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

Abstract

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.

Photocatalysis and perovskite oxide-based materials: a remedy for a clean and sustainable future

“…The reported evolution of the hydrogen in the presence of Ag as a dopant in KTaO 3 is 2072 μmol g −1 h −1 . 240–243 The lanthanum and ferrite-based perovskite material are also reported. 244 Table 3 lists various perovskite materials used for water splitting.…”

Local Structure of the Impurity Site in Nd:LaF3 by X‐Ray Fluorescence Holography

“…Furthermore, the good agreement between experimental data for the Nd and La emitters and the computationally generated model demonstrate that no significant lattice distortions take place around the dopant site. In the light of previous XFH characterizations of the local structure around dopant sites (examples are given in previous studies [ 5–9 ] ), this is a rather unusual situation and may be explained by the stability of the LaF 3 lattice and the same oxidization state of the lanthanides (nominally Nd 3+ and La 3+ ).…”

“…Prominent recent examples where XFH was used to investigate the impurity site are the characterization of lattice distortions around Ga impurity atoms in InSb single crystals, [ 4 ] the determination of the spontaneous formation of suboxidic coordination around Co in ferromagnetic rutile, [ 5 ] the determination of different dopant sites in topological insulators like Mn:Bi 2 Te 3 [ 6 ] and In:Bi 2 Se 3 , [ 7 ] the atomic environment around Ca cations in a doped KTaO 3 wafer, [ 8 ] or studying the effect of heavy element doping of Ta in a Fe 2 VAl Heusler‐type thermoelectric material. [ 9 ] XFH is, however, not limited to impurities, but can also be useful for the investigation of bulk samples, in particular for chalcogenide superconductors [ 10 ] and also in general for determining positional fluctuations of atoms.…”

X-ray Fluorescence Holography Measurement of Oxynitride Thin Film of CaTaO₂N

“…In the previous XAFS study on BaCa 0.03 Ti 0.97 O 2.97 [28], it was found that the doped Ca ion can occupy the B site despite having the same valence as that of the Ba 2+ ion at the A sites. In addition, the XFH measurements on Ca-doped KaTaO 3 indicated that both A and B sites are substituted by Ca [29]. A similar situation might occur in the case of perovskite oxynitrides CaTaO 2 N. Taking such a structural feature into account, we use the chemical formula (Ca 1−x Ta x )(Ta 1−x Ca x )O 2 N, where x is the fraction ratio of Ca at the B site and that of Ta at A site.…”