As

ZC

ARRAA

KCIDN

1

irdadhclsastdet[itnc

0h

Sensors and Actuators B 185 (2013) 512– 522

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h om epage: www.elsev ier .com/ locate /snb

comparative theoretical study of CO2 sensing using inorganic AlN, BN and SiCingle walled nanotubes

abiollah Mahdavifar ∗, Nasibeh Abbasi, Ehsan Shakerzadehomputational Chemistry Group, Department of Chemistry, Faculty of Science, Shahid Chamran University, Ahvaz, Iran

a r t i c l e i n f o

rticle history:eceived 21 December 2012eceived in revised form 10 April 2013ccepted 1 May 2013vailable online 21 May 2013

eywords:

a b s t r a c t

The adsorption of CO2 gas molecule on the armchair (4, 4) aluminum nitride (AlN), boron nitride (BN)and silicon carbide (SiC) nanotubes are investigated using DFT calculations. The combining processes ofgas adsorption on all different sites of AllNT, BNNT and SiCNT are exothermic and the relaxed geome-tries are stable. Our results reveal that the interaction between CO2 molecule and BNNT and SiCNT areweak, so that the adsorption of CO2 onto the BNNT and SiCNT is physisorption process. However, theAlNNT exhibits strong affinity toward the CO2 gas molecule. Compared with the weak adsorption on

O2 sensingnorganic nanotubesFT calculationBO analysis

BN and SiC nanotubes, CO2 molecule tends to be strongly chemisorbed to the AlNNT with appreciableadsorption energy (about −117 kJ/mol). The adsorption of CO2 molecule onto the AlNNT would affect theelectronic conductance and mechanical properties, which could serve as a signal of gas sensor. It seemsthat the considerable charge transfer from AlNNT to CO2 molecule is occurred due to the reduction of theHOMO–LUMO energy gap. Based on the obtained results, it is expected that AlNNT could be a promising

evice

candidate in gas sensor d

. Introduction

Since the discovery of carbon nanotubes [1], new branchesn material science and technology have opened and grown upapidly. Nanoscale materials have attracted considerable interestue to their novel properties as well as extreme different function-lity compared to their bulk counterparts. In the context of sensorevelopment work, gas sensors based on carbon nanotube (CNT)ave attracted intensive attention due to their excellent sensingapabilities, such as high sensitivity, fast response, small size andow operating temperature [2–5]. Two types of sensors based oningle walled nanotubes (SWNTs) that have received the mostttention are electromechanical sensors and gas/chemical/bio sen-ors. The basis of sensing mechanism of the both sensors relatedo the change in the electrical conductance. The electrical con-uctance changes can be directly related to either changes in thelectronic band structure due to mechanical perturbation, or par-ial electron charge transfer between gas molecule and nanowires6]. Semiconducting single walled nanotubes (SWNTs) as chem-cal sensors could be detected pollutant and toxic gases [7]. In

he recent years, significant attempts have made to investigateew nanotube sensors from both experimental and theoreti-al point of views. Semiconducting nanotube such as aluminum

∗ Corresponding author. Tel.: +98 9163015227; fax: +98 611 3331042.E-mail addresses: zb.nojini@gmail.com, zb nojini@scu.ac.ir (Z. Mahdavifar).

925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.snb.2013.05.004

s for detecting the CO2 molecule.© 2013 Elsevier B.V. All rights reserved.

nitride (AlN), boron nitride (BN), silicon carbide (SiC) nanotubesand doped nanotubes could provide very high sensitivity due totheir large surface to volume ratios and their unique electronicproperties.

Boron nitride nanotubes (BNNT) were first grown in 1995, andpossess a similar structure to CNTs [8]. Although they have wideband gaps (∼5 eV), theoretically, their band gaps could be tunedand even eliminated by transverse electric fields via the giant dcStark effect [9–11]. In addition, BNNTs are resistant to oxidation upto 800 ◦C [12], have excellent piezoelectricity [13,14] and poten-tial hydrogen storage capability [15]. These properties make BNNTswell recognized as a good complement to CNTs in future nanotech-nology. Similar to the BNNT, SiCNT is a semiconductor with a largeband gap, and is of technological interest for devices operate at hightemperatures, high frequency, and in harsh environment [16]. TheSiCNT could be applied as chemical gas sensors because of theirhigher reactivity. Theoretical studies show that O2 [17], H2 [18],CO and HCN [19], NO and N2O [20] could be chemisorbed on theexterior surface of SiCNT with large binding energy which indi-cate that SiCNT could act as sensor. However, group III nitridessemiconductor nanostructures such as boron nitride nanotubes(BNNTs) have attracted more attention because of their wide bandgaps. Similar to BNNTs, aluminum nitride nanotube (AlNNT) isone-dimensional III–V semiconductor nanostructure, which has

been an important research target because of its unique charac-ters. Aluminum nitride nanostructures have been the subject ofintensive experimental and theoretical studies due to their out-standing properties such as direct wide band gap (∼6.2 eV), high

d Act

tp[iAna[sid[

tbtit

octat

F

Z. Mahdavifar et al. / Sensors an

hermal conductivity [21,22], superior mechanical strength, highiezoelectric response and small or even negative electron affinity23–25]. Note that the synthesis of AlNNTs have been a challeng-ng task, but using DFT calculations Zhang et al. predicted thatlNNTs are energetically favorable and arrange in a hexagonaletwork adopting an sp2 hybridization [25]. Recently, the inter-ctions of an open ended single walled AlNNT with CO2 and N226] molecules are investigated using DFT calculations. In addition,ome theoretical studies reported the ability of AlNNT for adsorb-ng molecules such as H2O, N2 and O2 [21], CH4 [27]. Furthermore,oped SWCNTs have proven to be highly sensitive to small gases28–32].

Carbon dioxide (CO2) is the main greenhouse gas contributing tohe climate change, which is increased drastically due to the com-ustion of fossil fuels and chemical processing. Therefore, it seemshat finding fast and simple methods for capturing and monitor-ng of this gas is necessary to prevent environmental disasters, andhese have stimulated research activities in this field.

