The suggestion of efficient strategies for monitoring and reduction of environmental pollutants are very essential in biological and industrial processes. In the past few years, a huge surge of attention has been devoted to the design and improvement of appropriate gas sensing materials and harmful chemical remover devices [1-3]. Sulfur dioxide (SO2) is one of the main contributors to air pollution, which comes from exhaust products. Ozone (O3) has been also considered as harmful gas to the respiratory tissues and ocular mucosa, being mostly introduced to the human body by inhalation. The main target of tropospheric ozone is the lung. Also, it causes detrimental impacts on the eyes and the nervous system . Consequently, ozone has long been considered one of the most dangerous air pollutants. Under solar light, the NOx gases emitted from vehicle engines, power plants and volatile organic compounds in factory waste gas contribute to a photochemical reaction to create photochemical smog and ozone molecule. Therefore, the quick and precise detection of ozone to control its emission at the atmosphere is a key subject that should be resolved and fixed . Thus, it is of eminent importance to decrease or remove toxic SO2 and O3 molecules from the atmosphere. The most effective way to decrease or eliminate these air pollutants is the use of some gas sensors or removers. In this regard, BN nanotubes have been considered to be efficient sensing materials, which can be utilized for monitoring the concentrations of these gases in the air.
Carbon nanotubes (CNTs) were first discovered by Iljima  and were demonstrated to be used in a wide range of applications [7-11]. Boron nitride nanotubes which possess a similar geometry with carbon nanotubes have attracted significant attention in the past decades [12-21]. Owing to the unique mechanical properties, satisfied thermal conductivity, and high chemical stability , BN nanotubes have been proven to be promising material candidates for applications in nano-electronic devices and nanomedicine [23, 24]. Their properties can be modified by introducing different functional groups to wrap around with covalent or non-covalent interactions, which leads to the exceptional properties of BN nanotubes .
In comparison with carbon nanotubes, BN nanotubes have more advantages as they possess small toxicity. Besides, their electronic properties are independent of the diameter and chirality. The characteristics such as great surface to volume ratios and the high chemical stability make semiconducting BN nanotubes sensitive materials. Gas sensing materials, which were constructed from BN nanotubes have aroused significant attention because of their outstanding sensing capabilities [26-30]. Since the discovery of BN nanotubes, a large number of theoretical and experimental studies are carried out to examine the gas sensing capability of BN nanotube-based sensors [30, 31]. Furthermore, element doped BN nanotubes show advantages such as improved conductivity and chemical reactivity and sensor properties compared to the pristine ones [32-34]. Performing DFT calculations, Wang et al. suggested that the adsorption performance of BN nanotubes towards CH2O can be amended by Si doping . Several works have been carried out, describing the adsorption behaviors of TiO2 semiconductor based nanoparticles and nanocomposites [36-45] and two-dimensional stanene nanosheets . Besides, the adsorption energy of both SO2 and O3 on BNNT were compared with those of other adsorbents [47-54]. The exceptional sensor properties of BN nanotubes encourages us to examine the sensor properties of these nanotubes towards SO2 and O3 detection. In this paper, we performed a systematical first-principles study to investigate the interactions of SO2 and O3 gases with the pristine single-walled BN nanotubes. The main aim of this study is to explore the favorable materials for detecting harmful SO2 and O3 molecules in the atmosphere. The structural and electronic properties of the adsorption systems were mainly analyzed in view of the adsorption energies, charge density difference, molecular orbitals and Kohn-Sham, Hartree-Fock potentials.
METHODOLOGY AND MODEL SYSTEMS
All calculations were performed using the density functional theory (DFT) [55, 56], as implemented in the open source package for material eXplorer (OPENMX3.8) package . The generalized gradient approximation (GGA) in the Perdew-Burke-Ernzerhof (PBE) format was used to describe the exchange-correlation potentials . The pseudo-atomic orbitals (PAOs) centered on atomic sites were used as basis sets. The chosen basis sets were specified by S8.0-s3p3, O5.0-s2p2, B7.0-s2p2, and N5.0-s2p2, in generation by a confinement scheme. In our calculations, the energy cutoff was set to 150 Ry. For the considered BN nanotube, we have considered a (6, 6) nanotube. The convergence criterion for the self-consistent electronic minimization was set to 10−6 Hartree, and the force acting on each atom converges to 0.01 eV Å-1. The open-source program XCrysDen  was used for the visualization of adsorption configurations of BN nanotubes with adsorbed SO2 and O3 molecules. Also, VESTA (visualization for electronic and structural analysis) program was employed for visualization of the volumetric data such as electron/nuclear densities . Besides, the adsorption energy is estimated based on a difference between the total energy of the complex BN+Adsorbate system and individual BN and gas molecule components.
