In this research, sodium tantalate (NaTaO 3 ) photocatalyst was doped with Sr 2+ cations via solid-state or hydrothermal reactions. NaTaO 3 − SrSr 1/3 Ta 2/3 O 3 solid solutions that had been doped through the solid-state reaction with a Sr-rich shell covering a Sr-poor core appeared. The lattice mismatch at the heteroepitaxial core−shell interface induced surface reconstruction with regularly separated steps. The steady-state population of electrons excited with Hg−Xe lamp irradiation increased by up to 180 times in the solid-solution photocatalysts in which A sites and B sites of the perovskite-structured lattice were doped with Sr 2+ . On the other hand, A sites were selectively doped with the same cations through the hydrothermal reaction. Surface reconstruction and an enhanced electron population were absent in the A-site-doped photocatalysts. Compatible results were obtained by doping with Ca or Ba instead of Sr. Tuning of doping sites was essential to restricting recombination in NaTaO 3 photocatalysts doped with the alkaline-earth metals.

Electron–hole recombination always competes with desired reactions on semiconductor photocatalysts. Reducing recombination probability is essential for increasing the quantum efficiency of the reactions. Previous studies demonstrated that doping with lanthanoid or alkaline-earth metal cations reduced recombination probability in NaTaO3 photocatalysts for artificial photosynthesis. The motivation behind this study was to reveal how the guest metal cations reduced recombination probability. NaTaO3 photocatalysts were doped with Sr cations through crystallization in molten NaCl flux to produce 50–100 nm sized particles of NaTaO3–Sr­(Sr1/3Ta2/3)­O3 solid solution. Intraparticle distribution of Sr cations was sensitive to immersion time in the hot flux with a fixed Sr concentration of 2 mol % relative to Ta. Extended immersion for 60 h resulted in a homogeneous Sr distribution. Curtailed immersion for 1 h yielded particles capped with a 3 nm thick Sr-accumulated layer. The population of electrons bandgap-excited under Hg–Xe lamp irradiation was enhanced in the 1-h immersed photocatalyst by 160 times relative to that in a Sr-free NaTaO3 photocatalyst. In the 60-h immersed photocatalyst, population enhancement was not more than 9 times. We interpreted the large population enhancement in the 1-h immersed photocatalyst with a concentration gradient of Sr cations from the surface to bulk. The concentration gradient induced an energy gradient of conduction-band minimum. Photoexcited electrons were driven on the energy gradient to be separated from holes. The overall water splitting reaction rate was evaluated on the photocatalysts to show a 4-times enhancement on the 1-h immersed photocatalyst relative to the rate on the Sr-free photocatalyst. The reaction-rate enhancement less than the electron population enhancement was ascribed to a limited fraction of electrons overriding the energy gradient and returning back to the surface.

Sodium tantalate, NaTaO 3 , is one of the best semiconductors for photocatalytic water splitting and CO 2 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 NaTaO 3 , was characterized by extended X-ray absorption fine structure spectroscopy. The results reveal the presence of two Sr−O shells in the Srdoped NaTaO 3 photocatalysts. The small shell with an unexpectedly short Sr− O bond length of 1.96 Å corresponds to SrO 6 octahedra embedded in the corner-shared network of TaO 6 octahedra. The other shell with a Sr−O bond length of 2.60 Å corresponds to SrO 12 cuboctahedra with Sr cations at positions previously occupied by Na cations. Rietveld analysis of the X-ray diffraction data confirmed the formation of a NaTaO 3 −Sr(Sr 1/3 Ta 2/3 )O 3 solid solution to accommodate the two Sr−O shells in NaTaO 3 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.

Sodium tantalate (NaTaO 3 ) photocatalysts doped with Sr 2+ produce core− shell-structured NaTaO 3 −SrSr 1/3 Ta 2/3 O 3 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. Article pubs.acs.org/JPCC

Perovskite-structured NaTaO 3 films were epitaxially deposited on centimeter-sized SrTiO 3 (001) wafers by hydrothermal and solvothermal reactions to examine the surface science for an efficient photocatalytic water-splitting reaction. The addition of Ba cations in the starting solutions afforded Badoped NaTaO 3 films. X-ray fluorescence holography was employed to investigate and confirm the heteroepitaxial relationship between the Ba-doped film and substrate.  Hydrothermal and solvothermal preparation of NaTaO 3 heteroepitaxial films on SrTiO 3 (001).  Doping with Ba 2+ cations up to 4 mol% relative to Ta.  Feasibility of X-ray fluorescence holography for NaTaO 3 film characterization.