Computational investigation of the electronic and structural propertiesof CN radical on the pristine and Al-doped (6, 0) BN nanotubes

Alireza Soltani a,n, Ali Varasteh Moradi b, Mahsa Bahari a, Anis Masoodi b, Shamim Shojaee a

a Young Researchers and Elite Club, Gorgan Branch, Islamic Azad University, Gorgan, Iranb Department of Chemistry, Gorgan Branch, Islamic Azad University, Gorgan, Iran

a r t i c l e i n f o

Article history:Received 7 April 2012Received in revised form21 July 2013Accepted 30 July 2013Available online 26 August 2013

Keywords:BNNTAdsorptionBinding energyDFTQuantum molecular descriptors

a b s t r a c t

We have performed first-principle calculations to investigate the adsorption behavior of the CN radical(CRN) on the external surface of H-capped Al-doped (6, 0) zigzag single-walled BN nanotubes (BNNT).We calculated the bond length, gap energies, dipole moments, and electronic properties of the dCN onthe exterior surface of SWBNNT. Binding energy corresponding to the most stable configuration of CNradical on AlN-doped BNNT is found to be �471.73 kJ mol�1. The calculated density of states (DOS)reveals that there is a significant orbital hybridization between dCN and Al-doping species in theadsorption process being evidence of an exothermic process. The results indicate that BNNT could be asuitable sensor.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

CNT is one of the best materials based on its interaction withother molecules such as N2O, dCN, CO, NO, Cl2, NH3, and H3COH sinceits discovery by Iijima [1]. It also have numerous applications likestorage, chemical sensors, electronic devices [2–5] owing to itsextraordinary chemical, physical, geometrical properties, in additionto a large surface making CNTs a promising candidate in nanoelec-tronics, nanoscaling, biotechnology, and biosensors [6,7]. Some otherinvestigations have been performed on the adsorption of alcoholsand hydrocarbons on CNTs [8,9] and detection of gas molecules[10,11], organic vapors [12,13], various ions, and biomolecules [14–17] are other empirical researches which have been done on theadsorption capacity of CNTs. Considering electronic properties ofCNTs, tubular diameter and chirality are two restrictions, causing awide range of investigations that have been performed to createnanotubes independent of these factors. Moreover, nanotubes con-taining atoms of groups III and V elements are considered to be asuitable substitute for C atoms of CNTs [18–23]. Ultimately, for thefirst time BNNTs were stabilized computationally in 1994 [24], thensynthesized in 1995 [25]. Subsequently, numerous experimental andtheoretical investigations have been dedicated to consider theelectronic properties and different structures of BNNTs [26–29].Moreover, the BNNTs are considered to be a polar material due tothe slight positive charges of boron (B) atoms and the slight negativecharges of nitrogen (N) atoms; while there is no polarity in CNTs, this

could be a reason to apply BNNTs in electronic and mechanicaldevices [30,31]. Recently, Baei et al. [3] have investigated theadsorption of dCN on the pure (6, 0) zigzag CNTs via first-principlestheory based on DFT calculations and they reported that theinteraction of CN radical on CNTs can improved the �CN storagecapacity. Sensitivity of BNNT to CN radical (dCRN) has beenindicated by quantum mechanics calculations. In recent years,theoretical chemists have studied the reaction mechanisms of thedCN with CH3SH, OCS, CH2CO, HCNO, O2 [32–34], and providedtheoretical results on parameters such as dipole moments, geome-trical structures and electronic properties of the dCN on (6, 0) BNNT,and Al-doped (6, 0) zigzag BNNT surfaces with four molecularorientations, N-side and C-side. In this paper, we theoreticallyinvestigate the dCN adsorption capability on the zigzag BNNT andthe effects of this adsorption on the electronic properties of BNNT toelucidate the adsorption behavior for different configurations of thedCN approaching to the outer surface of the BNNT. The optimizedgeometrics, electronegativity (χ), electronic chemical potentiality (m),global softness (S), global hardness (η), and electrophilic index (ω)[35–39] were calculated and compared with the pristine (6, 0), andAl-doped (6, 0) zigzag BNNT. We hope our results can provideexperiments with useful information to design BN-based sensors.

