Computational Materials Science 48 (2010) 655–657
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Computational Materials Science
journal homepage: www.elsevier.com/locate/commatsci
DFT study of NH3(H2O)n=0,1,2,3 complex adsorption on the (8, 0) single-walled
carbon nanotube
Bahram B. Shirvani a, Javad Beheshtian a, Gholamabbas Parsafar b, Nasser L. Hadipour a,*
a
b
Department of Chemistry, Tarbiat Modares University, P.O. Box 14115-175, Tehran, Iran
Department of Chemistry, Sharif University of Technology, Tehran, Iran
a r t i c l e
i n f o
Article history:
Received 16 December 2009
Received in revised form 18 February 2010
Accepted 23 February 2010
Available online 25 March 2010
Keywords:
Carbon nanotube
NH3 adsorption
DFT
a b s t r a c t
Theoretical study of NH3(H2O)n=0,1,2,3 adsorption on (8, 0) carbon nanotube was performed at the X3LYP/
6–31G level of density functional theory (DFT). The tube–NH3 interaction was discussed in the terms of
binding energy (EB), coupling energy (EC), charge density, molecular orbitals, and dipole moments. The
results reveal that addition of water molecules to tube–NH3 system increases the interaction between
tube and ammonia molecule.
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Computational details
Since their discovery in 1991, carbon nanotubes (CNTs) have
been the subject of several comprehensive studies [1]. Many
researchers have worked on the gas adsorption on CNTs. Especially,
adsorption of Xe [2,3], CF4 [4], H2 [5–8], CH4 [9], NO [10], NH3, NO2,
and O2 [11–16] on single-walled carbon nanotubes (SWCNTs) has
been widely studied. However, SWCNTs show high sensitivity toward gaseous molecules which makes them remarkable as chemical sensors [17–19]. Many works have been conducted on very
weak interactions of gases with SWCNTs. The interaction between
ammonia and SWCNTs is likely to be dominated by the dispersion
forces, but in comparison with some other gases such as CH4 and
H2, the ammonia molecule carries a permanent dipole moment.
Thus dipole-induced dipole and dipole–quadrupole interactions
will also contribute. In the case of NH3, pure NH3 is only weakly adsorbed on pristine CNTs by van der Waals (vdW) interaction. Such
physisorption leads to a small charge transfer and bond modification, but apparently no change in the conductance of pristine CNTs
[17,20] occurs. On the other hand CNTs are sensitive to ammonia
gas only when the water vapor is present, indicating that the
ammonia–water solution (instead of NH3 alone) changes the conductance of CNTs [21,22]. This is supported by other experimental
and computational studies [23–25].
The present work is a theoretical study on how the nature of the
adsorbed NH3 and NH3(H2O)n=1,2,3 species changes the binding energy and electronic structure of CNT zigzag (8, 0).
Full-geometry optimizations are performed on the (8, 0) pristine and functionalized SWCNT using gamess (11 APR 2008 (R1))
suite of programs. All calculations and geometry optimizations
are performed with the DFT method using the hybrid exchange
X3LYP [26] functional, along with the 6–31G basis set. The
X3LYP functional is selected since it seems to give a better description of dispersion interactions, and hence, of the physical adsorption among all presently available functionals. The starting
geometries are generated using TubeGen tool [27] based on a hexagonal unit cell. Diameter
is 6.40 Å for (8, 0) zigzag CNT. The nano0
tubes0 have a 15.73 Å
A length, with an average C–C bond length of
1.43 Å
A. Finally, the bottom dangling bonds are saturated by hydrogen atoms.
* Corresponding author. Tel.: +98 218288 3495; fax: +98 218288 9730.
E-mail address: [email protected] (N.L. Hadipour).
0927-0256/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.commatsci.2010.02.035
3. Results and discussion
3.1. Interaction of ammonia with SWCNT
Interaction of (8, 0) zigzag SWCNT with a single NH3 molecule
and NH3(H2O)n=1,2,3 complexes is considered. To find the adsorption behavior of NH3 on SWCNTs, one ammonia molecule is moved
toward the empty or bridge site of the tube wall, and the equilibrium geometries are then found (Fig. 1). To evaluate the interaction
behavior between the NH3(H2O)n=0,1,2,3 complexes and CNT, we use
the binding energy (EB) and the coupling energy (EC), respectively.
