Water splitting into H₂ and O₂ over niobate and titanate photocatalysts with (111) plane-type layered perovskite structure

Photophysical and photocatalytic properties of A₅Nb ₄O₁₅ (A = Sr and Ba), Ba₃LaNb₃O ₁₂, ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba), and La₄Ti₃O₁₂ with layered perovskite structures, in which a plane in parallel with (111) of a simple perovskite structure was exposed at interlayer, were investigated. These oxides were obtained by a polymerizable complex method at 973-1473 K though only A ₅Nb₄O₁₅ (A = Sr and Ba) were prepared by a solid state reaction even at 1673 K. The shapes of these complex metal oxides were plate-like derived from the perovskite layered structure. These band gaps were estimated to be 3.7-4.1 eV from the onsets of diffuse reflection spectra. These oxides showed photoluminescence at 77 K. These oxides loaded with NiO cocatalysts showed activities for water splitting under UV irradiation. NiO _x/BaLa₄Ti₄O₁₅ and NiO _x/Ba₅Nb₄O₁₅ showed the highest activities among the titanates and niobates tested in the present study. NiO_x/BaLa₄Ti₄O₁₅ and NiO _x/Ba₅Nb₄O₁₅ gave 15% and 17% of quantum yields at 270 nm, respectively. Photocatalytic activities of ALa ₄Ti₄O₁₅ (A = Ca, Sr, and Ba) strongly depended on the alkaline earth metal ion. Pt, Au, Ni, and PbO₂ were selectively photodeposited on basal or edge plane of the BaLa₄Ti ₄O₁₅ plate-like powder while these were randomly loaded on CaLa₄Ti₄O₁₅. It was suggested that this difference in the surface property was the one of the important factors affecting photocatalytic ability for ALa₄Ti₄O₁₅ (A = Ca, Sr, and Ba).