Single-Crystal Model of Highly Efficient Water-Splitting Photocatalysts: A KTaO₃ Wafer Doped with Calcium Cations

Alkali tantalates, NaTaO3 and KTaO3, are known as highly efficient semiconductor photocatalysts for the overall water-splitting reaction when properly doped with foreign metal cations. Characterizing surface reaction sites is needed for further development. In this study, (001)-oriented KTaO3 wafers were doped with Ca cations thorough a solid-state reaction to provide platforms for surface science studies. X-ray diffraction showed a Ca-rich perovskite-structured surface layer that covers the Ca-poor or pristine KTaO3 substrate. The lattice of the surface layer was contracted and rotated relative to the substrate lattice. The heteroepitaxial junction of the surface layer on the substrate mimicked the core–shell structure that is found in real photocatalyst particles. Nanometer-scale islands spontaneously appeared on the surface to correct the lattice mismatch across the surface–substrate junction. X-ray fluorescence holography was applied to determine the local, atom-scale structure around Ca cations in the host lattice. Atom distributions reconstructed from Ca Kβ fluorescence holograms suggested that K and Ta cations were simultaneously exchanged with Ca cations. Local lattice deformation was quantitatively deduced around the Ca cations occupying the two different sites. The major features of real photocatalysts—heteroepitaxial surface–bulk junctions, surface restructuring to correct lattice mismatches, and simultaneous cation exchange—were reproduced in the doped wafers. The ability of X-ray fluorescence holography to determine a local structure around doping cations was also demonstrated.