Journal of the Korean Physical Society, Vol. 42, February 2003, pp. S267∼S271
Adsorption of H2 O Molecules at the Open Ends of Singlewalled Carbon
Nanotubes
Yong Gyoo Hwang∗
Department of Physics, Wonkwang University, Iksan 570-749
Young Hee Lee
Department of Physics and Center for Nanotubes and Nanostructured Composites,
Sungkyunkwan University, Suwon 440-746
The adsorption of H2 O molecules on the open ends of carbon nanotubes (CNTs) have been
investigated using the self-consistent-charge density functional tight binding (SCC-DFTB) method.
An H2 O molecule dissociates into H and OH fragments at the armchair nanotube edge with an
adsorption energy of −3.52 eV and relatively small activation barrier height of less than 0.35 eV.
The Fermi level shifts upward by 0.14 eV, and the density of states near the Fermi level does not
change appreciably. In the case of the adsorption on the zigzag nanotube edge, an H2 O molecule
dissociates into H and OH fragments with an activation barrier height of 0.12 eV. They adsorb onto
two nearby carbon atoms at the open end. The adsorption energy is −4.84 eV, larger in magnitude
than that on the armchair edge and the distortion of the local geometry under H and OH fragments
is very small. The Fermi energy shifts upward by 0.17 eV, and the density of states at the Fermi
level is enhanced. After the adsorption of H2 O, the Fermi energy shifts upward in the most stable
configurations over both CNTs. This will decrease the work function of the nanotubes and the
turn-on voltage of the field emission current. The enhancement of the field emission current is
expected to be larger for the zigzag nanotube, since the shift of the Fermi energy is larger and the
density of states is enhanced near the Fermi energy.
PACS numbers: 31.15.Ew, 71.15.Mb, 79.70.+q
Keywords: Density functional theory, Nanotube adsorption, Field emission
I. INTRODUCTION
Since the discovery of carbon nanotubes (CNT’s)
formed by arc discharge [1], there have been tremendous efforts to investigate their physical properties and
to utilize their unique properties in various applications.
CNT’s have high aspect ratio and are very stable due
to the strong carbon-carbon bondings, which opened a
new possibility to be applied as field emitters. CNT field
emitters have been reported to have low-threshold voltages and good emission stability [2]. Adsorption of gas
molecules change the field emission properties of carbon
nanotubes [3,4]. Adsorption of ambient gases such as O2
and H2 O has been reported to instantly modify the emission current and long-term exposure to these gases results in the irreversible current degradation [3,5–8]. Theoretical models have been proposed to understand the
emission properties of the clean open-end single-walled
nanotubes (SWNTs) [10], of capped SWNTs with adsorbed molecules [11,12], and of open-end SWNTs with
adsorbed molecules [13,14]. However, the details of the
∗ E-mail:
adsorption procedure have not been completely understood yet.
In this paper, we study the adsorption of H2 O
molecules on the open ends of (5,5) armchair and (9,0)
zigzag carbon nanotubes by using the self-consistentcharge density functional tight-binding method (SCCDFTB) [15]. A H2 O molecule dissociates into H and OH
fragments on both edges with relatively large adsorption
energies. The adsorption on the edge of the armchair
CNT has an activation barrier of 0.35 eV for chemisorption, whereas that on the edge of the zigzag CNT is
exothermic without an activation barrier. We find that
the field emission current is enhanced for both tubes due
to the decrease of the work function, in agreement with
experimental data [3,5–8]. On the zigzag CNT, the slope
of the voltage-current characteristic curve is expected to
increase due to the increase of the density of states, in
good agreement with experimental data [3].
II. THEORETICAL APPOACHES
[email protected]
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Journal of the Korean Physical Society, Vol. 42, February 2003
For our calculations we use the SCC-DFTB method.
