Versatile Surfactant/Swelling-Agent Template for Synthesis of LargePore Ordered Mesoporous Silicas and Related Hollow Nanoparticles

A surfactant/swelling-agent pair suitable for templating a variety of well-defined large-pore nanoporous silicas was identified. The pair includes a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO, block copolymer surfactant (Pluronic F127, EO106PO70EO106) with a large fraction of long hydrophilic PEO blocks and a swelling agent (toluene) that strongly solubilizes in micelles of the PEO-PPO-PEO surfactant family. Such a combination affords micellar templates for both spherical and cylindrical mesopores with potential to hinder cross-linking of micelle-templated nanostructures due to stabilization of nanoparticles by long PEO chains. Under low-temperature conditions (11-12 degrees C), the Pluronic F127/toluene pair affords ultralarge-pore FDU-12 (ULP-FDU-12) silica with face-centered cubic structure of spherical mesopores and related hollow nanospheres, as well as large-pore SBA-15 (LP-SBA-15) with two-dimensional hexagonal structure of cylindrical mesopores and related silica nanotubes. ULP-FDU-12 reaches the unit-cell parameter of 69 nm, which is very large. LP-SBA-15 has a unit-cell parameter up to 26 nm and pore diameter up to similar to 20 nm and is exceptionally well ordered. The hollow nanospheres and nanotubes are attainable through lowering of the silica-precursor/surfactant ratio. The materials templated by spherical micelles form when the surfactant/swelling-agent solution is kept under stirring for extended periods of time before the addition of the silica precursor. The sizes of entrances to the hollow nanospheres can be continuously tuned by adjusting the hydrothermal treatment temperature. The ordered mesoporous silicas can be converted from open-pore to closed-pore materials through the thermally induced pore closing. The diversity in morphology, pore size, and pore connectivity makes the proposed surfactant/swelling-agent templating system unprecedented in the large mesopore domain.