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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 7 of 35

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Figure 3. Schematics of the main “bottom-up” synthesis strategies of 2D nanosheets. (A) Chemical vapor deposition . Copyright
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2017, Springer Nature. (B) Physical vapor deposition . Copyright 2021, John Wiley & Sons, Inc. (C) Wet-chemical synthesis .
Copyright 2021, American Chemical Society.
The “top-down” strategy involves the exfoliation of 2D nanosheets from bulk-layered materials by breaking
weak interlayer van der Waals, π-π stacking, and/or hydrogen bonding interactions [Figure 2].
Novoselov et al. pioneeringly achieved a single layer of graphite using the micromechanical cleavage
method [Figure 2A], which enables the preparation of a variety of 2D nanosheets with a clean surface and
high quality on a macroscopic scale [33,34] . However, it is worth noting that this method has the drawback of
low efficiency. Paton et al. took the lead in conducting shear force-assisted [Figure 2B] and sonication-
[35]
assisted [Figure 2C] exfoliations in liquids, where bulk graphene and TMDs crystals can be efficiently
[36]
dispersed in common solvents to obtain large quantities of mono- and few-layer nanosheets. Another
representative shear force-assisted and sonication-assisted mixed work is demonstrated by Peng et al., who
developed a soft-physical process of preparing 2D MOF nanosheets with large lateral sizes by low-energy
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wet ball milling coupled with ultrasonication in methanol/propanol mixtures . Electrochemical (E-Chem)
exfoliation [Figure 2D] is a promising bulk method for producing graphene from graphite, where an applied
voltage drives ionic species to intercalate into graphite, forming gaseous species that expand and exfoliate
individual graphene sheets. Achee et al. reported a method for sustained graphene output within a
permeable and expandable containment system, indicating both high yield (65%) and extraordinarily large
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lateral size ( > 30 μm) of graphene .

The use of specific ions or molecules intercalation [Figure 2E] to weaken interactions within the layers by
exchanging or reacting can further enhance the efficiency of liquid-phase exfoliations, which have been
widely employed for preparing various types of 2D nanosheets such as GO, MXene, TMDs, and LDHs [39-42] .
Notably, these intrinsically nonporous 2D nanosheets still require a subsequent perforation process to
fabricate nanochannels (as discussed in the next section). For example, intrinsically porous 2D nanosheets
can be exfoliated from their related layered materials with high purity via melt blending with a polymer
matrix, as demonstrated by Varoon et al. . And the synthesis of g-C N nanosheets can be achieved by
[43]
4
3
thermal oxidation [Figure 2F] with long-time heating and etching, which was shown by Ren et al., to
produce a series of g-C N nanosheets with a thinner layer thickness, larger BET surface area, and higher
3
4
graphic nitrogen ratio . They proposed that the higher activity of the g-C N from long-time thermal
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4
3
oxidative etching might be ascribed to the enlarged specific surface, pore volume, and higher graphic
nitrogen ratio with the loose and soft laminar morphology .
[44]
However, it should be noted that the “top-down” strategy for exfoliation is limited by the availability of
layered materials, which may result in incidental structural deterioration and morphological damage during