Local Environment of Strontium Cations Activating NaTaO3 Photocatalysts

Abstract

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(Sr1/3Ta2/3)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.