Investigation of selective etching mechanism and its dependency on the particle size in preparation of hollow silica spheres

In this paper, the core–shell SiO₂@mSiO₂/CTAB spheres were fabricated by choosing the solid silica spheres of different sizes as the internal cores, then a series of etching experiments were conducted. For different etching time and particle size, different rattle-type or hollow silica spheres were obtained. Investigation showed that CTAB in the mesoporous shell played a very important role to the selectivity of the etching. Both the rattle-type and hollow structures were successfully fabricated by adjusting the etching time in the presence of CTAB but without obvious etching of mesoporous silica shell. More importantly, the morphology transformation depended on the particle size of the solid silica cores. Generally, when the particle size of the internal cores was very small (d < 170 nm), the hollow structure was first obtained even after being etched for a short time. When the particle size of the internal cores was slightly larger (200 nm < d < 500 nm), the rattle-type microspheres with double-shell structure were first obtained after being etched for a short time, then the hollow structure was obtained with prolonging etching time. But when the particle size of the internal cores was far larger (d > 1 μm), the rattle-type microspheres with at least double-shell structure were first obtained after a certain period of etching time, then the hollow spheres with multi-shell structure were obtained with prolonging etching time. It is further revealed that both the rattle-type and hollow silica spheres with multi-shell structure could be prepared successfully if the particle size of the solid silica core was large enough.