phys. stat. sol. (b) 243, No. 13, 3472 – 3475 (2006) / DOI 10.1002/pssb.200669166
Structural and electronic properties of Ptn (n = 3, 7, 13) clusters
on metallic single wall carbon nanotube
Nguyen Thanh Cuong1, Dam Hieu Chi*, 2, 3, Yong-Tae Kim1, and Tadaoki Mitani1
1
2
3
Materials Science School, Japan Advanced Institute of Science and Technology, 1-1, Asahidai,
Tatsunokuchi, Ishikawa, Japan
Center for Strategic Development of Science and Technology, Japan Advanced Institute of Science
and Technology, 1-1, Asahidai, Tatsunokuchi, Ishikawa, Japan
Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam
Received 20 April 2006, revised 17 August 2006, accepted 18 August 2006
Published online 11 October 2006
PACS 61.46.Bc, 61.46.Fg, 71.15.Mb, 73.22.–f
A systematic study of Ptn (n = 3, 5, 7) clusters adsorbed on the metallic (5, 5) single wall carbon nanotube
was carried out using theoretical calculations within Density Functional Theory. The geometrical and
electronic structure and interaction between the Pt clusters and the single wall carbon nanotube were investigated. The bridge adsorption sites on the outer wall of the carbon nanotube are found favorable for Pt
atom. We found that the average C – Pt and Pt – Pt bond length, binding energy, and the amount of charge
transfers from the Pt cluster toward the nanotube increase with the size of cluster. The calculated densityof-states suggest a mixing of ionic and covalent character for the binding nature of this system.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Nowadays, the catalysis plays an innovative role in the development of new technologies and electrocatalyst designs, a key factor for enhancing catalysis performances, become big issues accompany the
industrialization. Nanotechnology is believed to be important in heterogeneous catalysis due to its peculiar properties and potential applications. The carbon nanotube with beautiful tubular structure and a
large effective surface which could facilitate the adsorption of small catalyst particles, has received much
attention recently. Several studies [1, 2] were carried out that deal with the applications of carbon nanotubes as supports for catalyst in fuel cell. In these researches, Pt and Pt–Ru materials are found to be the
best catalysts of fuel cell to enhance the cathode oxygen reduction reaction, and it is believed that dispersity and cluster size of catalyst particles mainly affect the properties of the electro-catalyst. Therefore,
many consequent studies [3, 4] on dispersion and size control of clusters on carbon nanotube supports for
electrocatalysts have been conducted to investigate the effect of cluster size on electro-catalytic activity.
Recently, we have succeeded in establishing a new method based on a fundamental bottom-up approach to synthesize highly dispersed and size controlled Pt clusters on carbon nanotube supports, which
is called atom-to-cluster (SAC) approach [5]. The introduction of thiol groups to the surface of carbon
nanotube resulted in the extreme single-atom dispersion, and a finite size control of clusters from the
dispersed single atoms was archived by heating process.
In this paper, continuing the flow of our previous papers which reported experimental results, we
report our first-principles study on the interaction between a Pt single atom and Ptn (n = 3, 7, 13) clusters, and a metallic (5, 5) single wall carbon nanotube (SWNT). We will focus on the adsorption of the Pt
*
Corresponding author: e-mail: [email protected]
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Original
Paper
phys. stat. sol. (b) 243, No. 13 (2006)
3473
clusters on the SWNT, particularly the charge transfer and the binding nature between Pt and SWNT,
which could help us to better understand the experimental observations and could benefit the experimental study on catalysis.
2 Computational methods
We performed periodic calculations based on the density functional theory (DFT) [6, 7] by employing a
linear combination of localized pseudoatomic orbitals (LCPAO) method [8]. For the exchange and correlation terms, the generalized gradient approximation (GGA) is used as described by Perdew, K. Burke,
and M. Ernzerhof (PBE) [9]. Double valence plus single polarization orbitals were used as a basis set
with cutoff radii of 7.0 a.u. and 4.5 a.u. for Pt and C respectively [10]. Troullier–Martins type pseudopotentials [11] with a partial core correction are used to replace the deep core partial by the normconserving soft potentials in a factorized separable form with multiple projectors proposed by Blochl.
