Chemistry Reference
In-Depth Information
rich N-CNTs. It was further proved by the electrons transfer based on the
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wdin charge analysis. The charge of Au, Ag was 0.25 and 0.53 on B-
CNTs, which was much bigger than 0.07 and 0.13 on N-CNTs, respect-
ively. For the Pt system, the most favorable binding site was the bridge of
carbon-carbon on three kinds of CNTs. The binding energies of Pt on
pristine and B-CNTs were nearly the same due to the lack of direct
interaction between Pt and B. The PDOS of Pt and carbon on pristine and
B-CNTs were similar. However, N-doping slightly increased the binding
energies than that on pristine. The PDOS on N-CNTs were more dis-
persed and more anti-bonding empty states than that on pristine and
B-CNTs. The charge density differences showed that the covalent bond
between Pt and two neighboring carbon formed on three kinds of CNTs.
Moreover, the stronger covalent bond formed between Pt and electron
rich N-CNTs than electron deficient B-CNTs. The charges of Pt on three
kinds of CNTs were all about 0.12. Overall, the binding energy of Pt was
always the biggest among three metal monomers on the same CNTs
because of more anti-bonding empty states for Pt than that for Au and Ag.
2.2
The melting and freezing of metal nanoparticles confined in the
CNTs
The CNTs filled with different materials are of great interest in science
and technology of nanomaterials due to their novel structures and
properties. Especially, the metal-filled CNTs have potential applications,
such as nanocatalysts, semiconductor devices, nanomagnetic recording
media, fuel cells and so on. Under the influence of a substrate, nano-
particles are different from free nanoparticles.
26,27
Their properties rely
not only on the structure, particle size, and composition but also on the
nature of metal-substrate interactions. One of the important properties,
solid-liquid phase transition, has a significant influence on the synthesis
of nanoparticles. The melting point of metal nanoparticle is lower than
that of the bulk counterpart, and it increases with the increase of particle
size.
28
Although many experimental techniques have been developed to
investigate the melting process of nanoparticles, the understanding of
this process is limited and not satisfactory due to the size and compli-
cated structure of the nanoparticles.
29
Especially, the experimental
studies about phase transition and nucleation dynamics for metal
nanoparticle encapsulated in CNTs have proven elusive and dicult.
Fortunately, MD simulations can provide physical insights into metals
supported on substrates and have been widely applied to investigate
metal-filled CNTs.
By using the steepest descent method and MD simulation, Choi et al.
30
found that the cylindrical ultrathin copper nanowires in CNTs have
multi-shell packing structures. As the diameter of CNTs increases, the
encapsulated copper nanowires have the face-centered-cubic structure as
the bulk. The results obtained by Hwang et al.
31
using a classical MD
method suggested that induced by periodic energy barriers in (5, 5) CNTs,
the encapsulated copper nanoclusters tend to move swiftly along the tube
axis, with the diffusion speeds showing the Arrhenius relation. Add-
itionally, the copper nanowires even grow in the ultra-thin CNTs with
