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234
Adsorption of H2O, CO2, O2, Ti and Cu on Graphene:
A molecular modeling approach
Hilal S Wahaba,* , Salam H. Alib , Adi M.Abdul Husseinb
a
Department of Chemistry, College of Sciences, Al-Nahrain University,
P.O.Box 64090, Baghdad, Iraq
b
School of Applied Sciences, University of Technology, Baghdad, Iraq
Tel: +964 7801 620 707
[email protected]; [email protected]
Abstract-- In the present work, we investigate the adsorption of
small molecules namely, H2O, CO 2, O 2 and deposition of Ti and
Cu on graphene substrate using molecular modeling calculations.
The adsorption of small molecules has very little effect on the
electronic properties of graphene. The deposition of metalli c
atoms presented high molecular doping, i.e., charge transfer and
consequently better adsorption energies and stronger dipole
moments.
Nakamura et al.[10] have studied the variation in the structural
and electronic properties of graphene upon oxygen adsorption.
In this contribution we employ here a computational
method to simulate the adsorption of some small molecules
namely water, oxygen and carbon dioxide and further the
Index Term-- Graphene, Molecular calculations, Adsorption,
deposition of titanium and cupper atoms and probe their
Deposition
impacts on the atomic and electronic properties of the pristine
1.
INT RODUCT ION
The synthesis of monolayer graphene and the experimental
observation of Dirac charge carriers in this model have
graphene. The method is presented briefly in the next section
and the obtained results follow. Finally, in the conclusions we
summarize our results.
attracted immense attention and deepened the interest in this
two dimensional molecule [1,2]. Since graphene, which is the
mother of all known graphitic forms, namely buckyballs,
rolled nanotubes and stacked graphite, is distinguished with its
unusual structural and electronic flexibility, it could be tailored
chemically and structurally in numerous ways; deposition of
metal atoms [3] or molecules [4] on top; incorporation of
nitrogen or boron in its structure [5] and using different
substrates
that
modify
the
electronic
structure
[6].
Furthermore, the main resource for various electrical and
optical applications is stemmed due to the interactions between
graphene and various chemical molecules [7]. Therefore, this
one–atom thin 2D material [8] and the deposition and
adsorption of different atoms and molecules on its surface have
been the subject of numerous experimental [2] and theoretical
Wehling et al.[9] have investigated the water adsorption on
and
computational
the
first
impacts
of SiO2
principles
with the MSINDO software package [12, 13].This method is
based on a semiempirical simplification of complex molecular
matrix elements and permits accurate and efficient molecular
orbital (MO), calculations for many-atom configurations [14].
The method has been extensively documented for the first-,
second- and third-row main group elements and first-row
transition metal elements [12, 13, 15]. The MO calculations
were carried out at the level of the self-consistent field (SCF)
method and energy was calculated with the structure optimized
alongside, with energy convergence criterion below 10-9
Hartree.
Additionally, the inner shell electrons are taken into
account through the use of Zerner ps eudo potentials [16].
studies since its discovery in 2004 [9-11].
graphene
2. COMPUT AT IONAL MET HOD
The quantum chemical model calculations were performed
substrate
methodology.
using
Further,
Besides, MSINDO is a reliable method for studies of surface
properties and further, reproduces adsorption energy values
comparable to higher-level density functional theory (DFT)
calculations [17].
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International Journal of Basic & Applied Sciences IJBAS-IJENS Vol:12 No:06
For modeling graphene using the supercell technique, we
considered a 8x8x1 unit cell containing 128 carbon atoms. The
in-plane lattice parameter a = 2.42 Ả for graphene agrees well
with other theoretical reported values [7,18]. The adsorption
energies, Eads
of the molecules on graphene were computed according to the
following expression;
Eads = Emolecule/graphene – Epristine
–
graphene
Emolecule
(1)
Where Emolecule/graphene , Epristine graphene and Emolecule are the total
energies
of
the
molecule/graphene,
fully
relaxed
pristine
configurations
graphene
and
of
the
molecule,
respectively.
3. RESULT S AND DISCUSSION
3.1 Geometry
Graphene is composed of carbon atoms arranged in hexagonal
structure as presented in Fig. 1a. The structure begins with six
carbon atoms, tightly bound together chemically in the shape
of what is known as a large assembly of benzene rings that are
linked in a sheet of hexagons, i.e.; honeycomb structure. The
geometry is made out triangular lattice with a basis of two A-B
atoms per unit cell [19] (Fig. 1a). The C-C distance and C-C-C
angle have been computed in this work and equal 1.40 Ả and
119.97 degrees, respectively. These findings are in an excellent
accordance with the results of Malard et al. [20], 1.42 Ả and
Rosas et al. [21], 1.41 Ả and 119.95 degrees. The 8x8x1 unit
cell which contains 128 carbon atoms has presented a binding
