In the present work, the influence of different morphologies on the photocatalytic activity of a semiconducting transition metal oxide was investigated. The quaternary photocatalyst caesium tantalum tungstate (CsTaWO6) was therefore used as a model system. This compound exhibits a number of advantages compared to other photocatalysts such as titanium dioxide (TiO2) due to its crystallization in just one cubic crystal structure and its beneficial band positions for the test reaction of photocatalytic hydrogen generation. CsTaWO6 was nanostructured via hydrothermal and sol-gel processes to investigate the influence of surface area, crystallinity and pore sizes on the photocatalytic activity. It was possible to synthesize single crystal CsTaWO6 nanoparticles with different crystallite sizes and mesoporous materials with a number of pore sizes, pore ordering and resulting surface areas. When looking at the photocatalytic hydrogen evolution of all the investigated samples, it could be shown that an increase in surface area does not correlate with an increase in activity. In fact, the optimum crystallite size was found to be the most important aspect in nanostructured photocatalyst, whereas for CsTaWO6 this optimum lies at 12 to 13 nm. Furthermore, it could be shown that larger pores lead to an enhanced hydrogen production in mesoporous photocatalysts. In general, a decrease of the synthesis temperature resulted in an increase in the defect concentration (estimated from strain parameters) and therefore a lower activity. The optimum morphology for the CsTaWO6 system was found to be a mesoporous material with large pores (up to 40 nm), thin pore walls and an optimum crystallite size of approximately 12 nm.
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