104
M. H. Köhler et al.
is to build up from the molecular scale, using suitable precursor molecules. In 2010,
Li and co-workers [25] implemented this bottom-up method to synthesize for the first
time graphdiyne films with 1 μm thick. They used a cross-coupling reaction on Cu
foil surfaces and hexaethynylbenzene (HEB) as precursors to produce uniform multilayer films. Since then, a tremendous effort has been made to improve graphdiyne
synthesis. Even a β-graphdiyne-containing thin film with a thickness of about 25 nm
was fabricated [115]. However, experimental efforts have failed to produce structures other than graphdiynes. This is of particular concern because the best theoretical desalination results have been obtained for γ-graphyne-3, while graphdiyne
is impermeable. Improved surface engineering of substrates, rational design, and
synthesis of new precursors as well as the development of fundamentally new preparation technologies are needed to realize γ-graphyne-3 and others experimentally.
Theoretical works can also contribute to guiding microscopic processes, clarifying
mechanisms involved in the growth of large-area graphynes.
5 Conclusions, Challenges, and Perspectives
The reduction of GO is a delicate process, which can produce environmentally undesirable residues. Conventional strategies include thermal, chemical, and solvothermal
reduction. Despite thermal annealing being highly effective, it produces a large
amount of CO and CO 2 as a result of the elimination of oxygen (from the oxygencontaining functional groups) and carbon (from the pristine graphene regions in GO).
Thermal methods are usually very energy-consuming because of the elevated temperatures involved. On the other hand, chemical reducing agents can be more efficient,
but many chemical reagents are toxic and non-environmentally friendly. The search
for green chemical reagents (e.g., ascorbic acid (vitamin C), sugars, green tea) is a
current challenge that is bringing together large theoretical and experimental groups.
Hydrothermal reduction is also an emerging, green route that can produce stable rGO
suspensions in graphene hydrogels.
It is the transition from bulk to a nanoconfined fluid that makes possible
every membrane-based separation plant. Thus, separation efficiency in GO/rGO
membranes is closely related to their nano or sub-nanometer channels. If the
membrane is not stable enough, the channels do not hold and the entire process
is compromised. For instance, the interlayer channels can become wider when
converting from dry to hydrated states, in a process called swelling—which is
more prominent in GO rather than rGO. On the other hand, external pressure often
compacts the membrane, especially lamellar membranes. In this scenario, the pathways available for water molecules to cross the membrane are diminished and there
goes the membrane’s permeability. One alternative is the use of guest materials such
as nanotubes, molecules, or ions. These cross-linkers tend to amplify attractive forces
between host nanosheets, which significantly contribute to their structural stability.
The host material can even be another 2D nanosheet. For example, pristine graphene
and rGO with GO nanosheets can be blended to reduce intersheet repulsion and
M. H. Köhler et al.
is to build up from the molecular scale, using suitable precursor molecules. In 2010,
Li and co-workers [25] implemented this bottom-up method to synthesize for the first
time graphdiyne films with 1 μm thick. They used a cross-coupling reaction on Cu
foil surfaces and hexaethynylbenzene (HEB) as precursors to produce uniform multilayer films. Since then, a tremendous effort has been made to improve graphdiyne
synthesis. Even a β-graphdiyne-containing thin film with a thickness of about 25 nm
was fabricated [115]. However, experimental efforts have failed to produce structures other than graphdiynes. This is of particular concern because the best theoretical desalination results have been obtained for γ-graphyne-3, while graphdiyne
is impermeable. Improved surface engineering of substrates, rational design, and
synthesis of new precursors as well as the development of fundamentally new preparation technologies are needed to realize γ-graphyne-3 and others experimentally.
Theoretical works can also contribute to guiding microscopic processes, clarifying
mechanisms involved in the growth of large-area graphynes.
5 Conclusions, Challenges, and Perspectives
The reduction of GO is a delicate process, which can produce environmentally undesirable residues. Conventional strategies include thermal, chemical, and solvothermal
reduction. Despite thermal annealing being highly effective, it produces a large
amount of CO and CO 2 as a result of the elimination of oxygen (from the oxygencontaining functional groups) and carbon (from the pristine graphene regions in GO).
Thermal methods are usually very energy-consuming because of the elevated temperatures involved. On the other hand, chemical reducing agents can be more efficient,
but many chemical reagents are toxic and non-environmentally friendly. The search
for green chemical reagents (e.g., ascorbic acid (vitamin C), sugars, green tea) is a
current challenge that is bringing together large theoretical and experimental groups.
Hydrothermal reduction is also an emerging, green route that can produce stable rGO
suspensions in graphene hydrogels.
It is the transition from bulk to a nanoconfined fluid that makes possible
every membrane-based separation plant. Thus, separation efficiency in GO/rGO
membranes is closely related to their nano or sub-nanometer channels. If the
membrane is not stable enough, the channels do not hold and the entire process
is compromised. For instance, the interlayer channels can become wider when
converting from dry to hydrated states, in a process called swelling—which is
more prominent in GO rather than rGO. On the other hand, external pressure often
compacts the membrane, especially lamellar membranes. In this scenario, the pathways available for water molecules to cross the membrane are diminished and there
goes the membrane’s permeability. One alternative is the use of guest materials such
as nanotubes, molecules, or ions. These cross-linkers tend to amplify attractive forces
between host nanosheets, which significantly contribute to their structural stability.
The host material can even be another 2D nanosheet. For example, pristine graphene
and rGO with GO nanosheets can be blended to reduce intersheet repulsion and