Ultrathin Hf-MOF with Dinitrogen Chelating Sites Stabilizing and Inducing Generation of Single-Rod Cs3Bi2Br9 Nanocrystals for Efficient Photocatalytic CO2 Reduction

The ordered growth of semiconductor quantum dots (QDs) in confined environments remains a critical challenge in photocatalysis. Herein, Cs₃Bi₂Br₉ (CBB) QDs were covalently anchored as single-rod nanocrystals (SRNCs) within Hf-based metal-organic framework (MOF) nanosheets (Hf₁₂-bpy, H₂bpy = 2,2'-bipyridine-5,5'-dicarboxylic acid), forming a series of host-guest photocatalysts CBB@Hf₁₂-bpy. By modulation of the thickness of MOF nanosheets, the length of CBB SRNCs was effectively shortened to 18 nm, exhibiting strong quantum confinement effects. Mechanistic studies reveal that the bpy-CBB dual-nitrogen chelation effect induces a discrete distribution of CBB within Hf₁₂-bpy pores, and the well-matched interlayer spacing of H₂bpy (7.95 Å ≈ d_Bi···Bi) guides the anisotropic growth of CBB along the [001] direction into SRNCs. Such a long-range-ordered SRNC architecture significantly improves the bulk-to-surface charge separation efficiency, enabling ultrafast electron supply (average charge excitation rate: 5.320 mV). Additionally, the chelated N-Bi-N moieties work as covalent electron-transfer bridges to markedly reduce charge-transfer resistance (7.75 Ω) and interfacial charge-transfer barriers (100.5 meV), accelerating interfacial charge migration kinetics. These synergistic advantages endow CBB@Hf₁₂-bpy(18 nm) with an exceptional electron accumulation rate (1.54 g^-1·min^-1) and record-breaking CO₂-to-CO conversion efficiency (15,982.1 μmol·g^-1·h^-1) with 100% selectivity. The durability, stability, and potential photocatalytic mechanisms were also systematically investigated.