Density Functional study of Carbon Monoxide Adsorption on Pt–doped closed–end (3,3)
armchair Single–Wall Carbon Nanotube
Charinee Kaewkhonkaen1,*, Vithaya Ruangpornvusuti2
1,2
Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330,
Thailand
*e-mail: charinee.ka@gmail.com
Abstract
Structure of four different sizes of closed–end (3,3) armchair single–walled carbon
nanotubes (SWCNTs) of which lengths are 2, 4, 6, and 8 carbon layers, denoted by 2L, 4L,
6L and 8L,respectively and theirs platinum doping structures were obtained using density
functional theory (DFT). Binding abilities of Pt on bare SWCNTs structure are in order: 2L>
4L> 6L> 8L. The strongest binding abilities of Pt on the top of the axial C-C bond center at
the cap of various sizes of SWCNTs were found, and their strongest binding energies are 133.12, -123.45, -114.99 and -101.45 kcal/mol for 2L, 4L, 6L and 8L, respectively. CO
adsorption on Pt-doped SWCNTs were studied and their adsorption energies are strong and
within the range of -30.07 to -39.90 kcal/mol. Thermodynamic properties of Pt binding on
selected 2L SWCNT and their CO adsorption were also obtained. DOSs (density of states)
and energy gaps of Pt-doped SWCNTs, their CO adsorption structures and their bare tube are
reported. The changes in binding energy and conductance of Pt-doped SWCNTs after
adsorption of CO, may lead to the different response in the metal-doped CNT-based
CO sensors.
Keywords: carbon monoxide, carbon nanotube, DFT, Platinum, dope
Introduction
Since, Iijima discovered single–walled carbon nanotubes (SWCNTs) [1] and multi–
walled carbon nanotubes (MWCNTs) [2], there have been many studies of their of CNT i.e.
molecular electronic, optic, nanomechanic, magnetic and sensor properties [3, 4, 5]. Recently,
there has been growing interest in carbon nanotube material because of its specific chemical
and physical properties such as chemical stabilities and high specific surface area. There have
been several investigations into the adsorption of small gases on single–walled carbon
nanotube (SWCNTs) for instance water [6], acetone [7], glycine [8], ammonia [9,10],
nitrogen dioxide [9,10], oxygen [11], methane [12] and hydrogen [13]. The results from
experimental and theoretical study revealed that interaction energies are weak. There have
been effort to increase efficiency of gas adsorption on other materials. Hrdlicka et al.[14]
found that silver and palladium which were doped on a surface of metal oxide can increase
interaction energy and encourage interaction between gas molecules on the surface; likewise,
the SWCNTs which were doped by metal [15]. The geometrical, physical, electronic and
chemical properties of SWCNTs were changed due to metal–doped SWCNTs [3]; theirs have
been studied on both experimental and theoretical methods. Recently, Ti, Ni and Pd atoms
coated into the surface of SWCNTs were reported in experimental results [16] and Cu, Co,
Zn, Cd, Mn or Pb ions [17] can occur a continuous coating on the SWCNTs with binding
strength. Many techniques have been used to detect small gases. NO2, H2S and NH3 adsorbed
on Pt and Pd–nanoclusters functionalized on MWCNTs were studied using Radiofrequency
plasma enhanced chemical vapor deposition (RF–PECVD) technique [18]; and CO, NH3,
CO2, CH4 and CO sensor on Pt-doped SWCNT were demonstrated by Electron–beam
evaporation method [19, 20]. Theoretical calculations carried out for Zn–,Pd–and Os
SWCNTs doped on the C atom on the surface of SWCNT. The adsorption is obviously
stronger than on the undoped SWCNT and their adsorption abilities are in the same order:
NO2>NH3>H2O>CO2 >H2 [21]. The results reveal that the gas molecules can be chemisorbed
on metal–doped SWNT [22]. However, little information on computational details of reaction
on Pt–doped SWCNTs have been widely studied. In this work, structures of four different
sizes of closed-end (3,3) armchair SWCNTs, their Pt-doping and CO adsorption
configurations on Pt-doped SWCNTs have been obtained using DFT method at the
B3LYP/Gen level. Binding energies of Pt on SWCNTs, and adsorption energies of CO on Ptdoped SWCNTs were obtained. Thermodynamic properties of Pt binding on selected 2
carbon layers SWCNT and their CO adsorption were obtained. DOSs and energy gaps of Ptdoped SWCNTs, their CO adsorption structures and their bare tube are determined.
