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A first-principles study of the SCN− chemisorption on the surface of AlN, AlP, and BP nanotubes
Applied Surface Science 259 (2012) 637–642
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A first-principles study of the SCN− chemisorption on the surface of AlN, AlP, and BP nanotubes Alireza Soltani a,∗ , Mohammad Ramezani Taghartapeh a , Hossein Mighani b , Amin Allah Pahlevani a , Reza Mashkoor c a
Young Researchers Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Faculty of science, Golestan University, Gorgan, Iran c Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran b
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 8 July 2012 Accepted 18 July 2012 Available online 24 July 2012 Keywords: Aluminum nitride nanotubes Aluminum phosphide nanotubes Boron phosphide nanotubes SCN− DFT
a b s t r a c t We have performed first-principles calculations to explore the adsorption behavior of the SCN− on electronic properties of AlN, AlP, and BP nanotubes. The adsorption value of SCN− for the most stable formation on the AlPNT is about −318.16 kJ mol−1 , which is reason via the chemisorptions of SCN anion. The computed density of states (DOS) indicates that a notable orbital hybridization take place between SCN− and AlP nanotube in adsorption process. Finally, the AlP nanotube can be used to design as useful sensor for nanodevice applications. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction After the discovery of single-walled carbon nanotubes (SWCNTs) [1] numerous studies have been devoted to consider different kinds of physisorptions and chemisorptions of various gases such as SCN− , OCN, CO, NO, H2 , Cl2 , NH3 , NO2 , O2 , H2 O, and H3 COH. Also, other applications have been reported by scientists, like storage, chemical sensors, and electronic devices [2–6]. But SWCNTs have some certain drawbacks such as tubular diameter and chiralities which have encouraged scientists to think about modeling and synthesizing a substitute for it to finally stabilize BNNTs computationally [7], and afterwards experimentally synthesize it [8]. And after these outstanding discoveries, most of researchers found interest in some other Group III-nitrides materials which exhibit a prominent capability to produce tubular structures due to their excellent properties, e.g., hexagonal aluminum nitride (AlN) with a band gap of 6.2 eV, which is an interesting material for electronic substrates and field emitters [9–11]. It is also well known for its exclusive properties like, superior mechanical strength, high thermal conductivity, and a high piezo electric response [12,13]. A study by Zhang and Zhang, using an ab initio method in 2003, showed that AlN nanotubes (AlNNTs) are energetically favorable
∗ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani).
and arrange in a hexagonal network adopting a sp2 hybridization for both Al and N atoms [14]. There have been other particular investigations supposing that AlN could form conventional singlewalled nano-tubes resembling carbon nanotubes (CNTs). The strain energy needed to wrap up a graphitic sheet into an AlNNT is lower than that required to form BNNTs, GaNNTs and CNTs with the similar diameters [15–17]. The faceted single-crystalline hexagonal AlNNTs is in a horizontal tube furnace under high temperature using a highly non-equilibrium dc-arc plasma technique (synthesized by Wu et al. [18]) and is then synthesized with typically a few micro-meters in length with the diameters from 30 to 80 nm, in gram quantities by Tondare et al. [19]. The bond gaps of AlNNTs are wide and their gaps are slightly dependent on their chiralities [20], zigzag configuration of AlNNTs is seen to be energetically more eligible than the armchair one [21–29]. It has been found that the surface atoms attribute their electronic states close to the upper valence band edge as well as the lower conduction band edge as defect states. Moreover, the calculated imaginary section of dielectric functions (ε2 ) curves exhibited that the AlNNTs optical properties are dominantly dependent on size [22]. Some prominent applications of AlNNTs are LEDs for low energy consumption light emission, blue lasers for multimedia disks, field effect transistors, and high electron mobility transistors (HEMT) [21]. Due to its importance in the atmosphere and environmental issues, Studies on the reaction of thiocyanate anion (SCN− ) and its radical (SCN− ) with various redox species have been carried out
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.07.089
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A. Soltani et al. / Applied Surface Science 259 (2012) 637–642
