Chiral Mesostructured Silica Nanofibers of MCM‐41

Abstract

Mesostructured silica MCM-41 has been one of the most extensively studied mesostructured materials since its first synthesis by Mobil scientists in 1992.[1] Many important applications [2] of mesostructured silica MCM-41 in catalysis, separation, and nanoengineering are closely correlated to its ordered two-dimensional (2D) hexagonal mesostructure/ mesopore. Besides the usual straight 2D hexagonal mesostructure, [3] various curved mesostructures of MCM-41 have also been reported by several research groups in the last decade, [4] which has aroused great academic interest in their enigmatic morphogenesis. Recently our research group has investigated the topological transformation of a series of vesicular MCM-41 compounds with different mesostructures in an alkaline synthesis system [5] that was initialy developed by Rathouský and co-workers.[6] The self-assembly of sodium silicate (SS) and cetyltrimethylammonium bromide (CTAB) into a hexagonal mesostructure in such a method is driven by the hydrolysis of ethyl acetate (EA). Herein we report that chiral mesostructured silica nanofibers of MCM-41 can be fabricated in this SS/CTAB/EA/H 2 O system by simply lowering the SS and CTAB concentrations below 0.5 mol per 1000 mol H 2 O. It is remarkable that two types of chiral mesostructures with different symmetries were synthesized from the usual achiral materials in this study. Moreover, a relationship between the chiral and ordinary achiral mesostructures of MCM-41 was revealed through a systematic investigation of the synthesis system.The first type of chiral nanofibers of MCM-41 (Figure 1 a, b) has a single twist axis. The XRD pattern of such a single-axis nanofiber (Figure 1 c) reveals a highly ordered 2D hexagonal mesostructure with a lattice constant of 4.5 nm. The N 2 sorption isotherms of the calcined product show a steep capillary condensation at a P/P 0 ratio of 0.2:1-0.3:1, which corresponds to a BJH pore size of 2.4 nm (Figure 1 d). The BET surface area and mesopore volume of the single-axis nanofiber are 960 m 2 g À1 and 0.63 cm 3 g À1 , respectively. Analysis of the chiral mesostructure of the single-axis nanofibers by electron microscopy showed: 1) The twisted crystal facets could be distinguished from their field-emission SEM images (see Supporting Information); 2) periodic fringes along the axis of the nanofiber in the TEM image (Figure 1 b); and 3) the observed fringes moved along the axis when the

Entropy‐Driven Helical Mesostructure Formation with Achiral Cationic Surfactant Templates

“…2b), indicating the presence of spiral channels within the rods. [7][8][9][10][11] The helical mesostructure was present in all CMS-25 particles, regardless of the morphologies (see Supporting Information Fig. S1).…”

“…Very recently, the successful use of achiral cationic surfactant cetyltrimethylammonium bromide for the synthesis of helical mesoporous silica was also reported. [11] The authors attributed the origin of the helical mesostructures to particle morphology transformation that would reduce the surface free energy. An entropically driven model has been proposed to understand the helical conformation of long molecular chains in the crowded environment of a cell.…”

“…This was because twisting a bundle of rod-like micelles into helical structures would lead to an increase in energy and the breaking of hexagonal symmetry, [11] which might have competed with the increase of entropy caused by the excluded volume overlap. The formation of a less compact helical mesostructure might be a compromise between these two types of change in energy.…”

Challenges and breakthroughs in recent research on self-assembly

“…TEM observation confirmed the presence of hexagonally aligned mesoscopic channels with diameters of 2.2 nm wound together in a particular direction. This finding attracted some researchers to this field who have further developed the synthesis of chiral mesoporous objects [301][302][303]. As one of the other significant breakthrough materials for this area, PMO materials were introduced by three groups working independently: Inagaki group [304], Ozin group [305], and Stein group [306]) all in 1999.…”

Synthesis of Mesoporous Silica Materials by Using Anionic Surfactants as Template

“…Achiral surfactants can also form helical mesoporous silicas by optimizing the synthesis conditions, but the left-/right-handed ratio of helical particles remained at 50/50 58), 59) . The synthesis of helical mesoporous materials with tunable pitch size is possible in the presence of achiral surfactants, and a simple and purely interfacial interaction meschanism was proposed to explain the spontaneous formation of helical mesostructures 60) .…”