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M. H. Köhler et al.
one can clearly see the edges scattered in dark-field light. However, atomically thin
grain boundaries cannot be resolved by either imaging technique, because the inplane linear dielectric response of TMD monolayers is often isotropic, making the
scattered light intensity near grain boundaries indistinguishable. To circumvent this
limitation, Carvalho et al. [94] have proposed a dark-field nonlinear characterization
technique combining a second harmonic generation microscopy and a spatial filter
to further enhance the second harmonic contrast of 1D imperfections in 2D systems
(MoS 2 or WS 2 ), thus allowing detailed large-area spatial mapping of grain boundaries
and edges regardless of their local atomic and electronic structures. There is still room
for technological innovations in 2D materials characterization. New and improved
nanometer-scale imaging techniques are going to rise in decades to come, enabling
advancements toward both applications and fundamental 2D materials discoveries.
(d) High Intensive Energy Use—Nanostructured membranes represent a breakthrough in membrane technology because they allow to shift the trade-off between
permeability and salt rejection [62]. It also makes working with less membrane
surface than conventional technology possible. Still, energy consumption at equal
operating conditions is very similar to that of traditional saltwater RO (limited to
a reduction of almost 15% by theoretical calculations based on phenomenological
models [95]). In fact, the RO itself is just one stage among others. Because of that, a
15% reduction does not represent much in the overall cost saving. Improvements in
pressure bomb technology and pressure recovery systems allowed to reduce energy
demand associated with desalination plants over the last decades, but in the end,
the costs are ultimately limited by the osmotic pressure in the system. The benefits of using an ultra-permeable membrane are more significant for brackish water
RO: a reduction of 46% in energy consumption could in principle be achieved [95].
Besides that, an ultra-permeable membrane opens the window of possibilities to
shift the operating conditions in order to optimize the desalination system. Another
breakthrough aimed at minimizing energy demand is challenging and necessary to
face both the world’s water scarcity and climate emergency [96].
(e) Simulations—Despite a large number of studies, there is still a long way to go
when it comes to fully understand the mechanisms behind 2D membrane desalination. Most of the difficulties are associated with the fact that in the vicinity of an
environment as complex as a nanoporous membrane, both water and especially ions
can assume completely unpredictable behaviors. For instance, Cohen-Tanugi and
Grossman [64] investigated the role of chemical functional groups bonded to the
edges of graphene pores to suggest that commonly occurring hydroxyl groups can
roughly double the water flux thanks to their hydrophilic character. It has been shown
that a nozzle-like structure of protein channels and other nanoscale membranes also
influences water transport [97]. TMDs such as MoS 2 and WS 2 offer the possibility
to craft the pore’s edge with Mo(W), S, or both, which allows for the design of functional nanopores. Additionally, the fish-bone structure of these TMDs [14] makes it
promising as a nozzle-like sub-nanometer pore to be efficiently used in desalination
membranes. In their computational study, Heiranian et al. [14] revealed that pore
chemistry can significantly impact fluid transport, hence the ion rejection rate. It
M. H. Köhler et al.
one can clearly see the edges scattered in dark-field light. However, atomically thin
grain boundaries cannot be resolved by either imaging technique, because the inplane linear dielectric response of TMD monolayers is often isotropic, making the
scattered light intensity near grain boundaries indistinguishable. To circumvent this
limitation, Carvalho et al. [94] have proposed a dark-field nonlinear characterization
technique combining a second harmonic generation microscopy and a spatial filter
to further enhance the second harmonic contrast of 1D imperfections in 2D systems
(MoS 2 or WS 2 ), thus allowing detailed large-area spatial mapping of grain boundaries
and edges regardless of their local atomic and electronic structures. There is still room
for technological innovations in 2D materials characterization. New and improved
nanometer-scale imaging techniques are going to rise in decades to come, enabling
advancements toward both applications and fundamental 2D materials discoveries.
(d) High Intensive Energy Use—Nanostructured membranes represent a breakthrough in membrane technology because they allow to shift the trade-off between
permeability and salt rejection [62]. It also makes working with less membrane
surface than conventional technology possible. Still, energy consumption at equal
operating conditions is very similar to that of traditional saltwater RO (limited to
a reduction of almost 15% by theoretical calculations based on phenomenological
models [95]). In fact, the RO itself is just one stage among others. Because of that, a
15% reduction does not represent much in the overall cost saving. Improvements in
pressure bomb technology and pressure recovery systems allowed to reduce energy
demand associated with desalination plants over the last decades, but in the end,
the costs are ultimately limited by the osmotic pressure in the system. The benefits of using an ultra-permeable membrane are more significant for brackish water
RO: a reduction of 46% in energy consumption could in principle be achieved [95].
Besides that, an ultra-permeable membrane opens the window of possibilities to
shift the operating conditions in order to optimize the desalination system. Another
breakthrough aimed at minimizing energy demand is challenging and necessary to
face both the world’s water scarcity and climate emergency [96].
(e) Simulations—Despite a large number of studies, there is still a long way to go
when it comes to fully understand the mechanisms behind 2D membrane desalination. Most of the difficulties are associated with the fact that in the vicinity of an
environment as complex as a nanoporous membrane, both water and especially ions
can assume completely unpredictable behaviors. For instance, Cohen-Tanugi and
Grossman [64] investigated the role of chemical functional groups bonded to the
edges of graphene pores to suggest that commonly occurring hydroxyl groups can
roughly double the water flux thanks to their hydrophilic character. It has been shown
that a nozzle-like structure of protein channels and other nanoscale membranes also
influences water transport [97]. TMDs such as MoS 2 and WS 2 offer the possibility
to craft the pore’s edge with Mo(W), S, or both, which allows for the design of functional nanopores. Additionally, the fish-bone structure of these TMDs [14] makes it
promising as a nozzle-like sub-nanometer pore to be efficiently used in desalination
membranes. In their computational study, Heiranian et al. [14] revealed that pore
chemistry can significantly impact fluid transport, hence the ion rejection rate. It