Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular templates have been intensively investigated. However, their integration into device architectures still remains a challenge. Recent efforts have focused on the synthesis of MCM (Mobil Crystalline Material) and SBA (Santa Barbara amorphous) type materials inside porous supports characterized by straight, aligned pores oriented normal to the surface. For this purpose, porous alumina matrices, either ordered or disordered, have predominantly been employed. The hybrid systems thus obtained are promising components for device architectures in the fields of catalysis and separation, where macroscopic membranes that consist of aligned pores with diameters of a few nanometers and high aspect ratios are required. The mesoporous rods can be released by a wet-chemical-etching step that destroys the alumina matrix. When the solvent of the suspension thus obtained is evaporated, capillary forces that act between the mesoporous rods result in the occurrence of large aggregates in otherwise disordered powders. The preparation of ordered arrays of freestanding mesoporous microrods by means of recyclable macroporous silicon templates is reported here. The microrods are removed from the template by a simple mechanical lift-off process or by the shrinkage of a macroscopic sol layer on the template surface upon calcination. Since no wet-chemical-etching step is involved, condensation of the rods is avoided. It is assumed that such hierarchical systems, which combine features at the micrometer and nanometer scales, can be used in flow reactors, for the storage of low-molar-mass species, for size-exclusion chromatography, as a sensor component, and as an array of artificial chaperones guiding the folding of proteins. Macroporous Si is prepared by photoelectrochemical etching of lithographically prepatterned n-type silicon wafers. The pores form either a hexagonal or quadratic monodomain that may extend several square centimeters and exhibits a sharp pore diameter distribution. The pore diameters can be adjusted to any value between 370 nm and several micrometers, and the pore depth is only limited by the thickness of the wafer used. 1D nanoand microstructures that consist of various materials have been fabricated using macroporous Si as a template, by adapting a strategy initially introduced by Martin. The lateral arrangement of the mesoporous microrods that form inside the macroporous Si templates is determined by the lithographic prepatterning of the Si wafers into which the macropores are etched. Therefore, it should be possible to adjust the wetting properties of the microrod arrays by varying the size, shape, and lattice constants, as well as to implement self-cleaning behavior. An important aspect of the procedure reported here is that the mesoporous microrods are pulled out of the pores. The silicon template is therefore conserved and can be recycled. This is a prerequisite for upscaling this process and potentially enables the economic integration of mesoporous materials into device structures. The macroporous Si is modified by a two-step procedure to minimize adhesion between the microrods and the pore walls. At first, a well-defined, smooth silica layer with a thickness of about 5 nm and a high density of hydroxyl groups is grown by treatment with a boiling H2SO4/H2O2 mixture. Subsequently, 1H,1H,2H,2H-perfluorodecyltrichlorosilane is grafted onto the pore walls by adapting protocols reported elsewhere to render them into low-energy surfaces. Mesoporous SBA-15 precursors are prepared following the procedures described in the literature, using a mixture that contains poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer (EO20PO70EO20) as a structure-directing agent, and tetraethyl orthosilicate (TEOS) as a silica source under acidic conditions (1.0 M aqueous HCl). The modified macroporous Si templates are covered with the SBA-15 precursor solutions thus prepared, and the SBA-15 sols are gelated at room temperature for two days and then at 60 °C for one day. After removal of the EO20PO70EO20 by calcination at 550 °C for 6 h, mesoporous silica microrods connected to a silica film on the surface of the template are obtained inside the macroporous Si. A simple and crude lift-off procedure is applied to pull the mesoporous rods out of the pores. The macroporous Si templates are turned upside down, and the silica film that covers the surface of the macroporous Si is glued onto a scanning electron microscopy (SEM) sample holder. The macroporous Si is then torn away with the aid of an adhesive tape. To reC O M M U N IC A IO N S
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Mar 20, 2023
Narasimharao Kitchamsetti + 8
Open Access
