A 2D‐Rectangular p2gg Silica Mesoporous Crystal with Elliptical Mesopores: An Intermediate Phase of Chiral and Lamellar Mesostructures

Abstract

Since the pioneering work of Kresge and co-workers, [1,2] much effort has been devoted to surfactant-templated mesoporous materials that display topologies closely related to those observed for lyotropic liquid crystals. Enhanced longrange order introduced by using both charged surfactants and controlling polymerization of the inorganic precursors benefits the further identification and characterization of their periodic mesostructures. So far, various mesophases with three-dimensional (3D) symmetry, such as orthorhombic (Pmmm [3] etc.), tetragonal (P4 2 /mnm, [4] P4/mmm [3] ), hexagonal (P6 3 /mmc [5] ), micellar cubic (Pm 3n, [6,7] Fd 3m, [8,9] Im 3m, [7,10] Fm 3m, [11] Pm 3m, [10] etc.), bicontinuous cubic (Ia 3d, [1,2] Pn 3m [12] ), two-dimensional (2D) symmetry, such as rectangular (c2mm [13] ) and hexagonal (p6mm [1,2,14] ), and onedimensional (1D) symmetry, such as lamellar, [1,2] have been discovered as mesoporous crystals in different synthesis systems. Very recently, chiral mesoporous silica (CMS), with a uniform twisted rodlike morphology and hexagonally ordered chiral channels, was also obtained by using chiral, [15] achiral anionic, [16] and achiral cationic [17][18][19] surfactants as the structure-directing agent. However, the formation mechanism, especially the structural relationship between the chiral and other mesostructures, including 2D-hexagonal, 2D-rectangular and lamellar structures, is less well understood because of the complication of the organic-inorganic co-assembling process and lack of information, such as the "synthesis-space diagram". Additionally, the synthesis of mesoporous crystals with controlled mesostructures is of great importance for fundamental research and development of new catalysts, adsorbents, drug delivery hosts, and nanoreactors. Within the range of liquid-crystal phases in surfactantwater systems, an "intermediate phase" has been used to represent those noncubic phases occurring at compositions between hexagonal and lamellar phases. [20][21][22] Ribbon phases are the intermediate phases that exhibit rodlike surfactant aggregates (micelles) with noncircular cross sections, arranged on a 2D-rectangular lattice. [20][21][22] There have been several reports on the structure of the ribbon phases, mainly suggesting they have the primitive-rectangular (plane group p2gg) and centered-rectangular (plane group c2mm) symmetry. [21,23] However, it is hard to make their small structural difference clear: X-ray scattering techniques give us very limited information about the structures. Using direct observation techniques, such as high-resolution transmission electron microscopy (HRTEM), we have found that a systematic structural change from 2D-hexagonal chiral to 2D-rectangular p2gg to lamellar mesostructure can be controlled by precisely adjusting the basicity of the reaction mixture. HRTEM gave direct evidence for the new particular 2D-rectangular p2gg mesostructural constructions with peculiar packing of elliptical mesopores arranged on a 2D-hexagonal lattice. These have ...

Preparation of rectangular and 2D-hexagonal mesostructured silica at neutral conditions using poly(oxyethylene) cholesteryl ethers and a water-soluble silica precursor

“…The structural features change with the EO chain length of the surfactant. In the ChEO 10 and ChEO 15 systems rectangular and hexagonal structures are respectively obtained (mimicking the surfactant aqueous liquid crystals) whereas disordered structures are present for ChEO 20 , ChEO 24 and ChEO 30 systems. Similar structures for the system with ChEO 10 and ChEO 15 are obtained in the corresponding binary surfactant aqueous systems, which gives some hints about the possible structural analogies.…”

“…In fact, ChEO 10 forms a liquid crystal with such a structure in the binary surfactant-water system [35], indicating that there is a templating effect. As the EO chain is increased to 15, there is a change in structure indicated by a different spectrum with reflections at [10], [11] and [20], corresponding to a 2D-hexagonal structure, which is also present in the binary ChEO 15 -water system [35], namely, the surfactant acts as a structure directing agent in this case. The structural change is attributed to an increase in the curvature of aggregates with longer EO chains, due to an increased repulsion between surfactant heads, and consequently larger specific surface areas.…”

“…For practical applications, the mesostructure of the materials is one of the most important parameters since it determines many properties such as channel connectivity and nanodomain distribution. Several mesostructures have been reported, such as 2D-hexagonal [1][2][3][4][5][6][7][8], 3D-hexagonal P63/mmc [9], 2D-rectangular p2gg [10], cubic Ia3d [1,11], cubic Pm3n [2,3], cubic Im3m [4,5,12], cubic Fm3m [13], cubic Pn3m [14] and disordered, so called ''worm-hole" structure [15]. In addition there is a report of a rectangular structure composed of cylindrical holes with non-spherical cross section obtained from a bolaform surfactant [16].…”

“…where d 10 is the interlayer spacing corresponding to the reflection by the [10] planes (see Scheme 1). Interlayer spacings of the rectangular and the hexagonal mesostructures as well as parameters a and b are listed in Table 1.…”

A Mesoporous Superlattice Consisting of Alternately Stacking Interstitial Nanospace within Binary Silica Colloidal Crystals

“…Mesoporous silica synthesized by the replication of surfactant micelles has periodically arranged uniform nanospace [3] and various ordered structures,s uch as lamellar,2 D hexagonal, gyroid, and 3D cage-type structures,h ave been reported. [3a] Recently,nonclassical nanospace structures,such as chiral, [4] intergrowth, [5] and quasicrystalline [6] structures, have also been generated, whereas an ordered superlattice of nanospace (namely mesoporous superlattice) has not been reported.…”