Phase-Separation-Induced Titania Monoliths with Well-Defined Macropores and Mesostructured Framework from Colloid-Derived Sol-Gel Systems Junko Konishi, ² Koji Fujita,* ,² Kazuki Nakanishi, ‡ and Kazuyuki Hirao ² Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, and DiVision of Chemistry, Graduate School of Science, Kyoto UniVersity, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502, Japan ReceiVed September 26, 2005 ReVised Manuscript ReceiVed NoVember 18, 2005 Titania has attracted considerable attention because of its potential uses in diverse areas such as photocatalysis 1 , sensor devices 2 , electrode materials 3 , chromatographies 4 , and optical devices 5 . The performance of these devices can be dramati- cally improved by structural control over the nanometer to micrometer ranges. A key step in the synthesis of transition metal oxide with well-defined porous structure continues to be the use of organic templates that spatially pattern the deposition of the corresponding alkoxide or colloidal suspen- sions. For instance, mesoporous titania can be prepared by using self-organized arrays of long-chain surfactants or amphiphilic block copolymers as the templates. 6 Stabilized- emulsion or latex-sphere templating has been shown to extend the pore size up to several micrometers. 7 In applica- tions that utilize liquid-phase reactions such as catalyst supports, macroporous structures with mesotexture are highly desirable, because the interconnected macroporous channel facilitates the material transport to mesoporous internal regions where the reactions can take place. Dual templating techniques using surfactants and latex spheres can produce titania with bimodal pore size distribution. 8 Recent reports demonstrated that macro-mesoporous structures can be realized in the presence of a single surfactant 9 and even under a template-free condition. 10 Despise the drastic progress in the synthesis of porous inorganic materials, however, the integration of porous structure into large monoliths still remains a challenging task; templating approaches ensure the formation of well-defined porous structures, but three-dimensional structural buildup of inorganic network, including the morphological control, is difficult to achieve in parallel. Only a few techniques have been reported to obtain the bimodal pore structure in a titania monolith with dimensions of several centimeters. 11 In this paper, we demonstrate a simple and reproducible method based on the sol-gel route accompanied by the phase separation, which enables the fabrication of titania monoliths with well-defined macropores and accessible mesostructure. Special attention is paid to the systematic control of macroporous structures as well as the spontaneous formation of completely interconnected macroporous morphology. A lot of benefits are expected to arise not only from the pore structures integrated in monoliths but also from the novel functions of titania. The sol-gel process on the basis of polymerization- induced phase separation has been proven successful for designing well-defined macroporous monoliths in various silica-based systems. 12 The bicontinuous structure, in which each separated phase is interconnected in three-dimensional space, is spontaneously formed when the transient structure of the phase separation is frozen by the sol-gel transition. Time evolution of the transient structure of the phase separation is schematically illustrated in Figure 1. The characteristic size of the bicontinuous structure, or domain size, is defined by the sum of the thickness of the gel-phase domain and the width of the solvent-phase domain, the latter of which becomes macropores after drying. The formation of phase domains in the course of the phase separation includes the coarsening process in which the domain size grows from shorter to larger length scale as time elapses. 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