phys. Monogr. Am. Geophys. Union 32 (1985)], p. 546. 37. H. M. Stoll and D. P. Schrag, Science 272, 1771 (1996). 38. We thank the Canadian Polar Continental Shelf Project for their invaluable field support; R. Lundgren, O. Lib- man, J. Totten, and K. Weaver for help in the field; G. Kloc, S. Wilkison, and S. Pruss for assistance in sample preparations; and J. Massare and D. Woodrow for helpful discussions. Supported by the U.S. NSF Arctic Natural Sciences Division. 21 September 1998; accepted 10 November 1998 Hierarchically Ordered Oxides Peidong Yang, Tao Deng, Dongyuan Zhao, Pingyun Feng, David Pine, Bradley F. Chmelka, George M. Whitesides, Galen D. Stucky* Porous silica, niobia, and titania with three-dimensional structures patterned over multiple length scales were prepared by combining micromolding, poly- styrene sphere templating, and cooperative assembly of inorganic sol-gel spe- cies with amphiphilic triblock copolymers. The resulting materials show hier- archical 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. Several approaches are currently available for the preparation of ordered structures at dif- ferent length scales. For example, organic molecular templates can be used to form crystalline zeolite-type structures with order- ing lengths less than 3 nm (1); mesoporous materials with ordering lengths of 3 to 30 nm can be obtained using surfactants or am- phiphilic block copolymers as structure-di- recting agents (2–7 ); the use of latex spheres yields macroporous materials with ordering lengths of 100 nm to 1 m(8 –13); and soft lithography can be used to make high-quality patterns and structures with lateral dimen- sions of about 30 nm to 500 m(14 –16 ). Despite all of these efforts in nanostructuring materials, the fabrication of hierarchically or- dered structures at multiple length scales, such as seen in nature in diatoms (17 ), has remained an experimental challenge. Such materials are important both for the system- atic fundamental study of structure-property relations and for their technological promise in applications such as catalysis, selective separations, sensor arrays, wave guides, min- iaturized electronic and magnetic devices, and photonic crystals with tunable band gaps. Previously, micromolding has been used to form patterned mesoporous materials (18, 19). These studies, however, used acidic aqueous conditions to carry out the cooperative self-as- sembly (20), which is disadvantageous because of the limited processibility of the aqueous solu- tions. Either noncontinuous films were formed (18) or an electric field was needed to guide pattern formation, which requires a nonconduct- ing substrate (19). Latex spheres have also been used to make disordered macro- and mesoporous silica (9). We have developed a simple procedure for preparing hierarchically ordered structures by concurrently or sequentially combining micro- molding, latex sphere templating, and coopera- tive assembly of hydrolyzed inorganic species (metal alkoxides, metal chlorides) and amphiphi- lic block copolymers. The materials generated from this process exhibit structural ordering at multiple discrete length scales (in this case, 10, 100, and 1000 nm). Patterned macro- and meso- porous materials of various compositions, in- cluding silica, niobia, and titania, were synthe- sized. Such multiple-scale structural organiza- tion makes it possible to tune the physical properties of the materials over a wide range of chemical compositions. The scheme in Fig. 1A illustrates the proce- dure that was used to fabricate materials with two-scale ordering. Gelation of a self-assem- bling sol-gel precursor solution was carried out in the confined space of a poly(dimethylsilox- ane) (PDMS) mold (14 ). The precursor solution has the same composition as used in the prep- aration of mesoporous silica films (3, 21 )—that is, expressed as molar ratios, 0.008 to 0.018 poly(ethyleneoxide)-b-poly(propyleneoxide)-b- poly(ethyleneoxide) (EO n PO m EO n ); 1 tetra- ethoxysilane (TEOS); 20 to 60 ethanol (EtOH); 0.01 to 0.04 HCl; and 5 to 10 H 2 O. When Pluronic F127 (EO 106 PO 70 EO 106 ) was used as the structure-directing block copoly- mer species, a cubic mesophase resulted, whereas a hexagonal mesophase was ob- tained when Pluronic P123 (EO 20 PO 70 EO 20 ) was used (21). This sol-gel mesophase chem- istry has recently been extended to the prep- aration of diverse thermally stable mesostruc- tured transition metal oxides, including Nb 2 O 5 , TiO 2 , ZrO 2 , WO 3 , AlSiO 3.5 , and SiTiO 4 , by slowing the hydrolysis of inorgan- ic chloride precursor species in alcohol solu- tions (4). These materials were molded by placing a drop of the precursor solution on a freshly cleaned substrate (such as a silicon wafer), after which the mold was placed face down to cover the drop on the surface of the substrate. A pressure of roughly 1  10 5 to 2  10 5 Pa was applied to the PDMS mold. The area of the patterned surface was typi- cally 1 to 5 cm 2 , with molded feature sizes in P. Yang, D. Zhao, G. D. Stucky, Department of Chem- istry, University of California, Santa Barbara, CA 93106, USA. T. Deng and G. M. Whitesides, Depart- ment of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA. P. Feng, D. Pine, B. F. Chmelka, Department of Chemical Engi- neering, University of California, Santa Barbara, CA 93106, USA. *To whom correspondence should be addressed. Fig. 1. Schematic diagrams of the molding methods used to fabricate hierarchically ordered structures on a substrate. (A) For patterning of mesoporous solids, a droplet of sol-gel– block copolymer precursor so- lution was compressed be- tween the silicone mold and the substrate by applying a pressure of roughly 1  10 5 to 2  10 5 Pa. The high interfacial free energy of the solution caused the precursor to dewet the substrate where the mold and the substrate were in con- tact. (B) A sequential process for producing hierarchical or- dering over three discrete and independent length scales. R EPORTS 18 DECEMBER 1998 VOL 282 SCIENCE www.sciencemag.org 2244 on August 5, 2016 http://science.sciencemag.org/ Downloaded from