following experimental and theoretical methods [6–23,23–27]. It has been studied that thiocyanate anion, a particularly stable species can be produced by the reaction between CS2 and NH2 − species and also reaction between CH3 SCN + e− → SCN− + CH3 • . Moreover, the photoelectron spectrum of thiocyanate anion has been studied reporting the adiabatic electron affinity of SCN radical [23,28]. The SCN− can play notable roles in determining the structure of polymeric transition-metal complexes [24–27]. The SCN− anion is a very adaptable ambidentate ligand with two donor atoms. However, SCN− can coordinate either through the N and S atoms, or both, attaining rise to linkage isomers and polymers [23]. Recently, we have shown [28] the adsorption properties of SCN− on (6, 0), (8, 0), (10, 0), (5, 5), and Al/Ga-doped (6, 0) BNNTs via first-principles theory based on DFT calculations. In this Letter, we are attempting to consider the chemisorptions of SCN− on the exterior surface of AlN, AlP, and BP nano tubes. 2. Computational methods In this paper, we considered the adsorption behavior of the SCN− on the pristine zigzag (6, 0) and (4, 4) AlNNT, (6, 0) AlPNT and BPNT in which the ends of the AlN, AlP, BP nanotubes are saturated by hydrogen atoms. Structure optimizations, density of states (DOS), and energy analyses are performed using Gaussian 98 program package [33] at the level of density functional theory (DFT) with B3LYP/6–31G* [34]. The hydrogenated (6, 0), (4, 4) AlNNT and (6,
0) AlPNT and BPNT have 60 (Al24 N24 H12 ), 72 (Al28 N28 H16 ), 60 and (Al24 P24 H12 ) and 60 (B24 P24 H12 ) atoms, respectively. The adsorption energy of SCN− on the AlN, AlP, and BP nanotubes are defined through as follows: Ead = EAlNNT−SCN− − (EAlNNT + ESCN− )
(1)
Ead = EAlPNT−SCN− − (EAlPNT + ESCN− )
(2)
Ead = EBPNT−SCN− − (EBPNT + ESCN− )
(3)
where E(AlNNT−SCN− ) , E(AlPNT−SCN− ) , and E(BPNT−SCN− ) are the total energy of the AlNNT, AlPNT, and BPNT interacting with the SCN− . EAlNNT , EAlPNT , and EBPNT are total energy of the pristine AlNNT, AlPNT, and BPNT. And ESCN is the total energy of an isolated SCN− . Natural charge analysis with full NBO calculations were performed by using DFT/B3LYP level with 6–31G* basis set for optimized structures. The electrophilicity concept was stated for the first time in 1999 by Parr et al. [34]. is defined according to the following equation [21]: =
−(EHOMO + ELUMO ) 2
(4)
where EHOMO is the energy of the Fermi level and ELUMO is the first given value of the valance band. is defined as the negative of , as follows: = −. Furthermore, can be approximated using the
Fig. 1. The optimized structure for the most stable configuration (N-side) of (a) SCN− /(6, 0) AlNNTs, (b) SCN− /(4, 4) AlNNTs, (c) SCN− /(6, 0) AlPNTs, and (d) SCN− /(6, 0) BPNTs complexes.
A. Soltani et al. / Applied Surface Science 259 (2012) 637–642
Koopmans’ theorem [22] as: = (ELUMO – EHOMO )/2. S [23] and ω [24] are defined as the following equations, respectively. S=
1 2
(5)
ω=
2 2
(6)
3. Results and discussion 3.1. The SCN− adsorbed on AlNNT We first optimize the structure of SCN− and (6, 0), (4, 4) AlNNT, (6, 0) AlPNT and BPNT that were used for the SCN− adsorption which are shown in Fig. 1. We exhibit four adsorption states with the nitrogen atom of SCN− for the most stable configurations via its expected active sites (N-side) directed downward toward the Al atom in (6, 0) and (4, 4) AlNNTs and (6, 0) AlPNT, and B atom in (6,0) BPNT surfaces. The optimized geometrics of perfect (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT were found to be with Al N, Al P, and B P bond lengths ˚ and with the diameter of the are 1.923, 1.872, 2.306, and 1.891 A, ˚ respectively. The nanotubes are 6.038, 6.618, 7.200, and 5.993 A, Al N Al and N Al N bond angles in (6, 0) AlNNTs are 114.018◦ and 113.629◦ , while the Al N Al and N Al N angles in (4, 4) AlNNTs
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are 115.094◦ and 118.085◦ , respectively. The Al P Al and P Al P angles in (6, 0) AlPNTs are 110.914◦ and 116.809◦ , respectively. The B P B and P B P angles in (6, 0) BPNTs are 114.321◦ and 118.883◦ , respectively. The computed C S and C N equilibrium bond lengths are 1.676 and 1.180 A˚ in the pure SCN− , relatively. Our previous reported indicate [23] that the bond lengths of C S and C N are slightly shorter than that of SCN− on the BNNT. In contrast, the configuration of SCN− on the various BNNTs via the N atom is more notable than that via its S atom of a SCN− [28]. To clarify these results, the potential energy surfaces with the distance between the SCN− and the AlNNTs surface were computed [23,32]. The binding energy for the most stable chemisorbed configurations (N-side) of SCN− on zigzag (6, 0) AlPNT, AlNNT, and BPNT are about −318.163, −262.145, and −255.845 kJ mol−1 respectively. The separation distances between the AlPNT, AlNNT, and BPNT and the ˚ respectively. We have also comSCN− are 1.853, 1.765 and 1.488 A, puted the adsorption energy of single SCN− on the armchair (4, 4) AlNNT for the most stable state is −257.131 kJ mol−1 with an ˚ This is slightly lower than the equilibrium distance of 1.526 A. adsorption energies of SCN− on zigzag AlPNT, AlNNT, and BPNT. Noteworthy, AlPNTs are a good candidate for SCN− storage [29,30]. On the other hand, the observe diversity in adsorption energy for formations show that the binding energies of SCN− on the perfect (6, 0) AlPNTs are exothermic and notable in comparison with CNTs, BNNTs, BPNTs, and AlNNTs [26,27]. The NBO analysis demonstrated
Fig. 2. Charge distribution of HOMO and LUMO orbitals on the (6, 0) AlPNTs and AlNNTs loaded with one SCN− at the B3LYP/6-31G* level.
