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10 Electrochemical Manufacturing Methods Based on Surface …
the highly empirical nature of the field. Although propylene carbonate exhibits cointercalation together with Li into graphite, other organic carbonates do not, although
the difference in the molecular structure is really minor. Therefore, neither theoretical
predictions for the probability of intercalation nor the required electrode potential
can be assessed from elementary physico-chemical principles.
The cell configuration for the electrochemical exfoliation is very simple. A twoelectrode system is applied nearly exclusively. A symmetrical electrode configuration
is also customary, although non-reactive electrodes (like Pt) are often used for counter
electrode. Due to the two-electrode configuration, the cell voltage is the parameter
reported, and the electrode potential in a classical potential scale is not defined. Since
the cell voltage is typically between 1.5 and 15 V, it is obvious that the polarization
limit is much beyond the stability regime of the solvent, even for an ionic liquid,
and the reaction of the solvent is an important factor in the delamination process.
Although the high cell voltage commonly used makes the exfoliation-based topdown synthesis method similar to those to be discussed in Chap. 13, a few aspects
rationalize its discussion together with other site-preferred synthesis methods. These
reasons are: (i) the occurrence of the layers already in the starting material; (ii) the
difference between the intra- and interlayer bond strength, (iii) the obvious role of
the already existing step edge position in the layer cleavage process, and (iv) the fact
that intercalation and exfoliation processes can take place also at moderate voltages,
unlike the processes to be mentioned later, and the goal of the application of the
relatively high voltage is often the speeding up of the synthesis process.
It is also because of the relatively high cell voltage why functionalization reactions
in parallel to the exfoliation take place in essentially all cases, and the electrochemical exfoliation leads to a product that is rather ill-defined concerning its chemical
structure, especially concerning the regularity of the defect positions. Despite the
chemical structure of the product of the exfoliation process is indefinite, the formation of the functional groups of the mostly apolar lamellae is an important factor of
the stabilization of their suspension that prevents the fast coagulation of the exfoliated nanosheets. Since no exact electrochemical reaction is associated with either
the delamination process itself or the accompanying functionalization, the faradaic
efficiency of the electrochemical exfoliation process cannot be defined.
The exfoliation process can be either cathodic or anodic. The driving force of
the process is the uptake of the ions into the lamellar solid structure, which is
accompanied with the occurrence of a large mechanical stress. The stress-induced
delamination is also possible via bubble formation between the layer of interest
and its substrate, which establishes electrochemistry-based transfer procedures of
nanosheets. In both cases, the atomically smooth nanosheets can exhibit a lateral
dimension up to 50 μm, hence resulting in objects with extremely large aspect ratio.
The aspect ratio is illustrated in Fig. 10.11 for two kinds of electrochemically exfoliated nanosheets where the height measurement method with AFM is also presented
with representative step height profile functions.
10 Electrochemical Manufacturing Methods Based on Surface …
the highly empirical nature of the field. Although propylene carbonate exhibits cointercalation together with Li into graphite, other organic carbonates do not, although
the difference in the molecular structure is really minor. Therefore, neither theoretical
predictions for the probability of intercalation nor the required electrode potential
can be assessed from elementary physico-chemical principles.
The cell configuration for the electrochemical exfoliation is very simple. A twoelectrode system is applied nearly exclusively. A symmetrical electrode configuration
is also customary, although non-reactive electrodes (like Pt) are often used for counter
electrode. Due to the two-electrode configuration, the cell voltage is the parameter
reported, and the electrode potential in a classical potential scale is not defined. Since
the cell voltage is typically between 1.5 and 15 V, it is obvious that the polarization
limit is much beyond the stability regime of the solvent, even for an ionic liquid,
and the reaction of the solvent is an important factor in the delamination process.
Although the high cell voltage commonly used makes the exfoliation-based topdown synthesis method similar to those to be discussed in Chap. 13, a few aspects
rationalize its discussion together with other site-preferred synthesis methods. These
reasons are: (i) the occurrence of the layers already in the starting material; (ii) the
difference between the intra- and interlayer bond strength, (iii) the obvious role of
the already existing step edge position in the layer cleavage process, and (iv) the fact
that intercalation and exfoliation processes can take place also at moderate voltages,
unlike the processes to be mentioned later, and the goal of the application of the
relatively high voltage is often the speeding up of the synthesis process.
It is also because of the relatively high cell voltage why functionalization reactions
in parallel to the exfoliation take place in essentially all cases, and the electrochemical exfoliation leads to a product that is rather ill-defined concerning its chemical
structure, especially concerning the regularity of the defect positions. Despite the
chemical structure of the product of the exfoliation process is indefinite, the formation of the functional groups of the mostly apolar lamellae is an important factor of
the stabilization of their suspension that prevents the fast coagulation of the exfoliated nanosheets. Since no exact electrochemical reaction is associated with either
the delamination process itself or the accompanying functionalization, the faradaic
efficiency of the electrochemical exfoliation process cannot be defined.
The exfoliation process can be either cathodic or anodic. The driving force of
the process is the uptake of the ions into the lamellar solid structure, which is
accompanied with the occurrence of a large mechanical stress. The stress-induced
delamination is also possible via bubble formation between the layer of interest
and its substrate, which establishes electrochemistry-based transfer procedures of
nanosheets. In both cases, the atomically smooth nanosheets can exhibit a lateral
dimension up to 50 μm, hence resulting in objects with extremely large aspect ratio.
The aspect ratio is illustrated in Fig. 10.11 for two kinds of electrochemically exfoliated nanosheets where the height measurement method with AFM is also presented
with representative step height profile functions.