In the present study, it is attempted to scrutinize the propertiesf CO2 adsorption onto pristine AlNNT, BNNT and SiCNT using DFT

alculations through periodic boundary conditions (PBC). In addi-ion, the electronic properties of CO2 adsorbed onto the nanotubesre investigated. The obtained results may provide a new insight tohe gas sensing and monitoring nanotechnology.

ig. 1. Unit cell of (a) AlNNT, (b) BNNT and (c) SiCNT nanotubes, four possible sites for ga

uators B 185 (2013) 512– 522 513

2. Computational details

The interactions of CO2 gas molecules with AlNNT, BNNT andSiCNT are evaluated within the framework of density functionaltheory. The generalized gradient approximation (GGA) in formof Perdew–Burke–Ernzerhof (PBE) correction [33] is applied fordescribing the exchange-correlation term. The calculations wereperformed at PBEPBE/6-31G level of theory for all atoms. A single-wall armchair [4,4] AlNNT, BNNT and SiCNT, depicting in Fig. 1,is considered in this study. One-dimensional periodic boundarycondition (PBC) is applied along the tube axis. The number of cellalong the tube axis is selected in such a way that the gradient ofabsolute value of the total dipole moment and the total energybecome zero by increasing the number of unit cells. In this case, sixk points sampling in the Brillouin zone are employed for AlNNT,BNNT and SiCNT nanotubes. In addition, the length of the consid-ered unit cell is about 6.41 A (see Fig. 1). The gas molecule is locatedalong the tube axis direction in each unit cell as lateral distancebetween gas molecules is at least 6.414 A to eliminate the inter-actions between gas molecules in neighboring cells. Furthermore,

Natural Bond Orbital (NBO) analysis is performed by PBEPBE/6-31G without using PBC method. The length of tubes in this caseis ∼22 A. In addition, the two ends of AlNNT, BNNT and SiCNTare terminated with hydrogen atoms in the NBO calculations.

s adsorption at Al (top of the Al), N (top of the N), H (top of the center of hexagon).

5 nd Act

A[

imBit(

E

waariegbabe

E

E

E

war

ctt

cwcta

U

U

wrtp

qnmbtaJn

�

�

14 Z. Mahdavifar et al. / Sensors a

ll calculations are performed using Gaussian03 package34].

The different adsorption sites of AlNNT, BNNT and SiCNT includ-ng top of the each atom, top of the hexagonal ring (H site). CO2

olecule was oriented parallel or perpendicular to the AlNNT,NNT and SiCNT surfaces were also investigated. The details of the

nitial structures are available in supplementary material. In ordero understand the gas–tube interactions, the adsorption energyEads) of the gas onto nanotubes is defined as:

ads = Etube–gas − (Etube + Egas) (1)

here Etube–gas denotes the total energy of the adduct AlNNT, BNNTnd SiCNT with the corresponding gas molecule and Etube and Egas

re the total energies of the isolated nanotubes and gas molecule,espectively. According to the Eq. (1), negative adsorption energyndicates the formed complex is stable and positive adsorptionnergy belongs to the local minimum in which the adsorption ofas molecule onto the nanotubes is prevented by a barrier. It shoulde mentioned that the adsorption energy encompasses both inter-ction (Eint) and deformation (Edef) energy contributions, which areoth occurred during the adsorption process. Hence, the followingquations are applied to calculate these contributions

ads = Edef + Eint (2)

int = Etube–gas − (Etube in complex + Egas in complex) (3)

def = Edef gas + Edef tube (4)

here Etube in complex/Egas in complex is the total energy of tube/gasnd Edef tube/Edef gas is the deformation energy of tube/gas in itselaxed geometry.

The electronic structure of the most energetically favorableomplexes was analyzed according to the partial and net chargeransfers, which obtained from Natural Bond Orbital (NBO) analysiso estimate the sensing capability.

Based on PBEPBE/6-31G calculations, the adsorption energyurves (see Fig. 2) as well as the intermolecular tube–gas distanceere also investigated for each system. The adsorption energy

urves are fitted to Morse and Corrected-Morse Potential equa-ions (Eqs. (5) and (6), respectively) in order to obtain the Morsend Corrected-Morse parameter of these potentials.

i = 2D[x2 − 2x], x = exp(

−�

2

(ri

re− 1

))(5)

i = 2D[x2 − 2x] + a, x = exp(

−�

2

(ri

re− 1

))(6)

here ri is the intermolecular distance of CO2 from nanotubes, 2D,e, and � denote the dissociation energy, the equilibrium bond dis-ance, and the adjustable parameter and a is the Corrected-Morseotential, respectively.

During these years, many attempts have been made to use theuantitative chemical concepts in density functional theory [35],amely chemical potential (�) and hardness (�), in the study ofolecular systems. Parr et al. [36] interpreted that � and � could

e considered as the first and second partial derivatives of elec-ronic energy (E) with respect to the number of electrons (N) at

fixed external potential (�(r)), respectively. According to theanak’s approximation [37], their analytical and operational defi-itions are given as follows:

=(

∂E

∂N

)�

∼= (εL + εH)2

(7)

�(r),T

= 12

(∂2E

∂N2

)�(�r),T

∼= (εL − εH)2

(8)

uators B 185 (2013) 512– 522

where εH and εL are the obtained energies of the highest occupiedmolecular orbital (HOMO) and the lowest unoccupied molecularorbital (LUMO) from density functional theory calculations, respec-tively.

Parr et al. [38] have defined an index for the global electrophilic-ity power of a system in terms of the chemical potential andhardness as:

ω = �2

2�(9)

In fact, electrophilicity index is intended to be a measure of theenergy lowering of the chemical species due to maximum electronflow from the environment and could be considered as a mea-sure of the capacity of species to accept arbitrary electronic charge.Many authors have demonstrated the usefulness of this index intheoretical studies [39–41].

3. Results and discussion

The structural optimization of pristine armchair [4,4] single-walled AlNNT, BNNT and SiCNT is performed at PBEPBE/6-31Glevel of theory, and the calculated average Al N, B N and Si Cbond lengths are found to be 1.84, 1.46 and 1.83 A, respectivelywhich are in well agreement with the previous research works[42,43]. Moreover, the obtained band gaps are nearly 2.41, 4.39and 1.77 eV for AlNNT, BNNT and SiCNT, respectively, imply to theirsemiconductor characters. To testify the validity of our calculationmethods (PBEPBE/6-31G), our results were compared with our pre-vious research work [27] and it is found out that the electronicproperties of nanotubes such as band gaps are well reproduced. Itis noteworthy that the hexagonal nanotube that has a somewhatdifferent physical properties with respect to the AlN nanotube.