EB measures the average interaction between an ammonia molecule and its surroundings, including the intermolecular interaction
within the water molecules and the interaction between ammonia
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B.B. Shirvani et al. / Computational Materials Science 48 (2010) 655–657
energy in the hybrid system. Table 1 displays that EB and EC are
functions of n (number of water molecules) and they decrease
upon increasing it. The coupling energy EC is not sensitive enough
to the number of water molecules, except for the first water molecule added to the system in which the energy is reduced from
2.0 kcal/mol to 4.1 kcal/mol. However, Table 2 shows that when
the n is increased, the contribution of each water molecule in energy reduction is lessened. The equilibrium tube–NH3 distance
(R) also exhibits sensitivity to the n. Results reveal that R is shortened to 0.25 Å by changing the n from zero to three.
3.2. Electronic properties
Fig. 1. Equilibrium geometries and pictorial view for the adsorption sites of
NH3(H2O)n=0,1,2,3 on (8, 0) CNT. The green, red and blue cylinders indicate hydrogen,
oxygen and nitrogen atoms, respectively. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
where ENH3 ðH2 OÞn is the total energy of NH3(H2O)n in vacuum, with
the same configuration as the one on the CNT. The bond angle and
the bond length for the molecular geometry of NH3 molecule in gasphase are 116.25° and 1.005 Å, respectively. Comparing the NH3
molecule setting on outer wall of the CNT with the one in the
gas-phase shows small changes (less than 1%) in the bond angle
and bond length. The equilibrium tube–NH3 distance (R) is 3.50 Å.
When one water molecule was placed on NH3–tube system, the
two molecules (NH3, H2O) form a hydrogen bonded dimers. The
theoretical N–O distance for the water–ammonia complex is
3.096 Å. As compared to NH3–tube system, N–H distance in NH3
(H2O)–tube system increased with the average N–H distance of
1.015 Å. This difference is more for the N–H adjacent to water molecule (1.019 Å). Subsequently, when two water molecules are
added to the NH3–tube system, the calculated N–O and N–H average distances are 3.14 Å and 1.023 Å, respectively. Three water
molecules are added to the NH3–tube system, the N–O and N–H
average distances are 3.083 Å and 1.019 Å, respectively.
Table 1 shows the binding energy (EB), coupling energy (EC) and
the equilibrium tube–NH3(H2O)n=0,1,2,3 distance (R) calculated by
X3LYP. The binding energy EB describes the interaction between
NH3 molecule and its overall environment (tube, H2O molecules).
The coupling energy (EC) is the remainder of the intermolecular
interaction energies of a gas-phase NH3(H2O)n subtracted from EB
of confined NH3(H2O)n which measures the NH3–tube interaction
Influence of NH3 adsorption on the electronic properties such as
on-site charge transfer, charge density, molecular orbitals, and dipole moments are computed to get more details about the interaction between NH3 and tube in each system (Table 3).
The interaction between ammonia and SWCNTs is expected to
be dominated by dispersion forces, while NH3 molecule carries a
permanent dipole moment. Thus, in a tube–molecule system,
when NH3 molecule and CNT interact, a common chemical potential is produced due to the charge-transfer phenomenon. In Table 3,
Lowdin population analysis shows that the negative charge value
for nitrogen atom of the ammonia molecule increases by addition
of water molecules to the NH3–tube system. Therefore, the interaction between the electrons of nitrogen lone pair with the vacant
anti-bond (BD*: the lowest unoccupied orbital) C–C is increased
while the hydrogen atoms of the ammonia with p electrons in
the CNT is increased. These results are in agreement with the previous findings obtained experimentally by Mark D. Ellison et al.
[28]. They have studied the adsorption of NH3 and NO2 on SWCNTs.
The FTIR data suggests that NH3 interacts with the nanotubes via
both the lone pair of electrons and the H atoms.
To get better insight into the tube–NH3 interaction, Fig. 2 shows
the total electron density and charge densities of HOMO-8 (the
eight orbital lower than Highest Occupied Molecular Orbital) wave
functions for the tube–NH3 and HOMO-4 (the four orbital lower
than Highest Occupied Molecular Orbital) ones for tube–
NH3(H2O)n=1,2,3 stats. The coupling between the p electrons of
the CNT and the valence electrons of the NH3 molecule are
increased. However, there is still weak coupling between the p
electrons of the CNT and the valence electrons of the ammonia
molecules. However increasing the n in the NH3(H2O)n cluster
leads to a dissociated (ion-pair) structure for large n
[NHþ
4 (H2O)n OH ] as a local minimum where a proton is
transferred from a water molecule to the ammonia molecule
[29–31]. At the ammonia–water solution (instead of NH3 alone)
this changes the conductance of CNTs.
