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Abstract

In this thesis, the design and nano-engineering of tantalum-based materials are studied to develop more efficient photocatalysts for water splitting. Structural parameters of the catalyst materials are modified by using different synthesis methods including hydrothermal synthesis, soft- and hard-templating, and an in-situ preparation method. The influences of the synthetic methodology on the structure of the catalyst including its morphology, crystal and energetic structure, and its activity towards hydrogen production are evaluated, highlighting the positive effect of composites and amorphous phases on the photocatalytic activity. Furthermore, a novel in-situ synthesis method for nanostructured materials is presented, allowing an efficient synthesis of highly active photocatalysts.
Using hydrothermal synthesis, a series of crystalline sodium tantalates samples with NaTaO₃ and Na₂Ta₂O6 phases was prepared. A Na₂Ta₂O6 sample with the highest surface area showed the highest activity for water splitting. Furthermore, composite samples also showed a positive effect on the water splitting activity. In a second series, amorphous and crystalline sodium tantalates were obtained, revealing a positive effect on the water splitting activity through the combination of amorphous and crystalline NaTaO₃ .
Soft-templating yielded amorphous tantalum oxide, which was Na-loaded using different concentrations of NaOH during synthesis. The Na-loading led to a band gap widening and an increase in photocatalytic activity. As a comparison to the soft-templated samples, Ta₂O₅ and NaTaO₃ were synthesized via hard-templating, exhibiting highly crystalline and nanosporous samples with a large surface area. As a result, hard-templated NaTaO₃ has the highest activity towards hydrogen production observed from all the samples reported in this thesis.
The development of an effective in-situ synthesis method for tantalum-based photocatalysts is presented. Via hydrolysis of alkoxide precursors, tantalum and sodium tantalum based oxo hydroxo compounds are prepared that exhibit an amorphous structure. The obtained sample exceeds the water splitting activity observed for the highly crystalline reference samples; furthermore, its efficient catalyst preparation method can make a significant contribution to its large scale production.