3 perovskite is a potential gas sensor for detecting as well as identifying NO and SO 2 toxic gases.
The synthesis and gas sensitivity of RbSnCl 3 perovskite can be determined experimentally.The gas sensitivity of various perovskite structures can be studied both theoretically and experimentally.Various environmental factors, i.e., temperature, pressure, and presence of harmless environmental gases may affect the sensitivity of the gas sensor, which can be investigated in future studies.

Table 1
Lattice parameters of the optimized RbSnCl 3 before and after gas adsorption

Table 2
The average bond lengths (Å) in the gases and perovskite structure

Table 4
Average Mulliken charges of the elements in the unit of jej

Table 5
Average Hirshfeld charges of the elements in the unit of jej