Page 72 - Read Online

Page 12 of 35 Xing et al. Microstructures 2023;3:2023031 https://dx.doi.org/10.20517/microstructures.2023.11

Besides filtrating, various coating methods, such as spin-coating, spray-coating, and casting, have been
reported to assemble 2D laminar membranes. During the spin-coating process [Figure 5C], a solution
containing 2D materials to be deposited is spread out uniformly over the substrate under centrifugal force,
forming the ultrathin and laminar membranes. Kim et al. demonstrated two different spin-coating methods
to prepare GO membranes on polymeric substrates . When contacting the substrate surface to the air-
[85]
liquid interface of the GO solution followed by spin-coating, the repulsive edge-to-edge electrostatic
interactions lead to an island-like assembly of GO nanosheets, resulting in a relatively heterogeneous GO
stacking structure. In contrast, when contacting the substrate surface to GO solution only during spin-
coating, the face-to-face attractive capillary forces created by the spin-coating overcome the repulsive
interactions between GO edges, leading to a considerably dense GO laminar deposition. Chi et al. found
that a speed balance between deposition and solvent (water) evaporation is crucial to obtain smooth GO
membranes with uniformly aligned GO nanosheets via spin coating . Faster deposition may lead to
[86]
overflow of the solution, while more rapid evaporation will cause uneven distribution of nanosheets. By
matching up deposition and evaporation speeds, they obtained a uniform ultrathin GO membrane with a
thickness of 20 nm that grants high gas fluxes for efficient gas separation. For spray-coating [Figure 5D],
Guan et al. carefully controlled spraying times and evaporating rates to achieve facile structure
[87]
manipulation of GO membrane from disordered-to-ordered and porous-to-compact . Ibrahim et al.
further verified that using dilute concentration GO suspensions in spray-coating can help minimize the
[88]
edge-to-edge interactions and reduce extrinsic wrinkles formation . Casting [Figure 5E] also belongs to
coating methods that allow the continuous production of large-scale membranes. Akbari et al. utilized the
flow properties of a nematic GO fluid in developing an industrially adaptable process to produce GO
membranes with large in-plane order and stacking periodicity by shear-induced alignment of liquid crystals
of GO . Zhong et al. showcased a universal, scalable, efficient continuous centrifugal casting method to
[89]
produce highly aligned and compact 2D laminar membranes with impressive performances . Fluid
[90]
mechanics analyses indicated that the simultaneous generation of shear force and centrifugal force during
the continuous centrifugal casting process could be responsible for the alignment and compaction of 2D
nanosheets, respectively.

Layer-by-layer assembly is a flexible construction process involving alternate deposition of different
materials, often using one or more of the previously described methods. Such a flexible layer-by-layer
construction process enables precise control over the thickness of the selective layer by varying the number
of deposition cycles and is beneficial for introducing various interlayer stabilizing forces, including covalent,
electrostatic, or hydrogen bindings between adjacent 2D nanosheets, thereby producing membranes with
robust and ordered laminar structures . Hu et al. reported a 1,3,5-benzenetricarbonyl trichloride cross-
[81]
linked GO membrane made via layer-by-layer deposition . The covalent cross-linking enhanced the
[91]
stability of the stacked GO nanosheets, overcoming their inherent dispensability in the water environment
while fine-tuning their charges, functionality, and spacing . In addition, layer-by-layer assembly has been
[91]
used to create stable GO membranes through the electrostatic bonding of negatively charged GO
[92]
nanosheets and positively charged polyallylamine hydrochloride . These methods provide a gateway to
rationally design the charge properties and other functionalities of interlayer nanochannels of 2D laminar
membranes by carefully choosing the intercalated molecules, polyelectrolytes, or nanomaterials. For
instance, Song et al. developed both positively charged polyallylamine hydrochloride@GO and negatively
charged polystyrene sulfonate@GO, which they alternately deposited on polycarbonate substrates by the
layer-by-layer assembly, producing polyelectrolyte intercalated GO membranes with tunable charge-gating
ion exclusion effects . Apart from a single interaction, Zhao et al. fabricated GO-based hybrid membranes
[93]
via layer-by-layer self-assembly of gelatin molecules and GO nanosheets driven by multiple interactions,
including electrostatic attraction, hydrogen bond, and hydrophobic interaction . Electrostatic attractions
[94]
were formed between ionized carboxyl groups on GO and protonated amino groups on gelatin, hydrogen