Also the real space grid techniques are used with the energy cut-off of 150 Ry in numerical integrations
and the solution of the Poisson equation using the Fast Fourier Transformations (FFT) technique. All the
DFT calculations were performed by using OpenMX code [12], which is designed for the realization of
large-scale DFT calculations.
The infinite one-dimensional (5, 5) metallic single wall carbon nanotube is simulated. We have applied the supercell with 25.0 Å for a lattice and 16.0 Å for b lattice, which are large enough to neglect the
interaction between the nanotube and its periodic images. The c lattice (17.07 Å) aligned with the axis of
the nanotube is tuned to match the periodic condition. The structure optimization was stopped when the
forces due to displacements of an atom in the unit cell converged within 0.002 Ha/Å.
3 Results and discussion
We carefully optimized the geometrical structures, from initial structures constructed by putting a Pt
single atom and Ptn (n = 3, 7, 13) cluster on all the nonequivalent high symmetry sites of the outer wall
of the (5, 5) SWNT. The best adsorption configuration structures are presented in Fig. 1. When a Pt single
atom adsorb on the SWNT, the bridge sites is most preferred. The underlying C–C bonds are drawn
upward slightly, and the lengths are elongated consequently. Similarly, Pt3 and Pt7 adsorbed in the form
in which the contact Pt atoms (to the SWNT) are located on the bridge sites. In the case Pt13 cluster, there
are 3 Pt atoms have contact with outer wall of nanotube. Two of these are stabilized at the bridge sites,
while the other Pt atom is located at nearly the bridge sites with some stress due to the geometrical structure of Pt13. Consequently, the adsorption led to a slight deformation in the geometrical structure of Pt13.
a
c
b
d
Fig. 1 (online colour at: www.pss-b.com) Optimized geometrical structures of the Pt atom/Pt clusters adsorbed on the
(5, 5) SWNT. Small grey balls represent carbon atoms, and
small blue balls represent Pt atoms. (a) Pt single atom, (b) Pt3,
(c) Pt7, and (d) Pt13 clusters adsorbed on the (5, 5) SWNT.
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
N. T. Cuong et al.: Structural and electronic properties of Ptn (n = 3, 7, 13) clusters
3474
Table 1 Binding energies (eV) of the free Pt clusters and Pt clusters adsorbed on the (5, 5) SWNT and
distances (Å) of the closest C – Pt and Pt – Pt in the clusters adsorbed on the (5, 5) SWNT.
Eb1
Eb2
dC–Pt
dPt–Pt
Pt3
Pt7
Pt13
2.17
2.94
2.17
2.57
2.99
2.99
2.18
2.57–3.42
3.32
4.81
2.2
2.61–3.91
For further understanding of the binding nature of the adsorption, we adopted formulas
Eb1 =
nEPt - EPt n
;
n
Eb2 = EPt n + ESWNT - EPt n /SWNT
DOS (states/eV)
to evaluate the cohesive energy of free Pt clusters, and the binding energy of the Pt clusters on the (5, 5)
SWNT. Here, EPt , EPt n, and ESWNT are the total energies for a free standing Pt single atom, Pt clusters, and
a bare SWNT, respectively.
