In order to search for novel nanomaterial to adsorb methane (CH4) molecule, the adsorption of CH4
on the hydrogenated edges of armchair (4,4) aluminum nitride (AlN) and silicon carbide (SiC) nano-tubes has been systemically investigated using two different density functional theory (DFT) methods.Two MPW1PW91 and PBEPBE functional are employed in all calculations for the considered structures,adsorption energy, and NBO analysis. The CH4 molecule physisorbed on Tn (n = 1 and 2) sites on the surfaceof pure AlN and SiC nanotubes with an adsorption energy of about −4.42 and −1.43 kJ/mol respectively.The properties of CH4 molecule adsorbed on Ni-doped AlN and SiC nanotubes are also investigated. Theadsorption energy rises to about −60.36 and −39.26 kJ/mol for AlN Ni CH4 and SiC Ni CH4 respec-tively, when the CH4 adsorbed to Ni-doped nanotubes. Compared with the weak adsorption on pureAlN and SiC nanotubes, CH4 molecule tends to be strongly physisorbed to the Ni-doped AlN and SiCnanotubes with appreciable adsorption energy. The increase in adsorption energy is due to the chargetransfer from Ni-doped AlN and SiC nanotubes to the CH4 molecule. A considerable amount of charge
transfer during the adsorption process on Ni-doped AlN and SiC nanotubes may account for the changesof the electronic properties. With the adsorption of CH4 molecule on functionalized nanotubes, the bandgap of AlN Ni CH4 and SiC Ni CH4 systems are increased, thus leading to decreased reactivity of thesystems, the stability of the systems increased. These observations show that functionalized AlN and SiCnanotubes are highly sensitive toward CH4 molecule. Moreover, the present results may be useful for thedesign of AlN and SiC nanotubes based nanomaterials candidates such as adsorbent and storage.
. Introduction
Since the discovery of carbon nanotubes (CNTs) by Ijima [1],anotube as one-dimensional (1D) nanostructures have attractedxtensive attention due to their unique electric [2,3], optical [4], andechanical [5] properties in science and technology. Significant
fforts have been taken to study nano materials both experi-entally and theoretically [6]. A number of nanotubes, such asaN [7], BN [8], WC [9], SiC [10], and AlN [11] nanotubes, haveeen predicted theoretically. A variety of nanotubes such as boronitride nanotube (BNNT) [12] and hexagonal aluminum nitrideanotube (AlNNT) which are actually nano-rod with hallow cen-er [13] have been experimentally synthesized by various methodsuch as arc-discharge [14], metal-boride-catalyzed chemical vaporeposition [15], continuous laser heating at super high [16] or
mbient pressure [17] and so on. In general, III–V compounds, espe-ially the boron nitride (BN) are found to be important source ofanoscale one-dimensional (1D) tubular materials [18]. 1D III–V
semiconductors nanostructures of boron nitride nanotube (BNNT)have attracted much attention due to their large band gap.Aluminum nitride nanotube (AlNNT) like BNNT show unique prop-erties such as large band gap (∼6.2 eV), high thermal conductivity,and so on [6]. The Pauling electronegativity of the nitrogen atomis 3.04 which is much larger than that of aluminum atom [19] andindicate that a significant charge transfer from aluminum atom tonitrogen atom was occurred. Although AlNNT has not yet been syn-thesized, Zhao and Ding have investigated the strain energy andstability of single walled aluminum nitride using density functionaltheory [20]. Recently, Liu et al. [6] studied the interaction of an openended single walled AlNNT with H2, N2, and O2 molecules using DFTcalculations. Also, Jiao et al. [21] reported the interactions of CO2and N2 with AlNNT.
The interaction of AlNNTs with gases, excepting hydrogen [22],ammonia [23], and carbon dioxide [21] has seldom been investi-gated and remains largely an unexplored area.
