Local Environment of Strontium Cations Activating NaTaO3 Photocatalysts

Sodium tantalate, NaTaO3, is one of the best semiconductors for photocatalytic water splitting and CO2 reduction. Doping with metal cations is crucial to enhancing the quantum efficiency of the desired reaction. Nevertheless, details related to the doping of the host metal oxide and activation by guest metal cations are not sufficiently known. The most fundamental question is the increase in the quantum efficiency via doping with guest cations that are impurities in the host lattice. In this study, the local environment of Sr cations, which are the typically used guest cations in NaTaO3, was characterized by extended X-ray absorption fine-structure spectroscopy. The results reveal the presence of two Sr–O shells in the Sr-doped NaTaO3 photocatalysts. The small shell with an unexpectedly short Sr–O bond length of 1.96 Å corresponded to SrO6 octahedra embedded in the corner-shared network of TaO6 octahedra. The other shell with a Sr–O bond length of 2.60 Å corresponded to SrO12 cuboctahedra with Sr cations at positions previously occupied by Na cations. Rietveld analysis of the X-ray diffraction data confirmed the formation of a NaTaO3–Sr(Sr1/3Ta2/3)O3 solid solution to accommodate the two Sr–O shells in NaTaO3 with no requirement for creating oxygen anion vacancies. Mechanisms of increasing the quantum efficiency via doping with Sr cations are discussed on the revealed environment

Effect of Etching on Electron–Hole Recombination in Sr-Doped NaTaO₃ Photocatalysts

Sodium tantalate (NaTaO₃) photocatalysts doped with Sr²⁺ produce core–shell-structured NaTaO₃–SrSr_1/3Ta_2/3O₃ solid solutions able to split water efficiently, when prepared via the solid-state method. In this study, the photocatalysts were chemically etched to examine the different roles of the core and shell with respect to the recombination of electrons and holes. Under excitation by Hg–Xe lamp irradiation, the steady-state population of electrons in the core–shell-structured photocatalyst with a bulk Sr concentration of 5 mol % increased by 130 times relative to that of the undoped photocatalyst. During etching for the first 10 min, the shell detached from the top of the core, and the electron population in the uncovered core further increased by 40%. This population enhancement indicates that electrons are excited in the core and recombined in the shell. Etching up to 480 min resulted in the reduction of the electron population. To interpret the population reduction in this stage of etching, a Sr concentration gradient that controls the electron population in the core is proposed

Local Environment of Strontium Cations Activating NaTaO₃ Photocatalysts

Sodium tantalate, NaTaO₃, is one of the best semiconductors for photocatalytic water splitting and CO₂ reduction. Doping with metal cations is crucial for enhancing the quantum efficiency of the desired reactions. Nevertheless, details related to the doping of the host metal oxide and activation by guest metal cations are not sufficiently known. The most fundamental question concerns the increase in the quantum efficiency via doping with guest cations that are impurities in the host lattice. In this study, the local environment of Sr cations, which are the typically used guest cations in NaTaO₃, was characterized by extended X-ray absorption fine structure spectroscopy. The results reveal the presence of two Sr–O shells in the Sr-doped NaTaO₃ photocatalysts. The small shell with an unexpectedly short Sr–O bond length of 1.96 Å corresponds to SrO₆ octahedra embedded in the corner-shared network of TaO₆ octahedra. The other shell with a Sr–O bond length of 2.60 Å corresponds to SrO₁₂ cuboctahedra with Sr cations at positions previously occupied by Na cations. Rietveld analysis of the X-ray diffraction data confirmed the formation of a NaTaO₃–Sr­(Sr_1/3Ta_2/3)­O₃ solid solution to accommodate the two Sr–O shells in NaTaO₃ with no requirement for creating oxygen anion vacancies. Mechanisms for increasing the quantum efficiency via doping with Sr cations are discussed in the context of the revealed environment