C

Ja

b

c

d

a

ARR2AA

KNDBA

1

qspbaCarsabe

tacwsrt

0h

Applied Surface Science 264 (2013) 699– 706

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j our nal ho me p age: www.elsev ier .com/ loc ate /apsusc

arbon nitride nanotube as a sensor for alkali and alkaline earth cations

avad Beheshtiana, Mohammad T. Baeib, Zargham Bagheri c, Ali Ahmadi Peyghand,∗

Department of Chemistry, Shahid Rajaee Teacher Training University, P.O. Box: 16875-163, Tehran, IranDepartment of Chemistry, Azadshahr Branch, Islamic Azad University, Azadshahr, Golestan, IranPhysics Group, Science Department, Islamic Azad University, Islamshahr Branch, P.O. Box: 33135-369, Islamshahr, Tehran, IranYoung Researchers Club, Islamic Azad University, Islamshahr Branch, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 10 June 2012eceived in revised form6 September 2012ccepted 18 October 2012

a b s t r a c t

Adsorption of several alkali (Li+, Na+, and K+) and alkaline earth (Be2+, Mg2+, and Ca2+) cations on thesurface of a zigzag (9, 0) carbon nitride nanotube has been investigated using density functional theory.It has been found that almost all of the cations prefer to be strongly chemisorbed at the center of poroussite of the tube surface. The adsorption of alkaline cations much more influences the electronic propertiesof the tube, in comparison with the alkali ones, so that it is transformed from an intrinsic semiconductor

vailable online 26 October 2012

eywords:anostructuresFTand gap

with HOMO/LUMO energy gap of 4.02 eV to extrinsic p-type one with the gap of 0.54–1.94 eV. The alkalinecation adsorption may significantly raise potential barrier of the electron emission from the tube surface,hence impeding the field emission. It has been also concluded that the electrical sensitivity of the tubetoward the cations may be in the order: Be2+ � Mg2+ � Ca2+ � Li+ ∼ Na+ ∼ K+.

© 2012 Elsevier B.V. All rights reserved.

dsorption

. Introduction

Ever since the discovery of carbon nanotubes (CNTs) [1],uasi-one-dimensional tubular nanostructures have attracted con-iderable interests due to their excellent electronic properties andromising applications in fabricating nanodevices [2–4]. CNTs cane metallic or semi-conductive owing to their different helicitiesnd diameters of the rolled graphitic sheet [5]. Technologically,NTs are often functionalized by atoms or groups via differentpproaches in order to tune their electronic structures and thus theelevant electronic or optoelectronic properties [6,7]. Moreover, thetructures of non-carbon based nanotubes have been recognized,mong them the tubular structures of boron nitride (BN) and car-on nitride (CN) nanotubes have been studied by calculations andxperiments [8–11].

CN nanotubes (CNNTs) have attracted increasing interest due toheir interesting electronic, chemical, and mechanical propertiess well as large number of methods available for synthesis of CNompounds [11,12]. Theoretical studies have predicted that CNNTsith CN or C3N4 stoichiometry may exist [13]. Cao et al. have

ynthesized porous CNNTs with the CN stoichiometry through the

eaction of cyanuric chloride (C3N3Cl3) with sodium at 230 ◦C inhe presence of NiCl2 as catalyst [14]. Recently, Koh et al. [15] have

∗ Corresponding author. Tel.: +98 912 5061827.E-mail address: ahmadi.iau@gmail.com (A.A. Peyghan).

169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2012.10.100

used density functional theory (DFT) to show that CNNTs could beattractive hydrogen sorbent.

Ionic interaction with nanostructure surfaces is a subfield ofgreat interest due to potential applications such as chemical sensorsand electronic devices [15,16]. Fundamental sensing mechanism inthese materials is the modulation of their conductivity as a resultof charge transfer with adsorbates [17]. The phenomenon of spe-cific adsorption of metal ions on nanostructure surfaces has beenthe subject of a number of theoretical papers [18,19]. Most of themhave been focused on the interactions of alkali ions (AI) with sev-eral surfaces, such as CNT and graphene [20,21]. There are also a fewstudies on interactions of alkaline earth ions (AEI) with the surface[22].

