Three-Dimensional and Lamellar Graphene Oxide Membranes …
95
Although the difference between permeability and separation performance remains a
challenge, there have been some huge advancements. For instance, a recent work by
Thebo et al. [44] evaluated a class of rGO membranes, where the distance between the
lamellae was controlled using the amino acid theanine and tannic acid as a reducing
and cross-linking agent. They found a water permeability above 10,000 L · m
−2 · h
−1
· bar
−1 , which is up to three orders of magnitude times higher than the rates described
for similar materials and commercial membranes. The material also showed high
selectivity, retaining 100% of some dyes, such as rhodamine B and methylene blue,
with no sign of damage or delamination in water, acid, and basic media, even after
several months. It strikes that we can make use of small organic molecules in order
to design enhanced 2D nanofluidic channels based on GO lamellar membranes.
Adjusting GO’s degree of reduction can also be a viable strategy to improve
permeability. Membranes that are prepared using different types of GO, with varying
degrees of reduction or originating from different graphite sources, have very
different purification performances. These differences are related to the oxygenated
functional groups—hydroxyl (−OH), epoxy (−COC−), and carboxyl (−COOH).
However, the role of these different functional groups is not yet fully understood.
Yu et al. [45] prepared and systematically studied three types of GO membranes.
It included dominant oxygenated functional groups: dominant carboxyl, dominant
hydroxyl GO, and dominant epoxy GO. They noted that the functional groups
containing oxygen with different hydrophilicity and electricity controlled the spacing
between membrane layers, resulting in different water permeability and ion retention
rates. A combination of hydrophilic and electrostatic interactions was then appointed
as the main ingredient affecting those properties.
The ionic permeability rate can be controlled by adapting the nanochannel size
within a sub-nanometer range. This can be achieved by controlling the reduction
degree of the GO nanosheets exposing them to hydroiodic acid vapor, based on time
variation. As reduction increases, the size of the nanometric channels decreases,
enhancing ionic retention in the GO membrane [46].
GO processing also plays a crucial role in the membrane’s main characteristics. For
example, the sonication time during preparation affects the size of the GO nanosheet,
the amount and distribution of defects, the surface morphology, the degree of oxidation, and consequently, the spacing between lamellae. These characteristics directly
influence the permeability and the ability of the membrane to retain molecules and
ions of interest [47].
The path length that water must travel along the membrane, specifically through
nanosheet junctions, is currently one of the main challenges in GO membranes.
Shortening this distance is one of the most effective ways to improve membrane’s
permeability. To this end, some treatments (e.g., thermal treatments) can be used
to open pores in GO membranes in a controlled manner. These holey-GO (hGO)
membranes have been shown to undergo up to 3.8 times higher water permeability
relative to regular GO membranes, despite being up to 4 times thicker [48]. The
water path can also be facilitated by an interlayer spacing control, such as the one
presented by Dong et al. [49]. They prepared a new kind of stimuli-responsive GObased membrane with reversible, gas-tunable water permeability: water flux remains
95
Although the difference between permeability and separation performance remains a
challenge, there have been some huge advancements. For instance, a recent work by
Thebo et al. [44] evaluated a class of rGO membranes, where the distance between the
lamellae was controlled using the amino acid theanine and tannic acid as a reducing
and cross-linking agent. They found a water permeability above 10,000 L · m
−2 · h
−1
· bar
−1 , which is up to three orders of magnitude times higher than the rates described
for similar materials and commercial membranes. The material also showed high
selectivity, retaining 100% of some dyes, such as rhodamine B and methylene blue,
with no sign of damage or delamination in water, acid, and basic media, even after
several months. It strikes that we can make use of small organic molecules in order
to design enhanced 2D nanofluidic channels based on GO lamellar membranes.
Adjusting GO’s degree of reduction can also be a viable strategy to improve
permeability. Membranes that are prepared using different types of GO, with varying
degrees of reduction or originating from different graphite sources, have very
different purification performances. These differences are related to the oxygenated
functional groups—hydroxyl (−OH), epoxy (−COC−), and carboxyl (−COOH).
However, the role of these different functional groups is not yet fully understood.
Yu et al. [45] prepared and systematically studied three types of GO membranes.
It included dominant oxygenated functional groups: dominant carboxyl, dominant
hydroxyl GO, and dominant epoxy GO. They noted that the functional groups
containing oxygen with different hydrophilicity and electricity controlled the spacing
between membrane layers, resulting in different water permeability and ion retention
rates. A combination of hydrophilic and electrostatic interactions was then appointed
as the main ingredient affecting those properties.
The ionic permeability rate can be controlled by adapting the nanochannel size
within a sub-nanometer range. This can be achieved by controlling the reduction
degree of the GO nanosheets exposing them to hydroiodic acid vapor, based on time
variation. As reduction increases, the size of the nanometric channels decreases,
enhancing ionic retention in the GO membrane [46].
GO processing also plays a crucial role in the membrane’s main characteristics. For
example, the sonication time during preparation affects the size of the GO nanosheet,
the amount and distribution of defects, the surface morphology, the degree of oxidation, and consequently, the spacing between lamellae. These characteristics directly
influence the permeability and the ability of the membrane to retain molecules and
ions of interest [47].
The path length that water must travel along the membrane, specifically through
nanosheet junctions, is currently one of the main challenges in GO membranes.
Shortening this distance is one of the most effective ways to improve membrane’s
permeability. To this end, some treatments (e.g., thermal treatments) can be used
to open pores in GO membranes in a controlled manner. These holey-GO (hGO)
membranes have been shown to undergo up to 3.8 times higher water permeability
relative to regular GO membranes, despite being up to 4 times thicker [48]. The
water path can also be facilitated by an interlayer spacing control, such as the one
presented by Dong et al. [49]. They prepared a new kind of stimuli-responsive GObased membrane with reversible, gas-tunable water permeability: water flux remains