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Page 4 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

The fouling index (FI) is calculated using the formula (6), where J is the flux of the clean membrane, and
clean
J fouled is the flux of the fouled membrane.

The interlayer spacing of the 2D-material-based nanochannel membranes can be calculated using the Bragg
equation for X-ray diffraction (XRD), which is given by formula (7), where d is the interlayer spacing, λ is
the wavelength of the X-ray, and θ is the diffraction angle.

Through a comprehensive analysis of membrane performance using the equations outlined above, valuable
insights can be gained into the potential of nanochannel membranes based on 2D materials with densely
packed channel arrays. These membranes are anticipated to offer several distinct advantages for nanoscale
separation applications, including the following: (1) The membranes possess simple and tunable
nanochannels that facilitate quantitative modeling and probing of nanofluids transport mechanisms; (2)
The membranes allow flexible modification of nanochannels with favorable functionality to meet various
application requirements; (3) The membranes enable ultra-thin structures down to single-atom thickness
and ultra-high throughput by simple and scalable methods; and (4) The membranes can provide
nanochannels with specific sizes, and the size can be precisely controlled from tens of nanometers to sub-
nanometer scale to achieve high selectivity [5,26,27] .

Although their numerous theoretical advantages, 2D-material-based nanochannel membranes still face
many practical challenges. One such challenge is that most 2D materials, including GO and MXene, contain
numerous hydroxyl groups, carboxyl groups, Ti-O, and other hydrophilic functional groups on their
[28]
surfaces, which can lead to severe swelling problems and thus reduce the selectivity of the membranes .
Additionally, interfacial instability of 2D materials makes membrane structures very prone to collapse,
[29]
resulting in generally shorter membrane lifespans that cannot meet practical needs . Besides, high
operating pressures are still required for some membranes to achieve high permeability, which leads to
increased energy consumption, and other issues such as poor anti-fouling ability and relatively expensive
[30]
construction costs must be addressed . Given these challenges, there is no consensus on whether
developing innovative, large-scale reproducible strategies for constructing and regulating nanochannels is a
top priority for solving the aforementioned problems and ultimately enabling the widespread use of
membranes in industrial applications. Most of the reviews in the literature focus more on the intricate
details of membrane construction, regulation, and the role of nanochannels in separation performance. In
contrast, this work seamlessly links the separation mechanisms, addressing the nanochannel separation and
collapsing issues, selection of suitable membranes, and various practical applications.

This comprehensive review delves into the intricate details of membrane construction and regulation,
highlighting the critical role of nanochannels in separation performance. This insight into the mechanism
behind nanochannel collapse also provides invaluable insight into choosing the most suitable membrane for
a particular application. As the fundamental building “bricks” of membrane construction, obtaining the
ideal 2D material is a crucial starting step for achieving high-performance membranes. Therefore, the