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Adsorption phenomena of gas molecules upon Ga-doped BN nanotubes: A DFT study
Applied Surface Science 295 (2014) 18–25
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Adsorption phenomena of gas molecules upon Ga-doped BN nanotubes: A DFT study Mohammad T. Baei a , Yasser Kanani b , Vahid Joveini Rezaei c , Alireza Soltani c,∗ a b c
Department of Chemistry, Azadshahr Branch, Islamic Azad University, Azadshahr, Golestan, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 15 December 2013 Accepted 25 December 2013 Available online 3 January 2014 Keywords: BN nanotubes Ab initio calculations Interaction energies Electronic properties
a b s t r a c t The sensitivity of Ga-doped BN nanotubes to prevalent gas molecules (H2 S, HS− , H2 O2 , and H2 CO) has been studied using B3LYP density functional theory (DFT) method. The chemisorption of H2 S, HS− , H2 O2 , and H2 CO molecules on (8, 0) BN nanotubes are estimated to be −1.61, −4.05, −1.27, and −1.28 eV, respectively. The obtained results of binding energies imply that the Ga-doped (8, 0) BN nanotubes can be a promising candidate for modification of sensor materials for the detection of H2 S, HS− , and H2 CO molecules compared with the Ga-doped (5, 5) BN nanotubes. Also, these results represent that the Gadoped (8, 0) BN nanotubes have high recovery time and high sensitivity to the presence of H2 S, HS− , and H2 CO molecules. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Boron nitride nanotubes (BNNTs) have been an outstanding source of research as a typical representative of III–V compound semiconductors, partially because they have the morphology of honeycomb analogous to carbon nanotubes (CNTs). Although between CNTs and BNNTs have many similarities, but unlike to CNTs, BNNTs are semiconducting with a very stable wide energy gap, excellent mechanical properties, strong piezoelectrical properties, chemical stability, thermal conductivity, and oxidation resistance at high temperatures, performance thus valuable in corrosive and high-temperature environments[1–4], and their electronic properties are independent of the tube diameter, chirality and whether a nanotube is single-walled or multi-walled. However, there has been much investigation that approved BNNTs can be the best candidate because of their similarities to CNTs [5,6]. Moreover, studies have revealed that CNTs, BNNTs are the most important nanotubes because of the alternative properties to carbon nanotubes. Boron nitride nanotubes (BNNTs) are one of the composite nanotubes [4], which have been predicted theoretically in 1994 [7] and then experimentally synthesized in 1995 as being mainly semiconductor materials with wide band gaps [8]. Similar to carbon nanotubes, both covalent and non-covalent functionalization plans have been adopted for solubilizing BNNT. Due to the
∗ Corresponding author. Tel.: +98 938 4544921. E-mail address: [email protected] (A. Soltani). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.136
electronic property and the delocalized network of the nanotube are retained, non-covalent functionalization is of major theoretical interest. All in all, after the discovery of boron nitride nanotubes (BNNTs) [9], as structural analogs of the CNT, there have been a greater part of the experimental and theoretical studies directed toward the utilization of this novel material in displacement of CNTs [10]. In the previous studies, First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of SCN− and CO molecules on the zigzag and armchair single-walled Al- and Ga-doped BN nanotubes [4,11]. Their results of investigation revealed that these molecules are strongly bound to the outer surface of Al- and Gadoped BNNTs. Also, they studied the effect of the NH3 adsorption on the geometries and electronic properties of related BNNT that the calculations shown there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction [12]. In 2012, Ahamdi et al. performed first principles calculations using density functional theory on the structural and electronic properties of AlNNTs as a catalyst for S–H cleavage of the H2 S molecule using the B3LYP functional and 6-31G* basis set implemented in GAMESS program [13]. Moreover, Wang et al. considered the dissociative adsorption of first and second H2 S molecule on the structural properties of Si (1 1 1) surface [14]. Anota and Cocoletzi have reported first-principles simulations of SH and OH groups on the surface of (5, 5) BN nanotube [15]. As BNNT gas sensors represent high sensitivity to some small gaseous molecules [16], they are doomed to detect the presence of some other gases such as CO2 and COCl [17,18]. This is based on the weak adsorption
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of these molecules upon BNNT and therefore insufficient charge transfer to affect the electrical conductivity. The Ga-doped BNNT is applied to overcome this drawback. In this area some studies exhibited that the higher sensitivity of BNNT toward gas molecules could be resulted by doping metal atoms [19]. The Ga-doped BNNT can meaningfully change its electronic properties that greatly enhance its sensing capability. The aim of the studies were to investigate whether the interaction of H2 S, HS, H2 O2 , and H2 CO molecules upon the Ga-doped (8, 0) and (5, 5) BN nanotubes can be strong enough to introduce them as suitable gas sensors using DFT simulations.
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2. Computational details Full geometry optimizations on the Ga-doped (8, 0) and (5, 5) BN nanotubes in the presence and in the absence of H2 S, HS, H2 O2 , and H2 CO molecules were carried out by using free software GAMESS package [20] with density functional theory (DFT), using B3LYP functional with 6-31G (d) basis set [11,12]. The length and diameter of optimized pure (8, 0) BNNT were calculated to be about 9.34 ˚ respectively. For (5, 5) BNNT, the length and diameter and 6.32 A, ˚ respectively. The point charge of the Ga, of tube are 6.9 and 6.84 A,
Fig. 1. Different adsorption configurations of H2 S/Ga doped (8, 0) and (5, 5) BNNTs.
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B, and N atoms in the Ga-doped (8, 0) BNNT are 0.494, 0.634 and −0.689 e and are 0.480, 0.636 and −0.671 e for the Ga-doped (5, 5) BNNT, respectively. Spin multiplicity of the gas molecules were set to one with relevance to its molecular orbital of ground state (its ground electronic state is 1 + ). The hydrogenated (8, 0) and (5, 5) Ga-doped BNNTs have 80 (B31 N32 H16 Ga) and 70 (B24 N25 H20 Ga) atoms, respectively. The binding energy (Ead ) of H2 S, HS, H2 O2 , and H2 CO molecules on the Ga-doped BNNTs is determined through the following equation: Ead = EGa−BNNT−Molecule − (EGa−BNNT + EMolecule )
and H2 CO. The quantum molecular descriptors were calculated [21,22] by the following equations: I = −EHOMO and A = −ELUMO =
I−A 2
= − = −
(2) (3)
(I + A) 2
(4)
(1)
where EGa−BNNT and EGa−BNNT−Molecule are the total energies of the (8, 0) and (5, 5) Ga-doped BNNTs interacting with the molecules. EGa−BNNT is total energies of the pristine (8, 0) and (5, 5) Ga-doped BNNTs. EMolecule is the total energies of an isolated H2 S, HS, H2 O2 ,
ω=
2 2
(5)
where I (−EHOMO ) is the energy of the Fermi level and A (−ELUMO ) is the first given value of the conduction band. Elctronegativity ()
Fig. 2. Calculated molecular electrostatic potential plots for H2 S, H2 O2 , HS− , and H2 CO adsorbed upon Ga-doped (8, 0) BNNTs. The surfaces are defined by the 0.0004 electrons/b3 contour of the electronic density. Color ranges, in a.u.: blue, more positive than 0.020; red, more negative than −0.020.
