Stability of highly ordered nanostructures with uniformly cylindrical mesochannels

Highly ordered, optically transparent mesoporous silica monoliths (HOM) were fabricated by using a simple, reproducible strategy in which lyotropic and microemulsion phases of P123 (EO20-PO70-EO20) copolymer were used as templates in an instantly direct templating method. Our results show that hexagonal p6mm architectures with uniformly shaped cylindrical mesopore channels (HOM-2) were successfully fabricated by addition of the hydrocarbons to the lyotropic hexagonal mesophase structures formed at tetrannethylorthosilicate/P123 composition domains of 50 wt%. However, the addition of alkanes of long alkyl chain length and a large amount of trimethylbenzene to the HOM-2 mesophase led to the formation of cubically spherical Im3m mesopores (HOM-1). These designs of hexagonally (HOM-2) and cubically (HOM-1) cylindrical pore architectures allowed control of the internal pore geometries using this synthetic manipulation. At the hexagonal silica/P123 mesophase domains, the degree of solubility of the hydrocarbons crucially affected the formation of true microemulsion liquid crystal phases, the expansion of cylindrical pore sizes with uniform constrictions, and the changes in the geometrical shape of the phase (hexagonal P6mm-cubic Im3m structures). These cylindrical HOM structures have unprecedented hydrothermal stability in boiling water. There was significant retention of the hierarchical mesoporosity for a month or even longer. Our findings provide the first evidence that the instant direct templating strategy is a preferable method for achieving precise control of the mesophase geometry, of the developing constrictions in the cylindrical pore structure, and of the size and shape of the internal pore architecture. Such monoliths with high-quality textural properties, hydrothermal stability, and structural ordering might find widespread applications in mesoscopic fields. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.