Molybdenum Disulfide and Tungsten Disulfide as Novel …
207
CNTs mixed with polymeric materials are well guided to be applied in RO systems.
High-performance RO CNT/Polyamide membranes can be achieved with CNTs
dispersion in typical polymeric matrices [81]. The technique of CNT incorporation into polymers is well known and represents an excellent strategy to conceive
membranes that are reasonable in terms of water flux (compared to vertically aligned
CNTs) but extraordinary in terms of salt permeation, which makes them ideal to
improve current RO technology.
Though promising, all of these membrane technologies are still far from reaching
the desired production and commercialization stage. The next generation of desalination membranes is about to face the challenge of keeping the same (high) levels
of water permeability, salt rejection, and stability while also becoming industrially
scalable. In the meantime, there are several pitfalls to overcome such as chlorine tolerance, fouling/scaling, acid/base, and cleaning. The following section is focused on
the gaps and possibilities of MoS 2 and WS 2 as viable alternatives for 2D desalination
membranes.
5 Research Gaps
(a) Nanopore Opening—The available synthesis of large-scale MoS 2 and WS 2 single
layers by CVD makes them suitable to perform as water desalination 2D membranes.
However, nanoporous membranes (Fig. 2a) require well-defined and distributed
openings with pores of ≤ 1 nm radius. This is also true for layered membranes
(Fig. 2b), where superficial nanopores lead to improved filtration and selective ion
transport channels. From methods relying on plasma treatment or chemical etching
to irradiation with energetic particles, i.e., electrons or ions, there are different ways
to open nanopores in 2D membranes. Intrinsic defects in CVD-grown MoS 2 and
WS 2 membranes stand as a challenge regarding electron irradiation techniques.
While high-energy electrons are able to perforate freestanding MoS 2 and WS 2 , single
or double vacancies are created during the process [82], limiting their application.
Exposing the membrane to a high flux of electrons can also result in uncontrollable
size distribution and pore density. Nevertheless, a combination of methods can be
used to achieve the desired scalability. A recent experiment combining focused electron beams with an in situ heating holder was able to drill nanopores in WS 2 bilayers
(See Fig. 4) with precise control over spatial distributions with 5 nm accuracy of
patterning and the width of nanowells adjustable by dose-dependent parameters [83].
In contrast to electron beams, we can use ions (or heavy ions) to bombard MoS 2 and
WS 2 nanosheets, which involves a wider range of experimental parameters to be
explored during the defect creation mechanism. For example, swift heavy ions [84]
and highly charged ions [85] have been used to manufacture well-defined openings in
freestanding MoS 2 . In this way, pores with radii ranging from 0.3 to 3 nm have been
created. Some groups have also exploited the electrochemical activity of MoS 2 and
WS 2 . Debatably, electrochemical reaction (ECR) techniques were used to open wellcontrolled nanopores in a scalable way [86]. But again, the high density of intrinsic
207
CNTs mixed with polymeric materials are well guided to be applied in RO systems.
High-performance RO CNT/Polyamide membranes can be achieved with CNTs
dispersion in typical polymeric matrices [81]. The technique of CNT incorporation into polymers is well known and represents an excellent strategy to conceive
membranes that are reasonable in terms of water flux (compared to vertically aligned
CNTs) but extraordinary in terms of salt permeation, which makes them ideal to
improve current RO technology.
Though promising, all of these membrane technologies are still far from reaching
the desired production and commercialization stage. The next generation of desalination membranes is about to face the challenge of keeping the same (high) levels
of water permeability, salt rejection, and stability while also becoming industrially
scalable. In the meantime, there are several pitfalls to overcome such as chlorine tolerance, fouling/scaling, acid/base, and cleaning. The following section is focused on
the gaps and possibilities of MoS 2 and WS 2 as viable alternatives for 2D desalination
membranes.
5 Research Gaps
(a) Nanopore Opening—The available synthesis of large-scale MoS 2 and WS 2 single
layers by CVD makes them suitable to perform as water desalination 2D membranes.
However, nanoporous membranes (Fig. 2a) require well-defined and distributed
openings with pores of ≤ 1 nm radius. This is also true for layered membranes
(Fig. 2b), where superficial nanopores lead to improved filtration and selective ion
transport channels. From methods relying on plasma treatment or chemical etching
to irradiation with energetic particles, i.e., electrons or ions, there are different ways
to open nanopores in 2D membranes. Intrinsic defects in CVD-grown MoS 2 and
WS 2 membranes stand as a challenge regarding electron irradiation techniques.
While high-energy electrons are able to perforate freestanding MoS 2 and WS 2 , single
or double vacancies are created during the process [82], limiting their application.
Exposing the membrane to a high flux of electrons can also result in uncontrollable
size distribution and pore density. Nevertheless, a combination of methods can be
used to achieve the desired scalability. A recent experiment combining focused electron beams with an in situ heating holder was able to drill nanopores in WS 2 bilayers
(See Fig. 4) with precise control over spatial distributions with 5 nm accuracy of
patterning and the width of nanowells adjustable by dose-dependent parameters [83].
In contrast to electron beams, we can use ions (or heavy ions) to bombard MoS 2 and
WS 2 nanosheets, which involves a wider range of experimental parameters to be
explored during the defect creation mechanism. For example, swift heavy ions [84]
and highly charged ions [85] have been used to manufacture well-defined openings in
freestanding MoS 2 . In this way, pores with radii ranging from 0.3 to 3 nm have been
created. Some groups have also exploited the electrochemical activity of MoS 2 and
WS 2 . Debatably, electrochemical reaction (ECR) techniques were used to open wellcontrolled nanopores in a scalable way [86]. But again, the high density of intrinsic