10.5 Top-Down Electrochemical Synthesis of Nanosheets
345
Since the process is highly empirical, there is a wide platform to test new and new
experimental circumstances.
Concerning the compounds used in the electrolyte solution, simple inorganic salts
are very common. Sulphate ion is one of the species whose incorporation and the
oxidation/decomposition to SO 2 is thought to play a key role in the anodic exfoliation;
therefore, various compounds containing sulphate ions can be used (dilute [62, 63]
or concentrated [75] H 2 SO 4 , (NH 4 ) 2 SO 4 [70, 74, 76, 79], alkaline metal sulphates
[68, 76] or oxone [66]). A feasible pursuit in the choice of the solute is that it should
be oxidized easier than graphene, hence reducing the oxygen content of the product.
For this reason, hydrogen peroxide is often applied as solution component [70, 74],
and sodium halides as electrolyte were also used successfully [67, 77]. Nevertheless,
the exfoliation works also with various types of inorganic and organic acids [64] as
well as basic solutions like NaOH [74].
One of the most important quality indicators of the exfoliation products is their
content of foreign elements, mostly oxygen. The oxygen content is often given as one
of the indicators of the success of the sample preparation. The oxygen weight percent
of the delamination products falls between 0 and 15%. It remains mostly hidden
which kinds of functional groups are produced upon the oxidation and how their ratio
depend on the exfoliation circumstances. From a work in which the oxygen content
of the exfoliation process was systematically studied, one can learn that an excessive
amount of hydrogen peroxide leads to an increase of the oxygen content, while
elevated temperature favours the exfoliation and reduces the graphene oxidation
[70].
If the goal is the intentional doping of the graphene framework in parallel with
the exfoliation, targeted addition of compounds containing the element to be doped
proved to be a successful strategy. Examples range to mostly N and S doping. Some
6% N content can be achieved by using glycine [65] as N-doping agents, while a
high nitrogen and oxygen content (about 8 and 16%, respectively) were achieved by
using the reaction product of melamine and formaldehyde [71]. Parallel inclusion of
S and N was achieved by using melamine and alteration of the electrode potentials
[72]. Thiourea was effective in the deposition of additional sulphur without allowing
the oxidation of graphene [73].
We must note that the mechanism of the anodic exfoliation is by far not as clear and
well-studied as the cathodic intercalation. It can be assumed that the anodic treatment
leads to the formation of oxygen-containing groups mostly at the edge of the graphene
sheets [64, 78]. A severe damage of the graphene sheets can lead to the breakage of
the C–C bonds far from the graphene plane edge, which can lead to the occurrence
of additional locations for foreign atom incorporation. The relatively large oxygen
or nitrogen content of the anodically treated graphite makes the break of the in-plane
C–C bonds very likely. As an analogy of the cathodic intercalation, an alternative
route can be the partial oxidation (i.e., electron release without the incorporation of
foreign species) of the graphene plane in parallel to the anion incorporation. Since
anion residues have not been reported for anodically exfoliated graphene, it can be
assumed that the resulting adduct of the oxidized graphene sheet and the anions
decomposes when electrochemical control is no longer applied.
345
Since the process is highly empirical, there is a wide platform to test new and new
experimental circumstances.
Concerning the compounds used in the electrolyte solution, simple inorganic salts
are very common. Sulphate ion is one of the species whose incorporation and the
oxidation/decomposition to SO 2 is thought to play a key role in the anodic exfoliation;
therefore, various compounds containing sulphate ions can be used (dilute [62, 63]
or concentrated [75] H 2 SO 4 , (NH 4 ) 2 SO 4 [70, 74, 76, 79], alkaline metal sulphates
[68, 76] or oxone [66]). A feasible pursuit in the choice of the solute is that it should
be oxidized easier than graphene, hence reducing the oxygen content of the product.
For this reason, hydrogen peroxide is often applied as solution component [70, 74],
and sodium halides as electrolyte were also used successfully [67, 77]. Nevertheless,
the exfoliation works also with various types of inorganic and organic acids [64] as
well as basic solutions like NaOH [74].
One of the most important quality indicators of the exfoliation products is their
content of foreign elements, mostly oxygen. The oxygen content is often given as one
of the indicators of the success of the sample preparation. The oxygen weight percent
of the delamination products falls between 0 and 15%. It remains mostly hidden
which kinds of functional groups are produced upon the oxidation and how their ratio
depend on the exfoliation circumstances. From a work in which the oxygen content
of the exfoliation process was systematically studied, one can learn that an excessive
amount of hydrogen peroxide leads to an increase of the oxygen content, while
elevated temperature favours the exfoliation and reduces the graphene oxidation
[70].
If the goal is the intentional doping of the graphene framework in parallel with
the exfoliation, targeted addition of compounds containing the element to be doped
proved to be a successful strategy. Examples range to mostly N and S doping. Some
6% N content can be achieved by using glycine [65] as N-doping agents, while a
high nitrogen and oxygen content (about 8 and 16%, respectively) were achieved by
using the reaction product of melamine and formaldehyde [71]. Parallel inclusion of
S and N was achieved by using melamine and alteration of the electrode potentials
[72]. Thiourea was effective in the deposition of additional sulphur without allowing
the oxidation of graphene [73].
We must note that the mechanism of the anodic exfoliation is by far not as clear and
well-studied as the cathodic intercalation. It can be assumed that the anodic treatment
leads to the formation of oxygen-containing groups mostly at the edge of the graphene
sheets [64, 78]. A severe damage of the graphene sheets can lead to the breakage of
the C–C bonds far from the graphene plane edge, which can lead to the occurrence
of additional locations for foreign atom incorporation. The relatively large oxygen
or nitrogen content of the anodically treated graphite makes the break of the in-plane
C–C bonds very likely. As an analogy of the cathodic intercalation, an alternative
route can be the partial oxidation (i.e., electron release without the incorporation of
foreign species) of the graphene plane in parallel to the anion incorporation. Since
anion residues have not been reported for anodically exfoliated graphene, it can be
assumed that the resulting adduct of the oxidized graphene sheet and the anions
decomposes when electrochemical control is no longer applied.