8.2 Conversion of Carbon Nanostructures into Nanodiamonds
149
of these carbon nanostructures are also shown in Fig. 8.5. The DSC thermogram of
the electrolytic carbon indicates the occurrence of two exothermic events with peaks’
maximum temperature at 511 and 637 °C.
The electrolytic carbon was heated to the temperatures just before and after the
first exothermic peak and then cooled down to the room temperature. Heating of the
electrolytic carbon up to the temperatures below the first peak (e.g., 400 °C) did not
affect the morphology of the material, and CNTs and nanoparticles could still be
clearly seen in the SEM image of the heated samples (Fig. 8.6a). The sample heated
to 570 °C, which is beyond the first exothermic peak, experienced a massive weight
loss. Moreover, no carbon nanomaterial could be identified in the SEM image of the
heated sample (Fig. 8.6b). It indicates that the first and the second exothermic peaks
in DSC curve of the electrolytic carbon shown in Fig. 8.5 relate to the oxidation of the
nanometer-sized and the micrometer-sized fractions of the nanostructured carbon,
respectively.
Figure 8.5 shows the DSC and TG analyses of the CVD-MWCNTs heated at
the rate of 20 °C min
−1 under ambient airflow of 100 mL min
−1 . The DSC curve
exhibits one small exothermic peak at 454 °C, which is related to the oxidation
of the amorphous fraction of the sample, followed by a large exothermic peak at
650 °C corresponding to the oxidation of the whole remaining material. It is known
that the oxidation temperature of carbon nanomaterials depends on their degree of
graphitization; the greater the crystallinity, the higher the oxidation temperature [40].
However, despite its higher degree of crystallinity, the electrolytic carbon exhibits
a lower oxidation temperature than the CVD-MWCNTs. It can be explained by the
presence of Li 2 CO 3 nanocrystals in the nanostructure of the electrolytic carbon,
which can catalyze the oxidation of the material at lower temperatures [38].
It is known that molten carbonates of alkali and alkaline metals are able to act
as a solvent–catalyst for diamond formation from graphite at typical HPHT conditions of 5–8 GPa and 1600–2150 °C [4]. Subsequently, several other inorganic
melts, including metal halides such as LiCl and multicomponent systems, have also
been shown to catalyze the conversion of graphite to diamond at very high temperatures and pressures. The presence of nanometer-sized catalyst crystals embedded
in the graphitic nanostructure of the electrolytic carbon materials makes the molten
salt-produced material an attractive precursor for the production of diamond.
According to Fig. 8.5, the oxidation of CNTs and nanoparticles in the electrolytic
carbon material happens at the temperature window of 420–550 °C. It was observed
that if the electrolytic carbon is heated to specific temperatures within this oxidation window, the carbon nanomaterials are transformed into nanodiamonds. For
example, the electrolytic carbon was heated to 515 °C and then cooled down to the
room temperature. The sample, after the heat treatment process, contained octagonal
nanodiamonds from 5 nm to 1 μm, as shown in Fig. 8.6c, d [39].
Figure 8.6e shows a TEM micrograph of the diamond product as well as a selected
area diffraction pattern, in which the spots corresponded to the (111) plane of diamond. It was proposed that by producing carbon nanomaterials electrochemically
by intercalation, lithium salts are incorporated between the graphene sheets and
these are able to catalyze the transformation to diamond by simply heating of the
149
of these carbon nanostructures are also shown in Fig. 8.5. The DSC thermogram of
the electrolytic carbon indicates the occurrence of two exothermic events with peaks’
maximum temperature at 511 and 637 °C.
The electrolytic carbon was heated to the temperatures just before and after the
first exothermic peak and then cooled down to the room temperature. Heating of the
electrolytic carbon up to the temperatures below the first peak (e.g., 400 °C) did not
affect the morphology of the material, and CNTs and nanoparticles could still be
clearly seen in the SEM image of the heated samples (Fig. 8.6a). The sample heated
to 570 °C, which is beyond the first exothermic peak, experienced a massive weight
loss. Moreover, no carbon nanomaterial could be identified in the SEM image of the
heated sample (Fig. 8.6b). It indicates that the first and the second exothermic peaks
in DSC curve of the electrolytic carbon shown in Fig. 8.5 relate to the oxidation of the
nanometer-sized and the micrometer-sized fractions of the nanostructured carbon,
respectively.
Figure 8.5 shows the DSC and TG analyses of the CVD-MWCNTs heated at
the rate of 20 °C min
−1 under ambient airflow of 100 mL min
−1 . The DSC curve
exhibits one small exothermic peak at 454 °C, which is related to the oxidation
of the amorphous fraction of the sample, followed by a large exothermic peak at
650 °C corresponding to the oxidation of the whole remaining material. It is known
that the oxidation temperature of carbon nanomaterials depends on their degree of
graphitization; the greater the crystallinity, the higher the oxidation temperature [40].
However, despite its higher degree of crystallinity, the electrolytic carbon exhibits
a lower oxidation temperature than the CVD-MWCNTs. It can be explained by the
presence of Li 2 CO 3 nanocrystals in the nanostructure of the electrolytic carbon,
which can catalyze the oxidation of the material at lower temperatures [38].
It is known that molten carbonates of alkali and alkaline metals are able to act
as a solvent–catalyst for diamond formation from graphite at typical HPHT conditions of 5–8 GPa and 1600–2150 °C [4]. Subsequently, several other inorganic
melts, including metal halides such as LiCl and multicomponent systems, have also
been shown to catalyze the conversion of graphite to diamond at very high temperatures and pressures. The presence of nanometer-sized catalyst crystals embedded
in the graphitic nanostructure of the electrolytic carbon materials makes the molten
salt-produced material an attractive precursor for the production of diamond.
According to Fig. 8.5, the oxidation of CNTs and nanoparticles in the electrolytic
carbon material happens at the temperature window of 420–550 °C. It was observed
that if the electrolytic carbon is heated to specific temperatures within this oxidation window, the carbon nanomaterials are transformed into nanodiamonds. For
example, the electrolytic carbon was heated to 515 °C and then cooled down to the
room temperature. The sample, after the heat treatment process, contained octagonal
nanodiamonds from 5 nm to 1 μm, as shown in Fig. 8.6c, d [39].
Figure 8.6e shows a TEM micrograph of the diamond product as well as a selected
area diffraction pattern, in which the spots corresponded to the (111) plane of diamond. It was proposed that by producing carbon nanomaterials electrochemically
by intercalation, lithium salts are incorporated between the graphene sheets and
these are able to catalyze the transformation to diamond by simply heating of the