(387f) Synthesis and Characterization of Nitrogen-Doped Sr4Nb2O9 and Sr4Ta2O9 photocatalyst By Mechanochemical Method

Perovskite-type oxynitride often exhibit intense colors and have potential for applications such as visible light driven photocatalyst. Wu et al. reported that nitrogen-doped Sr₄Nb₂O₉, which was known as perovskite-type oxynitride, demonstrated high oxygen generation efficiency for water splitting under visible light irradiation. However, these oxynitride have been synthesized by the direct nitridation method which required very severe reaction conditions such as high temperature (900-1100℃) and under ammonia flow for long time (e.g., 10 h). Therefore, a safer and more convenient synthesis method is required. As an alternative candidate, mechanochemical method has attracted attention. This method does not need to treat sample at high temperature under ammonia flow for long time to dope nitrogen. Instead, solid materials such as CO(NH₂)₂ can be utilized as nitrogen sources. Therefore, in this study, we attempted to develop a much safer and simple synthesis method of nitrogen-doped Sr₄Nb₂O₉ and Sr₄Ta₂O₉ by using the mechanochemical method.

Initially, Sr₄Nb₂O₉ or Sr₄Ta₂O₉ were synthesized by a solid-state reaction method at 1000℃ for 9 h in air. Afterward, nitrogen-doping to the resulting samples was carried out by the mechanochemical treatment. In the experiment, a planetary ball mill (Frisch, P-7) was used to grind Sr₄Nb₂O₉ or Sr₄Ta₂O₉ and CO(NH₂)₂ (15wt%) at speed of 100-700 rpm for 0.5 to 12 hour, and then the obtained samples were heated at 400℃ in the air for 1 hour to remove the unreacted CO(NH₂)₂.

The results of the X-ray diffraction (XRD) analysis show that after the nitrogen doping treatment using mechanohemical method, although the decrease of the peak intensity was observed, the original structures of Sr₄Nb₂O₉ or Sr₄Ta₂O₉ were maintained. In addition, the absorption spectrum for all the treated samples were shifted to the visible light region. Then, we examined the effect of rotation speed and time during the mechanochemical treatment on crystallinity and visible light absorption of the resulting samples. The results show that there is a trade-off relationship between the degree of crystallinity and the amount of visible light absorption in the change of rotation speed and time.

Subsequently, hydrogen generetion property of the resulting sample was tested under visible light irradiation condition.The results show that Sr₄Nb₂O₉-N prepared under the condition of the rotation speed of 600 rpm for 1 h showed the highest hydrogen generation activity among the prepared samples in this study. The results indicate that the mechanochemical methods is an effective method to synthesize visible light responsive photocatalyst with much safer and simpler condition compared to the conventional methods.