Synthesis and photocatalytic properties of tetragonal tungsten bronze type oxynitrides

•TTB oxynitrides are successfully synthesized using moist-NH3.•Photocatalytic property of Ba3-xLaxTa5O14-xN1+x is improved by La-substitution.•Activities of samples synthesized in moist-NH3 are improved by annealing treatment.

Synthesis of a series of oxynitride Ba3-xLaxTa5O14−xN1+x (x = 0-1.5) with a tetragonal tungsten bronze (TTB) structure was examined using moist-NH3 and 5% NH3-N2 as 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 H2 and O2 evolution under visible light in the presence of methanol and silver ions, respectively, as sacrificial reagents. Nitrogen poor surface in the samples synthesized under moist-NH3 atmosphere was modified by annealing treatment using 5% NH3, resulting in improvement of photocatalytic activity for H2 evolution. Evaluation using monochromatic light revealed that the apparent quantum efficiency of Ba2LaTa5O13N2 was higher than that of Ba3Ta5O14N.

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