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Betzler, Sophia (2015): 3D hierarchical Nb3O7(OH) superstructures: synthesis, structural characterization and photophysical properties. Dissertation, LMU München: Faculty of Chemistry and Pharmacy



Global warming caused by continuous emission of greenhouse gases is omnipresent. Anthropogenic CO2 resulting from the combustion of fossil fuels adds the largest mass to the scales. Therefore, new technologies for power generation and energy storage are required. This study focuses on the synthesis and characterization of novel materials to be used as photoelectrode in dye-sensitized solar cells or as photocatalyst for water splitting. Hydrothermal conditions feature the formation of 3D hierarchical Nb3O7(OH) superstructures which are composed of highly-ordered nanowire networks. Despite their complexity these superstructures form self-organized starting from amorphous hollow cubes. Advanced transmission electron microscopy is applied for the characterization of the crystallographic structure, atomic arrangement and bonding characteristics of the nanostructures. 3D reconstruction of the nanowire arrangement, based on a combination of local thickness measurements and electron tomography, indicates suitable charge transport paths. The stabilization of the superstructures is based on the nanowire junctions. Even though no complete interpenetration of the nanowires was observed these networks exhibit a very high thermal stability. The morphology remains stable for temperatures up to 850 °C despite of the phase transformation of Nb3O7(OH) to H-Nb2O5. This phase transformation was investigated in detail with ex situ and in situ experiments yielding a good understanding of the impact of temperature, atmospheric condition and electron beam on the crystal structure. The morphological and photophysical properties of the nanostructures determine their performance in functional devices and promising hydrogen production rates are observed for the superstructures. These rates can be further enhanced by the incorporation of titanium into the crystal lattice. The capacity of the Nb3O7(OH) crystal lattice to incorporate titanium is limited to about 12 at% and the formation of anatase TiO2 plates is observed for titanium excess. The presence of titanium in the crystal lattice has two main effects. It slows down the crystallization of Nb3O7(OH) leading to superstructures composed of smaller nanocrystals and furthermore it reduces the surface defects resulting in lower charge recombination rates. Therefore, the hydrogen production rate of titanium doped (5.5 at% Ti) superstructures was by a factor of two higher than the one observed for undoped Nb3O7(OH).