2. Computational methods

In this research, we examined the adsorption behavior of thedCN on pristine (6, 0) and Al-doped (6, 0) BNNT in which the endof BN nanotubes being saturated by hydrogen atoms. All theoptimizations and energy calculations are performed using

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0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.physb.2013.07.032

n Corresponding author. Tel.: þ98 938 4544921.E-mail address: alireza.soltani46@yahoo.com (A. Soltani).

Physica B 430 (2013) 20–26

Gaussian 98 program package [41] at the level of density func-tional theory (DFT) with B3LYP/6-31Gn [42]. The hydrogenated (6,0) and Al-doped (6, 0) zigzag BNNTs have 60 (B24N24H12), 60(B24N23H12Al), and 60 (B23N24H12Al) atoms, respectively. Thebinding energy (BE) of a CN radical on the pristine (6, 0) and Al-doped (6, 0) BNNT are determined through the following equation:

BE¼ EBNNT�dCN–ðEBNNTþEdCNÞ ð1Þ

BE¼ EAl�BNNT�dCN–ðEAl�BNNTþEdCNÞ ð2Þwhere EBNNT-CN is the total energy of the BNNTs interacting with theCN radical. EBNNT is total energy of the BNNT, and EdCN is the energyof an isolated dCN. Natural charge analysis with full NBO calcula-tions were performed using DFT/B3LYP method with 6-31Gn basisset for the optimized structures. The electrophilic index (ω) asdefined by Parr et al. [38] and is calculated by the followingequation:

ω¼ μ2=2η ð3ÞThus, the quantity of ω illustrates the tendency of the system toobtain additional electronic charge from the environment [37,40].The chemical potential (m) is defined according to the followingequation [36–38]:

m¼�ðEHOMOþELUMOÞ=2 ð4Þ

where EHOMO is the energy of the Fermi level and ELUMO is thefirst given value of the valance band. Global hardness (η) can beapproximated using the Koopmans' theorem [38,39] as: η¼(ELUMO–EHOMO)/2. S is defined by the following equation:

S¼ 1=2η ð5Þ

3. Results and discussion

3.1. The CN radical adsorbed on BNNTs

We evaluate the binding energies of one CN radical on theBNNT surface, the total energy of the configuration was deter-mined as a function of distance of the CN radical to the exteriorsurface of the BNNT. We fully optimized BNNT, dCN, and dCN/BNNTcomplexes. The optimized B–N bond length and bond angle of(6, 0) BNNT were found to be about 1.455 Å and 119.9 Å, respec-tively. And the average diameter is about 4.664 Å. These resultssuggest that the method used in the present calculations issuitable for describing the behavior of BNNT. Several sites areconsidered as the initial sites for the CN radical approaching to the(6, 0) BNNT via its expected active sites (i.e., N-side, C-sideapproaches to B and N atoms of BNNT). Fig. 1 represents con-sidered adsorption sites of the related BNNT, including the top

Fig. 1. Adsorption models of a dCN on the pure (6, 0) BNNT: C-side and N-side (A–F).

A. Soltani et al. / Physica B 430 (2013) 20–26 21

sites directly above the boron (B-site) and nitrogen atoms (N-site),of the related tubes.

The potential energy surfaces (PES) were calculated betweendCN and the considered surfaces of BNNT and the total energy ateach site. The distance between dCN and the surface are in therange of 1.0–2.0 Å. As represented in Fig. 1A, the calculated bindingenergy is about �129.82 kJ mol�1. The computed binding energyvalues indicate that the interaction between the CN and the (6, 0)BNNT is rather strong. The binding energies for all consideredcomplexes are summarized in Table 1.

The interaction studies indicate that the adsorption energy for allsystems is rather negative. The absolute values for these energies arein the range of 9–129 kJ mol�1. The calculated binding energy for CNradical from C-side of atom is more stable than that of N-side of atom[3]. The most stable configurations of CN radical for both N-side andC-side on the (6, 0) SWBNNT are presented in Fig. 1A and D, theperpendicular closeness of the CN radical to the (6, 0) SWBNNT wallon the upper hexagon, and the current calculation shows that theadsorption energy for this sites in C and N sides are �129.82 and�91.04 kJ mol�1 with an equilibrium distance (D) of 1.59 Å and1.54 Å, respectively (Fig. 1A and D). Therefore, the adsorption energyof the CN radical on (6, 0) BNNT at each particular sites of the

