Boosting photocatalytic Cr(VI) reduction activities of layered COF through regulating donor-acceptor units and the orientation of imine bonds

In this work, three isoreticular covalent-organic frameworks (COFs) were successfully synthesized at room temperature, namely C1, C2, and C3, respectively. Due to their wide absorption ranges and sufficient photoredox potentials, the constructed COFs exhibited good photocatalytic performance for reducing Cr(VI). Among them, C2 displayed the best photocatalytic performance, which may be attributed to the unique donor-acceptor structure, good light absorption, and excellent photoinduced carriers’ separation and migration efficiency. When the COFs were modified with gold nanoparticles (labeled as Au@C1, Au@C2, and Au@C3), their photocatalytic performance can be further improved. For Au@C2, 98.2% of Cr(VI) can be reduced within 120 min, and its photocatalytic activity can be maintained over five reaction cycles. The structure-activity correlation was studied via both experimental and DFT calculation methods. It is shown that although C3 displayed wider absorption range and smaller energy band gap, its photocatalytic activity was much lower than C2. This may be due to the rapid nonradiative electron-hole recombination in C3, leading to low light utilization. These results indicate that to construct excellent COF photocatalysts, it not only needs to pursue wider absorption range and smaller energy band gap, but also improve the utilization of light energy and photogenerated charge carriers. In addition, the active species and photocatalytic mechanism were also studied.