Dramatic Change of Morphological, Photophysical, and Photocatalytic H₂ Evolution Properties of C₃N₄ Materials by the Removal of Carbon Impurities

Three-dimensional (3D) carbon nitrides (C3N4) show excellent photocatalytic performance for the hydrogen evolution reaction (HER) under visible light irradiation. 3D C3N4 can be produced by the high-temperature polycondensation of small organic compounds, often generating impurities of carbon-based residues for a variety of experimental reasons. The presence of these impurities has resulted in a misunderstanding of the intrinsic properties of C3N4 materials. Therefore, most studies take this impurity for granted and do not attempt to control it. This paper reports that the HNO3 treatment not only removes the inevitable impurity during carbon nitride synthesis, enabling to analyze the intrinsic properties such as morphological, photophysical, and electrochemical characterizations, but also increases dramatically the photocatalytic activity for the HER under visible light irradiation. As a result, the C3N4 material, which is free from C impurity, has favorable crystallinity, surface areas, porosity, electrochemical resistance, and photoluminescence lifetimes for the photocatalytic HER despite the diminishing ability for absorbing visible light. Furthermore, the HNO3-treated C3N4 material shows excellent thermal stability and stable photocatalytic HER activity.