Molybdenum Disulfide and Tungsten Disulfide as Novel …
203
4 Feasibility of Free-Standing MoS 2 and WS 2 Membranes
in Filtration
Recently, free-standing membranes of 2D materials have attracted attention due to
their wide range of applications, including piezoelectricity in MoS 2 [51], singlemolecule DNA sensing [52], high Young modulus [9], and water desalination [11,
53]. They can also be used to investigate fundamental properties of 2D materials
since free-standing membranes are not influenced by underlying substrates [54–56].
Free-standing WS 2 membranes have also been produced using similar methods.
The most used approach to produce these membranes is to use a substrate containing
an array of circular or square holes, usually produced by chemical or e-beam lithography. After that, the desirable 2D material (MoS 2 or WS 2 ) is transferred to this
substrate’s surface. Since the transference process of CVD 2D materials can be
difficult, the use of exfoliated materials is preferred.
Eda et al. [57] have prepared MoS 2 nanosheets by chemical exfoliation using
organolithium intercalation and forced hydration. Li intercalates between MoS 2
layers, and when it reacts with water, it increases the plane spacing with hydrogen
gas. The resulting MoS 2 is then used to produce layer-stacked membranes. Using a
CVD approach, Waduge et al. [56] prepared micrometer-scale apertures in silicon
nitride membranes, which were placed above a boat containing MoO 2 . A second boat
containing sulfur was heated and the carrier gas (Ar) transported the sulfur vapor to
the silicon nitride membranes. The growth process takes place at 750
z C leading to
selective MoS 2 growth near the apertures.
4.1 MoS 2 and WS 2 Membranes Compared to Other 1D/2D
Materials
Figure 3 compares MoS 2 /WS 2 , graphene, and CNT membranes. One of the most
interesting discoveries of our time and hypothesized as fruitful for a large range
of future technological applications is the role of dimensionality in determining the
properties of a material. Ultrathin 2D nanosheets of layered TMDs are fundamentally
intriguing. Aside from presenting electronic properties diverse from the bulk, this
group of materials exhibits versatile chemistry in contrast with graphene’s chemically
inert behavior [35]. Furthermore, 1D materials have attracted a lot of attention once
theoretical and experimental observations led to the discovery of anomalous water
transport under certain conditions, enabling nanofluidic flow enhancement in CNTs
[58]. In this section, the MoS 2 and WS 2 membranes will be compared to other 1D/2D
materials from the computational and experimental perspective.
A general 2D material can be used to transform a very thick membrane into two
kinds of construction design: a nanoporous membrane or a layer-stacked membrane
[59]. In terms of desalination purposes, nanoporous membranes of MoS 2 and WS 2 are
very peculiar for they present a mix of hydrophobic and hydrophilic edges, which
203
4 Feasibility of Free-Standing MoS 2 and WS 2 Membranes
in Filtration
Recently, free-standing membranes of 2D materials have attracted attention due to
their wide range of applications, including piezoelectricity in MoS 2 [51], singlemolecule DNA sensing [52], high Young modulus [9], and water desalination [11,
53]. They can also be used to investigate fundamental properties of 2D materials
since free-standing membranes are not influenced by underlying substrates [54–56].
Free-standing WS 2 membranes have also been produced using similar methods.
The most used approach to produce these membranes is to use a substrate containing
an array of circular or square holes, usually produced by chemical or e-beam lithography. After that, the desirable 2D material (MoS 2 or WS 2 ) is transferred to this
substrate’s surface. Since the transference process of CVD 2D materials can be
difficult, the use of exfoliated materials is preferred.
Eda et al. [57] have prepared MoS 2 nanosheets by chemical exfoliation using
organolithium intercalation and forced hydration. Li intercalates between MoS 2
layers, and when it reacts with water, it increases the plane spacing with hydrogen
gas. The resulting MoS 2 is then used to produce layer-stacked membranes. Using a
CVD approach, Waduge et al. [56] prepared micrometer-scale apertures in silicon
nitride membranes, which were placed above a boat containing MoO 2 . A second boat
containing sulfur was heated and the carrier gas (Ar) transported the sulfur vapor to
the silicon nitride membranes. The growth process takes place at 750
z C leading to
selective MoS 2 growth near the apertures.
4.1 MoS 2 and WS 2 Membranes Compared to Other 1D/2D
Materials
Figure 3 compares MoS 2 /WS 2 , graphene, and CNT membranes. One of the most
interesting discoveries of our time and hypothesized as fruitful for a large range
of future technological applications is the role of dimensionality in determining the
properties of a material. Ultrathin 2D nanosheets of layered TMDs are fundamentally
intriguing. Aside from presenting electronic properties diverse from the bulk, this
group of materials exhibits versatile chemistry in contrast with graphene’s chemically
inert behavior [35]. Furthermore, 1D materials have attracted a lot of attention once
theoretical and experimental observations led to the discovery of anomalous water
transport under certain conditions, enabling nanofluidic flow enhancement in CNTs
[58]. In this section, the MoS 2 and WS 2 membranes will be compared to other 1D/2D
materials from the computational and experimental perspective.
A general 2D material can be used to transform a very thick membrane into two
kinds of construction design: a nanoporous membrane or a layer-stacked membrane
[59]. In terms of desalination purposes, nanoporous membranes of MoS 2 and WS 2 are
very peculiar for they present a mix of hydrophobic and hydrophilic edges, which