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Theoretical investigation of OCN− adsorption onto boron nitride nanotubes
Applied Surface Science 261 (2012) 262–267
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Theoretical investigation of OCN− adsorption onto boron nitride nanotubes Alireza Soltani a,∗ , Nasim Ahmadian b , Abolfazl Amirazami c , Anis Masoodi a , E. Tazikeh Lemeski c , Ali Varasteh Moradi c a b c
Young Researchers Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshar, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran
a r t i c l e
i n f o
Article history: Received 21 June 2012 Received in revised form 28 July 2012 Accepted 28 July 2012 Available online 4 August 2012 Keywords: Adsorption Nanostructures Ab initio calculations Chemisorption
a b s t r a c t First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of OCN− on H-capped zigzag and armchair single-walled BN nanotubes (BNNTs). The results indicate that OCN− is strongly bound to the outer surface of zigzag (6, 0) BNNTs in comparison with armchair (5, 5) BNNT. Binding energy and equilibrium distance corresponding to the most stable ˚ respectively being typical for the chemisorpconfiguration are found to be −486.79 kJ mol−1 and 1.526 A, tions. Energy gap, dipole moment, natural atomic orbital occupancies and global indices for most stable configuration are calculated. Furthermore, the effect of the OCN− adsorption on the geometries and electronic properties of related BNNT is also studied. The calculated density of states (DOS) reveals that there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction. Therefore, one can conclude that BNNTs play an important role as suitable sensor. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) [1], much attention has been devoted to this material due to their unique structures and brilliant properties [2–4] in the wide range of applications including molecular sensing [5–7], gas storage [8–10], catalysis [11,12], field emission displays [13], switching behaviors [14]. Since their tubular diameter and chirality have a significant effect on the electronic properties of CNTs, much effort have been paid to examine non-carbon based nanotubes showing electronic properties independent of these features [15–17]. On the other hand, after the discovery of boron nitride nanotubes (BNNTs) [18], as structural analogs of the CNT, there have been a large majority of the experimental and theoretical studies directed toward the utilization of this novel material in displacement of CNTs [19]. A sheet of hexagonal rings including boron and nitrogen atoms in equal proportions is rolled up to form BNNT [20,21], which exhibits its own different properties than to its tubular carbon counterparts. BNNTs are semiconductors with a constant band gap of ∼5.5 eV, being almost independent of tubular diameter and helicity [22]. Brilliant properties such as; large iconicity, high thermal conductivity, superior resistance to oxidation, and high mechanical strength [23–26], make BNNTs highly useful materials for the broad diversity of application [22]. For example, numerous studies both theoretically and
∗ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani).
experimentally have been allocated to examine their application in nano-electronic fields [27,28], force sensors [29–31], gas storage [32], particularly in hydrogen storage [33,34]. The need for detectors with high specificity and sensitivity has directed scientists to serve tubular structures. Electronic conductance changes in BNNTs upon exposure to gas molecules. Based on this feather, they can be served as nanotube molecular sensors [20]. On the other hand, the cyanato anion (OCN− ) is a functional group for many organic compounds, and also has a significant role as intermediate in the combustion of nitrogenous fuels [35]. The OCN− was found to be as an adsorbed product of the dissociative adsorption of hydrogen isocyanate (HNCO), and of the reaction between cyanogens (C2 N2 ) and oxygen on Cu (1 0 0), Cu (1 1 1) and Cu (1 1 0) surfaces [36–40]. The OCN− is also obtained over supported transition metal catalysts where CO is reacted with NO [41–43]. Therefore, the investigation of adsorption behavior of OCN− on tubular surfaces can provide valuable information about its bonding and reactivity in catalysis and other surface phenomena [44]. Very recently, based on density functional theory (DFT), Baei et al. [45] have investigated the adsorption behavior of OCN− on the exterior surface of H-capped semiconducting single-walled carbon nanotubes. They have shown that the binding of OCN− to CNTs generated stable complexes in the range of 280–315 kJ mol−1 . In this paper, we investigate theoretically the OCN− adsorption capability of (6, 0), (8, 0) zigzag and armchair (5, 5) BNNTs and also the effect of this adsorption on the electronic properties of the related BNNTs to elucidate the adsorption behavior for different configurations of the OCN anion approaching the exterior surface of the BNNTs. In the middle of our
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.158
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research, we found that very recently Baei [56] has theoretically investigated the adsorption behavior of OCN− on (6, 0), (7, 0), and (8, 0) BNNTs based on the single point energy (SPE) calculations. Although he has reported that bonding of OCN− to corresponding BNNTs causes to complexes with strong adsorption energy like our results, their binding energy results, quantum molecular descriptors and electronic properties do not provide substantial validity due to the lack of fully geometry optimization of most stable configuration. The optimized geometries, electronic chemical potential (), global softness (S), global hardness () [46,47], and electrophilicity index (ω) [48] were calculated for most stable configuration and compared with the pristine related BNNT. Hope our results can provide experiments with useful information on designing sensors employing the BN nano-materials. 2. Computational methods In this research, all computations were performed via Gaussian 98 package [49] at the level of density functional theory (DFT) [50] using the hybrid exchange-functional B3LYP method [51,52]. Since 6-31G* standard basis set is sufficient for geometry optimization of nanotubes [53], isolated zigzag (6, 0) and (8, 0) single-walled BNNTs in the ends of the BN nanotubes are saturated by hydrogen atoms were allowed to fully relax by the B3LYP/6-31G* level of theory. For the short BNNT models, the hydrogenated zigzag (6, 0), (8, 0), and armchair (5, 5) BNNTs have 60 (B24 N24 H12 ), 80 (B32 N32 H16 ), and 70 (B25 N25 H20 ) atoms, respectively. For the long BNNT models, the hydrogenated zigzag (6, 0) and (8, 0) BNNT have 72 (B30 N30 H12 ) and 112 (B48 N48 H16 ) atoms, respectively. The binding energy (BE) of an OCN− on the BNNT is determined through the following equation: BE = EBNNT−OCN− − (EBNNT + EOCN− )
(1)
−
where EBNNT-OCN is the total energy of the BNNTs interacting with the OCN anion, EBNNT is total energy of the pure BNNT and EOCN − is the total energy of an isolated OCN− . is defined according to the following equation [54]: = −(EHOMO − ELUMO )/2
(2)
where EHOMO is the energy of the Fermi level and ELUMO is the first eigenvalue of the valance band. can be approximated using the Koopmans’ theorem [55] as: = (ELUMO − EHOMO )/2. S [56] and ω [48] are defined as following equations, respectively. S=
1 2
ω=
2 2
(3)
(4)
Fig. 1. Models for various adsorption states for OCN− via N-side (a) and O-side (b) on the side wall of (6, 0) BNNT and via N-side (a) on the (8, 0) BNNT, directly above the B atom (B-top) at the B3LYP/6-31G* level.
˚ with those of for the experimental value, i.e. 1.206 and 1.200 A, respectively [57]. These results suggest that the method used in the present calculations is suitable for considering the adsorption behavior of OCN− onto the SWBNNTs. We have considered full structural optimization of the most stable configurations, namely, OCN− via it expected active site (N-side approached denoting an OCN− perpendicular to the tube axis via N and O atoms) approaching B atom of the (6, 0) BNNTs. Fig. 1(a–b) represents considered adsorption sites of the related BNNTs. For the stable adsorption configurations, our calculations indicate that adsorption energy for cyanato anion (OCN− ) on the exterior surface of pristine (6, 0) BNNTs for the most stable configuration (N-side and O-side) are about −486.79 and -420.31 kJ mol−1 , and the equilibrium distance between two species are 1.569 and ˚ respectively. The OCN− adsorption on the outer surface of 1.562 A, pristine (8, 0) and (5, 5) BNNTs for the most stable configuration (N-side) are −472.78 and −169.86 kJ mol−1 , and the equilibrium ˚ as shown in distance between two species are 1.572 and 1.547 A, Fig. 2. One could observe that for whole considered systems of zigzag and armchair BNNTs, adsorption energies are negative. Such binding energies suggest characterizes of chemisorption process for whole systems. When the cyanato anion chemisorbed via N-side on the (6, 0), (8, 0) zigzag and (5, 5) armchair BNNTs, the equilibrium ˚ respectively. B N bond lengths are about 1.539, 1.538, and 1.563 A, In comparison with the pristine form, the B N bond lengths are longer for the OCN− adsorbed on the BNNTs. On the other hand, the calculated BE for OCN− from N-side of atom is more than that
The maximum amount of electronic charge, Nmax , that the electrophone system may accept is given by Eq. (7) as [53]. Nmax =
−
(7)
3. Results and discussion 3.1. The OCN− adsorbed on BNNTs To evaluate the binding energy of one OCN− on the BNNTs, the total energy of the configurations was determined as a function of distance of the OCN− onto the exterior surface of the BNNTs. The optimized BNNTs and OCN− were used for the molecule adsorption. The optimized B N bond length of (6, 0), (8, 0), and (5, 5) BNNT was found to be about 1.455, 1.446, and 1.445 A˚ the average diameter about 4.80, 6.39, and 6.83, respectively. For OCN− , the bond ˚ respeclength of O C and C N were found to be 1.233 and 1.194 A, tively, reported by B3LYP/6-31G* method, which is very compatible
Fig. 2. Models for two stable adsorption states for OCN− via N-side (a–b) on the surfaces of (8, 0) and (5, 5) BNNT.
