Adsorption of CO , CO 2 , NO and NO 2 on Carbon Boron Nitride Hetero Junction : DFT Study

The adsorption of CO, CO2, NO and CO2 gas molecules on different diameters and chiralities of carbon nanotube-boron nitride nanotube (CNT-BNNT) heterojunctions is investigated, applying the density functional theory and using basis set 6 31 g (d,p). The energetic, electronic properties and surface reactivity have been discussed. We found that the best CNT-BNNT heterojunctions for adsorbing the CO, NO, CO2 and NO2 gas molecules is (5,0) CNT-BNNT heterojunction through forming C-N bonds with adsorption energy of −0.26, −0.41 eV, −0.33 and −0.63 eV, respectively. Also, the adsorption of CO, NO, CO2 and NO2 gas molecules on (5,5) and (6,6) CNT-BNNT heterojunctions does not affect the electronic character of the CNT-BNNT heterojunctions, however the adsorption of NO and NO2 gas molecules on (5,0) and (9,0)CNT-BNNT heterojunctions in case of forming C-B bonds increases the band gaps to 1.21 eV and 1.52 eV, respectively. In addition, it is reported that the values of dipole moment for armchair (5,5) and (6,6) CNT-BNNT heterojunctions are not affected by gas adsorption. Also, for the zig-zag (5,0) and (9,0) CNT-BNNT heterojunctions, the values of dipole moment increase through forming C-N bonds and decrease through forming C-B bonds. In addition, it is reported that the highest dipole moment is obtained for (9,0) CNT-BNNT heterojunctions. Therefore, the zig-zag CNT-BNNT heterojunctions can be selected as good candidate for gas sensors.


Introduction
Heterojunction gas sensors have been widely fabricated due to their low cost and simple processing, such as high the triethylamine-sensing of NiO/SnO 2 hollowsphere P-N heterojunction sensors [1], the CuO-ZnO heterojunction gas sensors [2] the polypyrrole (Ppy)/TiO 2 heterojunction operated LPG sensor [3] the In 2 O 3 -WO 3 heterojunction nanofibers [4] the SnO 2 /WO 3 heterojunction gas sensor [5] [6] and heterojunctions of B-C-N nanotubes [7]- [9].Since the discovery of carbon nanotubes (CNTs) in 1991 by Iijima [10], they have attracted much attention due to their remarkable electrical, mechanical and thermal properties [11]- [13].The single-walled CNTs (SWCNTs) can be metallic or semiconducting, depending on their chirality and diameter.Compared with CNTs, BNNTs are semiconducting with an uniform wide energy gap [14], and their electronic properties are independent of the tube chirality and diameter [15].Also, the boron nitride nanotubes (BNNTs) do not react with molten metals, and have higher oxidation resistance than CNTs [16] [17].By joining two NTs with same diameter and different materials, the heterojunction can be obtained [11] [18].The combined advantages of CNTs and BNNTs make their structures are ideal components for applications requiring high strength, chemical stability, high-temperature resistance or electrical insulation, such as nanocables [19] [20].The CNT-BNNT heterojunctions have investigated due to their unique electronic properties which can be controlled by adjusting the atomic composition and joint configurations [21]- [23].Using, ab initio and semi-empirical approaches, the electronic properties of the CNT-BNNT heterojunctions have been studied [24]- [26].
In this paper, the adsorption of CO, NO, CO 2 and NO 2 gas molecules on the surfaces of the heterojunctions with the same diameters and different chiralities for (5,0), (9,0), (5,5) and (6,6) CNT-BNNTs is studied, using the density functional theory and basis set 6 -31 g (d,p).Also, the electronic properties and surface reactivity of CNT-BNNT heterojunctions have been discussed.

Computational Methods
The density functional theory as implemented within G03W package [27]- [33], using B3LYP exchange-functional and applying basis set 6 -31 g (d,p) are performed.All CNT-BNNT heterojunctions of (5,0) and (9,0), (5,5) and (6,6) are fully optimized with spin average as well as the adsorption of CO, CO 2 , NO and NO 2 gas molecules.The adsorption energies of gas molecules on CNT-BNNT heterojunctions (E ads ) [34] are calculated from the following relations: ( ) where E (heterojunction+gas molecule) is the total energy of nanotube and gas molecules, E heterojunction is the energy of the CNT-BNNT heterojunction, and E gas molecules is the energy of gas molecules.
Table 1.The configuration structures of the studied CNT-BNNT heterojunctions.

