Nano-Porous Graphene as Free-Standing Membranes
49
The support layer (usually made of PSF) is significantly more porous and thicker
than the active layer with cavities around 0.1−0.5 µm and an overall layer thickness
>100 µm [20, 21, 88, 89]. Ultrafiltration membranes with pores as narrow as 10 nm
have also been utilized as support layers for RO membranes [92]. Like polyamide
active layers, nano-porous atomically thin membranes must also lie on top of a porous
substrate for the reasons explained earlier. Figure 3 schematically compares an ideal
nano-porous atomically thin membrane structure and its transport mechanism with
state-of-the-art-membranes. Note that for simplicity, the active layer in the latter is
depicted as non-porous while, there are porous versions employed in RO technology
as well. Furthermore, the structure of the support layer in this image is inspired by
an asymmetric ultrafiltration membrane with the finger-like macro-voids cast from
PSF in [93].
In an ideal situation, the atomically flat membrane as the active layer, with the least
thickness (~0.35 nm) would sit on top of a porous support layer. This layer has wide
pores and a much lower thickness than its counterpart in polymeric membranes but
still provides sufficient mechanical support for the active layer. Through any of the
mechanisms of perforation techniques (see Sect. 4.3), the active layer would become
nano-porous with ideal size and distribution of nanopores across the membrane.
Therefore, the transport mechanism of salts and water across this membrane would
obey the size exclusion process (Fig. 3). In contrast, the polymeric membrane with
an average thickness of 200 nm, with high surface roughness, sits on top of a thick,
porous support layer with narrow pores. The dominant transport mechanism in this
structure is based on the solution-diffusion process particularly if the active layer is
Fig. 3 Cross-section schematic illustration of ideal nano-porous atomically thin membrane (on
the left) versus state-of-the-art membranes structure (on the right) and a comparison between ion
rejection mechanisms in the two
49
The support layer (usually made of PSF) is significantly more porous and thicker
than the active layer with cavities around 0.1−0.5 µm and an overall layer thickness
>100 µm [20, 21, 88, 89]. Ultrafiltration membranes with pores as narrow as 10 nm
have also been utilized as support layers for RO membranes [92]. Like polyamide
active layers, nano-porous atomically thin membranes must also lie on top of a porous
substrate for the reasons explained earlier. Figure 3 schematically compares an ideal
nano-porous atomically thin membrane structure and its transport mechanism with
state-of-the-art-membranes. Note that for simplicity, the active layer in the latter is
depicted as non-porous while, there are porous versions employed in RO technology
as well. Furthermore, the structure of the support layer in this image is inspired by
an asymmetric ultrafiltration membrane with the finger-like macro-voids cast from
PSF in [93].
In an ideal situation, the atomically flat membrane as the active layer, with the least
thickness (~0.35 nm) would sit on top of a porous support layer. This layer has wide
pores and a much lower thickness than its counterpart in polymeric membranes but
still provides sufficient mechanical support for the active layer. Through any of the
mechanisms of perforation techniques (see Sect. 4.3), the active layer would become
nano-porous with ideal size and distribution of nanopores across the membrane.
Therefore, the transport mechanism of salts and water across this membrane would
obey the size exclusion process (Fig. 3). In contrast, the polymeric membrane with
an average thickness of 200 nm, with high surface roughness, sits on top of a thick,
porous support layer with narrow pores. The dominant transport mechanism in this
structure is based on the solution-diffusion process particularly if the active layer is
Fig. 3 Cross-section schematic illustration of ideal nano-porous atomically thin membrane (on
the left) versus state-of-the-art membranes structure (on the right) and a comparison between ion
rejection mechanisms in the two