68
A. S. Kazemi and M. A. Abdol
(iii) The guided-growth perforation method can produce pores either during or
after the nucleation process of graphitic nanostructures [26]. During the CVD
growth, the graphitic nucleation can be terminated for specific locations on the
substrate to control gaps between graphene islands and to define pores between
them [190, 192–194]. In this method, pores could potentially be created
by controlling the ratio of the growth precursor and etchant (for example,
CH 4 and H 2 ). It has been shown that guided-growth perforation routes can
generate pores with dimensions ranging from a few Å up to sub-nanometres
and densities of 10
3 –10
5 pores/cm
2 [26].
(iv) In contrast to the creation of pores in initially non-porous material, recent
advances in self-assembly techniques have the potential for creating a high
density of atomically precise pores, directly in covalently bonded single-layer
materials [41, 195–200]. For example, surface-assisted synthesis of nanoporous graphene by aryl–aryl coupling of polyphenylene-based precursors or
cyclodehydrogenation has been proposed [195]. Furthermore, graphdiyne, a
new two-dimensional (2D) carbon allotrope, has been created at the millimeter
scale by cross-linking of hexaethynylbenzene on copper under nitrogen atmosphere [201]. Nevertheless, these approaches face key challenges in the
synthesis of continuous layers over sufficiently large areas for membrane
applications [202].
4.4 Characterization Tools of Nano-Porous Graphene
Membranes
Once mono or few layer graphene is fabricated via the CVD method, transferred
onto an appropriate porous support layer and perforated through any of the methods
discussed in the previous part, and prior to RO measurements; it is necessary to
characterize the surface of the membranes and inspect various surface features and
properties.
For assessment of continuity of free-standing surface over large scales, optical
microscopy, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) would be safe options. These methods allow large area
inspection, without damaging the membrane. FESEM is carried out on conductive
surfaces or on non-conductive surfaces coated with few nm of a metal. Although
pristine graphene is conductive, polymeric residues left from the transfer process
on CVD graphene are non-conductive and resolved images could not be captured
with the electron beam and often, a deposition of Au is necessary. AFM inspection
for continuity or number of layers is not recommended with contact mode, again
due to the numerous polymeric residues left on the surface but if careful enough
with tapping or non-contact mode, one may be lucky to scan across free-standing
membranes without having the tip ripping the surface. Yet, evaluating the number
of layers is highly recommended with Raman spectroscopy with 2D peak intensity
indications [31, 158, 203].
A. S. Kazemi and M. A. Abdol
(iii) The guided-growth perforation method can produce pores either during or
after the nucleation process of graphitic nanostructures [26]. During the CVD
growth, the graphitic nucleation can be terminated for specific locations on the
substrate to control gaps between graphene islands and to define pores between
them [190, 192–194]. In this method, pores could potentially be created
by controlling the ratio of the growth precursor and etchant (for example,
CH 4 and H 2 ). It has been shown that guided-growth perforation routes can
generate pores with dimensions ranging from a few Å up to sub-nanometres
and densities of 10
3 –10
5 pores/cm
2 [26].
(iv) In contrast to the creation of pores in initially non-porous material, recent
advances in self-assembly techniques have the potential for creating a high
density of atomically precise pores, directly in covalently bonded single-layer
materials [41, 195–200]. For example, surface-assisted synthesis of nanoporous graphene by aryl–aryl coupling of polyphenylene-based precursors or
cyclodehydrogenation has been proposed [195]. Furthermore, graphdiyne, a
new two-dimensional (2D) carbon allotrope, has been created at the millimeter
scale by cross-linking of hexaethynylbenzene on copper under nitrogen atmosphere [201]. Nevertheless, these approaches face key challenges in the
synthesis of continuous layers over sufficiently large areas for membrane
applications [202].
4.4 Characterization Tools of Nano-Porous Graphene
Membranes
Once mono or few layer graphene is fabricated via the CVD method, transferred
onto an appropriate porous support layer and perforated through any of the methods
discussed in the previous part, and prior to RO measurements; it is necessary to
characterize the surface of the membranes and inspect various surface features and
properties.
For assessment of continuity of free-standing surface over large scales, optical
microscopy, scanning electron microscopy (SEM) and field emission scanning electron microscopy (FE-SEM) would be safe options. These methods allow large area
inspection, without damaging the membrane. FESEM is carried out on conductive
surfaces or on non-conductive surfaces coated with few nm of a metal. Although
pristine graphene is conductive, polymeric residues left from the transfer process
on CVD graphene are non-conductive and resolved images could not be captured
with the electron beam and often, a deposition of Au is necessary. AFM inspection
for continuity or number of layers is not recommended with contact mode, again
due to the numerous polymeric residues left on the surface but if careful enough
with tapping or non-contact mode, one may be lucky to scan across free-standing
membranes without having the tip ripping the surface. Yet, evaluating the number
of layers is highly recommended with Raman spectroscopy with 2D peak intensity
indications [31, 158, 203].