Instability of graphitic carbon nitride in relation to photocatalysis: A review and concept of oxidative passivation

Abstract

Graphitic carbon nitride (g-C3N4) has been intensively studied for more than 15 years, mostly for photocatalysis, and yet only a few articles have explored its instability during photocatalytic reactions. It can mostly degrade due to reactions with reactive oxygen species generated by the reaction of oxygen and photoinduced electrons. Based on the literature, degradation can be observed by the breakdown of heptazine structures and by the formation of hydroxyl, carbonyl, and carboxyl groups. Such post-synthetic modifications alter the surface of g-C3N4 and affect its physico-chemical and photocatalytic properties. Moreover, this chemical instability enables the intentional introduction of these functional groups together with others, such as methanesulfonyl, phosphoramidate, and methoxybenzoyl, to alter the surface properties of g-C3N4. A passivation concept is proposed, wherein the post-synthetic oxidation of g-C3N4 by reactive oxygen species, especially hydroxyl radicals, creates a self-limiting protective layer. This mechanism, analogous to the passivation of metals by their oxides, could paradoxically enhance long-term durability, offering a new perspective for designing stable g-C3N4 based systems. The nanometric thickness of this protective layer was estimated using a parabolic growth model. While conceptual, this theoretical framework has to be verified by additional experiments.