Elucidating the impact of A-site cation change on photocatalytic H2 and O2 evolution activities of perovskite-type LnTaON2 (Ln = La and Pr)

Abstract

Transition metal (oxy)nitrides with perovskite-type structures have been regarded as one of the promising classes of inorganic semiconductor materials that can be used in solar water splitting systems for the production of hydrogen as a renewable and storable energy carrier. The performance of transition metal (oxy)nitrides in solar water splitting is strongly influenced by the crystal structure-related dynamics of photogenerated charge carriers. Here, we have systematically assessed the influence of A-site cation exchange on the visible-light-induced photocatalytic H₂ and O₂ evolution activities, photoanodic response, and dynamics of photogenerated charge carriers of perovskite-type LnTaON₂ (Ln = La and Pr). The structural refinement results reveal the orthorhombic Imma and Pnma structures for LaTaON₂ and PrTaON₂, respectively; the latter has a more distorted crystal structure from the ideal cubic perovskite due to the smaller size of Pr³⁺ cations. Compared with LaTaON₂, PrTaON₂ exhibits lower photocatalytic H₂ and O₂ gas evolution activities and photoanodic response owing to an excessive amount of intrinsic defects associated with anionic vacancies and reduced tantalum species stemming from a long high-temperature nitridation process under reductive NH₃ atmosphere. Transient absorption signals evidence the faster decay of photogenerated electrons (holes) in Pt (CoO_x)-loaded LaTaON₂ than that in Pt (CoO_x)-loaded PrTaON₂, consistent with the photocatalytic and photoelectrochemical performance of the two photocatalysts. This study suggests that in addition to selecting a suitable A-site cation, it is prerequisite to synthesize LnTaON₂ (Ln = La and Pr) crystals with a low defect density to improve their photo-conversion efficiency for solar water splitting.