Effect of the Defect Modulator and Ligand Length of Metal-Organic Frameworks on Carbon Dioxide Photoreduction

The nature of defects and organic ligands can fine-tune the absorption energy (E_abs) of metal-organic frameworks (MOFs), which is crucial for photocatalytic reactions; however, the relevant studies are in their infancy. Herein, a series of typical MOFs of the UiO family (UiO-6x-NH₂, x = 8, 7, and 6) with ligands of varied lengths and amino-group-modified defects were synthesized and employed to explore their performance for photocatalytic CO₂ reduction. Sample UiO-66-NH₂-2ABA (2ABA = 3,5-diamino-benzoate) with the shortest dicarboxylate ligand and two amino-group-modified defects exhibits superior photocatalytic activity due to the lowest E_abs. The CO yield photocatalyzed by UiO-66-NH₂-2ABA is 17.5 μmol g^-1 h^-1, which is 2.4 times that of UiO-68-NH₂-BA (BA = benzoate) with the longest ligand and no amino group involved in the defects. Both the experiments and theoretical calculations show that shorter dicarboxylate ligands and more amino groups result in smaller E_abs, which is favorable for photocatalytic reactions. This study provides new insights into boosting the photocatalytic efficiency by modulating the defects and ligands in MOFs.