8.3 Conversion of CO 2 into Diamond Nanocrystals
155
Upon heating of the nanostructured hybrid material in air, first, outer carbon layers ignite at about 400 °C, generating a considerable amount of heat. The thermal
energy released from the partial oxidation process might be sufficient to cause melting and evaporation of the encapsulated Li 2 CO 3 crystals in nanoscale areas. In this
situation, the carbon layers around Li 2 CO 3 nanoparticles could act as high-pressure
nanovessels, providing sufficient energy for the occurrence of diamond nucleation
within the saturated lithium carbonate.
It should be noticed that the air oxidation of nanodiamonds with grain sizes of
about 5 nm, synthesized by detonation, occurs at temperatures more than 550 °C
[108]. The oxidation resistance of diamond increases with its particle size, so that
micrometer-sized diamond produced by HPHT technique is oxidized at temperatures not less than 800 °C [109]. The oxidation temperature of diamond films was
measured to be more than 950 °C [110]. In the core-shell nanostructure observed in
Fig. 8.7, the diamond nucleation and growth was observed at the external temperature of 530 °C. At this condition, carbon nanomaterials are not thermally stable, but
diamond crystallites are relatively more stable. The rapid growth of diamond crystallites observed in this study, therefore, can be attributed to the rapid diffusion of
carbon atoms from unstable graphitic structures into relatively more stable diamond
crystals.
Figure 8.8 shows a HRTEM micrograph of the C-encapsulated Li 2 CO 3 nanovessels heated to 440 °C. Two core-shell nanoparticles can be recognized in the upperpanel of Fig. 8.8. The lower-panel in Fig. 8.8 exhibits a higher magnification image
from one of the core-shell nanostructures observed in the upper-panel. Fast Fourier
transformation patterns recorded on the encapsulated particles demonstrated that the
lower nanoparticle, observed in the upper-panel, is crystalline Li 2 CO 3 . On the other
hand, the upper core-shell particle, observed in the upper-panel of Fig. 8.8, was found
to compose of Li 2 CO 3 and diamond crystallites. The lower panel in Fig. 8.8 shows
the latter in a higher magnification. It is an interesting micrograph which shows
a diamond crystallite has nucleated within Li 2 CO 3 nanoparticles encapsulated in
graphitic carbon shells [68].
As discussed in this section, the reactive interaction of CO 2 gas with molten
LiCl–2 wt% Li 2 O led to the formation of Li 2 CO 3 dissolved in the molten salt. The
dissolved Li 2 CO 3 is crystallized upon cooling of the molten salt to form Li 2 CO 3
nanosingle crystals. It was also mentioned that graphite electrodes can be electrolytically exfoliated in the same system to produce graphene nanosheets. The graphene
nanosheets produced had the capability of encapsulating the Li 2 CO 3 crystals.
Overall, the process described may be used for large-scale and low-cost production of Li 2 CO 3 and diamond crystals using CO 2 gas. However, there are few relevant
issues that need further clarification: The experimental setup shown in Fig. 4.15a
exhibits a batch-type process for the fabrication of Li 2 CO 3 and C-encapsulated
Li 2 CO 3 nanoparticles, in which incoming CO 2 gas was replaced by Ar after 60 min of
processing, in order to characterize the materials produced. In a continuous process,
the CO 2 gas and the mixture of graphene/Li 2 CO 3 saturated salt can be continuously
introduced into and removed from the system, respectively. As the other issue, the
155
Upon heating of the nanostructured hybrid material in air, first, outer carbon layers ignite at about 400 °C, generating a considerable amount of heat. The thermal
energy released from the partial oxidation process might be sufficient to cause melting and evaporation of the encapsulated Li 2 CO 3 crystals in nanoscale areas. In this
situation, the carbon layers around Li 2 CO 3 nanoparticles could act as high-pressure
nanovessels, providing sufficient energy for the occurrence of diamond nucleation
within the saturated lithium carbonate.
It should be noticed that the air oxidation of nanodiamonds with grain sizes of
about 5 nm, synthesized by detonation, occurs at temperatures more than 550 °C
[108]. The oxidation resistance of diamond increases with its particle size, so that
micrometer-sized diamond produced by HPHT technique is oxidized at temperatures not less than 800 °C [109]. The oxidation temperature of diamond films was
measured to be more than 950 °C [110]. In the core-shell nanostructure observed in
Fig. 8.7, the diamond nucleation and growth was observed at the external temperature of 530 °C. At this condition, carbon nanomaterials are not thermally stable, but
diamond crystallites are relatively more stable. The rapid growth of diamond crystallites observed in this study, therefore, can be attributed to the rapid diffusion of
carbon atoms from unstable graphitic structures into relatively more stable diamond
crystals.
Figure 8.8 shows a HRTEM micrograph of the C-encapsulated Li 2 CO 3 nanovessels heated to 440 °C. Two core-shell nanoparticles can be recognized in the upperpanel of Fig. 8.8. The lower-panel in Fig. 8.8 exhibits a higher magnification image
from one of the core-shell nanostructures observed in the upper-panel. Fast Fourier
transformation patterns recorded on the encapsulated particles demonstrated that the
lower nanoparticle, observed in the upper-panel, is crystalline Li 2 CO 3 . On the other
hand, the upper core-shell particle, observed in the upper-panel of Fig. 8.8, was found
to compose of Li 2 CO 3 and diamond crystallites. The lower panel in Fig. 8.8 shows
the latter in a higher magnification. It is an interesting micrograph which shows
a diamond crystallite has nucleated within Li 2 CO 3 nanoparticles encapsulated in
graphitic carbon shells [68].
As discussed in this section, the reactive interaction of CO 2 gas with molten
LiCl–2 wt% Li 2 O led to the formation of Li 2 CO 3 dissolved in the molten salt. The
dissolved Li 2 CO 3 is crystallized upon cooling of the molten salt to form Li 2 CO 3
nanosingle crystals. It was also mentioned that graphite electrodes can be electrolytically exfoliated in the same system to produce graphene nanosheets. The graphene
nanosheets produced had the capability of encapsulating the Li 2 CO 3 crystals.
Overall, the process described may be used for large-scale and low-cost production of Li 2 CO 3 and diamond crystals using CO 2 gas. However, there are few relevant
issues that need further clarification: The experimental setup shown in Fig. 4.15a
exhibits a batch-type process for the fabrication of Li 2 CO 3 and C-encapsulated
Li 2 CO 3 nanoparticles, in which incoming CO 2 gas was replaced by Ar after 60 min of
processing, in order to characterize the materials produced. In a continuous process,
the CO 2 gas and the mixture of graphene/Li 2 CO 3 saturated salt can be continuously
introduced into and removed from the system, respectively. As the other issue, the