The silicon carbide nanotubes (SiCNT), an analog of carbon nano-
tubes, have been synthesized via the reaction between the silicon(produced by disproportionation reaction of SiO) with multi walledcarbon nanotubes (MWCNTs) (as templates) at different tempera-tures [24] and exhibit 1D tubular forms [25]. SiC has a large energy
arrier (∼1.25 eV) for transition between sp2 and sp3 structures.his means that the synthesis of SiC nanotube (SiCNT) is restrictedue to this energy difference [26]. Similar to the BNNT, SiCNT is aemiconductor with a large band gap, and is of technological inter-st for devices operate at high temperatures, high-frequency, andn harsh environment [27]. SiCNTs with alternating Si C bonds are
ore stable than the structures which contain C C or Si Si bonds10]. More recently, theoretically studies [10,26,28] showed thathe structures and the stability of SiCNTs with Si C bond are moretable. Carbon nanotubes are highly aromatic systems. Replacingne-half of the C atoms in CNTs by Si atoms decreases the aro-aticity of each six-member ring. The decreasing of aromaticity
eads to the decrease in stability. Therefore, the exterior surface ofiCNTs has a higher reactivity than that of SWCNTs [29,30]. Becausef higher reactivity of exterior surface of SiCNT, the SiCNT can bepplied as chemical gas sensors. Theoretical studies show that O231], H2 [32], CO and HCN [33], NO and N2O [34] can be chemisorbedn the exterior surface of SiCNT with large binding energy.
Methane (CH4) gas at room temperature, hold much promise asources of clean and affordable energy because it is more environ-entally friendly than most hydrocarbon fuels and it is also readily
vailable [35]. The adsorption of Methane on carbonaceous mate-ials developed from activated carbon, fullerene, carbon nanotubesnd nano bundles [36–38] opens the new interesting applicationsf gas storage.
In this research work, we investigate the properties of methanedsorption onto pristine and functionalized SiCNT and AlNNT nano-ubes using DFT calculations. We first present the interaction of CH4ith pristine AlNNT and SiCNT and then the adsorption and elec-
ronic properties of CH4 adsorbed onto the Ni-doped AlNNT andiCNT were examined.
. Computational details
The calculations were carried out using G03 [39] package. Thepin-polarized generalized gradient approximation (GGA) with theodified Perdew-Wang91 exchange [40] plus the Perdew-Wang91
orrelation (MPW1PW91) [41] and the Perdew-Burke-ErnzerhofPBE-PBE) corrections [42] were selected. We have employed a
ixed basis set formed by the CEP-121G [43] for Ni atom in con-unction with the conventional 6-31G basis set for all other atoms.
e chose two model nanotube (NT) systems with comparableiameter, length and chirality, i.e., finite (4,4) AlN and SiC singlealled nanotubes with hydrogen atoms terminated on the two
nds. The (4,4) AlN and SiC nanotubes with 7.34 A and 7.31 A diam-ter, respectively, shown in Fig. 1. The bond lengths of Al N andi C are about 1.818 A and 1.813 A respectively. Furthermore, theength of the both nanotubes is a 16.3 A. AlNNT include totally 40luminum and 40 nitrogen atoms. Also, SiC includes 40 atoms sili-on and 40 carbon atoms. Firstly, we determined the sites that a CH4olecule can adsorb into the nanotubes. It is noteworthy that due
o the ionicity of Al N and Si C bonds, the electronic structuresf these nanotubes are almost independent of the tube chiralitynd diameter. Therefore, it seems that investigations on armchairlNNTs and SiCNTs could be transferred to zigzag AlNNTs and SiC-Ts. Five possible adsorption sites on AlN and SiC nanotubes areonsidered, shown in Fig. 1: T1 (top of the Al or C atoms), T2 (top ofhe N or Si atom), T3 (top of the Al N or Si C band and top of axialigzag Al N or Si C bond), H (top of the center of hexagon), and Cinside the nanotubes).
Conceptual density functional theory provides insights into the
opular qualitative chemical concepts such as electronic chemicalotential [44] (�), hardness [45] (�) and electronegativity [46] (�).sing successive derivatives of the energy with respect to either
he number of electrons or the external potential, both local and
Fig. 1. The structures of (a) SiCNT and (b) AlNNT nanotubes. For possible sites forCH4 adsorption at T1 (top of the Al or C atoms), T2 (top of the N or Si atom), T3 (topof the Al N or Si C band), H (top of the center of hexagon).
global descriptors have been defined. Those descriptors are able tomeasure the whole response of an electronic system to a chemicalperturbation [47]. For an N-electron system with total energy E andexternal potential �(r), electron negativity is defined [46]:
� = −(
∂E
∂N
)�(r),T
= −� = εL + εH
2(1)
� = 12
(∂2E
∂N2
)�(r),T
= 12
(∂�
∂N
)�(r),T
= εL − εH
2(2)
where � and � are the electronic chemical potential and chemicalhardness, respectively. Following Janaks theorem, � and � could beapproximated in terms of energies of HOMO and LUMO molecularorbitals using density functional calculations (Eqs. (1) and (2)).
