First principles study of the physisorption of hydrogen molecule on graphene and carbon nanotube surfaces adhered by Pt atom Akihiko Fujiwaraa, Dam Hieu Chia,b,c,* a Japan Advanced Insti
Trang 1First principles study of the physisorption of hydrogen molecule on graphene and carbon nanotube surfaces adhered by Pt atom
Akihiko Fujiwaraa, Dam Hieu Chia,b,c,*
a
Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
b Vietnam National University, 144 Xuan Thuy, Cau Giay, Hanoi, Vietnam
c
ERATO Shimoda Nano-Liquid Process Project, Japan Science and Technology Agency, 2-5-3-Asahidai, Nomi, Ishikawa 923-1211, Japan
a r t i c l e i n f o
Article history:
Received 8 August 2009
Received in revised form 12 February 2010
Accepted 12 February 2010
Available online 27 April 2010
Keywords:
DFT
Fuel cell
Carbon nanotube
Catalysts
a b s t r a c t
Adsorptions of hydrogen, oxygen and carbon monoxide molecules on surfaces of single wall carbon nano-tubes (SWNTs) and graphene adhered by a Pt atom have been investigated by density functional theory calculation (DFT) Our calculations show that the Pt adatom significantly promotes the physisorption of hydrogen in a region around it with radius of about 5 Å The physisorption configuration in which oxygen molecule aligned parallelly to the surfaces of SWNTs and graphene are most preferred In contrast, both of the physisorption configurations in which CO molecule aligned parallelly and perpendicularly with the carbon end towards the graphene and SWNTs surfaces were preferred The obtained results suggested that the modification of the electronic structure by adhesion of Pt atom on surfaces of the support mate-rials can modify their physisorption properties of gas molecules
Ó 2010 Elsevier B.V All rights reserved
1 Introduction
Poly electrolyte membrane fuel cells (PEMFCs) have been
con-sidered to be the most promising, among different types of fuel
cell, because they operate at low temperature and give high
spe-cific power and power density[7,2,1] In PEMFCs platinum
cata-lysts, as an active component, is the most important component
for electro-catalysts[7] This brings up a major barrier to
commer-cial application of fuel cells that suffer from high cost and low
stability
Carbon materials are considered as the best support materials
for electro-catalysts in fuel cells because of its conductivity, surface
area, corrosion resistance and low cost[2,1] Among various types
of carbon materials, carbon nanotubes with high surface area, good
electronic conductivity, and high chemical stability, have been
found to be an ideal support material for Pt clusters[4] Highly
dis-persed and size-controlled small Pt clusters (less than 1 nm) made
from dispersed single Pt atoms were achieved by using carbon
nanotube supports[16] The motion of Pt clusters on CNT was also
observed experimentally by high-resolution transmission electron microscopy Further, the superior electro-catalytic activity and the high tolerance to carbon monoxide poisoning of nanoparticle sup-ported on carbon nanotube have been confirmed by several studies
[20,22,12] In addition, our previous theoretical studies reveal sev-eral novel properties of Pt clusters on SWNTs, including the sub-strate mediated interaction and the structural fluxionality [8,9] Fundamental information regarding properties of Pt nano clusters
on SWNTs under gas environment is strongly required for design-ing catalyst for fuel cell
In this paper, we report our first principle study on the absorp-tions of gas molecules on surfaces of SWNTs and graphene adhered
by a Pt atom Physisorptions of hydrogen, oxygen and carbon mon-oxide molecules on the systems have been investigated by density functional theory calculation (DFT) Our calculations show that the
Pt adatom significantly promotes the adsorption of hydrogen in a region around it with radius of about 5 Å The adsorption configu-ration in which the oxygen molecule is aligned parallelly to the surfaces of SWNTs and graphene are significantly more stable than the others In contrast, both of the adsorption configurations in which the CO molecule was aligned parallelly and perpendicularly with the carbon end towards the graphene and SWNTs surfaces were preferred The obtained results suggested that the modifica-tion of the electronic structure by adhesion of Pt nano clusters
on surfaces of SWNTs and graphene can modify their physisorption properties of gas molecules
0927-0256/$ - see front matter Ó 2010 Elsevier B.V All rights reserved.
* Corresponding author at: Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Tel.: +81 76 151 1584; fax: +81 76
151 1535.
E-mail address: dam@jaist.ac.jp (D.H Chi).