Also, NBO analysis shows that the natural charges of N atomin AlNNT and in BNNT and C atom in SiCNT are about −1.84, −1.13and −1.72 esu, respectively, because of the difference between elec-tronegativity of N and C atoms with respect to Al, B and Si atoms.These results confirm the ionic character of Al N, B N and Si Cbonds in AlNNT, BNNT and SiCNT. The obtained data are sum-marized in Table 1. Note that the electronic structures of thesenanotubes are almost independent of tube diameter and chiraltiydue to the large ionicity of these bonds. Therefore, it seems thatinvestigations on armchair nanotube could be transferred to zigzagone.

In order to investigate the CO2 adsorption on [4,4] pris-tine AlNNT, BNNT and SiCNT, different possible adsorption sitesof the nanotubes including the top of the each atoms (Al, N,B, Si and C atoms), center of the hexagonal ring (H site) areconsidered. Note that in the case of H-site, the CO2 moleculehorizontally located above hexagonal ring (see supplementarymaterial).

To examine the adsorption properties of CO2 adsorbed ontothe nanotubes, the adsorption energies are calculated using Eq. (1)which is depicted in Fig. 2. This figure typically shows the poten-tial energy surfaces of the gas molecule adsorbed onto the AlNNT,BNNT and SiCNT nanotubes. The calculated adsorption energies andintermolecular distances between CO2 molecule and the tube wallsare also summarized in Table 2. Furthermore, the relaxed configu-rations are depicted in Fig. 3. It is clear from Table 2 that in all casesthe calculated adsorption energies are negative, imply to stablestructures. Evaluations of adsorption energies in different situa-tions give some interesting information. First, the most negativeadsorption energy of final optimization geometries is related to the

Al and N sites of AlNNT. In other words, when the CO2 moleculeapproach with C and O atoms to the N and Al atoms, the moststable structure is obtained. Interestingly the final optimizationstructures of these situations indicate that the position of the CO2

Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522 515

Table 1Electronic properties such as highest occupied molecular orbital (HOMO), lowest unoccupied orbital (LUMO), gap () energy, electronic chemical potential (�), hardness (�),softness (S), electrophilicity (ω) and structural parameters of pristine gas and nanotubes calculated by PBEPBE/6-31G method.

HOMO (eV) LUMO (eV) (eV) � (eV) � (eV) S (eV−1) ω (eV) � (eV)

CO2 −8.78 −1.03 7.75 −4.90 7.75 0.13 1.55 −8.78SiCNT −4.55 −2.78 1.77 −3.66 1.77 0.57 3.79 −4.55AlNNT −5.29 −2.88 2.41 −4.08 2.41 0.41 3.46 −5.29BNNT −5.75 −1.11 4.39 −3.31 4.39 0.23 1.25 −5.75

Partial charge (esu), bond order and bond length (Å)

Si C O Al N B Bond order Bond length (Å)

CO2 – 0.88 −0.44 – – – 1.90 –

mi(Aart[

FS

SiCNT 1.72 −1.72 –

AlNNT – – – 1.84

BNNT – – – –

olecule in both different sites is changed from vertical to hor-zontal orientation. Hence in the relaxed structures of two sitesabove Al and N atoms), CO2 molecule horizontally located abovelNNT surface with C close to the N atom and O close to the Altom which cause the formation of unconventional four member

ing (see Fig. 3). Note that covalent bond can be formed betweenhe nitrogen atoms of azabenzene and the carbon atom of CO226]. The implication of these phenomena is that a structure with

35

40

45

50

55

60

65

70

75

80

2 3

E ads

(Kj/m

ol)

-80

-60

-40

-20

0

20

40

60

80

100

120

1 2 3 4 5 6

E ad

s(Kjm

ol-1

)

r (Å)

(a)

(c)

ig. 2. Typically potential energy surfaces of the adsorption of CO2 molecule as functioniCNT using PBEPBE/6-31G method.

– – 1.03 1.83−1.84 – 0.83 1.84−1.13 1.13 0.94 1.46

incorporated nitrogen atoms possessing significant negative chargedensity might potentially form a chemical bond with CO2. Accord-ing to the adsorption energies in Table 2 the interaction betweenCO2 molecule and AlNNT is very strong. The adsorption energyof CO obtained on the outer surface of AlNNT is −117.1 kJ/mol

2for both Al and N sites, which implies to the chemisorptions pro-cess. Jiao et al. [26,44] showed that the strong binding of isolatedcarbon dioxide on AlN single walled nanotube and single-layer

4 5 6r (Å)

-8

-6

-4

-2

0

2

4

6

8

10

2 3 4 5 6Ead

s (K

j/m

ol)

r (Å)

(b)

of tube–CO2 distance on (a) Al site of AlNNT, (b) H site of BNNT and (c) H site of

516 Z. Mahdavifar et al. / Sensors and Act

Table 2Adsorption energy (Eads (kJ/mol)) and nearest intermolecular distance, r (Å), of CO2

adsorption on different sites of AlNNT, SiCNT and BNNT nanotubes.

Configuration of gas Site Eads (kJ/mol) Bond r (Å)

AlNNTPerpendicular Al −117.09 N(1) C(1) 1.527

Al(1) O(1) 1.919N −117.14 Al(1) N(1) 2.094

C(1) O(1) 1.352C(1) O(2) 1.237

Horizontal H −29.56 Al(1) C(1) 3.072Al(1) O(1) 2.405Al(1) O(1) 1.845C(1) O(1) 1.192C(1) O(2) 1.206

BNNTPerpendicular B −8.72 N(1) C(1) 3.288

B(1) O(1) 3.370N −5.14 N(1) O(1) 3.268

Horizontal H −11.51 N(1) C(1) 2.298

SiCNTPerpendicular Si −8.89 Si(1) O(1) 3.088

C −14.80 Si(1) O(1) 3.563C(1) O(2) 3.857

attoswsstmo1lottC

tmelagim

TP

Horizontal H −15.75 C(1) C(3) 3.504Si(1) O(2) 3.554

luminum nitride nanostructures were chemisorption process andhermodynamically favored at low temperature. It is noteworthyhat our obtained results are also confirmed this point. On thether hand, Zhao et al. [45] showed that the interaction betweeningle walled carbon nanotube (SWCNT) and CO2 gas molecule iseak and does not have a significant influence on the electronic

tructures of SWCNT. Therefore, it can be concluded that the con-idered nanotube (AlNNT) in our research work is more favorablehan the SWCNT as sensing device for CO2 gas molecule. Further-

ore, the obtained intermolecular distances between the surfacef the AlNNT and CO2 molecule are about ∼1.53 A for N(1) C(1) and.92 A for Al(1) O(1). All obtained intermolecular distances are col-

ected in Table 2. It is obvious from this table that the best positionf the AlNNT for the adsorption of CO2 molecule is the C approacho N and O close to Al atoms of AlNNT, respectively. It is concludehat the best sites of the AlNNT surface for the chemisorption ofO2 molecule are top of the Al and N atoms.

