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A. S. Kazemi and M. A. Abdol
from the backside of graphene and the polymer layer/graphene is fished out of the
water onto the substrate of interest, the polymer layer is removed by a combination of
cleaning methods (i.e., chemical and heating). However, this polymeric layer is never
fully removed [31, 33, 158, 159] and leaves behind some residues that undermine the
mechanical stability and continuity of the graphene sheet. For RO applications, the
substrate of interest is an appropriate porous support layer that allows graphene to
free-stand over a large area and together with the properties of graphene, accomplish
high selectivity in the presence of imperfections and defects [161].
Ideal support or coating for the active layer will add a transport resistance in
series to both selective pores and defects, thus limiting leakage without adversely
impacting the permeance [161]. This resistive layer must be thinner than the spacing
between non-selective defects in the atomically thin layer [6, 161]. More critically,
the support must have high surface porosity for efficient use of the surface area of the
active layer, have high mechanical strength, higher permeance than the active layer,
good chemical resistance, provide stable adhesion and facilitate transfer or coating of
the active layer [161]. Most experimental studies have used specialized supports for
RO applications. Polycarbonate track-etched [33], poly(1-methyl silyl-1-propyne)
[162], polypropylene [163] and polyvinyl idene difluoride [163] membranes and
microfabricated Si supports [29, 31, 158, 159] are some examples. Microfabricated
Si supports have low surface porosity, which results in high permeance with respect
to the graphene area but low permeance based on total membrane area. They are also
expensive and require several fabrication steps. Additionally, they are not flexible to
fit a spiral-wound module for practical large-scale separations [159]. Even the polymeric substrates are suboptimum [20, 33], thus there is a need for the design and fabrication of a new generation of supporting layers; layers that meet all the mentioned
requirements to match nano-porous atomically thin membranes for pressure-driven
separations.
4.3 Fabrication of Nanopores on the Active Layer
Nano-porous graphene membranes as the active layer in the RO process can be
fabricated by creating pores in an initially non-porous atomically thin material,
or by synthesis of an intrinsically porous material. The former attains large area
nano-porous graphene sheets useful for scale-up in industry and the latter, gives
lab-scale nano-porous sheets. An active layer of RO membrane requires nanopores
large enough that enable water passage and ion blockage when size-exclusion separation technique is employed. Although nanopores could be developed from localized
defective sites, precise control of the size and density distributions of the pores
involves specific perforation strategies [26, 164]. Most of Sect. 4.3 is reproduced
with permission from Ref. [26], Copyright 2019, Royal Society of Chemistry.
A. S. Kazemi and M. A. Abdol
from the backside of graphene and the polymer layer/graphene is fished out of the
water onto the substrate of interest, the polymer layer is removed by a combination of
cleaning methods (i.e., chemical and heating). However, this polymeric layer is never
fully removed [31, 33, 158, 159] and leaves behind some residues that undermine the
mechanical stability and continuity of the graphene sheet. For RO applications, the
substrate of interest is an appropriate porous support layer that allows graphene to
free-stand over a large area and together with the properties of graphene, accomplish
high selectivity in the presence of imperfections and defects [161].
Ideal support or coating for the active layer will add a transport resistance in
series to both selective pores and defects, thus limiting leakage without adversely
impacting the permeance [161]. This resistive layer must be thinner than the spacing
between non-selective defects in the atomically thin layer [6, 161]. More critically,
the support must have high surface porosity for efficient use of the surface area of the
active layer, have high mechanical strength, higher permeance than the active layer,
good chemical resistance, provide stable adhesion and facilitate transfer or coating of
the active layer [161]. Most experimental studies have used specialized supports for
RO applications. Polycarbonate track-etched [33], poly(1-methyl silyl-1-propyne)
[162], polypropylene [163] and polyvinyl idene difluoride [163] membranes and
microfabricated Si supports [29, 31, 158, 159] are some examples. Microfabricated
Si supports have low surface porosity, which results in high permeance with respect
to the graphene area but low permeance based on total membrane area. They are also
expensive and require several fabrication steps. Additionally, they are not flexible to
fit a spiral-wound module for practical large-scale separations [159]. Even the polymeric substrates are suboptimum [20, 33], thus there is a need for the design and fabrication of a new generation of supporting layers; layers that meet all the mentioned
requirements to match nano-porous atomically thin membranes for pressure-driven
separations.
4.3 Fabrication of Nanopores on the Active Layer
Nano-porous graphene membranes as the active layer in the RO process can be
fabricated by creating pores in an initially non-porous atomically thin material,
or by synthesis of an intrinsically porous material. The former attains large area
nano-porous graphene sheets useful for scale-up in industry and the latter, gives
lab-scale nano-porous sheets. An active layer of RO membrane requires nanopores
large enough that enable water passage and ion blockage when size-exclusion separation technique is employed. Although nanopores could be developed from localized
defective sites, precise control of the size and density distributions of the pores
involves specific perforation strategies [26, 164]. Most of Sect. 4.3 is reproduced
with permission from Ref. [26], Copyright 2019, Royal Society of Chemistry.