Molybdenum Disulfide and Tungsten Disulfide as Novel …
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prismatic coordination phase. In multi-layered TMDs, a large variety of polymorphic structures arise as each individual layer can possess any of the two coordination
phases. The three commonly found polymorphs are defined as 1T, 2H, and 3R, where
the digit is indicative of the number of layers in the crystallographic unit cell, and the
letter designates the symmetry: T for tetragonal (D 3d group), H for hexagonal (D 3h
group), and R for rhombohedral (C
5
3V group). The 1T form displays metallic behavior,
while both 2H and 3R forms exhibit semiconducting behavior. Each TMD polymorph
possesses unique structural and electronic properties, which can be further explored
to build efficient adsorbent devices and desalination membranes.
Recent works have reported high water permeability and selectivity of MoS 2
nanosheets, both desired features in desalination membranes. A flexible laminar
separation membrane prepared from MoS 2 sheets exhibited a water flux from 3 to 5
times higher than that reported for graphene oxide (GO) and rejected 89 and 98% of
Evans blue and cytochrome C molecules, respectively [11]. The possibility to craft
the pore edge with Mo, S, or both provides flexibility to design nanopores within the
membrane with the desired functionality. Another option for creating a desalination
membrane is to use stacking layers of MoS 2 instead of crafting a pore. A few-layer
MoS 2 membrane of only 7 nm thick, grown by Chemical Vapor Deposition (CVD)
technique, has allowed for an excellent combination of high water permeability (>322
L·m
−2 ·h
−1 ·bar
−1 ) and high ionic sieving capability (>99%) for various seawater
salts including Na
+ , K
+ , Ca
2+ , and Mg
2+ with a range of concentrations [12]. Near
100% of salt ion rejection rates for actual seawater obtained from the Atlantic coast
was also reported, significantly outperforming the previously developed 2D MoS 2
layer membranes of micrometer thickness as well as conventional reverse osmosis
(RO) membranes. These results confirmed previous theoretical and computational
predictions about desalination capacity and water permeation of 2D nanoporous
MoS 2 [13, 14].
Advancing the search for high-performance lamellar separation membranes, Sun
et al. [15] investigated the potential of WS 2 , a semiconductor material similar to
MoS 2 , with a Young’s modulus of ~270 GPa [16]. The bulk structure was exfoliated
and a thin film was constructed via filtration. The lamellar WS 2 membrane exhibited
water flux five times greater than GO membranes and two times greater than MoS 2
laminar membranes, rejecting 90% of Evans blue molecules. This impressive water
permeance further increased from 450 to 930 L·m
−2 ·h
−1 ·bar
−1 with the addition of
metal hydroxide nanostrands. They created additional channels between the WS 2
layers allowing for increased water transport without degrading the membrane’s salt
rejection properties.
Understanding the mechanisms and peculiarities of 2D membrane-based desalination is the ultimate frontier to reach industrial scale. So far, cutting-edge theoretical
work has been driving advances and pointing directions for experimental work with
some success. But we still need more. We need to merge theoretical advances with
new experimental approaches, such as the scalable method to controllably make
nanopores in single-layer TMDs or nanoscale water velocity profile mapping introduced by Secchi and collaborators [17], always sharing the goal of making largescale application of MoS 2 and WS 2 membranes in water purification possible. It is
195
prismatic coordination phase. In multi-layered TMDs, a large variety of polymorphic structures arise as each individual layer can possess any of the two coordination
phases. The three commonly found polymorphs are defined as 1T, 2H, and 3R, where
the digit is indicative of the number of layers in the crystallographic unit cell, and the
letter designates the symmetry: T for tetragonal (D 3d group), H for hexagonal (D 3h
group), and R for rhombohedral (C
5
3V group). The 1T form displays metallic behavior,
while both 2H and 3R forms exhibit semiconducting behavior. Each TMD polymorph
possesses unique structural and electronic properties, which can be further explored
to build efficient adsorbent devices and desalination membranes.
Recent works have reported high water permeability and selectivity of MoS 2
nanosheets, both desired features in desalination membranes. A flexible laminar
separation membrane prepared from MoS 2 sheets exhibited a water flux from 3 to 5
times higher than that reported for graphene oxide (GO) and rejected 89 and 98% of
Evans blue and cytochrome C molecules, respectively [11]. The possibility to craft
the pore edge with Mo, S, or both provides flexibility to design nanopores within the
membrane with the desired functionality. Another option for creating a desalination
membrane is to use stacking layers of MoS 2 instead of crafting a pore. A few-layer
MoS 2 membrane of only 7 nm thick, grown by Chemical Vapor Deposition (CVD)
technique, has allowed for an excellent combination of high water permeability (>322
L·m
−2 ·h
−1 ·bar
−1 ) and high ionic sieving capability (>99%) for various seawater
salts including Na
+ , K
+ , Ca
2+ , and Mg
2+ with a range of concentrations [12]. Near
100% of salt ion rejection rates for actual seawater obtained from the Atlantic coast
was also reported, significantly outperforming the previously developed 2D MoS 2
layer membranes of micrometer thickness as well as conventional reverse osmosis
(RO) membranes. These results confirmed previous theoretical and computational
predictions about desalination capacity and water permeation of 2D nanoporous
MoS 2 [13, 14].
Advancing the search for high-performance lamellar separation membranes, Sun
et al. [15] investigated the potential of WS 2 , a semiconductor material similar to
MoS 2 , with a Young’s modulus of ~270 GPa [16]. The bulk structure was exfoliated
and a thin film was constructed via filtration. The lamellar WS 2 membrane exhibited
water flux five times greater than GO membranes and two times greater than MoS 2
laminar membranes, rejecting 90% of Evans blue molecules. This impressive water
permeance further increased from 450 to 930 L·m
−2 ·h
−1 ·bar
−1 with the addition of
metal hydroxide nanostrands. They created additional channels between the WS 2
layers allowing for increased water transport without degrading the membrane’s salt
rejection properties.
Understanding the mechanisms and peculiarities of 2D membrane-based desalination is the ultimate frontier to reach industrial scale. So far, cutting-edge theoretical
work has been driving advances and pointing directions for experimental work with
some success. But we still need more. We need to merge theoretical advances with
new experimental approaches, such as the scalable method to controllably make
nanopores in single-layer TMDs or nanoscale water velocity profile mapping introduced by Secchi and collaborators [17], always sharing the goal of making largescale application of MoS 2 and WS 2 membranes in water purification possible. It is