Contents lists available atScienceDirect
Computational Materials Science
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 c a t e / c o m m a t s c i
Trang 22 Methodology
2.1 Evaluation of Van der Waals interaction
DFT methods are known as low computational cost methods,
and they are the most popular methods for calculating electronic
structure of many-atom systems The interaction between these
gas molecules and SWNT, graphene surfaces is mainly driven by
Van der Waals force Unfortunately, conventional DFT methods
do not describe well Van der Waals interaction, dispersion
interac-tion[14,19] It has been confirmed that local density
approxima-tion (LDA) funcapproxima-tionals seem to well describe the physisorpapproxima-tion of
H2on graphene and carbon nanotube surfaces, compared to
exper-imental data[19,6] Okamoto and Miyamoto[19]have confirmed
that local density approximation (LDA) functionals predict
physi-sorption of the hydrogen molecule on graphene plate, while some
generalized gradient approximation (GGA) and hybrid-DFT
functionals lead to repulsion interaction Further more they also
indicated that the potential energy surfaces, given by LDA
func-tionals, were very good agreement with the result of second order Møller–Plesset perturbation (MP2) calculation[19]
Calculations of the interaction between hydrogen molecule and
a small graphene plate (C24H12) using Hatree–Fock method and DFT method with several functionals were carried out in compar-ison with that using MP2/6-311++G method for choosing the most appropriate functional (Fig 1a) All DFT[15,17]calculations were carried out with triple numerical plus polarization basis set (TNP) by Dmol3 package[11,10], and molecular orbital methods were carried out by Gaussian 03 package[3]
Our calculations show that HF method almost predicts a repul-sive interaction between hydrogen and garphene, while MP2 method predicts attraction interaction (Fig 1b) This result indi-cates correlation energy correction plays an important role in sim-ulating Van der Waals force, since MP2 method takes into account the correlation energy correction as the perturbation On the other hand, results obtained from calculations using DFT methods strongly depend on employed functionals (Fig 1c) While LDA functionals are good agreement with MP2 result, GGA functionals
(a)
(c) (b)
-150 -100 -50 0 50
100
Molecular orbital method / 6-311++G**
HF
MP2
Separation / Angstrom
-150 -100 -50 0 50 100
Density functional theory
LDA-VWN
GGA-PW91
GGA-BLYP
Separation / Angstrom
(A) (B) (C) (D) (C)
(D)
(A) (B)
(d)
Fig 1 (a) Interaction model between graphene and hydrogen molecule (b) Potential energy surface derived by molecular orbital methods (HF and MP2/6-311++G ) (c) Potential energy surface derived by DFT methods: potential energy surfaces were estimated by taking potential energy at distance of 10 Å as zero (d) Adsorption sites of H 2 on
Trang 3tend to underestimate interaction energy, and some GGA
function-als even predict repulsion between hydrogen molecule and
graph-ene (Table 1) Therefore, in this research we mainly used LDA
functionals to evaluate the interaction between gas molecules
and graphene and SWNT surfaces
2.2 Calculation models
In this study, we used a periodic supper cell to simulate the
graphene sheet The super cell included 128 carbon atoms with
edge length of a and b of 17.07 and 19.71 Å, respectively,
corre-spond to the C–C bond length of 1.42 Å These lattice parameters
were considered to be large enough to neglect the interaction of
Pt, gas molecules with their periodic image The c lattice was of
30 Å, that was large enough to neglect the interaction between
graphene sheets
We also applied a periodic super cells to simulate SWNTs The c
lattices (which were aligned along the axes of the SWNTs) of these
super cells are of 17.04 Å and 19.68 Å for (10, 0)SWNT and
(5, 5)SWNT, respectively These values were chosen to correspond
to the C–C bond length of 1.42 Å and to match the periodic
condi-tion The edge lengths of both a and b lattices of these super cells
were of 25 Å which were large enough to ensure that there are
no interactions Pt, gas molecules and their periodic images The
super cells consisted of 160 carbon atom for both (10, 0)SWNT
and (5, 5)SWNT
We used local density approximation Vosoko–Wilk–Nusair
