Three-Dimensional and Lamellar Graphene Oxide Membranes …
91
works on lamellar and 3D membranes, while the computational studies are shown
in Sect. 4. Current research gaps, perspectives, and conclusions end the chapter in
Sect. 5.
2 Preparation Methods of Graphene Oxide and Reduced
Graphene Oxide
Graphene is a 2D material formed by sp
2 -hybridized carbon atoms densely packed in
a regular atomic-scale chicken wire or honeycomb (hexagonal) pattern. Thanks to this
structure, graphene shows a significant number of desirable properties such as high
electrical and thermal conductivities, mechanical strength, molecular barrier capability, high specific surface area, and many others. Due to all these amazing features,
it is natural to think of graphene as an excellent candidate for water purification
membranes. However, the use of crystalline graphene in aqueous environments has
been challenging because of its low solubility and the vdW interactions that lead to
aggregation. The problematic scheduling of bottom-up synthesis methods does not
help either. Alternatively, compounds structurally similar to graphene can be chemically obtained from graphite or even other carbon sources. These graphene-based
species present many of the advantages of pure graphene, while also having other
properties thanks to the presence of oxygen groups functionalizing their surfaces.
The chemical oxidation of graphite leads to the formation of graphite oxide, which
consists of several stacked layers of GO. In the current literature, most of the methods
used in the oxidation of graphite are based on the first preparation by Brodie in 1859
and the subsequently developed Staudenmaier and Hummers methods, all of which
depending on the homogeneous reaction of graphite with strong mixed oxidants [28,
29]. For example, routes based on the Hummers method—undoubtedly the most
employed—usually involve a combination of oxidizing agents such as KMnO 4 and
H 2 SO 4 , leading to the formation of a super oxidizing agent Mn 2 O 7 :
KMnO 4(s) + 6H
+
(aq) + 3SO
2−
4(aq) → K
+
(aq) + MnO
+
3(aq) + H 3 O
+
+ 3HSO
−
4(aq)
MnO
+
3(aq) + MnO
−
4(aq) → Mn 2 O 7(aq)
New approaches to obtain GO involve different methods, many of them seeking
to decrease the use of strong oxidizers, the risk of explosion, environmental pollution
(due to synthesis-generated residues), and shorten the long reaction time. Recently,
a group from the Chinese Academy of Sciences at Shenyang described a scalable,
green synthesis of GO from graphite in a timescale of seconds [30]. They used a
water electrolytic oxidation method, where the oxidation degree of GO sheets can
be modulated by the concentration of H 2 SO 4 in solution, and the number of sheets
and lateral size can be controlled by the sonication time.
The GO can be isolated through exfoliation (e.g., by mechanical stirring or sonication) of graphite oxide. It exhibits a hexagonal carbon structure similar to graphene.
91
works on lamellar and 3D membranes, while the computational studies are shown
in Sect. 4. Current research gaps, perspectives, and conclusions end the chapter in
Sect. 5.
2 Preparation Methods of Graphene Oxide and Reduced
Graphene Oxide
Graphene is a 2D material formed by sp
2 -hybridized carbon atoms densely packed in
a regular atomic-scale chicken wire or honeycomb (hexagonal) pattern. Thanks to this
structure, graphene shows a significant number of desirable properties such as high
electrical and thermal conductivities, mechanical strength, molecular barrier capability, high specific surface area, and many others. Due to all these amazing features,
it is natural to think of graphene as an excellent candidate for water purification
membranes. However, the use of crystalline graphene in aqueous environments has
been challenging because of its low solubility and the vdW interactions that lead to
aggregation. The problematic scheduling of bottom-up synthesis methods does not
help either. Alternatively, compounds structurally similar to graphene can be chemically obtained from graphite or even other carbon sources. These graphene-based
species present many of the advantages of pure graphene, while also having other
properties thanks to the presence of oxygen groups functionalizing their surfaces.
The chemical oxidation of graphite leads to the formation of graphite oxide, which
consists of several stacked layers of GO. In the current literature, most of the methods
used in the oxidation of graphite are based on the first preparation by Brodie in 1859
and the subsequently developed Staudenmaier and Hummers methods, all of which
depending on the homogeneous reaction of graphite with strong mixed oxidants [28,
29]. For example, routes based on the Hummers method—undoubtedly the most
employed—usually involve a combination of oxidizing agents such as KMnO 4 and
H 2 SO 4 , leading to the formation of a super oxidizing agent Mn 2 O 7 :
KMnO 4(s) + 6H
+
(aq) + 3SO
2−
4(aq) → K
+
(aq) + MnO
+
3(aq) + H 3 O
+
+ 3HSO
−
4(aq)
MnO
+
3(aq) + MnO
−
4(aq) → Mn 2 O 7(aq)
New approaches to obtain GO involve different methods, many of them seeking
to decrease the use of strong oxidizers, the risk of explosion, environmental pollution
(due to synthesis-generated residues), and shorten the long reaction time. Recently,
a group from the Chinese Academy of Sciences at Shenyang described a scalable,
green synthesis of GO from graphite in a timescale of seconds [30]. They used a
water electrolytic oxidation method, where the oxidation degree of GO sheets can
be modulated by the concentration of H 2 SO 4 in solution, and the number of sheets
and lateral size can be controlled by the sonication time.
The GO can be isolated through exfoliation (e.g., by mechanical stirring or sonication) of graphite oxide. It exhibits a hexagonal carbon structure similar to graphene.