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Fig. 3. Total density of states (DOS) for an isolated SCN− , SCN− /(6, 0) AlNNTs, SCN− /(4, 4) AlNNTs, SCN− /(6, 0) AlPNTs, and SCN− /(6, 0) BPNTs complexes.
a change in the hybridization of the Al atom from sp2 to sp3 together with a charge transfer from the lone pair of N atom of SCN− to the anti-bonding orbitals of three Al P and Al N bonds around Al site. This analysis shows that in these formations 0.26 and 0.24e charge transferred from SCN− to the (6, 0) AlPNT and AlNNTs surfaces. Thus, we believed that the nature of this interaction can be mainly covalent. 3.2. Quantum molecular descriptors The quantum molecular descriptors for SCN− on AlP and AlN nanotubes in the most stable formations are presented in Table 1. We find that by decreasing in global hardness for SCN on the tubes leads to decrease in stability chemical and increase in behavior of the species. We observe with SCN− adsorption on the tubes surface due to decrease in bond gap, hardness and ionization potential, while softness and electrophilicity will be increased of the structures. Our computations also show that a charge transfer to take place between SCN− and the outer surface of the AlP and AlN nanotubes, which shows that the stability between SCN− and nanotubes
are lowered and their reactivity increased. Therefore, when SCN− approaches AlP and AlN nanotubes electrons are transferred from higher chemical potential to the lower electronic chemical potential, until the electronic chemical potentials become identical [31]. Hence, electrons will flow from a definite occupied orbital in an Al atom of AlNNTs and will go into a definite empty orbital in a single SCN− . The global electrophilicity index determines the energy lowering of a ligand due to the maximum flow of electron from donor to acceptor species and provides information about structural stability, reactivity and toxicity of chemical species. 3.3. Electronic energies and relative stabilities To evaluate the interaction of SCN− adsorption on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces, we examined the electronic energies of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces. The adsorption energy of SCN− can be quickly confirmed via the fact of HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) on surface of nanotubes are studied (see Table 1). Table 1 presents the results in the LUMO and HOMO energies of the SCN−
Table 1 EHOMO (eV), ELUMO (eV), chemical hardness, (eV), chemical potential, (eV), ω (eV), and Dipole moment, D (Debye), for SCN− on (6, 0) AlPNTs and AlNNTs surface at the B3LYP/6-31G(d) level. Property
SCN−
(6, 0) AlNNT
(6, 0) AlPNT
SCN− /AlNNT
SCN− /AlPNT
EHOMO (eV) ELUMO (eV) [ELUMO − EHOMO ] (eV) [I = −EHOMO ] (eV) [A = −ELUMO ] (eV) [ = (I − A)/2] (eV) [ = −X = −(I + A)/2] (eV) [S = 1/2] (eV−1 ) [ω = 2 /2] (eV) D (Debey)
−0.41 7.41 7.82 0.41 −7.41 3.91 −3.50 0.13 1.57 1.65
−6.46 −2.06 4.40 6.46 2.06 2.20 −4.26 0.23 4.12 11.15
−6.52 −3.31 3.21 6.52 3.31 1.60 −4.19 0.31 5.46 4.27
−3.53 0.12 3.47 3.53 −0.12 1.73 −1.61 0.29 0.74 29.14
−4.05 −1.37 2.68 4.05 1.37 1.34 −2.71 0.37 2.74 36.39
A. Soltani et al. / Applied Surface Science 259 (2012) 637–642
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Fig. 4.
