Chemistry Reference
In-Depth Information
and 600 K)). Interestingly, there are layered structures in the melted Au
N
within CNTs (Fig. 5a (900 K) and b (860 K)). On the other hand, the peaks
are higher and sharper at lower temperatures, which represent a stronger
layering. During slow heating, the peaks of outer layers are higher and
sharper than those of inner layers. In other words, the inner layers of
Au
1522
become disordered at lower temperatures, while the outer layers
exhibit disordered at higher temperatures during heating process. The
outer layers first form during the cooling process, suggesting that
freezing starts from the outermost layer of confined Au
N
. On the other
hand, most peaks shift toward the center of the CNTs with temperature
increasing during heating process. However, the peaks closest to the
nanotube's long axis shift outward with increasing temperature.
The nucleation analysis is performed in terms of classical nucleation
theory.
38
The nucleation rates appear to decrease with the increasing
nanoparticle size. For confined Au nanoparticles, nucleation starts at the
interface of Au
N
cluster with CNTs. The larger the diameter of CNTs with
same tube length (36.885 Å), the less the surface-to-volume ratio, which
provides less sites where nucleation is likely to occur. On the other hand,
the nucleation rates tend to decrease with the increasing of temperature.
It is attributed to the fact that as the temperature increases from 550 to
700 K, the increasing of nucleation barrier energy dominates nucleation
process. The size effects of heat capacity and interfacial free energy also
influence the nucleation rates. The derived nucleation rates of confined
Au clusters are around 10
35
10
36
m
1
s
1
, which is close to those of free
Au nanoparticles,
39
although their structures and environments are dif-
ferent. The solid-liquid interfacial free energy of confined Au is esti-
mated to be 0.036-0.056 J/m
2
by the Turnbull relation.
40
The nucleation
energy barriers are about (5-9)
10
20
J.
Since the particle size and composition of the bimetallic nanoparticles
will affect their physical and chemical properties, they may exhibit dif-
ferent properties compared to their single-component metal nano-
particles.
41,42
We
43
then simulated the melting and freezing of Au-Pt
nanoparticles ((Au
1
x
Pt
x
)
N
(x
=
0.2, 0.4, 0.6, 0.8, 1.0) with N
=
818, 1522,
and 2230) confined in armchair CNTs ((n, n)-NTs, n
=
19, 25, 30). It was
found that Au-Pt cluster/(n, n)-CNTs exhibit cylindrical multishelled
structures, and the atoms of each layer possess the hexagonal lattice,
similar to the observations on Au cluster/(n, 0)-CNTs
34
and on Au cluster/
(n, n)-CNTs,
35
but are different from those of free Au-Pt clusters
44
or bulk
gold or platinum. Moreover, for the confined Au-Pt nanoparticles, Pt
atoms tend to distribute from the tube wall to the tube center with in-
creasing Pt composition, while Au atoms are inclined to distribute from
the tube center to the tube wall with increasing Au composition, which
may be ascribed to relatively stronger Pt-CNTs interactions. For some Au-
Pt or pure Pt nanoparticles, their atom arrangements along the tube axis
exhibit interesting thin-thick alternation, which is related to the tube
diameter and interactions between metals and CNTs. Figure 6 shows the
relationship between melting temperature and Pt composition for con-
fined Au-Pt nanoparticles. The melting temperatures exhibit an ap-
proximately linear increase within 20-80% Pt as platinum compositions
B
