Experiment Section
In this work, the detailed information regarding all of the reagents, the preparation process for ZnO and HSNs and characterization methods can be found in the Supporting Information.
Results and Discussion
As a template, the prepared ZnO nanoparticles have a spheroid morphology (Fig. 1a) with a narrow size distribution, and the average diameter of nanoparticles is about 9 nm (Fig. 1b), which can be monodispersed in water to form nanodispersion with high transparency (lower left of Fig. S1, Supporting Information). Clear and continuous lattice fringes of ZnO nanoparticles with 0.28 nm lattice spacing in the high resolution transmission electron microscopy image (the lower right inset in Fig. 1a) correspond to the (100) crystal plane of ZnO, which indicates the ZnO nanocrystals are well crystallized.18X-ray diffraction (XRD) patterns of ZnO nanoparticles (Fig. 1c) present all the peaks of unmodified and modified samples expectedly corresponding to hexagonal wurtzite crystal form (JCPDS No.361451).18, 19 All peaks of the modified sample are slightly broadened and weakened than unmodified one, because organic modifier molecules coated on the nanoparticles surface weaken the detection of ZnO crystal components.18 The FTIR spectrum of ZnO nanoparticles (Fig. 1d) shows that KH550 has been successfully grafted onto the surface of nanoparticles because its characteristic absorption peaks appear in the curve of modified ZnO such as -CH- (~2947 cm-1), -CH2 (~1220 cm-1) and -NH2 (~1587 cm-1).20 Without doubt, KH550-modified ZnO nanoparticles can be well dispersed in the aqueous phase, as shown in Fig. 1a. In addition, the absorption peaks at ~3428 cm−1 corresponds to the characteristic peaks of -OH stretching vibration. Thermo- gravimetric analysis (TGA) results furtherly confirm that ZnO nanoparticles were wrapped by the KH550 and the loading is 18.6% (Fig. S1, Supporting Information). It is noted that surface amination treatment makes ZnO nanoparticles positively charged up to 34.2 mv, which facilitates coating process of SiO2 with negative charge.21
HSNs were prepared by the reverse microemulsion method. As shown in Fig. 2a, the microemulsion is composed of water as the disperse phase and cyclohexane as the continuous oil phase, Igepal CO-630 as the emulsifier contributing to the formation of a stable water-in-oil emulsions. ZnO nanoparticles are monodispersed in water nuclear and methyl orthosilicate (TMOS) precursor is dissolved in cyclohexane. Moreover, NH3·H2O is selected as hydrolysis catalyst, and meanwhile as the etchant for template agent. TMOS in oil phase hydrolyzes at the oil-water interface under the catalytic effect of NH3·H2O in water and the hydrolysis products enter into water nuclear for heterogeneous nucleation on the surface of ZnO nanoparticles.22, 23 The amount of water in microemulsion is small, resulting in the achievement of the uniform coating of SiO2 on ZnO by controlling the hydrolysis reaction rate of TMOS. At the same time, ZnO nanoparticles are corroded slowly and gradually by NH3·H2O in water. After 2 h reaction, modifier such as polyethylene pyrro-