92
M. H. Köhler et al.
Still, it also contains functional groups such as hydroxyl (−OH), alkoxy (C−O−C),
carbonyl (C−O), the carboxylic acid (−COOH), and others with oxygen-based functions. At the basal level, GO is essentially hydrophobic, formed by a network of
unoxidized aromatic polycyclic benzene ring islands. Given these characteristics,
GO is seen as an amphiphilic molecule with a basal plane mostly hydrophobic and
hydrophilic edges [31].
The maximum dispersibility of GO varies from 1 to 4 mg per mL of water, an
essential factor for processing and subsequent application. This value depends on
both the solvent and the degree of surface functionalization during oxidation. Due
to these amphiphilic characteristics, GO has also been applied as an alternative to
surfactant materials.
The presence of oxygenated groups also allows GO to act as a platform for various
chemical reactions. From the functionalization—with the most different organic,
inorganic, and biomolecule groups—to the reduction of GO, the particular conditions
result in graphene materials with modified properties. Some of these properties are
fundamental in the construction of membranes for water purification, increasing their
permeability and selectivity.
2.1 Reduced Graphene Oxide
The reduction of GO, that is, the removal of oxygenated groups, results in the formation of rGO. GO reduction can be accomplished through chemical (with reducing
agents such as hydrazine, metal hydrides, NaOH, hydroiodic and ascorbic acids),
thermal, or electrochemical processes [32]. All of them lead to products that resemble,
in different degrees, the pristine graphene obtained by peeling the graphite, particularly in terms of electrical, thermal, and mechanical properties as well as its surface
morphology.
An important characteristic of graphene obtained by graphite oxidation methods
is the presence of a high number of structural defects. These defects consist mainly of
(i) the organization of carbon atoms in the form of pentagons and heptagons, known
as stone-walled structures, forming perfect graphene “islands” (carbons hexagonally
linked to other sp
2 -hybridized carbons) surrounded by defective regions (carbons
linked to other carbons with hybridizations other than sp
2 ) and (ii) in the remaining
oxygenated groups after reduction.
Topological defects, as well as vacancies, impurities, and functionalization, can
introduce important changes in the properties of the material. This, in turn, can
influence the membrane’s behavior [33, 34].
The form of the reduction is also essential to define the material’s structure.
For example, some thermochemical reductions lead to the formation of porous
three-dimensional macrostructures with fascinating characteristics for environmental
applications in the purification of water and gas separation [35]. These materials are
discussed further in Sect. 3.2.2.
M. H. Köhler et al.
Still, it also contains functional groups such as hydroxyl (−OH), alkoxy (C−O−C),
carbonyl (C−O), the carboxylic acid (−COOH), and others with oxygen-based functions. At the basal level, GO is essentially hydrophobic, formed by a network of
unoxidized aromatic polycyclic benzene ring islands. Given these characteristics,
GO is seen as an amphiphilic molecule with a basal plane mostly hydrophobic and
hydrophilic edges [31].
The maximum dispersibility of GO varies from 1 to 4 mg per mL of water, an
essential factor for processing and subsequent application. This value depends on
both the solvent and the degree of surface functionalization during oxidation. Due
to these amphiphilic characteristics, GO has also been applied as an alternative to
surfactant materials.
The presence of oxygenated groups also allows GO to act as a platform for various
chemical reactions. From the functionalization—with the most different organic,
inorganic, and biomolecule groups—to the reduction of GO, the particular conditions
result in graphene materials with modified properties. Some of these properties are
fundamental in the construction of membranes for water purification, increasing their
permeability and selectivity.
2.1 Reduced Graphene Oxide
The reduction of GO, that is, the removal of oxygenated groups, results in the formation of rGO. GO reduction can be accomplished through chemical (with reducing
agents such as hydrazine, metal hydrides, NaOH, hydroiodic and ascorbic acids),
thermal, or electrochemical processes [32]. All of them lead to products that resemble,
in different degrees, the pristine graphene obtained by peeling the graphite, particularly in terms of electrical, thermal, and mechanical properties as well as its surface
morphology.
An important characteristic of graphene obtained by graphite oxidation methods
is the presence of a high number of structural defects. These defects consist mainly of
(i) the organization of carbon atoms in the form of pentagons and heptagons, known
as stone-walled structures, forming perfect graphene “islands” (carbons hexagonally
linked to other sp
2 -hybridized carbons) surrounded by defective regions (carbons
linked to other carbons with hybridizations other than sp
2 ) and (ii) in the remaining
oxygenated groups after reduction.
Topological defects, as well as vacancies, impurities, and functionalization, can
introduce important changes in the properties of the material. This, in turn, can
influence the membrane’s behavior [33, 34].
The form of the reduction is also essential to define the material’s structure.
For example, some thermochemical reductions lead to the formation of porous
three-dimensional macrostructures with fascinating characteristics for environmental
applications in the purification of water and gas separation [35]. These materials are
discussed further in Sect. 3.2.2.