Phase engineering of covalent triazine frameworks to enhance photocatalytic hydrogen evolution performance.

Photocatalytic water splitting for hydrogen production has been considered as an effective approach to address the current energy crisis and environmental challenges. Among all materials for such applications, covalent triazine frameworks (CTFs) are regarded as ideal candidates owing to their conjugated structures with rich aromatic nitrogen atoms, which can provide abundant active sites, suitable bandgaps, good structural tunability, and high chemical stability. Although current research studies have shown that the modification of functional groups in CTFs can adjust the band structure and carrier flow characteristics of photocatalysts, leading to improved performance, the impact of the intrinsic structural characteristics of CTFs (e.g., stacking modes, hydrogen bonding) on their photocatalytic performance remains unclear. In this paper, we demonstrate that the photocatalytic hydrogen evolution performance of CTFs can be enhanced through tuning their stacking arrangement, because the stacking modes affect the bandgaps of materials as well as their carrier separation and transfer efficiency. Under visible light conditions, CTF-AA (AA stacking) exhibited a hydrogen evolution rate of 4691.73 μmol g^-1 h^-1, which is 37.4% higher than that of CTF-AB (AB stacking, 3415.30 μmol g^-1 h^-1). Clearly, the stacking modes significantly influence the cycling stability of CTFs. After eight cycles (over 32 h), CTF-AA maintains its photocatalytic activity and initial performance with a slight decline, while CTF-AB only retains 56.8% of its initial hydrogen evolution rate. Theoretical calculations and physical characterization confirm that the transition of the stacking mode from AB to AA enhances interlayer overlapping, increases the energy level of the lowest unoccupied molecular orbital, and improves the separation and mobility of carriers. These combined factors significantly enhance the photocatalytic performance of CTF-AA. This work offers new insights into the relationship between the photocatalytic performance of CTFs and their stacking patterns, providing new guidelines for designing CTF catalysts with improved activity.