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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 9 of 35

[176]
Figure 4. Schematics of perforation on 2D materials. (A) Low-dose e-beam patterning of ZIF-L with pyridine as an etchant .
[177]
Copyright 2021, American Chemical Society. (B) UV irradiation etching of MoS /WS nanosheets . Copyright 2016, Royal Society of
2 2
[178] [179]
Chemistry. (C) Focused ion beam etching process on GO . Copyright 2019, Elsevier Ltd. (D) Plasma etching process . Copyright
[180]
2020, MDPI. (E) The synthesis of porous Si/C composite nanosheets by chemical etching . Copyright 2019, American Chemical
Society.
densities exceeding 10 cm -2[62] . In another approach, Surwade et al. employed an oxygen plasma etching
12
process [Figure 4D] to produce nanopores with tunable diameters on a graphene monolayer, which
exhibited rapid water transport and excellent salt rejection . Despite the success of these methods in
[15]
achieving tailored pore sizes and densities, they can be relatively expensive due to the use of specialized
instruments.

Generally, physical etching is an effective method to prepare arrays of monodisperse nanopores with a
precise and tunable pore size distribution, whereas chemical etching [Figure 4E] is relatively easy and low-
cost to produce nanopores in the graphene plane on a large scale. KOH, HNO , and others are commonly
3
used to prepare porous graphene. Zhu et al. reported simple activations with KOH to generate nanoscale
pores on the exfoliated GO nanosheets . The result in highly conductive, free-standing, and flexible porous
[63]
GO paper possessed a very high specific surface area with excellent electrical conductivity and yielded
outstanding performance, making it ideal for high-power energy storage [61,63,64] . Zhao et al. introduced
carbon vacancy pores into graphene nanosheets using a facile solution method that the HNO reacted with
3
the coordinatively unsaturated carbon atoms to partial detachment and removal of carbon atoms from the
[65]
nanosheet . Wang et al. prepared porous graphene nanosheets by refluxing reduced GO (rGO) nanosheets
in a concentrated HNO solution . The diameters of nanopores can be readily modulated from several to
[66]
3
hundreds of nanometers by varying the acid treatment time . Besides, H O and metal oxides can also
[68]
[66]
[67]
2
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be applied as chemical reagents to etch graphene.