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Figure 5. Schematics of the main assembly methods for 2D-material-based nanochannel membranes. (A) Building van der Waals
heterostructures [72] . Copyright 2013, Springer Nature Limited. (B) Pressure/Vacuum filtration. (C) Spin-coating. (D) Spray coating [181] .
Copyright 2011, Royal Society of Chemistry. (E) Rob casting [181] . Copyright 2011, Royal Society of Chemistry.

The pressure/vacuum filtrating method [Figure 5B] is the most widely adopted to stack 2D laminar
membranes. The thicknesses of the membrane can be straightforwardly controlled by altering the loading
amount of 2D nanosheets via pressure control. For instance, Dikin et al. first reported the preparation of a
free-standing GO membrane with macroscopic flexibility and stiffness by vacuum filtration of colloidal
[82]
dispersions of individual GO nanosheets . The X-ray spectrum of a typical GO membrane showed a peak
corresponding to a layer-to-layer distance (d-spacing) of about 0.83 nm, which can be attributed to an
approximately one molecule-thick layer of water that is presumably hydrogen-bonded between the
interlocked GO nanosheets within the laminates. Preparation parameters such as driving force, deposition
rate, and substrate effect play critical roles in determining the structural formation of 2D laminar
membranes. For example, Tsou et al. induced the assembly of GO membranes with different laminar
microstructures ranging from highly ordered to highly random utilizing pressure, vacuum, and evaporation
[83]
filtrating techniques .

It is crucial to mention that due to the extremely thin feature, 2D laminates are usually supported by a
porous substrate to form practical membranes. The differences in substrate properties also lead to a distinct
assembly structure of resulting 2D laminar membranes with uneven performance. An example can be found
in the study carried out by Zhang et al., who investigated the effect of substrate on GO membrane
[84]
formation and separation . The surface morphology and chemical structure of substrates induced the
assembly of GO and determined its adhesion. Furthermore, the bulk pore structure of substrates dominated
the whole transport resistance of the GO membrane.