Herein, we have reported a theoretical study on the interactionof three AIs (Li+, Na+, and K+) and three AEIs (Be2+, Mg2+, and Ca2+)with CNNTs through DFT calculations. The main questions whichhave been tried to be explored in the present study include: (1)where are the cations placed upon the interaction? (2) How are thegeometric and electronic properties modified by cation adsorption?And (3) what is the effect of cation adsorption on the electronicproperties of the CNNT? Our results may help direct experimentalexplorations of new nanostructured materials for field emission orsensor applications.

2. Computational methods

We have considered adsorption of different cations on the sur-face of a CNNT which has been rolled up from corresponding

700 J. Beheshtian et al. / Applied Surface Science 264 (2013) 699– 706

) CNN

gspthasitfaMpt

E

waCMbtp

3

3

mupb

Fig. 1. Optimized structure of (9, 0

raphitic-CN. The structure of the graphitic-CN has been con-tructed by C and N atoms, and the C N and C C bonds form aorous structure as shown in Fig. 1. Alike CNTs, CNNTs can be inhe form of zigzag or armchair depending on how the graphitic-CNas been rolled up, and they are labeled by the chiral indexes (m, n)ccording to the notation given by White et al. [23]. Here, we haveelected a (9, 0) zigzag CNNT consisting of 72C and 72N atoms,n which the end atoms have been saturated by hydrogen atomso avoid boundary effects. Geometry optimizations have been per-ormed at B3LYP/6-31G(d) level of theory on a CNNT in the absencend in the presence of different ions including Li+, Na+, K+, Be2+,g2+, and Ca2+. The B3LYP is a commonly used functional for com-

utational study of nanostructured materials [24–28]. However,he adsorption energy (Ead) has been defined as follows:

ad = E(X − CNNT) − E(X) − E(CNNT) (1)

here E(X−CNNT) is the total energy of the ion–CNNT complexes,nd E(CNNT) and E(X) are referred to the energy of an isolatedNNT and corresponding ion, respectively. Density of states (DOS),ulliken charge analyses, and all of the energy calculations have

een performed using the same level of theory. All of the calcula-ions have been carried out using the GAMESS electronic structurerogram [29].

. Results and discussion

.1. Geometric and electronic properties

In Fig. 1, we have shown top and side views of the opti-

ized structure of the (9, 0) zigzag CNNT. The CNNT converges to

nsmooth tubular surface due to the repulsions of the N N loneairs as well as the porous structure. There are three types of C Nonds and two types of C C bonds in the CNNT. We have named

T and its DOS. Distances are in Å.

the bonds parallel to the tube axis as vertical (V) bonds and theothers as diagonal (D) ones. The DC N bonds can be in two differ-ent positions as shown in Fig. 1, including the bonds near the DC Cand VC C with equilibrium lengths of 1.33 and 1.34 A, respectively.The bond length of both DC C and VC C is about 1.52 A. The differ-ence in energies between the highest occupied molecular orbital(HOMO) and the lowest unoccupied molecular orbital (LUMO), Eg,has been calculated from DOS results, indicating that the CNNT is asemi-conductive material with a wide Eg of 4.02 eV.

3.1.1. Interaction of AI with the CNNTIn order to investigate adsorption behavior of Li+, N+, and K+ on

the CNNT, we have adopted different initial adsorption sites on thetube including top of one C or N atom; above the different bonds inCNNT; and above the center of a hexagonal ring. Interestingly, afterfull optimization, we have only obtained one stable configuration(Fig. 2) for all of the AIs, in which the ions have been moved andlocated in the center of the porous site and diffused into the surfaceof the CNNT, so that the six nearest nitrogen atoms of CNNT havebonded to the mentioned ions. The negative values of Ead (Table 1)indicate that the adsorption of ions on the CNNT is energeticallyfavorable. The distances between cation and the nearest nitrogenatom of the tube for the Li+, N+, and K+ are found to be about 2.59,2.60 and 2.67 A with corresponding charge transfers of 0.491, 0.459,and 0.356e respectively, from the tube to each ion. In the case of K+

adsorption, (Fig. 2c) the cation slightly moves outward of the tubesurface. This created lump may be due to the larger radius of K+, ascompared to Li+ and Na+.

The Ead values become more negative and therefore energeti-

cally favorable in the order of Li+ > Na+ > K+ (Table 1). It is knownthat the energy difference between the HOMO of the nucleophileagent and the LUMO of the electrophile is one of the most impor-tant factors in HOMO/LUMO interactions. Our FMO analysis shows

J. Beheshtian et al. / Applied Surface Science 264 (2013) 699– 706 701

Fig. 2. Optimized structure of alkali ions adsorbed on CNNT and related DOS. Distances are in Å.

