60
A. S. Kazemi and M. A. Abdol
and shear velocity on the mechanical responses of nano-porous graphene membranes
were examined in [131]. Their results showed that the wrinkling of the membrane
became more obvious with increasing strain. Fracture stress in the nano-porous
graphene membrane decreased with increasing porosity. In addition, the effect of
shear velocity on the shear modulus decreased with increasing porosity [131].
(d) Multilayer graphene
Single-layer nano-porous graphene has shown potential as a RO desalination
membrane, and multiple efforts have targeted the synthesis of large-scale graphene
films [132]. However, producing perfect monolayer graphene over large areas
remains challenging. The primary method for synthesizing graphene sheets, chemical vapor deposition (CVD) results in substantial multilayer coverage, defects,
tears and wrinkles [28]. However, multilayer graphene membranes can be synthesized more economically than the monolayer material and have higher mechanical
stability. Some of the physical implications of these multiple graphene layers on the
membrane’s performance have been studied recently [99, 133]. The major concerns
are whether these multilayers will maintain high water permeability? what is the
interplay between nanopores on different layers? How would they influence the
membrane’s water permeation and salt rejection ability? Do we have the tool to
create arrays of pores with diameters ~1 nm, with suitable distribution and deep
enough into all layers across large-scale structure?
MD simulation was employed to investigate the effects of layering graphene on
RO performance. This was done by taking a bilayer nano-porous membrane as a
model system [99]. The model assumed that the alteration from a monolayer to two
layers, demonstrated the most important effects of adding layers and that subsequent
layers have a qualitatively similar impact. It was shown that multilayer nano-porous
graphene membranes display similar promising desalination properties to monolayer membranes. However, manipulating layer separation and pore alignment is
essential to improve desalination performance [99]. Figure 6 shows a snapshot of
a typical bilayer simulation with effective parameters on the desalination performance. The key parameters that were explored in multilayer nano-porous graphene
sheet efficiency for water desalination studies were the feed pressure P, the offset O
between nanopores in the upstream and downstream graphene layer, and the spacing
H between graphene layers. The interlayer spacing can be as fine as H = 3.35 Å (∝
layer spacing in graphite) or can be much larger. Some physically relevant spacing
are 10 and 14 Å that correspond to two and three water layers, between the graphene
sheets [99].
Cohen-Tanugi et al. [99] demonstrated that for completely aligned pores in
bilayer graphene, the flow rate decays nonlinearly with increasing layer separation.
Free energy barrier studies reveal that a small separation between the pores results in
a system possessing a single barrier for transport. This height of this barrier is larger
than a monolayer nano-porous graphene membrane partially due to the stronger
surface adsorption of water on the bilayer nano-porous graphene surface. Conversely,
for maximally offset pores, the interspace region does not allow any water passage
A. S. Kazemi and M. A. Abdol
and shear velocity on the mechanical responses of nano-porous graphene membranes
were examined in [131]. Their results showed that the wrinkling of the membrane
became more obvious with increasing strain. Fracture stress in the nano-porous
graphene membrane decreased with increasing porosity. In addition, the effect of
shear velocity on the shear modulus decreased with increasing porosity [131].
(d) Multilayer graphene
Single-layer nano-porous graphene has shown potential as a RO desalination
membrane, and multiple efforts have targeted the synthesis of large-scale graphene
films [132]. However, producing perfect monolayer graphene over large areas
remains challenging. The primary method for synthesizing graphene sheets, chemical vapor deposition (CVD) results in substantial multilayer coverage, defects,
tears and wrinkles [28]. However, multilayer graphene membranes can be synthesized more economically than the monolayer material and have higher mechanical
stability. Some of the physical implications of these multiple graphene layers on the
membrane’s performance have been studied recently [99, 133]. The major concerns
are whether these multilayers will maintain high water permeability? what is the
interplay between nanopores on different layers? How would they influence the
membrane’s water permeation and salt rejection ability? Do we have the tool to
create arrays of pores with diameters ~1 nm, with suitable distribution and deep
enough into all layers across large-scale structure?
MD simulation was employed to investigate the effects of layering graphene on
RO performance. This was done by taking a bilayer nano-porous membrane as a
model system [99]. The model assumed that the alteration from a monolayer to two
layers, demonstrated the most important effects of adding layers and that subsequent
layers have a qualitatively similar impact. It was shown that multilayer nano-porous
graphene membranes display similar promising desalination properties to monolayer membranes. However, manipulating layer separation and pore alignment is
essential to improve desalination performance [99]. Figure 6 shows a snapshot of
a typical bilayer simulation with effective parameters on the desalination performance. The key parameters that were explored in multilayer nano-porous graphene
sheet efficiency for water desalination studies were the feed pressure P, the offset O
between nanopores in the upstream and downstream graphene layer, and the spacing
H between graphene layers. The interlayer spacing can be as fine as H = 3.35 Å (∝
layer spacing in graphite) or can be much larger. Some physically relevant spacing
are 10 and 14 Å that correspond to two and three water layers, between the graphene
sheets [99].
Cohen-Tanugi et al. [99] demonstrated that for completely aligned pores in
bilayer graphene, the flow rate decays nonlinearly with increasing layer separation.
Free energy barrier studies reveal that a small separation between the pores results in
a system possessing a single barrier for transport. This height of this barrier is larger
than a monolayer nano-porous graphene membrane partially due to the stronger
surface adsorption of water on the bilayer nano-porous graphene surface. Conversely,
for maximally offset pores, the interspace region does not allow any water passage