Adsorption of CO, CO2, NO and NO2 on Carbon Boron Nitride Hetero Junction: DFT Study

Abstract

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.

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El-Barbary, A. , Eid, K. , Kamel, M. , Taha, H. and Ismail, G. (2015) Adsorption of CO, CO2, NO and NO2 on Carbon Boron Nitride Hetero Junction: DFT Study. Journal of Surface Engineered Materials and Advanced Technology, 5, 169-176. doi: 10.4236/jsemat.2015.54019.

1. Introduction

Heterojunction gas sensors have been widely fabricated due to their low cost and simple processing, such as high the triethylamine-sensing of NiO/SnO2 hollowsphere P-N heterojunction sensors [1] , the CuO-ZnO heterojunction gas sensors [2] the polypyrrole (Ppy)/TiO2 heterojunction operated LPG sensor [3] the In2O3-WO3 heterojunction nanofibers [4] the SnO2/WO3 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, CO2 and NO2 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.

2. Computational Methods

The density functional theory as implemented within G03W package [27] - [33] , using B3LYP exchange-func- tional 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, CO2, NO and NO2 gas molecules. The adsorption energies of gas molecules on CNT-BNNT heterojunctions (Eads) [34] are calculated from the following relations:

where E(heterojunction+gas molecule) is the total energy of nanotube and gas molecules, Eheterojunction is the energy of the CNT-BNNT heterojunction, and Egas molecules is the energy of gas molecules.

3. Results and Discussion

We will investigate the adsorption of gas molecules, CO, CO2, NO and NO2 on the vacant site (above a center of a hexagon ring) of CNT-BNNT heterojunctions with different charilities and diameters (5,0) CNT-BNNT, (9,0) CNT-BNNT, (5,5) CNT-BNNT and (6,6) CNT-BNNT as shown in Figure 1 and Table 1. For zig-zag (5,0) CNT-BNNT and (9,0) CNT-BNNT heterojunctions have two ways of joining, first through forming C-B bonds and second through forming C-N bonds, see Figure 1. However, the (5,5) CNT-BNNT and (6,6) CNT-BNNT are joined through one way.

Table 1. The configuration structures of the studied CNT-BNNT heterojunctions.

Figure 1. The fully optimized structures of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. Carbon atoms (gray), Nitrogen atoms (blue), Boron atom (pink) and hydrogen atoms (white).

3.1. Adsorption of CO, CO2, NO and NO2 Gas Molecules on CNT-BNNT Heterojunctions

We have adsorbed CO, NO, CO2 and NO2 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, CO2 and NO2 are listed in Table 2. It is found that the best CNT-BNNT heterojunction for adsorbing the CO, NO, CO2 and NO2 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, CO2 and NO2 gas molecules.

3.2. 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, CO2 and NO2 gas molecules on (5,0), (5,5), (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 is increased to 1.21 eV and 1.36 eV, respectively.

3.3. HOMO-LUMO Orbitals of Adsorbing CO, CO2, NO and NO2 Gas Molecules on CNT-BNNT Heterojunctions

Our calculated band gaps show that the adsorption of CO, NO, CO2 and NO2 gas molecules on (5,0), (5,5), (6,6) CNT-BNNT heterojunctions does not affect the electronic character of the CNT-BNNT heterojunctions, however the adsorption of NO2 gas molecules on (9,0) CNT-BNNT heterojunctions increases the band gap the of (9,0) CNT-BNNTs to ~1.52 eV. To explain that the molecular orbitals of adsorbing CO, CO2, NO and NO2 gas molecules on (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions are investigated, see Figure 2 and Figure 3. The band gaps of the CNT-BNNT heterojunctions are calculated and are listed in Table 3. The HOMO and LUMO energy orbitals for (5,0) and (9,0) CNT-BNNT heterojunctions in case of forming C-N bonds are found to be (−4.08 eV, −2.99 eV), (−3.27 eV, −2.72 eV) and in case of forming C-B bonds are (−4.35 eV, −3.54 eV) and (−4.08 eV, −3.54 eV), respectively. The HOMO and LUMO energy orbitals for (5,5) and (6,6) CNT-BNNT heterojunctions are (−4.35 eV, −2.72 eV) and (−4.08 eV, −2.72 eV), respectively. Comparing the HOMO- LUMO energies of the CNT-BNNT heterojunctions with ones after the adsorption of CO and CO2 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 NO2 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 NO2 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.

Figure 2. HOMO and LUMO molecular orbitals of adsorbing CO and CO2 gas molecules on the (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. Energies of HOMO and LUMO are listed above the molecular orbitals and are given by eV.

Table 2. The calculated adsorption energies (Eads) of CO, NO, CO2 and NO2 above a vacant site of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. All energies are given by eV.

Figure 3. HOMO and LUMO molecular orbitals of adsorbing NO and NO2 gas molecules on the (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. Energies of HOMO and LUMO are listed above the molecular orbitals and are given by eV.

3.4. 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 CO2 gas molecules on CNT-BNNT heterojunctions does not change the band gaps of the CNT-BNNT heterojunctions but the adsorption of NO and NO2 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, CO2, NO and NO2 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

Table 3. The calculated energy gaps (Eg) of adsorbing CO, NO, CO2 and NO2 above a vacant site of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. All energies are given by eV.

Table 4. The calculated dipole moments of CNT-BNNT heterojunctions and after adsorbing CO, NO, CO2 and NO2 above a vacant site of (5,0), (9,0), (5,5) and (6,6) CNT-BNNT heterojunctions. All dipole moments are given by Debye.

calculated and is listed in Table 4. The dipole moments of (5,5) and (6,6) CNT-BNNT heterojunctions are found to be 1.92 Debye and 2.59 Debye, respectively. The dipole moments of (5,0) and (9,0) CNT-BNNT heterojunctions through forming C-N bonds are found to be 0.67 Debye and 12.47 Debye and through forming C-B bonds are 2.0 Debye and 0.52 Debye, respectively.

Comparing the dipole moments of the CNT-BNNT heterojunctions with ones that the CO, CO2, NO and NO2 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.

4. Conclusion

The gas sensing behavior of CNT-BNNT heterojunctions, considering a range of different diameters and chiralities is reported. The adsorption of CO, CO2, NO, and NO2 gas molecules on the (5,0), (9,0), (5,5) and (6,6) CNT-BNNTs are studied using B3LYP/6-31 g(d, p). It is found that the best CNT-BNNT heterojunction for adsorbing the CO, NO, CO2 and NO2 gas molecules is (5,0) CNT-BNNT heterojunctions with adsorption energy of −0.26, −0.41 eV, −0.33 eV and −0.63 eV, respectively. It is reported that 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. Also, it is noticed that the highest dipole moment is for (9,0) CNT-BNNT heterojunctions through forming C-N bonds.

NOTES

*Corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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