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

Abstract

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 (C 3 N 4, 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 C 3 N 4 to form cyameluric acid and further release nitrates into the aqueous environment. The ratios of released nitrogen from nanosheet-structured C 3 N 4 and bulk C 3 N 4 that finally exists in the form of NO 3 – 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 C 3 N 4 . On a positive note, in the presence of organic pollutants which compete against C 3 N 4 for • OH, the C 3 N 4 decomposition has been completely or partially blocked; therefore, the stability of C 3 N 4 under practical working conditions has been obviously preserved. This work supplements the missing knowledge of the chemical instability of C 3 N 4 toward • OH and calls for attention to the potential deactivation and secondary pollution of catalysts in • OH-involved water treatment processes.