Table 1Adsorption energy of alkali ions on CNNT (Ead, eV), Mulliken charge (QT), the HOMO, LUMO and gap (Eg) in between energies (eV) for different alkali ion/CNNT complexes,work function (˚) and energy of Fermi level (EFL, eV).

Systema Ead QT (e)b EHOMO EFL ELUMO Eg �Eg(%)c �˚d

CNNT − − −7.49 −5.48 −3.47 4.02 − 5.48 −Li+ −3.8 0.491 −9.22 −7.66 −6.11 3.11 22.6 7.66 2.18Na+ −3.4 0.459 −9.21 −7.64 −6.08 3.13 22.1 7.64 2.16K+ −2.5 0.356 −9.19 −7.59 −6.00 3.19 20.6 7.59 2.11

a See Figs. 1 and 2.b Transferred charge from tube to AIs.c Change of Eg of CNNT upon adsorption of AIs.d The change of work function of tube upon the AI adsorption.

702 J. Beheshtian et al. / Applied Surface

Fig. 3. Calculated molecular electrostatic potential surface of CNNT. The surfacesare defined by the 0.0004 electrons/b3 contour of the electronic density. The redcta

t−asLoeatntcNMnt

etCAo3sgfiwsatosfistc

3

tttpFAi

olor is referred to the high electron density on N atoms and the blue one is referredo the positive sites (C atoms). (For interpretation of the references to color in thertwork, the reader is referred to the web version of the article.)

hat the HOMO energy of the CNNT as a nucleophile agent is about7.49 eV and the LUMO energy of the Li+ and K+ as electrophilegents are −6.93 and −5.99 eV, respectively, suggesting that themall Ead value (larger exothermicity of the adsorption process) ofi+ may come from its closer LUMO energy to the HOMO energyf the CNNT. In addition, as it is shown by mapped molecularlectrostatic potential (MEP) of the CNNT in Fig. 3, the C atomsre positively charged (blue colors) while the N ones are nega-ively charged (red colors) in C N bonds, indicating partially ionicature of the bond. Also, the ionic radius of Li+ is smaller thanhat of the Na+ and K+ so the lithium ion has a higher positiveharge density and strongly tends to interact with electron rich

atoms of the CNNT and it may lead to more negative value of Ead.ulliken population charge analysis for the pristine CNNT shows

et charge of +0.41 and −0.41e on the C and N atoms, respec-ively.

In order to consider the influence of the AI adsorption on thelectronic properties of the tube, we have drawn DOS plots forhe ion-adsorbed tubes and compared them with that of the bareNNT. Comparing DOSs of the free CNNT (Fig. 1) and those of theI-adsorbed systems (Fig. 2), it can be concluded that the presencef cations slightly decreases the value of the Eg from 4.02 eV to.11–3.19 eV in the AI-adsorbed tube (by about 21% change). Table 1hows that the Fermi level (EFL) of the CNNT shifts to lower ener-ies upon the adsorption of AI cations. The canonical assumptionor Fermi level is that in a molecule (at T = 0 K) it lies approximatelyn the middle of the Eg. It is noteworthy to mention that, in fact,

hat lies in the middle of the Eg is the chemical potential, andince the chemical potential of a free gas of electrons, as tradition-lly defined, is equal to its Fermi level herein the Fermi level ofhe considered systems is at the center of the Eg. The change of EFLf a semiconductor upon the ion adsorption alters its field emis-ion currents. Since CNNTs are one of the promising candidates aseld emitters, the present results are of great importance [30]. Iteems that the AI cations will limited the electron emission fromhe CNNT surface due to the charge transfer from the tube to theation.

