Is C₃N₄ Chemically Stable toward Reactive Oxygen Species in Sunlight-Driven Water Treatment?

Reactive oxygen species (ROS) are key oxidants for the degradation of organic pollutants in sunlight-driven photocatalytic water treatment, but their interaction with the photocatalyst is easily ignored and, hence, is comparatively poorly understood. Here we show that graphitic carbon nitride (C₃N₄, a famous visible-light-responsive photocatalyst) is chemically stable toward ozone and superoxide radical; in contrast, hydroxyl radical (^•OH) can tear the heptazine unit directly from C₃N₄ to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured C₃N₄ and bulk C₃N₄ that finally exists in the form of NO₃^- reach 9.5 and 6.8 mol % in initially ultrapure water, respectively, after 10 h treatment by solar photocatalytic ozonation, which can rapidly generate abundant ^•OH to attack C₃N₄. On a positive note, in the presence of organic pollutants which compete against C₃N₄ for ^•OH, the C₃N₄ decomposition has been completely or partially blocked; therefore, the stability of C₃N₄ under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of C₃N₄ toward ^•OH and calls for attention to the potential deactivation and secondary pollution of catalysts in ^•OH-involved water treatment processes.