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is determined as the negative of chemical potential (), as follows: = −. In addition, the chemical hardness () can be determined using the Koopmans’ theorem. I (−EHOMO ) is the ionization potential and A (−ELUMO ) the electron affinity of the molecule.
3. Result and discussion Relaxed structure and geometry parameters of Ga-doped singlewalled (8, 0) and (5, 5) BNNT in the presence and in the absence of H2 S, H2 O2 , and H2 CO molecules were performed using density functional theory (DFT) with the hybrid B3LYP three-parameter exchange-correlation functional as implemented in GAMESS, as shown in Fig. 1, in which two types of Ga N bonds can be recognized; one with a bond length of 1.44 A˚ and parallel to the tube
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˚ not parallel to the axis, and another with a bond length of 1.46 A, tube axis (diagonal). The adsorption of H2 S upon the surface of Ga-doped (8, 0) BNNT is more notable if it approaches from its S atom toward the Ga site than Ga-doped (5, 5) BNNT (see Fig. 1). The calculated binding energies for the H2 S interacted across the Ga atoms of (8, 0) and (5, 5) BNNTs are −0.73 and −0.64 eV and the adsorption distances between the S atom of molecule and the Ga atoms of tubes are 2.57 ˚ respectively. The Mulliken population analyzes exhibits and 2.61 A, that the H2 S adsorption on Ga-doped (8, 0) and (5, 5) BNNTs leads to charge transfer of 0.1 and 0.09 electrons from the tubes to the H2 S molecule, respectively, and thus, H2 S functions as an electron acceptor while the nanotube functions as an electron donor. Therefore, it can be seen from molecular electrostatic potential (MEP) surfaces in Fig. 2.
Fig. 3. Relevant HOMO and LUMO plots of the most stable adsorption configurations of H2 S interacted with Ga doped (8, 0) BNNT. In this configuration the blue color exhibits the positive charge and the yellow color the negative charge.
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Besides, the interaction between the H2 S molecule and the nanotubes introduces local structural deformation in both H2 S and the Ga doped (8, 0) and (5, 5) BNNTs as the S H bond length changes ˚ while the H S H from 1.349 A˚ in the pure H2 S to 1.351 and 1.352 A, bond angles of H2 S slightly increased from 92.82◦ in free model to 92.84 and 92.52◦ in the adsorbed configurations, respectively (see Fig. 1). The point charges for Ga, B, and N atoms after the interaction of H2 S with Ga-doped (8, 0) BN nanotubes are 0.345, 0.603, and −0.829 e and are 0.312, 0.616, and 0.808 e for Ga-doped (5, 5) BN nanotubes, respectively. These above results are in agreement with previous theoretical reports mentioned by Beheshtian and Zhang [13,23]. Zhang and coworker [23] have shown that the adsorption of H2 S towards the B-, N-, Si-, Ca-, Co-, and Fe-doped graphene are −0.11, −0.14, 0.94, −0.66, −1.80, −1.92 eV while the interaction distances are 3.26, ˚ respectively. These results show 3.22, 2.53, 4.81, 3.77, and 3.70 A, that the H2 S adsorption on the Fe-doped graphene is stronger in comparison with other doping. In other words, the binding energies for H2 S molecule dissociation towards the Ga atoms of (8, 0) and (5, 5) BNNTs are about −1.61 and −1.42 eV, and the distances between the SH and the Ga atoms of the zigzag and armchair BN nanotubes are 2.236 and 2.242 A˚ and the distances between the H ˚ respectively. The atoms and N atoms of tubes are 1.027 and 1.026 A, Ga N bonds increased from 1.804 to 1.832 A˚ in the pristine zigzag and armchair BNNTs to 1.829 and 1.860 A˚ upon the H2 S dissociation,
while N Ga N bond angles significantly reduced from 113.89 and 116.34◦ in the pure system to 110.07 and 111.30◦ in the adsorbed models, see Table 1. The charge transfers of 0.15 and 0.18 electrons have occurred from Ga-doped zigzag and armchair BN nanotubes to the H2 S molecule. The charge analysis implies that the structural deformation can be attributed to the change of their hybridization from sp2 to sp3 , suggesting that the H2 S dissociation on the Gadoped (8, 0) and (5, 5) BNNTs are strong chemical adsorption in nature. The H2 S dissociation upon the Ga-doped (8, 0) BNNTs is energetically more remarkable than (5, 5) BNNTs. Beheshtian and co-author introduced the H2 S dissociation with -HS and -H coupled to (5, 0) AlNNT. They found that the adsorption energy of H2 S dissociation interacted with AlNNT at the B3LYP and B97D/6-31G* methods are −1.52 and −1.83 eV, respectively, suggesting that the B3LYP value of H2 S dissociation is somewhat less negative in comparison to B97D. In this section, the effects of H2 S adsorption on the electronic properties of the Ga-doped (8, 0) BNNT is investigated. In Fig. 1, the DOS analysis reveals that the H2 S dissociation on the Ga and N atoms of BNNT near Fermi level (EFl ) is affected, therefore, the dissociation of H2 S molecule upon Ga-doped BNNT leads to the changes in the electronic properties of tube. Recently, Anota et al. [24,25] have reported that the energy gaps of the pure BNNT at the different methods are about 4.59 (LDA) and 4.78 eV (GGA). Our calculations indicated that the energy gap of the pure (8, 0) and (5, 5) BNNTs are 5.69 and 6.31 eV at the B3LYP/6-31G* method.
Fig. 4. Different adsorption configurations of HS− /Ga doped (8, 0) and (5, 5) BNNTs.