In the case of BNNT, the obtained adsorption energies are ∼ −5.1о −11.5 kJ/mol. The most stable structure is formed when the CO2

olecule adsorbed onto the H-site of the BNNT with the adsorptionnergy about −11.5 kJ/mol. The calculated shortest intermolecu-ar distance between gas and tube surface is 2.298 A. These results

re summarized in Table 2. It should be noted that the relaxedeometries of CO2 adsorption on B-site changed to the H-site, whichndicate that the H-site is the best position for the adsorption of CO2

olecule onto the BNNT surface. The final optimized geometry of

able 3arameters of the Corrected-Morse and Morse Potentials for CO2 adsorption on different

Site D (kJ/mol)

Morse Corrected-Morse

AlNNT Al 29.33 26.91

N 9.34 11.49

H 21.43 26.54

BNNT B 2.62 3.06

N 1.52 1.79

H 3.33 4.01

SiCNT Si 11.00 15.40

C 4.03 4.97

H 12.16 14.18

uators B 185 (2013) 512– 522

the BNNT/CO2 system is depicted in Fig. 4. Similar to the BNNT,H-site of the SiCNT surface is the best site for the adsorption ofCO2 molecule with the adsorption energy about −15.75 kJ/mol. Theabove results indicate that the interaction between CO2 moleculeand BNNT and SiCNT are very weak and the adsorption of CO2onto the BNNT and SiCNT is physisorption process. In addition, theadsorption of CO2 onto the BNNT and SiCNT could not change themechanical and electrical properties of these nanotubes (see Fig. 4).According to the obtained results, it could be found that althoughCO2 molecule adsorbed onto the BNNT and SiCNT through weakVan der Waals interactions, it chemisorbs on the outer surface ofthe AlNNT with the formation of chemical bond between surface ofthe tube and the gas atoms. Therefore, it reveals that the pristineAlNNT is a promising candidate for the CO2 adsorption of due to theappreciable adsorption energies and high sensitivity of its surface.

In this part, all of the obtained adsorption energy curves forAlNNT, BNNT and SiCNT are fitted to two model potentials, Morseand Corrected-Morse potentials, in order to evaluate the parame-ters of these models. The obtained data are listed in Table 3, whichcould be applied to perform a molecular simulation study of gasadsorption onto the nanotubes. Moreover, Fig. 5 shows the typicallyfitted potential energy curves with Corrected-Morse potentials.These curves show the typical features of the real intermolecularinteractions and reflect the salient features of the real interactionsin general way. These potential energy curves also provide a rea-sonable description for the properties CO2–nanotube. The fittedpotential data show that the attractive and repulsive portions havemore correlation with the Corrected-Morse potential, therefore justthe results of the Corrected-Morse potential reported (see Fig. 5).

The sensing mechanism of a sensor is related to the change inthe relaxed geometry due to the deformation energy and electronicband structure due to partial electron charge transfer betweenadsorbent and nanotube. Therefore, the deformation energy, Nat-ural Bond Orbital (NBO) of pristine nanotubes and nanotube/gassystems are considered in this part. The deformation of nanotubesafter gas molecule adsorbed onto the surface of the tubes is investi-gated in the relaxed geometry. As it is stated, the adsorption energyencompasses both interaction (Eint) and deformation (Edef) energycontributions. Since CO2 molecule chemisorbed onto the AlNNTsurface, the deformation and interaction energies of AlNNT/CO2system are calculated using Eqs. (3) and (4). The obtained data aresummarized in Table 4. By considering the data in Table 4, it is clearthat the curvature in the geometry of the AlNNT is occurred whenthe CO2 molecule adsorbed onto the Al and N site of the AlNNTsurface. The obtained deformation energy values for the relaxedstructures of N and Al sites are ∼276 kJ/mol. On the other hand,the contribution of the interaction energies for the both N and Al

sites are about −393.6 kJ/mol, which indicate that CO2 moleculecould be chemisorbed onto the AlNNT surface by the formation ofnew chemical bond between gas molecule and nanotube surface.It should be noted that no significant curvature is observed in the

sites of AlNNT, SiCNT and BNNT.

� a (kJ/mol)

Morse Corrected-Morse Corrected-Morse

7.02 7.22 −4.547.42 6.91 3.776.33 5.87 8.63

8.56 8.13 0.8610.62 10.06 0.62

9.78 9.20 1.40

5.48 4.87 10.568.85 8.40 4.538.241 7.78 67.24

Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522 517

N sites and (b) H site of AlNNT calculated by PBEPBE/6-31G method.

gcitoo

sldfT

Table 4Interaction energy (Eint), deformation energy (Edeform) and Adsorption energy (Eads)of CO2 adsorbed on pristine AlNNT.

Configuration Site Eint (kJ/mol) Edeform (kJ/mol) Eads (kJ/mol)

Fig. 3. Relaxed structures of CO2 adsorbed onto the (a) Al and

eometry of the AlNNT by CO2 adsorbed onto the H-site. In con-lusion, AlNNT could be acted as sensor for CO2 molecule becauset fulfills the mechanism of the sensing condition. Furthermore, inhe case of the BNNT and SiCNT, no significant perturbations arebserved in the relaxed geometries of nanotubes by CO2 adsorptionnto the BNNT and SiCNT surfaces.

