Highly selective charge-guided ion transport through a hybrid membrane consisting of anionic graphene oxide and cationic hydroxide nanosheet superlattice units

The development of graphene-based functional membranes with the ability to effectively filter and separate molecules or ions in solutions based on a simple criterion (for example, the size or charge of solutes) is crucial for various engineering-relevant applications, ranging from wastewater purification and reuse to chemical refinement. Here, we report a hybrid membrane consisting of anionic graphene oxide (GO) and cationic Co-Al (or Mg-Al) layered double hydroxide (LDH) nanosheet (NS) superlattice units for high selectivity charge-guided ion transport. The hybrid membrane possesses a series of characteristics, including being easy to access, mechanically robust, freestanding, flexible and semitransparent as well as having a large area. The interlayer spacing of the hybrid membrane is insensitive to humidity variations, ensuring the structural stability in solution-based mass transport applications. The concentration gradient-driven ion transmembrane diffusion experiments show that the cations bearing various valences can be effectively separated strictly according to their charges, independent of the cationic and charge-balancing anionic species. The relative selectivity of the hybrid membranes toward monovalent and trivalent cations is as high as 30, which is not achievable by GO multilayer stacks, LDH-NS multilayer stacks or their bulk-stratified membranes, indicating that a synergistic effect originating from the molecular-level heteroassembly of GO and LDH-NS has a dominant role in the high-performance charge-guided ion filtration and separation processes. These excellent properties of GO/LDH-NS hybrid membranes make them promising candidates in diverse applications, ranging from wastewater treatment and reuse and chemical refinement to biomimetic selective ion transport. A membrane that combines ultrathin carbon films with metallic nanosheets may find use in wastewater treatment and biomimetic ion transport. Graphene oxide is a promising two-dimensional building block for purification technology because it naturally stacks into thin layers on solution processing. By tailoring the interlayer spacing with different functional groups, researchers can create nanochannels that permit water to flow while rejecting molecules above a certain size. Now, Hongwei Zhu of Tsinghua University in Beijing and co-workers have expanded the capabilities of such membranes by sandwiching minerals known as layered double hydroxides (LDHs) into the graphene oxide framework. The cationic metals within LDH nanosheets helped the membrane selectively filter impurities with different charge levels — for example, particles with triple positive charges were rejected at a ratio of 30:1 relative to singly charge ions. We demonstrate our recent progress in the newly emerging and intriguing research field of developing graphene-based functional membranes with the ability to effectively filtrate and separate molecules or ions in solutions based on a simple criterion (for example, the size or charge of solutes) for various engineering-relevant applications ranging from wastewater purification and reuse to chemical refinement.