Hydroxylated metal–organic-layer nanocages anchoring single atomic cobalt sites for robust photocatalytic CO₂ reduction

Assembly of two-dimensional (2D) metal–organic layers (MOLs) based on the hard and soft acid–base theorem represents an exquisite strategy for the construction of photocatalytic platforms in virtue of the highly exposed active sites, much improved mass transport, and greatly elevated stability. Herein, nanocages composed of MOLs are produced for the first time through a cosolvent approach utilizing zirconium-based UiO-66-(OH)₂ as the structural precursor. To endow the catalytic activity for CO₂ conversion, single atomic Co²⁺ sites are appended to the Zr-oxo nodes of the MOL cages, demonstrating a remarkable CO yield of 7.74 mmol·g⁻¹·h⁻¹ and operational stability of 97.1% product retention after five repeated cycles. Such an outstanding photocatalytic performance is mainly attributed to the unique nanocage morphology comprising enormous 2D nanosheets for augmented Co²⁺ exposure and the abundant surface hydroxyl groups for local CO₂ enrichment. This work underlines the tailoring of both metal–organic framework (MOF) morphology and functionality to boost the turnover rate of photocatalytic CO₂ reduction reaction (CO₂RR).