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.
Mono-dispersed mesoporous silica spheres with ordered hexagonal regularity were synthesized from tetramethoxysilane and dodecyltrimethylammonium bromide as a surfactant under very specific conditions. By adjusting the optimum concentrations of the reactants, silica particles with diverse morphology were obtained at higher initial concentrations. Dilution of the reactants led to the formation of silica spheres with various sizes and relatively less mesoporous regularity. Mono-dispersed silica spheres could not be obtained when hexadecyltrimethylammonium bromide and tetradecyltrimethylammonium bromide were used as surfactants.
Particle size development of monodispersed mesoporous silica spheres (MMSS) was investigated by laser scattering measurement, TEM and XRD in situ. Smaller particles suddenly appeared after the commencement of the experiment, then growing homogeneously to larger particles. It is assumed that residual silica precursors in solution preferentially reacted with existing particle surface silanol, preventing generation of new particles. This leads to the formation of monodispersed particles. Based on the mechanism, monodispersed mesoporous silica spheres with core/shell structure have been newly synthesized.
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