(VWN)[21]functional to treat exchange–correlation energy
Dou-ble numerical plus polarization function (DNP) basis set was used
for all calculation Brillouin-zone integrations were performed by
using (1 1 4) k-point mesh, and (4 4 1) k-point for SWNTs
and graphene respectively with Monkhorsh-Pack scheme[18] All
calculations were performed by DFT using Dmol3package
3 Results and discussion
3.1 Physisorption of hydrogen
Our calculations as well as several previous publications
[14,6,5]indicated that H2prefers to be physisorbed at the center
of hexagon and aligned parallelly to the surface of graphene and
(10, 0)SWNT In this research, we used parallel adsorption
configu-ration of H2on the surface at the center of hexagon The adsorption
energies of H2on graphene and (10, 0)SWNT surfaces were
calcu-lated by formula:
Ead¼ EH 2þ EGraphene or SWNT EH2=Graphene or SWNT ð1Þ
The adsorption energy of hydrogen on graphene, 112.29 meV, is
close to the value found by Arelano et al., 86 meV with planar
peri-odic graphene layer[5]and by Henwood et al., 93.10 meV with the
hexagonal plate consisting of 96 carbon atoms[6] The adsorption
energy of H2 on (10, 0)SWNT, 107.36 meV, is close to the value
found by Henwood and David Carey[14], 89.98 meV, and larger
than the result found by Han and Lee [13], 34 meV, by
GGA-PW91 functional
For the physisorption of hydrogen on SWNTs and graphene
ad-hered by a Pt atom, our careful examination by full geometry
optimization calculation confirmed that no stable physisorption configuration of hydrogen was found in a region with radius of about 5 Å around Pt adatom When H2move into this region, it is pulled toward the Pt adatom We also considered several adsorp-tion sites of hydrogen around this area (Fig 1d) Our calculations clearly show that at these adsorption sites, H2prefers to be ad-sorbed parallel to the surfaces at the center of hexagon Adsorption energies and equilibrium distance from center of mass of H2to graphene and SWNT surfaces are summarized inTable 2 It means that outside of this region the effect of the Pt adatom is not clear
3.2 Physisorption of oxygen
Oxygen reduction plays an essential role in the performance of fuel cells, as oxygen reduction reaction is four-electrons-transfer reaction in which the first step is the adsorption of oxygen mole-cule on catalysts In this section we mainly focus on the adsorption
of oxygen on carbon support materials, including graphene, (10, 0)SWNT, and (5, 5)SWNT, as well as the effect of the Pt adatom
on it
We considered several adsorption configurations of oxygen on graphene, (10, 0)SWNT and (5, 5)SWNT surfaces, including top, center, and parallel (Fig 2a) The interaction energies between O2
and the surfaces were estimated by the depth of the potential wells
on potential energy surfaces The potential energy surfaces were calculated by changing the distance between O2 and the surface and calculating total energy at each point The depth of potential wells was estimated by taking the potential energy as zero when
O2was placed at the center of super cells.Fig 2b shows potential energy surfaces of the singlet and triplet states of O2 molecule when it approaches to graphene surface It is apparent that in a physisorption on a graphene surface, the distance between O2
and the surface is in a range from 2.5 to 3.0 Å, the triplet sate is more stable than the singlet state Similar results were obtained for the physisorptions of O2on (10, 0) and (5, 5)SWNT surfaces Therefore, in this research we used triplet potential energy surfaces
to evaluate interaction between O2and the surfaces
Typical triplet potential energy surfaces of O2on the surfaces are showed inFig 2c It is apparent that the adsorption energies
of O2on the surfaces strongly depend on adsorption configurations, and the configuration in which oxygen molecule aligned parallelly
to the surfaces of SWNTs and graphene are most preferred ( Ta-ble 3) This result can be explained by the interaction betweenp
electrons of O2andpelectrons of the surfaces The distance from
O2molecule to the surface seems not to depend significantly on
Table 1
Interaction energy (E ad ) and equilibrium distance (D e ) between H 2 and graphene surface.
Table 2 Adsorption energy (E ad ) of hydrogen on graphene and distance between hydrogen and graphene surface (D e ).