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Table 1 Relative values of Ead , EHOMO , ELUMO , EF , Eg and the distances (Å) between H2 S and Ga-doped BNNTs. All the energy values are considered as eV unit. Property
(8,0) BNNT
(8,0) BNNT/Ga
(8,0) BNNT/Ga-HSH
(5,5) BNNT
(5,5) BNNT/Ga
(5,5) BNNT/Ga-HSH
lB N lB N B lGa N lN Ga N lS H lN H Ead /eV D/Å EHOMO /eV ELUMO /eV Eg /eV Eg (%) DM/Debye EF /eV /eV /eV ω/eV
1.450 119.9 – – – – – – −6.54 −0.85 5.69 – 11.45 −3.70 −3.70 2.85 2.40
– – 1.804 113.89 – – – – −6.53 −1.84 4.69 – 11.69 −4.18 −4.19 2.35 3.73
– – 1.858 99.77 1.352 1.027 −1.61 2.236 −6.36 −0.86 5.5 17.27 14.10 −3.61 −3.61 2.75 2.37
1.446 118.31 – – – – – – −6.40 −0.09 6.31 – 0.006 −3.25 −3.25 3.16 1.67
– – 1.832 116.34 – – – – −6.39 −1.33 5.06 – 0.815 −3.86 −3.86 2.53 2.94
– – 2.10 100.9 1.351 1.026 −1.42 2.242 −6.29 −0.41 5.88 16.20 3.56 −3.35 −3.35 2.94 1.91
Upon the interaction of H2 S with Ga-doped (8, 0) and (5, 5) BNNT, the energy gaps (Eg ) of the nanotubes have dramatic changes. As shown in Fig. 1, the energy gaps increase from 4.69 and 5.06 eV in the pure Ga-doped (8, 0) and (5, 5) BNNT to 5.5 and 5.88 eV, respectively. DOS plots reveal remarkable changes of energy gap (Eg ) about 17.27 and 16.20%, respectively. Thus, the H2 S adsorption further diminishes the energy gap of Ga-doped (8, 0) and (5, 5) BNNT and meliorates the electrical resistivity of the nanotubes. The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) for all of the applied systems are investigated using B3LYP/631G* method. The results represent that HOMO is more localized on the S atom of molecule as is also situated upon the N atoms of the tube while the LUMO is more localized on the B N bonds of the tube and is also slightly situated upon the H atom of molecule. Accordingly, the pure Ga-doped BNNT are sensitive to the H2 S adsorption, suggesting the Ga-doped BNNT can be used as a suitable sensor for H2 S detection, see Fig. 3. We explore the stable adsorption geometry of HS− on the outer walls of the Ga-doped (8, 0) and (5, 5) BNNTs as shown in Fig. 4. The calculated adsorption energies for the most stable configuration (S atom of HS− ) attached to Ga-doped BNNTs. The values of binding energy for HS− interacted with (8, 0) and (5, 5) BNNTs are about −4.05 and −3.63 eV and the corresponding distances between the ˚ and S atom of HS− and the Ga atoms of BNNTs are 2.93 and 2.75 A, 0.64 and 0.72 electrons are transferred from the Ga atom of (8, 0) and (5, 5) BNNTs to the HS− , respectively. MEP plot reveals that the HS− functions as an electron acceptor (see Fig. 2). The binding energy for HS− coupled to (5, 5) SWCNT at the PBE level was reported about −0.98 eV by Denis, while this value for HS− / (10, 0) SWCNT system is −0.1 eV [26]. Curran and Cech [27,28] indicated an experimental study of thiolated SWCNTs. They reported 0.6% of atomic volume of sulfur. The length of H S bond in free HS− decreases from 1.363 A˚ to 1.350 and 1.352 A˚ in the interaction between the HS− and (8, 0) and (5, 5) BNNTs. Anota and Cocoletzi have shown that the adsorption energy of the HS− adsorbed upon (5, 5) BNNT surface is about −0.06 eV [15]. In the configurations described in Fig. 4, the Ga N bonds of HS− / (8, 0) and (5, 5) BNNTs ˚ respectively. Therefore, the Ga complexes are 1.920 and 1.919 A, doped (8, 0) BNNTs is compatible for HS− sensing owing to large binding energy. We also consider the effect of H2 O2 adsorption toward the electronic properties of Ga-doped (8, 0) and (5, 5) BN nanotubes, as shown in Fig. 5. The binding energy values for H2 O2 adsorbed upon the Ga doped (8, 0) and (5, 5) BN nanotubes were calculated to be about −1.27 and −1.21 eV meanwhile 0.16 and 0.15 electrons are transferred from the H2 O2 molecule to the nanotubes. As represented on Fig. 2, MEP plot indicates that the H2 O2
molecule functions as an electron donor with the positive charge (blue color) and tube functions as an electron acceptor with negative charge (red color). The binding distances between the Ga doped (8, 0) and (5, 5) BN nanotubes and the H2 O2 molecule are about ˚ respectively. The high negative binding energy 2.08 and 2.09 A, of H2 O2 interacted with Ga-doped BNNTs implies strong adsorption in nature. The length of the O O and O H bonds in the pure form of H2 O2 molecule is 1.455 and 0.973 A˚ while the H O O bond angle and dihedral angle are 99.67◦ and 118.6◦ at the B3LYP/6-31G* level of theory, respectively. As represented in Table 2, through the interaction of H2 O2 molecule, the largest O O and O H distances are increased to 1.463 and 1.0 A˚ in the zigzag configuration and 1.456 and 1.0 A˚ in the armchair configuration, respectively. Upon the interaction of H2 O2 molecule, the point charges on the Ga, B, and N atoms of Ga-doped (8, 0) BN nanotubes are 0.437, 0.646, and −0.761 e and are 0.451, 0.652, and −0.763 e for Ga-doped (5, 5) BN nanotubes, respectively. Our calculations are in agreement with the theoretical study by Ramachandran and co-worker [29]. According to the experimental studies by Koput and Redington [30,31], the length of the O O bond in free H2 O2 is 1.464 and 1.467 A˚ and the O H bond is 0.965 and ˚ respectively. In previous report [16], we indicated that the 0.950 A, H2 O2 physisorption on the surface of (5, 0) BNNT is about −0.47 eV and the binding distance of H atoms of H2 O2 on the N and B atoms ˚ respectively. Beheshtian and co-worker [32] are 1.94 and 2.39 A,
Table 2 Relative values of Ead , EHOMO , ELUMO , EF , Eg and the distances (Å) for (8, 0) BNNT/H2 O2 , (5, 5) BNNT/Ga/ H2 O2 , and (8, 0) BNNT/H2 CO systems. All the energy values are considered as eV unit. Property
(8,0) Ga-BNNT/H2 O2
(5,5) BNNT/Ga/H2 O2
(8,0) Ga-BNNT/H2 CO
lGa N lN Ga N lO H lO O lC H lC O Ead /eV D/Å EHOMO /eV ELUMO /eV Eg /eV Eg (%) DM/Debye EF /eV /eV /eV ω/eV
1.822 110.21 0.978 1.462 – – −1.27 2.07 −6.46 −1.55 4.91 4.69 11.24 −4.00 −4.00 2.46 3.27
1.881 112.72 0.977 1.456 – – −1.21 2.08 −6.31 −1.23 5.08 0.40 2.27 −3.77 −3.77 2.54 2.80
1.820 110.33 – – 1.099 1.225 −1.28 2.05 −6.34 −3.59 2.75 −41.36 16.70 −4.96 −4.97 1.38 8.96
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Fig. 5. Different adsorption configurations of H2 O2 and H2 CO adsorbed upon Ga-doped (8, 0) and (5, 5) BNNTs.
