Relationship between cation arrangement and photocatalytic activity for Sr-Al-Nb-O double perovskite

The relationship between the photocatalytic activity and the arrangement of metal cations was investigated with Sr-Al-Nb-O double perovskite (SAN) synthesized at 1400 °C for various calcination times using a solid state reaction. Transmission electron microscopic observation revealed that SAN particles had a domain structure of completely B-site ordered (Sr ₂AlNbO₆) and disordered (SrAl_0.5Nb _0.5O₃) phases. The ordered phase fraction was determined using a newly proposed mixed-phase model for the Rietveld refinement and a method using the relative intensity of the superlattice line of powder X-ray diffraction. It turned out that the mass fraction of the ordered phase in SAN calcined at 1400 °C could be controlled by the calcination time as 33% (10 h), 37% (20 h), 44% (30 h), and 48% (50 h). Photocatalytic activities of SAN for the evolution of H₂ and O₂ respectively from aqueous solutions of methanol and AgF decreased with increasing the calcination time, that is, with increasing the fraction of the ordered phase. These results suggested that the photocatalytic activity of ordered Sr₂AlNbO ₆ should be lower than that of disordered SrAl_0.5Nb _0.5O₃. This is practically the first report to reveal the photocatalytic activity of SAN as well as the effect of cation ordering in oxides on the photocatalytic activity.