Structural and photocatalytic properties of perovskite-type (La,Ca)Ti(O,N)3 prepared from A-site deficient precursors

Abstract

Two series of oxide precursors for perovskite-type (La,Ca)Ti(O,N)₃ were prepared by adding Ca²⁺ to A-site deficient LaTiO_3.5 heterogeneously (Ca²⁺-backfilling) or by substituting Ca²⁺ for La³⁺ in stoichiometric LaTiO_3.5 homogeneously (Ca²⁺-substitution). Activity of the resultant (La,Ca)Ti(O,N)₃ for photocatalytic O₂ evolution was tested in the presence of an electron acceptor (Ag⁺) and a superior activity of Ca²⁺-backfilled LaTiO₂N compared to Ca²⁺-substituted LaTiO₂N and unsubstituted LaTiO₂N was demonstrated. X-ray diffraction patterns of the precursor oxides revealed a higher degree of crystallinity in La_1−xTiO_3.5−3x/2 compared to La_1−xCa_xTiO_3.5−x/2. The higher crystallinity in La_1−xTiO_3.5−3x/2 resulted in lower Ti³⁺ defect formation during the ammonolysis reaction. This was evidenced by the lower background absorption of the diffuse reflectance spectra in Ca²⁺-backfilled compared to Ca²⁺-substituted LaTiO₂N. Structural refinement of the diffraction patterns revealed growing crystal sizes with the Ca²⁺ content, which was more pronounced for Ca²⁺-backfilled than for Ca²⁺-substituted LaTiO₂N. Thus, the enhanced photocatalytic activity of Ca²⁺-backfilled LaTiO₂N was related to the quality of the precursor oxides, which influenced the defect concentrations and crystallite sizes of the resulting oxynitrides.