Sol-gel chemistry is an important element in the fabrication of functional materials, since it provides a powerful way toward a controlled and accessible synthesis of metal oxides. Starting from a soluble precursor, hydrolysis under various chemical conditions leads to colloidal dispersions of polymeric networks and condensed particles. These dispersions can be deposited and dried to achieve powders or films of the respective metal oxide. The chosen sol-gel and drying conditions have a high impact on the resulting metal oxide’s morphology and crystallinity and hence on the material properties. Furthermore, by addition of sol-gel assisting diblock copolymers, the morphology can be tailored to match desired structural requirements, e.g. porosity, surface-to-volume ratio, and size distributions. This tunability of material properties together with large-scale deposition techniques, like roll-to-roll printing and spray deposition, make the sol-gel synthesis of metal oxides a powerful tool for functional materials. Mesoporous TiO₂ in combination with organic semiconductors can be used in photoactive layers of hybrid solar cells. Absorbed light creates uncharged quasi-particles in the organic phase which are split to charge carriers at the TiO₂ interfaces and transported within the TiO₂ phase to generate a current flow. The performance of this crucially depends on the porosity and size distribution of the TiO₂ phase and can be optimized by applying diblock copolymer templated sol-gel synthesis. Nonetheless, the involved flammable and toxic organic solvents are of reduced environmental compatibility. To promote green synthesis, synthetic polymers can be replaced by water-soluble biopolymers. In this work, we investigate the templating influence of the water-soluble bovine whey protein ß-lactoglobulin (ß-lg) on the morphology and crystallinity of TiO₂. Hydrolysis of ß-lg causes unfolding and peptization of the protein, which are used as nucleation seeds and morphological templates for the TiO₂ precursor. In situ grazing incidence small-angle and wide-angle X-ray scattering (GISAXS/GIWAXS) are carried out during spray deposition of ß-lactoglobulin:TiO₂ dispersions obtained by sol-gel synthesis, in order to reveal the kinetics during the formation of mesoporous TiO₂ on the short nanometer length scale. GISAXS and GIWAXS data is complemented by real-space imaging via scanning electron microscopy.

Julian Eliah Heger

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