Electrochemically Engineered Janus Membranes Using CuI and ZIF-8 Nanoparticles as High-Performance Liquid Diodes for Efficient Separation of Oil and Water

We report a sequential electrochemical strategy for fabricating CuI/ZIF-8 Janus membranes featuring asymmetric nanocoatings. These membranes consist of superhydrophilic CuI nanoparticles on one face and hydrophobic ZIF-8 nanocrystals on the opposite, creating a precisely engineered bifunctional architecture. The fabrication process utilizes a protective film to mask alternate sides during stepwise electrodeposition stages. Notably, transmembrane transport during ZIF-8 growth, resulting in rhombic dodecahedral ZIF-8 nanocrystals (∼610 × 710 nm average facet side), modulates the surface topography of the primary CuI coating. The latter features distinct triangular crystalline nanoparticles (∼400 nm per side), which evolve into a textured surface with a nanoscale roughness below 10 nm. This structural evolution significantly enhances the underwater superoleophobicity of the CuI layer. Comprehensive surface, structural, and wetting characterization confirmed the successful asymmetric modification of each face and the formation of the Janus architecture. The resulting membranes achieved exceptional separation efficiency (>99%) for both oil-in-water and water-in-oil mixtures. Remarkably, they exhibited a “liquid diode” effect─directional selectivity in water/oil transport analogous to an electrical diode─achieving complete rectification ( J reverse = 0). Furthermore, the CuI/ZIF-8 coatings displayed superior chemical stability─enduring extreme pH, salinity, and harsh organic solvents─alongside mechanical robustness confirmed by ultrasonication tests and antibiofouling properties. Combining low toxicity with high durability, these membranes represent a high-performance solution for demanding industrial oily wastewater treatment.