Chemistry Reference
In-Depth Information
section due to many published reviews and topics.
1-7
Herein, we focused
on the recent computational methodologies including: MD simulations
to investigate the melting and freezing of confined metals; DFT calcula-
tions to investigate the interfacial and reactive properties of metal sup-
ported CNTs.
2.1 The adhesion of noble metals outside the CNTs
The dispersion of metal nanoparticles on CNTs is a vital factor in de-
termining the catalytic activity of metal/CNTs systems. The structural
defects such as topological defects, vacancies, and chemical modification
can substantially introduce novel physicochemical properties of CNTs.
19
The interaction between the metals and the CNTs is usually described by
the binding energy (E
B
), which is calculated by the following formula:
E
B
=
E
metal
þ
E
CNTs
- E
metal
þ
CNTs
, where E
metal
, E
CNTs
, E
metal
þ
CNTs
repre-
sent the total energies of the most stable gas phase metals, the CNTs, and
the combined systems of metals and CNTs, respectively. The binding
behaviors between metal clusters and CNTs are explained in the aspects
of density of states (DOS), deformation charge differences and popu-
lation analysis.
We
20
investigated the comparative adhesion properties of Pt nano-
particles on (5, 5) CNTs surface with and without the point defect. On the
defect free CNTs, a single Pt
1
monomer formed the bridge configuration
with two neighboring carbon atoms with E
B
of 2.30 eV. The Pt
2
dimmers
were on the top sites of both C atoms with E
B
of 2.19 eV. In the Pt
3
trimer
system, two Pt atoms located at the bridge sites of the CNTs with E
B
of
2.69 eV. When one or three atoms of Pt
3
trimer were directly binding to
the outer wall of CNTs, the E
B
values were found to be 1.77 and 2.05 eV,
respectively. The Pt
4
tetrahedron clusters with three or two Pt atoms
directly binding to the outer wall of CNTs had stronger binding ability
than the Pt
4
plane configuration with four Pt atoms located on the top
site of the CNTs with respective E
B
of 2.76, 2.75 and 2.10 eV. Pt
5
and Pt
6
polyhedron clusters were built on either three (triangle) or four (rhom-
bus) Pt atoms directly contacting the CNTs. It was always observed that
Pt
5
and Pt
6
in which four Pt atoms in the first layer were slightly more
stable than another configuration in which three Pt atoms were directly
bonded with CNTs. The Pt-C bond distance was about 2.08 or 2.15 Å
depending on the top or bridge sites. The Pt-Pt distance varied from 2.50
to 3.22 Å, strongly dependent of cluster size and shape. The binding
energies of Pt
n
clusters on the defect free CNTs surface were between 2.19
and 3.22 eV. The bridge between the Pt atom in the Pt
n
cluster and the
carbon atom in CNTs proves stronger binding due to the stronger Pt-C
chemical absorption involved.
With a point (a single atom) defect, we
21
investigated several small
clusters Pt
n
(n
=
1-4) on (5, 5) CNTs. The Pt
1
atom was located on the
hollow site of CNTs with Pt-C bond distances of 1.94, 1.94, and 2.03 Å,
providing E
B
of 7.38 eV. The E
B
became only 2.28 eV when Pt was located
on the bridge site far from the point vacancy. For Pt
n
(n
=
2-4), one of the
Pt atoms in the clusters always adsorbed on the hollow site. For Pt
2
/CNTs,
the largest E
B
of 7.38 eV was obtained when Pt
2
dimer was located
