Unique double-shelled hollow silica microspheres: template-guided self-assembly, tunable pore size, high thermal stability, and their application in removal of neutral red

Abstract

A novel type of monodisperse and double-shelled hollow silica microspheres was reported. This unique double-shelled structure was fabricated using a consecutive template-guided self-assembly. Cationic poly(styrene) (CPS) particles prepared by emulsifier-free polymerization were first used as the template to coat with tetraethylorthosilicate (TEOS) in the presence of coupling agent methacryloxypropyltrimethoxysilane (MPS), forming the core/shell type of CPS/SiO₂ particles. The resulting CPS/SiO₂ particles were further used as the template and in situ polymerized with styrene, a cationic co-monomer 2-(methacryloyl)ethyltrimethyl ammonium chloride (DMC), followed by electro-statically guided self-assembly and polymerization of TEOS on the surface, generating the sandwich-like CPS/SiO₂/CPS/SiO₂ particles. Finally, the unique double-shelled hollow silica spheres were produced by one-step removal of the CPS core and CPS layer from the sandwich-like particles. The structure of the monodisperse and intact double-shelled hollow silica microspheres was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET), Elemental Analysis (EA), and ²⁹Si Nuclear Magnetic Resonance (²⁹Si-NMR). Unlike previously reported hollow silica microspheres which have one single shell, the novel silica microspheres have two shells, enabling improved thermal stability and larger BET surface area. Notably, the interior cavity and shell-to-shell distance can be effectively tuned simply by controlling the interior CPS core diameter and the outer CPS template layer. The absorption and separation of the neutral red (NR) in synthesized hollow silica spheres showed favorable adsorption behavior, such as higher absorbent amount of dye and lower rate of dye desorbed for NR in comparison to single-shelled hollow silica, which thereby makes it potentially more applicable in environmental separation and adsorption.