adsorption on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT top site obtained by the DFT calculations. We find that the adsorption of SCN− on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surface due to reduce ELUMO and EHOMO for all complexes, and also indicate that groups of HOMO and LUMO are more stable than those for AlPNTs. Hence, HOMO and LUMO orbitals are uniformly distributed between them, which demonstrates that covalent bond is notable throughout the nanotubes (see Figs. 2 and 3). The distribution of frontier orbital in the (6, 0), (4, 4) AlNNTs and (6, 0) AlPNT complexes, the HOMO are localized on the most electronegative N and P atoms in the center of the nanotube axis which corresponds to the lone pair of electrons on the N and P atoms. While the LUMO are more localized on the S C N orbitals and are less on the nitrogen atoms in the center of the nanotube axis (see Figs. 2 and 3). The chemical activity of the AlPNTs can be determined via the HOMO LUMO bond gap that is a notable Consequence trusting on the HOMO and LUMO energy levels. The computed bond gaps for (6, 0), (4, 4) zigzag and armchair AlNNTs are in the range of 4.76–4.55 eV at the B3LYP/6-31G* method (see Table 1). The experimental band gap of AlNNTs is larger (6.2 eV) than that the theoretical value [9]. When SCN− adsorbs on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces in stable configurations (N-side), the gap energies reduced from 4.40–4.55–3.21–2.27 to 3.47–3.53–2.68–1.93 eV, respectively. To better understand the
sensitivity of the SCN− toward the electronic property of the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT, we plotted the density of states (DOS) for SCN− adsorption on this complexes, as summarized in Fig. 4. To examine the sensitivity of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT for the most stable configurations in comparison with the perfect AlNNTs, AlPNTs, and BPNTs. It can be seen from the figures that the DOS of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT displays more reasonable changes than to the DOS of the pure (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT, indicating a bond strong of SCN− in the electronic conductivity of AlNNTs. Therefore, they results show due to strong chemical interaction of SCN− on AlNNTs, AlPNTs, and BPNTs systems the DOS near the Fermi level are affected via the adsorption process. Our results unveil after adsorption of SCN− on AlNNT, AlPNT, and BPNT surfaces, the HOMO–LUMO energy gaps of tubes has notable changes. Hence, it is concluded that (6, 0) AlPNTs can be a better candidate as sensor for SCN− . 3.4. Dipole moment The computed values of the size and structures of the dipole moment (D ) for SCN− on the (6, 0) AlPNT, AlNNT, and BPNT surfaces are summarized in Table 1. The dipole moment indicates a
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A. Soltani et al. / Applied Surface Science 259 (2012) 637–642
special property of a molecule that considers data in the case electronic and geometrical properties. Our computations value show when SCN− approaches the surface of (6, 0) AlNNT, (6, 0) AlPNT, and (6, 0) BPNT, the size and direction of the electric dipole moment vector are changed depending on the adsorption configurations. The studies of the electric dipole moment for SCN− and sites of the adsorption energy are highest, while calculates demonstrate that during SCN− adsorption for all systems, total dipole moment increases. Our studies indicate that the dipole moment for (6, 0) AlPNT, AlNNT, and BPNT are 4.27, 11.15, and 2.08 Debye, respectively. The values of D of the SCN− adsorption on (6, 0) AlPNT, AlNNT, and BPNT are about 36.39, 29.14, and 29.11 Debye, respectively. These results indicated that significantly changes due to increase of the D for SCN− adsorption on the AlPNT in comparable with AlNNT and BPNT. 4. Conclusion We have examined of SCN− adsorption on outer surface of (6, 0), (4, 4) AlNNT, (6, 0) AlPNT and BPNT through DFT calculations at B3LYP/6-31G* level. The SCN− adsorption on surfaces of the AlNNT, AlPNT, and BPNT shows that geometrical and electronic properties, quantum molecular descriptors, and density of state were computed. In contrast, our results showed that the charge transfer and the sensitivity of SCN− to AlPNTs is a significant factor in the change in the electrical conductance of the AlP nanotubes. Our calculations indicated that SCN− can be chemisorbed on AlPNTs surface. And, it can be used for SCN− storage. Decrease in ionization potential, bond gap, and global hardness via the adsorption energies of SCN− on the AlPNT lowers stability than that of the SCN− on the AlNNT and increase in chemical reactivity in the systems. Acknowledgments We would like to thank the Nanotechnology Working group of Young Researchers Club of the Islamic Azad University, Gorgan Branch, Iran. References [1] S. Ijima, Nature 354 (1991) 56. [2] J.A. Talla, Chemical Physics 392 (2012) 71. [3] W. An, C.H. Turner, Chemical Physics Letters 482 (2009) 274.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
