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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 15 of 35

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Figure 6. Regulating the size of nanochannels. (A) Physical confinement. (B) The stable, optimized geometries of Na -(H O) @GO
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clusters from density functional theory computation [101] . Copyright 2017, Springer Nature. (C) The fabrication procedure of free-
standing rGO membranes using reduction control [104] . Copyright 2015, John Wiley & Sons, Inc. (D) Original GO membrane (left) and
the composite GO-framework prepared by p-phenylenediamine cross-linking GO membrane (right) [105] . Copyright 2014, American
Chemical Society. (E) Graphene/carbon nanotube composite membranes are prepared by assembling refluxed GO (rGO) and multi-
walled carbon nanotubes [120] . Copyright 2015, American Chemical Society.
Apart from channel size regulation based on external forces, cationic control is a representative method of
channel size regulation based on internal forces. Chen et al. demonstrated cationic control of the interlayer
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spacing of GO membranes with angstrom precision using K , Na , Li , Ca , or Mg ions [Figure 6B] . The
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[101]
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confinement of interlayer spacing is mainly due to the interaction between hydrated cations and aromatic
rings (cation-π interactions) on the GO nanosheet and the interaction between hydrated cations and the
oxygenated groups on the GO nanosheet. As a result, the interlayer spacing controlled by one type of cation
can efficiently and selectively exclude other cations with larger hydrated volumes. Especially the K -
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controlled GO membranes can even reject K itself owing to the comparable interaction energy between K
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and GO nanosheets concerning the dehydration energy of K .
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Reduction is another effective method to control the small channel size for the solvated 2D laminates. These
2D nanosheets possess abundant functional oxygenated groups that open the interlayer spacing between
adjacent nanosheets and allow solvent molecules to intercalate into 2D laminates. Partially removing these
oxygenated functional groups through various thermal or chemical reductions can enhance interlayer π-π
interactions, thereby producing narrowed and stabilized nanochannels. Qiu et al. reported that
[102]
hydrothermal treatment could readily control the reduction of GO nanosheets in water . Meanwhile, Han
et al. obtained an rGO dispersion by base-refluxing to generate moderate holes and create a large number of