Precise preparation of highly monodisperse ZrO2@SiO2 core–shell nanoparticles with adjustable refractive indices

Abstract

This paper describes a straightforward method to precisely prepare monodisperse ZrO₂@SiO₂ core–shell nanoparticles (CSNs) by sol–gel polymerization of TEOS in the presence of ZrO₂ cores without additional capping agents. We studied the morphology and composition of CSNs by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). It has been found that the citric acid groups adsorbed onto the ZrO₂ surface during the preparation of ZrO₂ as well as the slow hydrolysis and condensation of TEOS in isopropanol are crucial for the selective growth of silica on the surface of ZrO₂ cores. The thickness of the silica shell can be facilely controlled from about 4 to 30 nm by varying the concentration of TEOS, with the refractive indices of the prepared CSNs tuned accordingly from about 1.2 to 1.9. The coated silica shell, which can be well dispersed in solvents such as methanol and ethanol, can improve the dispersibility of pure ZrO₂ nanoparticles which can only be dispersed in water. Moreover, the formation of the silica shell improved the chemical reactivity of pure ZrO₂ nanoparticles. After modifying with methyltriethoxysilane (MTES), the CSNs can be well dispersed in various organic solvents, such as dimethylbenzene, chloroform and isoamyl alcohol, which may find applications in optically transparent resins with controllable refractive index or organic silicon packaging materials with high refractive index for LED and so on. Additionally, other silane coupling agents can also be applied to modify the surface chemical properties of CSNs, which may provide more potential applications.