EPt n /SWNT is the total energy for the configuration with the Pt clusters adsorbed on the SWNT. Eb1 are
cohesive energies and Eb2 are binding energies computed with respect to isolated metal clusters. Table 1
shows the binding energies for the best adsorption sites for Ptn (n = 3, 7, 13) clusters. One can see that
the cohesive energies and binding energies increases with the size of Pt cluster. On the other hand, the
average bond length of C–Pt and Pt–Pt also increase with the size of Pt cluster, in consistent with our
previous experimental observations [5]. The binding energies of Pt clusters adsorbed on graphene sheet
were also carried out for comparison, and revealed that the curvature of the tube has an obvious effect on
the interaction between the Pt and the tube. This fact raises the question of the nature of the interaction
between Pt clusters and the SWNT. One can postulate three possibilities for the binding nature in this
system. One is the ionic binding in which the Coulomb interaction plays the central role. The second is
the covalent binding in which strong covalent bonding between Pt and C is essential in the adsorption
process, and the third is a mixing of ionic and covalent for the bonding character. One can easily find
100
(a)
Ef = –5.98 eV
Ef = –5.05 eV
80
60
40
20
DOS (states/eV)
0
100
(b)
Fig. 2 (online colour at: www.pss-b.com)
Density of states for the isolated Pt13 cluster (a –
blue line) and the bare (5, 5) SWNT (a – red
line), and the projected DOS for the adsorbed
Pt13 cluster (b – blue line) and the (5, 5) SWNT
(b – red line). The red, blue, and black vertical
dotted lines denote the Fermi levels for the bare
(5, 5) SWNT, the isolated Pt13 cluster, and the
adsorbed system, respectively.
Ef = –5.63 eV
80
60
40
20
0
-12
-10
-8
-6
-4
-2
Energy (eV)
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Original
Paper
phys. stat. sol. (b) 243, No. 13 (2006)
Table 2
3475
Total electron transfer from the Pt clusters to the (5, 5) SWNT.
total charge transfer
Pt
Pt3
Pt7
Pt13
0.53
0.81
1.23
1.86
that the binding energy is depending much on the number of C–Pt bonding, which suggests the covalence in the binding nature. For clarifying, the binding nature in this system, we investigated the electronic structures of the Pt clusters adsorbed on the (5, 5) SWNT.
The density of state (DOS) for the adsorbed Pt clusters and the isolated metal clusters with the same
structure were analyzed (Fig. 2). The Mulliken charge analyses also were carried out for the evaluation
of the total electron transfers from the Pt clusters toward the carbon nanotube (Table 2). The DOS of the
adsorbed Pt13 cluster was slightly deformed from the DOS of the isolated Pt13 cluster, and shifted by ca.
0.6 eV toward a lower energy region. This shift can be explained as the increase in effective Coulomb
potential due to the loss of charge. These results not only confirm the tendency of charge transfer from
the Pt clusters toward the carbon nanotube, but also suggest the ionicity in binding nature. Finally, the
deformation of the DOS of adsorbed clusters, compare with that of isolated clusters, strongly suggests an
orbital mixing between metal cluster and carbon nanotube at near Fermi level. From all of evidence, we
can conclude that the binding between the Pt clusters and the SWNT in this system has both ionic and
covalent characters.
4
Conclusion
We performed a first-principles study on the interaction between a single Pt atom and Ptn (n = 3, 7, 13)
clusters, and a metallic (5, 5) SWNT. The best adsorption site of single Pt atom on the outer wall of
SWNT is the bridge-type site and the curvature of the carbon nanotubes does affect the adsorption. The
binding energies between the Pt clusters and the SWNT increases with the size of Pt cluster. Charge
density analyses confirm a charge transfer from Pt clusters toward the carbon nanotube, and the amount
of charge transfers increases linearly with the size of Pt cluster. The calculated density-of-states suggest a
mixing of ionic and covalent characters for the binding nature of the systems.
Acknowledgements This work has been partly supported by the 21st COE (Center of Excellence) Program “Study
of Scientific Knowledge Creation” of JAIST, funded by Ministry of Education, Culture, Sports, Science and Technology (MEXT, Japan) and the HJK Computation for Materials Science project, funded by JAIST. And one of us,
N. T. Cuong, thanks Komatsu Seiren Co., Ltd. for the financial support.
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim