10.5 Top-Down Electrochemical Synthesis of Nanosheets
351
[116]. The experimental conditions were identical as reported above for the semiconductor particles. The lateral size of the resulting flakes was 1.27 μm on the average,
with a thickness distribution of 8.4 nm ± 3.3 nm.
10.6 Molten Salt Electrolysis Methods for the Synthesis
of Nanostructures
10.6.1 Carbon Nanostructures Prepared from Graphitic
Cathode
Although the intercalation-induced exfoliation process is the common initial step
in room-temperature and high-temperature electrolytic process of the carbon nanostructure preparation, the different experimental conditions rationalize the discussion
of the procedures with molten salts in a separate chapter. A wide scope of the experimental conditions and a variety of nanostructures produced with these methods have
been described in a few short review papers [117–119]. Below, the common features
of the processes based on molten salt electrolysis are summarized. The specific
features of the carbon nanotubes will be given in Chap. 13 where their arc discharge
synthesis processes are presented.
The construction of the electrolysis cells used in the molten salt processes changed
little since the discovery of the carbon nanotube formation in the high-temperature
intercalation processes [120]. Briefly, the container of the molten salt is a graphite
pot manufactured from a rod by drilling its central part. The container serves as
anode as well. The cathode rod is immersed into the molten salt from upwards to a
controlled depth. This setup makes it possible to apply a current-controlled synthesis
with fixed anode and cathode. Later, various modifications were made: (i) For the
sake of a classical electrochemical polarization study, the cell was equipped with
a quasireference electrode (a Mo wire [121] or a glassy carbon rod [122]). (ii) For
the operation of the cell with alternating polarity, two identical carbon rods were
immersed into the molten salt pool, and the container was no longer a part of the
circuit [123]. The arrangement of the electrodes in two basic cell configurations is
depicted in Fig. 10.12.
By using either of the cell types, the temperature control was maintained by an
external heating unit and a thermocouple measuring the temperature of the molten
salt. The flushing of the cell with a non-reactive gas (Ar) is a prerequisite for avoiding
oxidation processes.
The salt applied for the electrolysis process was mostly either LiCl [120–122, 124,
125] or NaCl [122, 126–128], although comparative studies with LiBr [120] and KCl
[126] are also available. The melting point of the salt used determines the temperature
range applied (>615 or 810 °C for LiCl and NaCl, respectively). The anode reaction is
the evolution of the halogen gas, while the cathode reaction is primarily the discharge
351
[116]. The experimental conditions were identical as reported above for the semiconductor particles. The lateral size of the resulting flakes was 1.27 μm on the average,
with a thickness distribution of 8.4 nm ± 3.3 nm.
10.6 Molten Salt Electrolysis Methods for the Synthesis
of Nanostructures
10.6.1 Carbon Nanostructures Prepared from Graphitic
Cathode
Although the intercalation-induced exfoliation process is the common initial step
in room-temperature and high-temperature electrolytic process of the carbon nanostructure preparation, the different experimental conditions rationalize the discussion
of the procedures with molten salts in a separate chapter. A wide scope of the experimental conditions and a variety of nanostructures produced with these methods have
been described in a few short review papers [117–119]. Below, the common features
of the processes based on molten salt electrolysis are summarized. The specific
features of the carbon nanotubes will be given in Chap. 13 where their arc discharge
synthesis processes are presented.
The construction of the electrolysis cells used in the molten salt processes changed
little since the discovery of the carbon nanotube formation in the high-temperature
intercalation processes [120]. Briefly, the container of the molten salt is a graphite
pot manufactured from a rod by drilling its central part. The container serves as
anode as well. The cathode rod is immersed into the molten salt from upwards to a
controlled depth. This setup makes it possible to apply a current-controlled synthesis
with fixed anode and cathode. Later, various modifications were made: (i) For the
sake of a classical electrochemical polarization study, the cell was equipped with
a quasireference electrode (a Mo wire [121] or a glassy carbon rod [122]). (ii) For
the operation of the cell with alternating polarity, two identical carbon rods were
immersed into the molten salt pool, and the container was no longer a part of the
circuit [123]. The arrangement of the electrodes in two basic cell configurations is
depicted in Fig. 10.12.
By using either of the cell types, the temperature control was maintained by an
external heating unit and a thermocouple measuring the temperature of the molten
salt. The flushing of the cell with a non-reactive gas (Ar) is a prerequisite for avoiding
oxidation processes.
The salt applied for the electrolysis process was mostly either LiCl [120–122, 124,
125] or NaCl [122, 126–128], although comparative studies with LiBr [120] and KCl
[126] are also available. The melting point of the salt used determines the temperature
range applied (>615 or 810 °C for LiCl and NaCl, respectively). The anode reaction is
the evolution of the halogen gas, while the cathode reaction is primarily the discharge