The SCC-DFTB method uses a basis of numerical s
and p atomic orbitals for carbon and oxygen atoms and
s orbital for hydrogen atom. Hamiltonian and overlap matrix elements are evaluated by a two-center approach. Charge transfer is taken into account through
the incorporation of a self-consistency scheme for Mulliken charges based on the second-order expansion of the
Kohn-Sham energy in terms of charge density fluctuations. The diagonal elements of the Hamiltonian matrix employed are then modified by the charge-dependent
contributions in order to describe the change in the
atomic potentials due to the charge transfer. The
off-diagonal elements have additional charge-dependent
terms due to the Coulomb potential of ions. They decay
as 1/r and thus account for the Madelung energy of the
system. Further details of the SCC-DFTB method have
been published elsewhere [15].
Open-ended single-walled nanotubes are chosen in this
study, since the edge can be opened easily during the
purification procedure [16,17] and even multiwalled nanotubes can be opened by the Joule heating during the
high-voltage annealing process [3,18]. We choose supercells of (5,5) armchair and (9,0) zigzag nanotubes in our
calculations for the sake of simplicity. The diameters of
the nanotubes are 6.8 and 7.0 Å, respectively, with an
average bond length of 1.42 Å, similar to that of C60 .
For an open edge, we use 10 layers for each nanotube,
where the dangling bonds at the bottom layer are saturated by hydrogen atoms to minimize the effect of dangling bonds. A water molecule is placed at a distance
above the nanotube, and the structures are fully relaxed
using the conjugate gradient method. The bottom hydrogen layer and the bottom carbon layer are fixed during the relaxation. Adsorption energies are defined as
Etot (CNT+H2 O) - Etot (CNT) - Etot (H2 O).
III. RESULTS AND DISCUSSION
We first consider an adsorption of an H2 O molecule
at the open end of the (5,5) nanotube. Fig. 1 shows
(meta) stable geometries of a H2 O molecule adsorbed on
an armchair nanotube. An H2 O molecule is placed at
various distances and orientations above the edge.
When a H2 O molecule is located over a carbon dimer
such that the oxygen atom points toward the center of
the dimer at the distance of 2.0 aB (Bohr radius), the
whole structure is relaxed to Fig. 1(a), called a top site.
The adsorption energy is only -57 meV and the distance
between the hydrogen atom and carbon atom at the top
edge is 2.44 Å. This suggests that the H2 O molecule is
physisorbed. When a H2 O molecule is relaxed with one
of the O-H bond parallel to the dimer after being placed
2.0 aB away from the carbon dimer, the H2 O molecule
dissociates into H and OH fragments with an adsorption
energy of −3.52 eV, as shown in Fig. 1(b). The details
Fig. 1. Relaxed geometries after the adsorption of a H2 O
molecule at the open end of a (5,5) nanotube: (a) top, (b)
top-split, (c) seat-bridge, (d) seat-split, (e) seat-bridge II, and
(f) top-split II.
of the geometrical parameters are listed in Table 1. The
bond angles around the carbon dimer atoms, C1 and C2 ,
are pretty close to 120 degrees, and the C1 -C2 bond is
elongated to 1.39 Å from 1.25 Å, indicating the weakening of the bond. The fragments are stabilized by a
complete bonding with the dimer, leading to a relatively
strong chemisorption.
When a H2 O molecule is put over a carbon atom at
2.3 aB with the oxygen atom pointing the carbon atom
and with the H2 O plane parallel or perpendicular to the
circumference of the tube, the H2 O molecule does not
dissociate but the structure relaxes to a metastable geometry in the seat site, as shown in Fig. 1(c). The
adsorption energy is −0.85 eV, the C-O bond length is
1.47 Å, relatively weak, and the C1 -C2 bond length is
1.38 Å, indicating a double bond. The H2 atom forms a
weak bridge between the oxygen atom and the C5 atom
in the tube.
When a H2 O molecule is placed 2.3 aB away from the
carbon dimer with the oxygen atom directly above the
C2 atom and one of the O-H bonds parallel to the dimer,
while the hydrogen atom points to the seat site, we obtained the stable structure shown in Fig. 1(d). The
adsorption energy is −1.92 eV. Although this geometry
is similar to Fig. 1(b), the local structures around C1 ,
C2 , C5 , and C6 are distorted significantly, making the
adsorption energy smaller.