Thus, the adsorption energy (∆Ead) is estimated using the following formula:
∆Ead = E molecule/nanotube - E molecule – E nanotube (1)
where, E molecule/nanotube, E molecule, and E nanotube are the total energies of BN nanotube with the adsorbed gas molecule, free gas molecule and bare BN nanotube, respectively.
RESULTS AND DISCUSSION
To identify the most stable adsorption configurations of SO2 and O3 molecules on the surface of BN nanotubes, we have examined three orientations of each of the SO2 and O3 molecules towards the nanotube surface. The relaxed structures of the considered BN nanotubes with adsorbed gas molecules were shown in Fig. 1.
The different atoms of the BN nanotubes were considered to be the active adsorption sites on the surface. Therefore, three types of initial adsorption structures of SO2 on the nanotube were considered: (A) SO2 molecule is perpendicular to the BN nanotube with two oxygen atoms positioned downward, which are located towards the boron atoms of the BN nanotube.
In this configuration, oxygen atoms of SO2 molecule were initially placed at the top of the boron atoms with a maximum distance of 2.02 Ǻ with respect to the nanotube surface; (B) SO2 molecule is perpendicular to the nanotube with two oxygen atoms positioned at the top of the boron and nitrogen atoms of the nanotube, and this configuration presents a downward orientation of SO2 over the surface, the average distance between the nanotube and SO2 molecule is about 2.05 Ǻ.; (C) SO2 molecule is perpendicular to the surface with oxygen atoms oriented at the top of the nitrogen atoms of the nanotube, the oxygen atom was weakly adsorbed on the surface of BN nanotube. In this configuration, we can see that SO2 stands in a distance of 2.08 Ǻ above the nanotube surface. For O3adsorption on the surface of BN nanotube, we can also consider three configurations denoted by adsorption types D-F. Among three configurations for SO2 adsorption on the BN nanotubes, configuration A has the lowest value of adsorption distance, representing the relative orientation of SO2 over the boron atoms of the nanotube, whereas the highest value of distance belongs to configuration C. The variation trends of adsorption energies show inverse relation with adsorption distances. The average value of adsorption energy for SO2 adsorption on the BN nanotube is about 0.32 eV.
(D) O3 molecule is perpendicular to the surface of BN nanotube with side oxygen atoms located downward, which are placed towards the boron atoms of the BN nanotube. The central oxygen atom of the nanotube does not contribute to the adsorption anymore. Instead, the side oxygen atoms were tested to be the most favorable adsorption sites. The average distance of O3 molecule towards the BN nanotube is about 2.03 Ǻ, which makes it the most favorable and strongest adsorption configuration.; (E) O3 molecule is suited perpendicularly to the nanotube surface with side oxygen atoms located at the top of the boron and nitrogen atoms of the nanotube, Similarly, in this configuration, the central oxygen atom of the O3 molecule does not interact with the nanotube; and the distance is about 2.06 Ǻ.; (F) O3 molecule is perpendicular to the surface with side oxygen atoms adapted to the nitrogen atoms of the nanotube. In this configuration, the side oxygen atoms weakly interact with the nitrogen atoms of the nanotube, and the resultant distance between the nanotube and O3 molecule is about 2.11 Ǻ. Of the three configurations for O3 adsorption on the BN nanotube, configuration D presents the smallest distance between the nanotube and O3 molecule, while the largest distance belongs to configuration F. The average adsorption energy is about 0.25 eV. Configuration D represents the strongest interaction between the nanotube and O3 molecule since it shows the least distance of O3 towards the nanotube surface.
All of these configurations represent the physical adsorption (physisorption) of the gas phase SO2 and O3 molecules on the nanotube surface as the substrate weakly interact with the adsorbing gas molecules. For all configurations, we found that the S-O and O-O bonds of the SO2 and O3 molecules were slightly elongated after the adsorption process. The reason can be probably attributed to the transfer of electronic density from the nanotube surface to the gas molecules, making their bond lengths stretched.