Adsorption of CO, CO2, NO and NO2 Gas Molecules on CNT-BNNT Heterojunctions
We have adsorbed CO, NO, CO 2 and NO 2 gas molecules vertically on the vacant site (above a center of a hexagon ring) of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions.The calculated adsorption energies of CO, NO, CO 2 and NO 2 are listed in Table 2.It is found that the best CNT-BNNT heterojunction for adsorbing the CO, NO, CO 2 and NO 2 gas molecules is (5,0) CNT-BNNT through forming C-N bonds with adsorption energy of −0.26 eV, −0.41 eV, −0.33 eV and −0.63 eV, respectively.Therefore, the (5,0) CNT-BNNT is considered to be the best heterojunction for adsorbing CO, NO, CO 2 and NO 2 gas molecules.

Energy Gaps of Adsorbed CO, CO2, NO and NO2 Gas Molecules on CNT-BNNT Heterojunctions
From Table 3, it is found that the adsorption of CO, NO, CO 2 and NO 2 gas molecules on (5,0), ( CNT-BNNT heterojunctions does not affect the electronic character of the CNT-BNNT heterojunctions, however the adsorption of NO and NO 2 gas molecules on (5,0) and (9,0) CNT-BNNT heterojunctions in case of forming C-B bonds is increased to 1.21 eV and 1.36 eV, respectively.
The band gaps of the CNT-BNNT heterojunctions are calculated and are listed in Comparing the HOMO-LUMO energies of the CNT-BNNT heterojunctions with ones after the adsorption of CO and CO 2 gas molecules, it is clear that the energy values are so close.Also, it is noticed that there is not any contribution from the gas molecules at the molecular orbitals and the electron density of HOMO and LUMO is located at the carbon terminals except the (5,0) CNT-BNNT heterojunctions the electron density is distributed over all (5,0) heterojunctions, see Figure 2. Comparing the HOMO-LUMO energies of the (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions with ones after the NO and NO 2 gas molecules are adsorbed, it is clear that the values of energy gap are similar, except for (5,0) and (9,0) CNT-BNNT heterojunctions in case of forming C-B bonds the energy gap increases to 1.21 eV and 1.52 eV, respectively.Also, it is noticed that there are representations from the NO and NO 2 gas moleculeson the LUMO of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions and the electron density is localized on carbon terminals, see Figure 3. Table 2.The calculated adsorption energies (E ads ) of CO, NO, CO 2 and NO 2 above a vacant site of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions.All energies are given by eV.

The Reactivity of CNT-BNNT Heterojunction Surfaces before and after Adsorbing Gas Molecules
Our calculated band gaps and molecular orbitals show that the adsorption of CO and CO 2 gas molecules on CNT-BNNT heterojunctions does not change the band gaps of the CNT-BNNT heterojunctions but the adsorption of NO and NO 2 gas molecules strongly alters the band gaps and the molecular orbitals of (5,0) and (9,0) CNT-BNNT heterojunctions through forming C-B bonds.To clear that the reactivity of CNT-BNNT heterojunction surfaces before and after adsorbing CO, CO 2 , NO and NO 2 gas molecules on (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions is studied, see Table 4.The surface reactivity of the CNT-BNNT heterojunctions is  Comparing the dipole moments of the CNT-BNNT heterojunctions with ones that the CO, CO 2 , NO and NO 2 gas molecules are adsorbed, it is clear that the values of dipole moment do not change for armchair (5,5) and (6,6) CNT-BNNT heterojunctions.Also, for the zig-zag (5,0) and (9,0) CNT-BNNT heterojunctions, the values of dipole moment increase through forming C-N bonds and decrease through forming C-B bonds, see Table 4. Finally, one can report that the highest dipole moment is for (9,0) CNT-BNNT heterojunctions through forming C-N bonds.

Table 3 .
The