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Table 1 ELUMO and EHOMO , dipole moment, D (Debye), energy of Fermi level (EF ), the change of bond gap (Eg ) values (eV) of OCN (N-Side) adsorption on (6, 0) and (8, 0) zigzag BNNT at the B3LYP/6-31G* level. Property
OCN−
(6, 0) BNNT
OCN/BNNT (6, 0)
(8, 0) BNNT
OCN/BNNTs (8, 0)
EHOMO /eV ELUMO /eV [ELUMO − EHOMO ]/eV D /Debye EF /eV Eg /eV
0.38 10.09 9.71 1.53 0.10 –
−6.68 −1.79 4.89 7.96 −4.23 –
−3.42 0.20 3.62 20.40 −1.61 0.78
−6.54 −0.85 5.69 11.44 −3.70 –
−3.42 1.49 4.91 40.87 −0.96 0.55
of O-side atom [45,56]. It has been found that the OCN− prefers to be adsorbed directly above the boron atom (B-top) of the considered nanotubes via its N-side with molecular axis perpendicular to the tube axis. Furthermore, it is observed that as the BNNT diameter increases, the adsorption energy of the OCN− decrease very slightly. These results can be explained due to the interaction between electrons of OCN− and -electrons of the surfaces. Additionally, the results reveal that absolute zigzag BNNTs can detect the OCN− , because the considered anion can be considerably adsorbed on the pure zigzag BNNT surfaces. Baei et al. have reported that the adsorption amount for the most stable configuration of OCN anion on (6, 0) and (8, 0) BNNT are about −312.3 and −311.0 kJ mol−1 based on single point energy (SPE) calculations. These findings highlight binding results based on SPE calculations, used by Baei in a recent work [56], cannot provide considerable validity. Fig. 1 shows the schematic representation of the optimized geometric structure for the adsorption states of OCN− for N-side (a) and O-side (b) interacting with the (6, 0) BNNT. Natural charge analysis (NBO) shows that in this configurations 0.54 and 0.48 e charge transferred from OCN− to the (6, 0) and (5, 5) BNNTs. NBO analysis shows a strong charge transfer between two species in adsorption process. Our computations result exhibits that, a strongly chemical adsorption would occur as a result of strong van der Waals interaction between OCN− and BNNT [34,53]. Considering the effects of tube length increase on the (6, 0) BNNT for the most stable configuration from 10 to 12 A˚ and for (8, 0) BNNT from 10 to 15 A˚ caused our next calculation stage, which the results exhibited that the binding energy of OCN− for N-side on surfaces of (6, 0) and (8, 0) are found to be −229.45 and −206.46 kJ mol−1 , and the equilibrium distance ˚ respectively, between two species are about 1.526 and 1.536 A, which is an indication of the important effect of increasing tube length on adsorption binding in this study, see Fig. 3(a–b).
The chemical activity of BNNTs can be characterized by the HOMO–LUMO energy gap that is a significant parameter relying on the HOMO and LUMO energy levels. In general, a small HOMO–LUMO energy gap means a high chemical activity and a low chemical stability. The calculated gap energies for the isolated (6, 0), (8, 0), and (5, 5) BNNTs at the B3LYP/6-31G* method are 4.89, 5.69, and 6.31 eV. When OCN− is adsorbed on tubes in the energy gap reduces from 4.89, 5.69, and 6.31 to 4.18, 4.91, and 5.35 eV, thus chemical stability of zigzag BNNTs will be decreased and hence chemical activity of such system will be slightly increased. For better elucidating the adsorption nature of interaction between the OCN− and the (6, 0) and (8, 0) BNNTs, we also paid attention to the electronic density of states (DOS) for the most stable complex. We
3.2. Electronic energies and relative stabilities To further investigate the adsorption phenomenon of the OCN− on the (6, 0), (8, 0), and (5, 5) BNNTs, we examined the electronic energies of the most stable system. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the OCN− and pristine (6, 0) and (5, 5) are studied, see Table 1. The data indicate that when the OCN− is absorbed on BNNT, ELUMO and EHOMO for whole systems are increased therefore both groups of occupied and unoccupied molecular orbital are more unstable than those for the pristine BNNT. For OCN− , the HOMO and LUMO are localized throughout the O C N bond indicating the electron conduction through this system. For the pure zigzag (6, 0) BNNT with equal number of B and N atoms, HOMO is localized on the nitrogen atoms of the nanotube while LUMO are more dominant along the B N bonds. For (6, 0) BNNT/OCN− system, we found that the HOMO is localized on the OCN− and nitrogen orbitals in the vicinity of the OCN− ; while the LUMO is localized on B N bonds along the center and end of the BNNT, as illustrated in Fig. 4.
