Generation of self-assembled 3D mesostructured SnO2 thin films with highly crystalline frameworks

Crack-free, mesoporous SnO2 films with highly crystalline pore walls are obtained by evaporation-induced self-assembly using a novel amphiphilic block-copolymer template ("KLE" type, poly(ethylene-co-butylene)-block-poly(ethylene oxide)), which leads to well-defined arrays of contracted spherical mesopores by suitable heat-treatment procedures. Because of the improved templating properties of these polymers, a facile heat-treatment procedure can be applied whilst keeping the mesoscopic order intact up to 600-650 degrees C. The formation mechanism and the mesostructural evolution are investigated. by various state-of-the-art techniques, particularly by a specially constructed 2D small-angle X-ray scattering setup. It is found that the main benefit from the polymers is the formation of an ordered mesostructure under the drastic conditions of using molecular Sn precursors (SnCl4), taking advantage of the large segregation strength of these amphiphiles. Furthermore, it is found that the crystallization mechanism is different from other mesostructured metal oxides such as TiO2. In the case of SnO2, a significant degree of crystallization (induced by heat treatment) already starts at quite low temperatures, 250-300 degrees C. Therefore, this study provides a better understanding of the general parameters governing the preparation of mesoporous metal. oxides films with crystalline pore walls.