induced etching, corroded surface of graphene is obtained by the interaction of
electron beam with pressured gas on the graphene layer which results in the growth
of nanopores (Killingsworth 2012). By various etching techniques, controlled and
stabilized nanopores with high selectivity can be developed on the graphene membrane, but extremely precise-sized pores cannot be obtained through this process.
Highly accurate methods like helium ion beam drilling and diblock copolymer
templating (Killingsworth 2012) need to be investigated to achieve precise-sized
nanopores. Nanoporous graphene membrane with high-density pores contains only
uniform-sized nanopores with high salt rejection rate (O’Hern et al. 2015).
Nanometer-sized pores can be introduced on graphene sheets by focused electron
beam irradiation (Huang et al. 2015). However, this method is inefficient and not
suitable for fabricating high-density porous graphene for practical application
(Huang et al. 2015). Selection of nanoporous graphene synthesis technique must
be based on influencing parameters in order to manufacture thin-layer graphene
membrane with size-controlled, high-density pores. Though graphene membrane is
efficient and suitable for desalination than other membranes, it has some limitations
like creation of size-tunable nanopores in membrane with narrow size distribution
and development of a continuous process for its synthesis. Future research works
focusing on these obstacles and resolving those uplift the utilization of graphene in
desalination process and make it a cost-effective nanoporous membrane.
4.10 Application of Nanofibers in Desalination
Nanofibers are one-dimensional nanomaterial with a diameter of 100 nanometers or
less. Nanofibers have been widely used in research and commercial applications due
to their outstanding physicochemical properties and characteristics. Among different
nanomaterials with promising potential applications, nanofibers stand out from the
rest of the materials. Most unique features of nanofibers include high surface-areato-volume ratio and high porosity, making them robust and attractive material for
advanced applications (Lim 2017). In recent decades nanofibers serve as attractive
material for various environmental applications including liquid filtration and particulate separation in water treatment (Lim 2017). Nanofibrous structures are used as
a filtration material which results in better particulate removal capacity, high separation flux, and lower operational energy (Lim 2017). One of the environmental
friendly and cost-effective techniques for removing salt ions from saline water is
capacitive deionization which works based on the principles of an electric doublelayer capacitor. Hollow carbon nanofibers are used as an effective electrode for
brackish water desalination using capacitive deionization process (El-Deen et al.
2014). The specific capacitance of hollow carbon nanofiber is four times greater
compared to the specific capacitance of solid carbon nanofibers. Moreover, the
surface area of the hollow nanofiber is ten times higher than the solid carbon
nanofibers which results in enhanced salt rejection (El-Deen et al. 2014). In addition
to hollow nanofibers, nanofiber membrane tailored with carbon nanotubes also
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
153
electron beam with pressured gas on the graphene layer which results in the growth
of nanopores (Killingsworth 2012). By various etching techniques, controlled and
stabilized nanopores with high selectivity can be developed on the graphene membrane, but extremely precise-sized pores cannot be obtained through this process.
Highly accurate methods like helium ion beam drilling and diblock copolymer
templating (Killingsworth 2012) need to be investigated to achieve precise-sized
nanopores. Nanoporous graphene membrane with high-density pores contains only
uniform-sized nanopores with high salt rejection rate (O’Hern et al. 2015).
Nanometer-sized pores can be introduced on graphene sheets by focused electron
beam irradiation (Huang et al. 2015). However, this method is inefficient and not
suitable for fabricating high-density porous graphene for practical application
(Huang et al. 2015). Selection of nanoporous graphene synthesis technique must
be based on influencing parameters in order to manufacture thin-layer graphene
membrane with size-controlled, high-density pores. Though graphene membrane is
efficient and suitable for desalination than other membranes, it has some limitations
like creation of size-tunable nanopores in membrane with narrow size distribution
and development of a continuous process for its synthesis. Future research works
focusing on these obstacles and resolving those uplift the utilization of graphene in
desalination process and make it a cost-effective nanoporous membrane.
4.10 Application of Nanofibers in Desalination
Nanofibers are one-dimensional nanomaterial with a diameter of 100 nanometers or
less. Nanofibers have been widely used in research and commercial applications due
to their outstanding physicochemical properties and characteristics. Among different
nanomaterials with promising potential applications, nanofibers stand out from the
rest of the materials. Most unique features of nanofibers include high surface-areato-volume ratio and high porosity, making them robust and attractive material for
advanced applications (Lim 2017). In recent decades nanofibers serve as attractive
material for various environmental applications including liquid filtration and particulate separation in water treatment (Lim 2017). Nanofibrous structures are used as
a filtration material which results in better particulate removal capacity, high separation flux, and lower operational energy (Lim 2017). One of the environmental
friendly and cost-effective techniques for removing salt ions from saline water is
capacitive deionization which works based on the principles of an electric doublelayer capacitor. Hollow carbon nanofibers are used as an effective electrode for
brackish water desalination using capacitive deionization process (El-Deen et al.
2014). The specific capacitance of hollow carbon nanofiber is four times greater
compared to the specific capacitance of solid carbon nanofibers. Moreover, the
surface area of the hollow nanofiber is ten times higher than the solid carbon
nanofibers which results in enhanced salt rejection (El-Deen et al. 2014). In addition
to hollow nanofibers, nanofiber membrane tailored with carbon nanotubes also
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
153