Fig. 3. The potential energy surfaces with two length; the (6, 0) BNNT is exhibited via (a) and the (8, 0) BNNT is exhibited via (b) for OCN− by N-side at the B3LYP/6-31G* level.
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Fig. 4. The molecular orbital of HOMO and LUMO for OCN− , pristine (6, 0) and (5, 5) BNNT, and OCN− /(6, 0) and (5, 5) BNNT complex at the B3LYP/6-31G* level.
now compare the DOS of OCN− adsorbed on the pristine (6, 0), (8, 0), and (5, 5) BNNT for the most stable configurations with those of pure (6, 0) and (8, 0) BNNT to examine the binding nature in these systems. As depicted in Fig. 5, the DOS of (6, 0), (8, 0), and (5, 5) BNNT/OCN− complex are significantly changed when the OCN− adsorbed on the BNNT surface. These results indicate that DOS near the Fermi level are affected between the OCN− and the (6, 0), (8, 0), and (5, 5) BNNT. These findings show that boron nanotubes can be applied as suitable sensor for practical applications. 3.3. Electric dipole moment When a particle likes OCN− approach to the surface BNNT the size and direction of the electric dipole moment vector of BNNT are important properties that characterized its dependence to the adsorption configurations. The electric charges of molecules similar BNNT with extended -systems are easily changed during adsorption gases and lead to changes of dipole moments. The result shows that dipole moment (t ) are 7.96 and 11.44 Debye for pristine (6, 0) and (8, 0) BNNT while, for the most stable configuration are 20.40 and 40.87 Debye, respectively.
3.4. Global indices The global indices of reactivity in the context of the DFT for the most stable configuration of OCN− approaching BNNTs are presented in Table 2. Since the global hardness of a species is defined as its resistance towards deformation in presence of an electric field, increase in global hardness leads to increase in stability and decrease in reactivity of the species [53]. There exists an inverse relationship between global hardness and global softness. When OCN− is adsorbed on related BNNT, energy gap (ELUMO − EHOMO ) decrease thus hardness, electrophilicity and electronic chemical potentials of bare BNNT decrease while softness increases [57]. Hardness () is found to be 1.61 eV for the most stable complex, which is close to the value (2.44 eV) found in the case of a free (6, 0) BNNT. Such trend lowers the stability of the tube and thus increases its reactivity. In calculations obtained with single point energy [56], was reported that decreases from 2.44 eV to 0.6 eV, when OCN− approaching on the (6, 0) BNNTs in a similar approach (the most stable configuration). Such discrepancy between the results confirms again the necessity of fully structural optimization of considered complexes.
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Fig. 5. The total density of states (DOS) for OCN− , pristine the (6, 0), (8, 0), and (5, 5) BNNT, and the OCN− adsorbed on the (6, 0), (8, 0), and (5, 5) BNNT. Table 2 Chemical potential (), hardness (), softness (S), Nmax (a.u.), and electrophilicity (ω) of OCN adsorptions on the (6, 0) and (8, 0) BNNT surface at the B3LYP/6-31G* level. All parameters are in units of eV. Property
OCN−
(6,0) BNNT
OCN/BNNT (6, 0)
(8,0) BNNT
OCN/BNNTs (8, 0)
[I = −EHOMO ]/eV [A = −ELUMO ]/eV [ = (I − A)/2]/eV [ = −(I + A)/2]/eV [S = 1/2]/eV−1 [ω = 2 /2]/eV [Nmax = −/]/a.u.
−10.1 −0.38 4.88 5.26 0.10 2.83 −1.07
6.68 1.79 2.44 −4.23 0.20 3.67 1.73
3.42 0.20 1.61 −1.81 0.31 1.01 1.12
6.54 0.85 2.84 −3.69 0.17 2.40 1.30
3.42 −1.49 2.45 −0.96 0.20 0.19 0.39
4. Conclusion In conclusion, we theoretically studied the adsorptions of the OCN− on zigzag and armchair (6, 0), (8, 0), and (5, 5) single-walled BNNTs through DFT calculations. After fully structural optimization of most stable configuration, equilibrium distance and binding energy are found to be 1.526 A˚ and −486.79 kJ mol−1 , which is an evidence of strong interaction between two species. The electronic structure, HOMO/LUMO is surfaces and charge analysis indicates that there exists noteworthy orbital hybridization between
OCN− and related tube. However, interestingly, our binding studies reveal that increasing the tube can noticeably reduce the adsorption capability of such systems and introduced on OCN− inside the tube causes to complexes with low adsorption energy being typical for physisorption. Acknowledgments We greatly appreciate of the nanotechnology working group of young research club, Islamic Azad University, Gorgan, Iran.
