Active BaTaO2N photocatalysts prepared from an amorphous Ta2O5 precursor for overall water splitting under visible light

Abstract

Barium tantalum oxynitride (BaTaO₂N), a photocatalyst active during one-step-excitation overall water splitting under visible light, was synthesized by NH₃-based nitridation of a mixture of BaCO₃ and amorphous Ta₂O₅·3H₂O. H₂ and O₂ were generated from water in stoichiometric amounts and in a stable manner using this material modified with Rh (or Ru), Cr₂O₃ and IrO₂ cocatalysts. This photocatalyst was responsive to visible light up to a wavelength of 540 nm for the water splitting reaction. Trials using nitridation temperatures from 1023 to 1273 K and durations from 0.5 to 20 h indicated that mild nitridation conditions such as 1123 K and 5 h provided high-performance specimens, whereas prolonged nitridation (15–20 h) dramatically decreased the photocatalytic activity. Transient absorption spectroscopy showed that BaTaO₂N generated abundant photoexcited free electrons with long lifetimes due to a low concentration of defects such as oxygen vacancies, but was deactivated after prolonged nitridation. X-ray photoelectron spectra indicated that overly long nitridation generated recombination sites such as Ta³⁺. The mechanism by which active BaTaO₂N was formed was examined, and the high reactivity of the amorphous Ta₂O₅·3H₂O nanoparticles was determined to be important. The use of highly reactive nanoparticles under mild nitridation conditions could allow the future development of high-performance oxynitride photocatalysts for overall water splitting.