.1.2. Interaction of AEI with the CNNTWe have also probed a number of different adsorption sites on

he tube surface for each AEI cation (Be2+, Mg2+, and Ca2+) similar tohe AI case in order to find stable configurations (local minima) forhe AEI/CNNT complexes. Full structural relaxation has been then

erformed for each initial cation-tube configuration. As shown inig. 4, adsorption behaviors of the Mg2+ and Ca2+ are similar to theI cations and they have located in the center of the porous site

n CNNT surface with Ead values of −11.8 and −9.4 eV and their

Science 264 (2013) 699– 706

distances to N atom are calculated as 1.45 and 2.60 A, respectively.One unusual case is Be2+ which is migrated from the center of hol-low site to near of two N atoms of porous site in CNNT, so that anew pentagonal ring including Be is formed (Fig. 4a). The high Ead(−15.7 eV) and small distance to N atoms (1.62 and 1.72 A) impliesthat the Be2+ is strongly chemisorbed on the CNNT. As it can beseen in Fig. 4a, the adsorption induces a local structural deforma-tion to the CNNT; the bond length of C N of the tube surface issignificantly increased from 1.33 A in the free CNNT to 1.36 and1.38 A, and N C C angles are also changed from 119.4◦ and 112.2◦

to 111.3◦ and 108.6◦ in Be2+-adsorbed form.According to the obtained results, the Ead of AEIs on the CNNT

is higher than that of AIs. This prediction is expected based on thePearson’s hard–soft acid–base theory (HSAB) which states that thesoft acids react strongly with the soft bases and in contrary, thehard acids react strongly with the hard bases, when all other factorsbeing equal. The AEIs (with two positive charge) is a hard acid withsmaller size, and is weakly polarizable compared to the AIs (withone positive charge). Therefore, they tend to interact more prefer-ably with the nitrogen which is known as a hard base. It has beenalso justified that the charge transfer to the ions are in the order:Be2+ (1.068e) � Mg2+ (0.945e) > Ca2+ (0.690e) � Li+ (0.491e) > Na+

(0.459e) > K+ (0.356e), indicating that lower charge transfer to theions causes less negative Ead value.

In order to consider the sensitivity of the CNNT toward the AEIions, the DOS plots of different AEI/CNNT complexes have beendrawn. Since a small change of Eg may modify the electrical conduc-tivity of CNNT, these changes (�Eg) upon the adsorption processare related to the sensitivity of the tube for a particular analyte. Asshown in Fig. 4, the DOSs of the AEI-adsorbed configurations havea distinct change near the conduction and valence levels comparedto that of the pristine tube, so that the HOMO, EFL, and especiallythe LUMO of the tube significantly shifts to lower energies ascribedto the charge transfer from the tube to the cations. Therefore, uponthe adsorption of AEI, the Eg of the tube is decreased from 4.02 eVin the bare CNNT to 0.54-1.94 eV in adsorbed forms and the tube istransformed from an intrinsic semiconductor to a p-type one. Thelargest decrement of Eg belongs to the case of Be2+ ion in which theEg reduces to 0.54 eV (by about 86.5% change) which would resultin an electrical conductivity change of the nanotube according tothe following equation:

� ∝ exp(−Eg

2kT

)(2)

where � is the electric conductivity, and k is the Boltzmann’s con-stant [31]. According to the equation, smaller values of Eg at a giventemperature lead to larger electric conductivity. The considerablechange in the Eg value demonstrates the high sensitivity of theelectronic properties of the CNNTs toward the AEIs. However, theconventional sensing mechanism is such that upon exposure to thetarget cation charge transfer between the adsorbed cation speciesand the surface of the semiconductor occurs. This charge trans-fer will either increase or decrease the concentration (or mobility)of the major carriers in the semiconductor, resulting in increase ordecrease of the electrical conductivity of the sensor under operatingconditions.

Table 2 shows that in the case of AEI adsorption, the Fermi levelmuch more shifts to lower energies in comparison with the case ofAI ones which may significantly leads to increased work function ofthe tube. The work function of a semiconductor is the least amount

of energy required to remove an electron from the Fermi level toa point far enough not to feel any influence from the material. Thechange of work function of an adsorbent upon the gas adsorptionalters its field emission properties. The emitted electron current

J. Beheshtian et al. / Applied Surface Science 264 (2013) 699– 706 703

Fig. 4. Optimized structures of alkaline earth ions adsorbed on CNNT and related DOSs. Distances are in Å.

Table 2Adsorption energy of alkaline earth cations on CNNT (Ead, eV), Mulliken charge (QT), the HOMO, LUMO and gap (Eg) in between energies (eV) for different alkaline earthion/CNNT complexes, work function (˚) and energy of Fermi level (EFL, eV).

