Recent advances in metal-free covalent organic frameworks for photocatalytic applications in energy and environmental fields

Covalent organic frameworks (COFs) are an emerging class of multivacancy organic polymers with a large specific surface area, stable pore size, high crystallinity, and good stability. Moreover, the tailorability of the structure provides a theoretical basis for the preparation of various COF-based materials. However, many COFs rely on metal ions to enhance their photocatalytic performance. Therefore, considering environmental factors and the necessity for developing COF applications, an in-depth review on metal-free COF photocatalysts from an environmental perspective is urgently needed. This review provides a comprehensive overview of the synthesis strategy of COFs, and the applications of metal-free COFs as photocatalysts in environmental fields, including water splitting, CO 2 reduction, pollutant degradation, organic synthesis and environmental remediation. Otherwise, the review summarizes the current and future development of the field and provides perspectives on trends for future challenges and outlooks. A review on metal-free COF photocatalysts from an environmental perspective is presented.