Fig. 2 displays the electronic band structures of the considered unit cell and supercell of (6, 6) BN nanotube, which represents that BN nanotube acts as a large bandgap semiconductor. It is well known to the sensor community that the semiconductor characteristics of the sensor material are an essential feature. Thus, the considered (6, 6) BN nanotube behaves as an appropriate sensor material due to its wide band gap and semiconducting nature. Figs.s 3 and 4 show the band structures for SO2 and O3 adsorbed BN nanotubes. It can be seen from these Figures that the electronic structure of the nanotubes was not significantly changed upon adsorption of gas molecules. The only change is the creation of a small line above the valence band edge of nanotubes. Therefore, the electronic band structures of the BN nanotubes remain unaffected after the adsorption of gas molecules. This is in accordance with the weak physisorption of the gas molecule on the nanotube surface.
The isosurface plots of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) for SO2 adsorption on the considered BN nanotubes were displayed in Fig. 5. As can be seen from this figure, the LUMOs of the adsorption systems are dominant over the adsorbed gas molecules, whereas the electronic density in the HOMOs is mainly distributed on the nanotube surface. The molecular orbitals of the SO2 molecule on the BN nanotube are in reasonable agreement with the calculated Kohn-Sham and Hartree-Fock potentials for the considered systems (Fig. 6). Similarly, Fig. 7 presents the isosurface plots of HOMOs and LUMOs for O3 adsorption on the considered BN nanotubes. These figures also show that the LUMOs are mostly located over the adsorbed O3 molecule, while the electronic densities in the HOMOs are high at the nanotube surface. The calculated potentials for O3 molecule adsorption on the nanotube were also displayed in Fig. 8, which indicates the potential distribution of the whole system consisting of the BN nanotube and its adsorbed gas phase O3 molecule.
In order to further examine the electronic structure of the interaction of gas molecules with BN nanotubes, we have presented the isosurfaces of the charge density difference plots for O3 adsorption on the considered nanotubes. As can be seen from these figures, there is a charge accumulation on the adsorbed O3 molecule after the adsorption process. Thus, O3 adsorption affects the electronic properties of the complex system by concentrating the electronic charges on the adsorbed gas molecule.
The natural bond orbital (NBO) analysis was conducted in this work in order to gain further insights into the charge transfer between adsorbent and adsorbate. The adsorption of O3 and SO2 makes substantial changes in the electronic behaviors of BN nanotube-based sensors. The SO2 molecule gives charges to the BN nanotube, whereas O3 molecule acts as a charge acceptor and accepts a charge from the BN nanotube. NBO analysis reveals a noticeable average charge transfer of about -0.32 |e| (e, the electron charge) from BN nanotube to O3 molecule. For SO2 adsorption on the nanotube, the average charge transfer from SO2 to the BN nanotube is calculated to be -0.28 |e|
In this paper, we have performed density functional theory calculations to investigate the adsorption behaviors of gas phase SO2 and O3 molecules on the considered BN nanotubes. We have examined different adsorption positions of the gas molecules on the nanotube surface. The results suggest that the adsorption of SO2 and O3 molecules on the pristine BN nanotube is an energy-favorable process. After the adsorption process, the S-O and O-O bonds of the adsorbed SO2 and O3 molecules were lengthened. The reason is that the electronic density transfers from the old bonds of the adsorbed SO2 and O3 molecules to the nanotube surface, making their bond lengths elongated. Molecular orbital calculations indicate that the electronic densities in the LUMOs were mainly distributed over the adsorbed gas molecules, whereas the HOMOs were high at the BN nanotube, as evidenced by the isosurface plots of Kohn-Sham potentials. NBO analysis reveals that SO2 acts as a charge donor, whereas O3 molecule behaves as a charge acceptor from the BN nanotube. The charge density difference calculations indicate the accumulation of electronic density over the adsorbed O3molecules, which suggest that O3 acts as an acceptor agent from the BN nanotube. This work aims at providing a theoretical basis and understanding of the adsorption behaviors of BN based chemical sensors for the detection of harmful gas molecules in the environment
This work has been supported by Azarbaijan Shahid Madani University.
CONFLICT OF INTEREST
The authors declare that there are no conflicts of interest regarding the publication of this manuscript.