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A theoretical study of the adsorption behavior of N2O on single-walled AlN and AlP nanotubes
Superlattices and Microstructures 58 (2013) 178–190
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A theoretical study of the adsorption behavior of N2O on single-walled AlN and AlP nanotubes Alireza Soltani a,⇑, Mohammad Ramezani Taghartapeh a, E. Tazikeh Lemeski b, Mehdi Abroudi a, Hossein Mighani c a
Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran c Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran b
a r t i c l e
i n f o
Article history: Received 8 November 2012 Received in revised form 19 February 2013 Accepted 27 February 2013 Available online 16 March 2013 Keywords: Adsorption AlNNT N2O DFT Global indices
a b s t r a c t We have performed first-principles computations to investigate the adsorption properties of the N2O on the outer surfaces of H-capped single-walled AlN and AlP nanotubes (SWAlNNTs and SWAlPNTs). Binding energy corresponding to the most stable configuration (O-side) of N2O on the AlNNTs is found to be 25.37 kJ mol1, which is not typical for the chemisorption process. For the N2O/AlNNTs complexes, the energy gaps, dipole moments, natural atomic orbital occupancies and global indices are calculated. The computed density of states (DOSs) reveals that there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction. Finally, we clarify that AlNNTs plays an important role as a suitable sensor. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the discovery of single-walled carbon nanotubes (SWCNTs) by Ijima [1] numerous studies have been conducted on determining the properties of physisorptions and chemisorptions of various gases such as SCN, OCN, N2O, CO, NO, H2, Cl2, NH3, NO2, O2, H2O, and H3COH. However, further applications have been shown 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
⇑ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.02.015
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discoveries, most of the researchers’ interests are directed to 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]. In addition AlNNT is famous because of some unique properties such as, superior mechanical strength, high thermal conductivity, and a high piezo electric response [12,13]. A research by Zhang, using an ab initio method in 2003, predicted that AlN nanotubes are energetically notable and arranged in a hexagonal network adopting an sp2 hybridization for both Al and N atoms [14]. Other investigation supposing that AlN could form conventional single-walled nanotubes similar to carbon nanotubes (CNTs) and showed that the strain energy required in order wrapping up a graphitic sheet into an AlNNT is lower than that required to form BNNTs, GaNNTs and CNTs with similar diameters [15–17]. In 2002, Tondareand co-author by using a highly non-equilibrium DC-arc plasma method, successfully synthesized the AlNNTs with diameter ranging from 30 to 80 nm, in gram quantities [18]. 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 and co-worker in 2003 [19]). Although other papers have reported on the synthesis of AlNNTs through different processes [17–19]. Although to develop the applications of nanotubes mostly dependent on their successful manipulation but application and processing of nanotubes have been limited by their insolubility in most common solvents because they tend to clump together into bundles due to intertube dispersion interactions. The calculated imaginary section of dielectric functions (e2) curves exhibited that the AlNNTs optical properties are dominantly dependent one size [22]. Some noticeable applications of AlNNTs are LEDs for low energy consumption light emission, blue lasers for multimedia disks, field effect transistors, and high electron mobility transistors (HEMTs) [21]. One of the most ruinous greenhouse gases is N2O, which is approved to be as an ozone demolishing agents in the atmosphere [23]. In consideration of global warming, the role of N2O is much more influential than those of CO2 and CH4, with approximately (ca.6%). N2O is known as a major stratospheric source of NOx. Annual atmospheric rate of N2O has a quick increase of (0.2–0.3%), resulting from human Anthropogenic activities [24]. Hence, the check of N2O emission due to various kinds of combustion has become a prominent issue of worry, like; lean-burn automobiles, coal-fired power plants, electric power generators, nitric acid factories, and biomass burning [25–29]. The most suitable thermodynamically preferred temperature for decomposition of N2O is under 1000 K. The Gibbs free energy at the room temperature energy of reaction comes up to 175 kJ mol1 [30]. There is also kinetically reaction retardation due to the very high activation energy of about 300 kJ mol1 [31]. The amount of N2O is not obviously recognized but it is assessed that about 2.4 Tg N2OAN per year or almost 30% of the global production of N2O is originated from wet tropical forest soils [32]. Wet tropical forest soils are considered to be source of 2.2 Tg NOAN emission per year. However, there is not a great deal of investigation on decomposition, adsorption, and formation mechanisms of N2O on absorbent materials like; CNTs, and BNNTs [32]. Recently, Hadipour et al. have studied adsorption properties of NH3 and HCOH on the AlN nanotubes. Also, Ahmadi et al. have reported the properties of CO adsorption on the surface of BN, AlN, BP, and AlP nanotubes [39]. Also, Zhao and Chi have reported that the Al-doped graphene has high sensitivity to formaldehyde molecule in comparison with the pure graphene [41]. Moreover, He has considered the interaction and charge transfer between graphene and organic molecules [42]. In our recent report, we have studied the adsorption properties of NH3 on the sidewalls of Al-, Ga-doped BN nanotubes. The results indicate that the adsorption between NH3 and Al-doped BNNT is typically more than that of Ga-doped BNNT [43]. Therefore, the investigation of adsorption behavior of N2O on tubular surfaces can provide valuable information about its bonding and reactivity in catalysis and other surface phenomena [33]. In the current article we are attempting to consider the adsorption properties of N2O on the exterior surfaces of AlNNTs and AlPNTs.
2. Computational methods In this lecture, we studied the adsorption properties of the N2O on the pristine zigzag (6, 0) and armchair (4, 4) AlNNTs and zigzag (6, 0) AlPNTs in which the ends of the AlN and AlP nanotubes are saturated by hydrogen atoms. All the geometrical optimizations and energy calculations are
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performed using Gaussian 98 program package [36] at the level of density functional theory (DFT) with B3LYP/6-31G basis set [37,38]. The hydrogenated (6, 0), (4, 4) zigzag and armchair AlNNTs and zigzag (6, 0) AlPNTs have 60 (Al24N24H12), 72 (Al28N28H16), and 60 (Al24P24H12) atoms, respectively. The binding energy (Ead) of N2O on the AlNNTs is determined through the following equation:
DEad ¼ EAlNNTN2 O ðEAlNNT þ EN2 O Þ
ð1Þ
DEad ¼ EAlPNTN2 O ðEAlPNT þ EN2 O Þ
ð2Þ
where EAlNNTN2 O and EAlPNTN2 O are the total energies of the AlNNT and AlPNT interacting with the N2O. EAlNNT and EAlPNT are total energies of the pristine AlNNT and AlPNT. EN2 O is the total energy of an isolated N2O. Natural charge analysis with full NBO calculations were performed by using DFT/B3LYP method with 6-31G basis set for optimized structures. The electrophilicity concept was stated for the first time in 1999 by Parr et al. [20]. l is defined according to the following equation [21]:
l ¼ ðI þ AÞ=2
ð3Þ
I (EHOMO) is the ionization potential and A (ELUMO) the electron affinity of the molecule. Where EHOMO is the energy of the Fermi level and ELUMO is the first given value of the conduction band. v is defined as the negative of l, as follows: v = l. Furthermore, g can be approximated using the Koopmans’ theorem [22] as: g = (ELUMO – EHOMO)/2. S [23] and x [24] are defined as the following equations, respectively.
