Formation mechanism for monodispersed mesoporous silica spheres and its application to the synthesis of core/shell particles

A formation mechanism for monodispersed mesoporous silica spheres was investigated from the viewpoint of both particle growth and the progress of the condensation of the silica precursor. The particle growth of monodispersed mesoporous silica spheres was studied by TEM observation. The development of the mesopores was examined by in situ X-ray diffraction measurements. The condensation reaction of the silica precursor was analyzed by silicic acid titration and pH-conductivity measurements during the synthesis. It was found that small particles emerged suddenly after the commencement of the synthesis, and then the residual silica precursors reacted preferentially with the surface silanols on these existing particles. This led to the formation of monodispersed mesoporous silica spheres. Conversely, new small particles emerge throughout the synthesis in the case of a heterogeneous system. To confirm the preferential reaction of the silica precursors with the surface silanols, expansion of the existing particles was carried out by adding more silica precursor after the completion of the synthesis. It was found that the sizes of the particles were enlarged by the successive addition of more of the silica precursor. Surprisingly, the radial alignment of the hexagonal mesopores was still retained in the expanded particles. In addition, by the addition of a different silica precursor to the initial one, highly monodispersed core/shell mesoporous silica spheres possessing a hydrophilic core and a hydrophobic shell were successfully obtained for the first time.