Fig. 2. (a) Reaction mechanism diagram for TMOS in reverse
microemulsion. (b-e) TEM of SiO2 nanoparticles prepared
in RPB at different molar ratio of NH3·H2O to ZnO. (b)
7.7, (c) 6.9, (d) 6.1, (e) 3.7. (f) TEM of SiO2nanoparticles prepared in STR. (g-h) Histogram of size distribution for
SiO2 nanoparticles prepared in RPB and STR. (g) Particle
size distribution. (h) Size distribution of inner cavity.
Processing capacity of RPB reactor
for synthesis of such HSNs mentioned above is 1.5 g per batch. The same
scale of preparation in STR were studied for comparison. The products
are solid and hollow SiO2 nanoparticles existing
simultaneously (Fig. 2f) under the optimum operation conditions in STR,
and further prolongation of reaction time in STR would not improve their
hollow structure. The particle size distribution of SiO2nanoparticles and their cavity size synthesized in RPB or STR have been
analyzed (Fig. 2g and 2h). The average particle size of
SiO2 nanoparticles is only 19.8 nm with a narrow
distribution from 10 to 30 nm for RPB, compared with average particle
size of 34.0 nm with a broad distribution from 10 to 50 nm for STR.
Furthermore, the average diameter of inner cavity is about 8 nm for RPB,
while it is far less than 8 nm for STR due to more than half of
nanoparticles without the hollow structure. More importantly, the
reaction time in RPB is 3 h as half as that in STR, resulting from the
formation of uniform water-in-oil emulsions with smaller size of water
phase and faster mass transfer at oil-water interface in
RPB.25 Moreover, the yields (The mass percentage of
the well-dispersed nanoparticles in the total output) of HSNs in RPB and
STR with the morphology as shown in Fig. 2d and 2f have a huge
difference. The yield in RPB is beyond 70%, which is twice as much as
that in STR. All in all, the high gravity technology provides the
possibility for the large scale-up preparation of ultrafine HSNs, which
is important for the industrial application of HSNs as antireflection
functional additive in optical polymer matrices.
From the FT-IR spectra of HSNs (Fig. 3a), it can be seen that unmodified
HSNs has no C-H (~2969cm-1), C=O
(~1644cm-1) and
C-N (~1398cm-1) peaks (characteristic absorption peak
for PVP), which appear in the modified ones, indicating PVP are
successfully wrapped on HSNs. However, there is no new characteristic
absorption peak, illustrating only existence of physical interaction
between PVP and HSNs in modified HSNs. Moreover, there are a large
number of –OH on the surface of HSNs, resulting in the appearance of
-OH stretching vibration peaks at ~3384 cm−1. XRD
patterns of HSNs (Fig. 3b) exhibits only one wide diffraction peak at
about 25°, indicating that the HSNs present an amorphous state.
In the TGA curves of HSNs (Fig.
3c), the weight loss of unmodified and modified HSNs are 5.9 wt% and
21.8 wt % at 800°C respectively. Combined with the results of FT-IR,
they are mainly due to elimination of -OH bonds and decomposition of PVP
on the surface of nanoparticles respectively, and the coating amount of
PVP is about 15.9 wt%.
The refractive index (n) of HSNs aqueous dispersions has a linear
relationship with the HSNs volume fraction (V%) in water (Fig. 3d),
which follows equation (1).16, 26
\(n_{t}=n_{1}V\%+n_{2}\left(1-V\%\right)=\left(n_{1}-n_{2}\right)V\%+n_{2}\ \)(1)
Where \(n_{t},n_{1}\text{\ and\ }n_{2}\) are refractive index of HSNs
aqueous dispersion, HSNs and H2O. V% is volume fraction
of HSNs. As described in Fig. 3d, \(n_{2}\) is about 1.333 and the slope
of the line is 0.009. The function of \(n_{t}\) and V% is fitted
linearly (R2=0.999), namely \(n_{t}\)=0.009V%+1.333.
So the refractive index of HSNs (\(n_{1}\)) is the sum of 1.333 and
0.009, namely 1.342. Moreover, the refractive index of HSNs also can be
calculated from their size, following the equation (1) too, but\(n_{t},n_{1}\text{\ and\ }n_{2}\) are refractive index of HSNs, solid
SiO2 and air. As we know n1 and
n2 are about 1.46 and 1 respectively. The shell of HSNs
has high porosity. According to the literatures, 27,
28 pore volume of the amorphous SiO2 is generally about
0.12 cm3/g. And the ratio of pore volume of the HSNs
shell to the whole volume of HSNs can be calculated at 17%, based on
the specific volume of 0.73cm3/g (density of 1.37
g/cm3) for HSNs. Considering inner cavity volume
fraction of 7 %, the total porosity of HSNs is about 24% and
theoretical refractive index is 1.350, which shows the
theoretical calculation result is
in good agreement with experimental one. This also indicates that cavity
of HSNs is fully filled with air instead of water. Otherwise, the
theoretical and measured value of refractive index for HSNs should be
1.430. In addition, it is satisfactory that the modified HSNs in water
are endowed with excellent dispersion stability, still retaining
transparent without settlement after placed at half month, due to strong
charge repulsion effect among HSNs (-19.2 mv).