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Page 16 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

[103]
permeation “gates”, ultimately increasing the permeability of the rGO membrane . However, the pre-
treatment of reduction may lead to the worse dispersion of nanosheets that is not conducive to uniformly
assembling them into well-stacked laminates. Liu et al. prepared a GO membrane beforehand and placed it
[104]
above a hydrogen iodide (HI) solution [Figure 6C] . The HI steam acts as a reducing agent, which reduces
GO and triggers the initial delamination between the rGO membrane and substrate. Fumagalli et al. found
that GO laminates reduced using thermal, HI, and VC reductions are highly impermeable to strong
chemicals and salt solutions, owing to a high degree of graphitization of the laminates, causing
[79]
nanochannels collapse during the reduction process .
The enlarged channel size can be achieved by cross-linking or intercalating cross-linkers, such as specific
molecules, polyelectrolytes, or nanomaterials, to reinforce transport efficiency. The most commonly used
cross-linkers for GO membranes are diamines , including ethylenediamine (EDA), butylenediamine
[105]
(BDA), and phenylenediamine (PDA), which can chemically bond with adjacent GO nanosheets through
condensation and nucleophilic addition reactions [Figure 6D]. The spatial configuration of amine
monomers primarily determines the channel size of cross-linked GO membranes in a dry state, whereas the
bonding strength of amine monomers within the GO laminate likely determines their channel size in a wet
state . Other molecules holding characteristic terminal groups, such as dicarboxylic acids , tannic
[107]
[106]
[110]
[112]
[111]
acid , borate , isophorone diisocyanate , porphyrins , and polybenzimidazole , have also been
[108]
[109]
proved to cross-link 2D-material membranes to address 2D channel size control chemically. Yang et al.
demonstrated the preparation of thiourea covalently linked GO membrane where thiourea bridged GO
laminates periodically through the reactions of amino and thiocarbonyl groups with the functional groups
of GO, leading to mechanically stable and structurally well-defined 2D channels . These molecule cross-
[113]
linkers usually yield covalent bonds through a limited number of active sites. In contrast, polyelectrolytes
possess long polymer chains with abundant functional groups, which can serve as polymer cross-linkers to
be incorporated into 2D laminates through sufficient active sites to produce a dense composite structure.
The commonly used polymer cross-linkers are polyvinyl amine (PEI) [114,115] , polyvinyl alcohol (PVA) ,
[116]
polyethylene glycol (PEG) , and so on. Ran et al. utilized imidazolium-functionalized brominated poly
[117]
(2,6-dimethyl-1,4-phenylene oxide) (Im-PPO) and sulfonated poly (2,6-dimethyl-1,4-phenylene oxide)
(S-PPO) to connect neighboring GO nanosheets via non-covalent π-π, electrostatic, and hydrogen bonding
[118]
interactions . These cross-linking strategies may pave the way to access highly stable and efficient
transport of 2D lamellar membranes.
Besides cross-linking, intercalating nanomaterials of specific sizes is another means to significantly improve
the permeability of 2D-material membranes, as it effectively creates large amounts of broadened pathways
for nanofluidic transport. Carbon-based nanomaterials, metal oxide nanoparticles, MOFs, and COFs are
among the representative intercalated nanomaterials. By embedding carbon nanodots of controllable sizes,
Wang et al. were able to tune the permeability of GO membranes . Han et al. expanded the interlayer
[119]
spacing of GO membranes by intercalating multi-walled carbon nanotubes (CNTs) [Figure 6E] . Goh et
[120]
al. further proposed that intercalating of CNTs of different diameters can effectively inhibit the restacking
(or aggregation) of GO nanosheets and thus create plenty of nanochannels for water transport while
retaining the molecular sieving capability of the ensuing membranes. Moreover, CNTs can also act as
anchors to interact and interconnect with the adjacent GO nanosheets due to the perfect compatibility of
carbon-based materials to reinforce the membrane stability . Intercalation of metal oxide nanoparticles is
[121]
another general and scalable approach to expand 2D nanochannels significantly. Zhang et al. prepared
nanoparticles@GO membranes using a simple in-situ solvothermal synthesis method, where size- and
[122]
density-controllable nanoparticles were uniformly grown on GO nanosheets through coordination .
Compared to the membrane produced by filtering GO solutions mixed directly with nanoparticles, the in-