Hard templating (also called nanocasting) has emerged as a powerful method for constructing new solid-state materials with periodic order. [1][2][3] Although silica can be prepared with a variety of periodic structures (for example, lamellar, hexagonal, cubic) and pore sizes by aqueous sol-gel methods using a surfactant template, [4,5] this method cannot generally be applied to other materials. [6,7] This limitation stems from the fact that the required precursor for many materials either condenses too quickly, disrupts the organization of the surfactant, or is not compatible with aqueous chemistry (for example, in the preparation of metal nitrides). Sometimes these problems can be overcome by using chelating ligands or by a judicious choice of solvent and template. [8][9][10][11][12] On the other hand, by using porous silica as a hard template, diverse nanostructured materials may be obtained with a wide variety of compositions (carbon, polymers, noble metals, and metal oxides) after etching of the silica. [13][14][15][16][17][18] The thermal stability of silica also allows for the use of high-temperature treatments to generate highly crystalline mesoporous products that may be otherwise difficult to obtain.The hard-templating approach has been applied to the synthesis of various novel mesoporous materials. Yue et al. recently reported the synthesis of mesoporous rutile and anatase TiO 2 using SBA-15 silica as the hard template, [19] and other hard templates have been employed in the synthesis of nanostructured titania. [20] High-surface-area nanocrystalline TiO 2 is of particular interest for applications, such as dyesensitized solar cells, [21] photocatalysts, [22] gas sensors, [23] and batteries. [24] The incorporation of high-surface-area anatase TiO 2 into photonic structures is a further challenge that has recently garnered attention. [25,26] By using titania in colloidal crystals and inverse opals, the high refractive index of TiO 2 (n = 2.5-2.9) can impart a complete photonic bandgap in these materials.To date, the synthesis of mesoporous materials has been mainly limited to the ordered pore structures obtained from the lyotropic liquid crystalline phases of surfactants and block copolymers. We have recently developed a new class of mesoporous materials with chiral nematic pore structures templated by the lyotropic liquid-crystalline phase of nanocrystalline cellulose (NCC). [27][28][29] Condensation of a silica precursor (Si(OMe) 4 in the current study) in the presence of nanocrystalline cellulose affords a composite material in which the SiO 2 surrounds NCC with a chiral nematic organization. Upon removal of the cellulose template, a porous silicate is obtained as a free-standing film that has a long-range chiral nematic structure, resulting in photonic properties. By varying the pitch of the chiral nematic composites, mesoporous materials with tunable photonic properties are obtained.Herein, we present the synthesis of mesoporous anatase TiO 2 using chiral nematic mesoporous silica films as a hard te...