S ¼ 1=2g
ð4Þ
x ¼ ðl2 =2gÞ
ð5Þ
The maximum amount of electronic charge, DNmax, that the electrophone system may accept is given by Eq. (7) as [25]:
DNmax ¼ l=g
ð6Þ
3. Results and discussion 3.1. The N2O adsorbed on the pristine AlNNTs We first optimize the structure of N2O and pure AlNNTs including (6, 0), (4, 4) zigzag and armchair AlNNTs were used for the N2O adsorption are shown in Fig. 1. We studied two adsorption state with the nitrogen and oxygen atoms of N2O for the most stable configurations via its expected active sites (N-side and O-side) directed downward toward the Al atom in (6, 0) and (4, 4) AlNNTs surfaces. The optimized geometrics of perfect (6, 0), (4, 4) zigzag and armchair AlNNTs were found to be with AlAN bond lengths are about 1.923 and 1.872 Å, and with the diameter of the nanotubes are about 6.038 and 6.676 Å, respectively. The AlANAAl and NAAlAN bond angles in (6, 0) AlNNTs are 114.018° and 113.629°, while the AlANAAl and NAAlAN bond angles in (4, 4) AlNNTs are 115.094° and 118.085°, respectively. After the N2O adsorbed on the (6, 0) and (4, 4) AlNNTs, the equilibrium AlAN bond lengths are about 1.820 and 1.827 Å, and with the diameter of the nanotubes are about 6.216 and 6.856 Å, respectively. In comparison with the pristine form, the AlAN bond lengths and diameters are shorter and longer for the N2O adsorbed on the (6, 0) and (4, 4) AlNNTs, respectively. The discrepancy in Ead for different configurations is reflected in the fact that the binding energies between the N2O molecule and corresponding AlNNTs are rather dependent on orientations and locations of the N2O. On the other hand, the computed Ead for N2O from O-side of atom is more than that of N-side atom [33,34]. The binding energy of a single N2O on the exterior surface of pristine (6, 0) and (4, 4) AlNNTs for the most stable configurations (O-side) are 25.37 and 22.33 kJ mol1, and the equilibrium distance between the closest atoms of two species are 2.289 and 2.37 Å, respectively. While the adsorption energy for N2O on (6, 0) and (4, 4) AlNNT in N-side
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Fig. 1. The optimized structure for the most stable configuration (N-side and O-side) of (a) N2O/(6, 0) AlNNT, and (b) N2O/(4, 4) AlNNT complexes.
are 16.05 and 12.67 kJ mol1, and the equilibrium distance between the closest atoms of two species are 1.566 and 1.546 Å, respectively. These results demonstrate a physisorption process because of weak van der Waals interaction between the N2O and the nanotube. Our computation results indicate that the pristine (6, 0) AlNNT can be slightly more sensitive to N2O molecule than that of (4, 4) AlNNT. In case of parallel model of N2O adsorption on (6, 0) AlNNT, the interaction distance and the adsorption energy between two species are 25.19 kJ mol1 and 2.29 Å, respectively (Fig. 2a). In Fig. 2b, the adsorption energy and interaction distance between the O atom of N2O and the Al atom of (6, 0) AlNNT in the end of tube are 24.15 kJ mol1 and 2.27 Å, respectively. Besides, we investigated the adsorption energy of N2O on sidewalls of (6, 0), (7, 0), and (8, 0) BNNT at the B3LYP/631G and MP2/6-311 + G methods [33]. Recently, Baei et al. have reported an ab initio study of N2O physisorption upon the (6, 0) magnesium oxide nanotube [40]. For the linear structure of N2O, the NAO and NAN bond lengths are 1.134 and 1.192 Å, respectively. And the N@N@O bond angle is
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Fig. 2. The optimized structure for the most stable configurations (N-side and O-side) of (a and b) N2O/(6, 0) AlPNT complexes.
about 180°, and this molecule have C 1 V symmetry. While with the N2O adsorption on AlN nanotubes (6, 0), the average N@N and N@O bond lengths gradually reduces from 1.134 and 1.192 Å an isolate N2O, to (NBN) 1.131 and (N@O) 1.184 Å in (6, 0) AlNNTs, then 1.133 and 1.185 Å in (4, 4) AlNNTs. The structure changes of N2O on (4, 4) AlN nanotubes in comparison with (6, 0) AlN nanotubes is gradually bent inward, away from the Al atom. On the other hand, the slightly difference in Ead for configurations shows that the binding energies of N2O on the pristine (6, 0) AlNNTs increase in comparison with (4, 4) AlNNTs. Also, this computed for the most stable configurations (O-side) shows that the binding energy slightly increases in comparison with CNTs and BNNTs [33,34]. The difference in adsorption energy between configurations can be attributed to the polarization of N2O molecule. Since oxygen atom is more electronegative than nitrogen atom thus for configuration, in which oxygen atom orients toward the AlNNT surfaces, interaction between N2O and surfaces is more than that of configuration, in which nitrogen atom orients toward the surfaces. On the other hand, the results show that the N2O molecule weakly physical adsorption on the pure AlNNT, due to weak van der Waals interaction between the AlNNT and N2O molecule [33]. The electron density difference (Dq) is also calculated to a better understanding of the relevant adsorption process between N2O and AlNNT [44] by natural bond orbitals (NBOs) and Mulliken Population Analysis (MPA). NBO analysis shows that in this configurations 0.094 and 0.079e charge transferred from (6, 0) and (4, 4) AlNNT to N2O molecule (N-side). About 0.085e is transferred from the pristine AlNNT to the N2O molecule (O-side), showing that the interaction nature between the N2O molecule and the pristine AlNNT is mainly electrostatic. MPA analysis indicates that the net charges transferred from (6, 0) AlNNT and AlPNT to the N2O molecule (O-side) are about 0.093 and 0.079e, respectively. The results reveal that absolute AlNNT cannot detect the N2O molecule. We examine the binding energies of N2O molecule for the most stable state on the (6, 0) AlPNT surface in comparison to (6, 0) AlNNT, the total energy of the configuration was determined as a function of distance of the N2O molecule to the outer surface of the AlPNT and then consider the interaction of N2O molecule directed downward toward the Al atom in (6, 0) AlPNT via its N and O sides, as shown in Fig. 3. The optimized geometrics of perfect (6, 0) zigzag AlPNT was found to be with AlAP bond length is about 2.306 Å, and with the diameter of the AlPNT is about 6.618 Å, respectively. The AlAPAAl and PAAlAP bond angles in (6, 0) AlPNTs are 110.914° and 116.809°, respectively. The binding energy of a single N2O on the outer surface of perfect (6, 0) AlPNT for Al site in N- and O-sides show that the adsorption energies for these sites are 7.68 and 8.26 kJ mol1 with an equilibrium distance (D) of 2.69 and 2.64 Å, respectively (Fig. 3a and b). After the N2O adsorbed on the (6, 0) AlPNTs, the equilibrium AlAP bond lengths is about 2.320 Å, and with the diameter of the nanotubes is about 7.384 Å. When the N2O adsorption on (6, 0) AlPNT, the average N@N and N@O bond lengths gradually reduces from 1.134 and 1.192 Å an isolate N2O, to (NBN) 1.134 and (N@O) 1.185 Å in (6, 0) AlPNTs. As shown in Fig. 3c, when N2O (O-side) approaches to the surface of AlPNT
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Fig. 3. Optimized geometries of N2O adsorptions on (6, 0) AlPNT.