also indicated that the interactions of H2 O2 on the pristine and Sidoped BC3 graphenes are weak with binding energies about −0.05 and −0.23 eV, respectively. This result reveals that the Ga-doped (8, 0) BNNT plays considerable role in the determination of adsorption energy of H2 O2 molecule. In this report, we investigate the sensitivity of Ga-doped BNNTs to H2 O2 molecule. Hence, the adsorption of H2 O2 upon Ga-doped (8, 0) and (5, 5) BNNT cannot influence the electronic properties of nanotubes (see Fig. 3). The DOS plots reveal that the energy gaps of (8, 0) and (5, 5) BNNTs are diminished from 4.69 and 5.06 eV to 4.91 and 5.09 eV during the adsorption processes, introducing low sensitivity of Ga-doped BNNT to H2 O2 molecule. We explore the adsorption behavior of H2 CO upon Ga-doped (8, 0) BNNT, in the most stable configuration, as shown in Fig. 5. The chemisorption energy and the interaction distance of H2 CO
˚ molecule on the Ga-doped (8, 0) BNNT are −1.28 eV and 2.05 A, respectively. The interaction of H2 CO molecule on the Ga-doped BNNT leads to a large electron transfer (about 0.19 e) from the molecule to the tube. As shown in Fig. 2, computed MEP plot for this system, indicates that the H2 CO molecule functions as an electron donor (blue color) and the tube functions as an electron acceptor (red color). The result of binding energy represents a strong chemisorption between H2 CO molecule and Ga-doped BNNT. Charge analysis implies that considerable hybridization occurs between the H2 CO and Ga-doped BNNT. The interaction of H2 CO toward Ga-doped BNNT leads to a significant structural distortion and as the Ga N bond was slightly lengthened by 0.025 A˚ (Table 2). Upon the adsorption process, the C O bond length in H2 CO molecule increases from 1.206 to 1.225 A˚ in H2 CO/Ga-doped BNNT system, and the bond angles of Ga O C and O C H are
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122.17 and 118.94◦ , respectively. After the adsorption of H2 CO molecule, the point charges for Ga, B, and N atoms of (8, 0) BNNT are about 0.452, 0.616, and −0.683 e, respectively. Zhang et al. [33] indicates that the binding energy of H2 CO on the pristine BNNT is about −0.064 eV owing to weak Van der Waals interaction between two species. For the most stable state, the chemisorption of H2 CO molecule interacts with SiB -doped (8, 0) BNNT is about −1.824 eV. These results reveal that the interaction of H2 CO on the Si-doped (8, 0) BNNT is much stronger than the Ga-doped (8, 0) BNNT. The DOS plot indicates that H2 CO adsorption can influence on the electronic property of Ga-doped (8, 0) BNNT as the energy gap of this system reduced from 4.69 eV in the pristine state to 2.7 eV in the presence of H2 CO molecule, and thus, the doping of Ga atom can increases the conductivity of the BNNT. Thus, the Ga-doped (8, 0) BNNT can be a promising sensor for H2 CO molecule. 3.1. Quantum molecular descriptors The quantum molecular descriptors for the most stable formations of H2 S, H2 O2 , and H2 CO molecules interacted with (8, 0) and (5, 5) BN nanotubes at the B3LYP/6-31G* method were studied. As shown in Table 1, the chemical potential () for (8, 0) and (5, 5) BNNTs are −3.70 and −3.25 eV, respectively, this result implies that the zigzag BNNT is more reactive than that of the armchair BNNT. The chemical potential are −4.19 and −3.86 eV in the Ga-doped (8, 0) and (5, 5) BNNTs, respectively. Increase in reactivity and decrease in global hardness of the applied systems stem from the reduction of energy gaps from 5.69 and 6.31 eV in the pristine (8, 0) and (5, 5) BN nanotubes to 4.69 and 5.06 eV in the Ga-doped (8, 0) and (5, 5) BN nanotubes. 4. Conclusions The adsorption phenomena of H2 S, H2 O2 , HS− , and H2 CO molecules towards Ga-doped (8, 0) and (5, 5) BN nanotubes have been theoretically investigated using B3LYP/6-31G* density functional theory level. The following conclusions can be drawn. (1) Due to the higher chemical reactivity of the Ga-doped (8, 0) BNNT, the altered exterior wall of tube facilities that interacts with H2 S, H2 O2 , and H2 CO molecules. It is founded that H2 S, H2 O2 , and H2 CO molecules are strongly chemisorbed upon the surface of the Ga-doped (8, 0) BNNT with high negative adsorption energies and small interaction distances than that of the Ga-doped (5, 5) BNNT. (2) The results demonstrate that H2 S and H2 CO molecules can be adsorbed upon the outer wall of Ga-doped (8, 0) BNNT with remarkable adsorption energies and large charge transfers leading to significant changes in the electrical conductance of Ga-doped (8, 0) BNNTs. Therefore, the Ga-doped (8, 0) BNNT can be introduced as a promising sensor for H2 S and H2 CO detections.
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(3) Decrease in the global hardness and energy gap is due to the doping of Ga, and consequently, the stability of the Ga-doped (8, 0) and (5, 5) BN nanotubes are lowered while their reactivity increased. Acknowledgments We would like to thank the Jaber Ebne Hayyan Unique Industry researchers Company. We should also thank the Nanotechnology Working Group of Young Researchers and Elite Club of Islamic Azad University, Gorgan Branch, Iran. References [1] Y. Yu, H. Chen, Y. Liu, T. White, Y. Chen, Mater. Lett. 80 (2012) 148–151. [2] T. Ikuno, T. Sainsbury, D. Okawa, J.M.J. Fréchet, A. Zettl, Solid State Commun. 142 (2007) 643–646. [3] A. Soltani, N. Ahmadian, A. Amirazami, A. Masoodi, E. Tazikeh Lemeski, A. Varasteh Moradi, Appl. Surf. Sci. 261 (2012) 262–267. [4] A. Soltani, N. Ahmadian, Y. Kanani, A. Dehnokhalaji, H. Mighani, Appl. Surf. Sci. 258 (2012) 9536–9543. [5] P.N. D’yachkov, D.V. Makaev, J. Phys. Chem. Solids 70 (2009) 180–185. [6] J-x. Zhao, Y-h. Ding, Diam. Relat. Mater. 19 (2010) 1073–1077. [7] A. Rubio, J.L. Corkill, M.L. Cohen, Phys. Rev. B 49 (1994) 5081. [8] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, et al., Science 269 (1995) 966–967. [9] N. Saikia, S.K. Pati, R.C. Deka, Appl. Nanosci. 2 (2012) 389–400. [10] A. Soltani, A. Ahmadi Peyghan, Z. Bagheri, Physica E 48 (2013) 176–180. [11] A. Ahmadi Peyghan, A. Soltani, A.A. Pahlevani, Y. Kanani, S. Khajeh, Appl. Surf. Sci. 270 (2013) 25–32. [12] A. Soltani, S. Ghafouri Raz, V. Joveini Rezaei, A. Dehno Khalaji, M. Savar, Appl. Surf. Sci. 263 (2012) 619–625. [13] J. Beheshtian, A. Ahmadi Peyghan, Z. Bagheri, Physica E 44 (2012) 1963–1968. [14] C. Ren, X. Wang, Y. Miao, L. Yi, X. Jin, Y. Tan, J. Mol. Struct. Theochem 949 (2010) 96–100. [15] E.C. Anota, G.H. Cocoletzi, J. Mol. Model. 19 (2013) 2335–2341. [16] A. Soltani, M. Ramezani Taghartapeh, E. Tazikeh Lemeski, M. Abroudi, H. Mighani, Superlattice. Microst. 58 (2013) 178–190. [17] J. Beheshtian, A. Ahmadi Peyghan, Z. Bagheri, Sensor. Actuat. B 171–172 (2012) 846–852. [18] H. Choi, Y.C. Park, Y.