Computational details
Closed–end (3,3) armchair SWCNTs capped by half a C24 of sodalite like cage (SLC)
structure were employed for all various size of SWCNTs. Four different sizes of SWCNTs of
which lengths are 2, 4, 6 and 8 carbon layers, (denoted as 2L, 4L, 6L and 8L are consist of
48, 72, 96,120 carbon atoms, respectively as shown in Figure 1), their Pt-doping and their CO
adsorption structures were optimized using B3LYP/Gen method ,using 6–31G(d) basis set for
C, O atoms and LANL2DZ basis set (Los Alamos ECP plus double zeta for heavier element)
for Pt atom in gas phase. The optimized structure of complexes are shown that were at
stationary point corresponding to local minima without imaginary frequencies. Four
SWCNTs, 2L, 4L, 6L and 8L doped by platinum atom on T1-T7 of SWCNTs were
investigated. We defined T1–T2 as center positions and T3–T7 as end position of Pt atom on
the SWCNT, see Figure 2. The average bond lengths of C–C are equal to 1.486 and 1.348 Å
of bare structure for broken (T2, T4, T5, T6) and unbroken (T1, T3, T7) C-C bond, the
selected geometrical data are listed in Table 1. The average C-C bond lengths are 2.564 and
1.484 Å, when Pt atom doped on the SWCNT. Pt–C distances are equal to 1.968 and 2.019 Å
and the bond angles of C–Pt–C (81.3o, 43.1o) for broken and unbroken C-C bond,
respectively. CO gas molecule was adsorbed on Pt–doped (3,3) SWCNT. Adsorption energy
between CO gas and the Pt–doped (3,3) SWCNT was defined as
Eads = E(CO/Pt/SWCNT) – E(Pt/SWCNT) – ECO
where E(CO/Pt/SWCNT) is total energy of CO adsorbed on Pt–doped (3,3) SWCNT, E(Pt/SWCNT)
and ECO are the total energy of Pt–doped SWCNT and CO molecule, respectively. The
electronic density of state of Pt/SWCNT and CO adsorption were investigated and plotted by
the GaussSum 2.1.4 program. The energy gaps referred to the energy difference between the
highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO). Thermodynamic property changes i.e., enthalpy (H) and Gibbs free energy (G)
changes were studied and reported at 298.15 K and standard pressure. All calculations were
carried out using Gaussian 03 quantum chemical package.
Results and discussion
3.1 Pt–doped closed–end (3,3) armchair SWCNTs
The optimized structures of Pt–doped SWCNTs show that Pt atom is more favorable
on the top of the axial C–C bond as defined T1–T7 as illustrate in Figure 2. It is found that
C–C bond of SWCNT is broken when Pt atom was on SWCNTs at T2, T4, T5, T6 positions
different from T1, T3, T7 positions. The studies on the number of C layers in all positions, as
results at Table 2 reveal that binding energies decreased when the number of C layer
increased, except T1. The binding abilities of Pt onto various position of 2L SWCNT are in
order: T4> T7> T5> T6> T2> T3> T1 binding energies (EZPE) are –60.72, –60.33, –53.12, –
51.11, –46.61, –42.07, –30.20 kcal/mol, respectively. The energetic and thermodynamic
properties of binding reaction of Pt atoms doped on various sizes of SWCNTs are listed in
Table 3. The affinities of binding reaction of single atom of Pt onto various position of 2L
SWCNTs are in the same order of binding abilities that are T4> T7> T5> T6> T2> T3> T1
and their Gibbs free energy changes (Go) are –51.82, –51.50, –44.10, –42.34, –37.82, –
33.45, –22.20 kcal/mol, respectively. It is concluded that Pt atom which doped on SWCNT at
T4 and T7 positions are more favorable than other positions and binding energies are –133.12
and –70.27 kcal/mol, respectively. The Pt single atom binding on SWCNT at T1 is the
weakest binding reaction (–37.37 kcal/mol). It can refer that Pt atom is energetically
favorable on the top of the axial C–C bond center at the ended–cap of closed–end SWCNTs.