in the parallel position, the physisorption energy is about 12.64 kJ mol1 and interaction distance is about 2.53 Å, indicating that the parallel adsorption is slightly stable than the above-mentioned case. Upon interaction of N2O at the end of tube (Fig. 3d), the adsorption energy is calculated to be 10.17 kJ mol1 and the distance between the two species is about 2.58 Å. On the other hand, the computed Ead for N2O molecule from O-side of atom is more than that of N-side atom. The lower adsorption energy shows that N2O adsorption capability of AlPNTs is lower than that of AlNNTs. On the other hand, the results indicate that the N2O molecule weakly physical adsorption on the perfect AlPNT. The results reveal that pure AlPNT cannot detect the N2O molecule. We calculated the molecular electrostatic potential (MEP) plots for N2O on walls of (6, 0) AlNNT and AlPNT (see in Fig. 6). The MEP is the interaction energy of a positive point charge with the nuclei and electrons of a molecule. Almost all the positive and negative charges of an adsorbed molecule symbolize as nucleophiles and electrophiles sites once they are initially approached to an adsorbent surface, and can also be correlated with hydrogen-bond-donating and accepting tendencies [32]. Fig. 6 demonstrates the N2O molecule functions as an electron donor and the Al atoms of (6, 0) AlNNT and AlPNT as an electron acceptor. The MEP plots show a strong positive electrostatic potential (electron poor blue color1) and weak negative electrostatic potential (electron rich – red color) occur at the end and opposite end regions of nanotubes, respectively, where the hydrogens are located. Moreover, the results of MEP show that the regions of negative electrostatic potential (denoted by red) are usually applied to identify hydrogen-bond-acceptor sites, while for describing the hydrogen-bond-donating sites the regions of positive (denoted by blue) are significant.
1
For interpretation of colour in Fig. 6, the reader is referred to the web version of this article.
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To overcome this weakness of pristine AlNNT, the Ga-doped AlNNT could be better candidate for adsorption process of N2O molecule. In the case of Ga-doped (6, 0) AlNNT, the Ga-N distance is 1.89 Å. Also, the NAGaAN bond angle in the interaction between two species is about 118.88°. For N2O/Ga-doped (6, 0) AlNNT, the NAN and NAO bond lengths of N2O are about 1.129 and 1.999 Å, respectively. The binding energy of N2O on Ga-doped AlNNT is computed to be 35.80 kJ mol1, exhibiting a physisorption characteristic. The interaction distance between the oxygen atom of N2O and the Ga atom of tube is about 2.57 Å and a slight charge (0.067e) is transferred from the Ga-doped AlNNT to the N2O (Fig. 3) by NBO analysis. After the adsorption of N2O on Ga-doped AlNNT, the charges on Ga, Al, and N atoms decreased from 1.685, 1.914 and 1.853e to 0.755, 0.890 and 0.852e, respectively. The NBO analysis indicates that with the adsorption of N2O molecule, Ga atom is less positively charged because of essential electron transfer from Ga atom to N2O. The MPA analysis demonstrates that the net charge of 0.07e is transferred from the Ga-doped AlNNT to the N2O. Our calculated study indicates that the adsorption of Ga-doped AlNNT with the N2O is slightly stronger than
Fig. 4. Optimized structures and the electronic density of states of N2O adsorption on the Ga-doped AlNNT.
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the pure tube. The electronic properties were calculated for pure Ga-doped (6, 0) AlNNT and N2O/Gadoped (6, 0) AlNNT are shown in Fig. 4 for comparison. As shown in Fig. 4, the results of DOS indicate that the electronic structure of the Ga-doped AlNNT is not sensitive to the N2O adsorption. The physisorption of N2O in the Ga-doped AlNNT show no significant changes because of N2O adsorption in the gap region of the TDOS plots. 3.2. Electronic energies and relative stabilities To further investigate the adsorption properties of N2O on (6, 0) and (4, 4) AlNNTs, we examined the electronic energies of N2O on (6, 0) and (4, 4) AlNNTs. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the N2O adsorption on the (6, 0) and (4, 4) AlNNTs are studied (see Table 1). Table 2 presents the results in the LUMO and HOMO energies of the N2O adsorption on the (6, 0) and (4, 4) AlN nanotubes top site obtained by the DFT calculations. The study indicate that the adsorption of N2O on the AlNNTs surface, ELUMO and EHOMO for whole models are decreased therefore both groups of occupied and unoccupied molecular orbital are more stable than those for AlNNTs. Furthermore, HOMO and LUMO orbitals are uniformly distributed throughout nanotube axis, which illustrates that covalent functionalization is preferable throughout the nanotubes (see Fig. 5). The distribution of frontier orbital in the perfect (6, 0) AlNNTs model, the HOMO is localized on the most electronegative nitrogen atoms in center and edge of the nanotube axis which is corresponds to the lone pair of electron on the nitrogen atoms. The LUMO is more localized on the center of the nanotube axis. In N2O molecule, the HOMO and LUMO are uniformly distributed on the N@N@O orbitals which are summarized in Fig. 5. We found that the HOMO is localized on the nitrogen orbitals at the center, edge, and opposite end of the nanotube axis; while the LUMO are more localized on Al and N atoms at the center and edge of the tube, and also is on the N@N@O orbital in the AlNNT. The chemical activity of the AlNNTs can be characterized by the HOMO–LUMO energy gap that is a significant parameter relying on the HOMO and LUMO energy levels. Typically, a small HOMO–LUMO energy gap means a high chemical activity and a low chemical stability. The calculated gap energies for (6, 0), (4, 4) zigzag and armchair AlNNT are in the range of 4.40–4.55 eV at the B3LYP/6-31G level
Table 1 EHOMO (eV), ELUMO (eV), chemical hardness, g (eV), chemical potential, l (eV), electrophilicity, x and maximum amount of electronic charge transfer, DNmax (a.u.), in atomic units, for N2O on (6, 0) and (4, 4) AlNNT surface at the B3LYP/6-31G(d) level. Property
N2O
(6,0) AlNNT
N2O/(6,0) AlNNTs (O-side)
(4,4) AlNNT
N2O/(4,4) AlNNT (O-side)
EHOMO (eV) ELUMO (eV) Eg (eV) DEg (eV) lD (Debye) Ead (kJ/mol1) EF (eV)
9.31 0.52 8.79 – 0.01 – 4.91
6.46 2.06 4.4 – 11.14 – 4.26
6.35 1.93 4.38 0.02 0.52 25.37 4.14
6.46 1.91 4.55 – 0.003 – 4.18
6.38 1.84 4.54 0.01 0.53 22.33 4.11
Table 2 Chemical potential (l), hardness (g), softness (S), DNmax (a.u.), and electrophilicity (x) of N2O adsorption on (6, 0) and (4, 4) AlNNT surfaces at the B3LYP/6-31G level. The parameters are in units of eV. Property
N2O
(6,0) AlNNT
N2O/(6,0) AlNNTs (O-side)
(4,4) AlNNT
N2O/(4,4) AlNNT (O-side)
[I = EHOMO] (eV) [A = ELUMO] (eV) [g = (I A)/2] (eV) [l = (I + A)/2] (eV) [S = 1/2g] (eV) [x = l2/2g] (eV) [DNmax = l/g] (a.u.)