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Theoretical investigation of OCN− adsorption onto boron nitride nanotubes Alireza Soltani a,∗ , Nasim Ahmadian b , Abolfazl Amirazami c , Anis Masoodi a , E. Tazikeh Lemeski c , Ali Varasteh Moradi c a b c
Young Researchers Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Qaemshahr Branch, Islamic Azad University, Qaemshar, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran
a r t i c l e
i n f o
Article history: Received 21 June 2012 Received in revised form 28 July 2012 Accepted 28 July 2012 Available online 4 August 2012 Keywords: Adsorption Nanostructures Ab initio calculations Chemisorption
a b s t r a c t First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of OCN− on H-capped zigzag and armchair single-walled BN nanotubes (BNNTs). The results indicate that OCN− is strongly bound to the outer surface of zigzag (6, 0) BNNTs in comparison with armchair (5, 5) BNNT. Binding energy and equilibrium distance corresponding to the most stable ˚ respectively being typical for the chemisorpconfiguration are found to be −486.79 kJ mol−1 and 1.526 A, tions. Energy gap, dipole moment, natural atomic orbital occupancies and global indices for most stable configuration are calculated. Furthermore, the effect of the OCN− adsorption on the geometries and electronic properties of related BNNT is also studied. The calculated density of states (DOS) reveals that there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction. Therefore, one can conclude that BNNTs play an important role as suitable sensor. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction Since the discovery of carbon nanotubes (CNTs) [1], much attention has been devoted to this material due to their unique structures and brilliant properties [2–4] in the wide range of applications including molecular sensing [5–7], gas storage [8–10], catalysis [11,12], field emission displays [13], switching behaviors [14]. Since their tubular diameter and chirality have a significant effect on the electronic properties of CNTs, much effort have been paid to examine non-carbon based nanotubes showing electronic properties independent of these features [15–17]. On the other hand, after the discovery of boron nitride nanotubes (BNNTs) [18], as structural analogs of the CNT, there have been a large majority of the experimental and theoretical studies directed toward the utilization of this novel material in displacement of CNTs [19]. A sheet of hexagonal rings including boron and nitrogen atoms in equal proportions is rolled up to form BNNT [20,21], which exhibits its own different properties than to its tubular carbon counterparts. BNNTs are semiconductors with a constant band gap of ∼5.5 eV, being almost independent of tubular diameter and helicity [22]. Brilliant properties such as; large iconicity, high thermal conductivity, superior resistance to oxidation, and high mechanical strength [23–26], make BNNTs highly useful materials for the broad diversity of application [22]. For example, numerous studies both theoretically and
∗ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani).
experimentally have been allocated to examine their application in nano-electronic fields [27,28], force sensors [29–31], gas storage [32], particularly in hydrogen storage [33,34]. The need for detectors with high specificity and sensitivity has directed scientists to serve tubular structures. Electronic conductance changes in BNNTs upon exposure to gas molecules. Based on this feather, they can be served as nanotube molecular sensors [20]. On the other hand, the cyanato anion (OCN− ) is a functional group for many organic compounds, and also has a significant role as intermediate in the combustion of nitrogenous fuels [35]. The OCN− was found to be as an adsorbed product of the dissociative adsorption of hydrogen isocyanate (HNCO), and of the reaction between cyanogens (C2 N2 ) and oxygen on Cu (1 0 0), Cu (1 1 1) and Cu (1 1 0) surfaces [36–40]. The OCN− is also obtained over supported transition metal catalysts where CO is reacted with NO [41–43]. Therefore, the investigation of adsorption behavior of OCN− on tubular surfaces can provide valuable information about its bonding and reactivity in catalysis and other surface phenomena [44]. Very recently, based on density functional theory (DFT), Baei et al. [45] have investigated the adsorption behavior of OCN− on the exterior surface of H-capped semiconducting single-walled carbon nanotubes. They have shown that the binding of OCN− to CNTs generated stable complexes in the range of 280–315 kJ mol−1 . In this paper, we investigate theoretically the OCN− adsorption capability of (6, 0), (8, 0) zigzag and armchair (5, 5) BNNTs and also the effect of this adsorption on the electronic properties of the related BNNTs to elucidate the adsorption behavior for different configurations of the OCN anion approaching the exterior surface of the BNNTs. In the middle of our
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.158