-H. Kim, Y.S. Lee, J. Am. Chem. Soc. 133 (2011) 2084–2087. [19] A. Ahmadi Peyghan, M.T. Baei, M. Moghimi, S. Hashemian, Computational and Theoretical Chemistry. [20] M. Schmidt, et al., General atomic and molecular electronic structure system, J. Comput. Chem. 14 (1993) 1347. [21] A. Soltani, A. Varasteh Moradi, M. Bahari, A. Masoodi, S. Shojaee, Physica B 430 (2013) 20–26. [22] A. Soltani, S. Ghafouri Raz, M. Ramezani Taghartapeh, A. Varasteh Moradi, R. Zafar Mehrabian, Comp. Mater. Sci. 79 (2013) 795–803. [23] Y.-H. Zhang, L.-F. Han, Y.-H. Xiao, D.-Z. Jia, Z.-H. Guo, F. Li, Comp. Mater. Sci. 69 (2013) 222–228. [24] E.C. Anota, H.H. Cocoletzi, M.S. Villanueva, D.G. Toral, Superlattice. Microst. 63 (2013) 298–305. [25] E.C. Anota, G.H. Cocoletzi, Physica E 56 (2014) 134–140. [26] P.A. Denis, J.S. Gancheff, J. Mater. Sci 45 (2010) 1039–1045. [27] S.A. Curran, J. Cech, D. Zhang, J.L. Dewald, A. Avadhanula, M. Kandadai, S. Roth, J. Mater. Res. 21 (2006) 1012. [28] S.A. Curran, J. Cech, D. Zhang, J.L. Dewald, A. Avadhanula, M. Kandadai, S. Roth, Phys. Stat. Sol. 243 (2006) 3221–3225. [29] C.N. Ramachandran, D.D. Fazio, N. Sathyamurthy, V. Aquilanti, Chem. Phys. Lett. 473 (2009) 146–150. [30] J. Koput, J. Mol. Spectrosc. 115 (1986) 438. [31] R.L. Redington, W.B. Oslon, P.C. Cross, J. Chem. Phys. 36 (1962) 1311. [32] J. Beheshtian, A. Ahmadi Peyghan, M. Noei, Sensor. Actuat. B: Chem. 181 (2013) 829–834. [33] R. Wang, R. Zhu, D. Zhang, Chem. Phys. Lett. 467 (2008) 131–135.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Adsorption phenomena of gas molecules upon Ga-doped BN nanotubes: A DFT study Mohammad T. Baei a , Yasser Kanani b , Vahid Joveini Rezaei c , Alireza Soltani c,∗ a b c
Department of Chemistry, Azadshahr Branch, Islamic Azad University, Azadshahr, Golestan, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran
a r t i c l e
i n f o
Article history: Received 29 September 2013 Received in revised form 15 December 2013 Accepted 25 December 2013 Available online 3 January 2014 Keywords: BN nanotubes Ab initio calculations Interaction energies Electronic properties
a b s t r a c t The sensitivity of Ga-doped BN nanotubes to prevalent gas molecules (H2 S, HS− , H2 O2 , and H2 CO) has been studied using B3LYP density functional theory (DFT) method. The chemisorption of H2 S, HS− , H2 O2 , and H2 CO molecules on (8, 0) BN nanotubes are estimated to be −1.61, −4.05, −1.27, and −1.28 eV, respectively. The obtained results of binding energies imply that the Ga-doped (8, 0) BN nanotubes can be a promising candidate for modification of sensor materials for the detection of H2 S, HS− , and H2 CO molecules compared with the Ga-doped (5, 5) BN nanotubes. Also, these results represent that the Gadoped (8, 0) BN nanotubes have high recovery time and high sensitivity to the presence of H2 S, HS− , and H2 CO molecules. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Boron nitride nanotubes (BNNTs) have been an outstanding source of research as a typical representative of III–V compound semiconductors, partially because they have the morphology of honeycomb analogous to carbon nanotubes (CNTs). Although between CNTs and BNNTs have many similarities, but unlike to CNTs, BNNTs are semiconducting with a very stable wide energy gap, excellent mechanical properties, strong piezoelectrical properties, chemical stability, thermal conductivity, and oxidation resistance at high temperatures, performance thus valuable in corrosive and high-temperature environments[1–4], and their electronic properties are independent of the tube diameter, chirality and whether a nanotube is single-walled or multi-walled. However, there has been much investigation that approved BNNTs can be the best candidate because of their similarities to CNTs [5,6]. Moreover, studies have revealed that CNTs, BNNTs are the most important nanotubes because of the alternative properties to carbon nanotubes. Boron nitride nanotubes (BNNTs) are one of the composite nanotubes [4], which have been predicted theoretically in 1994 [7] and then experimentally synthesized in 1995 as being mainly semiconductor materials with wide band gaps [8]. Similar to carbon nanotubes, both covalent and non-covalent functionalization plans have been adopted for solubilizing BNNT. Due to the
∗ Corresponding author. Tel.: +98 938 4544921. E-mail address: [email protected] (A. Soltani). 0169-4332/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.12.136
electronic property and the delocalized network of the nanotube are retained, non-covalent functionalization is of major theoretical interest. All in all, after the discovery of boron nitride nanotubes (BNNTs) [9], as structural analogs of the CNT, there have been a greater part of the experimental and theoretical studies directed toward the utilization of this novel material in displacement of CNTs [10]. In the previous studies, First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of SCN− and CO molecules on the zigzag and armchair single-walled Al- and Ga-doped BN nanotubes [4,11]. Their results of investigation revealed that these molecules are strongly bound to the outer surface of Al- and Gadoped BNNTs. Also, they studied the effect of the NH3 adsorption on the geometries and electronic properties of related BNNT that the calculations shown there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction [12]. In 2012, Ahamdi et al. performed first principles calculations using density functional theory on the structural and electronic properties of AlNNTs as a catalyst for S–H cleavage of the H2 S molecule using the B3LYP functional and 6-31G* basis set implemented in GAMESS program [13]. Moreover, Wang et al. considered the dissociative adsorption of first and second H2 S molecule on the structural properties of Si (1 1 1) surface [14]. Anota and Cocoletzi have reported first-principles simulations of SH and OH groups on the surface of (5, 5) BN nanotube [15]. As BNNT gas sensors represent high sensitivity to some small gaseous molecules [16], they are doomed to detect the presence of some other gases such as CO2 and COCl [17,18]. This is based on the weak adsorption
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of these molecules upon BNNT and therefore insufficient charge transfer to affect the electrical conductivity. The Ga-doped BNNT is applied to overcome this drawback. In this area some studies exhibited that the higher sensitivity of BNNT toward gas molecules could be resulted by doping metal atoms [19]. The Ga-doped BNNT can meaningfully change its electronic properties that greatly enhance its sensing capability. The aim of the studies were to investigate whether the interaction of H2 S, HS, H2 O2 , and H2 CO molecules upon the Ga-doped (8, 0) and (5, 5) BN nanotubes can be strong enough to introduce them as suitable gas sensors using DFT simulations.
