Insight Into the Defects of Cage-Type Silica Mesoporous Crystals with Fd3m Symmetry: TEM Observations and a New Proposal of "Polyhedron Packing" for the Crystals

publication · 7 years ago
by Lu Han, Yasuhiro Sakamoto, Shunai Che, Osamu Terasaki (Shanghai Jiao Tong University, Stockholm University)
The many faces of crystals: Silica mesoporous crystals were synthesized by using a co-structure directing method. Structural defects in the crystals with Fdequation imagem symmetry—described as tetrahedrally close-packed and consisting of 512 and 51264 polyhedra—were found (see figure). These defects were explained through the use of simulated TEM images. The defect formation is also discussed. 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 micelles—face-centered cubic (Fmequation imagem) and hexagonal close-packed (P63/mmc)—to Fdequation imagem structures with an increase of HCl and 2) a few structural defects in the crystals with Fdequation imagem symmetry. The structure of a crystal with Fdequation imagem 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 13–15 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 minimum-area packing.
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