High-Resolution Cryogenic-Electron Microscopy Reveals Details of a Hexagonal-to-Bicontinuous Cubic Phase Transition in Mesoporous Silica Synthesis Liora Omer, §,‡ Sharon Ruthstein, ‡ Daniella Goldfarb, ‡ and Yeshayahu Talmon* ,§ Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel, and Department of Chemical Physics, Weizmann Institute of Science, RehoVot 76100, Israel Received April 21, 2009; E-mail: ishi@tx.technion.ac.il Abstract: We studied the structural evolution during the formation of large-pore cubic Ia3 j d silica-based mesoporous materials, synthesized with Pluroinc P123 and butanol as structure directing agents. We used cryogenic high resolution scanning electron microscopy (cryo-HRSEM) and freeze-fracture-replication (FFR) transmission electron microscopy (TEM). Typically a silica precursor is added to an acid-catalyzed solution of Pluronic P123 and butanol. The latter serves as a cosolute, which can be added either at the beginning of the reaction, or after precipitation and the formation of a hexagonal phase. In this study we focused on the structural evolution from the hexagonal phase to the final cubic phase in the two different reactions. The same structural evolution with different kinetics was detected for both reactions. Cryo-HRSEM and FFR-TEM images revealed that from the hexagonal phase a perforated layer (PL) phase is formed, which later evolves into a bicontinuous structure. The final cubic phase forms within the layers, maintaining their orientation. We suggest a formation mechanism involving cylinder merging for the hexagonal to PL transition. Upon additional polymerization of the silica, the PL phase relaxes into the stable Ia3 j d cubic phase. Another minor mechanism detected involves the direct transition between the hexagonal to the final cubic phase through cylinder branching. Introduction Ordered mesoporous materials with large-pore diameters of 2-30 nm and various symmetries can be synthesized using triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers (Pluronics, PEO x PPO y PEO x ). 1,2 These commercially available block-copolymers are low-cost, nontoxic and biode- gradable surfactants. The mesoporous products display high structure regularity, thick inorganic walls, and excellent thermal and hydrothermal stability. 1 The most studied mesoporous material synthesized using Pluronics is the two-dimensional (2- D) hexagonal SBA-15, 2 which has been tested for many applications, some of these have been summarized in a number of studies and reviews. 3-8 Three-dimensional (3-D) large-pore mesostructures are highly attractive for applications requiring easily accessible and uniform large pores. The Ia3 j d double-gyroid structure with its highly branched and interwoven bicontinuous structure is particularly attractive for applications which are susceptible to pore blockage. 9,10 The gyroid morphology of the Ia3 j d cubic phase is composed of a matrix constructed over the Schoen G minimal surface and two interpenetrated networks embedded in this matrix. 11 Wei and Hilhouse showed that the diffusion through a mesoporous thin film of cubic symmetry is more efficient than through 2D hexagonal and rhombohedral films. 12 Furthermore, they found that the diffusion through the double-gyroid phase was the highest among the cubic thin films by over an order of magnitude. 12 Large-pore cubic Ia3 j d mesostructured silica can be obtained with Pluronics using additives such as inorganic salts, 13,14 hydrophobic silica precursor (organosiloxane), 15,16 or § Technion-Israel Institute of Technology. ‡ Weizmann Institute of Science. (1) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. 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