Photocatalytic technology, often termed as the “Holy Grail of science”, has gained significant attention for addressing energy shortages and environmental crises. This technology is widely applied in environmental purification due to its ability to absorb solar energy, convert it into chemical energy, promote reactions, all while offering a non-polluting, highly effective means of mineralizing pollutants. However, despite extensive studies, photocatalysis has not yet to meet the practical demands for widespread application. One main key challenge that hinders the effective use of photocatalysis in environmental applications is catalyst deactivation. Therefore, a systematic review of the literature on photocatalysis in environmental applications is presented, with a classification based on photocatalytic materials and pollutant types. The review paper begins by summarizing the primary mechanisms of catalyst deactivation, followed by a summary of regeneration strategies tailored to these mechanisms. Various methods for assessing the degree of photocatalyst deactivation are also discussed, with emphasis on their relationship to photocatalytic reaction mechanisms. The review then highlights recent advances in the development of anti-deactivation photocatalysts and their applications in environmental purification. Finally, the current status of the field, the challenges that remain, and potential directions for future research are outlined to enhance the efficacy of photocatalytic processes in environmental applications.

Crystallinity and crystal structure greatly influence the photocatalytic behavior of photocatalysts. Pristine g-C₃N₄ produced by traditional thermal-induced polycondensation reaction bears low crystallinity and thus poor photoactivity, which originates from the incomplete polymerization of the precursor containing amine groups, abundant hydrogen bonds, and unreacted amino, as well as cyanide functional groups in the skeleton. During photocatalytic process, these residual functional groups often work as electron trap sites, which may hinder the transfer of electrons on the plane, resulting in low photoactivity. Fortunately, crystalline carbon nitride (CCN) was reported as a promising photocatalyst because its increased crystallinity not only reduces the number of carriers recombination centers, but also increases charge conductivity and improves light utilization due to extended π-conjugated systems and delocalized π-electrons. As such, we summarize the recent studies on CCN-based photocatalysts for the photoactivity enhancement. Firstly, the unique structure and properties of CCN materials are presented. Next, the preparation methods and modification strategies are well outlined. We also sum up the applications of CCN-based materials in the environmental purification and energy fields. Finally, this review concerning CNN materials ends with prospects and challenges in the obtainment of high crystallinity by effective techniques, and the deep understanding of photocatalytic mechanism.