“Brick and Mortar” Strategy for the Formation of Highly Crystalline Mesoporous Titania Films from Nanocrystalline Building Blocks Johann M. Szeifert, ‡,† Dina Fattakhova-Rohlfing, ‡,† Dimitra Georgiadou, † Vit Kalousek, § Jiri Rathouský, § Daibin Kuang, | Sophie Wenger, | Shaik M. Zakeeruddin, | Michael Gra ¨tzel, | and Thomas Bein* ,† Department of Chemistry and Biochemistry and Center for NanoScience (CeNS), UniVersity of Munich (LMU), Germany, J. HeyroVsky ´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, and Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fe ´de ´rale de Lausanne, Switzerland ReceiVed October 26, 2008. ReVised Manuscript ReceiVed December 16, 2008 We present a novel “brick and mortar” strategy for creating highly efficient transparent TiO 2 coatings for photocatalytic and photovoltaic applications. Our approach is based on the fusion of preformed titania nanocrystalline “bricks” through surfactant-templated sol-gel titania “mortar”, which acts as a structure- directing matrix and as a chemical glue. The similar chemical composition of both bricks and mortar leads to a striking synergy in the interaction of crystalline and amorphous components, such that crystallization is enhanced upon thermal treatment and highly porous and highly crystalline structures are formed at very mild conditions. Coatings with a broad variety of periodic mesostructures and thicknesses ranging from few nanometers to several micrometers are accessible using the same organic template, and the final structures are tunable by varying the fraction of the “bricks”. The beneficial combination of crystallinity and porosity leads to greatly enhanced activity of the films in photocatalytic processes, such as the photooxidation of NO. Acting as the active layers in dye-sensitized solar cells, films of only 2.7 µm in thickness exhibit a conversion efficiency of 6.0%. Introduction Nanostructured films of TiO 2 have an enormous potential for applications in photocatalysis, solar cells, sensors, and energy storage. 1 However, reaching this potential often requires films that simultaneously feature both large and easily accessible surface area and highly crystalline pore walls. Crystalline titania layers are most commonly as- sembled from crystalline particles by sintering. 2,3 This approach offers very good control over the phase composition and the degree of crystallinity, but lacks the possibility to tune the structure and the porosity. To overcome these shortcomings, researchers employ molecular/oligomeric pre- cursors (sols) in a templated sol-gel process. 4-7 Thus, precise control over the porosity by using structure directing agents 8-11 can be achieved, but the crystallinity of the resulting TiO 2 frameworks is usually only moderate and high calcination temperatures are needed for further crystallization. 12,13 Here we present a novel preparation strategy that not only combines the strong points of both above-mentioned tech- niques, but also extends the material’s functionality by introducing further control over physical properties beyond the scope of the classical techniques. 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