A first-principles study of the SCN− chemisorption on the surface of AlN, AlP, and BP nanotubes Alireza Soltani a,∗ , Mohammad Ramezani Taghartapeh a , Hossein Mighani b , Amin Allah Pahlevani a , Reza Mashkoor c a
Young Researchers Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Faculty of science, Golestan University, Gorgan, Iran c Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran b
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 8 July 2012 Accepted 18 July 2012 Available online 24 July 2012 Keywords: Aluminum nitride nanotubes Aluminum phosphide nanotubes Boron phosphide nanotubes SCN− DFT
a b s t r a c t We have performed first-principles calculations to explore the adsorption behavior of the SCN− on electronic properties of AlN, AlP, and BP nanotubes. The adsorption value of SCN− for the most stable formation on the AlPNT is about −318.16 kJ mol−1 , which is reason via the chemisorptions of SCN anion. The computed density of states (DOS) indicates that a notable orbital hybridization take place between SCN− and AlP nanotube in adsorption process. Finally, the AlP nanotube can be used to design as useful sensor for nanodevice applications. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction After the discovery of single-walled carbon nanotubes (SWCNTs) [1] numerous studies have been devoted to consider different kinds of physisorptions and chemisorptions of various gases such as SCN− , OCN, CO, NO, H2 , Cl2 , NH3 , NO2 , O2 , H2 O, and H3 COH. Also, other applications have been reported by scientists, like storage, chemical sensors, and electronic devices [2–6]. But SWCNTs have some certain drawbacks such as tubular diameter and chiralities which have encouraged scientists to think about modeling and synthesizing a substitute for it to finally stabilize BNNTs computationally [7], and afterwards experimentally synthesize it [8]. And after these outstanding discoveries, most of researchers found interest in some other Group III-nitrides materials which exhibit a prominent capability to produce tubular structures due to their excellent properties, e.g., hexagonal aluminum nitride (AlN) with a band gap of 6.2 eV, which is an interesting material for electronic substrates and field emitters [9–11]. It is also well known for its exclusive properties like, superior mechanical strength, high thermal conductivity, and a high piezo electric response [12,13]. A study by Zhang and Zhang, using an ab initio method in 2003, showed that AlN nanotubes (AlNNTs) are energetically favorable
∗ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani).
and arrange in a hexagonal network adopting a sp2 hybridization for both Al and N atoms [14]. There have been other particular investigations supposing that AlN could form conventional singlewalled nano-tubes resembling carbon nanotubes (CNTs). The strain energy needed to wrap up a graphitic sheet into an AlNNT is lower than that required to form BNNTs, GaNNTs and CNTs with the similar diameters [15–17]. The faceted single-crystalline hexagonal AlNNTs is in a horizontal tube furnace under high temperature using a highly non-equilibrium dc-arc plasma technique (synthesized by Wu et al. [18]) and is then synthesized with typically a few micro-meters in length with the diameters from 30 to 80 nm, in gram quantities by Tondare et al. [19]. The bond gaps of AlNNTs are wide and their gaps are slightly dependent on their chiralities [20], zigzag configuration of AlNNTs is seen to be energetically more eligible than the armchair one [21–29]. It has been found that the surface atoms attribute their electronic states close to the upper valence band edge as well as the lower conduction band edge as defect states. Moreover, the calculated imaginary section of dielectric functions (ε2 ) curves exhibited that the AlNNTs optical properties are dominantly dependent on size [22]. Some prominent applications of AlNNTs are LEDs for low energy consumption light emission, blue lasers for multimedia disks, field effect transistors, and high electron mobility transistors (HEMT) [21]. Due to its importance in the atmosphere and environmental issues, Studies on the reaction of thiocyanate anion (SCN− ) and its radical (SCN− ) with various redox species have been carried out
