Ordered Arrays of Mesoporous Microrods from Recyclable Macroporous Silicon Templates

Ordered mesoporous materials fabricated by exploiting self-assembled surfactants as molecular templates have been intensively investigated.[1–5] However, their integration into device architectures still remains a challenge. Recent efforts have focused on the synthesis of MCM (Mobil Crystalline Material)[6,7] and SBA (Santa Barbara amorphous) type[6,8–13] materials inside porous supports characterized by straight, aligned pores oriented normal to the surface. For this purpose, porous alumina matrices, either ordered[14] or disordered,[15] 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 [16] templates is reported here. The microrods are removed from the template by a simple mechanical liftoff process or by the shrinkage of a macroscopic sol layer on the template surface upon calcination. Since no wetchemical-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 lowmolar-mass species, for size-exclusion chromatography, as a sensor component, and as an array of artificial chaperones guiding the folding of proteins.[17] Macroporous Si is prepared by photoelectrochemical etching of lithographically prepatterned n-type silicon wafers.[16] 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,[18] by adapting a strategy initially introduced by Martin.[19] 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,[20] as well as to implement self-cleaning behavior.[21] 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,2Hperfluorodecyltrichlorosilane is grafted onto the pore walls