Establishing Efficient Cobalt-Based Catalytic Sites for Oxygen Evolution on a Ta3N5 Photocatalyst

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• 作者：Ela Nurlaela ; Samy Ould-Chikh ; Isabelle Llorens ; Jean-Louis Hazemann ; Kazuhiro Takanabe
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：August 25, 2015
• 年：2015
• 卷：27
• 期：16
• 页码：5685-5694
• 全文大小：514K
• ISSN：1520-5002

文摘

In a photocatalytic suspension system with a powder semiconductor, the interface between the photocatalyst semiconductor and catalyst should be constructed to minimize resistance for charge transfer of excited carriers. This study demonstrates an in-depth understanding of pretreatment effects on the photocatalytic O₂ evolution reaction (OER) activity of visible-light-responsive Ta₃N₅ decorated with CoO_x nanoparticles. The CoO_x/Ta₃N₅ sample was synthesized by impregnation followed by sequential heat treatments under NH₃ flow and air flow at various temperatures. Various characterization techniques, including X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), scanning transmission electron microscopy (STEM), and X-ray photoelectron spectroscopy (XPS), were used to clarify the state and role of cobalt. No improvement in photocatalytic activity for OER over the bare Ta₃N₅ was observed for the as-impregnated CoO_x/Ta₃N₅, likely because of insufficient contact between CoO_x and Ta₃N₅. When the sample was treated in NH₃ at high temperature, a substantial improvement in the photocatalytic activity was observed. After NH₃ treatment at 700 掳C, the Co⁰鈥揅oO_x core鈥搒hell agglomerated cobalt structure was identified by XAS and STEM. No metallic cobalt species was evident after the photocatalytic OER, indicating that the metallic cobalt itself is not essential for the reaction. Accordingly, mild oxidation (200 掳C) of the NH₃-treated CoO_x/Ta₃N₅ sample enhanced photocatalytic OER activity. Oxidation at higher temperatures drastically eliminated the photocatalytic activity, most likely because of unfavorable Ta₃N₅ oxidation. These results suggest that the intimate contact between cobalt species and Ta₃N₅ facilitated at high temperature is beneficial to enhancing hole transport and that the cobalt oxide provides electrocatalytic sites for OER.