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following experimental and theoretical methods [6–23,23–27]. It has been studied that thiocyanate anion, a particularly stable species can be produced by the reaction between CS2 and NH2 − species and also reaction between CH3 SCN + e− → SCN− + CH3 • . Moreover, the photoelectron spectrum of thiocyanate anion has been studied reporting the adiabatic electron affinity of SCN radical [23,28]. The SCN− can play notable roles in determining the structure of polymeric transition-metal complexes [24–27]. The SCN− anion is a very adaptable ambidentate ligand with two donor atoms. However, SCN− can coordinate either through the N and S atoms, or both, attaining rise to linkage isomers and polymers [23]. Recently, we have shown [28] the adsorption properties of SCN− on (6, 0), (8, 0), (10, 0), (5, 5), and Al/Ga-doped (6, 0) BNNTs via first-principles theory based on DFT calculations. In this Letter, we are attempting to consider the chemisorptions of SCN− on the exterior surface of AlN, AlP, and BP nano tubes. 2. Computational methods In this paper, we considered the adsorption behavior of the SCN− on the pristine zigzag (6, 0) and (4, 4) AlNNT, (6, 0) AlPNT and BPNT in which the ends of the AlN, AlP, BP nanotubes are saturated by hydrogen atoms. Structure optimizations, density of states (DOS), and energy analyses are performed using Gaussian 98 program package [33] at the level of density functional theory (DFT) with B3LYP/6–31G* [34]. The hydrogenated (6, 0), (4, 4) AlNNT and (6,
0) AlPNT and BPNT have 60 (Al24 N24 H12 ), 72 (Al28 N28 H16 ), 60 and (Al24 P24 H12 ) and 60 (B24 P24 H12 ) atoms, respectively. The adsorption energy of SCN− on the AlN, AlP, and BP nanotubes are defined through as follows: Ead = EAlNNT−SCN− − (EAlNNT + ESCN− )
(1)
Ead = EAlPNT−SCN− − (EAlPNT + ESCN− )
(2)
Ead = EBPNT−SCN− − (EBPNT + ESCN− )
(3)
where E(AlNNT−SCN− ) , E(AlPNT−SCN− ) , and E(BPNT−SCN− ) are the total energy of the AlNNT, AlPNT, and BPNT interacting with the SCN− . EAlNNT , EAlPNT , and EBPNT are total energy of the pristine AlNNT, AlPNT, and BPNT. And ESCN is the total energy of an isolated SCN− . Natural charge analysis with full NBO calculations were performed by using DFT/B3LYP level with 6–31G* basis set for optimized structures. The electrophilicity concept was stated for the first time in 1999 by Parr et al. [34]. is defined according to the following equation [21]: =
−(EHOMO + ELUMO ) 2
(4)
where EHOMO is the energy of the Fermi level and ELUMO is the first given value of the valance band. is defined as the negative of , as follows: = −. Furthermore, can be approximated using the
Fig. 1. The optimized structure for the most stable configuration (N-side) of (a) SCN− /(6, 0) AlNNTs, (b) SCN− /(4, 4) AlNNTs, (c) SCN− /(6, 0) AlPNTs, and (d) SCN− /(6, 0) BPNTs complexes.
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Koopmans’ theorem [22] as: = (ELUMO – EHOMO )/2. S [23] and ω [24] are defined as the following equations, respectively. S=
1 2
(5)
ω=
2 2
(6)
3. Results and discussion 3.1. The SCN− adsorbed on AlNNT We first optimize the structure of SCN− and (6, 0), (4, 4) AlNNT, (6, 0) AlPNT and BPNT that were used for the SCN− adsorption which are shown in Fig. 1. We exhibit four adsorption states with the nitrogen atom of SCN− for the most stable configurations via its expected active sites (N-side) directed downward toward the Al atom in (6, 0) and (4, 4) AlNNTs and (6, 0) AlPNT, and B atom in (6,0) BPNT surfaces. The optimized geometrics of perfect (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT were found to be with Al N, Al P, and B P bond lengths ˚ and with the diameter of the are 1.923, 1.872, 2.306, and 1.891 A, ˚ respectively. The nanotubes are 6.038, 6.618, 7.200, and 5.993 A, Al N Al and N Al N bond angles in (6, 0) AlNNTs are 114.018◦ and 113.629◦ , while the Al N Al and N Al N angles in (4, 4) AlNNTs
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are 115.094◦ and 118.085◦ , respectively. The Al P Al and P Al P angles in (6, 0) AlPNTs are 110.914◦ and 116.809◦ , respectively. The B P B and P B P angles in (6, 0) BPNTs are 114.321◦ and 118.883◦ , respectively. The computed C S and C N equilibrium bond lengths are 1.676 and 1.180 A˚ in the pure SCN− , relatively. Our previous reported indicate [23] that the bond lengths of C S and C N are slightly shorter than that of SCN− on the BNNT. In contrast, the configuration of SCN− on the various BNNTs via the N atom is more notable than that via its S atom of a SCN− [28]. To clarify these results, the potential energy surfaces with the distance between the SCN− and the AlNNTs surface were computed [23,32]. The binding energy for the most stable chemisorbed configurations (N-side) of SCN− on zigzag (6, 0) AlPNT, AlNNT, and BPNT are about −318.163, −262.145, and −255.845 kJ mol−1 respectively. The separation distances between the AlPNT, AlNNT, and BPNT and the ˚ respectively. We have also comSCN− are 1.853, 1.765 and 1.488 A, puted the adsorption energy of single SCN− on the armchair (4, 4) AlNNT for the most stable state is −257.131 kJ mol−1 with an ˚ This is slightly lower than the equilibrium distance of 1.526 A. adsorption energies of SCN− on zigzag AlPNT, AlNNT, and BPNT. Noteworthy, AlPNTs are a good candidate for SCN− storage [29,30]. On the other hand, the observe diversity in adsorption energy for formations show that the binding energies of SCN− on the perfect (6, 0) AlPNTs are exothermic and notable in comparison with CNTs, BNNTs, BPNTs, and AlNNTs [26,27]. The NBO analysis demonstrated
Fig. 2. Charge distribution of HOMO and LUMO orbitals on the (6, 0) AlPNTs and AlNNTs loaded with one SCN− at the B3LYP/6-31G* level.