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2. Computational details Full geometry optimizations on the Ga-doped (8, 0) and (5, 5) BN nanotubes in the presence and in the absence of H2 S, HS, H2 O2 , and H2 CO molecules were carried out by using free software GAMESS package [20] with density functional theory (DFT), using B3LYP functional with 6-31G (d) basis set [11,12]. The length and diameter of optimized pure (8, 0) BNNT were calculated to be about 9.34 ˚ respectively. For (5, 5) BNNT, the length and diameter and 6.32 A, ˚ respectively. The point charge of the Ga, of tube are 6.9 and 6.84 A,
Fig. 1. Different adsorption configurations of H2 S/Ga doped (8, 0) and (5, 5) BNNTs.
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B, and N atoms in the Ga-doped (8, 0) BNNT are 0.494, 0.634 and −0.689 e and are 0.480, 0.636 and −0.671 e for the Ga-doped (5, 5) BNNT, respectively. Spin multiplicity of the gas molecules were set to one with relevance to its molecular orbital of ground state (its ground electronic state is 1 + ). The hydrogenated (8, 0) and (5, 5) Ga-doped BNNTs have 80 (B31 N32 H16 Ga) and 70 (B24 N25 H20 Ga) atoms, respectively. The binding energy (Ead ) of H2 S, HS, H2 O2 , and H2 CO molecules on the Ga-doped BNNTs is determined through the following equation: Ead = EGa−BNNT−Molecule − (EGa−BNNT + EMolecule )
and H2 CO. The quantum molecular descriptors were calculated [21,22] by the following equations: I = −EHOMO and A = −ELUMO =
I−A 2
= − = −
(2) (3)
(I + A) 2
(4)
(1)
where EGa−BNNT and EGa−BNNT−Molecule are the total energies of the (8, 0) and (5, 5) Ga-doped BNNTs interacting with the molecules. EGa−BNNT is total energies of the pristine (8, 0) and (5, 5) Ga-doped BNNTs. EMolecule is the total energies of an isolated H2 S, HS, H2 O2 ,
ω=
2 2
(5)
where I (−EHOMO ) is the energy of the Fermi level and A (−ELUMO ) is the first given value of the conduction band. Elctronegativity ()
Fig. 2. Calculated molecular electrostatic potential plots for H2 S, H2 O2 , HS− , and H2 CO adsorbed upon Ga-doped (8, 0) BNNTs. The surfaces are defined by the 0.0004 electrons/b3 contour of the electronic density. Color ranges, in a.u.: blue, more positive than 0.020; red, more negative than −0.020.
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is determined as the negative of chemical potential (), as follows: = −. In addition, the chemical hardness () can be determined using the Koopmans’ theorem. I (−EHOMO ) is the ionization potential and A (−ELUMO ) the electron affinity of the molecule.
3. Result and discussion Relaxed structure and geometry parameters of Ga-doped singlewalled (8, 0) and (5, 5) BNNT in the presence and in the absence of H2 S, H2 O2 , and H2 CO molecules were performed using density functional theory (DFT) with the hybrid B3LYP three-parameter exchange-correlation functional as implemented in GAMESS, as shown in Fig. 1, in which two types of Ga N bonds can be recognized; one with a bond length of 1.44 A˚ and parallel to the tube
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˚ not parallel to the axis, and another with a bond length of 1.46 A, tube axis (diagonal). The adsorption of H2 S upon the surface of Ga-doped (8, 0) BNNT is more notable if it approaches from its S atom toward the Ga site than Ga-doped (5, 5) BNNT (see Fig. 1). The calculated binding energies for the H2 S interacted across the Ga atoms of (8, 0) and (5, 5) BNNTs are −0.73 and −0.64 eV and the adsorption distances between the S atom of molecule and the Ga atoms of tubes are 2.57 ˚ respectively. The Mulliken population analyzes exhibits and 2.61 A, that the H2 S adsorption on Ga-doped (8, 0) and (5, 5) BNNTs leads to charge transfer of 0.1 and 0.09 electrons from the tubes to the H2 S molecule, respectively, and thus, H2 S functions as an electron acceptor while the nanotube functions as an electron donor. Therefore, it can be seen from molecular electrostatic potential (MEP) surfaces in Fig. 2.
Fig. 3. Relevant HOMO and LUMO plots of the most stable adsorption configurations of H2 S interacted with Ga doped (8, 0) BNNT. In this configuration the blue color exhibits the positive charge and the yellow color the negative charge.
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Besides, the interaction between the H2 S molecule and the nanotubes introduces local structural deformation in both H2 S and the Ga doped (8, 0) and (5, 5) BNNTs as the S H bond length changes ˚ while the H S H from 1.349 A˚ in the pure H2 S to 1.351 and 1.352 A, bond angles of H2 S slightly increased from 92.82◦ in free model to 92.84 and 92.52◦ in the adsorbed configurations, respectively (see Fig. 1). The point charges for Ga, B, and N atoms after the interaction of H2 S with Ga-doped (8, 0) BN nanotubes are 0.345, 0.603, and −0.829 e and are 0.312, 0.616, and 0.808 e for Ga-doped (5, 5) BN nanotubes, respectively. These above results are in agreement with previous theoretical reports mentioned by Beheshtian and Zhang [13,23]. Zhang and coworker [23] have shown that the adsorption of H2 S towards the B-, N-, Si-, Ca-, Co-, and Fe-doped graphene are −0.11, −0.14, 0.94, −0.66, −1.80, −1.92 eV while the interaction distances are 3.26, ˚ respectively. These results show 3.22, 2.53, 4.81, 3.77, and 3.70 A, that the H2 S adsorption on the Fe-doped graphene is stronger in comparison with other doping. In other words, the binding energies for H2 S molecule dissociation towards the Ga atoms of (8, 0) and (5, 5) BNNTs are about −1.61 and −1.42 eV, and the distances between the SH and the Ga atoms of the zigzag and armchair BN nanotubes are 2.236 and 2.242 A˚ and the distances between the H ˚ respectively. The atoms and N atoms of tubes are 1.027 and 1.026 A, Ga N bonds increased from 1.804 to 1.832 A˚ in the pristine zigzag and armchair BNNTs to 1.829 and 1.860 A˚ upon the H2 S dissociation,
while N Ga N bond angles significantly reduced from 113.89 and 116.34◦ in the pure system to 110.07 and 111.30◦ in the adsorbed models, see Table 1. The charge transfers of 0.15 and 0.18 electrons have occurred from Ga-doped zigzag and armchair BN nanotubes to the H2 S molecule. The charge analysis implies that the structural deformation can be attributed to the change of their hybridization from sp2 to sp3 , suggesting that the H2 S dissociation on the Gadoped (8, 0) and (5, 5) BNNTs are strong chemical adsorption in nature. The H2 S dissociation upon the Ga-doped (8, 0) BNNTs is energetically more remarkable than (5, 5) BNNTs. Beheshtian and co-author introduced the H2 S dissociation with -HS and -H coupled to (5, 0) AlNNT. They found that the adsorption energy of H2 S dissociation interacted with AlNNT at the B3LYP and B97D/6-31G* methods are −1.52 and −1.83 eV, respectively, suggesting that the B3LYP value of H2 S dissociation is somewhat less negative in comparison to B97D. In this section, the effects of H2 S adsorption on the electronic properties of the Ga-doped (8, 0) BNNT is investigated. In Fig. 1, the DOS analysis reveals that the H2 S dissociation on the Ga and N atoms of BNNT near Fermi level (EFl ) is affected, therefore, the dissociation of H2 S molecule upon Ga-doped BNNT leads to the changes in the electronic properties of tube. Recently, Anota et al. [24,25] have reported that the energy gaps of the pure BNNT at the different methods are about 4.59 (LDA) and 4.78 eV (GGA). Our calculations indicated that the energy gap of the pure (8, 0) and (5, 5) BNNTs are 5.69 and 6.31 eV at the B3LYP/6-31G* method.
