In situ GISAXS investigation of different protein-templated titania nanostructures

Nanostructured titania thin films have been studied for a large variety of applications. An environmentally benign and scalable synthesis route for this material class could be of interest to many state-of-the-art devices, from solar cells to battery materials. Protein-assisted sol-gel synthesis is a low-temperature, low-cost, and highly scalable technique, that can be used to achieve a nanostructured titania thin film. Beta-lactoglobulin (β-Lg) is a bovine whey protein that is a prominent foaming agent in the food industry. It is non-toxic, abundant, and has excellent foamability and foam stability. Additionally, it has been shown that β-Lg forms differently shaped aggregates at different solution pH values. With simple changes to the solution chemistry, different domain sizes, porosities, and morphologies are possible. Therefore, it is a promising candidate to create tunable and mesoporous titania structures. In this work, we investigate the film formation with small-angle/wide-angle grazing incidence X-ray scattering (GISAXS/GIWAXS) techniques. We observe that films printed at acidic pH form significantly different final bulk morphologies than films printed at neutral pH. The crystallite phase is strongly reduced in average domain size and domain-domain distance. Agglomerate size is increased for the acidic template. In situ scattering experiments during printing of thin hybrid films provide insights into the film formation kinetics. We see a temporal shift in the condensing of the crystallite phase at higher protein concentrations, leading to a smaller window of rearrangement and, therefore, less pronounced differences in morphology at higher template concentrations. Scanning electron microscopy (SEM) reveals that the surface structure is also dependent on the pH value. A more dense and foamlike structure is found at neutral pH. From PL, UV-Vis, and GIWAXS, we see that the solution parameters do not significantly influence the optical properties, crystallinity, and crystal orientation.