Superlattices and Microstructures 60 (2013) 291–299

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o ca t e / s u p e r l a t t i c es

BeO nanotube bundle as a gas sensor

0749-6036/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.spmi.2013.04.028

⇑ Corresponding author at: Department of Physics, Razi University, Kermanshah, Iran.Tel./fax: +98 831 427 4556.E-mail address: a.fathalian@gmail.com (A. Fathalian).

Ali Fathalian a,b,⇑, Faramarz Kanjouri c, Jaafar Jalilian d

a Department of Physics, Razi University, Kermanshah, Iranb Institute for Studies in Theoretical Physics and Mathematics (IPM), Tehran, Iranc Physics Department, Faculty of Science, Kharazmi University, University Square, Karaj, Irand Young Researchers Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran

a r t i c l e i n f o

Article history:Received 19 February 2013Received in revised form 8 April 2013Accepted 25 April 2013Available online 16 May 2013

Keywords:Beryllium oxide nanotube bundleDensity functional theoryGas adsorption

a b s t r a c t

The structural and electronic properties of isolated and bundled of(8,0) Beryllium monoxide nanotube (BeONT) have been studied bythe first principles calculations in the framework of the densityfunctional theory (DFT). Results show that the inter-tube interac-tion in nanotube bundle changes the structural and electronicproperties of nanotubes. The effects of H2 molecule gas adsorptionon the electronic properties of BeONT bundle are investigation.Adsorption of H2 molecules in BeONT bundle reduced the semicon-ductor energy gap. Our results show that the BeONT bundle has agood candidate for hydrogen storage and a gas sensor.

Crown Copyright � 2013 Published by Elsevier Ltd. All rightsreserved.

1. Introduction

The nanostructures such as nanoparticles, nanowires and nanotubes have attracted considerableinterest due to their special electrical and mechanical properties. These novel materials have a widerange of application in nanoelectronics, nanoscaling biotechnology and biosensors [1,2]. Semiconduc-tor nanotubes such as ZnO [3,4], BN [5,6], SiC [7] and GaN [8] have attracted much attention in recentyears due to their unique structures and semiconductive properties. Other complex structures, very re-cently graphitic BeO nanofilm have been shown to be useful as precursors in the growth of wurtzitefilms [9,10] and BeO nanotubes have been investigated [11]. Moreover, BeO demonstrate many inter-esting nanostructures such as belt, helices [12] and nanotubes [13,14]. Nanotubes filled with chosenmaterials can lead to one dimensional structure with exciting new applications. Hydrogen as a pollu-tion-free energy resource has attracted much attention from material, physical and chemical scientists.

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How to efficiently store hydrogen has become a hotly concerned project, because it is a crucial factor forpractical utilization of hydrogen energy [15]. Many groups studied on hydrogen adsorption in carbonnanotubes bundle [16] and boron nitride nanotubes [17]. In 2007, Dapeng et.al studied on storage ofhydrogen in single wall carbon nanotubes bundle by grand canonical monte carlo method [18]. Theystudied the external surface effect on hydrogen storage in carbon nanotubes bundle. Theoreticallyhydrogen adsorption has been investigated by semi-empirical [19] and by first principles methods inBN cages [20]. In this paper, we study on adsorption of H2 gas in BeO nanotube bundle by the first prin-ciples study, due to the important of these materials in gas sensors devices.

2. Computational model and method

The present calculations are performed by using the full potential linearized augmented planewave method in the framework of the Density Functional Theory (DFT) [21] within the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA) [22], implemented the WIEN2kcode [23]. For beryllium and oxygen atoms the 1s state is the core state while 2s and 2p are the valancestates. We expand the basis function up to RMTKmax = 7.5, (RMT is the least muffin-tin sphere radii, KMAX

is the maximum modulus for the reciprocal vectors). The Fourier expansion charge density is trun-cated at Gmax = 14 Ry1/2 . The maximum angular quantum number for the expansion of wave functioninside the atomic spheres is used lmax = 10. The Brillouin zone integration is performed within theGamma centered Monkhorst–Pack scheme [24] using 4 � 4 � 18 k-point.

3. Results and discussion

3.1. Isolated and bundled (8,0) BeONTs

3.1.1. Structural propertiesWe obtained the equilibrium configurations of isolated and bundled (8,0) BeONT. It is found that

the Be–O bond length is about 1.56 Å, which is in agreement with other ab initio results [25]. As it

Fig. 1. Illustrate (a) isolated (8,0) BeONT, (b) structure of the (8,0) BeONT bundle in the cross section. The yellow and red ballsdenote beryllium and oxygen atoms respectively.

Fig. 2. Total valence charge density plot for Be–O atom bond along nanotube axis (before relaxation (red line) and afterrelaxation (blue line)).