energy of -31.856 a.u. (Table I) which confirms the stability of
the modeled geometry.
From Fig.2 a,
we observe the asymmetric bands of the Highest Occupied
Molecular Orbitals (HOMOs) and Lowest Occupied Molecular
Orbitals (LUMOs) and further, a broken zone on the minus
sign site. Neto et al.[19] have reported, in their study of the
electronic properties of graphene, that the plus sign applies to
the upper (π*) and the minus sign the lower (π) band, i.e.;
HOMO. In addition, they concluded that the electron -hole
symmetry was broken and the π and π* bands become
asymmetric. Another interesting result is that the broken zone
revealed computed gap energy of 3.27 eV (Fig. 2a). This
computational finding is in agreement with other reported
observations by Rosas et al. [21], 2.95 eV and Martinez et al.
[22], 2.8 eV. Nevertheless and in contrary to the findings of
Rosas et al. [21], this study has shown zero dipole moment for
the
modeled
graphene
(Table
I).
3.2 Adsorption of small molecules
After ensuring the description of the geometry, the feature
of small molecules viz. O2 , CO2 , H2 O adsorption onto graphene
235
surface was examined. In this simulation the CO2 and O2
molecules were initially positioned in a conformation parallel
to the graphene surface at 2.355 and 2.125 Ả distances,
respectively. Upon optimization of the adsorption models, a
very slight migration (0.1 Ả) of the considered molecules
towards the surface was observed (Figs. 1b and d). This
insignificant adsorption with low Eads values (Table I) confirms
low susceptibility of these small molecules towards the
graphene surface. The adsorption of H2 O molecule onto the
graphene surface via parallel and perpendicular configurations
was also examined. From Table I, we learn that the Eads value
for the parallel orientation of water is, to some extent, higher
than that of perpendicular arrangement and also than that of
O2 , CO2 adsorbates.
According to our MSINDO calculations, graphene behaves as
a semiconductor material with gap energy of 3.27 eV. This
behavior was also reported by other authors [21], for their
density functional theory (DFT) calculations. No impacts for
the adsorption of the studied small molecules H2 O, CO2 and O2
on the gap energy of graphene were observed, as illustrated in
Figs. 2d, e and f, respectively. For further scrutinization,
HOMO and LUMO magnitudes have been computed which
subsequently verified the insignificant influence of the
previously adsorbed molecules (Table II). Our findings are in
good agreement with other computational study for Leenaerts
et al. [23], who reported that graphene is highly hydrophobic
and further, the adsorbed water molecule has very little effect
on the electronic structure of graphene.
3.3 Deposition of metallic atoms
We proceed with the study of the metallic titanium (Ti)
and cupper (Cu) atoms deposition on single layered graphene
surface (Figs. 1f and e, respectively). According to the best of
our knowledge, the metallic deposition of Ti and Cu atoms on
graphene surface has not previously been reported. Calandra
and Mauri [24] have stated that the electronic flexibility
promotes the graphene to be tailored chemically for the
deposition of metal atoms. Fig. 1 shows some sort of distortion
in the carbon polygon upon metallic atoms deposition. The
high Eads values (Table I) for Ti (-341.3 kJ/mol ) and Cu (183.8 kJ/mol) which confirm high susceptibility of these atoms
towards the graphene surface could be the source of this slight
distortion.
The HOMO and LUMO energy levels of graphene,
graphene+Cu and graphene+Ti are presented in Table II. From
this table one can observe that in the deposition of Ti and Cu
onto graphene surface, the interaction of these two metal atoms
with HOMO of graphene predominates , and consequently
lower negative HOMO values are merged in comparison with
the HOMO values of other small molecules adsorption.
Accordingly, the former case would require lower activation
energy. Based on the above findings, the comparison of the
DOSs of these two models in Figs.2b and 2c, with all other
adsorption DOSs reveals explicitly lower band gaps of 1.25
and 2.66 eV for Ti and Cu, respectively. Moreover, along the
HOMO/LUMO regions of graphene, there is also some
titanium and copper character, which is probably due to the
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chemisorptions’ phenomenon. In contrast, H2 O, CO2 and O2
molecules have presented only a slight, contribution to solely
LUMO zone of graphene, which are solely physisorbed.
Hence, one can draw a conclusion that the contribution of both
Ti and Cu atoms to the HOMO and LUMO bands is anoth er
proof of the strong interaction between the graphene surface
and metal atoms. This finding is in accordance with the results
of Pi and colleagues for transition metals doping on graphene
[25].
Currently, a quantitative explanation for the binding of
graphene to various metal surfaces is not intensively available
in the literature [26]. Nevertheless, the so-called d-band model,
developed to elucidate trends in binding of adsorbed molecules
on transition metals [27], expects a stronger binding with
decreasing occupation of the d orbital. Accordingly, the
stronger binding or interaction of graphene with Ti than with