Graphene (10, 0)SWNT Graphene (10, 0)SWNT
Trang 4adsorption configurations and curvature of the surfaces Our
ob-tained result also indicates that the adsorption energies of oxygen
on SWNTs are slightly lower than that on graphene
To investigate the effect of the Pt adatom, we calculated the
adsorption energies of O2on the surfaces adhered by a Pt atom
Our calculations showed that on the surfaces adhered by a Pt atom,
triplet still is the most stable state The physisorption energies of
O2at triplet state on the surfaces of graphene and SWNTs adhered
by a Pt atom with parallel configuration are summarized inTable 4
It is apparent that adhesion of Pt promotes the interaction between
oxygen and the surfaces
3.3 Physisorption of carbon monoxide
In direct methanol fuel cell, CO is one of the most important
intermediate substance, therefore information about the
adsorp-tion of CO on catalysts is important for understanding properties
of catalysts In this section, we focus on the absorptions of CO on
graphene, (10, 0)SWNT, and (5, 5)SWNT surfaces as well as the
ef-fect of the Pt atom on these adsorptions
We considered several adsorption configurations of CO on graphene, (10, 0)SWNT, and (5, 5)SWNT surfaces (Fig 3a) We also evaluated the interaction between CO and the surfaces by the depth of potential wells on potential energy surfaces The potential energy surfaces were calculated by changing the distance between
CO and the surfaces, and calculating energy at each point (Fig 3b) These potential energy surfaces indicate that the adsorptions of CO molecules on graphene and SWNT surfaces strongly depend on the adsorption configurations We found that CO molecules prefer to
-200 -150 -100 -50
0
Parallel Center
Distance / Angstrom
2.2 2.4 2.6 2.8 3 3.2 3.4
-0.03 -0.025 -0.02 -0.015 -0.01 -0.005 0 0.005
2.2 2.4 2.6 2.8 3 3.2 3.4
Parallel Singlet Parallel Triplet
Distance / Angstrom
Fig 2 (a) Adsorption configuration of O 2 on graphene, (10, 0)SWNT surface, and (5, 5)SWNT surfaces (b) Singlet and triplet potential energy surface (c) Triplet potential energy surface with different adsorption configurations of O 2 on graphene surface (d) Adsorption site of O 2 and CO on graphene surface.
Table 3
Adsorption energy (E ad ) of O 2 on graphene and SWNT surfaces and distance between the center of mass of O 2 and the surfaces (D e ).
Table 4 Adsorption energy (E ad ) of O 2 on graphene and SWNT surfaces.
Adsorption configuration
Without Pt
With Pt Without Pt
With Pt Without Pt With Pt
Trang 5adsorb parallelly, or perpendicularly to the surfaces with the
car-bon end toward the surfaces (Table 5)
The difference in adsorption energy between configurations can
be attributed to the polarization of CO molecule For configuration,
in which oxygen atom orients toward the surfaces, the negative
charge of oxygen atom weakens the interaction between CO and
graphene or SWNT surfaces, while the positive charge of carbon
atom promotes the interaction between CO and the surfaces For
graphene surface, the adsorption energies of two most preferring
adsorption configurations, carbon center and parallel, are almost
the same For (10, 0)SWNT and (5, 5)SWNT surfaces carbon center
configuration seems to be more stable than parallel configuration
The distance between CO molecule and the surfaces does not seem
to significantly depend on the adsorption configuration
To investigate the effect of the Pt adatom, we also evaluated the adsorption of CO on graphene and SWNT surface adhered by a Pt atom with carbon center, oxygen center, and parallel configuration
at the adsorption site as inFig 2d The adsorption energies are summarized inTable 6 In contrast with the case of O2, adhesion
of Pt atom does not show a clear influence to the interaction be-tween CO and the surfaces
4 Conclusions Our calculations indicated that the adsorption of the Pt atom on graphene and (10, 0)SWNTs leads to the formation of an active re-gion with radius of about 5 Å for the adsorption of hydrogen atom
In this region the Pt adatom significantly promotes the adsorption
of hydrogen by creating a deep and wide potential well on the po-tential energy surface for hydrogen molecules For the adsorption
of oxygen, we found that oxygen molecule do not change it its spin state, the most stable state is triplet state Oxygen molecules prefer
to be adsorbed parallel at the center of hexagons on graphene and SWNT surfaces For the adsorption of CO, we found that CO mole-cules prefer to be aligned parallelly to the graphene and SWNT
Parallel
(a)
(b)
-150 -125 -100 -75 -50 -25 0
Carbon center Oxy center
Parallel
Distance /Angstrom
Fig 3 (a) Adsorption configurations of CO on graphene and SWNTs (b) Potential energy surface.
Table 5
Adsorption energy (E ad ) of CO on graphene and SWNT surfaces and distance between the center of mass of CO and the surfaces (D e ).
Table 6
Adsorption energy (E ad ) of CO on graphene and SWNT surfaces.
Adsorption
configuration
Without
Pt
With Pt Without Pt
With Pt Without Pt
With Pt Carbon center 143.49 142.92 137.92 137.47 136.12 130.41
Oxygen center 108.19 108.27 101.04 100.82 98.70 96.14
Trang 6surfaces, or perpendicularly to the surfaces with the carbon end
towards the surfaces
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