Adsorption of H2 O Molecules at the Open Ends of Singlewalled· · · – Yong Gyoo Hwang and Young Hee Lee
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Table 1. Structural parameters for the geometries, Fig.
1(b) and (d). The units of energy, length, and angles are eV,
Å, and degrees, respectively.
Structure
Ead
H1 -C1
H2 -O
H2 -C5
H1 -O
C1 -C2
C2 -O
C1 -C3
C2 -C4
C5 -C6
∠ H1 C1 C2
∠ H1 C1 C3
∠ H2 OC2
∠ H1 OC2
∠ OC2 C1
∠ H2 C5 C6
∠ C 3 C1 C 2
∠ C 1 C2 C 4
∠ C 6 C5 C 7
∠ C 5 C6 C 8
Fig. 1(b)
3.52
1.10
0.98
1.39
1.36
1.43
1.44
1.25
119.9
119.2
109.9
121.3
119.2
120.6
127.4
125.2
Fig. 1(d)
1.92
1.10
0.98
1.38
1.35
1.46
1.44
1.28
106.9
116.9
140.2
110.0
128.6
111.3
124.4
When an H2 O molecule is placed over the seat site
with the oxygen atom pointing down at 0 aB (i.e., with
O at the same height as that of the carbon dimer) the
H2 O molecule flips up to form a metastable structure in
Fig. 1(e). The adsorption energy is −0.25 eV, relatively
small with a C-H bond length of 1.54 Å.
We obtain the most stable geometry in Fig. 1(f) by
flipping down the H2 atom in Fig. 1(b). The adsorption
energy is −3.58 eV. The energy difference from that of
Fig. 1(b) is very small, and the barrier against changing
from one geometry to another is negligible. In the field
emission experiment, the geometry in Fig. 1(f) will flip
to that in Fig. 1(b) to maximize the dipole field strength
when the field is applied. Therefore, we consider here
only the detailed structure of Fig. 1(b).
The adsorption of an H2 O molecule at the open end
of the (9,0) nanotube is relatively simple, compared to
that of the (5,5) nanotube. When a H2 O molecule is
put such that two hydrogen atoms are at 1.0 aB above
the edge with the middle of the two hydrogen atoms directly above the middle of the two adjacent edge carbon
atoms, or such that O is pointing down at 1.0 aB above
the middle of the two adjacent edge carbon atoms, the
structures relaxed to the precursor state in Fig. 2(a),
where two weak bonds are formed between the hydrogen
atoms and two carbon atoms in a dimer without breaking the H2 O molecule. The adsorption energy is −0.34
eV with the bond length between a hydrogen atom in the
Fig. 2. Relaxed structures after the adsorption of H2 O on
the open end of a (9,0) nanotube: (a) top, (b) top-split.
H2 O molecule and a carbon atom in the dimer is 1.52 Å.
When an H2 O molecule is put above a carbon atom at
the edge such that the oxygen atom is at 2.3 aB above
the carbon atom, or such that one of the O-H bonds is
at 1.8 aB above the edge parallel to the edge plane and
the bond center is above the middle of the two adjacent
edge carbon atoms, the H2 O molecule dissociates into H
and OH fragments and chemisorbs exothermally on the
zigzag edge without an activation barrier, as shown in
Fig. 2(b). The adsorption energy is −4.84 eV, which
is 1.32 eV larger in magnitude than that on the (5,5)
nanotube. All bonds are stable by saturating the dangling bonds without serious distortions. The details of
the structure are listed in the Table 2.