Fig. 4. Different adsorption configurations of HS− /Ga doped (8, 0) and (5, 5) BNNTs.
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Table 1 Relative values of Ead , EHOMO , ELUMO , EF , Eg and the distances (Å) between H2 S and Ga-doped BNNTs. All the energy values are considered as eV unit. Property
(8,0) BNNT
(8,0) BNNT/Ga
(8,0) BNNT/Ga-HSH
(5,5) BNNT
(5,5) BNNT/Ga
(5,5) BNNT/Ga-HSH
lB N lB N B lGa N lN Ga N lS H lN H Ead /eV D/Å EHOMO /eV ELUMO /eV Eg /eV Eg (%) DM/Debye EF /eV /eV /eV ω/eV
1.450 119.9 – – – – – – −6.54 −0.85 5.69 – 11.45 −3.70 −3.70 2.85 2.40
– – 1.804 113.89 – – – – −6.53 −1.84 4.69 – 11.69 −4.18 −4.19 2.35 3.73
– – 1.858 99.77 1.352 1.027 −1.61 2.236 −6.36 −0.86 5.5 17.27 14.10 −3.61 −3.61 2.75 2.37
1.446 118.31 – – – – – – −6.40 −0.09 6.31 – 0.006 −3.25 −3.25 3.16 1.67
– – 1.832 116.34 – – – – −6.39 −1.33 5.06 – 0.815 −3.86 −3.86 2.53 2.94
– – 2.10 100.9 1.351 1.026 −1.42 2.242 −6.29 −0.41 5.88 16.20 3.56 −3.35 −3.35 2.94 1.91
Upon the interaction of H2 S with Ga-doped (8, 0) and (5, 5) BNNT, the energy gaps (Eg ) of the nanotubes have dramatic changes. As shown in Fig. 1, the energy gaps increase from 4.69 and 5.06 eV in the pure Ga-doped (8, 0) and (5, 5) BNNT to 5.5 and 5.88 eV, respectively. DOS plots reveal remarkable changes of energy gap (Eg ) about 17.27 and 16.20%, respectively. Thus, the H2 S adsorption further diminishes the energy gap of Ga-doped (8, 0) and (5, 5) BNNT and meliorates the electrical resistivity of the nanotubes. The HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) for all of the applied systems are investigated using B3LYP/631G* method. The results represent that HOMO is more localized on the S atom of molecule as is also situated upon the N atoms of the tube while the LUMO is more localized on the B N bonds of the tube and is also slightly situated upon the H atom of molecule. Accordingly, the pure Ga-doped BNNT are sensitive to the H2 S adsorption, suggesting the Ga-doped BNNT can be used as a suitable sensor for H2 S detection, see Fig. 3. We explore the stable adsorption geometry of HS− on the outer walls of the Ga-doped (8, 0) and (5, 5) BNNTs as shown in Fig. 4. The calculated adsorption energies for the most stable configuration (S atom of HS− ) attached to Ga-doped BNNTs. The values of binding energy for HS− interacted with (8, 0) and (5, 5) BNNTs are about −4.05 and −3.63 eV and the corresponding distances between the ˚ and S atom of HS− and the Ga atoms of BNNTs are 2.93 and 2.75 A, 0.64 and 0.72 electrons are transferred from the Ga atom of (8, 0) and (5, 5) BNNTs to the HS− , respectively. MEP plot reveals that the HS− functions as an electron acceptor (see Fig. 2). The binding energy for HS− coupled to (5, 5) SWCNT at the PBE level was reported about −0.98 eV by Denis, while this value for HS− / (10, 0) SWCNT system is −0.1 eV [26]. Curran and Cech [27,28] indicated an experimental study of thiolated SWCNTs. They reported 0.6% of atomic volume of sulfur. The length of H S bond in free HS− decreases from 1.363 A˚ to 1.350 and 1.352 A˚ in the interaction between the HS− and (8, 0) and (5, 5) BNNTs. Anota and Cocoletzi have shown that the adsorption energy of the HS− adsorbed upon (5, 5) BNNT surface is about −0.06 eV [15]. In the configurations described in Fig. 4, the Ga N bonds of HS− / (8, 0) and (5, 5) BNNTs ˚ respectively. Therefore, the Ga complexes are 1.920 and 1.919 A, doped (8, 0) BNNTs is compatible for HS− sensing owing to large binding energy. We also consider the effect of H2 O2 adsorption toward the electronic properties of Ga-doped (8, 0) and (5, 5) BN nanotubes, as shown in Fig. 5. The binding energy values for H2 O2 adsorbed upon the Ga doped (8, 0) and (5, 5) BN nanotubes were calculated to be about −1.27 and −1.21 eV meanwhile 0.16 and 0.15 electrons are transferred from the H2 O2 molecule to the nanotubes. As represented on Fig. 2, MEP plot indicates that the H2 O2
molecule functions as an electron donor with the positive charge (blue color) and tube functions as an electron acceptor with negative charge (red color). The binding distances between the Ga doped (8, 0) and (5, 5) BN nanotubes and the H2 O2 molecule are about ˚ respectively. The high negative binding energy 2.08 and 2.09 A, of H2 O2 interacted with Ga-doped BNNTs implies strong adsorption in nature. The length of the O O and O H bonds in the pure form of H2 O2 molecule is 1.455 and 0.973 A˚ while the H O O bond angle and dihedral angle are 99.67◦ and 118.6◦ at the B3LYP/6-31G* level of theory, respectively. As represented in Table 2, through the interaction of H2 O2 molecule, the largest O O and O H distances are increased to 1.463 and 1.0 A˚ in the zigzag configuration and 1.456 and 1.0 A˚ in the armchair configuration, respectively. Upon the interaction of H2 O2 molecule, the point charges on the Ga, B, and N atoms of Ga-doped (8, 0) BN nanotubes are 0.437, 0.646, and −0.761 e and are 0.451, 0.652, and −0.763 e for Ga-doped (5, 5) BN nanotubes, respectively. Our calculations are in agreement with the theoretical study by Ramachandran and co-worker [29]. According to the experimental studies by Koput and Redington [30,31], the length of the O O bond in free H2 O2 is 1.464 and 1.467 A˚ and the O H bond is 0.965 and ˚ respectively. In previous report [16], we indicated that the 0.950 A, H2 O2 physisorption on the surface of (5, 0) BNNT is about −0.47 eV and the binding distance of H atoms of H2 O2 on the N and B atoms ˚ respectively. Beheshtian and co-worker [32] are 1.94 and 2.39 A,
Table 2 Relative values of Ead , EHOMO , ELUMO , EF , Eg and the distances (Å) for (8, 0) BNNT/H2 O2 , (5, 5) BNNT/Ga/ H2 O2 , and (8, 0) BNNT/H2 CO systems. All the energy values are considered as eV unit. Property
(8,0) Ga-BNNT/H2 O2
(5,5) BNNT/Ga/H2 O2
(8,0) Ga-BNNT/H2 CO
lGa N lN Ga N lO H lO O lC H lC O Ead /eV D/Å EHOMO /eV ELUMO /eV Eg /eV Eg (%) DM/Debye EF /eV /eV /eV ω/eV
1.822 110.21 0.978 1.462 – – −1.27 2.07 −6.46 −1.55 4.91 4.69 11.24 −4.00 −4.00 2.46 3.27
1.881 112.72 0.977 1.456 – – −1.21 2.08 −6.31 −1.23 5.08 0.40 2.27 −3.77 −3.77 2.54 2.80
1.820 110.33 – – 1.099 1.225 −1.28 2.05 −6.34 −3.59 2.75 −41.36 16.70 −4.96 −4.97 1.38 8.96
24
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Fig. 5. Different adsorption configurations of H2 O2 and H2 CO adsorbed upon Ga-doped (8, 0) and (5, 5) BNNTs.
