In this work, we investigated the kinetic balance between ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) and subsequent condensation over the growth of silica particles in the Stöber method. Our results reveal that, at the initial stage, the reaction is dictated by TEOS hydrolysis to form silanol monomers, which is denoted as pathway I and is responsible for nucleation and growth of small silica particles via condensation of neighboring silanol monomers and siloxane network clusters derived thereafter. Afterward, the reaction is dictated by condensation of newly formed silanol monomers onto the earlier formed silica particles, which is denoted as pathway II and is responsible for the enlargement in size of silica particles. When TEOS hydrolysis is significantly promoted, either at high ammonia concentration (≥0.95 M) or at low ammonia concentration in the presence of LiOH as secondary catalyst, temporal separation of pathways I and II makes the Stöber method reminiscent of in situ seeded growth. This knowledge advance enables us not only to reconcile the most prevailing aggregation-only and monomer-addition models in literature into one consistent framework to interpret the Stöber process but also to grow monodisperse silica particles with sizes in the range 15-230 nm simply but precisely regulated by the ammonia concentration with the aid of LiOH.

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This work demonstrates a simple approach to rational fabrication of multishelled hollow silica spheres via periodic injections of a given amount of tetramethylammonium hydroxide (TMAH) to catalyze the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol/ water mixtures. Upon TMAH injection each time, the silica networks formed at the early stage of the sol−gel reaction of TEOS was found to possess noticeably lower condensation degree than these formed at the late reaction stage. The former could be readily and completely etched away in boiling water whereas the latter remained intact. The use of TMAH as basic catalyst led to temporally clear-cut separation between the early and late stages during the sol−gel reaction of TEOS and made the volume ratio of the loosely condensed silica gel networks, formed at the early reaction stage, to the densely condensed ones, formed at the late reaction stage well-defined around 3:2, enabling the formation of multishelled hollow spheres with precisely defined shell thickness and spacing between neighboring shells. The present template-free approach will open up new prospects in rational design and fabrication of structure-tailored multishelled silica spheres to accommodate to the demand of improved applications.

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