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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 21 of 35

Figure 9. Regulating surface physicochemical properties of nanochannels. (A) Intercalating metal ions. [a] A snapshot of the ab initio
molecular dynamics simulation at 27 ps. [b] The interaction energy between hydrated cations and graphene oxide sheets [labeled
cation-(H O) @GO] and the hydration energy of the cation [labeled cation-(H O) ]. [c–e] The most stable optimized geometries of
2 6 2 6
+
+
cation-(H O) @GO clusters from density functional theory computation, where the cations are Na , K , and Li + [101] . Copyright 2017,
2
6
Springer Nature. (B) A GO polymer cross-linked network composite membrane for forward osmosis desalination [158] . Copyright 2017,
The Royal Society of Chemistry. (C) Restacked Ti C T -based membrane by introducing reduced GO as the spacer [160] . Copyright 2019,
2 x
3
Springer Nature. (D) Oxidation process from graphite to graphene oxide [161] . Copyright 2021, MDPI.
Modifying the surface of 2D materials with a small number of metal ions, such as Mg and Ca , is
2+
2+
considered an effective strategy to improve the membrane performance significantly. This enhancement is
mainly due to the electrostatic bonding between metal ions and functional groups on the surface of 2D
materials . To further understand this mechanism, Chen et al. modified the GO membrane with hydrated
[156]
Na ions and found, through DFT calculations, that the hydrated ions were adsorbed through hydrogen
+
bonds in the region where the oxidized groups and aromatic rings coexisted, effectively adjusting and fixing
the interlayer distance . The interlayer distance is proportional to the hydration radius of the
[101]
+
+
corresponding Li > Na > K class ions [Figure 9A]. Ding et al. effectively reduced the d-spacing to
+
~ 1.5 nm. They effectively suppressed the swelling effect by intercalating Al into the Ti C T MXene film
3+
2 x
3
combined with abundant surface termini (such as ¼O, eOH, and eF functional groups) within the
nanosheets . Cross-linking 2D materials by organic molecules is another effective strategy to tune and fix
[157]
the interlayer distance in layered films. Epoxy, a commonly used chemical raw material, is employed to
modify the surface of 2D-material nanosheets. This strategy successfully prevented the swelling of GO and
produced nearly 97% NaCl repulsion . Hung et al. cross-linked GO with EDA monomer to make the
[99]