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).