interaction is slightly different. For Fig. 1B, our calculations indicatethat the binding energy and interaction distance of CN radical from itsC side on the B atom of (6, 0) BNNTs are �119.62 kJ mol�1 and1.522 Å while these values in configuration E are �119.59 kJ mol�1

and 1.611 Å, respectively. In contrast with configuration D, the valuesof binding energy in configurations C and F are about �90.39andþ9.60 kJ mol�1 with distances of 1.386 and 1.521 Å, respectively.When the dCN adsorbed upon the B atom of (6, 0) BNNT surface a localstructural deformation are observed as the B–N and CRN lengths inadsorbent and adsorbate are mounted from 1.449 and 1.174 Å to 1.525and 1.163 Å, respectively, attributing to a change from sp2 to sp3

hybridization between two species (see Table 1). Natural chargeanalysis shows that in this configuration 0.28 electron is transferredfrom (6, 0) BNNT to the CN radical. This result reveals that the dCN actsas an electron acceptor and the tube acts as an electron donor. Weexplore the adsorption of dCN for the most stable configuration on theB atoms of (8, 0) and (5, 5) BNNTs (see Fig. 2). The current calculationindicated that the adsorption of �CN towards (8, 0) and (5, 5) BNNTsare �90.96 and �82.21 kJ mol�1 and the adsorption distances are1.65 and 1.67 Å, respectively. We observed that when the tubediameter increases (see Fig. 2), the adsorption energy of dCN interactedwith BNNT decreases at the interaction site. These results reveal that

Table 1Binding energy (BE) value (kJ mol�1), equilibrium distance (R) (Å) of CN on (6, 0) BNNT surface at the B3LYP/6–31G* level of theory.

Property dCN (6, 0) BNNT dCN/BNNT AlB BNNT AlN BNNT dCN/AlN BNNT dCN/AlB BNNT

EHOMO, eV 1.29 �6.68 �6.56 �6.64 �5.41 �5.70 �6.81ELUMO, eV 8.40 �1.79 �2.15 �1.88 �2.92 �2.29 �2.15Eg, eV 7.12 4.89 4.41 4.76 2.49 3.40 4.66mD, Debye 0.014 7.96 10.81 8.65 8.29 10.56 7.40Ead, kJ mol�1 – – �129.82 – – �471.43 �295.43D, Å – – 1.598 – – 1.938 1.945DB–N, Å – 1.449 1.525 – – – –

DCRN, Å 1.174 – 1.163 – – 1.166 1.165DAl–N, Å – – – 1.767 – 1.938DAl–B, Å – – – 2.035 2.054 –

EF, eV 4.84 �4.23 �4.35 �4.26 �4.16 �3.99 �4.48ΔEg, eV – – 0.48 – – 0.91 0.1I¼�EHOMO, eV �1.29 6.68 6.56 6.64 5.41 5.70 6.81A¼�ELUMO, eV �8.40 1.79 2.15 1.88 2.92 2.29 2.15η¼(I�A)/2, eV 3.55 2.44 2.00 2.38 1.24 1.70 2.33m¼�(IþA)/2, eV 4.84 �4.19 �4.35 �4.26 �4.16 �3.99 �4.48S¼1/2η, eV 0.14 0.20 0.25 0.21 0.40 0.58 0.214ω¼m2/2η, eV 3.30 3.68 4.73 3.81 6.98 4.68 4.30

Fig. 2. Adsorption models of dCN on the pure (8, 0) and (5, 5) BNNT: C-side (A and B).

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(6, 0) BNNT is more reactive for CN adsorption than that of (8, 0) and(5, 5) BNNTs.

3.2. The CN radical adsorbed on the Al-doped BNNTs

We examined the effect of metal doping on the adsorptionbehavior of CN radical on Al-doped (6, 0) BNNTs system for themost stable configuration (one boron and nitrogen atoms sub-stituted by one Al atom), and the current calculation indicated thatCN radical can significantly be adsorbed on Al (AlN and AlB) sites,and the calculated binding energy (BE) for CN radical in C-side ismore than that in N-side, as represented in Fig. 3.