In order to investigate the electronic properties of nanotube/gasystems, NBO calculations are also considered. The electron popu-

ation analysis reveals that considerable charge transfer is occurreduring the adsorption process. The partial charges of atoms derivedrom the NBO calculations are summarized in Table 5. It is clear fromable 5 that the considerable charge transfers take place to the CO2

AlNNT–CO2 Al −393.42 276.33 −117.09N −393.92 276.72 −117.14H −31.64 2.09 −29.56

molecule from the AlNNT surface when CO2 molecule adsorbedonto AlNNT. The obtained results of the partial charge of atomsindicate that the charge of the N(1) and Al(1) atoms (atom num-bering is depicted in Fig. 3) of AlNNT were increased from −1.84 to

518 Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522

of (a)

−pcftAfs

Fig. 4. Relaxed structures of CO2 adsorbed onto the H site

1.55 esu and 0.84 to 1.89 esu, respectively. On the other hand, theartial charges of C(1), O(1) and O(2) atoms of gas molecule werehanged from −0.44 to −0.77, −0.44 to −0.52 and 0.88 to 0.83 esuor O(1), O(2) and C(1) atoms, respectively. These results indicate

hat the strong electrostatic interaction between gas molecule andlNNT is occurred. It seems that the considerable charge transfers

rom AlNNT to CO2 molecule. These results are in accordance withtrong adsorption energies of the AlNNT/CO2 systems in the Al and

BNNT and (b) SiCNT calculated by PBEPBE/6-31G method.

N-sites. Typically, electron density of pristine and AlNNT/CO2 sys-tem is shown in Fig. 6. Comparing the partial charges of Si, C, Band N atoms of pristine SiCNT and BNNT with considered atoms inSiCNT, BNNT/CO2 systems, show that no significant charge transfer

is occurred which is in agreement with low adsorption energies ofthis systems.

The Wiberg bond index (WBI) which demonstrate the strengthof the covalent character, is also considered in this study. The WBI

Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522 519

F n (a)

C

[oWamm

W

iaA0ssowtCoa

tion theory as follow [48]:

ig. 5. Typically fitted potential energy curves for the adsorption of CO2 molecule oorrected-Morse potentials using PBEPBE/6-31G method.

46,47] comes from the manipulation of the density matrix in therthogonal natural atomic orbital based on the NBO analysis. TheBI expresses the sum of squares of density matrix elements (pjk)

nd equals two times the charge density in the atomic orbital (pjj)inus the square of the charge density with the following mathe-atical definition:

BI =∑

k

p2jk = 2pjj − p2

jj (10)

WBI closely relates to the bond order character, the larger WBImplies to stronger covalent character. The analysis of WBI whichre collected in Table 6, show that the adsorption of CO2 onto thelNNT surface cause a decrease in the WBI of Al(1) N(1) bond from.88 to 0.36. On the other hand, CO2 molecule bonded to the AlNNTurface with C and O pointing to N(1) and Al(1) atoms of AlNNTurface, respectively (see Fig. 3). The bond order (quantified by WBI)f Al(1) O(1) and N(1) C(1) bonds are 0. 66 and 0.77 respectivelyhile the bond order of C(1) O(2) bond was decreased from 1.90

o 1.08. Natural Bond Orbital (NBO) analysis indicate that when theO2 molecule adsorbed onto the AlNNT surface, the bond-characterf C(1) O(1) is changed from 2 in pristine CO2 molecule to 1.68 indsorbed CO2 molecule on the AlNNT surface (see Table 6). Also, by

Al site of AlNNT (b) N site of AlNNT, (c) H site of BNNT and (d) H site of SiCNT with

adsorption of CO2 molecule on the AlNNT surface, the s-characterof C atom in the C(1) O(2) bond length is changed from 50% to34.44% which indicate that the sp hybridization of C atom in CO2is changed to nearly sp2 hybridization in the adsorbed CO2 on theAlNNT surface (see Table 1S in supplementary material). Also, thebond angle of the O(1) C(1) O(2) is reduced from 180◦ to 130◦

(see Fig. 3a). However unlike AlNNT, in the case of BNNT and SiCNTthe CO2 molecule adsorbs through weak Van der Waals interactionand the charge transfer between nanotubes and CO2 molecule isvery small (about 0.01 esu) and the WBI of Si C and B N bonds inthe SiCNT, BNNT/CO2 system was remained nearly constant. Theobtained data are collected in Table 6.

Furthermore, the second-order perturbative estimates ofdonor–acceptor (bond–antibond) interactions in the NBO basis areinvestigated. The energetic stabilization due to such i → ∗

jdonor

acceptor interactions could be estimated by 2nd-order perturba-

�E(2)i→j

= −2

⟨i|

�F |∗

j

⟩2

εj∗ − εi(11)

520 Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522

Table 5Calculated partial charges of Al, N, B, Si, C and O atoms for the adsorption of CO2

molecule on AlN, BN and SiC nanotubes (atom numbering is according to Figs. 3 and4).

Natural charge (esu)

AlNNT–CO2 Al-site N-site H-siteAl(1) 1.89 1.89 1.84Al(2) 1.89 1.88 1.83N(1) −1.55 −1.55 −1.85N(2) −1.86 −1.86 −1.85C(1) 0.83 0.83 0.94O(1) −0.77 −0.77 −0.48O(2) −0.52 −0.52 −0.39

BNNT–CO2 B-site N-site H-siteB(1) 1.19 1.14 1.13N(1) −1.19 −1.14 −1.13C(1) 0.98 0.89 0.88O(1) −0.48 −0.43 −0.44O(2) −0.48 −0.45 −0.44

SiCNT–CO2 Si-site C-site H-siteSi(1) 1.76 1.73 1.73C(1) −1.73 −1.74 −1.74C(2) −1.73 −1.74 −1.74C(3) 0.9 0.9 0.9

w⟨N

tn2

FP

Table 6Wiberg bond indices of the nanotube–CO2 systems using PBEPBE/6-31G method.

Bond order

AlNNT–CO2 Al-site N-site H-siteAl(1) N(1) 0.36 0.36 0.84Al(1) C(1) 0.02 0.02 –Al(1) O(1) 0.66 0.66 0.17N(1) C(1) 0.73 0.73 0.02C(1) O(1) 1.08 1.08 1.68C(1) O(2) 1.77 1.77 1.97

BNNT–CO2 B-site N-site H-siteB(1) N(1) 0.09 1.03 0.92B(1) C(1) 0.01 – 0.01B(1) O(1) 0.01 0.02 0.01N(1) C(1) 0.01 – 0.01N(1) O(1) 0.01 – –C(1) O(1) 1.85 1.86 1.87C(1) O(1) 1.85 1.91 1.87

SiCNT–CO2 Si-site C-site H-siteSi(1) C(1) 1.1 1.11 1.11Si(1) C(2) 0.01 0.02 0.02Si(1) O(1) 0.06 0.02 0.03C(1) C(2) – 0.01 –C(1) O(1) – – –

O(1) −0.45 −0.44 −0.44O(2) −0.43 −0.45 −0.45

here�F is the effective orbital Hamiltonian and εi =

⟨i|

�F |i

⟩, εj∗ =

∗j|�F |∗

j

⟩are the respective orbital energies of donor and acceptor

BOs.This analysis is carried out by examining all possible interac-

ions between filled (donor) Lewis-type NBOs and empty (acceptor)on-Lewis NBOs, and estimating their energetic importance bynd-order perturbation theory. Since these interactions lead to

ig. 6. Typically electron density of (a) AlNNT, (b) AlNNT–CO2 systems usingBEPBE/6-31G method.

