Innovative synthesis routes towards hierarchically organized nanostructured materials combined with in-situ SAXS characterization techniques
Final Report Abstract
The objective of the project was to develop innovative synthesis routes towards novel multifunctional materials for a wide field of application in our daily lives, e.g. as filters, catalysts, optical sensors, etc.. To improve the chemical and structural properties of such materials a good understanding of the underlying formation processes and the possibility of a deliberate tailoring of the porous structure is indispensable. The focus of the project was on sol-gel derived oxides (silica, silica/titania mixed oxides and titania) with a hierarchical network build-up of a cellular, open macropore network (500 nm to 2 µm) comprising well-ordered mesopores in the single struts of about 5-10 nm in diameter. Synthetic approaches were based on polymerization-induced phase separation during sol-gel reactions of glycolated precursor molecules, such as tetrakis(2- hydroxyethyl)orthosilicate (EGMS) for silica, bis(2-hydroxyethoxy)titanium (EGMT) for titania, or novel single-source precursors for the mixed oxides in the presence of structure-directing agents. Sol-gel processes are typically catalyzed by acids or bases. Interestingly, in the formation of silica-based materials the acid seems to be chosen rather randomly. In this work, we could show that not only acid strength but also the acid anions influence the sol-gel reactions and phase separation processes in the formation of hierarchically organized silica monoliths starting from EGMS to a large extent. Especially the build-up of the macroporous network is strongly dependent on the acid applied ranging from cellular networks to closed pore structures. The main challenge in the preparation of mixed-oxides is to overcome the problem of phase-separation into two metal-oxide rich domains due to the different reaction rates of the individual precursor compounds. One of the most effective methods in preparing mixed-metal oxides is the application of so-called single-source precursor molecules, where the (semi)metallic centres are already linked via an organic spacer. Within the frame of the project, three different mixed-metal oxide systems have been developed and extensively investigated with respect to sol-gel processing – structure relations in the final material. The deliberate development of porous titania materials is of interest with regard to applications, such as photocatalysis and electrochemical devices. Meso/macroporous TiO2 monoliths are hardly reported in literature and most synthesis procedures lead to X-ray amorphous materials and a calcination step is required to induce crystallization. For the first time, monolithic titania was fabricated from bis(2-hydroxyethoxy)titanium in hydrochloric acid with formaldehyde. The synthesis conditions and parameters were optimized to yield highly porous, particulate titania monoliths with high specific surface areas of > 500 m^2 g^−1 composed of ~2 nm anatase crystallites avoiding any heat treatment steps.
Publications
- Functional (Meso)Porous Nanostructures, The Sol-Gel Handbook Vol. 2 (Eds. D. Levy, M. Zayat), Wiley VCH, Weinheim, 2015
A. Feinle, N. Hüsing
