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with more reliable means and opportunities to explore the deeper mechanisms of structure and separation
and purification of 2D-material-based membranes [95-98] .

REGULATING NANOCHANNELS
The transport of nanofluids through nanochannel membranes based on 2D materials predominantly takes
place within the interlayer space. It is within this space that the transport behavior is influenced by various
physical and chemical factors. Moreover, addressing issues such as nanochannel instability, instability at the
2D-material interface, and swelling can be effectively tackled by adjusting the physicochemical factors of the
2D-material-based nanochannels. Therefore, rational regulation of the nanochannels is of utmost
importance. Controlling the physical factors of channel size, channel length, and channel morphology can
primarily modulate nanofluidic transport by achieving precise sieving, shortening the transport distance,
and reducing the transport resistance. Regulating the chemical factors of the surface chemistry effects can
specifically facilitate nanofluidic transport through various permeant-channel interactions. Many efforts
have been devoted to carefully designing precise geometries and favorable chemical features of
nanochannels for realizing fast and selective molecular/ionic transportation. This section presents typical
provisions for the physicochemical properties of nanochannels in 2D-material-based membranes. Based on
experimental and theoretical studies, various effects and in-depth structure-property relationships of
nanofluid transport are also discussed.

Regulating size of nanochannels
In 2D-material-based nanochannel membranes, nanofluidic transport occurs in interlayer capillaries and
wrinkles formed by adjacent 2D nanosheets and inter-edge gaps and intrinsic pores formed on planar 2D
nanosheets interconnect with each other to create numerous nanochannels. Firstly, the channel size
[Figure 6], one of the most prominent characteristics of nanochannels, firmly decides the entry of
[16]
nanofluids. Joshi et al. found that the interlayer spacing of a GO membrane was ~ 0.9 nm , allowing any
ion or molecule with a hydrated radius of 0.45 nm or less to enter the nanochannels and permeate at a speed
order of magnitude faster than would occur through simple diffusion, while all species larger than this are
sieved out. Such sharp size cutoff determined by the interlayer spacing has significant implications on a
myriad of occasions. By adjusting the channel size through diverse methods, such as confinement,
reduction, cross-linking, and intercalation, a broad range of different-sized nanochannels can be designed to
sieve target ions and molecules from the bulk solution precisely.

Due to the inevitable swelling effect when immersed, hydration would increase the size of nanochannels and
deprive their sieving capability. Achieving a small channel size for the 2D laminates immersed in solvents
has been a challenging task. Abraham et al. described a physical confinement method [Figure 6A] to control
[99]
the interlayer spacing from ~ 9.8 Å to ~ 6.4 Å to obtain accurate and tunable ion sieving . GO laminates
were stored at different relative humidities to yield controllable interlayer spacing attributed to
incorporating water molecules into various sites between GO nanosheets. Subsequently, epoxy was used to
encapsulate stacked GO laminates for preparing physically confined GO membranes since the epoxy
mechanically restricts laminate swelling upon water exposure. As ions and water permeate along the GO
nanochannel, the permeation rate for Na and K showed an exponential dependence, decreasing by two
+
+
orders of magnitude as interlayer spacing decreased from 9.8 Å to 7.4 Å. However, the water permeation
rate showed only a slight variation, reducing by a factor of ~ 2 within the same range of interlayer spacing.
The former is related to the partial clogging of graphene capillaries, and the latter is attributed to a low
barrier and a large slip length for water in graphene capillaries. Similarly, Li et al. reported an external
pressure regulation method for controlling the interlayer spacing of GO laminates against swelling .
[100]