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Sensors and Actuators B 161 (2012) 1025– 1029

Contents lists available at SciVerse ScienceDirect

Sensors and Actuators B: Chemical

journa l h o mepage: www.elsev ier .com/ locate /snb

heoretical study of aluminum nitride nanotubes for chemical sensing oformaldehyde

li Ahmadia, Nasser L. Hadipoura,∗, Mohammad Kamfiroozib, Zargham Bagheri c

Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, IranDepartment of Chemistry, Islamic Azad University, Shiraz Branch, Shiraz, IranPhysics Group, Science Department, Islamic Azad University, Islamshahr Branch, P.O. Box 33135-369, Islamshahr, Tehran, Iran

r t i c l e i n f o

rticle history:eceived 2 August 2011eceived in revised form0 November 2011ccepted 1 December 2011vailable online 9 December 2011

a b s t r a c t

Semiconductive carbon nanotubes (CNTs) have demonstrated great sensitivity toward molecules suchas NH3, NO, and NO2. Nevertheless, pristine CNTs cannot be used for detection of some highly toxicmolecules such as formaldehyde (HCOH). In the present study, we examined the possibility of usingaluminum nitride nanotubes (AlNNTs) as a potential gas sensor for HCOH detection by performing densityfunctional theory (DFT) calculation. It was found that HCOH molecule can be chemisorbed on the surfaceof AlNNTs with Gibbs free energies of −0.59 to −0.64 eV, at standard temperature and pressure (STP, 1 atm

eywords:ensorsormaldehydeluminum nitride nanotubesFT

and 298 K). In view of the high change of HOMO/LUMO energy gap of the tube during the chemisorption,it is expected that the process induce a significant change in its electrical conductivity. Hence, the AlNNTcan be used as a potential efficient gas sensor for HCOH detection. Furthermore, it was shown that H2Omolecules cannot significantly change the electronic properties of AlNNTs.

© 2011 Elsevier B.V. All rights reserved.

dsorption3LYP

. Introduction

An increasing interest in air quality control in the living envi-onment has arisen in the recent years. Formaldehyde (HCOH), asne of the substances causing sick house syndrome is a highly toxicolatile carcinogen [1,2], and can result in asthma, watery eyes, der-atitis, respiratory irritation, and pulmonary edema [2–4]. Thus, it

eems necessary to monitor and control its exposure in both indus-rial and residential environments. Recently, enormous efforts haveeen devoted for development of rapid, simple, and sensitive meth-ds for detecting HCOH [5–8]

Graphene and various nanotubes have attracted strong inter-sts as chemical sensors because of their high sensitivity anduick response time toward several molecules [9,10,7,11,12].or instance, in our previous study, it was shown that MgOanotubes selectively react with CO and NO gaseous molecules13]. Sensing mechanism is referred to the sensible conductancehange caused by charge transfer between adsorbents and gaseousolecules. Gaseous molecules affect electronic transport proper-

ies of graphene and tubes via physisorption or chemisorption.everal pristine nanotubes, however, cannot be used for detectionf some toxic gaseous molecules, since they cannot be adsorbed on

∗ Corresponding author. Tel.: +98 218288 3495; fax: +98 218288 9730.E-mail addresses: hadipour@modares.ac.ir, nasserhadipour@gmail.com

N.L. Hadipour).

925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.12.001

the surface of the tubes. Therefore, considerable experimental andtheoretical works have been focused on improving the sensing per-formance of the pristine tubes toward various desired molecules bydoping or functionalizing [14–16].

Performing density functional theory (DFT) calculations, Zhanget al. [17] have established that, unlike pristine carbon nano-tubes (CNT), the B-doped ones are good candidates for detectingHCOH molecules. Qin et al. have shown that graphene sheets withStone–Wales defects (SW-g), are more sensitive than the pristinetypes toward HCOH molecule. They also have emphasized thatunfortunately, the adsorption on the SW-g sheet is still very weak,due to its small binding energy, large binding distance, and smallnet charge transfer between the sheet and the HCOH, as well [18].Indicating a weak physisorption of HCOH molecule on the pris-tine boron nitride nanotubes (BNNTs), Zhang et al. [16] have foundthat the molecule presents strong chemisorption on silicon-dopedBNNTs.

