Physica E 43 (2011) 1717–1719

Contents lists available at ScienceDirect

Physica E

1386-94

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Interaction of NH3 with aluminum nitride nanotube:Electrostatic vs. covalent

Ali Ahmadi a, Javad Beheshtian b, Nasser L. Hadipour a,n

a Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iranb Department of Chemistry, Shahid Rajaee Teacher Training University, P.O. Box 16875-163, Tehran, Iran

a r t i c l e i n f o

Article history:

Received 14 April 2011

Received in revised form

18 May 2011

Accepted 25 May 2011Available online 1 June 2011

77/$ - see front matter & 2011 Elsevier B.V. A

016/j.physe.2011.05.029

esponding author. Tel.: þ98 218288 3495; fa

ail address: hadipour@modares.ac.ir (N.L. Had

a b s t r a c t

The interaction of NH3 with aluminum nitride nanotubes (AlNNTs) has been investigated on the basis of

density functional theory calculations. Unlike the case of carbon nanotubes, it was found that the NH3

can be chemically adsorbed on the top of the aluminum atom of AlNNTs. The NH3 adsorption energy of

AlNNTs is typically more than that of BNNTs. Despite the strong interaction, the chemical modification

of AlNNTs with the NH3 results in little changes in the electronic properties of AlNNTs. Morokuma, NBO

and density of states analyses reveal that the nature of this interaction is mainly electrostatic rather

than covalent.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Group III nitrides are important materials for producingtubular structures due to their excellent properties, e.g., hexago-nal aluminum nitride (AlN) with a band gap of �6.2 eV, is aninteresting material for field emitters and electronic substrates[1,2]. Moreover, AlN is well known for its unique properties suchas high thermal conductivity, superior mechanical strength and ahigh piezoelectric response [3,4].

In 2003, Zhang and Zhang, using an ab initio method, predictedthat AlN nanotubes (AlNNTs) are energetically favorable andarrange in a hexagonal network adopting an sp2 hybridizationfor both Al and N atoms [5]. Many theoretical investigationsassumed that AlN could form conventional single-walled nano-tubes similar to carbon nanotubes (CNTs) and showed that thestrain energy required in order to wrap up a graphitic sheet intoan AlNNT is lower than that required to form BNNTs, GaNNTs andCNTs with the similar diameters [6–14].

Nevertheless, synthesis of AlNNTs in bulk quantities is still achallenging problem. In 2003, Wu et al. synthesized the facetedsingle-crystalline hexagonal AlNNTs in a horizontal tube furnaceunder high temperature [15]. Using a highly non-equilibriumDC-arc plasma method, Tondare et al. successfully synthesizedthe AlNNTs with diameter ranging from 30 to 80 nm, in gramquantities [16]. Recently, other papers have reported on thesynthesis of AlNNTs through different processes [17–19].

ll rights reserved.

x: þ98 218288 9730.

ipour).

The opportunity to develop the applications of nanotubesrelies mostly on their successful manipulation. However, applica-tion and processing of nanotubes have been limited by theirinsolubility in most common solvents because they tend to clumptogether into bundles due to intertube dispersion interactions.This tendency of tube association posts an obstacle for furtherpurifying and processing of nanotubular materials.

Wrapping with polymers is a way to provide a supramolecularapproach to solubilization of nanotubes [18–20] but in manycases, sidewall chemical functionalization is an important firststep. Consequently, considerable efforts have been devoted to thechemical modification of nanotubes and semiconductor surfaces,which might pave the way to purification and their potentialapplications in molecule-based devices, drug delivery, biosensorsand so on [21–26].

Over the past decade, CNTs and BNNTs functionalization hasbeen extensively studied [27–29], while the functionalization ofAlNNTs has remained unexplored. Recently, Lim and Lin haveexplored the hydrogen chemisorption of single-walled AlNNTsusing density functional theory (DFT) [30]. In this work, we reportDFT study of AlNNTs functionalized with NH3. Our study can helpin applications of AlNNTs in gas sensor or biosensor industry ortheir purification and processing.

2. Computational methods

Geometry optimizations, the natural bond orbital (NBO) anddensity of states (DOS) analyses were performed on a (7, 0) zigzagAlNNT, constructing from 49 Al and 49 N atoms, and the NH3/AlNNTcomplex at the B3LYP/6-31Gn level of DFT as implemented in

Fig. 1. Geometric parameters for representative atoms in optimized aluminum

nitride nanotube, AlNNT, (bonds in angstroms and angles in degrees).

Fig. 2. The most stable configuration of NH3/AlNNT complex.

A. Ahmadi et al. / Physica E 43 (2011) 1717–17191718

Gaussian 98 suite of program [31]. Geometry parameters of theoptimized tube are shown in Fig. 1. The length and diameter ofoptimized pure AlNNT were computed to be about 17 and 7.34 A,respectively.

