GoKawiilPreparation of GO sheets with controllable lateral sizes
Aqueous dispersions of GO with broad size distributions, synthesized by means of a modified Hummer’s method37, were purchased from Nanjing XFNANO Materials Tech Co., Ltd. Temperature-controlled fractionated centrifugation was used to separate the polydisperse GO dispersions into several distinct size groups. To measure the lateral size of the GO nanosheets, droplets of the GO dispersion were deposited on silicon wafers and air dried. Lateral size distributions were determined from atomic force microscopy images (Supplementary Fig. 17) by analysing more than 500 nanosheets per sample. The average lateral size of irregularly shaped GO nanosheets was calculated as the mean of their largest and smallest transverse widths. The sorted GO fractions were immersed in water and stored at 4 °C to prevent aggregation. For this study, the roughly 2-μm GO size fraction was selected on the basis of its optimal performance characteristics (Supplementary Fig. 18).
Preparation of GO with varying oxidation degrees
GOs with different oxidation levels were prepared using a modified Hummer’s method37. Four GO samples with varying degrees of oxidation, covering a wide range of oxygen content, were synthesized by adjusting the amount of oxidant (KMnO 4 ) and oxidation time during the preparation process. It was observed that freshly prepared GO samples showed non-uniform sizes, with smaller GO sheets generally having higher oxidation degree. A patchwork GOM with fewer oxygen-containing functional groups showed higher permeability at the same interlayer spacing, whereas membranes with strong binding pillars required appropriate oxidation to provide robust linking and support, preventing swelling and collapse. On the basis of these characteristics, membranes with 25.4% oxygen content oxidation showed optimal performance in composite GOMs, compared with the typical 30.6% oxygen content (Supplementary Fig. 19). Furthermore, the GOs with varying oxidation degrees were subjected to fractionated centrifugation before membrane preparation to ensure optimal performance in composite GOMs.
Preparation of GOMs
The prepared size-graded GO sheets were diluted to a concentration of 2 μg ml−1. The diluted suspension was then filtered under a vacuum pressure of roughly 0.01–0.02 MPa through microporous substrates (cellulose, nylon, polyethersulfone or anodized aluminium oxide and so on) or alumina ceramic tubes, all with a pore size of roughly 0.22 μm, to form GO films. In the experiment, the operation with lower concentration and lower vacuum pressure provided sufficient time for the GO sheets to spread uniformly, resulting in flat and consistent membrane. The GOMs were subsequently freeze dried using a freeze dryer (LGJ-10E, Sihuankeyi Co., Ltd).
Preparation of the composite GOMs
The freeze-dried GOMs were immersed in aqueous solutions of reactive molecules (for example, DA) at various concentrations (0.03 mM, 0.06 mM, 0.13 mM, 0.33 mM, 0.65 mM and 1.3 mM, pH 7.5) for 1–3 min (Supplementary Fig. 1). The solution volume was carefully controlled to infiltrate the membrane sufficiently while preventing rapid swelling into disordered, stacked GO sheets. The membranes were then frozen at temperatures below −50 °C. Unlike traditional immersion strategies that result in uncontrolled swelling, the frozen surrounding bulk ice extracted some liquid water in confinement from the GOM interlayers, thereby minimizing the initial interlayer spacing. This process was followed by annealing below the bulk ice freezing point (for example, −10 °C) for various annealing times to achieve the desired GOM interlayer spacing. Then, the GOMs were cooled to below −30 °C for the reaction process. The GOMs were subsequently freeze dried to obtain the PDA-crosslinked GOM (composite GOM).
Building on this approach, the mixed solution of benzene-1,2,4,5-tetramine (P) and benzene-1,3,5-tricarboxylic acid (A) in a 3:2 ratio was used to fabricate the poly-PA-crosslinked GOM. During the reaction, the P and A molecules underwent copolymerization. For freeze-dried GOM treated with aqueous acrylamide solutions, the membrane was further exposed to ultraviolet light for 1 h, yielding poly-AM-crosslinked GOM membranes (Supplementary Fig. 6). In our approach, we introduced this mechanism to achieve the separation of reactive molecular assembly and subsequent reactions. It prevented the imprecise spacing regulation and channel blockage from excessive reacted molecules to flux reduction, which can possibly arise from simultaneous assembly and reaction or from relying solely on chemical reactions for interlayer stabilization6.
PVA-modified anodic aluminium oxide ceramic tube surface
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