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Fig. 3. Total density of states (DOS) for an isolated SCN− , SCN− /(6, 0) AlNNTs, SCN− /(4, 4) AlNNTs, SCN− /(6, 0) AlPNTs, and SCN− /(6, 0) BPNTs complexes.
a change in the hybridization of the Al atom from sp2 to sp3 together with a charge transfer from the lone pair of N atom of SCN− to the anti-bonding orbitals of three Al P and Al N bonds around Al site. This analysis shows that in these formations 0.26 and 0.24e charge transferred from SCN− to the (6, 0) AlPNT and AlNNTs surfaces. Thus, we believed that the nature of this interaction can be mainly covalent. 3.2. Quantum molecular descriptors The quantum molecular descriptors for SCN− on AlP and AlN nanotubes in the most stable formations are presented in Table 1. We find that by decreasing in global hardness for SCN on the tubes leads to decrease in stability chemical and increase in behavior of the species. We observe with SCN− adsorption on the tubes surface due to decrease in bond gap, hardness and ionization potential, while softness and electrophilicity will be increased of the structures. Our computations also show that a charge transfer to take place between SCN− and the outer surface of the AlP and AlN nanotubes, which shows that the stability between SCN− and nanotubes
are lowered and their reactivity increased. Therefore, when SCN− approaches AlP and AlN nanotubes electrons are transferred from higher chemical potential to the lower electronic chemical potential, until the electronic chemical potentials become identical [31]. Hence, electrons will flow from a definite occupied orbital in an Al atom of AlNNTs and will go into a definite empty orbital in a single SCN− . The global electrophilicity index determines the energy lowering of a ligand due to the maximum flow of electron from donor to acceptor species and provides information about structural stability, reactivity and toxicity of chemical species. 3.3. Electronic energies and relative stabilities To evaluate the interaction of SCN− adsorption on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces, we examined the electronic energies of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces. The adsorption energy of SCN− can be quickly confirmed via the fact of HOMO (highest occupied molecular orbital) and the LUMO (lowest unoccupied molecular orbital) on surface of nanotubes are studied (see Table 1). Table 1 presents the results in the LUMO and HOMO energies of the SCN−
Table 1 EHOMO (eV), ELUMO (eV), chemical hardness, (eV), chemical potential, (eV), ω (eV), and Dipole moment, D (Debye), for SCN− on (6, 0) AlPNTs and AlNNTs surface at the B3LYP/6-31G(d) level. Property
SCN−
(6, 0) AlNNT
(6, 0) AlPNT
SCN− /AlNNT
SCN− /AlPNT
EHOMO (eV) ELUMO (eV) [ELUMO − EHOMO ] (eV) [I = −EHOMO ] (eV) [A = −ELUMO ] (eV) [ = (I − A)/2] (eV) [ = −X = −(I + A)/2] (eV) [S = 1/2] (eV−1 ) [ω = 2 /2] (eV) D (Debey)
−0.41 7.41 7.82 0.41 −7.41 3.91 −3.50 0.13 1.57 1.65
−6.46 −2.06 4.40 6.46 2.06 2.20 −4.26 0.23 4.12 11.15
−6.52 −3.31 3.21 6.52 3.31 1.60 −4.19 0.31 5.46 4.27
−3.53 0.12 3.47 3.53 −0.12 1.73 −1.61 0.29 0.74 29.14
−4.05 −1.37 2.68 4.05 1.37 1.34 −2.71 0.37 2.74 36.39
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Fig. 4.