also indicated that the interactions of H2 O2 on the pristine and Sidoped BC3 graphenes are weak with binding energies about −0.05 and −0.23 eV, respectively. This result reveals that the Ga-doped (8, 0) BNNT plays considerable role in the determination of adsorption energy of H2 O2 molecule. In this report, we investigate the sensitivity of Ga-doped BNNTs to H2 O2 molecule. Hence, the adsorption of H2 O2 upon Ga-doped (8, 0) and (5, 5) BNNT cannot influence the electronic properties of nanotubes (see Fig. 3). The DOS plots reveal that the energy gaps of (8, 0) and (5, 5) BNNTs are diminished from 4.69 and 5.06 eV to 4.91 and 5.09 eV during the adsorption processes, introducing low sensitivity of Ga-doped BNNT to H2 O2 molecule. We explore the adsorption behavior of H2 CO upon Ga-doped (8, 0) BNNT, in the most stable configuration, as shown in Fig. 5. The chemisorption energy and the interaction distance of H2 CO
˚ molecule on the Ga-doped (8, 0) BNNT are −1.28 eV and 2.05 A, respectively. The interaction of H2 CO molecule on the Ga-doped BNNT leads to a large electron transfer (about 0.19 e) from the molecule to the tube. As shown in Fig. 2, computed MEP plot for this system, indicates that the H2 CO molecule functions as an electron donor (blue color) and the tube functions as an electron acceptor (red color). The result of binding energy represents a strong chemisorption between H2 CO molecule and Ga-doped BNNT. Charge analysis implies that considerable hybridization occurs between the H2 CO and Ga-doped BNNT. The interaction of H2 CO toward Ga-doped BNNT leads to a significant structural distortion and as the Ga N bond was slightly lengthened by 0.025 A˚ (Table 2). Upon the adsorption process, the C O bond length in H2 CO molecule increases from 1.206 to 1.225 A˚ in H2 CO/Ga-doped BNNT system, and the bond angles of Ga O C and O C H are
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122.17 and 118.94◦ , respectively. After the adsorption of H2 CO molecule, the point charges for Ga, B, and N atoms of (8, 0) BNNT are about 0.452, 0.616, and −0.683 e, respectively. Zhang et al. [33] indicates that the binding energy of H2 CO on the pristine BNNT is about −0.064 eV owing to weak Van der Waals interaction between two species. For the most stable state, the chemisorption of H2 CO molecule interacts with SiB -doped (8, 0) BNNT is about −1.824 eV. These results reveal that the interaction of H2 CO on the Si-doped (8, 0) BNNT is much stronger than the Ga-doped (8, 0) BNNT. The DOS plot indicates that H2 CO adsorption can influence on the electronic property of Ga-doped (8, 0) BNNT as the energy gap of this system reduced from 4.69 eV in the pristine state to 2.7 eV in the presence of H2 CO molecule, and thus, the doping of Ga atom can increases the conductivity of the BNNT. Thus, the Ga-doped (8, 0) BNNT can be a promising sensor for H2 CO molecule. 3.1. Quantum molecular descriptors The quantum molecular descriptors for the most stable formations of H2 S, H2 O2 , and H2 CO molecules interacted with (8, 0) and (5, 5) BN nanotubes at the B3LYP/6-31G* method were studied. As shown in Table 1, the chemical potential () for (8, 0) and (5, 5) BNNTs are −3.70 and −3.25 eV, respectively, this result implies that the zigzag BNNT is more reactive than that of the armchair BNNT. The chemical potential are −4.19 and −3.86 eV in the Ga-doped (8, 0) and (5, 5) BNNTs, respectively. Increase in reactivity and decrease in global hardness of the applied systems stem from the reduction of energy gaps from 5.69 and 6.31 eV in the pristine (8, 0) and (5, 5) BN nanotubes to 4.69 and 5.06 eV in the Ga-doped (8, 0) and (5, 5) BN nanotubes. 4. Conclusions The adsorption phenomena of H2 S, H2 O2 , HS− , and H2 CO molecules towards Ga-doped (8, 0) and (5, 5) BN nanotubes have been theoretically investigated using B3LYP/6-31G* density functional theory level. The following conclusions can be drawn. (1) Due to the higher chemical reactivity of the Ga-doped (8, 0) BNNT, the altered exterior wall of tube facilities that interacts with H2 S, H2 O2 , and H2 CO molecules. It is founded that H2 S, H2 O2 , and H2 CO molecules are strongly chemisorbed upon the surface of the Ga-doped (8, 0) BNNT with high negative adsorption energies and small interaction distances than that of the Ga-doped (5, 5) BNNT. (2) The results demonstrate that H2 S and H2 CO molecules can be adsorbed upon the outer wall of Ga-doped (8, 0) BNNT with remarkable adsorption energies and large charge transfers leading to significant changes in the electrical conductance of Ga-doped (8, 0) BNNTs. Therefore, the Ga-doped (8, 0) BNNT can be introduced as a promising sensor for H2 S and H2 CO detections.
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