Improving the sensing performance of the pristine tubes andgraphene sheets by manipulating their structure is too expen-sive, and thus, finding high sensitive pristine nanotubes is ahighly demanded interest. All the above mentioned problems, havemotivated us to verify heteropolar aluminum nitride nanotubes(AlNNTs) as a sensor, investigating their interactions with HCOH

molecule using DFT calculations.

AlNNTs are the inorganic type of quasi-one-dimensional nano-tubes. They are isoelectronic with CNTs, and have been successfullysynthesized by Tondare et al. and other research groups [19–21].

1 Actuators B 161 (2012) 1025– 1029

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Table 1Calculated adsorption energies (Ead), the bond reorganization energy (Ebr, calculatedas the energy difference between the geometry of HCOH after adsorption on AlNNTand the full relaxed molecule), �G and the charge transferred from the HCOH totube in the several studied configurations. Energies are in eV.

Configuration Ead Ebr QT (e) �G

A −1.30 1.89 0.17 −1.16B −0.37 1.51 0.15 −0.31

026 A. Ahmadi et al. / Sensors and

dsorptions of some gaseous molecules including NH3, H2, N2, andO2 on the AlNNTs have been reported so far [22–24]. Recently, weave shown that the NH3 adsorption capacity of AlN nanotubes isypically more than that of carbon and BN types [22].

. Computational details

Geometry optimizations, natural bond orbital (NBO), and den-ity of states (DOS) analyses were performed on a (7, 0) zigzaglNNT (constructed of 49 Al and 49 N atoms), and differentCOH/AlNNT complex configurations at the B3LYP/6-31G* level of

heory as implemented in Gaussian 98 suite of program [25]. Thisevel of theory is a popular approach which has been commonlysed for nanotube structures [26–28]. Vibration frequencies werelso calculated at the same level to confirm that all the stationaryoints correspond to true minima on the potential energy surface.eometry and DOS of the optimized tube is shown in Fig. 1, indi-ating that the tube is semi-conductive with HOMO/LUMO energyap (Eg) of 4.27 eV. The length and the diameter of the optimizedure AlNNT are computed to be about 17 A and 7.34 A, respectively.

n order to avoid the boundary effects, atoms at the open ends ofhe tube are saturated by hydrogen atoms. The adsorption energyEad) of HCOH molecule is defined as follows:

ad = E(

HCOHAlNNT

)− E(AlNNT) − E(HCOH) (1)

here E(HCOH/AlNNT) is the total energy of the adsorbed HCOHolecule on the AlNNT surface, and E(AlNNT) and E(HCOH) are

he total energies of the pristine AlNNT, and the HCOH molecule,espectively.

. Results and discussion

.1. Optimizations

For the AlNNT/HCOH complex, various possible adsorptioneometries was investigated by placing the O, C or an H atom ofCOH molecule on the Al or N atom of AlNNT, with the HCOH

Fig. 1. Structural model (panel A) and the electronic density of states (DO

Fig. 2. Optimized structures of chemically functionalized (7, 0) zigz

C −0.68 0.02 0.16 −0.59D −0.73 0.03 0.16 −0.64

molecular axis being vertical to the surface of the tube, and parallelto the latitudinal or zigzag Al N bond, respectively.

Herein, we obtained four most stable adsorption configurationsas shown in Figs. 2 and 3. The adsorption process was consid-ered in two forms, including the chemical functionalization (CF)in which a strong interaction changes significantly the geomet-rical parameters and rehybridization of atoms in adsorption site,and chemisorption, with an insignificant change in the mentionedparameters (with an Ead more than −0.60 eV).

3.1.1. Chemical functionalizationAs shown in panel A of Fig. 2, carbon and oxygen atoms of HCOH

molecule are close to the N1 and Al2 atoms (Fig. 1) of the AlNNT. Thecalculated value of Ead is about −1.3 eV (Table 1) and correspond-ing interaction distance between the C and O of the HCOH and N1and Al4 atom are 1.53 and 1.79 A, respectively. The adsorption ofHCOH molecule induces a locally structural deformation, both initself and in the AlNNT. The bond length of N1–Al2 (Figs. 1 and 2) ofthe tube is significantly increased from 1.81 A in the free AlNNT to2.12 A in the adsorbed form, indicating a bond weakening or evencleavage, due to the strong interaction. During this process, the NBOhybridization of N1 and Al2 atoms are changed from sp2 to sp3.