adsorption on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT top site obtained by the DFT calculations. We find that the adsorption of SCN− on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surface due to reduce ELUMO and EHOMO for all complexes, and also indicate that groups of HOMO and LUMO are more stable than those for AlPNTs. Hence, HOMO and LUMO orbitals are uniformly distributed between them, which demonstrates that covalent bond is notable throughout the nanotubes (see Figs. 2 and 3). The distribution of frontier orbital in the (6, 0), (4, 4) AlNNTs and (6, 0) AlPNT complexes, the HOMO are localized on the most electronegative N and P atoms in the center of the nanotube axis which corresponds to the lone pair of electrons on the N and P atoms. While the LUMO are more localized on the S C N orbitals and are less on the nitrogen atoms in the center of the nanotube axis (see Figs. 2 and 3). The chemical activity of the AlPNTs can be determined via the HOMO LUMO bond gap that is a notable Consequence trusting on the HOMO and LUMO energy levels. The computed bond gaps for (6, 0), (4, 4) zigzag and armchair AlNNTs are in the range of 4.76–4.55 eV at the B3LYP/6-31G* method (see Table 1). The experimental band gap of AlNNTs is larger (6.2 eV) than that the theoretical value [9]. When SCN− adsorbs on the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT surfaces in stable configurations (N-side), the gap energies reduced from 4.40–4.55–3.21–2.27 to 3.47–3.53–2.68–1.93 eV, respectively. To better understand the
sensitivity of the SCN− toward the electronic property of the (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT, we plotted the density of states (DOS) for SCN− adsorption on this complexes, as summarized in Fig. 4. To examine the sensitivity of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT for the most stable configurations in comparison with the perfect AlNNTs, AlPNTs, and BPNTs. It can be seen from the figures that the DOS of SCN− on (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT displays more reasonable changes than to the DOS of the pure (6, 0), (4, 4) AlNNTs, (6, 0) AlPNT, and (6, 0) BPNT, indicating a bond strong of SCN− in the electronic conductivity of AlNNTs. Therefore, they results show due to strong chemical interaction of SCN− on AlNNTs, AlPNTs, and BPNTs systems the DOS near the Fermi level are affected via the adsorption process. Our results unveil after adsorption of SCN− on AlNNT, AlPNT, and BPNT surfaces, the HOMO–LUMO energy gaps of tubes has notable changes. Hence, it is concluded that (6, 0) AlPNTs can be a better candidate as sensor for SCN− . 3.4. Dipole moment The computed values of the size and structures of the dipole moment (D ) for SCN− on the (6, 0) AlPNT, AlNNT, and BPNT surfaces are summarized in Table 1. The dipole moment indicates a
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special property of a molecule that considers data in the case electronic and geometrical properties. Our computations value show when SCN− approaches the surface of (6, 0) AlNNT, (6, 0) AlPNT, and (6, 0) BPNT, the size and direction of the electric dipole moment vector are changed depending on the adsorption configurations. The studies of the electric dipole moment for SCN− and sites of the adsorption energy are highest, while calculates demonstrate that during SCN− adsorption for all systems, total dipole moment increases. Our studies indicate that the dipole moment for (6, 0) AlPNT, AlNNT, and BPNT are 4.27, 11.15, and 2.08 Debye, respectively. The values of D of the SCN− adsorption on (6, 0) AlPNT, AlNNT, and BPNT are about 36.39, 29.14, and 29.11 Debye, respectively. These results indicated that significantly changes due to increase of the D for SCN− adsorption on the AlPNT in comparable with AlNNT and BPNT. 4. Conclusion We have examined of SCN− adsorption on outer surface of (6, 0), (4, 4) AlNNT, (6, 0) AlPNT and BPNT through DFT calculations at B3LYP/6-31G* level. The SCN− adsorption on surfaces of the AlNNT, AlPNT, and BPNT shows that geometrical and electronic properties, quantum molecular descriptors, and density of state were computed. In contrast, our results showed that the charge transfer and the sensitivity of SCN− to AlPNTs is a significant factor in the change in the electrical conductance of the AlP nanotubes. Our calculations indicated that SCN− can be chemisorbed on AlPNTs surface. And, it can be used for SCN− storage. Decrease in ionization potential, bond gap, and global hardness via the adsorption energies of SCN− on the AlPNT lowers stability than that of the SCN− on the AlNNT and increase in chemical reactivity in the systems. Acknowledgments We would like to thank the Nanotechnology Working group of Young Researchers Club of the Islamic Azad University, Gorgan Branch, Iran. References [1] S. Ijima, Nature 354 (1991) 56. [2] J.A. Talla, Chemical Physics 392 (2012) 71. [3] W. An, C.H. Turner, Chemical Physics Letters 482 (2009) 274.
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