Parr et al. [48] have recently introduced an electrophilicity index(w), as:
w = �2
2�(3)
which was proposed as a measure of the electrophilic power of amolecule.
3. Results and discussion
3.1. Electronic properties of pristine AlNNT and SiCNT
The geometries of the pristine AlN and SiC nanotubes were fullyoptimized in the framework of density functional theory using
Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562 555
Table 1Highest occupied molecular orbital (HOMO), lowest unoccupied orbital (LUMO), gap () energy, electronic chemical potential (�), hardness (�), softness (S) and electrophilicity(ω) of AlNNT, SiCNT, AlNNT–CH4 (T1 site) and SiCNT–CH4 (T2 site) systems.
f MPW1PW91 and PBEPBE functional methods. The calculatedtructural parameters of AlNNT and SiCNT such as Al N and Si Cond length (∼1.8 A) are in well agreement with the other researchorks [49,50]. To testify the validity of our calculation methods
MPW1PW91and PBEPBE), we compared our DFT results with theesults of the Menon et al. [10] and Zhao and Ding [51]. We foundut that the electronic properties of nanotubes such as band gapsre well reproduced by our present DFT schemes. These results areummarized in Table 1. It is well known that a material with a smallut nonzero band gap is referred to as a semiconductor Our DFT cal-ulation results show that the SiCNT and AlNNT are semiconductorsnd have band gap about 3.41 eV and 4.88 eV, respectively. Menont al. [10] using ab initio method showed that the band gap ofhe SiC nanotubes was 3.53 eV. Zhao and Ding [51] investigated
he electronic properties of SiC nanotube using PBE method andhey obtained the band gap of SiC is about 1.30 eV in which aren well agreement with our obtained results from PBEPBE method∼1.75 eV). In the case of the AlNNT, we compared our results with
-15
5
25
45
65
85
105
1.5 3.5 5.5 7.5 9.5
Eads(kJmol-1)
r(Å)
T1
T2
T3
H
-10
0
10
20
30
40
50
60
70
2 3 4 5 6 7 8 9 10
Eads(kJmol-1)
r(Å)
T1
T2
T3
H
(a)
(b)
ig. 2. Potential energy surfaces of the adsorption of CH4 molecule as function ofube–CH4 distance on (a) AlNNT and (b) SiCNT using MPW1PW91/6-31G method.
the results of research work which was done by Belkhir [52]. Belkhirshowed that the band gap of hexagonal AlNNT was 4.64 eV usingPBE method. Our results show that the band gap of AlNNT is about4.88 eV. Hence, we conclude that the results of the two calculationmethods were used in this research work are in good agreementwith the other previous research works [10,51,52].
Natural bond orbital (NBO) analysis was performed usingMPW1PW91/3-21G* and PBEPBE/3-21G*. NBO analysis shows thatthe natural charges of nitrogen and aluminum atoms in AlN nano-tube are about −1.68 and 1.68 esu respectively because of thedifference between electronegativity of Al and N atoms. This trendwas also obtained for SiC nanotube by two used calculation meth-ods. In addition, the charge analysis using NBO calculations indicatethat the natural charges of carbon and silicon atoms in SiC are about
−1.79 and 1.79 esu respectively. Because of more electronegativityof carbon, the charges transfer from silicon atom to the vicenty ofthe carbon atoms, indicating that the Si C bonds of the side wall
Fig. 3. Fully optimized geometrical structures of CH4 adsorption on outer surfaceof (a) AlNNT at T1 site and (b) SiCNT at T2 site using MPW1PW91/6-31G method.
