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Adsorption properties of N 2O on (6,0), (7,0), and (8,0) zigzag single-walled boron nitride nanotubes: a computational study

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Adsorption properties of N 2O on (6,0), (7,0), and (8,0) zigzag single-walled boron nitride nanotubes: a computational study
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  This article appeared in a journal published by Elsevier. The attachedcopy is furnished to the author for internal non-commercial researchand education use, including for instruction at the authors institutionand sharing with colleagues.Other uses, including reproduction and distribution, or selling orlicensing copies, or posting to personal, institutional or third partywebsites are prohibited.In most cases authors are permitted to post their version of thearticle (e.g. in Word or Tex form) to their personal website orinstitutional repository. Authors requiring further informationregarding Elsevier’s archiving and manuscript policies areencouraged to visit:http://www.elsevier.com/copyright  Author's personal copy Adsorption properties of N 2 O on  (6,0), (7,0) , and  (8,0)  zigzag single-walledboron nitride nanotubes: A computational study Mohammad T. Baei a, ⇑ , Ali Reza Soltani b , Ali Varasteh Moradi c , E. Tazikeh Lemeski c a Department of Chemistry, Azadshahr Branch, Islamic Azad University, Azadshahr, Golestan, Iran b Young Researchers Club, Gorgan Branch, Islamic Azad University, Gorgan, Iran c Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran a r t i c l e i n f o  Article history: Received 17 February 2011Received in revised form 13 May 2011Accepted 13 May 2011Available online 20 May 2011 Keywords: Boron nitride nanotubeAdsorptionBinding energyDFT a b s t r a c t The behavior of N 2 O adsorbed on the external surface of H-capped  (6,0), (7,0) , and  (8,0)  zigzag single-walled boron nitride nanotubes was studied by means of DFT and ab initio calculations. Geometryoptimizations were carried out at the B3LYP/6-31G  and MP2/6-311+G  levels of theory using theGaussian 03 suites of programs. We present the nature of the N 2 O interaction in selected sites of thenanotubes. Thecalculationsindicatedthat pristinetheBNNTscannot beusedasanN 2 Ostoragemedium.BindingenergiescorrespondingtoadsorptionoftheN 2 Oarecalculatedtobeintherange1–6kJmol  1 .Inall pathways for the zigzag configurations of   (6,0) ,  (7,0) , and  (8,0) , the N 2 O parallel to the BNNTs arethe most stable configurations. Comparison of the calculated binding energies of obtained at theB3LYP/6-31G  and MP2/6-311+G  levels of theory indicated that the calculated bonding energies forN 2 O at the B3LYP/6-31G  method are more than that at MP2/6-311+G  method. More efficient bindingenergies cannot be achieved by increasing the nanotube diameter. We also provide the effects of N 2 Oadsorption on the electronic properties of the nanotubes.   2011 Elsevier B.V. All rights reserved. 1. Introduction Synthesis of carbon nanotubes (CNTs) by Ijima [1] caused aburst of activity by both physical and chemical properties [1–3]andapplicationsasanovelmaterial[4,5].Theelectronicpropertiesof CNTs depend on their tubular diameter and chirality. Manyinvestigations have been undertaken to investigate non-carbonbased nanotubes, which exhibit electronic properties independentof these features. Among these, boron nitride nanotubes (BNNTs),which are made from the groups III and V elements neighboringC in the Periodic Table, are an interesting subject of many studies[6–8]. Boron nitride nanotubes (BNNTs) are inorganic proportionof carbon nanotubes (CNTs) and have good physical propertiesfor a broad variety of applications [9]. BNNTs are semiconductorswith almost the same band gaps of 5.5eV [10] and they are chem-ically and thermally stable [11–14]. Because of the slight positivecharge of B and slight negative charge of N, the polarity and ionic-ity of the nanotubes are increased. Therefore, BNNTs are consid-ered as more appropriate materials than CNTs for applications inspecific electronic and mechanical devices. BNNTs examined asnew materials for hydrogen storage [15]. Also, BNNTs have beenexperientially investigated for detection of hydrogen molecules[16]. Moreover, electronic conductance of a BNNT semiconductorcan be changed upon