72
A. S. Kazemi and M. A. Abdol
showed salt rejections from 85.2 to 93.4% towards Na 2 SO 4 , MgCl 2 , NaCl and KCl
while experiencing a range of pressure. 2–4 MPa of pressure for non-supported
stainless-steel mesh (30 mm) and 8–10 MPa for supported polycarbonate track etch
membrane (0.2 mm) were sustained, respectively. Further advancements in applying
efficient support layers for nano-porous graphene membranes were conducted by
Kazemi et al., [158] in 2019, using various TEM grids as porous primary mechanical
supports to maintain the mechanical integrity of the membranes. 1.27 × 10
6
µm
2 to
7.92 × 10
5
µm
2 spanned area monolayer CVD graphene, were transferred onto holethrough Ni TEM grids with aperture sizes of 6.5–70 µm. O 2 plasma etching was used
to create nanopores with sizes from 1 nm to 8 nm in every 100 × 100 nm
2 unit area.
TEM analysis showed that the distribution of 1 and 2 nm wide pores were several
times higher than 5–8 nm wide pores. The team used gravity driven configuration to
examine water desalination parameters at 10 kPa pressure. NaCl and KCl rejections
varied from 7 to 46% from the largest TEM aperture size to the lowest. To improve
the salt rejections and minimize the leakage from the defected graphene surface,
they used a secondary support provided by porous SiN wafer with an overall smaller
aperture than the entire TEM mesh above. This reduced the graphene effective area to
2.77 × 10
4
µm
2 but increased the NaCl rejection to ~76% while maintaining a very
high water permeability. In a more in-depth study, the same team investigated large
area mobile superimposed nano-porous monolayer graphene membrane/grid on to
different SiN/Si holes with distinct size (6.5, 10.5, 30 and 75 µm) and spacing (21.4,
60, 200 and 238 µm), experimentally and by modeling [159]. Their studies showed
NaCl rejection of 58–100% in different configurations of graphene/grid/holes with
very high water permeation ~5 × 10
7 L m
−2 h
−1 bar
−1 .
By synthesizing bilayer graphene membranes containing pores with diameters
from ~6 to 1000 nm, Buchheim et al. [133] investigated liquid permeation over a
wide range of viscosities and pressures. With the goal of assessing the thickness—
permeation paradigm, they revealed that nano-porous membranes with thicknesses
up to 90 nm demonstrate dominance of the entrance resistance for aspect ratios up
to one. Furthermore, they showed that liquid permeation across these atomically
thin pores was limited by viscous dissipation at the pore entrance independent of
thickness and universal for porous materials.
There are some recent advances in the literature that report large area mono or
bilayer graphene transfer over porous supports with high mechanical durability.
Although they have not directly used these membranes for water desalination but
exploiting new ideas from these reports is recommended for more efficient fabrication of RO membranes. Huang et al. [209] reported a novel nano-porous carbonassisted transfer technique, yielding the transfer of relatively large area (1 mm
2 )
monolayer CVD graphene onto macro-porous support (with 5 µm pores) with an
ultra-low density of the intrinsic defects (porosity of 0.025%) without generating
cracks. Wang et al. [210] directly investigated the ability of cm-scale monolayer CVD
graphene withstanding high pressure when placed on porous polycarbonate tracketched membranes with diameters in the range of 30 nm to 3 µm using electron
microscopy, AFM, and gas flow measurements. Surprisingly, non-wrinkled areas
withstood pressures exceeding 100 bar at which many kinds of membrane suffer
A. S. Kazemi and M. A. Abdol
showed salt rejections from 85.2 to 93.4% towards Na 2 SO 4 , MgCl 2 , NaCl and KCl
while experiencing a range of pressure. 2–4 MPa of pressure for non-supported
stainless-steel mesh (30 mm) and 8–10 MPa for supported polycarbonate track etch
membrane (0.2 mm) were sustained, respectively. Further advancements in applying
efficient support layers for nano-porous graphene membranes were conducted by
Kazemi et al., [158] in 2019, using various TEM grids as porous primary mechanical
supports to maintain the mechanical integrity of the membranes. 1.27 × 10
6
µm
2 to
7.92 × 10
5
µm
2 spanned area monolayer CVD graphene, were transferred onto holethrough Ni TEM grids with aperture sizes of 6.5–70 µm. O 2 plasma etching was used
to create nanopores with sizes from 1 nm to 8 nm in every 100 × 100 nm
2 unit area.
TEM analysis showed that the distribution of 1 and 2 nm wide pores were several
times higher than 5–8 nm wide pores. The team used gravity driven configuration to
examine water desalination parameters at 10 kPa pressure. NaCl and KCl rejections
varied from 7 to 46% from the largest TEM aperture size to the lowest. To improve
the salt rejections and minimize the leakage from the defected graphene surface,
they used a secondary support provided by porous SiN wafer with an overall smaller
aperture than the entire TEM mesh above. This reduced the graphene effective area to
2.77 × 10
4
µm
2 but increased the NaCl rejection to ~76% while maintaining a very
high water permeability. In a more in-depth study, the same team investigated large
area mobile superimposed nano-porous monolayer graphene membrane/grid on to
different SiN/Si holes with distinct size (6.5, 10.5, 30 and 75 µm) and spacing (21.4,
60, 200 and 238 µm), experimentally and by modeling [159]. Their studies showed
NaCl rejection of 58–100% in different configurations of graphene/grid/holes with
very high water permeation ~5 × 10
7 L m
−2 h
−1 bar
−1 .
By synthesizing bilayer graphene membranes containing pores with diameters
from ~6 to 1000 nm, Buchheim et al. [133] investigated liquid permeation over a
wide range of viscosities and pressures. With the goal of assessing the thickness—
permeation paradigm, they revealed that nano-porous membranes with thicknesses
up to 90 nm demonstrate dominance of the entrance resistance for aspect ratios up
to one. Furthermore, they showed that liquid permeation across these atomically
thin pores was limited by viscous dissipation at the pore entrance independent of
thickness and universal for porous materials.
There are some recent advances in the literature that report large area mono or
bilayer graphene transfer over porous supports with high mechanical durability.
Although they have not directly used these membranes for water desalination but
exploiting new ideas from these reports is recommended for more efficient fabrication of RO membranes. Huang et al. [209] reported a novel nano-porous carbonassisted transfer technique, yielding the transfer of relatively large area (1 mm
2 )
monolayer CVD graphene onto macro-porous support (with 5 µm pores) with an
ultra-low density of the intrinsic defects (porosity of 0.025%) without generating
cracks. Wang et al. [210] directly investigated the ability of cm-scale monolayer CVD
graphene withstanding high pressure when placed on porous polycarbonate tracketched membranes with diameters in the range of 30 nm to 3 µm using electron
microscopy, AFM, and gas flow measurements. Surprisingly, non-wrinkled areas
withstood pressures exceeding 100 bar at which many kinds of membrane suffer