We attribute the ultrahigh ion sieving performance to the vertical water channels in the AD-rGO membrane assembled on the substrate template, where AD-rGO flakes with suitable lateral sizes can be inserted and stacked within the substrate. By preparing the AD-rGO flakes with an average lateral size to approximately 450 nm (Fig. 2a), it is convenient to insert these flakes into the large pore side (approximately 2 µm) of the MCE substrate without leaking from the other small pore side (approximately 0.2 µm), allowing stable stacking of the AD-rGO flakes within the substrate. Such a simple structure greatly shortens the water pathway and increase the density of water channels (Fig. 1a).
In addition, compared with pure GO (content of oxygen-containing groups > 30%)26,27, the AD-rGO prepared by the amino-hydrothermal method has a low content of oxygen-containing groups (~ 22%) and additional C-N groups (~ 4%) as analyzed by the C1s X-ray photoelectron spectrum (Fig. 2b and Table S2). The low content of oxygen-containing groups will decreases transport resistance due to the nearly frictionless flow on graphene surface11 and improves water permeance. The cation-π interaction can be further enhanced due to the electrical negativity of doped nitrogen 28–31, which is beneficial for the rejection of heavy metals.
The effect of salt concentration on the ion sieving performance was further investigated, using CuSO4 as an example. 150 mL of CuSO4 solutions with a concentration of 25 ~ 200 mg L− 1 was added to the feed side and filtered through the membranes under a pressure of 1 bar. As shown in Fig. 3a, the water permeance gradually decreased from 1880 L m− 2 h− 1 bar − 1 to 899 L m− 2 h− 1 bar − 1 as the concentration of CuSO4 solution increased from 25 mg L− 1 to 200 mg L− 1, while the rejection rate remained stable at 99.9%. The results show that higher salt concentration can reduce the water permeance. The strong hydrated cation–π interactions between Cu2+ and GO flakes that reduce the interlayer spacing. The higher salt concentration will further reduce the interlayer spacing, causing enrichment and blockage of salt ions in interlayer spacing, leading to lower water permeance affected by the concentration polarization32,33. The water permeance of 899 L m− 2 h− 1 bar − 1 with a rejection rate of 99.9% still maintains a high efficiency, demonstrating the robust ion sieving performance of the AD-rGO membrane.
The water permeance is related to the amount of AD-rGO used for membranes performance. A gradient screening of the amount of AD-rGO have been performed (Fig. 3b). Notably, the membrane prepared with a mass of ~ 0.4 mg of AD-rGO reached ultrahigh water permeance of 2535 L m− 2 h− 1 bar − 1 with a sufficient high rejection rate of 99.9% for 50 mg L− 1 CuSO4 solutions. With a lower amount of AD-rGO, the membrane still maintains excellent ion sieving performance, which is superior to the typical layered membranes.
Furthermore, the membranes exhibit excellent stability during long-term filtration experiments. Taking a 50 mg L− 1 CuSO4 solution as the initial feed liquid, the filtrate continuously flows through the membrane and was collected every 35 mL for a total of 16 cycles using semi-automatic vacuum filtration method (See Supplementary Information section 3). As shown in Fig. 3c, the AD-rGO membrane exhibited high stability in both the water permeance and rejection rate during continuous operation.
The mechanical stability of AD-rGO membrane was also tested by ultrasonic treatment at 40 KHz for 100 minutes in aqueous solutions. Compared with the rapid breakdown and dissolution of the GO membrane under ultrasonic treatment, the AD-rGO membrane remained stable for over 100 minutes, indicating that it is a robust membrane (Fig. 3d). The insertion and stacking of AD-rGO flakes inside the asymmetric channels of the MCE substrate, coupled with the hydrophobicity of the flakes based on low oxygen-containing groups, greatly improve the stability of the AD-rGO membrane in aqueous solutions.
In summary, we have successfully achieved the vertical channels of AD-rGO membrane by inserting and stacking flakes in a custom-made substrate with asymmetric pores. The AD-rGO flakes with an average lateral size to approximately 450 nm can easily be inserted into the large pore side (approximately 2 µm) of the MCE substrate without leaking from the other small pore side (approximately 0.2 µm), guaranteeing stable stacking of the AD-rGO flakes within the substrate. This simple structure greatly shortens the water pathway and increase the density of water channels, resulting in the membrane exhibiting unprecedented ion sieving performance. Moreover, the membranes showed good stability in long-term filtration experiments.
Notably, based on the custom-made substrate with asymmetric channels, the vertically asymmetric channels of 2D nanofiltration membranes can be conveniently fabricated and easily extended to other 2D nanofiltration membranes, including MXene, MoS2, and MOFs. The high density vertical transport channels construction reported in this work provides new inspiration for the fields of nanofiltration and separation membranes.