The bond length between Al–N and Al–B of the Al-doped (6, 0)BNNT are about 1.938 and 2.228 Å, respectively. The most stableconfiguration indicates that AlN site is stronger than AlB site. We alsofind that doping of AlN is energetically more favorable than that of theAlB, and the current calculation for CN in C and N sides upon the AlNindicate that the adsorption energies for these sites are �471.43 and�461.29 kJ mol�1 with an equilibrium distance (D) of 1.94 and 1.83 Å,respectively. Adsorption energies for AlB site in C and N sides are�295.43 and �101.29 kJ mol�1 with an equilibrium distance (D) of1.95 and 1.83 Å, respectively. The current computations indicate forchemisorption phenomena [43]. These strong binding energies can beverified due to the interaction between π-electrons of CN and π-electrons of the BNNT surfaces. The distance of CN radical to thesurface seems to depend significantly on the adsorption configurations

and curvature of the surfaces [44]. The finding shows that theAlN-doped on (6, 0) BNNT is much more stable than AlB-doped on(6, 0) BNNT. These studies indicated when a dCN are chemisorbed onthe Al atom of BNNT, the binding energy notably increases incomparison with the pristine BNNT systems which reflect the sig-nificant role of Al atom in adsorption behavior of BNNTs as sensor [22].Our charge analyses for both corresponding complexes illustrate that0.53 and 0.56 electrons are transferred from the electron-rich AlN andAlB (6, 0) BNNT systems to the CN radical, respectively.

3.3. Electronic energies and relative stabilities

To further investigate the adsorption phenomenon of the CNradical on (6, 0) BNNT, we examined the electronic energies of theBNNT-dCN systems. The highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO) inthe CN radical, pristine, and Al-doped (6, 0) BNNT are studied (seeTable 1). Table 1 presents the results of the LUMO and HOMOenergies of the CN radical adsorbed on the mentioned sites thatare obtained by the DFT calculations. These findings indicate whenthe CN radical absorbed on (6, 0) pristine and Al-doped BNNT,ELUMO and EHOMO for whole systems were reduced, thereforemolecular orbitals of both occupied and unoccupied groups arelower and more stable than those of the (6, 0) BNNT (see Fig. 4).

We found that the HOMO is localized on the C–N bond and isslightly on nitrogen orbital, while the LUMO is localized at the

Fig. 3. Adsorption models of dCN on the Al-doped (6, 0) BNNT: C-side and N-side (A–D).

A. Soltani et al. / Physica B 430 (2013) 20–26 23

opposite end of the tube while the charges are distributed on theboron and nitrogen atoms in the (6, 0) BNNT. The distribution offrontier orbital in Al-doped (6, 0) BNNT systems exhibit that theHOMO is localized on C–N orbitals and domiciled on the AlN site, andalso slightly accommodated on the more electronegative nitrogenatoms at the center and end of nanotube axis in AlN and AlB sites,corresponding to the lone pair of electron on the nitrogen atoms. TheLUMO is more localized on the boron and nitrogen atoms in parallel,end, and center of the nanotube axis; it is localized at the opposite endof the nanotube axis, while the charge distribution in boron andnitrogen atoms are mainly visible at BNNT (see Fig. 5).

The results demonstrated that AlN-doped (6, 0)-BNNT is morereactive than AlB-doped (6, 0)-BNNT due to charge transfer whichoccurred on the BNNT layer. The chemical activity of SWBNNT canbe characterized by the LUMO–HOMO energy gaps that are asignificant parameter relying on the HOMO and LUMO energylevels. Typically, the small LUMO–HOMO energy gaps mean a highchemical activity and a low chemical stability [35]. The calculatedenergy gaps for the isolated (6, 0), and Al-doped zigzag BNNT atthe B3LYP/6-31Gn method are in the range of 4.89–4.76 eV (seeTable 1). When CN radical is adsorbed on the pristine (6, 0) and theAl-doped (6, 0) BNNT surfaces in different configurations, theenergy gaps reduced from 4.89–4.76 to 4.41–4.66 eV, respectively,

thus chemical stability of zigzag BNNTs will be decreased with asignificant increase in chemical activity.