Cu is consistent with this hypothesis. Winterlin and Bocquet
[27] reported that DFT calculations have revealed that Cu and
Ag with their filled d bands hardly interact with the graphene
layer, whereas Ni and Co interact strongly.
3.4 Electronic structure
The electronic structure of graphene due to the adsorption
of small molecules and deposition of metallic atoms is a
particularly important issue. It is substantial for understanding
the chemical interaction with other species and is even more
essential for the physical properties such as electron transport,
which turns free standing graphene such a unique material
[28,29].
A key issue to investigate the electronic structure is the charge
transfer between the substrate (graphene) and adsorbents or
deposited species. Giovannette et al. [30] have reported that
graphene will be p-doped (n-doped) if the transition metals
work function is larger (smaller) than graphene. Recently, DFT
calculations predict the n-doped doping of graphene [31].
Leenaerts et al. [2] concluded that there are two different types
of charge transfer mechanisms when molecules adsorb on
graphene. One is the due to orbital hybridization which occurs
for all molecules, but it results in small charge transfer in the
case of physisorption processes as it is evidently shown in
Table 3 for the small molecules, H2 O, O2 and CO2, adsorption
on graphene. Whilst, the second is due to the position of the
HOMO and LUMO of the atoms or molecules with respect to
the Dirac point of graphene. These charge transfers are
relatively large, as it is clearly presented in Table 3 for the
deposition of Ti and Cu. The above findings have been ratified
due to the relatively high dipole moments of Ti and Cu in
comparison to the dipole moments of the small molecules as it
is obviously illustrated in Table 1. Based on the above
findings, it is therefore, necessary to realize the difference
between the different mechanisms when calculating charge
transfers to get quantitatively reliable results. For further
analysis, one could also observe, from Table 3, the electronic
feature of adsorbent/adsorbate complex. The acceptor
character of parallel and perpendicular oriented water
molecules on graphene is in accordance with the experimental
findings of Schedin et al. [32], where they found that the
acceptor character is energetically favored on perfect graphene.
236
On contrary, the CO2 and O2 act as donor molecules. On the
other hand, the deposition of metallic atoms, Ti and Cu,
induces the charge transfer from graphene surface into their
partially empty 3d orbitals (Table III).
Also from Tables I and 3, we observe that the strength of
binding and adsorption energies are related to the values of the
computed Löwdin charge population changes i.e., charge
transfer. The high Löwdin charge population changes in the
case of Ti and Cu atoms deposition onto pristine graphene
illustrate explicitly the much higher values of adsorption
energy than the binding energy of the atoms under
consideration. Whilst, the binding and adsorption energy
values (Table I) are comparable in the case of H2 O, O2 and
CO2, adsorption on graphene. These findings are also in good
agreement with theoretical studies of the adsorption of
molecules on single wall carbon nano tubes [33]. This suggests
that some of the knowledge of adsorption on nanotubes could
be transferable to graphene.
4.
CONCLUSIONS
The current MSINDO calculations investigate the
electronic properties of graphene resulting from adsorption of
H2 O, CO2 and O2 and deposition of Ti and Cu.
The adsorption of the small molecules H2 O, CO2 and O2 have
exhibited a comparable binding and adsorption energies, while,
the adsorption energies of Ti and Cu deposition onto pristine
graphene is approximately an order of magnitude larger than
the binding energies of the deposited atoms. Charge transfers
between a graphene and the adsorbed molecules are very
small, whereas, the deposition of metallic atoms, Ti and Cu,
induces the charge transfer from graphene surface into their
partially empty 3d orbitals.
A CKNOWLEDGEMENT S
One of the authors (H.S.Wahab) thanks the IIE / SRF for the
partial financial support of the research stay.
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[8]
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[11]
[12]
[13]
[14]
[15]
[16]
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A ET
T ABLE I
A COMP UTED ENERGIES FOR THE ADSORBED MOLECULES ONTO THE GRAP HENE SURFACE
TOTAL ENERGY IN ATOMIC UNITS, A .U . ; ZPE ZERO P OINT ENERGY ; E B BINDING ENERGY ;
Energy
Graphene
CO2
O2
H2 O Par
H2 O Per
Cu
Ti
ET, a.u.
-752.227
-789.863
(-37.605)
-783.746
(-31.494)
-769.309
(-17.035)
-769.307
(-17.035)
-800.466
(-48.167)
-755.578
(-3.220)
ZPE, a.u.
0.896
0.914
0.906
0.927
0.927
0.898
0.897
ET,
a.u.
-751.331
-788.949
-782.840
-768.382
-768.380
-799.568
-754.681
Eb, a.u.
-31.856
-31.868
-31.870
-31.872
-31.871
-31.928
-31.988
Eads , kJ/mol
-
-34.2
-39.3
-42.0
-36.8
-183.8
-341.3
DM, Debye
0.000
1.018
1.055
1.016
1.019
3.804
8.842
corrected,
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T ABLE II
HOMO-LUMO ENERGIES FOR GRAP HENE AND THE ADSORBED MOLECULES ONTO
GRAP HENE SURFACE , IN ATOMIC UNITS
Molecule
LUMO
HOMO
Graphene(G)
-0.0275