556 Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562
Table 2Adsorption energy (Eads (kJ/mol)) and equilibrium distance, re (Å), of methaneadsorbed on pristine nanotubes.
re partially ionic. (see Tables S1 and S2 in Supporting Informa-ion).
.2. Methane adsorption on AlNNT and SiCNT
To examine the adsorption properties of CH4 adsorbed onto theanotubes, the adsorption energy curves (Fig. 2) of nanotube–CH4,s well as the pristine nanotube and CH4 molecule, were calcu-
ated. The adsorption energy, Eads, was calculated using the belowxpression:
ads = Etube−CH4− (Etube + ECH4 ) (4)
able 3alculated partial charges of Al, N, Si, C and H atoms for the adsorption of CH4
olecule on pristine and functionalized AlN and SiC nanotubes (atom numbering isccording to Figs. 5 and 6).
Fig. 4. Typically fitted potential energy curves (true energy) with Morse potentialfor adsorption of CH4 molecule on (a) T1 site of AlNNT and (b) T2 site of SiCNT usingMPW1PW91/6-31G method.
where Etube−CH4is the total energy of pure nanotubes with an
adsorbed CH4 molecule, and ECH4 and Etube is the total energy ofthe isolated methane and nanotube molecules respectively. Hence,according to Eq. (4), negative adsorption energy indicates that thesystem is stable. In some cases the adsorption energy is positivewhich correspond to the local minimum where the adsorption ofmethane onto the nanotubes is prevented by a barrier.
First, we have explored a CH4 molecule adsorbed onto thebare AlNNT and SiCNT. For the AlNNT and SiCNT different adsorp-tion sites including T1 (top of the Al or C atoms), T2 (top ofthe N or Si atom), T3 (top of the AL N or Si C band and thetop of axial zigzag Al N or Si C bond), H (top of the center ofhexagon), and C (inside the nanotube,), were considered. In thecase of AlNNT and SiCNT, the CH4 molecule interacts with thesurface of the nanotubes with interaction distance about 3.2 and3.9 A for AlNNT–CH4 and SiCNT–CH4, respectively. The calculatedadsorption energies and the nearest intermolecular distances fordifferent adsorption sites are collected in Table 2. The obtainedresults indicate that CH4 molecule adsorbed onto the AlNNT andSiCNT through weak Van der Waals interaction, which means thatthe process is physisorption. For this weak adsorption, the chargetransfer between nanotubes and CH4 molecule is very small (about0.01 esu).
The data summarized in Table 2 contain some interesting fea-tures. Firstly, in the case of AlNNT, the calculated adsorptionenergies, obtained from two different DFT schemes, for the T1 and
T3 sites are negative and have the same values means the adsorp-tion of CH4 molecule on these positions are favorable and thegeometries are more stable than those others. Furthermore, thefinal optimization structure of both of T3 site (top of the AL N
Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562 557
F rface
ambtiosomvFnmieitt
wacHtacoaa
c
Fig. 4 shows the typically fitted potential energy curves (correctedadsorption energy or true energy) with Mores potential. Thesecurves show the typical features of the real intermolecular inter-actions and reflect the salient features of the real interactions in
Table 4The well depth, 2D, and adjustment parameter, � in Morse potential equationfor methane adsorption on different sites on AlNNT and SiCNT calculated byMPW1PW91 and PBEPBE methods.
ig. 5. Fully optimized geometrical structures of Ni metal atom bonded on outer su
nd the axial zigzag Al N) indicates that the position of the CH4olecule is changed from T3 to T1 site, leads to the T1 site is the
est site for adsorption of CH4 molecule on the outer surface ofhe AlNNT. This result can be considered due to the high polar-zibility of aluminum than that of nitrogen atom [53]. In the casef SiCNT, all the calculated adsorption energies except for the T2ite are positive which means that the adsorption of CH4 moleculento the SiCNT in the T2 site is favorable and the geometries areore stable than those others (see Fig. 3). Moreover, the positive
alues of adsorption energy of CH4 on C site (inside the tube, seeigs. S1 and S2 in Supporting Information) of both AlN and SiCanotubes (about 48 and 63 kJ/mol, calculated from MPW1PW91ethod) showed that the CH4 molecule cannot adsorbed onto the
nner surfaces of both nanotubes. Due to the adsorption energy andlectronic properties of CH4 adsorbed on T1 site of AlN nanotubes,t reveals that the pristine AlN nanotube is favorable for the adsorp-ion of CH4 molecule than that of pristine SiC nanotubes. In addition,he low-energy-gain is an indication of a physisorption process.
