Interfacial Coordination and Dimensional Confinement Enable Continuous Thin MOF-on-MOF Membranes for Ion Sieving.

Metal-organic frameworks (MOFs) with crystallographically defined pores and chemically programmable internal structures are attractive candidates for regulating ion transport at the molecular scale. Yet, it is still difficult to grow MOF-on-MOF structures as continuous thin membranes because the secondary phase usually forms either thick coatings or discontinuous particles, both of which hinder ion transport. In this work, we found that Cu-O-Zr coordination at the interface can guide UiO-66 to grow continuously on the CuOx-BDC scaffold even under severe lattice mismatch, producing a thin integrated membrane. The resulting architecture also seals an interfacial void, which serves as an electrolyte reservoir during operation, improves access to the internal surface, and helps protect the chemically unstable core. More importantly, the confined overgrowth allows the mismatch-induced tensile strain to be accommodated as elastic lattice distortion in the UiO-66 layer. The resulting strain-expanded pore structure lowers the kinetic barrier for nanofluidic ion transport through field-assisted partial dehydration. Consequently, the membrane achieves a high Na+ adsorption capacity of 1.36 g m-2 with 95.75% retention after 500 cycles. These results demonstrate the feasibility of redirecting MOF-on-MOF growth from discontinuous particulate deposition to continuous functional membranes for ion sieving.