Nano-Porous Graphene as Free-Standing Membranes
67
The generation of pores across pristine graphene or other graphitic materials
can be achieved by various perforation routes usually classified into four categories: stochastic etching, guided-etching, guided growth techniques [165] and selfassembly. Among them, irradiation etching, and guided growth are ideal for RO
membranes.
(i) Etching routes are conducted via removing a few carbon clusters from the
graphitic basal plane of graphene sheets by providing the minimum ablating
energy to break the C = C sp
2 bonds and generate a pore behind in the graphene
structure [26]. This activation energy can be delivered by several excitation
sources, such as irradiation [166, 167], thermal [168] and chemical/plasma
[27, 31, 158, 169–171] processes. Irradiation itself can be via gamma [172],
electron [173, 174], ion [33, 166, 173, 175], electric pulses [176, 177] and
ultraviolet [27] routes. Etching based on irradiation techniques displayed a
pore size range of 0.5–1.2 nm and pore densities across graphitic structures
between 10
3 and 10
6 pore/cm
2 based on treatment conditions [178, 179].
Control of power and time of oxygen plasma which also induces nanometrescale pores in suspended single-layer graphene; has resulted in 1–10 nm wide
pores at a high density [31, 158, 171, 180, 181]. Etching based on chemical etchants and thermal activations were dependent on the development of
intrinsic defective sites at different etchant concentrations, operating pH and
temperatures [182–184]. These techniques exhibited a wide range of pore
sizes between 10–100 nm and a limited surface density <10
2 , with kinetic
rates varying from a few minutes to several hours [27, 168]. While these
etching methods produce a distribution of pore sizes and possibly work better
in single-layer materials, Focused Ion Beams (FIB) can precisely machine a
large number of individual pores down to ~8 nm in diameter [29] and electron beams focused below 1 nm can generate precise sub-nanometer to fewnanometer pores [185, 186]. One of the early works that showed that nanopores
can be introduced onto 2D suspended graphene was reported by Fischbein and
Drndic in 2008, who used focused electron to drill nanometer sized holes in
multilayer graphene [187]. However, machining using FIB is less scalable and
appropriate for smaller membranes, whereas electrochemical methods and
machining using atomic force microscopy (AFM) or tightly focused electron
beams are suitable for the creation of a few pores for microscale membranes
[170].
(ii) Guided-etching perforation routes involve a temporary porous template via
lithography [188] and pore mediators [189]. Several templates were used
that may include continuous porous structures in addition to nanospheres and
nanoparticles [189, 190] for localizing the oxidative energies during the perforation process. This method offered a pore size distribution of 20–200 nm and
pore density distribution between 10
4 and 10
1 pores/µm
2 . However, functionalization of such pore mediators for patterning into ordered arrays via
linkers [191] may increase the porosity level and provide better distribution
and anchoring across graphitic surfaces.
67
The generation of pores across pristine graphene or other graphitic materials
can be achieved by various perforation routes usually classified into four categories: stochastic etching, guided-etching, guided growth techniques [165] and selfassembly. Among them, irradiation etching, and guided growth are ideal for RO
membranes.
(i) Etching routes are conducted via removing a few carbon clusters from the
graphitic basal plane of graphene sheets by providing the minimum ablating
energy to break the C = C sp
2 bonds and generate a pore behind in the graphene
structure [26]. This activation energy can be delivered by several excitation
sources, such as irradiation [166, 167], thermal [168] and chemical/plasma
[27, 31, 158, 169–171] processes. Irradiation itself can be via gamma [172],
electron [173, 174], ion [33, 166, 173, 175], electric pulses [176, 177] and
ultraviolet [27] routes. Etching based on irradiation techniques displayed a
pore size range of 0.5–1.2 nm and pore densities across graphitic structures
between 10
3 and 10
6 pore/cm
2 based on treatment conditions [178, 179].
Control of power and time of oxygen plasma which also induces nanometrescale pores in suspended single-layer graphene; has resulted in 1–10 nm wide
pores at a high density [31, 158, 171, 180, 181]. Etching based on chemical etchants and thermal activations were dependent on the development of
intrinsic defective sites at different etchant concentrations, operating pH and
temperatures [182–184]. These techniques exhibited a wide range of pore
sizes between 10–100 nm and a limited surface density <10
2 , with kinetic
rates varying from a few minutes to several hours [27, 168]. While these
etching methods produce a distribution of pore sizes and possibly work better
in single-layer materials, Focused Ion Beams (FIB) can precisely machine a
large number of individual pores down to ~8 nm in diameter [29] and electron beams focused below 1 nm can generate precise sub-nanometer to fewnanometer pores [185, 186]. One of the early works that showed that nanopores
can be introduced onto 2D suspended graphene was reported by Fischbein and
Drndic in 2008, who used focused electron to drill nanometer sized holes in
multilayer graphene [187]. However, machining using FIB is less scalable and
appropriate for smaller membranes, whereas electrochemical methods and
machining using atomic force microscopy (AFM) or tightly focused electron
beams are suitable for the creation of a few pores for microscale membranes
[170].
(ii) Guided-etching perforation routes involve a temporary porous template via
lithography [188] and pore mediators [189]. Several templates were used
that may include continuous porous structures in addition to nanospheres and
nanoparticles [189, 190] for localizing the oxidative energies during the perforation process. This method offered a pore size distribution of 20–200 nm and
pore density distribution between 10
4 and 10
1 pores/µm
2 . However, functionalization of such pore mediators for patterning into ordered arrays via
linkers [191] may increase the porosity level and provide better distribution
and anchoring across graphitic surfaces.