Structural Study of Mesoporous MCM-48 and Carbon Networks Synthesized in the Spaces of MCM-48 by Electron Crystallography

Abstract

Recently, we have developed a new electron crystallography (EC) method for study of three dimensional (3D) structures of silica-mesoporous materials, and the 3D-structural solutions of MCM-48 and SBA-1, -6, and -16 were briefly reported. The method gives a unique structure solution through the Fourier sum of the 3D-structure factors, both amplitudes and phases, which are obtained from Fourier analyses of a set of high-resolution electron microscope (HREM) images. The method was fully described in an application for structure analyses of two MCM-48 crystals with different crystal morphologies. Little structural difference was observed between the two crystals, although small differences in the structure factors were observed. The space group of MCM-48 was determined to be Ia 3̄ d, and the wall surface of the two crystals followed exactly the periodic minimal surface of gyroid (G). The wall separated two interpenetrating and noninterconnecting channel systems with different chiralities. After structural analysis of MCM-48, the structures of two different carbon networks, CMK-1 and CMK-4, which were synthesized within the channels of MCM-48 from different carbon sources, were studied by electron microscopy (EM). It was observed that in both cases carbon networks were equally formed in the two channels of MCM-48 without changing the space-group symmetry and that the symmetry of Ia 3̄ d was retained after the dissolution of silica mesoporous MCM-48 for CMK-4 but changed to I 4 1 / a for CMK-1. The simplest model for structure change in CMK-1 was proposed on the basis of the observations of extra reflections in ED patterns and domain structures in HREM images as that the carbon networks equally formed in two noninterconnecting channels of MCM-48 were displaced during the dissolution relative to each other without rotation along the [001] axis by keeping each network rigidly. It is stressed that the method must be extended further for structural study of new materials with orders in two different lengths scales, atomic and mesoscopic scales.