Changes in the charge-transfer properties of TP-CYANO-COF photocatalysts and enhancement of hydrogen generation activity through control of synthesis conditions

Hydrogen energy has attracted significant attention as a clean and sustainable alternative to fossil fuels. Photocatalytic hydrogen production using solar energy is considered a promising approach because of its low environmental impact. Covalent Organic Frameworks (COFs) are attractive photocatalysts due to their tunable structures, large surface area, and high stability. In particular, introducing cyano groups into COFs can improve visible-light absorption and charge-transfer properties. However, the effects of synthesis conditions on the photocatalytic performance of TP-CYANO-COF remain unclear. In this study, TP-CYANO-COF photocatalysts were synthesized under different temperatures and reaction times to optimize synthesis conditions and investigate their effects on charge-transfer behavior and hydrogen evolution activity.

Preparation of photocatalyst
TP-CYANO-COF was synthesized using TP and BD-CYANO with mixed solvents and acetic acid under various temperatures and reaction times. For photocatalytic evaluation, the catalyst was dispersed in an aqueous solution containing sodium ascorbate and hexachloroplatinic acid, purged with nitrogen, and irradiated with visible light (λ ≥ 420 nm). Hydrogen production was analyzed by gas chromatography.

Results and Conclusion
The photocatalytic hydrogen evolution activity strongly depended on synthesis temperature and reaction time. The sample synthesized at 180 °C for 72 h showed the highest hydrogen evolution rate of 4510 μmol g⁻¹ h⁻¹. Photoluminescence results indicated reduced electron–hole recombination, while diffuse reflectance spectra showed enhanced visible-light absorption with a slight band-gap reduction. Transient photocurrent measurements also confirmed improved charge separation efficiency. These results suggest that optimizing synthesis conditions effectively enhances charge-transfer properties and photocatalytic activity, whereas excessively high temperatures may cause structural deterioration and reduced performance.