Budded, Mesoporous Silica Hollow Spheres: Hierarchical Structure Controlled by Kinetic Self‐Assembly

Abstract

Recently, mesoporous materials with hierarchical structures and well-defined morphologies have attracted much attention. Besides possible practical applications, studies of mesoporous materials may also shed light on fundamental mechanisms of biomineralization. Among the many types of mesoporous materials, hollow spheres are of significance because of their potential uses in encapsulation, drug-delivery, and controlled-release applications. Although many articles have been published describing the interior templating of mesoporous hollow particles using hard templates, vesicles, or emulsions, the fabrication of hierarchical structures on the shell has rarely been reported. In this Communication, we report a novel, single-step, emulsion-templating method to fabricate a unique, hierarchical morphology. Our products consist of budded, mesoporous silica hollow spheres. What distinguishes these structures from the traditional ones is that they combine two separate, distinct mesophases with different-sized mesovoids in a single hollow sphere. Specifically, the shell of the sphere is based on wormholelike mesoporous structures, while the protruding buds are formed from lamellar (vesicular) mesostructures. Our process utilizes the sodium salt of the anionic surfactant N-lauroylsarcosine (Sar-Na), which is commonly used in cosmetic and pharmaceutical industries due to its biodegradability and low toxicity. Instead of employing the usual emulsion-templating methods with nonpolar fluids such as trimethylbenzene, we employ a novel process in which the emulsion is generated by the acidification of a Sar-Na solution. As shown in Scheme 1, the addition of acid converts a portion of the Sar anions to Sar-H, which is an amphiphilic polar oil that behaves as droplets. The condensation of the added silica precursors (3aminopropyltrimethoxysilane (APMS) and tetraethylorthosilC O M M U N IC A TI O N