146
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
peak was confirmed at 1332 cm
−1 . A solid–gas–solid mechanism was suggested to
be responsible for the transformation of CNTs to nanodiamonds [36].
It should be mentioned that CNTs used in these investigations were produced by
chemical vapor deposition (CVD) with the aid of catalysts. Another route of CNT and
nanoparticle synthesis uses the intercalation of lithium from molten lithium chloride
into graphite by electrolysis, as discussed in previous chapters. The mechanism by
which this occurs is that the lithium-ions discharge on the cathode and pass into the
graphite between the layers of graphite/graphene under the influence of the cathodic
potential. The electrolysis reaction can be expressed as:
2Li
+
+ Cl
−
= 2Li (at the cathode) + Cl 2(at the ande) G
◦
800
◦ C = 650.8 kJ
(8.1)
Although the diameter of the lithium atoms is similar to that of interlamellar
spacing in graphite, there is sufficient stress to extrude sheets of graphite into the
melt where they roll up to minimize the surface area exposed to the salt. The formation
of either CNTs or nanoparticles depends upon the temperature and the crystallite size
of the graphite which become detached from the graphite surface and accumulate in
the molten salt bath from which separation can be achieved. The presence of moisture
in the atmosphere of the reactor leads to the formation of Li 2 O with a solubility of
more than 11 mol% in molten LiCl, according to the reaction (8.2):
H 2 O + 2LiCl = 2HCl + Li 2 O G
◦
800
◦ C = 182 kJ
(8.2)
Although the Gibbs free energy of reaction (8.2) is positive, yet the reaction can
proceed at a finite rate as a result of dissolution of products in the molten salt. Oxygen
anions formed in the molten LiCl can be oxidized on the graphite anode to produce
CO 2 , which subsequently reacts with the Li 2 O dissolved in the molten salt to form
Li 2 CO 3 :
CO 2 + Li 2 O = Li 2 CO 3 G
◦
800
◦ C = −66.0 kJ
(8.3)
The X-ray diffraction pattern of the post-electrolysis material taken before washing treatment (Fig. 8.4a) shows the presence of LiCl · H 2 O, Li 2 CO 3 and carbon in
graphite structure. The presence of LiCl · H 2 O can be attributed to the salt which
covers the carbon product. It is known that LiCl crystals easily absorb water from
atmosphere to form LiCl · H 2 O. In addition to the reaction (8.3), the formation of
Li 2 CO 3 may also be related to the reaction of intercalated lithium with carbon in
the presence of oxygen donor materials like impurities and binders available in the
graphite cathode.
The XRD analysis of the carbon product obtained after washing and drying treatments is also shown in Fig. 8.4a. It demonstrates the effective removal of the lithium
chloride-based salt from the carbon product by the water washing treatment. However, a considerable amount of Li 2 CO 3 is left after the treatment. It can be attributed
to the solubility of Li 2 CO 3 and LiCl in water, which is about 13 and 832 g L
−1 at
20 °C, respectively. The unknown peaks in the XRD pattern may be attributed to the
8 Molten Salt-Assisted Preparation of Nanodiamonds at Atmospheric …
peak was confirmed at 1332 cm
−1 . A solid–gas–solid mechanism was suggested to
be responsible for the transformation of CNTs to nanodiamonds [36].
It should be mentioned that CNTs used in these investigations were produced by
chemical vapor deposition (CVD) with the aid of catalysts. Another route of CNT and
nanoparticle synthesis uses the intercalation of lithium from molten lithium chloride
into graphite by electrolysis, as discussed in previous chapters. The mechanism by
which this occurs is that the lithium-ions discharge on the cathode and pass into the
graphite between the layers of graphite/graphene under the influence of the cathodic
potential. The electrolysis reaction can be expressed as:
2Li
+
+ Cl
−
= 2Li (at the cathode) + Cl 2(at the ande) G
◦
800
◦ C = 650.8 kJ
(8.1)
Although the diameter of the lithium atoms is similar to that of interlamellar
spacing in graphite, there is sufficient stress to extrude sheets of graphite into the
melt where they roll up to minimize the surface area exposed to the salt. The formation
of either CNTs or nanoparticles depends upon the temperature and the crystallite size
of the graphite which become detached from the graphite surface and accumulate in
the molten salt bath from which separation can be achieved. The presence of moisture
in the atmosphere of the reactor leads to the formation of Li 2 O with a solubility of
more than 11 mol% in molten LiCl, according to the reaction (8.2):
H 2 O + 2LiCl = 2HCl + Li 2 O G
◦
800
◦ C = 182 kJ
(8.2)
Although the Gibbs free energy of reaction (8.2) is positive, yet the reaction can
proceed at a finite rate as a result of dissolution of products in the molten salt. Oxygen
anions formed in the molten LiCl can be oxidized on the graphite anode to produce
CO 2 , which subsequently reacts with the Li 2 O dissolved in the molten salt to form
Li 2 CO 3 :
CO 2 + Li 2 O = Li 2 CO 3 G
◦
800
◦ C = −66.0 kJ
(8.3)
The X-ray diffraction pattern of the post-electrolysis material taken before washing treatment (Fig. 8.4a) shows the presence of LiCl · H 2 O, Li 2 CO 3 and carbon in
graphite structure. The presence of LiCl · H 2 O can be attributed to the salt which
covers the carbon product. It is known that LiCl crystals easily absorb water from
atmosphere to form LiCl · H 2 O. In addition to the reaction (8.3), the formation of
Li 2 CO 3 may also be related to the reaction of intercalated lithium with carbon in
the presence of oxygen donor materials like impurities and binders available in the
graphite cathode.
The XRD analysis of the carbon product obtained after washing and drying treatments is also shown in Fig. 8.4a. It demonstrates the effective removal of the lithium
chloride-based salt from the carbon product by the water washing treatment. However, a considerable amount of Li 2 CO 3 is left after the treatment. It can be attributed
to the solubility of Li 2 CO 3 and LiCl in water, which is about 13 and 832 g L
−1 at
20 °C, respectively. The unknown peaks in the XRD pattern may be attributed to the