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shown in Fig. 1, after relaxation Be atoms moved radially toward the tube axis while O atoms moved inthe opposite direction after relaxation. Thus, atomic positions changes after relaxation because of dif-ference in electronegativity of Be and O atoms. The values of electronegativity for Be and O atoms is1.57 and 3.44, respectively [26]. Be atoms moved inward while more electronegative O atoms moveoutward. Thus, the radial geometry of tubular structures is characterized by two concentric cylindricaltubes. All the Be atoms forming the inner cylinder and all the O atoms forming the outer cylinder. Theradial buckling b is defined by [27]:

Fig. 3.states f

b ¼ rO � rBe ð1Þ

where rO and rBe are the mean radii of O and Be cylinders, respectively. Here the radial buckling is cal-culated for isolated and bundled (8,0) BeONTs. The value of radial buckling for isolated (8,0) BeONTs is0.2 Å and for bundled (8,0) BeONTs is 0.4 Å. Thus, the value of radial buckling in (8,0) BeONTs bundleis larger than isolated (8,0) BeONTs. The reason of this phenomenon is attractive interaction between

(a)

(b)

(a) The total density of states for hexagonal (8,0) BeONT bundle and isolated (8,0) BeONT. (b) The partial density ofor beryllium and oxygen atoms in isolated and bundled (8,0) BeONT.

(a)

(b)

Fig. 4. Show the band structures of: (a) isolated (8,0) BeONT and (b) (8,0) BeONT bundle along the R � C � D � X �M � Csymmetry directions of the first Brillouin zone.

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BeONTs walls in BeONTs bundle. In order to explain the importance of the radial buckling on structuraland electronic properties of BeONTs, the valence charge density on sites O and Be atoms along thenanotube axis has been plotted. As it can be seen from the Fig. 2, the valence charge density is distrib-uted more around the O atoms, which is because of more electronegativity of O atom. The contribu-tions of valence charge density is changed after relation, as the contributions of valence charge densityis increased for O atoms and decreased for Be atoms. Therefore the bonding property is sensitive toradial buckling.

3.2. Electronic properties

The armchair and zigzag BeONTs are semiconductor and they have a wide energy gap. The isolated(8,0) BeONTs has the energy gap about Eg = 5.2 eV. Each of BeONT in bundle interacts with its sixneighboring tubes. To determine the stable ground structure state, the unit cell total energy have beencalculated in the terms of the inter-tube separation distance. In addition the equilibrium inter-tubeseparation distance about d = 3 Å. The inter-tube interaction energy, EI, for each nanotube bundle isdefined by [6]:

EI ¼ Etotalbundle � Etotal

isolated ð2Þ

where Etotalbundle and Etotal

isolated are total energy of bundled nanotube and total energy of isolated nanotubeper unit cell, respectively. The obtained value of inter-tube interaction energy is EI = �2 eV, which rep-resents that the bundle nanotube is more stable than isolated nanotube. To investigate the electronicproperties, the density of states (DOS) and the energy band structure have been calculated. The bun-dling of the isolated nanotubes to crystalline bundle changes the electronic properties of isolated

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nanotubes. The Fig. 3-a shows the isolated (8,0) BeONT and (8,0) BeONT bundle total DOS. As it can beseen from the figure, both structures are semiconductor. In nanotube bundle, the inter-tube interac-tion between the BeONTs walls with nearest neighbors causes a very small deformation, such as bandsplitting in conduction band and reduction of semiconducting energy gap (Fig. 4). The semiconductingenergy gap for isolated (8,0) BeONT is about Eg = 5.2 eV while the obtained value for energy gap of(8,0) BeONT bundle is Eg = 4.5 eV. Therefore, the energy gap in bundle decreases about 0.7 eV dueto the inter-tube interactions. In order to exactly investigation of electronic properties, the partial den-sity of states for beryllium and oxygen atoms compared in isolated and bundled (8,0) BeONTs to-gether. It can be seen from the Fig. 3b the conduction bands edge shifts toward Fermi level in (8,0)BeONT bundle.

4. Adsorption of H2 molecules

In this section the effects of adsorption of H2 molecules on electronic properties of (8,0) BeONTbundle are investigated. The Fig. 5 shows the snapshot for the adsorbed H2 molecules in BeONT bundleobtained from our calculations. The number of inserted H2 molecules are 8, 12 and 15 per unit cell,that distributed randomly inside and between BeONT walls. First, the systems are relaxed (H2 mole-cules + BeONT bundle) for all calculations. The Fig. 6 shows the DOS for H2 molecules adsorbed on

Fig. 5. Top views of the optimized structures of H2 molecules adsorbed on (8,0) BeONT bundle. The yellow, red and blue ballsdenote beryllium, oxygen and hydrogen atoms respectively.

(a)(b)

(d)(c)

Fig. 6. Illustrate: (a) The total density of states for pristine (8,0) BeONT bundle. The total density of states for H2 moleculesadsorbed on (8,0) BeONT bundle with various concentrations (b) 8H2, (c) 12H2 and (d) 15H2 molecules per unit cell.