C(2) O(1) 1.85 1.86 1.86C(2) O(2) 1.91 1.83 1.83

loss of occupancy from the localized NBOs of the idealized Lewisstructure into the empty non-Lewis orbitals (and thus, to depar-tures from the idealized Lewis structure description), they arereferred to as delocalization corrections to the zeroth-order nat-ural Lewis structure. For each donor NBO (i) and acceptor NBO(j), the stabilization energy E(2) associated with delocalization(2e-stabilization) is estimated. The strongest interaction is inthis situation (adsorbed CO2 onto the Al or N atoms of AlNNT)identified for the interaction of �-bond Al(2)-N(1) localized onAlNNT as donor with the adjacent �* C(1) O(2) bond of gasmolecule as acceptor. The energy of this charge transfer is obtained61.46 kJ/mol. Table 7 shows the donor–acceptor (bond–antibond)interactions for the adsorption of CO2 molecule onto the AlNNTsurface.

By considering, the NBO analysis showed that the net chargetransfer is occurred from the nanotubes to the CO2 molecule. Thepartial atomic charges of tube/CO2 systems are summarized in

Table 7The second-order perturbation energy E(2), the donor–acceptor (bond–antibond)interactions for the adsorption of CO2 molecule onto the AlNNT surface associatedwith delocalization.

Donor NBO Acceptor NBO E(2) (kJ/mol)

Al-site�(Al1 N1) �*(C1 O1) 40.86�(N1 Al2) �1*(C1 O2) 23.91�(N1 Al2) �2*(C1 O2) 61.46�(N1 Al3) �1*(C1 O2) 24.12�(Al1 O1) �*(C1 O2) 51.58

N-site�(Al1 N1) �*(C1 O2) 40.82�(Al1 N1) �*(C1 O1) 29.60�(Al1 O1) �*(C1 O2) 51.54�(Al3 N1) �1*(C1 O2) 24.07�(Al3 N1) �2*(C1 O2) 60.96

H-site�(Al1 N3) �1*(C1 O2) 0.21�(Al1 N2) �*(C1 O1) 0.21�(Al1 N1) �*(C1 O1) 0.21�1(Al2 N2) �*(C1 O2) 0.96�1(Al2 N2) �*(C1 O2) 0.42�1(C1 O2) �*(Al1 N3) 0.25

Z. Mahdavifar et al. / Sensors and Actuators B 185 (2013) 512– 522 521

e of B

TtdcttsoaCit(ast

THge

Fig. 7. Typically contour plots of HOMO of the (a) Al-site of AlNNT–CO2 (b) H-sit

able 5. In comparison, the electronic properties of tube/CO2 sys-em such as HOMO–LUMO gap energy with pristine nanotubeemonstrate a considerable change (see Tables 1 and 8) in thease of AlNNT. By adsorption of CO2 molecule onto the AlNNT,he band gap of the pristine nanotube was decreased from 2.41o 1.96 eV. These results are in well agreement with the obtainedtrong adsorption energy. Typically, contour plots of HOMO orbitalsf AlNNT/CO2 system are shown in Fig. 7. In the cases of the SiCNTnd BNNT, no significant change is observed for the adsorption ofO2 on the tube surface, which means that the adsorption process

s occurred through weak Van der Waals interactions. According tohe global reactivity indices of the pristine nanotubes and tube/gaspresented in Table 8), it is found that when the CO2 molecule

dsorbed onto the surface of nanotubes the hardness values of theystems were slightly decreased and the electrophilicity of the sys-ems were slightly increased, which indicated that the reactivity of

able 8ighest occupied molecular orbital (HOMO), lowest unoccupied orbital (LUMO),ap () energy, electronic chemical potential (�), hardness (�), softness (S) andlectrophilicity (ω) of CO2 adsorption on ALNNT, BNNT and SiCNT.

Site HOMO LUMO Band gap � � S ω

AlNNTAl −5.03 −3.08 1.96 −4.06 1.96 0.51 4.20N −5.03 −3.08 1.95 −4.05 1.95 0.51 4.21H −5.27 −2.85 2.41 −4.06 2.41 0.41 3.42

BNNTB −6.75 −0.10 6.65 −3.42 6.65 0.15 0.88N −5.48 −1.12 4.37 −3.30 4.37 0.23 1.25H −5.52 −1.13 4.39 −3.32 4.39 0.23 1.27

SiCNTSi −4.49 −2.90 1.60 −3.70 1.60 0.63 4.28C −4.52 −2.93 1.60 −3.73 1.60 0.63 4.36H −4.52 −2.93 1.60 −3.72 1.60 0.63 4.35

NNT–CO2 and (c) H-site of SiCNT–CO2 systems calculated by PBEPBE functional.

the systems were increased. A comparison between HOMO–LUMOgap for the pristine and nanotubes/gas system imply to the energygap for the nanotubes/gas system is lower than that for pristinenanotubes, which imply to metallic character of the considered sys-tem and increasing its reactivity. Therefore, the obtained data showthat CO2 molecule chemisorbed onto the AlN surface with consider-able change in the structural parameters and electronic propertiesof AlNNT. According to the obtained results from the global reac-tivity indices as well as the NBO calculations and high sensitivity,AlNNT could be a promising CO2 sensor. Since the metals deco-rated of single walled nanotubes (SWNTs) with transition metalsallow the fabrication of single-chip device, the adsorption of CO2molecule onto the metal-decorated of AlNNT, BNNT and SiCNT is inunderway in our research group.

