Tetragonal tungsten bronze-type nanorod photocatalysts with tunnel structures: Ta substitution for Nb and overall water splitting

Abstract

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 NiO_x (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 NiO_x. Furthermore, Ta substitution effects on the photocatalytic behaviour were systematically investigated for hydrogen production by aqueous methanol reforming. The average H₂ formation rates of Sr₂KNb_5−xTa_xO₁₅ 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.