Preparation of single-hole silica hollow microspheres by precipitation-phase separation method

Silica hollow microspheres with a single hole in the shell have been prepared by the precipitation-phase separation method. The synthesis was performed by mixing a tetrahydrofuran (THF)–ethanol solution of poly(styrene-co-methyl methacrylate) (P(St-co-MMA)) and tetraethyl orthosilicate (TEOS) with water, adding ammonia to catalyze the hydrolysis of TEOS and finally removing P(St-co-MMA) by calcination. The effects of the THF-to-ethanol volume ratio, the P(St-co-MMA)-to-TEOS mass ratio, the organic solvent-to-water volume ratio, and the amount of ammonia on the final morphology, size and hole size of the microspheres have been investigated. Generally, the morphology of the microspheres ranged from single-hole to bowl-like, depending on the P(St-co-MMA)-to-TEOS mass ratio; the size of the microspheres decreased with increasing THF-to-ethanol volume ratio; and the size of the holes increased with decreasing THF-to-ethanol volume ratio and decreasing organic solvent-to-water volume ratio. The formation process of single-hole silica/P(St-co-MMA) composite microspheres was monitored with transmission electron microscopy (TEM). The TEOS and P(St-co-MMA) microphases separated from each other to form a hemispherical structure, which transformed to a single-hole structure after ammonia was added. A plausible mechanism for the formation of the single-hole hollow microspheres is proposed.