Analysis of NBO calculation indicates that the net charge transferas occurred from the nanotubes to the CH4 molecule. The partial
tomic charges of tube–CH4 systems are summarized in Table 3. Inomparison, the electronic properties of tube–CH4 system such asOMO–LUMO gap energy with pristine nanotube demonstrate that
here is no significant change is observed (see Table 1). These resultsre in well agreement with the low adsorption energy gained. Inonclusion, the obtained data show that CH4 molecule adsorbednto the AlN and SiC nanotubes through weak physisorption and
lso pristine AlN and SiC nanotubes are not good candidates fordsorption of CH4 molecule.
Secondly, on the basis of our calculations, the adsorption energyurves for the best position of the CH4 molecule on the outer surface
of (a) AlNNT at T2 site and (b) SiCNT at H site using MPW1PW91/6-31G method.
of the AlN and SiC nanotubes were fitted with the Morse potential(Eq. (5)) to obtain the parameters of Morse potential.
Ui = 2D[x2 − 2x], x = exp(
−�
2
(ri
re− 1
))(5)
where ri is the intermolecular distance of methane from nanotubes,2D, re, and � denote the dissociation energy, the equilibrium bonddistance, and the adjustable parameter respectively The resultsobtained are listed in Table 4, the data can be used in the molecularsimulation study of adsorption CH4 into the AlN and SiC nanotubes.
558 Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562
(a) N
gas
3
mmStNlnT[
wab
E
wErm
To investigate the electronic properties of Ni-doped AlN and SiCnanotubes, NBO calculations were considered. The electron popu-lation analysis reveals that considerable electron transfer from Niatom to the AlN and SiC nanotubes occurred. Analysis of the NBO
Table 5Binding energy, Eb , equilibrium distance, re (tube–Ni), HOMO, LUMO, gap () energy,electronic chemical potential (�), hardness (�), softness (S) and electrophilicity (ω)of functionalized AlNNT and SiCNT.
Fig. 6. Fully optimized geometrical structures of CH4 molecule adsorbed on
eneral way. These potential energy curves also provide a reason-ble description for the properties of CH4–nanotube, via computerimulation, if obtained parameters were used.
.3. Functionalized AlN and SiC nanotubes with Ni metal atom
The above results indicate that the interaction between CH4olecule and AlN and SiC nanotubes are very weak and that the CH4olecule cannot strongly adsorb on pristine AlN and SiC nanotubes.
o, the influence of doping Ni metal atom on the AlN and SiC nano-ubes on the adsorption of CH4 molecule is considered. Althoughi metal atom can occupy different sites of nanotubes, our calcu-
ation proves that the most stable site for Ni metal adsorption onanotubes are T2 and H site for AlN and SiC respectively (see Fig. 5).hese results are in accordance with the results of Zhao and Ding51].
The geometries of Ni metal doped on AlN and SiC nanotubesere fully optimized on the basis of our calculations (MPW1PW91
nd PBEPBE). The binding energy (Eb) in this case was computed aselow
b = Etube−Ni − (Etube + ENi) (6)
here Etube–Ni is the total energy of Ni-doped nanotube system;Ni and Etube are the energy of isolated Ni metal and nanotubesespectively. As reported in Table 5 the interaction between the Nietal atom and nanotube is quite strong. The binding energies of
i-doped AlNNT and (b) Ni-doped SiCNT using MPW1PW91/6-31G method.
Ni into the outer surface of AlN and SiC nanotubes indicate thatthe chemisorptions were occurred (−359.5 and −354.55 kJ/molobtained from MPW1PW91 calculation for AlN Ni and SiC Nisystems respectively). This trend is also observed for the PBEPBEfunction. In these cases, the nearest distances between the surfacesof the nanotubes and Ni atom are about 1.8 A and 1.37 A for AlN andSiC nanotubes respectively (see Table 5).
Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562 559
Table 6NBO analysis of the ALNNT–Ni and SiCNT–Ni nanotubes (atom numbering is accord-ing to Fig. 5a and b).
SiC Ni
Bond order Ni C2 Ni Si1 Ni C3 Ni Si4 Ni C5 Ni Si6MPW1PW91 0.39 0.35 0.17 0.40 0.40 0.24PBEPBE 0.46 0.38 0.46 0.39 0.16 0.31
AlN Ni
booRrotico
AtuorHTistorsa
3
oiCFta
TAa
Table 8Highest occupied molecular orbital (HOMO), lowest unoccupied orbital (LUMO),gap () energy, electronic chemical potential (�), hardness (�), softness (S) andelectrophilicity (ω) of AlN Ni CH4 and SiC Ni CH4 systems.
The adsorption energy (Eads) in this case was defined as below:
Eads = Etube−Ni−CH4− (Etube−Ni + ECH4 ) (7)
Bond order N1 Al1 N1 Al2 N1 Al3 N1 NiMPW1PW91 0.65 0.84 0.64 0.73PBEPBE 0.62 0.83 0.63 0.80
ond order calculations indicate that the Ni atom bonded to theuter surface of the AlN and SiC nanotubes with the total bondrders about 1.5 and 2.7 for SiC and AlN nanotubes, respectively.esults of the NBO calculations are also in good agreement with theesults of the binding energies. As results, the adsorption of Ni atomnto the surfaces of the AlN and SiC nanotubes are exothermic andhe Ni–nanotube systems are more stable. The data are reportedn Table 6. In comparison, results of the binding energies and NBOalculations show that the AlN Ni system is more stable than thatf SiC Ni system.
By considering the global reactivity indices of the AlN, SiC,lN Ni and SiC Ni systems (presented in Tables 1 and 5), it is found
hat when the Ni atom bonded to the nanotubes the hardness val-es of the systems were slightly decreased and the electrophilicityf the systems were slightly increased, which indicated that theeactivity of the systems were increased. A comparison betweenOMO–LUMO gap for the pristine and doped nanotubes (seeables 1 and 5) imply to the energy gap for the doped nanotubess lower than that for pristine nanotubes, which means that theystems behaves like a metal systems and the reactivity of the sys-ems were increased. In conclusion, the stronger binding energiesf Ni–nanotubes, as well as the results obtained from the globaleactivity indices, and the NBO calculations show that the formedtructures are of higher reactivity and stability than pristine AlNnd SiC nanotubes.
.4. Adsorption of methane on Ni-doped AlN and SiC nanotubes
We now turn to discussion of the adsorption of CH4 moleculen the Ni-doped AlN and SiC nanotubes. First, the Ni atom is keptndividually above the outer surface of the nanotubes, and then the
H4 molecule is allowed to interact with the Ni atom as shown inig. 6a and b, which means that the CH4 molecule inserted intohe tube–Ni systems. Next, we discuss the adsorption properties ofCH4 molecule adsorbed on the Ni-doped AlN and SiC nanotubes.
able 7dsorption energy, Eads (kJ/mol), and equilibrium distance, re (Å), of methanedsorption on Ni-doped nanotubes.
The geometries of tube–Ni–CH4 system were fully optimized on thebasis of our two methods of calculations without any restrictions.
Fig. 7. Typically contour plots of HOMO of the (a) SiCNT–CH4 system, (b) SiCNT–Nisystem and (c) SiCNT–Ni CH4 system calculated by MPW1PW91 functional.
560 Z. Mahdavifar, M. Haghbayan / Applied Surface Science 263 (2012) 553–562
F AlNNr nt Feri to the
waa
(
ig. 8. Comparison of the electronic density of states (DOS) for (a) AlNNT, (b) Ni–epresent virtual orbitals, green lines represent occupied orbitals, FL lines representerpretation of the references to color in this figure legend, the reader is referred
here Etube−Ni−CH4is the total energy of the Ni–tube with CH4
dsorbed, Etube−Ni and ECH4 are the total energies of Ni doped tubend methane molecule respectively.
After CH4 molecule inserted into the 5a and 5b structuresmeans tube–Ni system), the new geometries were optimized using
T, (c) AlNNT–Ni CH4, (d) SiCNT, (e) Ni–SiCNT and (f) SiCNT–Ni CH4. Blue linesmi Level, and the red line represents DOS spectra calculated by MPW1PW91. (Forweb version of this article.)