9.31 0.52 4.39 4.91 0.11 2.75 1.12
6.46 2.06 2.20 4.26 0.23 4.12 1.93
6.35 1.93 2.21 4.14 0.22 3.88 1.887
6.46 1.91 2.27 4.18 0.22 3.84 1.84
6.38 1.84 2.27 4.11 0.22 3.72 1.81
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Fig. 5. Charge distribution of HOMO and LUMO orbitals on the (6, 0) AlNNT loaded with one N2O at the B3LYP/6-31G level.
(see Table 1). When N2O adsorbs on surfaces of (6, 0) and (4, 4) AlNNTs in stable configuration (O-side), the gap energies reduced from 4.4–4.55 to 4.38–4.54 eV, thus chemical stability of zigzag (6, 0) and armchair (4, 4) AlNNT will be decreased and hence chemical activity of such system will be slightly increased. Fig. 7 shows the influence of N2O on the electronic properties of the (6, 0) and (4, 4) AlNNTs, we have computed the density of state (DOS) plots corresponding to the adsorption of the N2O and the (6, 0) and (4, 4) AlNNTs. It can be seen that the DOS of N2O on (6, 0) and (4, 4) AlNNTs displays more reasonable changes than to the DOS of the pure (4, 4) AlNNTs, revealing slightly effect of N2O molecule in the electronic conductivity of AlNNTs. Therefore, this information indicates for N2O on (6, 0) and (4, 4) AlNNT systems the DOS near the Fermi level is not affected via the N2O adsorption. On the one hand, there exists the slightly difference of the DOS between three combined systems of N2O/AlNNTs. This results show after adsorption of N2O on AlNNT, the HOMO–LUMO energy gap of nanotube has no notable change. This is an evidence of the weak interaction between N2O and related to the AlNNTs.
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Fig. 6. The molecular electrostatic potential of N2O/(6, 0) AlNNT and N2O/(6, 0) AlPNT complexes.
3.3. Electric dipole moment The electric dipole moment vector of species has important properties that show the charge distribution when gases on the system adsorbed. While a N2O approaches to the surface of (6, 0) and (4, 4) AlNNTs, the size and direction of the electric dipole moment vector are change dependence to the adsorption configurations. Our computation indicates that during N2O adsorb for all complexes, total dipole moment increases. We consider that dipole moment for (6, 0) and (4, 4) AlNNTs are 11.15 and 0.003 Debye, respectively. In the case of the N2O adsorption on (6, 0) and (4, 4) AlNNTs are 12.00 and 1.84 Debye, respectively. Substantially, both electron transfers and dipole moment implies the
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Fig. 7. Total density of states (DOS) for an isolated N2O, N2O/(6, 0) AlNNT,N2O/(4, 4)AlNNT complexes.
concept that AlNNTs with absorption of the N2O are caused to increase the polarization and change dipole moments.
3.4. Global indices The global hardness of a species is defined as its resistance towards deformation in the presence of an electric field. Increasing in global hardness leads to increase in stability and decrease in reactivity of the species. The global indices of reactivity in the context of the DFT for N2O adsorption on (6, 0), (4, 4) zigzag and armchair AlNNTs are presented in Table 2. When N2O adsorbs on AlNNTs, hardness, electrophilicity and electronic chemical potentials of AlNNTs will be decreased, also softness will be increased. The results also indicate that when N2O is chemisorbed on the outer surface of the AlNNTs, a slight charge transfer to the N2O could occur, which suggest that their electronic transport properties could be slightly changed upon adsorptions of N2O. The direction of electron flow will be distinguished by electronegativity or electronic chemical potential. When N2O approaches on AlNNTs electrons is transferred from higher chemical potential to the lower electronic chemical potential, until the electronic chemical potentials become identical [35]. As a result, electrons will flow from a definite occupied orbital in an Al atom of AlNNTs and will go into a definite empty orbital in single N2O. The global electrophilicity index determines the energy lowering of a ligand due to maximum flow of electron from donor to acceptor species and provides information about structural stability, reactivity and toxicity of chemical species. On the other hand, the electrophilicity index determines maximum flow of electron from donor to acceptor species and supplies data connected to structural stability, reactivity and toxicity of chemical species.
