Magnetic TiO2–SiO2 hybrid hollow spheres with TiO2 nanofibers on the surface and their formation mechanism

Abstract

TiO₂–SiO₂ hollow spheres with new hierarchical structures and magnetic response were prepared by etching mesoporous SiO₂/TiO₂/Fe₂O₃ hybrid particles in alkaline solution. Different from the conventional synthetic approaches to hollow particles which were always based on template methods, the shell layer of the SiO₂/TiO₂/Fe₂O₃ hybrid particles were solidified firstly by controlling the calcination temperature, which resulted in the removal of particle core and reservation of the shell layer during the etching process. The mechanism proposed was the dual porous structures in SiO₂/TiO₂/Fe₂O₃ spheres facilitated the diffusion of the alkaline solution into the particle interior and led to the destruction and removal of Si–O–Si bonds, and the residual Ti–O–Ti structures remained as TiO₂ nanofibers on the Ti–O–Si shell of the hollow spheres. Furthermore, encapsulation of Fe₂O₃ in the fibers induced the magnetic response of the hollow sphere. The hollow capsule with robust Ti–O–Si shell in the extreme pH solution made it applicable in many fields such as water treatment, bioseparation, microreactor and drug delivery systems. This work provided a new concept for the preparation of functional hollow spheres from its porous precursor. We believed that the present facile route was a significant breakthrough in the synthesis of hollow spheres and could be extended to the preparation of other hollow particles.