Monodisperse polystyrene-silica core-shell particles and silica hollow spheres prepared by the Stöber method

Monodisperse polystyrene (PS) bead particles (diameter approximately 750 nm) have been synthesized with an initiator, 2,2'-azobis(2-methylpropionamidine) dihydrochloride (AIBA), using a surfactant-free emulsion polymerization. The PS particles charged positively by the cationic initiator exhibit uniform size with a narrow size distribution. The Stöber method was adopted to coat silica on the surface of the PS particles. The silica coatings were performed on different conditions; pH (10.0-12.5), tetraethylorthosilicate (TEOS) concentration (10-40 mM), and reaction time (1-5 h). The reaction rates are too low at low pH (10.0) so that no or little silica is coated on the PS bead particles. At high pH condition (approximately 12.5) relatively rough silica shell and heterogeneous nucleation of silica colloids were observed due to high reaction rate of TEOS hydrolysis and condensation reaction. Smooth surface with uniform thickness of silica shells were obtained at a pH value around 11. Also, the thickness of silica coated on the PS spheres increases with increase of TEOS concentration and reaction time. Silica hollow spheres were also obtained by calcination of the PS-silica core-shell particles.