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4. Conclusion In conclusion, we theoretically studied the adsorption properties of N2O molecule on AlNNTs and AlPNT and through DFT calculations. For N2O adsorption on the AlNNT and AlPNT, electronic properties of the tubes will be changed. Our calculations show that the N2O adsorption on (6, 0) AlNNTs for the most stable configuration (O-side) is about 25.37 kJ mol1. For AlNNT, the calculated binding energy for N2O in O-side is a little more than that in N-side. And the adsorption of N2Oon (6, 0) zigzag is more stable than N2O on (4, 4) armchair AlNNTs. Our computations show that N2O could not be chemisorbed on AlNNTs and AlPNTs surfaces. Decrease in HOMO–LUMO energy gaps, ionization potential, and global hardness with the adsorption of N2O on AlNNTs surface due to increase chemical reactivity and also lead to lowering of stability in the complexes. Acknowledgment We would like to thank the International Research Academy of Nanotechnology of Golestan (IRANG). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
[37] [38]
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Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
A theoretical study of the adsorption behavior of N2O on single-walled AlN and AlP nanotubes Alireza Soltani a,⇑, Mohammad Ramezani Taghartapeh a, E. Tazikeh Lemeski b, Mehdi Abroudi a, Hossein Mighani c a
Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran c Department of Chemistry, Faculty of Science, Golestan University, Gorgan, Iran b
a r t i c l e
i n f o
Article history: Received 8 November 2012 Received in revised form 19 February 2013 Accepted 27 February 2013 Available online 16 March 2013 Keywords: Adsorption AlNNT N2O DFT Global indices
a b s t r a c t We have performed first-principles computations to investigate the adsorption properties of the N2O on the outer surfaces of H-capped single-walled AlN and AlP nanotubes (SWAlNNTs and SWAlPNTs). Binding energy corresponding to the most stable configuration (O-side) of N2O on the AlNNTs is found to be 25.37 kJ mol1, which is not typical for the chemisorption process. For the N2O/AlNNTs complexes, the energy gaps, dipole moments, natural atomic orbital occupancies and global indices are calculated. The computed density of states (DOSs) reveals that there is a significant orbital hybridization between two species in adsorption process being an evidence of strong interaction. Finally, we clarify that AlNNTs plays an important role as a suitable sensor. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the discovery of single-walled carbon nanotubes (SWCNTs) by Ijima [1] numerous studies have been conducted on determining the properties of physisorptions and chemisorptions of various gases such as SCN, OCN, N2O, CO, NO, H2, Cl2, NH3, NO2, O2, H2O, and H3COH. However, further applications have been shown 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
⇑ Corresponding author. Tel.: +98 9113702973. E-mail address: [email protected] (A. Soltani). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.02.015
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discoveries, most of the researchers’ interests are directed to 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]. In addition AlNNT is famous because of some unique properties such as, superior mechanical strength, high thermal conductivity, and a high piezo electric response [12,13]. A research by Zhang, using an ab initio method in 2003, predicted that AlN nanotubes are energetically notable and arranged in a hexagonal network adopting an sp2 hybridization for both Al and N atoms [14]. Other investigation supposing that AlN could form conventional single-walled nanotubes similar to carbon nanotubes (CNTs) and showed that the strain energy required in order wrapping up a graphitic sheet into an AlNNT is lower than that required to form BNNTs, GaNNTs and CNTs with similar diameters [15–17]. In 2002, Tondareand co-author by using a highly non-equilibrium DC-arc plasma method, successfully synthesized the AlNNTs with diameter ranging from 30 to 80 nm, in gram quantities [18]. 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 and co-worker in 2003 [19]). Although other papers have reported on the synthesis of AlNNTs through different processes [17–19]. Although to develop the applications of nanotubes mostly dependent on their successful manipulation but application and processing of nanotubes have been limited by their insolubility in most common solvents because they tend to clump together into bundles due to intertube dispersion interactions. The calculated imaginary section of dielectric functions (e2) curves exhibited that the AlNNTs optical properties are dominantly dependent one size [22]. Some noticeable applications of AlNNTs are LEDs for low energy consumption light emission, blue lasers for multimedia disks, field effect transistors, and high electron mobility transistors (HEMTs) [21]. One of the most ruinous greenhouse gases is N2O, which is approved to be as an ozone demolishing agents in the atmosphere [23]. In consideration of global warming, the role of N2O is much more influential than those of CO2 and CH4, with approximately (ca.6%). N2O is known as a major stratospheric source of NOx. Annual atmospheric rate of N2O has a quick increase of (0.2–0.3%), resulting from human Anthropogenic activities [24]. Hence, the check of N2O emission due to various kinds of combustion has become a prominent issue of worry, like; lean-burn automobiles, coal-fired power plants, electric power generators, nitric acid factories, and biomass burning [25–29]. The most suitable thermodynamically preferred temperature for decomposition of N2O is under 1000 K. The Gibbs free energy at the room temperature energy of reaction comes up to 175 kJ mol1 [30]. There is also kinetically reaction retardation due to the very high activation energy of about 300 kJ mol1 [31]. The amount of N2O is not obviously recognized but it is assessed that about 2.4 Tg N2OAN per year or almost 30% of the global production of N2O is originated from wet tropical forest soils [32]. Wet tropical forest soils are considered to be source of 2.2 Tg NOAN emission per year. However, there is not a great deal of investigation on decomposition, adsorption, and formation mechanisms of N2O on absorbent materials like; CNTs, and BNNTs [32]. Recently, Hadipour et al. have studied adsorption properties of NH3 and HCOH on the AlN nanotubes. Also, Ahmadi et al. have reported the properties of CO adsorption on the surface of BN, AlN, BP, and AlP nanotubes [39]. Also, Zhao and Chi have reported that the Al-doped graphene has high sensitivity to formaldehyde molecule in comparison with the pure graphene [41]. Moreover, He has considered the interaction and charge transfer between graphene and organic molecules [42]. In our recent report, we have studied the adsorption properties of NH3 on the sidewalls of Al-, Ga-doped BN nanotubes. The results indicate that the adsorption between NH3 and Al-doped BNNT is typically more than that of Ga-doped BNNT [43]. Therefore, the investigation of adsorption behavior of N2O on tubular surfaces can provide valuable information about its bonding and reactivity in catalysis and other surface phenomena [33]. In the current article we are attempting to consider the adsorption properties of N2O on the exterior surfaces of AlNNTs and AlPNTs.
2. Computational methods In this lecture, we studied the adsorption properties of the N2O on the pristine zigzag (6, 0) and armchair (4, 4) AlNNTs and zigzag (6, 0) AlPNTs in which the ends of the AlN and AlP nanotubes are saturated by hydrogen atoms. All the geometrical optimizations and energy calculations are
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performed using Gaussian 98 program package [36] at the level of density functional theory (DFT) with B3LYP/6-31G basis set [37,38]. The hydrogenated (6, 0), (4, 4) zigzag and armchair AlNNTs and zigzag (6, 0) AlPNTs have 60 (Al24N24H12), 72 (Al28N28H16), and 60 (Al24P24H12) atoms, respectively. The binding energy (Ead) of N2O on the AlNNTs is determined through the following equation:
DEad ¼ EAlNNTN2 O ðEAlNNT þ EN2 O Þ
ð1Þ
DEad ¼ EAlPNTN2 O ðEAlPNT þ EN2 O Þ
ð2Þ
where EAlNNTN2 O and EAlPNTN2 O are the total energies of the AlNNT and AlPNT interacting with the N2O. EAlNNT and EAlPNT are total energies of the pristine AlNNT and AlPNT. EN2 O is the total energy of an isolated N2O. Natural charge analysis with full NBO calculations were performed by using DFT/B3LYP method with 6-31G basis set for optimized structures. The electrophilicity concept was stated for the first time in 1999 by Parr et al. [20]. l is defined according to the following equation [21]:
l ¼ ðI þ AÞ=2
ð3Þ
I (EHOMO) is the ionization potential and A (ELUMO) the electron affinity of the molecule. Where EHOMO is the energy of the Fermi level and ELUMO is the first given value of the conduction band. v is defined as the negative of l, as follows: v = l. Furthermore, g can be approximated using the Koopmans’ theorem [22] as: g = (ELUMO – EHOMO)/2. S [23] and x [24] are defined as the following equations, respectively.