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research, we found that very recently Baei [56] has theoretically investigated the adsorption behavior of OCN− on (6, 0), (7, 0), and (8, 0) BNNTs based on the single point energy (SPE) calculations. Although he has reported that bonding of OCN− to corresponding BNNTs causes to complexes with strong adsorption energy like our results, their binding energy results, quantum molecular descriptors and electronic properties do not provide substantial validity due to the lack of fully geometry optimization of most stable configuration. The optimized geometries, electronic chemical potential (), global softness (S), global hardness () [46,47], and electrophilicity index (ω) [48] were calculated for most stable configuration and compared with the pristine related BNNT. Hope our results can provide experiments with useful information on designing sensors employing the BN nano-materials. 2. Computational methods In this research, all computations were performed via Gaussian 98 package [49] at the level of density functional theory (DFT) [50] using the hybrid exchange-functional B3LYP method [51,52]. Since 6-31G* standard basis set is sufficient for geometry optimization of nanotubes [53], isolated zigzag (6, 0) and (8, 0) single-walled BNNTs in the ends of the BN nanotubes are saturated by hydrogen atoms were allowed to fully relax by the B3LYP/6-31G* level of theory. For the short BNNT models, the hydrogenated zigzag (6, 0), (8, 0), and armchair (5, 5) BNNTs have 60 (B24 N24 H12 ), 80 (B32 N32 H16 ), and 70 (B25 N25 H20 ) atoms, respectively. For the long BNNT models, the hydrogenated zigzag (6, 0) and (8, 0) BNNT have 72 (B30 N30 H12 ) and 112 (B48 N48 H16 ) atoms, respectively. The binding energy (BE) of an OCN− on the BNNT is determined through the following equation: BE = EBNNT−OCN− − (EBNNT + EOCN− )
(1)
−
where EBNNT-OCN is the total energy of the BNNTs interacting with the OCN anion, EBNNT is total energy of the pure BNNT and EOCN − is the total energy of an isolated OCN− . is defined according to the following equation [54]: = −(EHOMO − ELUMO )/2
(2)
where EHOMO is the energy of the Fermi level and ELUMO is the first eigenvalue of the valance band. can be approximated using the Koopmans’ theorem [55] as: = (ELUMO − EHOMO )/2. S [56] and ω [48] are defined as following equations, respectively. S=
1 2
ω=
2 2
(3)
(4)
Fig. 1. Models for various adsorption states for OCN− via N-side (a) and O-side (b) on the side wall of (6, 0) BNNT and via N-side (a) on the (8, 0) BNNT, directly above the B atom (B-top) at the B3LYP/6-31G* level.
˚ with those of for the experimental value, i.e. 1.206 and 1.200 A, respectively [57]. These results suggest that the method used in the present calculations is suitable for considering the adsorption behavior of OCN− onto the SWBNNTs. We have considered full structural optimization of the most stable configurations, namely, OCN− via it expected active site (N-side approached denoting an OCN− perpendicular to the tube axis via N and O atoms) approaching B atom of the (6, 0) BNNTs. Fig. 1(a–b) represents considered adsorption sites of the related BNNTs. For the stable adsorption configurations, our calculations indicate that adsorption energy for cyanato anion (OCN− ) on the exterior surface of pristine (6, 0) BNNTs for the most stable configuration (N-side and O-side) are about −486.79 and -420.31 kJ mol−1 , and the equilibrium distance between two species are 1.569 and ˚ respectively. The OCN− adsorption on the outer surface of 1.562 A, pristine (8, 0) and (5, 5) BNNTs for the most stable configuration (N-side) are −472.78 and −169.86 kJ mol−1 , and the equilibrium ˚ as shown in distance between two species are 1.572 and 1.547 A, Fig. 2. One could observe that for whole considered systems of zigzag and armchair BNNTs, adsorption energies are negative. Such binding energies suggest characterizes of chemisorption process for whole systems. When the cyanato anion chemisorbed via N-side on the (6, 0), (8, 0) zigzag and (5, 5) armchair BNNTs, the equilibrium ˚ respectively. B N bond lengths are about 1.539, 1.538, and 1.563 A, In comparison with the pristine form, the B N bond lengths are longer for the OCN− adsorbed on the BNNTs. On the other hand, the calculated BE for OCN− from N-side of atom is more than that
The maximum amount of electronic charge, Nmax , that the electrophone system may accept is given by Eq. (7) as [53]. Nmax =
−
(7)
3. Results and discussion 3.1. The OCN− adsorbed on BNNTs To evaluate the binding energy of one OCN− on the BNNTs, the total energy of the configurations was determined as a function of distance of the OCN− onto the exterior surface of the BNNTs. The optimized BNNTs and OCN− were used for the molecule adsorption. The optimized B N bond length of (6, 0), (8, 0), and (5, 5) BNNT was found to be about 1.455, 1.446, and 1.445 A˚ the average diameter about 4.80, 6.39, and 6.83, respectively. For OCN− , the bond ˚ respeclength of O C and C N were found to be 1.233 and 1.194 A, tively, reported by B3LYP/6-31G* method, which is very compatible
Fig. 2. Models for two stable adsorption states for OCN− via N-side (a–b) on the surfaces of (8, 0) and (5, 5) BNNT.