MPWPW91/6-31G and PBEPBE/6-31G methods with in conjunction
with CEP-121G basis set for Ni atom. Final optimized geometriesindicate that while Ni atom bonded to the outer surface of tubesthe CH4 molecule directly bonded to the Ni atom without any dis-sociation (see Fig. 6a and b).
lied Su
tnssbrstAn
aftTfanNSs(aMcseAfh
pnlNcnnaawNooa(cc
kaHPnastdwCSot[C
Z. Mahdavifar, M. Haghbayan / App
The structural parameters of adsorbed CH4 on Ni-doped nano-ubes such as equilibrium distance of CH4 adsorbed on Ni-dopedanotube (rNi C, rNi H) and equilibrium distance between Ni andurface of nanotubes (rNi AlN, rNi SiC) are summarized in Table 7. Ithould be noted that the C and H atoms of CH4 molecule is directlyonded to the Ni atom with bond length about 2.22 A and 1.80 Aespectively. On the other hand, the Ni atom bonded to the outerurface of the nanotubes with Ni Al and Ni N bonds in AlN nano-ubes and Ni Si and Ni C bonds in SiC nanotubes (see Table 7).s a result, it seems CH4 molecule strongly adsorbed on Ni-dopedanotubes.
Compared the adsorption energy of tube–Ni–CH4 systemss defined by Eq. (7) and tube–CH4 systems indicate that theunctionalized nanotubes with Ni atom are more favorable thanhat of pristine AlN and SiC nanotubes (compared the data inables 2 and 7) for the methane adsorption. In other words, theunctionalized AlN and SiC nanotubes with Ni metal atom improvebility of nanotubes for adsorption of CH4 molecule. Furthermore,o bond dissociation was observed for the adsorption of CH4 oni-doped nanotubes, which means that functionalized AlN andiC nanotubes can act as a storage device for methane safetytorage. Consequently, the adsorption energy of CH4 on AlN Ni−60.36 kJ/mol, obtained by MPW1PW91) system is more neg-tive than that of CH4 on SiC Ni (−39.26 kJ/mol, obtained byPW1PW91) system, which imply to the strong physisorption pro-
ess. These results are well in agreement with the results of thetructural parameters. In comparison, the calculated adsorptionnergies of AlN Ni CH4 with SiC Ni CH4 show that the Ni-dopedlN nanotube is more favorable than that Ni-doped SiC nanotube
or the adsorption of CH4 molecule. This is probably related to theigh reactivity of the Ni-doped AlN nanotubes.
To better understand the adsorption process, the electronicroperties of CH4 molecule adsorbed on Ni-doped AlN and SiCanotubes using MPW1PW91/3-21G* and PBEPBE/3-21G* calcu-
ations were considered separately. The charge transfer from thei atom to the AlN and SiC nanotubes make the Ni atom positivelyharged. Bonding of Ni atom to the outer surface of AlN and SiCanotubes leads to substantial charge transfer from Ni atom to theanotubes. The partial charges, calculated from NBO, on Al, N, Si, Cnd Ni atoms are listed in Table 3. Upon the CH4 adsorption, the neg-tive charge on Ni atom and C atom of CH4 molecule were observedhich means that there is a charge transfer from nanotubes to thei atom and CH4 molecule was occurred. In other words, becausef inserted CH4 molecule to the Ni-doped nanotubes, the directionf the charge transfer was inverted (from nanotubes to Ni atomnd then to the CH4 molecule) compared with Ni-doped nanotubesfrom Ni to nanotubes). The obtained data of partial charges indi-ate that the electrostatic interaction is one of the major factorsontributed to the overall stabilities of the adsorption process.