Systema Ead QTb (e) EHOMO EFL ELUMO Eg �Eg(%)c

�˚d

CNNT − − −7.49 −5.48 −3.47 4.02 − 5.48 −Be2+ −15.7 1.068 −10.77 −10.50 −10.23 0.54 86.5 10.50 5.02Mg2+ −11.8 0.945 −10.80 −9.92 −9.05 1.75 56.4 9.92 4.44Ca2+ −9.4 0.690 −10.76 −9.79 −8.82 1.94 51.7 9.79 4.31

a See Fig. 4.b Transferred charge from tube to AEIs.c Change of Eg of CNNT upon adsorption of AEIs.d The change of work function of tube upon the AEIs adsorption.

704 J. Beheshtian et al. / Applied Surface Science 264 (2013) 699– 706

) CNNT

dc

j

wtc

˚

wswusftn

TAf

TA(

Fig. 5. Optimized structure of (9, 0

ensities in vacuum are theoretically described by the followinglassical equation:

= AT2 exp(−˚

kT

)(3)

here A is called the Richardson constant (A/m2), T is the tempera-ure (K) and (eV) is the material’s work function. Work functionan be calculated using the following equation:

= Einf − EFL (4)

here Einf is the electrostatic potential at infinity. If the electro-tatic potential at infinity assumed to be zero, the work functionill equal to negative value of EFL. Therefore, as shown in Table 2,pon the AEI adsorption on the tube its work function will be con-

iderably increased which may be arisen from a donation of chargerom the tube to the cations. However, as can be seen from Eq. (3),he emitted electron current density is exponentially related withegative value of ˚. Therefore, the emitted electron current density

able 3dsorption energy of a Ca2+ cation on the longer CNNT (Ead, eV), Mulliken charge (QT), the

unction (˚) and energy of Fermi level (EFL, eV).

System Ead QT (e)b EHOMO EFL

CNNT(38.2a) − − −7.50 −5.52

Ca2+/CNNT −9.2 0.671 −10.70 −9.76

a The length of tube in Å.b Transferred charge from tube to Ca2+.c Change of Eg of CNNT upon adsorption of Ca2+.d The change of work function of tube upon the Ca2+ adsorption.

able 4dsorption energy (per cation) of three Na+ ions and also three Mg2+ ones on the CNNT (

eV) for the ion/CNNT complexes, work function (˚) and energy of Fermi level (EFL, eV).

Systema Ead QT (e)b EHOMO EFL

CNNT − − −7.49 −5.48

3Na+ −3.0 1.428 −12.36 −11.46

3Mg2+ −10.3 0.912 −17.50 −17.41

a See Fig. 6.b Average transferred charge from tube to a cation.c Change of Eg of CNNT upon adsorption of mentioned cations.d The change of work function of tube upon the Na+ and Mg2+ adsorptions.

with length of 38.2 A and its DOS.

from the tube surface will be significantly decreased upon AEIadsorption which confirms its high sensitivity of the tube towardthese cations.

Finally, we have investigated the possibility effect of the tubelength on the Ead and electronic properties of the nanotube. Tothis end, a CNNT has been selected with optimized length of about40.2 A, being 19.2 A longer that the former studied tube. This tube isconstituted of 126C and 126N atoms, in which the end atoms havebeen saturated with hydrogen atoms (Fig. 5). The date of Table 1and DOS spectrum in Fig. 5 indicate that HOMO, LUMO, Eg, Fermilevel energies and work function of this tube is nearly equal to thoseof the shorter one. Ead of a Ca2+ on the longer tube and the changeof Eg (Table 3) are calculated to be as same as those on the formertube. Therefore, one can conclude that the effects of increasing the

length of the tube may be negligible on the adsorption process andelectronic properties of the tube.

We also have investigated simultaneous adsorption of three Na+

ions and also three Mg2+ ones on the former CNNT. Calculated data

HOMO, LUMO and gap (Eg) in between energies (eV) for Ca2+/CNNT complex, work

ELUMO Eg �Eg(%)c �˚d

−3.53 3.97 − 5.52 −−8.79 1.91 51.9 9.76 4.24

Ead, eV), Mulliken charge (QT), the HOMO, LUMO and gap (Eg) in between energies

ELUMO Eg �Eg(%)c �˚d

−3.47 4.02 − 5.48 −−10.56 1.80 55.2 11.46 5.98−17.33 0.17 95.7 17.41 11.93

J. Beheshtian et al. / Applied Surface Science 264 (2013) 699– 706 705

Mg2+

idtsnso4t

4

Abelat(tftAssBb

R

[

[

[

[

[

[

[

[

Fig. 6. Optimized structures of 3Na+ and 3

n Table 4 and DOS in Fig. 6 demonstrate that the Ead is slightlyepended on the ion numbers, while the electronic properties ofhe tube are significantly altered by increasing the number of ions. Ituggests that the Eg of the tube is more decreased by increasing theumber of either Na+ or Mg2+ ion, indicating that the tube cannotelectively function between AIs and AEIs. Interestingly, in the casef Mg2+ adsorption, the Eg of tube is noticeably decreased from.02 eV to 0.17 eV which transforms the tube from a semiconductoro a semimetal material.

