Synthesis and photocatalytic properties of tetragonal tungsten bronze type oxynitrides

Synthesis of a series of oxynitride Ba_3-xLa_xTa₅O_14−xN_1+x(x = 0–1.5) with a tetragonal tungsten bronze (TTB) structure was examined using moist-NH₃and 5% NH₃-N₂as nitriding atmosphere. The samples with x = 0–1 were successfully obtained as phase pure TTB oxynitride whereas the x = 1.5 samples gave a mixture of TTB oxynitride, Ta₃N₅, and LaTaO₄. Absorption of TTB oxynitrides in visible region was enhanced as increasing the content of nitrogen. TTB oxynitrides exhibited both of H₂and O₂evolution under visible light in the presence of methanol and silver ions, respectively, as sacrificial reagents. Nitrogen poor surface in the samples synthesized under moist-NH₃atmosphere was modified by annealing treatment using 5% NH₃, resulting in improvement of photocatalytic activity for H₂evolution. Evaluation using monochromatic light revealed that the apparent quantum efficiency of Ba₂LaTa₅O₁₃N₂was higher than that of Ba₃Ta₅O₁₄N.