4. Conclusion

Adsorption of CO2 molecule on three types of inorganic nano-tubes (AlNNT, BNNT and SiCNT) has been explored through densityfunctional theory method. The geometrical structures, electronicproperties, the Wiberg bond index and the donor–acceptor inter-actions in the NBO basis are analyzed to predict the adsorptionproperties and mechanism of these complexes. The obtainedresults indicate that the CO2 molecule could be adsorbed on theBN and SiC nanotubes through weak Van der Waals interactions,which implies to physisorption process. Moreover, the adsorptionof CO2 onto the BNNT and SiCNT surfaces could not affect on themechanical and electrical properties of these nanotubes. In the caseof AlNNT, CO2 molecule tends to be strongly chemisorbed to theAlNNT with considerable adsorption energy (about −117 kJ/mol).

Upon the adsorption of CO2 to the AlNNT, the HOMO–LUMO energygap of the AlNNT is decreased from 2.41 to 1.96 eV, thus theadsorption of CO2 molecule could change the conductivity and thesensitivity of AlNNT. Based on the obtained results, it is expected

5 nd Act

tf

A

t

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[[

[

[

[

[

[

[

[

[

[

[

[

[

[

Ehsan Shakerzadeh was born in Fasa, Iran (1984). In 2008 received his M.Sc.

22 Z. Mahdavifar et al. / Sensors a

hat AlNNT could be a promising candidate in gas sensor devicesor detecting the CO2 molecule.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at http://dx.doi.org/10.1016/j.snb.2013.05.004.

eferences

[1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58.[2] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Nanotube

molecular wires as chemical sensors, Science 287 (2000) 622–625.[3] P.G. Collins, K. Bradley, M. Ishigami, A. Zettl, Extreme oxygen sensitivity of

electronic properties of carbon nanotubes, Science 287 (2000) 1801–1804.[4] E.S. Snow, F.K. Perkins, J.A. Robinson, Chemical vapor detection using single

walled carbon nanotubes, Chemical Society Reviews 35 (2006) 790–798.[5] D.R. Kauffman, A. Star, Carbon nanotube gas and vapor sensors, Angewandte

Chemie International Edition 47 (2008) 6550–6570.[6] M.A. Ryan, A.V. Shevade, C.J. Taylor, M.L. Homer, M. Blanco, Computational

Methods for Sensor Material Selection, Springer, 2009.[7] P. Pannopard, P. Khongpracha, M. Probst, J. Limtrakul, Gas sensing properties of

platinum derivatives of single-walled carbon nanotubes: a DFT analysis, Journalof Molecular Graphics and Modelling 28 (2009) 62–69.

[8] N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl,Boron-nitride nanotubes, Science 269 (1995) 966–967.

[9] K.H. Khoo, M.S.C. Mazzoni, S.G. Louie, Tuning the electronic properties of boronnitride nanotubes with transverse electric fields: a giant dc Stark effect, PhysicalReview B 69 (2004) 201401–201404.

10] C.W. Chen, M.H. Lee, S.J. Clark, Band gap modification of single-walled carbonand boron nitride nanotubes under a transverse electric field, Nanotechnology15 (2004) 1837–1843.

11] M. Ishigami, J.D. Sau, S. Aloni, M.L. Cohen, A. Zettl, Observation of the giantstark effect in boron-nitride nanotubes, Physical Review Letters 94 (2005)056804–056807.

12] Y. Chen, J. Zou, S.J. Campbell, G.L. Caer, Pronounced resistance to oxidation,Applied Physics Letters 84 (2004) 2430–2432.

13] J. Wang, V.K. Kayastha, Y.K. Yap, Z. Fan, J.G. Lu, Z. Pan, I.N. Ivanov, A.A. Puret-zky, D.B. Geohegan, Low temperature growth of boron nitride nanotubes onsubstrates, Nano Letters 5 (2005) 2528–2532.

14] S.M. Nakhmanson, A. Calzolari, V. Meunier, J. Bernholc, M.B. Nardelli, Sponta-neous polarization and piezoelectricity in boron nitride nanotubes, PhysicalReview B 67 (2003) 235406–235410.

15] S.H. Jhi, Y.K. Kwon, Hydrogen adsorption on boron nitride nanotubes: apath to room-temperature hydrogen storage, Physical Review B 69 (2004)245407–245410.

16] A. Fissel, B. Schroter, W. Richer, Low-temperature growth of SiC thin-films onSi and 6H-SiC by solid-source molecular-beam epitaxy, Applied Physics Letters66 (1995) 3182–3184.

17] F. Cao, X. Xu, W. Ren, C. Zhao, Theoretical study of O2 molecular adsorption anddissociation on silicon carbide nanotubes, Journal of Physical Chemistry C 114(2010) 970–976.

18] G. Mopurmpakis, G.E. Froudakis, G.P. Lithoxoos, J. Samios, SiC nanotubes: anovel material for hydrogen storage, Nano Letters 6 (2006) 1581–1583.

19] R.Q. Wu, M. Yang, H.Y. Lu, Y.P. Feng, Z.G. Huang, Q.Y. Wu, Silicon carbide nano-tubes as potential gas sensors for CO and HCN detection, Journal of PhysicalChemistry C 112 (2008) 15985–15988.

20] G.H. Gao, H.S. Kang, First principles study of NO and NNO chemisorption onsilicon carbide nanotubes and other nanotubes, Journal of Chemical Theoryand Computation 4 (2008) 1690–1697.

21] W.G. Liu, G.H. Chen, X.C. Huang, D. Wu, Y.P. Yu, DFT Studies on the interactionof an open-ended single-walled aluminum nitride nanotube (AINNT) with gasmolecules, Journal of Physical Chemistry C 116 (2012) 4957–4964.

22] K. Rezouali, M.A. Belkhir, A. Houari, J. Bai, Ab initio study of confinement andsurface effects in hexagonal AlN nanotubes, Computation Materials Science 45(2009) 305–309.

23] D. Zhang, R.Q. Zhang, Theoretical prediction on aluminum nitride nanotubes,Chemical Physics Letters 371 (2003) 426–432.

24] Y.F. Zhukovskii, N. Pugno, A.I. Popov, C. Balasubramanian, S. Bellucci, Influ-ence of F centres on structural and electronic properties of AlN single-wallednanotubes, Journal of Physics: Condensed Matter 19 (2007) 395021–395038.

