Morphology Development of Mesoporous Materials: a Colloidal Phase Separation Mechanism

Abstract

Synthetic conditions such as temperature, stirring rate, ionic strength, acidity, and reactant ratios that affect the mesostructures and macrostructures of mesoporous materials have been extensively studied in the nonionic block copolymer templating system. Highly ordered SBA-15 materials with ∼100% rodlike morphologies (∼1−2 μm long) have been obtained in the presence of inorganic salts. Synthesis conditions such as low temperature, low acidity, and low ionic strength that increase the induction time give rise to the morphologies of mesoporous silica with increased curvatures. The particle growth process of rodlike SBA-15 materials from solutions has been examined by directly observing the morphologies of particles as a function of time. A colloidal phase separation mechanism (CPSM) for the formation of mesoporous materials is proposed. It is suggested that the formation process of mesoporous materials involves three stages: (1) cooperative self-assembly of inorganic/organic composites; (2) formation of a new crystal-like phase rich in aggregates of block copolymer/silica species; and (3) phase separation of this liquid crystal-like phase from the solution and further growth of solid mesostructures driven by further condensation of silica species. The morphologies of mesoporous materials are developed after the phase separation stage and influenced by the competition mainly between the free energy of mesostructure self-assembly (Δ G ) and the colloidal surface free energy ( F ), as well as other interactions such as the shearing force. When the phase separation stage occurs early, Δ G is dominant and the macrostructure of mesoporous materials is developed together with the formation of mesostructure; therefore, mesoporous materials with crystal-like morphologies can be generated. The role of inorganic salts is well explained based on the CPSM by taking into account the colloidal interaction enhanced by inorganic salts. Cubic ( Im 3̄ m ) mesoporous single crystals have been obtained with exclusively rhombdodecahedron shapes (1−2 μm in size). Ultrathin microtoming TEM measurements confirm that such polyhedral particles are perfect single crystals. The different influences of hydrothermal treatments upon the mesostructures and macrostructures have been discussed in the hexagonal and cubic synthesis systems. By employing the CPSM, hexagonal mesoporous single crystals have been obtained in the absence of inorganic salts; the size limitation (∼1−2 μm) of mesoporous single crystals has also been discussed. Such understandings may be useful in the fabrication of mesoporous materials with well-defined, crystal-like morphologies.