3.4. Quantum molecular descriptors

The employed HOMO–LUMO energy gaps for obtaining globalparameters such as hardness, electronegativity, chemical potential,maximum amount of electronic charge, and electrophilicity index arecalculated. The global hardness of a species is defined as its resistancetowards deformation in presence of an electric field. Increase inhardness and decrease in softness will result in the increase of stabilityand decrease in reactivity of a system [35]. The global indexes ofreactivity in the context of DFT method for CN radical adsorption onthe pristine (6, 0) and Al-doped-BNNT are presented in Table 1. WhenCN radical is adsorbed on BNNT, hardness, electrophilicity andelectronic chemical potentials of bare BNNT are decreased, while thesoftness increased. The results also demonstrate, a fairly large chargetransfer to the CN radical occurs when CN radical is chemisorbed onthe outer surface of the Al-doped/BNNT, suggesting that their electro-nic transport properties are significantly changed upon the chemi-sorption of CN radical on the applied nanotubes. The direction ofelectron flow can be distinguished by electronegativity or electronicchemical potential. When the CN radical approaches to Al-doped/

Fig. 4. Charge distribution of HOMO and LUMO orbitals on the pristine (6, 0) BNNTs loaded with one dCN at the B3LYP/6-31G* level.

A. Soltani et al. / Physica B 430 (2013) 20–2624

BNNT, electrons are transferred from higher chemical potential to thelower electronic chemical potential, until the electronic chemicalpotentials become identical [35]. As a result, electrons can flow froma definite occupied orbital in Al-doped-BNNT and will go into adefinite empty orbital in a CN radical.

3.5. Electric dipole moment

The electric dipole moment vector of the species is importantproperties that exhibit the charge distribution when the gases areadsorbed on system. While a dCN approaches to the surface (6, 0) andAl-doped (6, 0) BNNT, the size and direction of the electric dipolemoment vector is changed owing to the adsorption configurations.The results of electric dipole moment for CN radical show that theadsorption energies are at its highest value (see Table 1). The resultdemonstrates that during CN radical adsorption, total dipole moment(mD) increases. We also consider dipole moment from pristine BNNTs(7.96 Debye) to the CN radical/(6, 0) BNNT system about 10.81 Debye,and the dipole moment for Al doped in AlN and AlB sites (6, 0) BNNTare about 8.23 and 8.65 Debye, respectively. The dipole momentincreases to 10.56 Debye for CN radical absorbed on the Al-doped-BNNT, (AlN site), while it decreases to 7.40 Debye for CN radical/Al-doped-BNNT system, (AlB site). Substantially, both electron transferand dipole moment indicate for the increase in polarization andchange of dipole moment of the applied systems.

3.6. The densities of states (DOSs)

To investigate the adsorption influence of (6, 0) and Al-doped (6, 0)on the electronic properties of the BNNTs, we have computed theelectronic density of states (DOSs) for all of the applied complexes. TheDOS of these complexes were analyzed to understand the chemicalbinding through electronic structures. Consequently, the DOS for theCN radical on (6, 0) BNNT system was compared with that of thelonely CN radical and pristine BNNTs. The results are depicted in Fig. 6.

The DOS results indicate a strong orbital mixing near the Fermilevel and involvement of covalent adsorption between the CN and AlN-doped (6, 0) BNNT, as a result, CN radical approaching on the AlN-doped (6, 0) BNNT may changes the electronic conductivity in theconsidered system (see Fig. 6).

4. Conclusions

In this research, computational studies are performed to evalu-ate the binding energy of CN radical on the zigzag configurations of(6, 0) and Al-doped (6, 0) BNNT using DFT methods. On the basis ofour calculations, it seems that Al-doped-BNNT can be used as a CNradical detector in comparison with (6, 0) BNNT. For the Al-doped(AlN site) BNNT the calculated adsorption energy for CN radical in C-side is a little more than N-side. For CN radical adsorption on zigzagAlN-doped (6, 0) BNNTs, electronic properties of the tubes are

Fig. 5. Charge distribution of HOMO and LUMO orbitals on the pristine Al-doped (6, 0) BNNTs loaded with one dCN at the B3LYP/6-31G* level.

A. Soltani et al. / Physica B 430 (2013) 20–26 25

changed after the adsorption process. The current calculationindicates that decrease in the energy gap, global hardness, ioniza-tion potential are due to the adsorption of CN radical on (6, 0), Al-doped (6, 0) BNNT, while the lowering of stability is due to anincrease in chemical reactivity.

Acknowledgments

We would like to thank the Nanotechnology WorkgroupResearchers and Elite Club of the Islamic Azad University, Gorganbranch, Iran.

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Fig. 6. The total density of states (DOS) for an isolated dCN on (6, 0) and Al-doped (6, 0) BNNT for the most stable configuration.

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