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.