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Page 20 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

nanosheets [15,19,58-71] . The creation of nanopores can provide a greater number of transport nanochannels for
nanofluids and sharply reduce the average transport distances. Ying et al. reported GO membranes with
introduced in-plane mesopores by a reoxidation process, demonstrating 2-3 folds enhancement of water
[146]
permeance as that of the pristine GO membranes . Li et al. also proposed thermally reduced nanoporous
GO membranes through a combination of mild H O oxidation and moderate thermal treatment, increasing
2
2
[147]
the water permeability by 26 times . Those intrinsically porous 2D materials are more attractive as
building blocks to assemble 2D laminar membranes. Their uniform nanopores consisting of angstrom-sized
windows and nanometer-sized cavities can provide numerous cross-layer shortcuts and ensure precise size
exclusions. Peng et al. reported ultra-permeable and super selective molecular sieve membranes made of 2D
MOF nanosheets, which achieved H permeance of up to several thousand GPU with H /CO selectivity
2
2
2
greater than 200 [37,148] . Shinde et al. demonstrated ultrathin 2D COF membranes with a well-defined ordered
porous structure . These membranes displayed remarkable permeabilities for polar and nonpolar organic
[149]
solvents, which were approximately 100 times higher than the amorphous polymer membranes .
[149]
Besides the tortuous transport pathways through internal nanochannels of 2D laminar membranes
extending in horizontal and vertical directions, nanowrinkled morphologies [Figure 8C] also exist in 2D
laminar membranes, whose effects in transmembrane transport should not be neglected. Xi et al. proposed
[150]
an rGO membrane with 2D nanochannels uniformly confined with a space size of ~ 8 Å . The mild
reduction avoids the hydrothermal corrugation of GO nanosheets and thus enables the creation of highly
parallel 2D nanochannels for precise sieving of mono-/multi-valent metal ions . Li et al. also
[150]
demonstrated a mild-thermal annealing for preparing rGO membrane for nanofiltration since the mild
reduction condition might favor the formation of a more ordered and better-controlled transport
[151]
nanochannel . Contrary to these efforts to eliminate nanowrinkles, Saraswat et al. hypothesized that the
imperfect stacking in the 2D laminates could lead to voids, wrinkles, and disordered microstructures that
could provide alternative non-ideal transport pathways for nanofluids, resulting in a higher effective
permeance . Kang et al. further revealed the roles of nanowrinkles in mass transport across GO
[152]
membranes . They found nanowrinkles by themselves serve as fast transporting ways while their
[153]
[153]
connection with narrow interlayer channels can form a selective network . Huang et al. developed a
nanostrand-channeled GO membrane, whose permeance offered a 10-fold enhancement without sacrificing
the rejection rate compared with that of pristine GO membrane, attributing to the generation of more
nanofluidic channel networks in the membrane . The nanostrand-channeled concept is also extendable to
[154]
[155]
[17]
other 2D laminar membranes, such as MXene and WS membranes , for increased permeability. These
2
findings corroborate that nanowrinkles serve as fast tracks for nanofluids to enhance membrane
permeability. Considering nanowrinkles broadly existing within flexible 2D nanosheets, they are expected to
show critical transportcontroling effects in nanofluidic transport in 2D-material membranes.
Regulating surface physicochemical properties of nanochannels
The 2D-material nanosheets have abundant functional groups. During the filtration process, the 2D
material-based membrane is prone to swelling effect, resulting in the collapse of nanochannels and loss of
sieving performance. Targeted modification of 2D-material surfaces is, therefore, significant, and a number
of methods have been developed for modifying the chemical properties of membrane surfaces [Figure 9].
One commonly used strategy is the Chemical intercalation of cross-linking agents (e.g., ions, small
molecules, or macromolecules), which enhances the interlayer force and improves the mechanical stability
of 2D nanosheet membranes. At the same time, this strategy modulates the affinity of water/ions to the
nanosheet surfaces, thereby regulating their mobility as they pass through the channels.