62
A. S. Kazemi and M. A. Abdol
Cohen-Tanugi et al. [99] summarized the most efficient scenarios for multilayer
experimental situation based on their simulation findings in four points:
(1) If both pore alignment and layer separation could be precisely controlled, a
multilayer nano-porous graphene membrane with the smallest possible layer
separation and fully aligned pores would represent the most promising choice.
(2) If only layer separation could be precisely controlled, sufficiently large layer
separation (i.e., 8 Å) would be preferable, to avoid fully obstructing water
passage across misaligned pores.
(3) If only pore offset could be controlled, a membrane with fully aligned pores is
desirable.
(4) If neither pore neither alignment nor layer separation could be controlled,
having a nano-porous graphene membrane with the greatest possible pore
density is recommended to enhance the likelihood of having aligned pores.
In an innovative MD study, Abdol et al. [136] created aligned pores in multilayer
graphene using focused ion beam irradiation and strengthened the stability of the
whole structure simultaneously. They proposed that incident ion beams, besides
creating pores, can reinforce the lateral strength of multilayer graphene by creating
some permanent covalent bonds around the pores between graphene layers. They
also showed that by changing the kinetic energy of the incident beams the density
of the covalent bonds can be tuned. Graphene layers in a multilayer structure have a
great tendency to be delaminated or swelled in an aquatic environment. Considering
that, the proposed technique could be a promising approach to overcome the weak
points of multilayer graphene structures and make them suitable candidates for use
as water purification membranes. However, these ideas are yet to be explored in the
experiment.
(e) Ion transport and its implications
Membranes functioning over different length scales, achieve selective transport
through a variety of mechanisms [20]. At the smallest scale, dense polymeric
non-porous RO or gas separation membranes, such as RO membranes for water
desalination, operate by a solution-diffusion mechanism [5, 61]. In these systems,
differences in solubilities and diffusivities of the species in the membrane material
trigger selectivity. While solubility relates to the molecular structure, porosity and
chemical affinity of the membranes; diffusivity is regulated by thermally activated
rearrangements of the polymer chains [20]. More permeable materials typically
provide less selectivity when selectivity is only governed by diffusion, and result
in a trade-off between permeability and selectivity. The incorporation of additional
mechanisms, such as chemical affinity or molecular sieving assists in overcoming
this trade-off [13].
The atomic thickness and composition in 2D membranes induce several discrepancies with solid-state nanopores. Sahu and Zwolak [137] have comprehensively
discussed ionic phenomena in nanoscale pores through 2D materials. For pores in
2D membranes with diameters above ~2 nm, ‘access resistance’ is dominant over
A. S. Kazemi and M. A. Abdol
Cohen-Tanugi et al. [99] summarized the most efficient scenarios for multilayer
experimental situation based on their simulation findings in four points:
(1) If both pore alignment and layer separation could be precisely controlled, a
multilayer nano-porous graphene membrane with the smallest possible layer
separation and fully aligned pores would represent the most promising choice.
(2) If only layer separation could be precisely controlled, sufficiently large layer
separation (i.e., 8 Å) would be preferable, to avoid fully obstructing water
passage across misaligned pores.
(3) If only pore offset could be controlled, a membrane with fully aligned pores is
desirable.
(4) If neither pore neither alignment nor layer separation could be controlled,
having a nano-porous graphene membrane with the greatest possible pore
density is recommended to enhance the likelihood of having aligned pores.
In an innovative MD study, Abdol et al. [136] created aligned pores in multilayer
graphene using focused ion beam irradiation and strengthened the stability of the
whole structure simultaneously. They proposed that incident ion beams, besides
creating pores, can reinforce the lateral strength of multilayer graphene by creating
some permanent covalent bonds around the pores between graphene layers. They
also showed that by changing the kinetic energy of the incident beams the density
of the covalent bonds can be tuned. Graphene layers in a multilayer structure have a
great tendency to be delaminated or swelled in an aquatic environment. Considering
that, the proposed technique could be a promising approach to overcome the weak
points of multilayer graphene structures and make them suitable candidates for use
as water purification membranes. However, these ideas are yet to be explored in the
experiment.
(e) Ion transport and its implications
Membranes functioning over different length scales, achieve selective transport
through a variety of mechanisms [20]. At the smallest scale, dense polymeric
non-porous RO or gas separation membranes, such as RO membranes for water
desalination, operate by a solution-diffusion mechanism [5, 61]. In these systems,
differences in solubilities and diffusivities of the species in the membrane material
trigger selectivity. While solubility relates to the molecular structure, porosity and
chemical affinity of the membranes; diffusivity is regulated by thermally activated
rearrangements of the polymer chains [20]. More permeable materials typically
provide less selectivity when selectivity is only governed by diffusion, and result
in a trade-off between permeability and selectivity. The incorporation of additional
mechanisms, such as chemical affinity or molecular sieving assists in overcoming
this trade-off [13].
The atomic thickness and composition in 2D membranes induce several discrepancies with solid-state nanopores. Sahu and Zwolak [137] have comprehensively
discussed ionic phenomena in nanoscale pores through 2D materials. For pores in
2D membranes with diameters above ~2 nm, ‘access resistance’ is dominant over