The NBO analysis also shows a rehybridization of both the C and

O atoms of the HCOH from sp2 to sp3, in this configuration. Geomet-ric parameters, such as the increased C O bond length (1.38 A ascompared to 1.21 A in the isolated HCOH), reduced H C H bondangle (106.3◦ as compared to 115.4◦ in the isolated HCOH), and

Ss) (panel B) for the (7, 0) AlNNT. The unit of DOS is electrons/eV.

ag AlNNT with formaldehyde. The distances are in angstrom.

A. Ahmadi et al. / Sensors and Actuators B 161 (2012) 1025– 1029 1027

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Fig. 3. Optimized structures and density of states (DOSs) of chem

eviation of the hydrogen atoms from the original molecular plane,learly indicate high sp3 character of the C and O atoms. Furtherndication of the deformation degree in the Geometry of HCOH,s a result of the adsorption process, is given by the bond reorga-ization energy (Ebr, Table 1), calculated as the energy differenceetween the full relaxed HCOH and the adsorbed molecule. Ebr ofhe HCOH for the configuration is 1.89 eV, which is the confirmationf a strong interaction.

The strong adsorption of the HCOH on the AlNNT in this configu-ation can be easily rationalized by the fact of the highest occupiedolecular orbitals (HOMOs) of AlNNT being located on the N atoms,

nd its lowest unoccupied molecular orbitals (LUMOs) on the Alnes. As the result, the HOMO of HCOH, more locating on C atom,onates electrons preferably to the LUMO centered on the Al atomf AlNNT, and the HOMO of AlNNT, more locating on N atoms,onates electrons preferably to the LUMO centered on the C atomf HCOH. All above mentioned evidences, indicate that the AlNNT ishemically functionalized by the HCOH molecule, in configuration.

A similar trend was found for the configuration B (Fig. 2, panel B)n which, the C and O atom of the HCOH is close to N1 and Al4 atomsf the AlNNT (Figs. 1 and 2). The interaction distances between the Cnd O of the HCOH and N1 and Al4 atoms of the AlNNT are 1.63 and.89 A, respectively. As configuration A, a locally structural defor-ation is observed for both the HCOH molecule and the AlNNT.

he distances between N1 and Al4 (Figs. 1 and 2) of the tube is con-iderably decreased from 3.57 A in the free AlNNT to 3.24 A in thedsorbed form. The calculated Ebr of the HCOH is about 1.51 eV, con-rming a strong structure deformation. Accordingly, it is concludedhat the CF process has taken place in this configuration.

CF is not the appropriate process, however, in gas detection dueo the followings:

(a) Despite the thermodynamic feasibility of these interactions,they cannot take place easily in the room temperature because

of the high activation barriers for structure deformation.

b) One of the most important characteristics of the gas sensors isrecovery of the device. However, strong CF implies that the des-orption of the adsorbate could be difficult and the device may

adsorptions of HCOH on AlNNT. The unit of DOS is electrons/eV.

suffer from long recovery times. Much longer recovery timeis expected, if the adsorption energy is significantly increased.Based on the conventional transition state theory, the recoverytime, �, can be expressed as

� = �−10 exp

(−Ead

kT

)(2)

where T is temperature, k is the Boltzmann’s constant, and �0 is theattempt frequency. According to Eq. (2), an increase in the adsorp-tion energy Ead, will prolong the recovery time in an exponentialmanner.

3.1.2. ChemisorptionAs shown in panel C of Fig. 3, in configuration C, the O and one

of the H atoms of the HCOH are close to N1 and Al4 atoms of theAlNNT, respectively. The distances between the H and O atoms ofthe HCOH, and N1 and Al4 atoms of the AlNNT are 2.60 and 2.07 A,respectively. The Ead value of this configuration is approximately−0.68 eV, with a charge transferred of 0.16 e from the HCOH to thetube.

The locally structural deformation of both the HCOH moleculeand the AlNNT are not comparable to those of the configurationsA and B. The distance of Al1 and N1 atoms (Figs. 1 and 3) showsan insignificant change of 0.02 A. Additionally, the length of threeAl N1 bonds, around the N1 atom (adsorption site), is very slightlyincreased from 1.81 A in the free AlNNT to 1.83 A in the adsorbedform. The NBO hybridization of N1 and Al4 atoms are not signif-icantly changed during this process, and the value of Ebr of theHCOH is about 0.02 eV, indicating that there is a slight structuredeformation compared to that of CF.

