Showing papers by "Peidong Yang published in 1998"

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

Abstract: 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 interactions1,2,3,4,5,6,7,8. This approach to mesostructured materials has been extended, with sporadic success, to non-silica oxides5,6,7,8,9,10,11,12,13,14,15,16,17, 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 140 A) 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 silica1 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.

Hierarchically ordered oxides

Abstract: Porous silica, niobia, and titania with three-dimensional structures patterned over multiple length scales were prepared by combining micromolding, polystyrene sphere templating, and cooperative assembly of inorganic sol-gel species with amphiphilic triblock copolymers. The resulting materials show hierarchical ordering over several discrete and tunable length scales ranging from 10 nanometers to several micrometers. The respective ordered structures can be independently modified by choosing different mold patterns, latex spheres, and block copolymers. The examples presented demonstrate the compositional and structural diversities that are possible with this simple approach.

Continuous Mesoporous Silica Films with Highly Ordered Large Pore Structures

Abstract: Mesoporous silica films can be grown at solid±liquid and liquid±vapor interfaces through an interfacial silica±surfac-tant self-assembly process, [1±8] and more recently, the formation of continuous mesoporous silica films by sol-gel dip-coating has been reported by Lu et al. [9] All of these films were formed using acidic conditions, [10] and low molecular weight surfactants. Many of them are granular and have 2-dimensional (2D) hexagonal structures with pore channels aligned within the substrate plane, an orientation that does not provide the easy accessibility to the substrate, as required for many separation and catalysis applications. [2±9] Few reports have been published on mesoporous films that might have accessible pores. [6,9,11] Furthermore, these films generally have quite small pore sizes (~20 Š) and relatively low porosities (<60 %). Here we report the formation of continuous mesoporous silica films with large periodic cage and pore structures using low-cost commercially available poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers and poly-(ethylene oxide) non-ionic surfactants [12±15] as the structure directing agents in conjunction with dip-coat processing. [9] These structures can be made with either continuous channel or ordered cage arrays. The films exhibit highly ordered and oriented large periodic mesostructures, which can be 3D cubic (Im3m, Pm3m space group), hexagonal (P6 3 /mmc, p6mm) structures with variable pore sizes (34± 90 Š) and porosities to 75 %. These films exhibit high thermal stability upon calcination at 450 C and are crack-free when the thickness is less than 1 mm. The relatively large porosities of the films result in dielectric constants (k = 1.45±2.1) that are very low. The mesoporous silica films are deposited by dip-coating on polished (100) silicon wafers and glass slides. The coating solutions are prepared by the addition of an ethanol solution of PEO-PPO-PEO triblock copolymers or alkyl poly(ethylene oxide) oligomeric non-ionic surfactants to silica sol-gels made by an acid-catalyzed process [10,14,15] with stirring for 2 h at room temperature. The silica sol-gels are prepared by heating the mixture of a calculated amount tetraethoxysiliane (TEOS), ethanol, water, and HCl at 50± 80 C for 1 h. Besides ethanol, other organic solvents, such as methanol, 1,4-dioxane, tetrahydrofuran (THF), CH 3 CN, and propanol, can be used as the solvents. The silica films are transparent and continuous with smooth surfaces and mesoscopic structures (Fig. 1). The thickness of the film can be uniform and varied from 300 nm to several hundred micrometers by adjusting the coating …

Triblock-Copolymer-Directed Syntheses of Large-Pore Mesoporous Silica Fibers

Abstract: Mesoporous silica fibers with accessible, highly ordered large periodic pores are directly drawn from a highly viscous surfactant/silicate solution. Poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymers are used as the structure-directing agents under acidic conditions during the syntheses. These fibers have uniform exterior diameters of submicron to several hundred micron and no apparent limit on aspect ratios. They are made up of either supercage arrays or 2-dimensional channel arrays with pore sizes as large as 63 A.

Block polymer processing for mesostructured inorganic oxide materials

Abstract: Mesoscopically ordered, hydrothermally stable metal oxide-block copolymer composite or mesoporous materials are described herein that are formed by using amphiphilic block copolymers which act as structure directing agents for the metal oxide in a self-assembling system.

Nitrogen-driven sp 3 to sp 2 transformation in carbon nitride materials

Abstract: The coordination of carbon as a function of nitrogen concentration in energetically deposited carbon nitride films has been systematically studied. A structural transformation from primarily ${\mathrm{sp}}^{3}$-bonded to ${\mathrm{sp}}^{2}$-bonded carbon and a density decrease from 3.3 to $2.1 {\mathrm{g}/\mathrm{c}\mathrm{m}}^{3}$ were observed as the nitrogen concentration increases from 11 to 17 %. Calculations on nitrogen-substituted carbon clusters indicate that there is a preference to form ${\mathrm{sp}}^{2}$-bonded carbon when the nitrogen concentration is larger than 12%. The implications for these results to the synthesis of superhard carbon nitride materials are discussed.