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

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.