To explore the effect of entropy and temperature on the adsorp-tion processes, we also calculated Gibbs free energy (�G), atstandard temperature and pressure (STP, 1 atm and 298 K). The

results are shown in Table 1, indicating that they are somewhatsmaller than the Ead values because of the entropic effect. The HCOHmolecule can be chemisorbed on the surface of AlNNTs with �G val-ues in the range of −0.59 to −0.64 eV at the mentioned conditions.

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028 A. Ahmadi et al. / Sensors and

.2. Electronic properties

In order to considering the sensitivity of the AlNNT toward theCOH molecule, we plotted the DOS of this structure and com-ared it with that of the pristine AlNNT (Fig. 1). Fig. 3C shows theOS of the HCOH on AlNNT, dramatically different from that of

he corresponding pure AlNNT due to the strong interaction. Com-ared to the pristine tube, the DOS of the configuration C shows aew peak appeared just above the Fermi level. This peak indicateshat after the chemisorption of the HCOH molecule, the systemecomes more semiconductor-like, with a drop in the DOS nearhe Fermi level, and thus, a significant increase in electric conduc-ivity is expected, compared to the pristine tube. The phenomenonan be explained by the following relation [29]:

∝ exp(−Eg

2kT

)(3)

here � is the electric conductivity, and k is the Boltzmann’s con-tant. According to the equation, smaller Eg at a given temperatureeads to the larger electric conductivity. However, in configura-ion C, the Eg decreases from 4.27 eV in the free AlNNT to 3.08 eVy the chemisorptions process. The considerable change of about.19 eV in Eg value, demonstrates the high sensitivity of the elec-ronic properties of AlNNTs toward the adsorption of the HCOH

olecule. It should be noted that, the chemisorption of the HCOHnto the AlNNT causes the major band features to move toward theigher energies; in other words, the Fermi level is shifted towardhe lower energies. We think that AlNNT can transform the pres-nce of the HCOH directly into an electrical signal, therefore, coulde potentially used in HCOH sensors.

As mentioned before, the desorption of adsorbate is of greatmportance. It is expected that upon this process, the conductivityf the tube should be reduced according to the following equation:

= n�e (4)

here n, � and e are the carrier concentration, mobility and chargef electron, respectively [29]. Considering the role of the HCOHolecule as an electron donor in the tube, it is believed that the cor-

esponding reduction in amount of the carrier concentration afterhe desorption is responsible for the decrease in conductivity oflNNT. However, it has been shown that the electric conductiv-

ty of different adsorbents changes upon the desorption process byeveral experiments [30,31].

We found that the adsorption process of the HCOH moleculen the AlNNT in configuration D (Fig. 3, panel D) is similar to thatf the configuration C, both thermodynamically and electrically. Inonfiguration D, the O and one of H atoms of the HCOH are close

o the N3 and Al4 atoms of the AlNNT, respectively. The calculatedad is approximately −0.73 eV, and the distance between the H and

of the HCOH, and N3 and Al4 atoms of the AlNNT are 2.63 and.04 A, respectively. The locally structural deformation of the HCOH

Fig. 4. Geometrical structures of different H2O/AlNN

tors B 161 (2012) 1025– 1029

molecule and the AlNNT is small, so that the Ebr of the HCOH is about0.03 eV.

As shown in panel D of Fig. 3 (configuration D), similar to the con-figuration C, the adsorption of the HCOH onto AlNNT causes a clearincrease of the DOS in the region just above the Fermi level, so thata virtual sate appears in the energy level of −3 eV (in comparisonto the free AlNNT); which is also expected to cause an increase inthe conductance. Meanwhile, the Fermi level shifts slightly towardthe lower energies after the adsorption.

It is understood that, when AlNNT interacts with an electron-rich HCOH molecule, charge transfer of 0.16 e to the tube occurs,which dramatically enhances the conductivity of the HCOH onAlNNT system. Based on Eq. (3), the change in DOS, especiallynear the Fermi level, is expected to bring about obvious changes inthe corresponding electric conductivity. Thus, it is concluded thatAlNNTs may be suitable for sensing applications for toxic HCOH.

It is noteworthy to mention that the adsorption of HCOH onAlNNTs in configurations C and D, are not physisorption due to thepartly large values of Ead. However, we think that the chemisorp-tions is taken place preferably in room temperature comparing tothe CF, due to its low energy barriers and slight structure deforma-tions.

