Sol–gel processes at the droplet interface: hydrous zirconia and hafnia nanocapsules by interfacial inorganic polycondensation

Abstract

We report a simple method to prepare nanocapsules of hydrous zirconia and hafnia by interfacial precipitation in water-in-oil miniemulsions. We show that the sol–gel precipitation of transition metal hydroxides, exemplified for the cases of zirconium and hafnium, can be driven to the droplet interface by addition of an organic base. The prompt addition of triethylamine generates hydroxide ions at the interface, which initiate a polycondensation of the metal hydroxo species. Electron and X-ray diffractions indicate that the resulting materials are amorphous, but can be transformed to monoclinic ZrO₂ or HfO₂ upon thermolytic treatment. Mixed oxides of Hf and Zr with different compositions (Hf_xZr_1−xO₂) can also be formed by using the desired ratios of precursors. Furthermore, the ability of the hollow particles to encapsulate efficiently hydrophilic materials without leakage has been proven with the example of a water-soluble dye (fluorescein isothiocyanate).