As shown in Table 3, the computed energy-gap (Eg) is insensitive to the n except for tube–NH3(H2O)3 system in which a slight
change of about 0.003 eV is observed. It is known that the changes
in the dipole moments affect the intermolecular interactions. In
Table 3, the amplitude of dipole moments for the tube–
NH3(H2O)n=0,1,2,3 complexes are listed. With the exception of
adding the first water molecule, the system dipole moments are
increased by addition of more water molecules. However, dipole
Table 1
Binding energy (EB), coupling energy (EC) and equilibrium tube–molecule distance (R)
of the tube–NH3(H2O)n=0,1,2,3 systems as calculated by X3LYP.
Table 2
Binding energy (EB) and coupling energy (EC) per water molecule as a function of
number of water molecules (0,1,2,3).
molecule and the nanotube. EB can be calculated using the following equation:
EB ¼ ðEcomplex Etube ENH3 nEwater Þ n ¼ 0; 1; 2; 3
where Ecomplex is the total energy of the NH3(H2O)n/tube hybrid
complex; Etube is the total energy of tube; Ewater is the total energy
of an individual water molecule; ENH3 is the total energy of an individual ammonia molecule and n is the number of water molecules.
To differentiate between the ammonia–water molecular interaction
and the tube–ammonia coupling interaction, here we define the
coupling energy (EC) as:
EC ¼ Ecomplex Etube ENH3 ðH2 OÞn
n ¼ 0; 1; 2; 3
System
EB (kcal/mol)
EC (kcal/mol)
R (Å)
System
EB (kcal/mol)
EC (kcal/mol)
NH3
NH3(H2O)
NH3(H2O)2
NH3(H2O)3
2.0
9.2
13.2
18.7
2.0
4.1
4.4
4.9
3.50
3.42
3.33
3.25
NH3
NH3(H2O)
NH3(H2O)2
NH3(H2O)3
2.0
9.2
6.6
6.3
2.0
4.1
2.2
1.6
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B.B. Shirvani et al. / Computational Materials Science 48 (2010) 655–657
Table 3
Energy-gap (Eg), dipole moment |l|, and nitrogen atom charge of the tube–NH3(H2O)n=0,1,2,3 systems as calculated by X3LYP.
System
Eg (eV)
|l| (debye)
Nitrogen atom charge
Average hydrogen atom charge
NH3
NH3(H2O)
NH3(H2O)2
NH3(H2O)3
0.220
0.220
0.220
0.223
2.16
1.83
3.89
6.51
0.628
0.645
0.659
0.688
+0.213
+0.205
+0.198
+0.197
4. Conclusion
DFT calculations with X3LYP method are performed for hybrid
systems NH3 ðH2 OÞn¼0;1;2;3 on outer wall of a finite zigzag (8, 0) carbon nanotube. The tube–NH3 interactions are evaluated in terms of
the binding energy (EB), coupling energy (EC), charge density,
molecular orbitals, and dipole moments. The energy values and
equilibrium distances between NH3 molecule and tubes obtained
from DFT calculations are typical of physisorption. The current results clearly indicate that addition of water molecules to tube–NH3
system increases the interaction between tube and ammonia
molecule.
References
Fig. 2. Electron charge density for (a) tube–NH3, (b) tube–NH3(H2O), (c) tube–
NH3(H2O)2, (d) tube–NH3(H2O)3. Red (blue) shape corresponds to positive (negative) values of the wave function. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Dipole moment orientation for (a) pristine tube (8, 0), (b) tube–NH3, (c)
tube–NH3(H2O) (d) tube–NH3(H2O)2 (e) tube–NH3(H2O)3.
moments direction from tube–NH3 to tube–NH3(H2O)n=1,2,3
systems are inverted (Fig. 3).
[1] S. Ijima, Nature 354 (1991) 56.
[2] A. Kuznetsova, J.T. Yates, J. Liu, R.E. Smalley, J. Chem. Phys. 112 (2000) 9590.