25] M. Zhao, Y. Xia, Z. Tan, X. Liu, F. Li, B. Huang, Y. Ji, L. Mei, Strain energy and ther-

mal stability of single-walled aluminum nitride nanotubes from first-principlescalculations, Chemical Physics Letters 389 (2004) 160–164.

26] Y. Jiao, A. Du, Z. Zhu, Victor Rudolph, C. Smith Sean, A density functional theorystudy of CO2 and N2 adsorption on aluminium nitride single walled nanotubes,Journal of Materials Chemistry 20 (2010) 10426–10430.

uators B 185 (2013) 512– 522

27] Z. Mahdavifar, M. Haghbayan, Theoretical investigation of pristine and func-tionalized AlN and SiC single walled nanotubes as an adsorption candidate formethane, Applied Surface Science 263 (2012) 553–562.

28] S. Liu, Q. Shen, Y. Cao, L. Gan, Z. Wang, M.L. Steigerwald, X. Guo, Chemicalfunctionalization of single-walled carbon nanotube field-effect transistors asswitches and sensors, Coordination Chemistry Reviews 254 (2010) 1101–1116.

29] J. Beheshtian, M.T. Baei, A.A. Peyghan, Theoretical study of CO adsorption on thesurface of BN, AlN, BP and AlP nanotubes, Surface Science 606 (2012) 981–985.

30] M. Yu, X. Liu, Y. Wang, Y. Zheng, J. Zhang, M. Li, W. Lan, Q. Su, Gas sensing prop-erties of p-type semiconducting vanadium oxide nanotubes, Applied SurfaceScience 258 (2012) 9554–9558.

31] K. Li, W. Wang, D. Cao, Metal (Pd, Pt)-decorated carbon nanotubes for CO andNO sensing, Sensors and Actuators B 159 (2011) 171–177.

32] Y. Kwon, H. Kim, S. Lee, I.J. Chin, T.Y. Seong, W.I. Lee, C. Lee, Enhanced ethanolsensing properties of TiO2 nanotube sensors, Sensors and Actuators B 173(2012) 441–446.

33] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation madesimple, Physical Review B 77 (1996) 3865–3868.

34] M.J. Frisch, et al., Gaussian03, Revision B. 03, Gaussian, Inc., Pittsburgh, PA, 2003.35] P. Geerlings, F. De Proft, W. Langenaeker, Conceptual density functional theory,

Chemical Reviews 103 (2003) 1793–1873.36] R.G. Parr, R.A. Donelly, M. Levy, W.E. Palke, Electronegativity-density functional

viewpoint, Journal of Chemical Physics 68 (1978) 3801–3807.37] J.F. Janak, Proof that ∂E/∂ni = ε in density-functional theory, Physical Review B

18 (1978) 7165–7168.38] R.G. Parr, L. Szentpaly, S. Liu, Electrophilicity index, Journal of the American

Chemical Society 121 (1999) 1922–1924.39] A. Chakraborty, S. Gir, P.K. Chattaraj, Analyzing the efficiency of Mn–(C2H4)

(M = Sc, Ti, Fe, Ni; n = 1, 2) complexes as effective hydrogen storage materials,Structural Chemistry 22 (2011) 823–837.

40] P.K. Chattaraj, D.R. Roy, P. Geerlings, M. Torrent-Sucarrat, Local hardness: acritical account, Theoretical Chemistry Accounts 118 (2007) 923–930.

41] A.S. Özen, F. De Proft, V. Aviyente, P. Geerlings, Interpretation of hydrogen bond-ing in the weak and strong regions using conceptual DFT descriptors, Journalof Physical Chemistry A 110 (2006) 5860–5868.

42] M. Zhao, Y. Xia, X. Liu, Z. Tan, B. Huang, C. Song, L. Mei, First-principlescalculations of AlN nanowires and nanotubes: atomic structures, ener-getics, and surface states, Journal of Physical Chemistry B 110 (2006)8764–8768.

43] R.L. Zhou, L. Wang, B.C. Pan, Elastic and melting properties of crystalline SiCnanotubes, Journal of Physical Chemistry C 114 (2010) 8199–8205.

44] Y. Jiao, A. Du, Z. Zhu, Victor Rudolph, C. Smith Sean, Adsorption of carbondioxide and nitrogen on single-layer aluminum nitride nanostructures stud-ied by density functional theory, Journal of Physical Chemistry C 114 (2010)7846–7849.

45] J. Zhao1, A. Buldum, J. Han, J. Ping Lu, Gas molecule adsorption in carbon nano-tubes and nanotube bundles, Nanotechnology 13 (2002) 195–200.

46] A.W. Ehlers, E.J. Baerends, K. Lammertsma, Nucleophilic or electrophilic phos-phinidene complexes MLnPH; what makes the difference? Journal of theAmerican Chemical Society 124 (2002) 2831–2838.

47] K.K. Pandey, G. Frenking, The nature of the M E bond: theoretical investiga-tion of the molecules [(RO)3M E] (M = Mo, W; E = N, P, As, Sb, Bi; R = H, Me)and [(Me3CO)3Mo P], European Journal of Inorganic Chemistry 2004 (2004)4388–4395.

48] A.E. Reed, L.A. Curtiss, F. Weinhold, Intermolecular interactions from a nat-ural bond orbital, donor–acceptor viewpoint, Chemical Reviews 88 (1988)899–926.

Biographies

Zabiollah Mahdavifar received his Ph.D. in Computational & Physical Chemistryunder the supervisor of Prof. Amir Abbas Rafati in 2008 from Bu-Ali Sina Universityin Iran. His Ph.D. thesis was molecular simulation and computational study of gasadsorption on nanotubes. Now, his research is focused on molecular simulation andcomputational chemistry with special interest in prediction of novel nanomaterialfor rechargeable battery, prediction of nanomaterial for solar cells and gas sensing,adsorption and separation.

Nasibeh Abbasi was born in Ahvaz, Iran in 1986. She completed her B.Sc. in Physics in2008 from Chamran University and received her M.Sc. in Physical Chemistry underthe supervision of Assist. Prof. Zabiollah Mahdavifar from Chamran University in2011. Now, she is working at Chamran University.

degree and in 2012 his Ph.D. degree in Physical Chemistry, both at the ChemistryDepartment of Shahid Chamran University. His main scientific interests are the com-putational nanochemistry, molecular modeling, and different aspects of aromaticity.His hobbies are classical music and philosophy books.