Tetragonal tungsten bronze-type nanorod photocatalysts with tunnel structures: Ta substitution for Nb and overall water splitting
Tetragonal tungsten bronze-type tantalum (Ta) substituted Sr2KNb5O15 nanorod photocatalysts with tunnel structures were prepared by a facile and low-cost molten salt method using potassium chloride (KCl) at 850 °C for only 2 h. Although all native photocatalysts did not possess any detectable activity in pure water splitting, after deposition of NiOx (double-layered Ni/NiO) as co-catalysts, samples of Sr2KNb5O15 and Sr2KTa5O15 can split pure water into H2 and O2 in a stoichiometric amount (≈2 : 1), which can be ascribed to the improved charge carrier separation and transfer in the presence of NiOx. Furthermore, Ta substitution effects on the photocatalytic behaviour were systematically investigated for hydrogen production by aqueous methanol reforming. The average H2 formation rates of Sr2KNb5−xTaxO15 first decrease with tantalum substitution for x < 2.5, presumably due to a decreased amount of absorbed photons and an obvious reduction of their exposed surface areas, whereas the activity is significantly improved for samples containing more Ta (x > 2.5) and especially the fully substituted Sr2KTa5O15. This can be explained by a stronger driving force for photogenerated conduction band electrons to reduce water.