[3] A. Kuznetsova, J.T. Yates, V.V. Simonyan, J.K. Johnson, C.B. Huffman, R.E.
Smalley, J. Chem. Phys. 115 (2001) 6691.
[4] O. Byl, P. Kondratyuk, S.T. Forth, S.A. FitzGerald, L. Chen, J.K. Johnson, J.T. Yates,
J. Am. Chem. Soc. 125 (2003) 5889.
[5] A.C. Dillon, M.J. Heben, Appl. Phys. A 72 (2001) 133.
[6] G.E. Froudakis, J. Phys. Condens. Matter 14 (2002) 453.
[7] I. Efremenko, M. Sheintuch, Langmuir 21 (2005) 6282.
[8] J.S. Arellano, L.M. Molina, A. Rubio, M.J. López, J.A. Alonso, J. Chem. Phys. 117
(2002) 2281.
[9] A. Lubezky, L. Chechelnitsky, M. Folman, J. Chem. Soc. Faraday Trans. 92 (1996)
2269.
[10] M. Fastow, Y. Kozirovski, M. Folman, J. Heidberg, J. Phys. Chem. 96 (1992)
6126.
[11] J. Kong, N.R. Franklin, C. Zhou, M.G. Chapline, S. Peng, K. Cho, H. Dai, Science
287 (2000) 622.
[12] S. Peng, K. Cho, P.F. Qi, H.J. Dai, Chem. Phys. Lett. 387 (2004) 271.
[13] G.U. Sumanasekera, C.K.W. Adu, S. Fang, P.C. Eklund, Phys. Rev. Lett. 85 (2000)
1096.
[14] J. Li, Y.J. Lu, Q. Ye, M. Cinke, J. Han, M. Meyyappan, Nano Lett. 3 (2003) 929.
[15] L. Valentini, I. Armentano, J.M. Kenny, C. Cantalini, L. Lozzi, S. Santucci, Appl.
Phys. Lett. 82 (2003) 961.
[16] S. Chopra, A. Pham, J. Gaillard, A. Parker, A.M. Rao, Appl. Phys. Lett. 80 (2002)
4632.
[17] C.W. Bauschlicher Jr., A. Ricca, Phys. Rev. B 70 (2004) 115409–115411.
[18] A. Goldoni, R. Larciprete, L. Petaccia, S. Lizzit, J. Am. Chem. Soc. 125 (2003)
11329.
[19] D. Sung, S. Hong, Y.H. Kim, N. Park, S. Kim, S.L. Maeng, K.C. Kim, Appl. Phys.
Lett. 89 (2006) 243110.
[20] J.J. Zhao, A. Buldum, J. Han, J.P. Lu, Nanotechnology 13 (2002) 195.
[21] V. Derycke, R. Martel, J. Appenzeller, P.H. Avouris, Appl. Phys. Lett. 80 (2002)
773.
[22] K. Bradley, J-C.P. Gabriel, M. Briman, A. Star, G. Grüner, Phys. Rev. Lett. 91
(2003) 218301–218311.
[23] X. Feng, S. Irle, H. Witek, K. Morokuma, R. Vidic, E. Borguet, J. Am. Chem. Soc.
127 (2005) 1533.
[24] G. Grüner, Anal. Bioanal. Chem. 384 (2006) 322.
[25] D. Pantarotto, C.D. Partidos, R. Graff, J. Hoebeke, J.P. Briand, M. Prato, A. Bianco,
J. Am. Chem. Soc. 125 (2003) 6160.
[26] X. Xu, W.A. Goddard III, Proc. Natl. Acad. Sci. USA 101 (2004) 2673.
[27] J.T. Frey, D.J. Doren, TubeGen 3.3; University of Delaware: Newark, DE, 2005,
<http://turin.nss.udel.edu/research/tubegenonline.html>.
[28] M.D. Ellison, M.J. Crotty, D. Koh, R.L. Spray, K.E. Tate, J. Phys. Chem. B 108
(2004) 7938.
[29] U. Buck, F. Huisken, Chem. Rev. 100 (2000) 3863.
[30] K. Nauta, R.E. Miller, Science 287 (2000) 293.
[31] A. Lenz, L. Ojamae, J. Phys. Chem. A 110 (2006) 13388.