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H. Jiang et al. / Chemical Physics Letters 429 (2006) 492–496
1
1
1
1
1
08
06
04
02
00
a
b
1
0
.0
.5
0.0
9
9
8
6
o
-0.5
230 C
0
100
200
300
400
500
600
160
180
200
220
240
o
o
Temperature ( C)
Temperature ( C)
Fig. 2. TGA (a) and DSC (b) curves of the as-synthesized Sn nanoparticles.
mal oxidation of the pure Sn nanoparticles. The TGA
results showed that the Sn nanoparticles were covered with
the surfactants by ꢁ4 wt% to the particle weight. And these
surfactants can protect the Sn nanoparticles from oxida-
tion. Fig. 2b shows the thermal profiles of the as-synthe-
sized Sn nanoparticles obtained from DSC where the
perature range of 60 K for cluster size with the number of
atoms between 70 and 200 [7].
From Table 1, it can be seen that the larger molar ratios
between the surfactants and the precursors were used, the
smaller the particle size was. This is because a larger
amount of surfactants can restrict the growth of Sn nano-
particles. The surfactant molecules coordinate with the
nanoclusters, resulting in the capping effect to restrict the
particle growth. This was also found by Pal et al. on their
gold nanoparticle synthesis that increasing the concentra-
tions of surfactants would limit the particle size through
the restriction of particle growth [16]. The DSC results
showed size dependent melting depression behavior and
size dependent latent heat of fusion.
Fig. 4 shows the size dependence of the melting points of
the synthesized Sn nanoparticle powders, which was com-
pared with Lai et al. model [2]. The solid line was calcu-
lated from Eq. (1). Lai et al. obtained this equation
based on the model of Hanszen [17] in which it was
assumed that the solid particle was embedded in a thin
liquid overlayer and the melting temperature was taken
to be the temperature of equilibrium between the solid
sphere core and the liquid overlayer of a given critical
thickness t0.
melting point (T ) was observed at 230 ꢁC. Compared to
m
the Tm of micron sized Sn particles, the Sn nanoparticles
exhibited the Tm depression by 2–3 ꢁC.
The oxide-free Sn nanoparticles were obtained by using
the different molar ratios between a precursor and a surfac-
tant. Thus, different sized Sn nanoparticles can be
obtained.
Fig. 3a,b shows the TEM image and DSC profile of the
Sn nanoparticles which were synthesized by using
ꢀ4
4
.2 · 10 mol tin (II) acetate as a precursor in the presence
of 0.045 mol surfactants. The average particle size was
around 52 nm. The melting point of the Sn nanoparticles
was around 228.0 ꢁC, which was 4 ꢁC lower than that of
micron sized Sn particles. Fig. 3c,d shows the TEM image
and DSC profile of the Sn nanoparticles which were syn-
ꢀ
3
thesized by using 1.1 · 10 mol tin (II) acetate as a precur-
sor in the presence of 0.045 mol surfactants. The average
particle size was around 85 nm. The melting point of the
as-synthesized Sn nanopartiles was 231.8 ꢁC, which was
still lower than the melting point of micron sized Sn parti-
cles. The TEM image of Sn nanoparticles which were syn-
ꢀ
ꢁ
rsl
1
T
r
¼ 232 ꢀ 782
ꢀ r
ð1Þ
1
5:8ðr ꢀ t0Þ
ꢀ
4
thesized by using 1.75 · 10 mol tin (II) acetate as a
precursor in the presence of 0.045 mol surfactants is shown
in Fig. 3e. The average particle size was around 26 nm. The
melting point of these particles was around 214.9 ꢁC, which
was 17.7 ꢁC lower than that of micron sized Sn particles.
The melting transition of this sample took place over a
temperature range of about 60 ꢁC, which was much wider
than the other three samples. This phenomenon can be
attributed to a broadening of the phase transition due to
the finite size effect [15] and wide size distribution of the
Sn nanoparticles. Schmidt et al. also found a melting tem-
where T (ꢁC) is the melting points of particles with a radius
r
˚
˚
of r (A). t (A) is the critical thickness of the liquid layer. r
0
sl
is the interfacial surface tension between the solid and li-
quid, the liquid and its vapor. From their experiment, rsl
is determined to be 48 ± 8 mN/m and the best fit for t is
0
˚
18 A.
It is found that our experimental results are in reason-
able agreement with the Lai et al. model and the melting
point depends nonlinearly on the cluster radius.
From Table 1, it could also be found that the latent heat
of fusion of the different sized Sn nanoparticles was smaller