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Table 1 ELUMO and EHOMO , dipole moment, D (Debye), energy of Fermi level (EF ), the change of bond gap (Eg ) values (eV) of OCN (N-Side) adsorption on (6, 0) and (8, 0) zigzag BNNT at the B3LYP/6-31G* level. Property
OCN−
(6, 0) BNNT
OCN/BNNT (6, 0)
(8, 0) BNNT
OCN/BNNTs (8, 0)
EHOMO /eV ELUMO /eV [ELUMO − EHOMO ]/eV D /Debye EF /eV Eg /eV
0.38 10.09 9.71 1.53 0.10 –
−6.68 −1.79 4.89 7.96 −4.23 –
−3.42 0.20 3.62 20.40 −1.61 0.78
−6.54 −0.85 5.69 11.44 −3.70 –
−3.42 1.49 4.91 40.87 −0.96 0.55
of O-side atom [45,56]. It has been found that the OCN− prefers to be adsorbed directly above the boron atom (B-top) of the considered nanotubes via its N-side with molecular axis perpendicular to the tube axis. Furthermore, it is observed that as the BNNT diameter increases, the adsorption energy of the OCN− decrease very slightly. These results can be explained due to the interaction between electrons of OCN− and -electrons of the surfaces. Additionally, the results reveal that absolute zigzag BNNTs can detect the OCN− , because the considered anion can be considerably adsorbed on the pure zigzag BNNT surfaces. Baei et al. have reported that the adsorption amount for the most stable configuration of OCN anion on (6, 0) and (8, 0) BNNT are about −312.3 and −311.0 kJ mol−1 based on single point energy (SPE) calculations. These findings highlight binding results based on SPE calculations, used by Baei in a recent work [56], cannot provide considerable validity. Fig. 1 shows the schematic representation of the optimized geometric structure for the adsorption states of OCN− for N-side (a) and O-side (b) interacting with the (6, 0) BNNT. Natural charge analysis (NBO) shows that in this configurations 0.54 and 0.48 e charge transferred from OCN− to the (6, 0) and (5, 5) BNNTs. NBO analysis shows a strong charge transfer between two species in adsorption process. Our computations result exhibits that, a strongly chemical adsorption would occur as a result of strong van der Waals interaction between OCN− and BNNT [34,53]. Considering the effects of tube length increase on the (6, 0) BNNT for the most stable configuration from 10 to 12 A˚ and for (8, 0) BNNT from 10 to 15 A˚ caused our next calculation stage, which the results exhibited that the binding energy of OCN− for N-side on surfaces of (6, 0) and (8, 0) are found to be −229.45 and −206.46 kJ mol−1 , and the equilibrium distance ˚ respectively, between two species are about 1.526 and 1.536 A, which is an indication of the important effect of increasing tube length on adsorption binding in this study, see Fig. 3(a–b).
The chemical activity of BNNTs can be characterized by the HOMO–LUMO energy gap that is a significant parameter relying on the HOMO and LUMO energy levels. In general, a small HOMO–LUMO energy gap means a high chemical activity and a low chemical stability. The calculated gap energies for the isolated (6, 0), (8, 0), and (5, 5) BNNTs at the B3LYP/6-31G* method are 4.89, 5.69, and 6.31 eV. When OCN− is adsorbed on tubes in the energy gap reduces from 4.89, 5.69, and 6.31 to 4.18, 4.91, and 5.35 eV, thus chemical stability of zigzag BNNTs will be decreased and hence chemical activity of such system will be slightly increased. For better elucidating the adsorption nature of interaction between the OCN− and the (6, 0) and (8, 0) BNNTs, we also paid attention to the electronic density of states (DOS) for the most stable complex. We
3.2. Electronic energies and relative stabilities To further investigate the adsorption phenomenon of the OCN− on the (6, 0), (8, 0), and (5, 5) BNNTs, we examined the electronic energies of the most stable system. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the OCN− and pristine (6, 0) and (5, 5) are studied, see Table 1. The data indicate that when the OCN− is absorbed on BNNT, ELUMO and EHOMO for whole systems are increased therefore both groups of occupied and unoccupied molecular orbital are more unstable than those for the pristine BNNT. For OCN− , the HOMO and LUMO are localized throughout the O C N bond indicating the electron conduction through this system. For the pure zigzag (6, 0) BNNT with equal number of B and N atoms, HOMO is localized on the nitrogen atoms of the nanotube while LUMO are more dominant along the B N bonds. For (6, 0) BNNT/OCN− system, we found that the HOMO is localized on the OCN− and nitrogen orbitals in the vicinity of the OCN− ; while the LUMO is localized on B N bonds along the center and end of the BNNT, as illustrated in Fig. 4.