. Conclusion

We have performed a DFT study on the adsorption of AIs andEIs on the surface of a (9, 0) zigzag CNNT. The cations prefer toe adsorbed in the center of the hollow site of the tube surfacexcept for Be2+ which attached to two nitrogen atoms of the hol-ow site. The adsorption energies of AEIs have been calculated to bebout −15.7, −11.8, and −9.4 eV for Be2+, Mg2+, and Ca2+, respec-ively, which are much higher than those of obtained for the AIsabout −3.8, −3.4 and −2.5 eV for Li+, Na+ and K+). The adsorp-ion AEIs may significantly decrease the electron emission currentrom the CNNT surface attributed to the charge transfer from theube to the cations. DOS plots reveal that upon the adsorption ofEIs, the intrinsic semi-conductive tube is transformed to an extrin-ic p-type one with higher electrical conductance. The electricalensitivity of the tube toward the cations may be in the order:e2+ � Mg2+ > Ca2+ � Li+ ∼ Na+ ∼ K+. We believe that the CNNT cane a work function-based or electronic sensor for AIs and AEIs.

eferences

[1] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56–58.

[

[

ions adsorbed on CNNT and related DOSs.

[2] L. Guo, J. Liang, S. Dong, Z. Xu, Q. Zhao, Property of carbon nanotube tip for sur-face topography characterization, Applied Surface Science 258 (2011) 766–773.

[3] K.P.S.S. Hembram, G. Mohan Rao, Structural and surface features of multiwallcarbon nanotube, Applied Surface Science 255 (2009) 5770–5774.

[4] G.N. Fursey, D.V. Novikov, G.A. Dyuzhev, A.V. Kotcheryzhenkov, P.O. Vassiliev,The field emission from carbon nanotubes, Applied Surface Science 215 (2003)135–140.

[5] T.W. Odom, J.L. Huang, P. Kim, C.M. Lieber, Structure and electronic propertiesof carbon nanotubes, Journal of Physical Chemistry B 104 (2000) 2794–2809.

[6] M. Burghard, Electronic and vibrational properties of chemically modifiedsingle-wall carbon nanotubes, Surface Science Reports 58 (2005) 1–109.

[7] C. Wongchoosuk, A. Udomvech, T. Kerdcharoen, The geometrical and electronicstructures of open-end fully functionalized single-walled carbon nanotubes,Current Applied Physics 9 (2009) 352–358.

[8] M. Menon, D. Srivastava, Structure of boron nitride nanotubes: tube closingversus chirality, Chemical Physics Letters 307 (1999) 407–412.

[9] Y. Miyamoto, M. Cohen, S. Louie, Theoretical investigation of graphitic carbonnitride and possible tubule forms, Solid State Communications 102 (1997) 605.

10] 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.

11] A. Ahmadi, J. Beheshtian, N. Hadipour, Chemisorption of NH3 at the open ends ofboron nitride nanotubes: a DFT study, Structural Chemistry 22 (2011) 183–188.

12] G. Koh, Y.W. Zhang, H. Pan, First-principles study on hydrogen storage bygraphitic carbon nitride nanotubes, International Journal of Hydrogen Energy37 (2012) 4170–4178.

13] G. Chai, C. Lin, M. Zhang, J. Wang, W. Cheng, First-principles study of CN carbonnitride nanotubes, Nanotechnology 21 (2010), 195702(1)-195702 (4).

14] C. Cao, F. Huang, C. Cao, J. Li, H. Zhu, Synthesis of carbon nitride nanotubesvia a catalytic-assembly solvothermal route, Chemistry of Materials 16 (2004)2515–5213.

15] C. Li, E.T. Thostenson, T.W. Chou, Sensors and actuators based on carbon nano-tubes and their composites: a review, Composites Science and Technology 68(2008) 1227–1249.

