Abstract
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.
Highlights
The results indicate that the permeations of various salts through these two types of control membranes and the corresponding relative selectivity are insignificant compared with graphene oxide (GO)/layered double hydroxide (LDH)-NS hybrid membranes (Figure 4) and GO membranes (Figure 3)
In summary, we have synthesized a novel hybrid membrane consisting of anionic GO and cationic Co-Al LDH-NS superlattice units by a simple vacuum-assisted assembly method
The vacuum-assisted processing of the hybrid membrane can be finished within 2–3 min, which is extremely promising for the development of industrial-scale production
Summary
The ever-deepening crisis of fresh water shortage and contamination worldwide and the increasingly high demand for chemical refinement have accelerated the pursuit of membrane-based filtration and separation technologies with the merits of being cost-effective, low in energy consumption, easy to operate and, more importantly, not entailing the use of chemicals and minimizing impact on the environment.[1,2,3] For this purpose, the development of membranes with novel separation functions is a key objective.Recently, the rapid advancement of nanostructured materials, especially carbon nanomaterials (for example, carbon nanotubes, graphene and its derivatives), has opened new avenues for fabricating next-generation membranes with precisely designed compositions and structures for achieving desired filtration and separation performances.[4,5,6,7,8,9] Among them, graphene oxide (GO), prepared by the oxidation and exfoliation of graphite, is an excellent graphene derivative with great promise as a two-dimensional (2D) building block for the construction of separation membranes toward practical applications owing to its characteristics, such as ease of large-scale synthesis and functionalization.[10,11,12,13] Because of the liquid-phaseprocessing feasibility, large-area and mechanically robust GO laminates with controllable thicknesses can be readily prepared via a series of liquid-phase membrane assembly techniques, such as drop-casting,[14,15] vacuum filtration[16,17] and spin-coating.[18,19] In view of the structure of GO, various oxygen-containing functional groups (for example, hydroxyl, epoxy, carbonyl and carboxyl) decorate the graphene basal plane and its edges, resulting in numerous sp[2] aromatic clusters isolated within the sp[3] C-O matrix.[20,21] On the basis of this unique structure, nanochannels for selective mass transport can be readily constructed by taking full advantage of the interlayer galleries between adjacent GO nanosheets (NS) within the lamellar membranes. The oxygen functionalities act as pillars to keep a relatively large interlayer spacing while the sp[2] clusters are connected across the stacking layers to form a nanocapillary network through
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