Three-Dimensional and Lamellar Graphene Oxide Membranes …
93
The low cost and the massive scalability are some of the advantages associated with graphene’s reduction. Furthermore, the starting material is pure graphite,
and the technique can easily be adapted for the production of “chemically modified graphenes”. An additional advantage is that chemical oxidation of graphite is
currently considered the only viable method of synthesis for obtaining industrialscale membranes for water purification based on graphene [36].
3 Experimental Works
3.1 GO/rGO Lamellar Membranes
The last decade witnessed significant advances in the experimental synthesis and
characterization of GO and rGO. There are different stacking possibilities for these
structures according to the preparation method we choose to use. In order to get their
maximum potential, a smart approach could be to organize them in a lamellar architecture. Lamellar GO and rGO are used as building blocks in advanced membrane
devices. Compared with others, these structures come up with many advantages such
as high energy efficiency and price-performance ratio, reduced dimensions, and easy
operation [37].
In addition, lamellar GO/rGO membranes present a highly active chemical
surface—favoring adsorptive processes and selectivity—and very stable porous
structures—with internal spaces of high permeability and the possibility of multifunctionality. Nevertheless, to obtain all these advantages, it is necessary to guarantee
an efficient, low-cost assembly methodology.
The most common preparation methods of graphene-based separation membranes
include vacuum-assisted/pressure-assisted, self-assembly, casting/coating, and layerby-layer (LBL). The vacuum filtration method is a straightforward process based
on the filtration of GO/rGO dispersions. It allows for almost parallel deposition of
GO/rGO sheets on top of each other over a porous base membrane (e.g., filtration or
ultrafiltration membranes), as depicted in Fig. 2a. In this process, the membrane’s
thickness can be easily controlled by parameters such as the dispersion concentration
and volume, and the degree of functionalization of the graphene species [38]. After
drying, the deposited GO/rGO lamellar membrane can be peeled off using tweezers
or, in exceptional cases, immersing the base membrane containing the GO/rGO
in a bath where the two membranes have different affinities with the solvent, in
such a way that they separate from each other (in a process called phase inversion). The organization of the sheets along the membrane depends on the filtration
process: while the pressure-assisted approach results in a highly ordered membrane,
only a partially organized membrane can be produced by vacuum-assisted filtration.
The self-assembly method, with solvent evaporation, leads to a very heterogeneous
membrane [39].
93
The low cost and the massive scalability are some of the advantages associated with graphene’s reduction. Furthermore, the starting material is pure graphite,
and the technique can easily be adapted for the production of “chemically modified graphenes”. An additional advantage is that chemical oxidation of graphite is
currently considered the only viable method of synthesis for obtaining industrialscale membranes for water purification based on graphene [36].
3 Experimental Works
3.1 GO/rGO Lamellar Membranes
The last decade witnessed significant advances in the experimental synthesis and
characterization of GO and rGO. There are different stacking possibilities for these
structures according to the preparation method we choose to use. In order to get their
maximum potential, a smart approach could be to organize them in a lamellar architecture. Lamellar GO and rGO are used as building blocks in advanced membrane
devices. Compared with others, these structures come up with many advantages such
as high energy efficiency and price-performance ratio, reduced dimensions, and easy
operation [37].
In addition, lamellar GO/rGO membranes present a highly active chemical
surface—favoring adsorptive processes and selectivity—and very stable porous
structures—with internal spaces of high permeability and the possibility of multifunctionality. Nevertheless, to obtain all these advantages, it is necessary to guarantee
an efficient, low-cost assembly methodology.
The most common preparation methods of graphene-based separation membranes
include vacuum-assisted/pressure-assisted, self-assembly, casting/coating, and layerby-layer (LBL). The vacuum filtration method is a straightforward process based
on the filtration of GO/rGO dispersions. It allows for almost parallel deposition of
GO/rGO sheets on top of each other over a porous base membrane (e.g., filtration or
ultrafiltration membranes), as depicted in Fig. 2a. In this process, the membrane’s
thickness can be easily controlled by parameters such as the dispersion concentration
and volume, and the degree of functionalization of the graphene species [38]. After
drying, the deposited GO/rGO lamellar membrane can be peeled off using tweezers
or, in exceptional cases, immersing the base membrane containing the GO/rGO
in a bath where the two membranes have different affinities with the solvent, in
such a way that they separate from each other (in a process called phase inversion). The organization of the sheets along the membrane depends on the filtration
process: while the pressure-assisted approach results in a highly ordered membrane,
only a partially organized membrane can be produced by vacuum-assisted filtration.
The self-assembly method, with solvent evaporation, leads to a very heterogeneous
membrane [39].