Light‐Programmed Localized Supersaturation Enables Ambient Aqueous Synthesis of Highly Crystalline Metal–Organic Frameworks

ABSTRACT Crystalline materials, particularly metal–organic frameworks (MOFs), are essential for applications in catalysis, gas storage, and energy conversion, but conventional crystallization methods typically require high precursor concentrations or elevated temperatures, such as those provided by hydrothermal or solvothermal approaches. In this study, a self‐assembled nanoreactor was developed to achieve localized supersaturation via the photooxidation of Cu 2 O nanoparticles under visible light irradiation. Unlike conventional methods, this strategy enables HKUST‐1 crystallization at ambient temperature in water without external heating. By modulating the Cu 2+ release response and ligand accessibility, we propose a two‐step process in which a metastable supersaturation regime forms a shell, followed by a labile supersaturation regime that favors predominantly inward crystal growth at or near the core‐shell interface. Transport perturbations, shell characterization, and a limiting reaction–diffusion analysis support this mechanistic interpretation, while local activities and the microscopic growth trajectory were not measured directly. X‐ray diffraction, Raman spectroscopy, and Brunauer–Emmett–Teller analyses confirmed the crystallinity and porosity of the resulting HKUST‐1. By generating localized supersaturation while maintaining dilute bulk conditions, this approach provides a programmable reaction environment for nucleation and growth, thereby opening a mild aqueous route to advanced functional materials.