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.