Mesoporous metal oxide thin films have attracted significant attention due to their potential use in photocatalysis, redox reactions, sensing etc. The most straightforward preparation method involves sol-gel chemistry in combination with a suitable structure-directing block copolymer (evaporation-induced self-assembly, EISA), and heat-treatment leads to the corresponding crystalline mesoporous oxide. The nanocrystals in the pore walls usually are randomly oriented with respect to the substrate and the mesopores, while it would be desirable to achieve preferred 3D orientation of the crystallites. So far, single-crystalline substrates are generally required to achieve oriented crystallization. Therefore, it is commonly not expected that mesoporous films with crystallographic orientation can be formed from solutions. Here we report a general, surfactant-mediated methodology for the preparation of mesoporous films with uniaxial crystallographical orientation of the nanocrystals (MoO₃, Nb₂O₅, etc.) on non-crystalline substrates (glass), by dip-coating of ethanolic solutions of a metal oxide precursor (e.g. MoCl₅) and block copolymers of the KLE family (poly(ethylene-co-butylene)-b-poly(ethylene oxide)) [1]. Subsequently, the films are heated in air to initiate crystallization, and the KLE block copolymers leads to ordered mesoporous, crystalline films with a uniaxial crystallographic orientation of the nanocrystals. For instance, for MoO₃ the [010] planes are orientated parallel to the substrate. Systematic studies revealed that the effect occurs for oxides with highly anisotropic unit cells (MoO₃, etc.), which leads to crystallization perpendicular to the lattice planes of maximum polarizability, and the role of the surfactant is to mediate the polarization induced by the surface (“soft epitaxy”) [1]. We anticipate this approach to provide the basis for the generation of layers of more complex metal oxides.

[1] Smarsly et al. Adv. Mater. 2006, 18, 1827.

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