13 research outputs found
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 SrâO shells in the Sr-doped NaTaO3 photocatalysts. The small shell with an unexpectedly short SrâO bond length of 1.96 Ă
corresponded to SrO6 octahedra embedded in the corner-shared network of TaO6 octahedra. The other shell with a SrâO bond length of 2.60 Ă
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 NaTaO3âSr(Sr1/3Ta2/3)O3 solid solution to accommodate the two SrâO 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 ElectronâHole Recombination in Sr-Doped NaTaO<sub>3</sub> Photocatalysts
Sodium
tantalate (NaTaO<sub>3</sub>) photocatalysts doped with
Sr<sup>2+</sup> produce coreâshell-structured NaTaO<sub>3</sub>âSrSr<sub>1/3</sub>Ta<sub>2/3</sub>O<sub>3</sub> 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 HgâXe
lamp irradiation, the steady-state population of electrons in the
coreâshell-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 NaTaO<sub>3</sub> Photocatalysts
Sodium
tantalate, NaTaO<sub>3</sub>, is one of the best semiconductors
for photocatalytic water splitting and CO<sub>2</sub> 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<sub>3</sub>, was characterized by extended X-ray absorption fine structure
spectroscopy. The results reveal the presence of two SrâO shells
in the Sr-doped NaTaO<sub>3</sub> photocatalysts. The small shell
with an unexpectedly short SrâO bond length of 1.96 Ă
corresponds to SrO<sub>6</sub> octahedra embedded in the corner-shared
network of TaO<sub>6</sub> octahedra. The other shell with a SrâO
bond length of 2.60 Ă
corresponds to SrO<sub>12</sub> 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<sub>3</sub>âSrÂ(Sr<sub>1/3</sub>Ta<sub>2/3</sub>)ÂO<sub>3</sub> solid solution to accommodate the two SrâO shells
in NaTaO<sub>3</sub> 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