Sodium Tantalate Photocatalysts Doped with Metal Cations: Why Are They Active for Water Splitting?

Sodium tantalate (NaTaO3 ) is the first semiconductor photocatalyst that produces hydrogen-oxygen mixed bubbles in the overall water splitting reaction, when appropriately doped with metal cations. A series of studies are reviewed herein to answer the question of why doping with metal cations raised the quantum efficiency of the reaction. Infrared absorption of band-gap-excited photocatalysts demonstrated that cation doping reduced the electron-hole recombination rate and the steady-state population of charge carriers accordingly increased. In-depth studies are focused on Sr cations incorporated through solid-state, flux, and hydrothermal reactions. The recombination rate was reduced when Ta cations were exchanged with Sr cations. Sodium cations were simultaneously exchanged to balance the cationic and anionic charges with no need for creating oxygen anion vacancies. NaTaO3 Sr(Sr1/3 Ta2/3 )O3 solid solution was formed as a result of the simultaneous doping. In addition to doping at the appropriate sites, the intraparticle distribution of Sr cations played an essential role to reduce the recombination rate. Strontium cations segregated to produce graded composition from the Sr-rich surface to the Sr-poor core. The bottom of the conduction band was raised at the Sr-rich surface and the excited electrons were driven to the Sr-poor core, leaving holes at the surface. However, the graded composition had a dual purpose; the excited electron population increased and its fractional contribution to the surface reaction decreased.