S ¼ 1=2g
ð4Þ
x ¼ ðl2 =2gÞ
ð5Þ
The maximum amount of electronic charge, DNmax, that the electrophone system may accept is given by Eq. (7) as [25]:
DNmax ¼ l=g
ð6Þ
3. Results and discussion 3.1. The N2O adsorbed on the pristine AlNNTs We first optimize the structure of N2O and pure AlNNTs including (6, 0), (4, 4) zigzag and armchair AlNNTs were used for the N2O adsorption are shown in Fig. 1. We studied two adsorption state with the nitrogen and oxygen atoms of N2O for the most stable configurations via its expected active sites (N-side and O-side) directed downward toward the Al atom in (6, 0) and (4, 4) AlNNTs surfaces. The optimized geometrics of perfect (6, 0), (4, 4) zigzag and armchair AlNNTs were found to be with AlAN bond lengths are about 1.923 and 1.872 Å, and with the diameter of the nanotubes are about 6.038 and 6.676 Å, respectively. The AlANAAl and NAAlAN bond angles in (6, 0) AlNNTs are 114.018° and 113.629°, while the AlANAAl and NAAlAN bond angles in (4, 4) AlNNTs are 115.094° and 118.085°, respectively. After the N2O adsorbed on the (6, 0) and (4, 4) AlNNTs, the equilibrium AlAN bond lengths are about 1.820 and 1.827 Å, and with the diameter of the nanotubes are about 6.216 and 6.856 Å, respectively. In comparison with the pristine form, the AlAN bond lengths and diameters are shorter and longer for the N2O adsorbed on the (6, 0) and (4, 4) AlNNTs, respectively. The discrepancy in Ead for different configurations is reflected in the fact that the binding energies between the N2O molecule and corresponding AlNNTs are rather dependent on orientations and locations of the N2O. On the other hand, the computed Ead for N2O from O-side of atom is more than that of N-side atom [33,34]. The binding energy of a single N2O on the exterior surface of pristine (6, 0) and (4, 4) AlNNTs for the most stable configurations (O-side) are 25.37 and 22.33 kJ mol1, and the equilibrium distance between the closest atoms of two species are 2.289 and 2.37 Å, respectively. While the adsorption energy for N2O on (6, 0) and (4, 4) AlNNT in N-side
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Fig. 1. The optimized structure for the most stable configuration (N-side and O-side) of (a) N2O/(6, 0) AlNNT, and (b) N2O/(4, 4) AlNNT complexes.
are 16.05 and 12.67 kJ mol1, and the equilibrium distance between the closest atoms of two species are 1.566 and 1.546 Å, respectively. These results demonstrate a physisorption process because of weak van der Waals interaction between the N2O and the nanotube. Our computation results indicate that the pristine (6, 0) AlNNT can be slightly more sensitive to N2O molecule than that of (4, 4) AlNNT. In case of parallel model of N2O adsorption on (6, 0) AlNNT, the interaction distance and the adsorption energy between two species are 25.19 kJ mol1 and 2.29 Å, respectively (Fig. 2a). In Fig. 2b, the adsorption energy and interaction distance between the O atom of N2O and the Al atom of (6, 0) AlNNT in the end of tube are 24.15 kJ mol1 and 2.27 Å, respectively. Besides, we investigated the adsorption energy of N2O on sidewalls of (6, 0), (7, 0), and (8, 0) BNNT at the B3LYP/631G and MP2/6-311 + G methods [33]. Recently, Baei et al. have reported an ab initio study of N2O physisorption upon the (6, 0) magnesium oxide nanotube [40]. For the linear structure of N2O, the NAO and NAN bond lengths are 1.134 and 1.192 Å, respectively. And the N@N@O bond angle is
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Fig. 2. The optimized structure for the most stable configurations (N-side and O-side) of (a and b) N2O/(6, 0) AlPNT complexes.
about 180°, and this molecule have C 1 V symmetry. While with the N2O adsorption on AlN nanotubes (6, 0), the average N@N and N@O bond lengths gradually reduces from 1.134 and 1.192 Å an isolate N2O, to (NBN) 1.131 and (N@O) 1.184 Å in (6, 0) AlNNTs, then 1.133 and 1.185 Å in (4, 4) AlNNTs. The structure changes of N2O on (4, 4) AlN nanotubes in comparison with (6, 0) AlN nanotubes is gradually bent inward, away from the Al atom. On the other hand, the slightly difference in Ead for configurations shows that the binding energies of N2O on the pristine (6, 0) AlNNTs increase in comparison with (4, 4) AlNNTs. Also, this computed for the most stable configurations (O-side) shows that the binding energy slightly increases in comparison with CNTs and BNNTs [33,34]. The difference in adsorption energy between configurations can be attributed to the polarization of N2O molecule. Since oxygen atom is more electronegative than nitrogen atom thus for configuration, in which oxygen atom orients toward the AlNNT surfaces, interaction between N2O and surfaces is more than that of configuration, in which nitrogen atom orients toward the surfaces. On the other hand, the results show that the N2O molecule weakly physical adsorption on the pure AlNNT, due to weak van der Waals interaction between the AlNNT and N2O molecule [33]. The electron density difference (Dq) is also calculated to a better understanding of the relevant adsorption process between N2O and AlNNT [44] by natural bond orbitals (NBOs) and Mulliken Population Analysis (MPA). NBO analysis shows that in this configurations 0.094 and 0.079e charge transferred from (6, 0) and (4, 4) AlNNT to N2O molecule (N-side). About 0.085e is transferred from the pristine AlNNT to the N2O molecule (O-side), showing that the interaction nature between the N2O molecule and the pristine AlNNT is mainly electrostatic. MPA analysis indicates that the net charges transferred from (6, 0) AlNNT and AlPNT to the N2O molecule (O-side) are about 0.093 and 0.079e, respectively. The results reveal that absolute AlNNT cannot detect the N2O molecule. We examine the binding energies of N2O molecule for the most stable state on the (6, 0) AlPNT surface in comparison to (6, 0) AlNNT, the total energy of the configuration was determined as a function of distance of the N2O molecule to the outer surface of the AlPNT and then consider the interaction of N2O molecule directed downward toward the Al atom in (6, 0) AlPNT via its N and O sides, as shown in Fig. 3. The optimized geometrics of perfect (6, 0) zigzag AlPNT was found to be with AlAP bond length is about 2.306 Å, and with the diameter of the AlPNT is about 6.618 Å, respectively. The AlAPAAl and PAAlAP bond angles in (6, 0) AlPNTs are 110.914° and 116.809°, respectively. The binding energy of a single N2O on the outer surface of perfect (6, 0) AlPNT for Al site in N- and O-sides show that the adsorption energies for these sites are 7.68 and 8.26 kJ mol1 with an equilibrium distance (D) of 2.69 and 2.64 Å, respectively (Fig. 3a and b). After the N2O adsorbed on the (6, 0) AlPNTs, the equilibrium AlAP bond lengths is about 2.320 Å, and with the diameter of the nanotubes is about 7.384 Å. When the N2O adsorption on (6, 0) AlPNT, the average N@N and N@O bond lengths gradually reduces from 1.134 and 1.192 Å an isolate N2O, to (NBN) 1.134 and (N@O) 1.185 Å in (6, 0) AlPNTs. As shown in Fig. 3c, when N2O (O-side) approaches to the surface of AlPNT
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Fig. 3. Optimized geometries of N2O adsorptions on (6, 0) AlPNT.
