Effect of Different Functional Groups on Photocatalytic Hydrogen Evolution in Covalent-Organic Frameworks

Covalent-organic frameworks (COFs) have been recognized as a new type of promising photocatalysts for hydrogen evolution. To investigate how different functional groups attached in the backbone of COFs affect the overall photocatalytic H-2 evolution, for the first time, we selected and synthesized a series of ketoenamine-based COFs with the same host framework as model system. It includes TpPa-COF-X (X=-H, -(CH3)(2), and -NO2) with three different groups attached in the backbone of TpPa-COF. We systematically investigated the differences in morphology, light-absorption intensity and band gap of these 2D COFs. The results of photocatalytic H-2 evolution measurements indicate that the TpPa-COF-(CH3)(2) shows the best activity, while the activity of TpPa-COF-NO2 is relatively low compared to that of other two COFs in the system. Moreover, the separation ability of photogenerated charge was also followed the order of TpPa-COF-(CH3)(2)>TpPa-COF>TpPa-COF-NO2. The best photocatalytic H-2 production performance of TpPa-COF-(CH3)(2) in these systems should be mainly attributed to the better electron-donating ability of -CH3 groups compared to -H or -NO2 group, which result in more efficient charge transferring in the inner of the material. This work demonstrates that reasonably adding electron-donating group in TpPa-COFs can lead to a better photocatalytic H-2 evolution activity, and which is meaningful for further design of efficient COF-based photocatalysts for H-2 evolution.