To find the activation barrier in the case of the (5,5)
nanotube, atoms in the H2 O molecule are moved from
the positions in the geometry in Fig. 1(a) to those in
Fig. 1(b) by a few percents and the whole structure is
relaxed while the positions of the oxygen atom and the
H fragment are fixed. The activation barrier height is
found to be 2.22 eV, rather high due to the dissociation
of H2 O molecule into H and OH fragments. When only
the position of the oxygen atom is fixed, the whole structure is relaxed to Fig. 1(c) with a barrier height of 0.09
eV. When the H2 O molecule approaches the CNT from
afar with an OH bond parallel to the carbon dimer and
with oxygen atom directly above the final position of the
oxygen atom in Fig. 1(d), the adsorption barrier height
is found to be 0.35 eV.
From these results, one may conclude that if an H2 O
molecule has relatively small kinetic energy to overcome
a barrier height of about 0.09 eV, it may adsorb dissociatively to form the structure in Fig. 1(d). If it has enough
kinetic energy to overcome the barrier height of about
0.35 eV, it may adsorb dissociatively to form the most
stable structure in Fig. 1(b) When five H2 O molecules
adsorb on the open-end of the tube, the final structure
is very similar to the repetition of Fig. 1(b). Therefore,
the configuration in Fig. 1(b) is considered to be the
most stable and is analyzed in detail.
To find an adsorption barrier in case of the (9,0) nanotube, the atoms in the H2 O molecule are moved from
the positions in the geometry in Fig. 2(a) to those in
Fig. 2(b) by a few percents and the whole structure is
relaxed while the positions of the oxygen atom and the
H fragment are fixed. The barrier height is found to be
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Journal of the Korean Physical Society, Vol. 42, February 2003
Table 2. Structural parameters for the geometries Fig.
2(a) and (b). The units of energy, length, and angles are eV,
Å, and degrees, respectively.
Structure
Ead
H1 -O
H1 -C1
H2 -C2
O-C1
C1 -C3
C1 -C4
C2 -C4
C2 -C5
∠ H1 C1 C3
∠ H1 C1 C4
∠ H1 OH2
∠ H1 OC1
∠ OC1 C3
∠ OC1 C4
∠ H2 C2 C4
∠ H2 C2 C5
∠ C 3 C1 C 2
∠ C 4 C2 C 5
Fig. 2(a)
0.34
1.03
1.52
1.52
1.42
1.42
1.42
1.42
117.7
101.8
99.6
101.8
117.6
118.0
118.0
Fig. 2(b)
4.84
0.99
1.33
1.10
1.33
1.43
1.44
1.42
1.41
107.1
119.0
119.4
119.4
119.1
119.3
119.4
0.12 eV. This is in good contrast with O2 adsorption on
zigzag edge, where no activation barrier exists [9].
In the most stable adsorption geometry in the case
of the (5,5) nanotube (Fig. 1(b)), the Mulliken charge
around the oxygen atom is −0.37 e, compared to −0.59 e
in the molecular state, indicating less ionicity in the CO
bond. The Mulliken charge around the hydrogen atom
H1 is +0.09 e, about the same as +0.07 e in CH4 , consistent with the fact that the length of the C1 -H1 bond
is the same as in CH4 . the Mulliken charge around C1
is −0.24 e, about the same as −0.28 e in CH4 . The
Mulliken charges around the H2 and oxygen atoms are
+0.32 e and −0.37 e, respectively, compared to +0.30 e
and −0.59 e in an H2 O, respectively. The strength of
the O-H2 bond is weaker than the O-H bond in H2 O
molecule due to the less buildup of charge around the
oxygen atom. The most significant changes occur around
the carbon atoms in the dimer, where H2 O adsorbs. The
charge around C1 and C2 atoms are −0.24 e and +0.29
e, compared with −0.08 e in the clean open-ended nanotube. The charge transfer in the latter is expected due
to the larger electronegativity of the adjacent oxygen
atom.