in the parallel position, the physisorption energy is about 12.64 kJ mol1 and interaction distance is about 2.53 Å, indicating that the parallel adsorption is slightly stable than the above-mentioned case. Upon interaction of N2O at the end of tube (Fig. 3d), the adsorption energy is calculated to be 10.17 kJ mol1 and the distance between the two species is about 2.58 Å. On the other hand, the computed Ead for N2O molecule from O-side of atom is more than that of N-side atom. The lower adsorption energy shows that N2O adsorption capability of AlPNTs is lower than that of AlNNTs. On the other hand, the results indicate that the N2O molecule weakly physical adsorption on the perfect AlPNT. The results reveal that pure AlPNT cannot detect the N2O molecule. We calculated the molecular electrostatic potential (MEP) plots for N2O on walls of (6, 0) AlNNT and AlPNT (see in Fig. 6). The MEP is the interaction energy of a positive point charge with the nuclei and electrons of a molecule. Almost all the positive and negative charges of an adsorbed molecule symbolize as nucleophiles and electrophiles sites once they are initially approached to an adsorbent surface, and can also be correlated with hydrogen-bond-donating and accepting tendencies [32]. Fig. 6 demonstrates the N2O molecule functions as an electron donor and the Al atoms of (6, 0) AlNNT and AlPNT as an electron acceptor. The MEP plots show a strong positive electrostatic potential (electron poor blue color1) and weak negative electrostatic potential (electron rich – red color) occur at the end and opposite end regions of nanotubes, respectively, where the hydrogens are located. Moreover, the results of MEP show that the regions of negative electrostatic potential (denoted by red) are usually applied to identify hydrogen-bond-acceptor sites, while for describing the hydrogen-bond-donating sites the regions of positive (denoted by blue) are significant.
1
For interpretation of colour in Fig. 6, the reader is referred to the web version of this article.
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To overcome this weakness of pristine AlNNT, the Ga-doped AlNNT could be better candidate for adsorption process of N2O molecule. In the case of Ga-doped (6, 0) AlNNT, the Ga-N distance is 1.89 Å. Also, the NAGaAN bond angle in the interaction between two species is about 118.88°. For N2O/Ga-doped (6, 0) AlNNT, the NAN and NAO bond lengths of N2O are about 1.129 and 1.999 Å, respectively. The binding energy of N2O on Ga-doped AlNNT is computed to be 35.80 kJ mol1, exhibiting a physisorption characteristic. The interaction distance between the oxygen atom of N2O and the Ga atom of tube is about 2.57 Å and a slight charge (0.067e) is transferred from the Ga-doped AlNNT to the N2O (Fig. 3) by NBO analysis. After the adsorption of N2O on Ga-doped AlNNT, the charges on Ga, Al, and N atoms decreased from 1.685, 1.914 and 1.853e to 0.755, 0.890 and 0.852e, respectively. The NBO analysis indicates that with the adsorption of N2O molecule, Ga atom is less positively charged because of essential electron transfer from Ga atom to N2O. The MPA analysis demonstrates that the net charge of 0.07e is transferred from the Ga-doped AlNNT to the N2O. Our calculated study indicates that the adsorption of Ga-doped AlNNT with the N2O is slightly stronger than
Fig. 4. Optimized structures and the electronic density of states of N2O adsorption on the Ga-doped AlNNT.
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the pure tube. The electronic properties were calculated for pure Ga-doped (6, 0) AlNNT and N2O/Gadoped (6, 0) AlNNT are shown in Fig. 4 for comparison. As shown in Fig. 4, the results of DOS indicate that the electronic structure of the Ga-doped AlNNT is not sensitive to the N2O adsorption. The physisorption of N2O in the Ga-doped AlNNT show no significant changes because of N2O adsorption in the gap region of the TDOS plots. 3.2. Electronic energies and relative stabilities To further investigate the adsorption properties of N2O on (6, 0) and (4, 4) AlNNTs, we examined the electronic energies of N2O on (6, 0) and (4, 4) AlNNTs. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) in the N2O adsorption on the (6, 0) and (4, 4) AlNNTs are studied (see Table 1). Table 2 presents the results in the LUMO and HOMO energies of the N2O adsorption on the (6, 0) and (4, 4) AlN nanotubes top site obtained by the DFT calculations. The study indicate that the adsorption of N2O on the AlNNTs surface, ELUMO and EHOMO for whole models are decreased therefore both groups of occupied and unoccupied molecular orbital are more stable than those for AlNNTs. Furthermore, HOMO and LUMO orbitals are uniformly distributed throughout nanotube axis, which illustrates that covalent functionalization is preferable throughout the nanotubes (see Fig. 5). The distribution of frontier orbital in the perfect (6, 0) AlNNTs model, the HOMO is localized on the most electronegative nitrogen atoms in center and edge of the nanotube axis which is corresponds to the lone pair of electron on the nitrogen atoms. The LUMO is more localized on the center of the nanotube axis. In N2O molecule, the HOMO and LUMO are uniformly distributed on the N@N@O orbitals which are summarized in Fig. 5. We found that the HOMO is localized on the nitrogen orbitals at the center, edge, and opposite end of the nanotube axis; while the LUMO are more localized on Al and N atoms at the center and edge of the tube, and also is on the N@N@O orbital in the AlNNT. The chemical activity of the AlNNTs can be characterized by the HOMO–LUMO energy gap that is a significant parameter relying on the HOMO and LUMO energy levels. Typically, a small HOMO–LUMO energy gap means a high chemical activity and a low chemical stability. The calculated gap energies for (6, 0), (4, 4) zigzag and armchair AlNNT are in the range of 4.40–4.55 eV at the B3LYP/6-31G level
Table 1 EHOMO (eV), ELUMO (eV), chemical hardness, g (eV), chemical potential, l (eV), electrophilicity, x and maximum amount of electronic charge transfer, DNmax (a.u.), in atomic units, for N2O on (6, 0) and (4, 4) AlNNT surface at the B3LYP/6-31G(d) level. Property
N2O
(6,0) AlNNT
N2O/(6,0) AlNNTs (O-side)
(4,4) AlNNT
N2O/(4,4) AlNNT (O-side)
EHOMO (eV) ELUMO (eV) Eg (eV) DEg (eV) lD (Debye) Ead (kJ/mol1) EF (eV)
9.31 0.52 8.79 – 0.01 – 4.91
6.46 2.06 4.4 – 11.14 – 4.26
6.35 1.93 4.38 0.02 0.52 25.37 4.14
6.46 1.91 4.55 – 0.003 – 4.18
6.38 1.84 4.54 0.01 0.53 22.33 4.11
Table 2 Chemical potential (l), hardness (g), softness (S), DNmax (a.u.), and electrophilicity (x) of N2O adsorption on (6, 0) and (4, 4) AlNNT surfaces at the B3LYP/6-31G level. The parameters are in units of eV. Property
N2O
(6,0) AlNNT
N2O/(6,0) AlNNTs (O-side)
(4,4) AlNNT
N2O/(4,4) AlNNT (O-side)
[I = EHOMO] (eV) [A = ELUMO] (eV) [g = (I A)/2] (eV) [l = (I + A)/2] (eV) [S = 1/2g] (eV) [x = l2/2g] (eV) [DNmax = l/g] (a.u.)