Fig. 3. The potential energy surfaces with two length; the (6, 0) BNNT is exhibited via (a) and the (8, 0) BNNT is exhibited via (b) for OCN− by N-side at the B3LYP/6-31G* level.
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Fig. 4. The molecular orbital of HOMO and LUMO for OCN− , pristine (6, 0) and (5, 5) BNNT, and OCN− /(6, 0) and (5, 5) BNNT complex at the B3LYP/6-31G* level.
now compare the DOS of OCN− adsorbed on the pristine (6, 0), (8, 0), and (5, 5) BNNT for the most stable configurations with those of pure (6, 0) and (8, 0) BNNT to examine the binding nature in these systems. As depicted in Fig. 5, the DOS of (6, 0), (8, 0), and (5, 5) BNNT/OCN− complex are significantly changed when the OCN− adsorbed on the BNNT surface. These results indicate that DOS near the Fermi level are affected between the OCN− and the (6, 0), (8, 0), and (5, 5) BNNT. These findings show that boron nanotubes can be applied as suitable sensor for practical applications. 3.3. Electric dipole moment When a particle likes OCN− approach to the surface BNNT the size and direction of the electric dipole moment vector of BNNT are important properties that characterized its dependence to the adsorption configurations. The electric charges of molecules similar BNNT with extended -systems are easily changed during adsorption gases and lead to changes of dipole moments. The result shows that dipole moment (t ) are 7.96 and 11.44 Debye for pristine (6, 0) and (8, 0) BNNT while, for the most stable configuration are 20.40 and 40.87 Debye, respectively.
3.4. Global indices The global indices of reactivity in the context of the DFT for the most stable configuration of OCN− approaching BNNTs are presented in Table 2. Since the global hardness of a species is defined as its resistance towards deformation in presence of an electric field, increase in global hardness leads to increase in stability and decrease in reactivity of the species [53]. There exists an inverse relationship between global hardness and global softness. When OCN− is adsorbed on related BNNT, energy gap (ELUMO − EHOMO ) decrease thus hardness, electrophilicity and electronic chemical potentials of bare BNNT decrease while softness increases [57]. Hardness () is found to be 1.61 eV for the most stable complex, which is close to the value (2.44 eV) found in the case of a free (6, 0) BNNT. Such trend lowers the stability of the tube and thus increases its reactivity. In calculations obtained with single point energy [56], was reported that decreases from 2.44 eV to 0.6 eV, when OCN− approaching on the (6, 0) BNNTs in a similar approach (the most stable configuration). Such discrepancy between the results confirms again the necessity of fully structural optimization of considered complexes.
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Fig. 5. The total density of states (DOS) for OCN− , pristine the (6, 0), (8, 0), and (5, 5) BNNT, and the OCN− adsorbed on the (6, 0), (8, 0), and (5, 5) BNNT. Table 2 Chemical potential (), hardness (), softness (S), Nmax (a.u.), and electrophilicity (ω) of OCN adsorptions on the (6, 0) and (8, 0) BNNT surface at the B3LYP/6-31G* level. All parameters are in units of eV. Property
OCN−
(6,0) BNNT
OCN/BNNT (6, 0)
(8,0) BNNT
OCN/BNNTs (8, 0)
[I = −EHOMO ]/eV [A = −ELUMO ]/eV [ = (I − A)/2]/eV [ = −(I + A)/2]/eV [S = 1/2]/eV−1 [ω = 2 /2]/eV [Nmax = −/]/a.u.
−10.1 −0.38 4.88 5.26 0.10 2.83 −1.07
6.68 1.79 2.44 −4.23 0.20 3.67 1.73
3.42 0.20 1.61 −1.81 0.31 1.01 1.12
6.54 0.85 2.84 −3.69 0.17 2.40 1.30
3.42 −1.49 2.45 −0.96 0.20 0.19 0.39
4. Conclusion In conclusion, we theoretically studied the adsorptions of the OCN− on zigzag and armchair (6, 0), (8, 0), and (5, 5) single-walled BNNTs through DFT calculations. After fully structural optimization of most stable configuration, equilibrium distance and binding energy are found to be 1.526 A˚ and −486.79 kJ mol−1 , which is an evidence of strong interaction between two species. The electronic structure, HOMO/LUMO is surfaces and charge analysis indicates that there exists noteworthy orbital hybridization between
OCN− and related tube. However, interestingly, our binding studies reveal that increasing the tube can noticeably reduce the adsorption capability of such systems and introduced on OCN− inside the tube causes to complexes with low adsorption energy being typical for physisorption. Acknowledgments We greatly appreciate of the nanotechnology working group of young research club, Islamic Azad University, Gorgan, Iran.
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