(a)(b)

(d)(c)

Fig. 7. (a) The energy band structure of (8,0) BeONT bundle. The energy band structure of H2 molecules adsorbed on (8,0)BeONT bundle with various concentrations, (b) 8H2, (c) 12H2, and (d) 15H2 molecules per unit cell.

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BeONT bundle. The results show that adsorption of H2 molecules on BeONT bundle reduced the semi-conducting energy gap. The semiconducting energy gap is decreased by increasing number of ad-sorbed H2 molecules. By comparing the DOS and band structures of pristine (8,0) BeONT bundle

H2 Concentration (atoms/per unit cell)

Ene

rgy

gap

(eV

)

2.6

3

3.4

3.8

4.2

4.6

0 4 8 12 16

Fig. 8. Show the semiconducting energy gap in terms of adsorbed hydrogen molecule concentrations.

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and (8,0) BeONT bundle+H2 molecules, a number of differences are obvious: (i) The degenerate bandsin the pristine BeONT bundle split into non-degenerate states in the H2 adsorbed at (8,0) BeONT bun-dle (Fig. 7). (ii) adsorption of H2 molecules in BeONT bundle creates some sharp localized states belowthe fermi level (Fig. 7). (iii) DOS and band structures are split by adsorption of H2 molecules. (iv) Thepristine BeONT bundle has a direct semiconducting energy gap along the C point while adsorption ofH2 molecules lead to a semiconductor indirect gap, i.e., the conduction band minimum (CBM) and va-lence band maximum (VBM) are in C and A points, respectively (Fig. 7). Generally, adsorption of H2

molecules on BeONt bundle reduced the energy gap due to two mechanism: (a) creation of localizedstates near the fermi level, which are dependence on H2 molecules states. (b) the fermi level shifts to-ward the conduction band.

Also, the values of the energy gap are plotted in the terms of H2 molecules concentrations (Fig. 8),which shows that the adsorption of H2 molecules reduced the semiconducting energy gap. The adsorp-tion energy (Eadsorption) is defined as the difference between the total energy of H2 adsorbed at (8,0)BeONT bundle and the sum of the energy of the pristine BeONT bundle and H2 molecules, and is cal-culated by [28]:

Eadsorption ¼EðbundleþH2Þ � Epristine � nEH2

nð3Þ

The obtained values for adsorption energy of 8H@, 12H2 and 15H2 molecules are �7, �3.45 and �3 eV,respectively. It can be seen that the adsorption energy is decreased by increasing the number of ad-sorbed H2 molecules, so the system would be less stable.

For investigation of interaction extremity between H2 molecules and BeONT bundle (Fig. 9) the twodimensional charge density of pristine BeONT bundle and the H2 adsorbed at (8,0) BeONT bundle isplotted. Partitioning the charge density can be used to define charge transfer as well (the charge den-sity refers to the valence electron charge density). The Fig. 9a shows that the BeONT walls have a weekinteraction together. However, the interaction between H2 molecules and BeONT bundle is strong andcharge transfer occurred between H2 molecules and BeONT walls. The interaction between H2 mole-cules and BeONT walls is increased and became stronger by increasing the number of adsorbed H2

molecules.

5. Conclusions

In this paper, the structural and electronic properties of isolated and bundled (8,0) BeONT and theeffects of adsorption of hydrogen molecules on electronic properties of BeONT bundle is investigatedby using the first principles calculations in the framework of the density functional theory. The mainresults are:

Fig. 9. Illustrate: (a) The two dimensional charge density of the pristine (8,0) BeONT bundle, the two dimensional chargedensity of H2 molecules adsorbed on (8,0) BeONT bundle with various concentrations (b) 8H2 molecules and (c) 15H2 molecules.Magnitude of the charge density is expressed by colors, with the blue at one end of the spectrum being the smallest and thewhite at the other end the largest.

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(I) The semiconducting energy gap reduced in (8,0) BeONT bundle by compared with the isolated(8,0) BeONT due to inter-tube interactions. the obtained value for inter-tube interaction energyshows that the nanotube bundle is more stable than isolated nanotubes.

(II) The value of radial buckling in BeONT bundle is larger than isolated BeONT which representsthat the interaction between BeONT walls in bundle is attractive. After relaxation the radialbuckling changes the contributions of valence charge for Be and O atoms.

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(III) The semiconducting energy gap for (8,0) BeONT bundle is decreased by adsorption of H2 mol-ecules in this system. Adsorption of H2 molecules in BeONT bundle causes very small changeson electronic properties of BeONT bundle, such as shifting the fermi level towards conductionband, creating the localized states below the fermi level due to H2 molecules states and splittingdegenerate bands in pristine BeONT bundle to non-degenerate states.

(IV) The adsorption energy is decreased by increasing the concentration of H2 molecules which rep-resents that the systems trend to less stability.

(V) The two dimensional charge density shows that the H2 molecules interac with BeONT walls andcharge transfer increases between H2 molecules and BeONT walls by increasing the number ofadsorbed H2 molecules. The results show that the BeONT bundle is a good candidate for hydro-gen storage and a gas sensor.

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