9.31 0.52 4.39 4.91 0.11 2.75 1.12
6.46 2.06 2.20 4.26 0.23 4.12 1.93
6.35 1.93 2.21 4.14 0.22 3.88 1.887
6.46 1.91 2.27 4.18 0.22 3.84 1.84
6.38 1.84 2.27 4.11 0.22 3.72 1.81
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Fig. 5. Charge distribution of HOMO and LUMO orbitals on the (6, 0) AlNNT loaded with one N2O at the B3LYP/6-31G level.
(see Table 1). When N2O adsorbs on surfaces of (6, 0) and (4, 4) AlNNTs in stable configuration (O-side), the gap energies reduced from 4.4–4.55 to 4.38–4.54 eV, thus chemical stability of zigzag (6, 0) and armchair (4, 4) AlNNT will be decreased and hence chemical activity of such system will be slightly increased. Fig. 7 shows the influence of N2O on the electronic properties of the (6, 0) and (4, 4) AlNNTs, we have computed the density of state (DOS) plots corresponding to the adsorption of the N2O and the (6, 0) and (4, 4) AlNNTs. It can be seen that the DOS of N2O on (6, 0) and (4, 4) AlNNTs displays more reasonable changes than to the DOS of the pure (4, 4) AlNNTs, revealing slightly effect of N2O molecule in the electronic conductivity of AlNNTs. Therefore, this information indicates for N2O on (6, 0) and (4, 4) AlNNT systems the DOS near the Fermi level is not affected via the N2O adsorption. On the one hand, there exists the slightly difference of the DOS between three combined systems of N2O/AlNNTs. This results show after adsorption of N2O on AlNNT, the HOMO–LUMO energy gap of nanotube has no notable change. This is an evidence of the weak interaction between N2O and related to the AlNNTs.
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Fig. 6. The molecular electrostatic potential of N2O/(6, 0) AlNNT and N2O/(6, 0) AlPNT complexes.
3.3. Electric dipole moment The electric dipole moment vector of species has important properties that show the charge distribution when gases on the system adsorbed. While a N2O approaches to the surface of (6, 0) and (4, 4) AlNNTs, the size and direction of the electric dipole moment vector are change dependence to the adsorption configurations. Our computation indicates that during N2O adsorb for all complexes, total dipole moment increases. We consider that dipole moment for (6, 0) and (4, 4) AlNNTs are 11.15 and 0.003 Debye, respectively. In the case of the N2O adsorption on (6, 0) and (4, 4) AlNNTs are 12.00 and 1.84 Debye, respectively. Substantially, both electron transfers and dipole moment implies the
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Fig. 7. Total density of states (DOS) for an isolated N2O, N2O/(6, 0) AlNNT,N2O/(4, 4)AlNNT complexes.
concept that AlNNTs with absorption of the N2O are caused to increase the polarization and change dipole moments.
3.4. Global indices The global hardness of a species is defined as its resistance towards deformation in the presence of an electric field. Increasing in global hardness leads to increase in stability and decrease in reactivity of the species. The global indices of reactivity in the context of the DFT for N2O adsorption on (6, 0), (4, 4) zigzag and armchair AlNNTs are presented in Table 2. When N2O adsorbs on AlNNTs, hardness, electrophilicity and electronic chemical potentials of AlNNTs will be decreased, also softness will be increased. The results also indicate that when N2O is chemisorbed on the outer surface of the AlNNTs, a slight charge transfer to the N2O could occur, which suggest that their electronic transport properties could be slightly changed upon adsorptions of N2O. The direction of electron flow will be distinguished by electronegativity or electronic chemical potential. When N2O approaches on AlNNTs electrons is transferred from higher chemical potential to the lower electronic chemical potential, until the electronic chemical potentials become identical [35]. As a result, electrons will flow from a definite occupied orbital in an Al atom of AlNNTs and will go into a definite empty orbital in single N2O. The global electrophilicity index determines the energy lowering of a ligand due to maximum flow of electron from donor to acceptor species and provides information about structural stability, reactivity and toxicity of chemical species. On the other hand, the electrophilicity index determines maximum flow of electron from donor to acceptor species and supplies data connected to structural stability, reactivity and toxicity of chemical species.
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4. Conclusion In conclusion, we theoretically studied the adsorption properties of N2O molecule on AlNNTs and AlPNT and through DFT calculations. For N2O adsorption on the AlNNT and AlPNT, electronic properties of the tubes will be changed. Our calculations show that the N2O adsorption on (6, 0) AlNNTs for the most stable configuration (O-side) is about 25.37 kJ mol1. For AlNNT, the calculated binding energy for N2O in O-side is a little more than that in N-side. And the adsorption of N2Oon (6, 0) zigzag is more stable than N2O on (4, 4) armchair AlNNTs. Our computations show that N2O could not be chemisorbed on AlNNTs and AlPNTs surfaces. Decrease in HOMO–LUMO energy gaps, ionization potential, and global hardness with the adsorption of N2O on AlNNTs surface due to increase chemical reactivity and also lead to lowering of stability in the complexes. Acknowledgment We would like to thank the International Research Academy of Nanotechnology of Golestan (IRANG). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36]
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