The binding energy of Pt atom on broken C–C bond SWCNT is more binding energy than
unbroken C–C bond SWCNT because its structure can be changed from sp2 to sp3 easily. So
that we chose one configuration for each cases (broken and unbroken C–C bond) to further
DOS study and CO adsorption which is the most stable structure that are Pt–doped 2L
SWCNT as T4, T7 positions. We look to calculate the DOS for both bare SWCNT and the
Pt–doped SWCNT. Figure 5 (a and b) show the same results from the DOS for bare and
doped SWCNT. The peak near the HOMO region contributes a small amount but the LUMO
has a much larger contribution where Pt is doped shifts down in comparison with a bare
SWCNT by 0.02 and 0.26 eV for T4 and T7 positions, respectively. This shift can be
described by charge transfer between C–Pt bonds. The calculated energy gaps of bare and Pt–
doped SWCNT as T4, T7 are 2.52, 2.50 and 2.26 eV, respectively.
3.2 CO adsorption
The optimized structure of CO adsorbed on Pt–doped SWCNT was displayed in
Figure 3 and 4 as T4 and T7, respectively. Previous work revealed that the adsorption of CO
by C atom pointing is more energetically favorable than O pointing to Pt. The average
distances of Pt–O and Pt–C are equal to 3.112 and 1.969 Å, respectively. It is indicated that
the Pt and C atoms form a strong bond and the binding between the Pt and O atoms is rather
weak. Adsorption energies of CO on Pt–doped (2L, 4L, 6L, 8L) SWCNTs at T4 are shown in
Table 4. Their adsorption energies are in range of –30.46 to –30.40 kcal/mol. It does not
differ significantly at the same position; likewise, adsorption energies at T7 position which
are in range of –39.90 to –39.04 kcal/mol. This results mention that increased the number of
C layer does not affect CO adsorption. The binding abilities of adsorption reaction of CO
molecule on Pt–doped 2L SWCNTs are strong binding at T7 is more than T4 and their
binding energies (EZPE) are –218.47 and –208.23 kcal/mol, respectively. The affinities of
adsorption reaction of CO molecule on Pt–doped 2L SWCNTs are in the same order of
binding abilities that are T7> T4 and their Gibbs free energy changes (Go) are –202.49
and–192.93 kcal/mol, respectively. Those energies indicate a good of the adsorption of CO
on the surface of nanotube. Significant changes can be seen in the DOS of Pt–doped SWCNT
when CO is adsorbed onto the surface as Figure 5c. A small peak in DOS arises near LUMO
region, and the energy gap is narrowed which are 2.13 and 2.08 eV for T4 and T7 positions,
respectively. This illustrate that Pt–doped SWCNT can be used as CO gas sensor.
Conclusions
Adsorption of CO on metal dopes SWCNTs has been studied within DFT. By doping
with Pt atom on closed–end (3,3) SWCNTs. The geometrical and DOS were analyzed to
predict the adsorption properties and theirs thermodynamic properties. Moreover, the
adsorption of CO gas, C pointing adsorption configuration is favorable. Strong adsorption
energy between CO and Pt–SWCNT indicates that the nature of chemisorption. We found
that the DOS and band gap of Pt–doped SWCNTs altered considerably after detecting CO
molecule.
Figure 1 Four sizes of closed–end (3,3) armchair SWCNTs as (a) 2L, (b) 4L, (c) 6L and (d) 8L.
Figure 2 Pt-doped on closed–end (3,3) SWCNTs defined as (a) Type1, (b) Type2, (c) Type3, (d)
Type4, (e) Type5, (f) Type6 and (g) Type7.
Figure 3 Adsorption structures of CO adsorbed Pt-doped SWCNTs (a) 2L (b) 4L, (c) 6L, and (d) 8L at
T4 position.
Figure 4 Adsorption structures of CO adsorbed Pt-doped SWCNTs (a) 2L (b) 4L, (c) 6L, and (d) 8L at
T7 position.
Figure 5 The density of states (DOSs) of (a) bare, (b) the most stable Pt-doped SWCNT and (c)
CO/Pt/SWCNTs.
Table 1 Selected geometrical data for bare and Pt–doped closed–end (3,3) armchair SWCNTs at T4 and T7
positions.