The Morokuma analysis was performed at the same level oftheory in GAMSS program. The atoms at the open ends of thenanotube were saturated by hydrogen atoms to avoid the bound-ary effect. The basis set superposition error (BSSE) for theinteraction energy was corrected using the counterpoise method.We define the adsorption energy (Ead) of NH3 as

Ead ¼ EtotðNH3þAlNNTÞ�EtotðAlNNTÞ�EtotðNH3ÞþEBSSE

where Etot is the total energy of a given system.

3. Results and discussion

Taking a zigzag (7, 0) AlNNT as sample (Fig. 1), we studied thestable structure of pure AlNNTs. As shown in Fig. 1, two types ofAl–N bonds can be found: one with the bond length of 1.810 A and inparallel with the tube axis, and another with the bond length of1.815 A, but not in parallel with the tube axis. The Al–N–Al andN–Al–N bond angles in Fig. 1, vary between 114.21 and 119.41, closeto 1201 (the angle between two sp2 hybrid orbitals).

Furthermore, the charge analysis using the NBO methodsuggests that about 0.64 e charges are transferred from theAl atom to its nearest N atoms in the tube, which are much largerthan those of carbon nanotubes (CNTs: 0 e) or boron nitridenanotubes (BNNTs: 0.21 e) [20]. Indicating semiconductor prop-erty, we computed the band gap of 4.27 eV for the pure single-walled AlNNT, which is in more agreement with former reportedby Zhao et al. [9]. The experimental band gap for AlN (hexagonalsheet) is much larger, 6.28 eV. However, contrary to the case ofcarbon nanotubes, the band gap of AlNNTs increases with thetubes diameter increasing and saturates at a value correspondingto the band gap of an AlN hexagonal sheet [32].

For the NH3/AlNNT complex, we scrutinized different possibleadsorption geometries, including the N and H atoms of NH3

molecule, nearest to the Al atom or N atom of the tube. The moststable configuration is such that the nitrogen atom of NH3 is close

to a sidewall Al atom with a distance of about 2.06 A in between,Fig. 2.

A local structural deformation at the adsorption site of nano-tube can be observed due to the adsorption of the NH3 molecule,where the sidewall Al atom is slightly pulled out of the surfaceand the corresponding three Al–N bonds within the sidewall(with bond length about 1.846, 1.855 and 1.855 A) are thusslightly lengthened.

The NBO analysis demonstrated a change in the hybridizationof the Al atom from sp2 to sp3 together with a charge transferfrom the lone pair of N atom of NH3 to the anti-bonding orbitalsof three Al–N bonds around Al site.

These results imply that the adsorption process is a stronginteraction in nature. The calculated adsorption energy is about�0.98 eV. The higher adsorption energy indicates that NH3

adsorption capability of AlNNTs is higher than that of CNTsand BNNTs.

To further understand the nature of interaction between NH3

and AlNNT, we analyzed the DOS for the NH3/AlNNT complex andcompared with the corresponding DOS for the individual parts,i.e. AlNNT and NH3 molecule separated.

Fig. 3 shows the total electronic DOS for the most stable stateof NH3, AlNNT and NH3/AlNNT complex. It can be seen that theDOS of NH3/AlNNT complex is almost exactly the superposition ofthe DOS of individual parts. This finding highlights that thecovalent interaction between NH3 and AlNNT is rather weak,and that no significant hybridization takes place between therespective orbitals of the two entities. Though, the DOS below thefermi level of AlNNT is not affected by the adsorption process(Fig. 3).

However, in spite of the strong interaction, the electronicproperties of the tube are not changed sensibly, so that theHOMO–LUMO energy gap of the tube is fairly equivalent beforeand after the adsorption process due to the less charge transferred(0.12 e obtained by NBO analysis) from NH3 to AlNNT. Thus, webelieved that the nature of this interaction may be mainlyelectrostatic, rather than sole covalent.

To further examine this issue, we performed the Morokumaanalysis on the NH3/AlNNT complex. Since the system is too largeto perform the Morokama analysis, we considered a small model,Fig. 4. Notice that our purpose was showing a qualitative featureof the nature of this interaction. Our results unveil that the highinteraction obtained quantitatively in terms of binding energieshas mainly electrostatic character.

Fig. 3. DOS for the most stable state of NH3, AlNNT and NH3/AlNNT complex.

Fig. 4. The model of NH3/AlNNT complex for Morokuma analysis.

A. Ahmadi et al. / Physica E 43 (2011) 1717–1719 1719

4. Conclusions

Density functional theory calculations have been used to inves-tigate the interaction of NH3 molecule with AlNNTs. The resultsimply that in comparison to the CNTs and BNNTs, interaction of NH3

with AlNNT is much stronger. The calculated adsorption energy isabout �0.98 eV. The higher adsorption energy indicates that NH3

adsorption capability of AlNNTs is higher than that of CNTs andBNNTs. The results of Morokuma, NBO and density of states analysesrevealed that the N–Al binding interaction in the NH3/AlNNT com-plex has more ionic character, rather than the sole covalent bond.

Acknowledgment

The authors would like to acknowledge the Iran NanotechnologyInitiative Council (INIC) for financial support.

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