Origin of extended visible light absorption in nitrogen-doped CuTa${}_2$O${}_6$ perovskites. The role of copper defects

The optical band gap of the semiconductor CuTa${}_2$O${}_6$, synthesised via solid-state reaction, can be greatly reduced by annealing in ammonia, which leads to a significant red-shift of the visible light absorption. Using X-ray photoelectron spectroscopy (XPS), we have shown that this absorption extension does not result from the incorporation of nitrogen, but can be attributed to copper defects formed under the reducing conditions of ammonia treatment. Photocatalytic hydrogen evolution experiments were used to investigate the influence of these defects on the photocatalytic performance. We have further shown that CuTa${}_2$O${}_6$ with similar increased visible light absorption can be prepared by annealing with an organic reducing agent - sodium citrate - in inert gas atmosphere.