Plasmon-Enhanced Photocatalytic CO₂ Conversion within Metal–Organic Frameworks under Visible Light

Materials development for artificial photosynthesis, in particular, CO₂ reduction, has been under extensive efforts, ranging from inorganic semiconductors to molecular complexes. In this report, we demonstrate a metal-organic framework (MOF)-coated nanoparticle photocatalyst with enhanced CO₂ reduction activity and stability, which stems from having two different functional units for activity enhancement and catalytic stability combined together as a single construct. Covalently attaching a CO₂-to-CO conversion photocatalyst Re^I(CO)₃(BPYDC)Cl, BPYDC = 2,2'-bipyridine-5,5'-dicarboxylate, to a zirconium MOF, UiO-67 (Re_n-MOF), prevents dimerization leading to deactivation. By systematically controlling its density in the framework (n = 0, 1, 2, 3, 5, 11, 16, and 24 complexes per unit cell), the highest photocatalytic activity was found for Re₃-MOF. Structural analysis of Re_n-MOFs suggests that a fine balance of proximity between photoactive centers is needed for cooperatively enhanced photocatalytic activity, where an optimum number of Re complexes per unit cell should reach the highest activity. Based on the structure-activity correlation of Re_n-MOFs, Re₃-MOF was coated onto Ag nanocubes (Ag⊂Re₃-MOF), which spatially confined photoactive Re centers to the intensified near-surface electric fields at the surface of Ag nanocubes, resulting in a 7-fold enhancement of CO₂-to-CO conversion under visible light with long-term stability maintained up to 48 h.