Hematite template route to hollow-type silica spheres

Hollow-type silica spheres with controlled cavity size were prepared from Fe2O3–SiO2 core–shell composite particles by selective leaching of the iron oxide core materials using acidic solution. The spherical Fe2O3 core particles with a diameter range of 20–400 nm were first prepared by the hydrolysis reaction of iron salts. Next, the Fe2O3–SiO2 core–shell particles were prepared by the deposition of a SiO2 layer onto the surface of Fe2O3 particles using a two-step coating process, consisting of a primary coating with sodium silicate solution and a subsequent coating by controlled hydrolysis of tetraethoxysilicate (TEOS). The Fe2O3 core was then removed by dissolving with acidic solution, giving rise to hollow-type silica particles. Scanning electron microscopy clearly revealed that the cavity size was closely related to the initial size of the core Fe2O3 particle. According to the cross-sectional view obtained by transmission electron microscopy, the silica shell thickness was about 10 nm. The porous texture of the hollow-type silica particles was further characterized by nitrogen adsorption–desorption isotherm measurements.