Bradley F. Chmelka

Generalized syntheses of large-pore mesoporous metal oxides with semicrystalline frameworks

Surfactants have been shown to organize silica into a variety of mesoporous forms, through the mediation of electrostatic, hydrogen-bonding, covalent and van der Waals interactions. This approach to mesostructured materials has been extended, with sporadic success, to non-silica oxides, which might promise applications involving electron transfer or magnetic interactions. Here we report a simple and versatile procedure for the synthesis of thermally stable, ordered, large-pore (up to 140u2009Å) mesoporous metal oxides, including TiO2, ZrO2, Al2O3, Nb2O5, Ta2O5, WO3, HfO2, SnO2, and mixed oxides SiAlO3.5, SiTiO4, ZrTiO4, Al2TiO5 and ZrW2O8. We used amphiphilic poly(alkylene oxide) block copolymers as structure-directing agents in non-aqueous solutions for organizing the network-forming metal-oxide species, for which inorganic salts serve as precursors. Whereas the pore walls of surfactant-templated mesoporous silica are amorphous, our mesoporous oxides contain nanocrystalline domains within relatively thick amorphous walls. We believe that these materials are formed through a mechanism that combines block copolymer self-assembly with complexation of the inorganic species.

Learning From Biological Systems: Novel Routes to Biomimetic Synthesis of Ordered Silica Structures

Biological systems have evolved mechanisms that precisely control inorganic structures on both the micro- and nanoscale, operating at ambient pressures and temperatures. In both the calcium carbonate, calcium phosphate and silicon dioxide utilizing organisms, proteins and polysaccharides have been found to play integral roles in the organization of these biominerals[1–3]. The organic constituents generally have been thought to act as direct templates or modulators for the deposition of the particular mineral. We have explored the synthesis and structural control of silica by the marine sponge, Tethya aurantia . Needles of amorphous silica comprise the skeletal system of this organism, representing 75% of the dry weight of the organism. These glassy needles, called spicules, are 2 mm in length and 30 μn in width[4,5]. We have characterized the structure, genetics and functions of the proteins that form an occluded axial filament within each silica spicule. Based on our discovery, a unique structure-directing catalytic mechanism exhibited by these protein filaments, and the structural determinants responsible for the catalytic activity, we have designed novel block copolypeptides that catalyze and spatially direct he condensation of silicon alkoxides to form organized silica structures ranging from transparent spheres to lath-like structures at ambient pressure, low temperature and neutral pH.