exposure to gas molecules, serving as a basisfor nanotube molecular sensors.Sensitivity of BNNTs to N 2 O has been indicated by means of quantum mechanics calculations. The determination of the struc-ture of adsorbed N 2 O on BNNTs surfaces is also important forunderstandingitsbondingandreactivityincatalysisandothersur-face phenomena. Nitrous oxide (N 2 O) has been generated as a by-product in nitric and adipic acids, and its decomposition into N 2 and O 2  is a topic of biotic interest for environmental chemistry[17–19]. Thestudy of the chemical reactionsof N 2 OonBNNTs sur-faces is of scientific importance because N 2 O has been recognizedas an environmental pollutant and a relatively strong greenhousegas [17,18,20,21], N 2 O is also a significant contributor to thedestruction of the ozone layers in the stratosphere. At present,the N 2 O concentration in the aerosphere is rising almost 0.25%,everyyear [22]. It is verydesirabletofindefficient andeconomicalmethods to convert harmful N 2 O into unharmful gases such as N 2 through catalytic surface reactions, i.e., N 2 O ? N 2  +O(s) (O(s) de-notesanOatomadsorbedonthesurface). Thereisincreasedinter-est in using BNNTs instead of noble metal in the environmentalcatalysis fields.The reactions of N 2 O with alkaline earth oxides [23–29], TiO 2 [30], molecular zeolite [31–37], metals [38], and isolated Cu + [39,40] have been largely studied in many fields. However, to our 2210-271X/$ - see front matter   2011 Elsevier B.V. All rights reserved.doi:10.1016/j.comptc.2011.05.021 ⇑ Corresponding author. Tel.: +98 9111751399. E-mail address:  Baei52@yahoo.com (M.T. Baei).Computational and Theoretical Chemistry 970 (2011) 30–35 Contents lists available at ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc  Author's personal copy knowledge, no experiments and theoretical investigation havebeen reported on the adsorption of N 2 O on BNNT surfaces. Theunderstanding of the physisorption of N 2 O on BNNTs surfaces isimportant for N 2 O storage. In this study, we report the results of computational calculations on the physisorption of N 2 O on  (6,0),(7,0) , and  (8,0)  zigzag single-walled BNNT surfaces with fourmolecular orientations, N- and O-down and parallel models at fivedistinct sites. 2. Computational methods In the present work, adsorption behaviors of the N 2 O on the sin-gle-walled BNNTs were studied by using the representative modelsof   (6,0), (7,0) , and  (8,0)  zigzag single walled BNNTs with fourmolecular orientations, N- and O-down and parallel models inwhich the ends of the nanotubes are saturated by hydrogen atoms.The hydrogenated (6,0), (7,0) , and  (8,0)  zigzag single-walled BNNTs Fig. 1.  (a) and (b) adsorption configurations of N 2 O on BNNTs: O-down (left) and N-down (right), (c) and (d) adsorption configurations of N 2 O in parallel models.  Table 1 Binding energy value (kJ mol  1 ), equilibrium distance (rd, Å), and band gap (eV) of N 2 O on zigzag of   (6,0) ,  (7,0) , and  (8,0)  BNNTs at the B3LYP/6-31G  method. Model Band gap (eV) R (Å) Binding energy (BE) SiteN B Z Center Parallel (6,0)  4.89 N A B1 = 1.449N A B2 = 1.458N-down BE   2.01   2.61   2.37   2.20   5.9rd 4.0 3.5 3.5 4.0 3.5O-down BE   2.74   2.58   2.75   2.85   6.02rd 3.5 3.5 3.5 3.5 3.5 (7,0)  5.35 N A B1 = 1.450N A B2 = 1.454N-down BE   1.04   1.50   1.15   1.09   2.98rd 4.0 3.5 3.5 4.0 3.5O-down BE   1.82   1.56   1.59   1.79   3.01rd 3.5 3.5 3.5 3.5 3.5 (8,0)  5.69 N A B1 = 1.449N A B2 = 1.452N-down BE   0.01   0.88   0.65   0.21   1.34rd 4.0 3.5 3.5 4.0 3.5O-down BE   0.95   0.96   1.01   0.89   1.58rd 3.5 3.5 3.5 3.5 3.5 M.T. Baei et al./Computational and Theoretical Chemistry 970 (2011) 30–35  31  Author's personal copy have 60 (B 24 N 24 H 12 ), 70 (B 28 N 28 H 14 ), and 80 (B 32 N 32 H 16 ) atoms. Inthe first step, the structures were allowed to relax by all atomicgeometrical optimization at the B3LYP/6-31G  and MP2/6-311 + G  methods.The use of localized basis sets reliability reduces the amount of computational work required when using with large vacuum re-gions in the unit cell. However, finiteness of the localized basis setsguidance to basis set superposition errors (BSSE) that described byTournusetal.instudyofbenzeneoncarbonnanotubes[41].Forpassof this problem, BSSE has been estimated for the calculated struc-turesbyB3LYPandMP2methods.Usingthesemethods,thebindingenergy (BE) of an N 2 O on the BNNTs wall was calculated as follows: BE ¼ E BNNT  N 2 O ð E BNNT þ E N 2 O Þþ d BSSE  ð 1 Þ  Table 2 Binding energy value (kJ mol  1 ), equilibrium distance (rd, Å), and band gap (eV) of N 2 O on zigzag of   (6,0) ,  (7,0) , and  (8,0)  BNNTs at the MP2/6-311 + G  method. Model Band gap (eV) R (Å) Binding energy (BE) SiteN B Z Center Parallel (6,0)  4.85 N A B1 = 1.430N A B2 = 1.448N-down BE 0.10   0.38 0.00 0.00   1.32rd 4.0 3.5 4.0 4.0 4.0O-down BE   0.98   0.92   0.94   0.86   1.34rd 3.5 3.5 3.5 4.0 4.0 (7,0)  5.25 N A B1 = 1.431N A B2 = 1.444N-down BE   0.09   0.32 0.03   0.08   0.65rd 4.0 3.5 3.5 4.0 3.5O-down BE   1.26   1.01   0.77   0.79   0.87rd 3.5 3.5 3.5 4.0 4.5 (8,0)  5.55 N A B1 = 1.429N A B2 = 1.445N-down BE   0.25   0.71   0.63   0.19   0.81rd 4.0 3.5 4.0 4.0 4.5O-down BE   1.33   1.45   1.47   0.84   0.92rd 3.5 3.5 3.5 4.0 4.5 Fig. 2.  Binding energy curves of N 2 O (O-down and N-down) adsorption at B, N, Z, center, and parallel sites on zigzag of   (6,0) ,  (7,0) , and  (8,0)  BNNTs at the B3LYP/6–31G  method.32  M.T. Baei et al./Computational and Theoretical Chemistry 970 (2011) 30–35  Author's personal copy where E BNNT  N 2 O  was obtained from the scan of the potential energyof the BNNTs-molecular nitrous oxide structure, E BNNT  is the energyof the optimized BNNT structure, E N 2 O  is the energy of an optimizedN 2 O, and  d BSSE  is the BSSE correction. All the calculations werecarried out by using the Gaussian 03 suite of programs [42]. 3. Results and discussion An N 2 O molecule can approach the nanotube walls from outside(out), which is the most common case, and from the inside (in).Zigzag configurations of   (6,0), (7,0) , and  (8,0)  SWBNNTs have twodifferent B A N bonds (N A B 1  and N A B 2 ) (see Fig. 1a and b; Tables 1 and 2). For the adsorption of the N 2 O (N-down and O-down)and parallel models: N @ N @ O parallel (Fig. 1c) and O @ N @ N parallel(Fig. 1d) on the BNNTs, we considered five possible sites (i.e., the Csite (center site) above the hexagon, the N, B sites above the nitro-gen and boron atoms, the Z site above the zigzag and axial B A Nbond, and parallel sites) as described in Fig. 1. The notation N downand O down denotes an N 2 O perpendicular to the surface via N andO.We limited our analysis to the interaction of N 2 O with the nano-tubes’ outer walls. Considering each site and configuration, weended up with 16 different approaches of N 2 O to the BNNTs walls.For each of these cases we investigated the BNNT-N 2 O potentialenergy surface (PES) at the B3LYP/6-31G  and MP2/6-311 + G  methods. We have also compared the B3LYP/6-31G  results withthe results of MP2/6-311 + G  study in interaction of the N 2 O withthe nanotubes. The binding energies of the N 2 O (N-down and O-down) and parallel models at the five sites on the zigzag configu-rations of   (6,0), (7,0) , and  (8,0)  SWBNNTs at the B3LYP/6-31G  and MP2/6-311 + G  methods are plotted in Figs. 2 and 3, andthe binding energy with the equilibrium distance in each case issummarized in Tables 1 and 2.In all pathways for the zigzag of  (6,0) ,  (7,0) , and  (8,0)  BNNTs, thepotential is not attractive; presenting a maximum of    6 kJ mol  1 ,which does not characterizes a chemisorption process. The bindingenergies obtained from these calculations are slightly dependenton orientations and locations of the N 2 O. The calculated BE(binding energy) of the BNNTs indicated that N 2 O cannot beabsorbed on the sites; the BEs for the different sites have verysmall differences in total energy ( < 2 kJ mol  1 ); and the calculatedBE for N 2 O in O-down is more than that in N-down, unlike of N 2 O adsorption on CNTs that BE in N-down is more than that inO-down [43]. In all pathways for the zigzag of   (6,0) ,  (7,0) , and (8,0) , the N 2 O parallel to the BNNTs are the most stable configura-tions. The current calculation shows that the adsorption energiesfor N @ N @ O parallel site (Fig. 1c) in zigzag of   (6,0), (7,0) , and  (8,0) Fig. 3.  Binding energy curves of N 2 O (O-down and N-down) adsorption at B, N, Z, center, and parallel sites on zigzag of   (6,0) ,  (7,0) , and  (8,0)  BNNTs at the MP2/6–311 + G  method. M.T. Baei et al./Computational and Theoretical Chemistry 970 (2011) 30–35  33
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