3.3. The H2O adsorption

Finally, the effect of water or humidity on the electronic prop-erties of the AlNNT has been investigated. We first considered theadsorption geometry of the H2O molecule on the outer wall of thetube. Here, various adsorption sites, including the center of thehexagon, the bridge of the Al N bond, and top of the Al and Natoms, are explored to determine the optimal adsorption geometry.Geometry optimizations converged to three adsorption geometries(Fig. 4) including:

(1) Configuration I (Fig. 4A) in which, an H atom of the H2O forms ahydrogen bond with an N atom of the tube surface with distanceof 1.96 A, and Ead of −0.40 eV.

(2) Configuration II (Fig. 4B) in which, the O atom of the H2O pointstoward an N atom of the tube with a distance of 2.01 A, releasingan energy of −0.89 eV.

(3) Configuration III (Fig. 4C) in which, both of the H atoms in H2Oare close to two N atoms of the tube and its O atom sits on an Alatom. The Ead of this configuration is approximately −1.01 eV,which is the largest among the all studied configurations as aconsequence of both hydrogen and covalent bond formation.

However, as shown in Table 2, the H2O cannot influence the Eg

value of AlNNT and the largest change of Eg belongs to the con-

figuration C, in which the Eg of the tube is reduced from 4.27 to4.18 eV, showing a reduction of 0.02% which is negligible. There-fore, it can be concluded that humidity may not affect the electricalconductivity of AlNNT due to having no effect on its Eg.

T complexes. The distances are in angstrom.

A. Ahmadi et al. / Sensors and Actua

Table 2Calculated adsorption energies (Ead) and HOMO–LUMO gap of H2O/AlNNT com-plexes. We note that the Eg of pristine tube is about 4.27 eV. Energies are in eV.

Configuration Ead Ega�Eg

I −0.40 4.26 0.01II −0.89 4.19 0.08

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. Conclusion

In summary, we performed density functional theory calcula-ions to explore the adsorption of HCOH molecule on AlNNTs. Ouresults suggest that HCOH molecule can be chemisorbed on theurface of the AlNNTs, both with significant adsorption energiesnd charge transfer, which could induce significant changes in thelectrical conductivity of the tube. Thus, AlNNTs may be a promisingandidate for detection of toxic HCOH. We also showed that the H2Oolecule cannot significantly influence the electronic properties of

he AlNNTs.

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Biographies

Ali Ahmadi was born in Tabriz, Iran, in 1983. He received the B.Sc. degree in theCollege of Chemistry, University of Tehran and received the M.Sc. degree in PhysicalChemistry from Tarbiat Modares University. Currently, he is a Ph.D. candidate ofTarbiat Modares University. His main research interests are in the computationalchemistry, gas sensors, nanostructured materials and classical thermodynamics.

Nasser L. Hadipour was born in Rasht, Iran, in 1948. He studied chemistry at theSharif University of Technology. He received the M.Sc. degree in Chemistry fromNortheastern University at Boston and the M.Sc. degree in Physics from Universityof Massachusetts at Amherst. He has got Ph.D. in Solid State Chemistry from Uni-versity of Massachusetts at Amherst and post-doc from City University of New York.Dr. Hadipour is currently working as head of laboratory and working group on com-putational chemistry at Tarbiat Modares University. His main research interests arein the Gas sensors, Hydrogen bonding, NMR & NQR parameters and nanomaterials.

Mohammad Kamfiroozi was born in Shiraz, Iran, in 1981. He received the B.Sc.degree in the College of Chemistry, University of Tehran and received the M.Sc.degree in Analytical Chemistry from Isfahan University of Technology. His mainresearch interests are in the computational chemistry, gas sensors and HPLC.

Zargham Bagheri was born in Tehran, Iran, in 1970. He graduated from the Univer-sity of Isfahan (Iran) with a M.Sc. degree in Physics in 1995. He was a research fellow

at the Islamic Azad University, Science and Technology Branch, Tehran, Iran andreceived his Ph.D. in Solid State Physics in 2000. Currently, he is assistant professorof Physics at Islamic Azad University, Islamshahr Branch, Iran. His research inter-ests include nanostructured materials, electrochemical gas sensors, ComputationalPhysics and solid state NMR.