Bond distance (Å)
TM–SWCNTs
Bare SWCNT(T4)
Pt_(2L)SWCNT
Pt_(4L)SWCNT
Pt_(6L)SWCNT
Pt_(8L)SWCNT
Bare SWCNT(T7)
Pt_(2L)SWCNT
Pt_(4L)SWCNT
Pt_(6L)SWCNT
Pt_(8L)SWCNT
C1–C2
1.486
2.556
2.563
2.567
2.568
1.372
1.485
1.484
1.483
1.483
C1–Pt
Bond angle (º)
C2–Pt
–
1.966
1.967
1.968
1.968
–
2.019
2.019
2.018
2.019
C1–M–C2
–
1.966
1.967
1.968
1.968
–
2.018
2.018
2.019
2.018
–
81.1
81.3
81.4
81.5
–
43.2
43.1
43.1
43.1
Table 2 Binding energies of platinum atom on various wall positions of various sizes of closed–end (3,3)
armchair SWCNTs.
Reaction
SWCNT + Pt(Tn)  Pt(Tn)_SWCNT
SWCNT + Pt(T1)  Pt(T1)_SWCNTb
SWCNT + Pt(T2)  Pt(T2)_SWCNTc
SWCNT + Pt(T3)  Pt(T3)_SWCNTb
SWCNT + Pt(T4)  Pt(T4)_SWCNTc
SWCNT + Pt(T5)  Pt(T5)_SWCNTc
SWCNT + Pt(T6)  Pt(T6)_SWCNTc
SWCNT + Pt(T7)  Pt(T7)_SWCNTb
a
In kcal/mol.
b
unbroken C–C bond.
c
broken C–C bond.
2L
–37.37
–125.18
–51.01
–133.12
–109.52
–115.26
–70.27
Ebindinga for SWCNTs
4L
6L
–41.62
–14.45
–115.56
–104.37
–37.31
–27.99
–123.45
–114.99
–99.77
–86.36
–104.40
–92.86
–58.89
–47.29
8L
–27.55
–96.81
–16.61
–101.45
–77.68
–83.11
–40.36
Table3 Energies and thermodynamic properties of Platinum adatom on 2L SWCNTs and CO adsorption on most
and second most stable structures.
SWCNT + Pt(Tn)  Pt(Tn)_SWCNT
SWCNT + Pt(T1)  Pt(T1)_SWCNT
SWCNT + Pt(T2)  Pt(T2)_SWCNT
SWCNT + Pt(T3)  Pt(T3)_SWCNT
SWCNT + Pt(T4)  Pt(T4)_SWCNT
SWCNT + Pt(T5)  Pt(T5)_SWCNT
SWCNT + Pt(T6)  Pt(T6)_SWCNT
SWCNT + Pt(T7)  Pt(T7)_SWCNT
CO + Pt(T4)_SWCNT  CO/Pt(T4)_SWCNT
CO + Pt(T7)_SWCNT  CO/Pt(T7)_SWCNT
a
In kcal/mol
EZPE a
–30.26
–46.68
–41.99
–60.72
–52.99
–51.12
–60.15
–208.23
–218.47
HO298 a
–30.20
–46.61
–42.07
–60.76
–53.12
–51.11
–60.33
–219.64
–229.11
GO298 a
–22.20
–37.82
–33.45
–51.82
–44.10
–42.34
–51.50
–192.93
–202.49
Table 4 Adsorption energies of CO on Pt atom of Pt-doped close–ended (3,3) armchair SWCNTs.
Pt–SWCNTs
CO + Pt_(2L)SWCNT  CO/Pt_(2L)SWCNT
CO + Pt_(4L)SWCNT  CO/Pt_(4L)SWCNT
CO + Pt_(6L)SWCNT  CO/Pt_(6L)SWCNT
CO + Pt_(8L)SWCNT  CO/Pt_(8L)SWCNT
a
In kcal/mol
Eadsa(T4)
Eadsa (T7)
–30.07
–30.45
–30.46
–30.40
–39.90
–39.86
–39.43
–39.04
Table 5 The HOMOs, LUMOs and energy gaps of Pt–doped closed–end (3,3) armchair SWCNTs and their CO
adsorption configurations compared with their bare tube.
Pt–SWCNTs
bare SWCNT
Pt(T4)_(2L)SWCNT
Pt(T7)_(2L)SWCNT
CO + Pt(T4)_(2L)SWCNT
CO + Pt(T7)_(2L)SWCNT
a
In eV.
ELUMOa
EHOMO–LUMOa
EHOMOa
–2.93
–3.02
–3.10
–3.37
–3.28
–5.44
–5.52
–5.36
–5.50
–5.36
2.52
2.50
2.26
2.13
2.08
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