10.5 Top-Down Electrochemical Synthesis of Nanosheets
341
10.5 Top-Down Electrochemical Synthesis of Nanosheets
10.5.1 Intercalation and Accompanying Exfoliation
Processes
Electrochemical exfoliation of nanolayers, especially of graphite, belongs to the
liquid-phase exfoliation methods [54]. It is a progressively expanding field of
graphene production with several tens of papers appearing every year; therefore,
it is not possible to strive for completeness. Reviews of various sub-topics of this
field are also available [55–58].
Nanosheet-forming methods by electrochemical processes are all based on the
ingress of ions or molecules into materials exhibiting a lamellar structure. In the case
of delamination processes, the penetration of the species between the nanosheets
mostly cannot lead to an intercalation compound of well-defined composition. In
order to obtain nanosheets with monoatomic thickness, the prerequisite is that the
bond strength connecting atoms within a sheet are much stronger than those acting
between the planes. Therefore, an attack will primarily delaminate the layers, and
bond breaking between the atoms connected within a plane will be of smaller, though
not negligible, importance.
Historically, the irreversible change of graphite cathodes in the presence of
teraalkylammonium salts was described already at the end of 1970s [59, 60]. The first
studies published in this field dealt with the electrochemical behaviour of the resulting
modified electrode. Although the elucidation of the intercalation phenomena can be
taken as scientifically plausible also nowadays, no structural study was performed at
the time of the above-mentioned works.
The first application of the intercalation process was the lithium uptake of graphitic
carbon. This process leads to maximum lithium content at the composition of LiC 6
with various intermediate compositions, all of them having a regular equilibrium
structure concerning the Li distribution. Since the electrochemical lithium intercalation and deintercalation both are reversible, it could be taken advantage of in the
construction of lithium ion batteries, where graphitic carbon is the cheapest negative
electrode material till nowadays.
At this point, it is worth of comparing Li-intercalated graphite with metals
dissolving hydrogen. In both cases, the outer-shell s
1 electron of the intercalated
(absorbed) species becomes a part of the delocalized electron system of the host
lattice. Although the negative charge delocalized cannot be assigned to any location
of the host lattice, the intercalated (absorbed) species are Li
+ (H
+ ) and they exhibit a
large mobility due to their shrinkage caused by the ionization and the expansion of
the lattice due to the presence of the foreign interstitial species.
The intercalation of molecular compounds was also reported in the field of
graphitic materials. In particular, propylene carbonate was shown to be detrimental
because it co-intercalates with Li, hence leading to electrode degradation. This
process of harmful impact on battery materials was turned into a productive one
by using it for nanosheet production. The example of propylene carbonate shows
341
10.5 Top-Down Electrochemical Synthesis of Nanosheets
10.5.1 Intercalation and Accompanying Exfoliation
Processes
Electrochemical exfoliation of nanolayers, especially of graphite, belongs to the
liquid-phase exfoliation methods [54]. It is a progressively expanding field of
graphene production with several tens of papers appearing every year; therefore,
it is not possible to strive for completeness. Reviews of various sub-topics of this
field are also available [55–58].
Nanosheet-forming methods by electrochemical processes are all based on the
ingress of ions or molecules into materials exhibiting a lamellar structure. In the case
of delamination processes, the penetration of the species between the nanosheets
mostly cannot lead to an intercalation compound of well-defined composition. In
order to obtain nanosheets with monoatomic thickness, the prerequisite is that the
bond strength connecting atoms within a sheet are much stronger than those acting
between the planes. Therefore, an attack will primarily delaminate the layers, and
bond breaking between the atoms connected within a plane will be of smaller, though
not negligible, importance.
Historically, the irreversible change of graphite cathodes in the presence of
teraalkylammonium salts was described already at the end of 1970s [59, 60]. The first
studies published in this field dealt with the electrochemical behaviour of the resulting
modified electrode. Although the elucidation of the intercalation phenomena can be
taken as scientifically plausible also nowadays, no structural study was performed at
the time of the above-mentioned works.
The first application of the intercalation process was the lithium uptake of graphitic
carbon. This process leads to maximum lithium content at the composition of LiC 6
with various intermediate compositions, all of them having a regular equilibrium
structure concerning the Li distribution. Since the electrochemical lithium intercalation and deintercalation both are reversible, it could be taken advantage of in the
construction of lithium ion batteries, where graphitic carbon is the cheapest negative
electrode material till nowadays.
At this point, it is worth of comparing Li-intercalated graphite with metals
dissolving hydrogen. In both cases, the outer-shell s
1 electron of the intercalated
(absorbed) species becomes a part of the delocalized electron system of the host
lattice. Although the negative charge delocalized cannot be assigned to any location
of the host lattice, the intercalated (absorbed) species are Li
+ (H
+ ) and they exhibit a
large mobility due to their shrinkage caused by the ionization and the expansion of
the lattice due to the presence of the foreign interstitial species.
The intercalation of molecular compounds was also reported in the field of
graphitic materials. In particular, propylene carbonate was shown to be detrimental
because it co-intercalates with Li, hence leading to electrode degradation. This
process of harmful impact on battery materials was turned into a productive one
by using it for nanosheet production. The example of propylene carbonate shows