Enhancing photocatalytic hydrogen generation by achieving long-lasting shallow trapping states in distorted covalent triazine frameworks

Abstract

Short-lived shallow trapping states in photocatalysts created from polymeric semiconductors impedes the effective use of charge carriers, leading to a decrease in the solar-to-hydrogen conversion efficiency of covalent triazine frameworks (CTF). In this study, methylene-modified covalent triazine frameworks (M−CTF) with increased structural distortion were synthesized through a dynamic trimerization reaction of cyano groups using the precursors of 1,4-terephthalonitrile and 1,4-phenylenediacetonitrile. The optimal M−CTF catalyst obtained in this approach showed persistent shallow trapping states through n-π* electronic transitions, leading to a noticeable red-shift in the light absorption edge to 600 nm. Femtosecond transient absorption spectroscopy verified the existence of shallow trapping states with extended lifetimes (1103.8 ps) in M−CTF. The distorted structure promote increased involvement of photo-induced electrons in reducing water to generate H2, leading to a substantial improvement in the ability of M−CTF to catalyze the production of H2. M−CTF sample loaded with Pt cocatalysts exhibited an impressive efficiency of 10.1 mmol·g−1·h −1 in producing H2 via photocatalysis under visible light (λ > 420 nm), marking a tenfold improvement compared to the original CTF. The findings underscore the significance of structural distortion in CTF for achieving long-lasting shallow trapping states, leading to improved photocatalytic efficiency.