16] Y.H. Li, J. Ding, Z. Luan, Z. Di, Y. Zhu, C. Xu, D. Wu, B. Wei, Competitive adsorptionof Pb2+, Cu2+ and Cd2+ ions from aqueous solutions by multiwalled carbonnan-otubes, Carbon 41 (2003) 2787–2792.

17] A. Ahmadi, N.L. Hadipour, M. Kamfiroozi, Z. Bagheri, Theoretical study of alu-minum nitride nanotubes for chemical sensing of formaldehyde, Sensors and

Actuators B: Chemical 161 (2012) 1025–1029.

18] J. Zheng, Z. Ren, P. Guo, L. Fang, J. Fan, Diffusion of Li+ ion on graphene: a DFTstudy, Applied Surface Science 258 (2011) 1651–1655.

19] P.E. Diaz-Flores, F. López-Urı’as, M. Terrones, J.R. Rangel-Mendez, Simulta-neous adsorption of Cd2+ and phenol on modified N-doped carbon nanotubes:

7 urface

[

[

[

[

[

[

[

[

[

[

06 J. Beheshtian et al. / Applied S

experimental and DFT studies, Journal of Colloid and Interface Science 334(2009) 124–131.

20] K. Nishidate, K. Sasaki, Y. Oikawa, M. Baba, M. Hasegawa, Density functionalelectronic structure calculations of lithium ion adsorption on defective carbonnanotube, Journal of Surface Science and Nanotechnology 3 (2005) 358–361.

21] J. Li, H. Li, X. Liang, S. Zhang, T. Zhao, D. Xia, Z. Wu, First principles study on thediffusion of alkali-metal ions on the armchair single-wall nanotubes, Journal ofPhysical Chemistry A 11 (2009) 791–796.

22] L.M. Costa, J.W.M. Carneiro, G.A. Romeiro, L.W.C. Paes, Interaction betweenalkaline earth cations and oxo-ligands. DFT study of the affinity of the Ca2+

cation for carbonyl ligands, Journal of Molecular Modeling 17 (2011) 243–249.23] C. White, D. Robertson, J.W. Mintmire, Helical and rotational symmetries of

nanoscale graphitic tubules, Physical Review B 47 (1993) 5485–5488.

24] J.-F. Jia, H. Wang, X.-Q. Pei, H.-S. Wu, Ab initio investigation of hydrogenation

of (BN)12, Applied Surface Science 253 (2007) 4485–4489.25] J. Beheshtian, A.A. Peyghan, Z. Bagheri, Detection of phosgene by Sc-doped

BN nanotubes: a DFT study, Sensors and Actuators B: Chemical (2012),http://dx.doi.org/10.1016/j.snb.2012.05.082.

[

[

Science 264 (2013) 699– 706

26] L. Guo, H.-s. Wu, Z.-h Jin, Magic behavior and bonding nature in hydro-genated aluminum nitride clusters, Applied Surface Science 242 (2005)88–96.

27] N.D. Yilmazer, M.F. Fellah, I. Onal, A density functional theory study of ethyleneadsorption on Ni10(1 1 1), Ni13(1 0 0) and Ni10(1 1 0) surface cluster models andNi13 nanocluster, Applied Surface Science 256 (2010) 5088–5093.

28] J. Beheshtian, H. Soleymanabadi, M. Kamfiroozi, A. Ahmadi, The H2 dissociationon the BN, AlN, BP and AlP nanotubes: a comparative study, Journal of MolecularModeling 18 (2012) 2343–2348.

29] M.W. Schmidt, K.K. Baldridge, J.A. Boatz, S.T. Elbert, M.S. Gordon, J.H. Jensen,S. Koseki, N. Matsunaga, K.A. Nguyen, S. Su, T.L. Windus, M. Dupuis, J.A. Mont-gomery, General atomic and molecular electronic structure system, Journal ofComputational Chemistry 14 (1993) 1347–1363.

30] Z. Zhong, G.I. Lee, C.B. Mo, S.H. Hong, J.K. Kang, Tailored field-emission propertyof patterned carbon nitride nanotubes by a selective doping of substitu-tional N(sN) and pyridine-like N(pN) atoms, Chemistry of Materials 19 (2007)2918–2920.

31] S. Li, Semiconductor Physical Electronics, 2nd ed., Springer, USA, 2006.