Consequently, HOMO–LUMO gap were considered since it isnown to be the index of both kinetic and stability (reactivity)nd electrical conductivity [54]. Table 8 presents the results of theOMO, LUMO and gap energies obtained by the MPW1PW91 andBEPBE calculations. The calculated gap energies for the Ni-dopedanotubes are in the range 3.12–3.91 eV. When CH4 moleculedsorbed on Ni-doped nanotubes indicate the gap energy of theystem was increased (the energy gap increased from 3.12–3.91 eVo 3.34–4.16 eV). These energy gap behaviors suggest that the con-uctivity of nanotubes will be significantly decreased. In otherords, the reactivity of nanotubes will be significantly decreased.ompared HOMO–LUMO energies gap of tube–CH4 (AlN CH4 andiC CH4) and tube–Ni–CH4 systems reveal that the energies gap
f tube–Ni–CH4 were decreased. This trend was also reported onhe Pd-doped single walled carbon nanotubes by Tian and Wang55]. A typically contour plot of the HOMO for the adsorption ofH4 on pristine and functionalized SiCNT were shown in Fig. 7.
rface Science 263 (2012) 553–562 561
Furthermore, the total density of state (DOS) was also calculatedfrom the eigen value generated by the MPWPW91 and PBEPBE func-tional (see Fig. 8). From the comparisons of DOS spectra before andafter adsorption of CH4 on the pure and functionalized AlN andSiC nanotubes is found that there is no significant change in theoverall feature of the DOS of the nanotubes when CH4 moleculeadsorption on AlN and SiC nanotubes. A comparison of the DOS ofpristine AlN and SiC nanotubes with AlN Ni CH4 and SiC Ni CH4systems show that there is a significant change in Fermi level wasoccurred. As can be seen in Fig. 8, some energy states cross Fermilevel, this may due to the magnetic behavior of the Ni-adsorbednanotube systems. One may note that the band gap was changedupon adsorption. This observation is consistent with the change inHOMO and LUMO levels.
The global reactivity indices in the conceptual of the DFT forCH4 adsorption on Ni-doped nanotubes are presented in Table 8.When CH4 molecule adsorbed on Ni-doped nanotubes, hardness(�) and electronic chemical potential (�) of the nanotubes willbe increased. The more harness of the systems, the more stabil-ity of the system. Furthermore, NBO calculations show that duringthe adsorption process a charge transfer from Ni-doped nanotubesto the CH4 molecule could be occurred, which suggest that theirelectronic transport properties could significantly changed uponadsorption of CH4 molecule. The direction of electron flow will bedistinguished by electronegativity or electronic chemical potential.
4. Conclusion
In summary, we have theoretically investigated structural, ener-getic and electronic properties for the adsorption of CH4 moleculeon the pristine and functionalized armchair (4,4) AlNNT andSiCNT using two DFT functional. On the basis of results of theadsorption energy and equilibrium distance considered, the CH4molecule adsorbed into the pristine AlNNT and SiCNT throughweak Van der Waals interactions, which means that the adsorptionis physisorption process. Also, we have investigated systemicallyhow supporting Ni atom on AlNNT and SiCNT can affect theircapability for adsorption of CH4 molecule onto AlNNT and SiCNTwithout any bond dissociation. The Ni-doped AlN and SiC nano-tubes are demonstrated to be a good candidate for safety adsorptionof CH4 molecule. Data on adsorption energy, structural parameters,electronic properties, and global reactivity indices were obtained,which gives some useful results for CH4 adsorption. We found thatthe adsorption energy of CH4 molecule on Ni-doped AlN and SiCnanotubes are more negative, which means that the systems aremore stable. In all calculations, CH4 shows strong adsorption toAlN Ni nanotube. This indicates that the potential of functional-ized AlN nanotubes based nanomaterial for safety storage of CH4molecule. Moreover, the present of Ni atom on AlN and SiC surfaceconsiderably enhances the capacity of AlN amd SiC to accept elec-trons. The population analysis reveals that a charge transfer fromNi atom to AlN and SiC has occurred. We also found evidence forthe adsorption of CH4 on Ni-doped nanotubes is accompanied witha charge transfer from Ni-doped nanotubes to the CH4 molecule. Inaddition, NBO calculations essentially show significantly change inband gap of Ni-doped nanotubes when CH4 molecule adsorbed ontothe Ni-doped nanotubes. In conclusion, this prediction can help usto develop AlNNT and SiCNT-based nanomaterial candidates suchas adsorbent and storage.
Appendix A. Supplementary data
Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.apsusc.2012.09.106.
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