-0.2131
CO2 + G
-0.0234

-0.2057
O2 + G
-0.0199

-0.2101
Par H2 O + G
-0.0230

-0.2052
Per H2 O + G
-0.0223

Cu + G
-0.0251

Ti + G
-0.0311

-0.2042
-0.1853
-0.1660
T ABLE III
Computed Löwdin charge population changes of selected atoms for the graphene and the adsorbed molecules onto graphene surface (atom numbering shown in
Fig.1)
Atom
Graphene
C3
C4
C7
C8
C9
C10
C12
C19
C21
C22
C,
CO2
O,
CO2
O,
CO2
O, O2
O, O2
O,
H2O
H,
H2O
H,
H20
Cu
Ti
4.000
4.000
4.000
4.000
4.000
4.000
4.000
4.000
4.000
4.000
3.294
CO2
H2 O
Par
H2 O
Per
Ti
3.800
3.803
3.903
3.849
3.847
3.865
4.095
3.992
3.987
3.997
4.047
3.294
6.325
6.353
6.321
3.903
3.908
5.953
5.997
6.569
6.566
0.721
0.759
0.747
0.721
0.758
0.747
11
4
Cu
3.924
6.355
6.000
6.000
6.558
O2
11.369
5.116
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C21
C12
C4
(a)
1.40
239
C3
C19
C9
C8
C7
C10
C22
A-B unit cell
OC O
2.255
(b)
H H
O
2.025
(c)
2.172
OO
(d)
Cu
Ti
(e)
(f)
Fig. 1. Adsorption modes of (b) CO 2 ; (c) H2 O; (d) O2 ; (e) Cu; (f) T i onto (a) Graphene surface
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240
DOS (arbitrary units)
(a)
Graphene
G
3.27
-10
-5
0
5
10
Energy, eV
(b)
(c)
Ti + G
Cu + G
1.25
2.66
(d)
H2 O + G
3.25
(e)
CO2 + G
3.27
(f)
O2 + G
3.26
Fig. 2. Density of states (DOS) for adsorption modes of (b) T i;(c) Cu;(d) H 2 O;(e) CO2 ; (f) O2 onto
(a) Graphene surface.
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