Insight Into the Defects of Cage-Type Silica Mesoporous Crystals with Fd3 m Symmetry: TEM Observations and a New Proposal of Polyhedron Packingfor the Crystals

摘要

Silica mesoporous crystals were synthesized by using a gemini cationic surfactant (C18-3-1) as the directing agent, carboxyethylsilanetriol sodium salt as the co-structure directing agent (CSDA), and varying amounts of HCl.By using transmission electron microscopy (TEM) we observed 1) a structural change from the close-packed structures of spherical micellesfacecentered cubic (Fm3 m) and hexagonal close-packed (P63/mmc)to Fd3m structures with an increase of HCl and 2) a few structural defects in the crystals with Fd3 m symmetry. The structure of a crystal with Fd3 m symmetry is described as one of the tetrahedrally close-packed (tcp) structures consisting of 512 and 51264 polyhedra. The observed TEM images of the structural defects were explained well through use of simulated TEM images by introducing new 13C15 polyhedra comprising 51262, 51263, 4151062, 425865, and 4151064, which have been observed in bubbles by Matzke. The mesostructural changes and defect formation are discussed in terms of the hardness of micelles composed of surfactant/CSDA/ silica species that have formed through a change of the interaction between the surfactant and CSDA, which causes the micelles to change from a regime of close-packing to one of minimumarea packing.