In the most stable adsorption geometry in the case
of the (9,0) nanotube (Fig. 2(b)), the Mulliken charge
around the oxygen atom is −0.34 e, compared to −0.59 e
in the molecular state. The Mulliken charge around the
hydrogen atom H2 is +0.09e, about the same as +0.07
e in CH4 . The Mulliken charge around the C2 atom is
Fig. 3. Local density of states for the most stable configurations: clean nanotube, nanotube after the adsorption, and
H2 O in each column. (a) (5,5) nanotube, (b) (9,0) nanotube.
-0.03 e, quite different from −0.28 e in CH4 molecules.
Although the length of the C2 -H2 bond is the same as in
CH4 , the Mulliken charge around the C2 atom is quite
different from that in CH4 . This indicates the difficulty
in obtaining the equilibrium geometry because there are
states quite close to the Fermi energy and a fictitious
electronic temperature must have been chosen, 300 K in
this work. Contrary to the case of the (5,5) nanotube,
Mulliken charges should be interpreted more carefully.
But the unmistakable changes in the charges are those
around the C1 and oxygen atoms. They are +0.34 e and
−0.34 e, compared to −0.14e in the clean nanotube and
−0.59 e in H2 O moelcule. There is significant charge
build-up around carbon atoms next to the C1 and C2
atoms. They are −0.36 e and −0.27 e, respectively, compared to −0.14 e in the clean nanotube.
Next we consider the shift of the Fermi level and the
change in the local density of states after the adsorption of an H2 O. The Fermi level shifts upward for the
most stable geometries in Fig. 1(b) and Fig. 2(b). This
will decrease the work function and enhance the field
emission current. Local density of states (LDOS) for
the most stable adsorption structures are shown in Fig.
3. The first and second columns show the LDOSs for
the adsorption onto the open ends of the (5,5) and (9,0)
nanotubes, respectively. The first panel in each column
represents LDOS of the clean nanotube, the second one
represents LDOS of the nanotube after the adsorption of
a H2 O molecule (excluding the contribution from H2 O),
Adsorption of H2 O Molecules at the Open Ends of Singlewalled· · · – Yong Gyoo Hwang and Young Hee Lee
and third one represents LDOS from the adsorbed H2 O
molecule. The shifts of the Fermi energy from those of
the clean CNT’s are +0.14 and +0.17 eV for the (5,5)
and (9,0) nanotubes, respectively. As one can see from
Fig. 3, there is no enhancement of LDOS near the Fermi
energy after the adsorption of H2 O molecule in the case
of the (5,5) nanotube, whereas there is a strong enhancement of LDOS near the Fermi energy in the case of the
(9,0) nanotube. Therefore the voltage-current characteristic curve is expected to be shifted to lower voltage for
both nanotubes due to the shift of the Fermi emergy,
and the slope of the curve will increase in the case of the
zigzag nanotube due to the enhancement of LDOS near
the Fermi energy.
IV. CONCLUSION
In summary, we have studied the adsorption of H2 O
molecule at the open ends of (5,5) and (9,0) carbon nanotubes using the SCC-DFTB method. On the open end
of the armchair nanotube, an H2 O molecule breaks into
H and OH fragments and chemisorbs with an adsorption
energy of −3.52 eV. The Fermi energy moves upward by
0.12 eV and the adsorption does not introduce any LDOS
near the Fermi energy, giving rise to the enhancement of
the field emission current mainly by lowering the threshold voltage. On the open end of the zigzag nanotube, an
H2 O molecule also breaks into H and OH fragments and
chemisorbs with the adsorption energy of −4.84 eV. The
Fermi energy moves upward by 0.17 eV and the LDOS
near the Fermi energy also increases. The enhancement
of the field emission current would be greater due to the
enhanced LDOS near the Fermi energy in addition to the
decrease of the work function. The slope of the voltagecurrent characteristic curve is expected to increase as
observed experimentally.
ACKNOWLEDGMENTS
This research was supported by Wonkwang University
in 2002. One of us (YHL) acknowledges the financial
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support from the MOST through CNNC at SKKU and
NRL programs. We thank the supercomputer center at
Chonbuk National University for allowing us to use their
computer.
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