Fig. 1. (a) TEM image and HRTEM image of ZnO nanoparticles (The upper
right corner lies the local enlarged view). (b) Particle size
distribution of ZnO nanoparticles. (c) FTIR spectra of ZnO
nanoparticles. (d) XRD patterns of ZnO nanoparticles.
lidone (PVP) dissolved in water is dropwise added and coated on the
surface of SiO2 by in-situ modification method. After
forming a firm SiO2 coating, the ZnO template disappears
and a hollow structure was formed. And thus the modified HSNs are
finally obtained. The whole preparation process is performed in RPB,
which can produce high gravity environment (tens to hundreds of g) by
centrifugal force. The process in the RPB reactor is thus significantly
intensified, helpful to providing a more even emulsion system and
reaction surrounding, which is suitable for the preparation of uniform
nanoparticles at a large-scale.24, 25
The effects of different preparation conditions on the morphology and
dispersibility of HSNs were investigated and the results show that molar
ratio of NH3·H2O to ZnO is the most
critical factor to form an obvious hollow structure in the preparation
process. TEM images of SiO2 nanoparticles prepared in
RPB at different molar ratio of NH3·H2O
to ZnO (Fig. 2b-e) have presented that SiO2nanoparticles undergo a transition from a solid structure to an
inconspicuous hollow structure, then to an obvious hollow structure and
final to a core-shell structure with the value decreasing from 7.7 to
3.7. More NH3·H2O will corrode ZnO
rapidly and SiO2 nucleates independently. Less
NH3·H2O cannot corrode ZnO to form
ZnO@SiO2 nanoparticles with a core-shell structure.
Under the optimum ratio of 6.1, the deposition rate matches the
corrosion rate of template, and the product can be prepared with high
uniformity and well-controlled morphology. In addition, other influence
factors such as reaction
temperature (Fig. S2, Supporting Information), reaction time (Fig. S3,
Supporting Information) and types of modifiers also have important
effects on the morphology and dispersion state of HSNs. The results show
that 20°C of reaction temperature
and 3 h of reaction time are necessary to attain ideal products. With
the modification of different kinds of modifiers (PVP,
3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane),
HSNs can be stably monodispersed in corresponding solvents (water,
methyl isobutyl ketone and MeOH) to form the transparent dispersions.