KTaO3 Wafers Doped with Sr or La Cations for Modeling Water-Splitting Photocatalysts: 3D Atom Imaging around Doping Cations

Potassium tantalate (KTaO₃) is a highly efficient semiconductor photocatalyst for the overall water-splitting reaction. Doping a semiconductor photocatalyst with foreign metal cations typically increases the apparent quantum yield of the splitting reaction. In this study, we constructed a single-crystalline model of cation-doped photocatalysts, which would be suitable for future investigation with advanced surface-sensitive methods. Centimeter-sized (001)-oriented KTaO₃ wafers were doped with Sr or La cations in KCl flux. X-ray diffraction (XRD) revealed Sr- and La-containing perovskite-structured layers epitaxially covering bulk KTaO₃. On the Sr-doped wafer, the surface-layer lattice was expanded by 2% relative to the bulk lattice. X-ray fluorescence holography (XFH) was employed to determine the 3D short-range ordered structure around the K and Sr cations. Holograms obtained with Sr Kα fluorescence confirmed the simultaneous settling of Sr cations in the A and B sites. The placement of the Sr cations in B sites was supported by the TaO6 breathing vibration observed in Raman scattering. These experimental results suggested that a KTaO₃–Sr(Sr₁/₃Ta₂/₃)O₃ solid solution is generated by doping. Two La-containing phases, one with lattice contraction by 2% and the other with expansion by 0.4%, were recognized on the La-doped wafer. La Lα fluorescence holograms indicated a complex manner of doping. The obtained atom distribution around La cations was interpreted by the simultaneous La cation occupation at the A-site, B-site, and an interstitial site. Local lattice deformation was quantitatively deduced around the La cations occupying the interstitial site. Element composition determined by X-ray photoelectron spectroscopy revealed the enrichment of doping elements on the wafer surface. Nanometer-scale topography observed by atomic force microscopy suggested that doping concentrations should be optimized to provide flat, crystalline surfaces.