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Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11 Page 5 of 35

review begins with a comprehensive description of 2D-material preparation strategies, including “top-
down” and “bottom-up” approaches. Nanochannel properties directly impact separation performances,
such as permeability and selectivity, and as such, the review outlines 2D nanosheet perforation methods
(i.e., drilling out-of-plane nanochannels, similar to creating “porous bricks”) and membrane assembly
methods (similar to building “house”) for obtaining in-plane nanochannels. Nanochannel regulation is
similar to “renovating a house”, giving the membrane improved structural and functional properties. Thus,
four aspects of such modification are briefly discussed (i.e., channel size, channel length, channel
morphology, and surface chemistry). Laboratory-scale application attempts are the first step towards
industrial applications (similar to “trial residence”), which are also reviewed. Afterward, to give readers a
more intuitive picture, the review also categorizes various representative application scenarios and describes
their specific needs for membranes, such as liquid molecular separation, gas separation, and ion sieving. A
visual summary of the main elements described above is provided in Figure 1. In the final section, we
conclude with a discussion of current challenges and an outlook on the eventual commercialization of
membranes. Overall, this comprehensive review presents readers with a detailed roadmap of the entire
process of membrane preparation and regulation from raw materials to final applications (“clay” to “bricks”
to “furnished houses”) and highlights the corresponding strategies employed in each step. By providing
clear and concise information, this review aims to inspire future research and the development of new and
innovative membrane preparation and regulation strategies, ultimately leading to the realization of highly
efficient and effective membrane technologies.

CONSTRUCTING NANOCHANNELS WITH 2D NANOSHEETS
2D nanosheets refer to a class of materials that consists of only one or a few atomic layers, exhibiting an
[31]
ultrathin sheet-like geometry beyond the nanoscale . These materials are characterized by strong interlayer
covalent bonds and weak interlayer van der Waals bonds and exhibit excellent electrical, optical, and
mechanical properties compared to their bulk counterparts, making them extremely attractive in the fields
of physics, materials science, and chemistry . The successful isolation of monolayer graphene by
[32]
Novoselov et al. in 2004 paved the way for the exploration of many other 2D materials, which have become
ideal building blocks for the development of membranes with nanochannels due to their atomic thickness
and unique physicochemical properties [30,33] .

When 2D materials are engineered into membranes, two basic forms of nanochannels can be created: out-
of-plane nanochannels and in-plane nanochannels. Out-of-plane nanochannels can be formed by utilizing
2D nanosheets with intrinsically porous or perforating intrinsically nonporous 2D materials. In-plane
nanochannels can be fabricated from 2D layered membranes in which several or multiple layers of
nanosheets are aligned parallel to form well-defined nanochannels. This section describes typical
construction strategies for creating nanochannels from 2D materials, including synthetic methods for
intrinsic nanopores by multilayer peeling or monolayer growth, perforation approaches for artificial
nanopores through physical and chemical etching, and various membrane assembly methods, including van
der Waals assembly of individual nanochannels through nanosheet extraction, solution-assisted assembly of
dense nanochannels by pressure/vacuum filtration, spin/spray coating, and other techniques.

Synthesis of 2D nanosheets
2D nanosheets serve as the foundational materials for constructing nanochannels from 2D materials. The
synthesis of 2D nanosheets can be generally classified as “top-down” and “bottom-up” strategies, as shown
in Figures 2 and 3.