Tuning the Structure and Orientation of Hexagonally Ordered Mesoporous Channels in Anodic Alumina Membrane Hosts: A 2D Small‐Angle X‐ray Scattering Study

Abstract

Periodic mesoporous materials have attracted considerable attention during the last decade because of their promising applications as catalyst supports and nanoreactors, or as hosts for nanostructured materials with appealing optoelectronic properties. [1,2] Many of these applications will benefit from arrangements of preferentially aligned, ordered arrays of certain mesostructures. The evaporation-induced self-assembly (EISA) method has been established as an efficient process for the preparation of thin films with mono-oriented mesostructured domains. [3,4] However, the most frequently obtained films display hexagonally ordered channels that are aligned parallel to the surface of the substrate. [5] Recently, the synthesis of mesoporous materials within the regular, larger channels of anodic alumina membranes (AAMs) has been explored, with the aim of attaining greater control over the morphology of the mesoporous system.[6] A first approach, through a sol-gel synthesis route using the triblock copolymer poly(ethylene oxide) 100 -b-poly(propylene oxide) 65 -b-poly(ethylene oxide) 100 (PEO 100 PPO 65 PEO 100 or Pluronic F-127) as a structure-directing agent, resulted in 2D hexagonal mesostructures with two different orientations that were found to coexist at different ratios depending on the concentration of the surfactant. [7] In one case, the long axes of the mesopores were aligned with the long axes of the AAM channels (columnar orientation). In another case, a circular orientation of the mesostructure was observed. Similar (freestanding) unusual mesophase structures are known to exist in cetyltrimethylammonium bromide (CTAB)-templated materials prepared by solvothermal methods and have been named "circulites" or circular crystals. [8,9] The efficient EISA method can also be used to prepare AAM mesoporous composite materials by applying coating solutions that are typically used for the deposition of mesoporous silica films. When using cationic CTAB as a template, partially ordered mesoporous materials with aligned, columnar mesopores only in the vicinity of the alumina walls were obtained that showed promising behavior as molecular separators.[10] Use of the triblock copolymer PEO 20 PPO 70 PEO 20 (Pluronic123 or P123) as a template resulted in striking mesostructures with concentric or helical mesopores and single chains of spherical mesopores, depending on the confinement conditions imposed by alumina nanochannels with diameters of less than 100 nm. [11] In contrast, columnar mesopores were reported when the same template (P123) was used in the sol-gel approach in larger Anopore channels.[12] However, when a slightly different protocol at the same surfactant/silica ratio was used in the solgel synthesis route, hexagonal mesophases with mixed orientations resulted.[13] The presence of water vapor in the ageing process was investigated in a related study using the P123 template. [14,15] In this case, the circular orientation was favored over the columnar one at higher water pressure; this selectivity was attrib...

Hierarchically structured biphenylene-bridged periodic mesoporous organosilica

“…[36][37][38][39] Moreover, these types of hierarchical nanostructures can offer significant advantages over self-supporting thin films, such as higher mechanical stability and high aspect ratios of the mesophase system. While inorganic mesoporous silica has been studied extensively in the confinement of AAM pores, [36][37][38][39][40][41][42] there are only a few reports on the synthesis of mesoporous organosilicas confined in AAM channels. Some of us have recently reported the synthesis of ethane-bridged PMO in anodic alumina membranes.…”

Fabrication and Analytical Applications of Hybrid Mesoporous Membranes

“…2(b)). [25][26][27][28][29] On the other hand, Yamaguchi et al 18,30 have proposed another method whereby an alumina membrane is set in an ordinary membrane filtration apparatus and a precursor solution is introduced into the columnar alumina pores under moderate aspiration (Fig. 2(c)).…”

“…However, the most critical disadvantage of these methods is the formation of mesoporous silica not only inside the columnar alumina pore but also at the membrane surface. 20,28 The use of the hybrid mesoporous membrane for nanofluidic systems is based on mass transport inside the silicananochannels formed inside the columnar alumina pores. The mesoporous silica formed at the membrane surface would prevent mass transport through the silica-nanochannels inside the columnar alumina pores.…”

“…Until now, the surfactants used for the fabrication of hybrid mesoporous membranes include cetyltrimethylammonium bromide (CTAB), 18,26,28 decaethylene glycol hexadecyl ether (Brij 56), 27,28 and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers such as PEO20PPO70PEO20 (Pluronic P123) 20,21,[24][25][26][28][29][30] and PEO106PPO70PEO106 (Pluronic F127). 19,22,23,30 Among these surfactants, CTAB has been well known to form mesoporous silica composed of 1D silicananochannels, 2 and the first achievement of hybrid mesoporous membrane with 1D silica-nanochannels was reported by using CTAB as the structural directing reagent.…”

Review. Fluorescence Microscopy Studies of Porous Silica Materials

“…A further approach to control the mesoporous structure is the confined synthesis of mesoporous silica inside porous anodic alumina membranes [48,62,63] via the EISA process. Feil et al [48] synthesized template-free, chemically functionalized mesoporous silica filaments with 4 nm pore diameter and used SMM to investigate the translational and orientational dynamics of TDI dye molecules (see chemical structure in Fig.…”