7.4 Molten Salt-Assisted Conversion of PET into Carbon …
123
number of graphitized locations can be depicted on the smooth surface of the carbon
particles produced in molten NaCl, particularly, on the edge sites. For example, the
presence of graphite flakes on the edge sites of carbon particles can be clearly seen
from the SEM micrograph shown in Fig. 7.8b. Figure 7.8c shows a lower magnification SEM image demonstrating that these graphitized zones have also occasionally
developed from the edge sites toward the surface of irregular-shaped carbon particles.
In some areas, the carbon material is highly crystallized into graphene nanosheets,
such as shown in the SEM micrograph of Fig. 7.8d. These findings are interesting
since it is demonstrated that amorphous carbon materials can be graphitized under
the influence of molten NaCl at the relatively low temperature of 1100 °C.
In another experiment, the mixture of PET and NaCl was heated to a higher temperature of 1300 °C, by the same heating rate of 10 °C min
−1 , and the carbon material
obtained was characterized after the system was cooled down to the room temperature, and the salt content was washed away. Upper panel (left) in Fig. 7.9 shows the
alumina crucible and the mixture of carbon material dispersed in the solidified NaCl.
In order to evaluate the distribution of the carbon phase in the solidified NaCl, the
alumina crucible was broken, and the mixture of solidified salt and carbon was easily
retrieved from the crucible, as can be seen from Fig. 7.9, the upper panel (right). As
can be observed, the carbon material is entirely distributed into the solidified NaCl,
demonstrating the high dispersibility of the carbon product in molten NaCl, which
is remarkable. The high dispersibility observed greatly promotes the influence of
molten NaCl on the structural evolution of the PET-derived carbon materials. The
carbon–salt mixture was washed with distilled water, in order to dissolve the NaCl
content of the mixture. It was observed that the carbon material floats on the surface
of water due to its low density. After stirring for 20 min, the suspension was filtered,
and the carbon material obtained was dried overnight. Figure 7.7, upper panel, shows
the XRD analysis of the carbon material obtained, from which the presence of the
(002) reflection of hexagonal carbon at the 2θ value of 25.9° is evident. This value
represents an interlayer spacing of 3.44 Å. The broad peak with the maxima at the 2θ
value of 42.5° corresponds to overlapping (100) and (101) reflections. Moreover, the
diffraction peaks related to crystalline NaCl nearly vanished from the XRD pattern,
demonstrating an effective salt removal achieved during the washing process, due
to the exfoliated nature of the sample. Figure 7.7, down panel, exhibits the Raman
spectrum of the carbon material, providing interesting and rather important information about the quality of the carbon material produced. First of all, the relatively
small defect-induced D band observed at 1364 cm
−1 , together with the sharp and
distinguished graphite G band at 1590 cm
−1 could produce a low I D /I G value of 0.47,
revealing the presence of crystalline carbon domains with a low level of defects.
Moreover, the 2D band observed at 2723 cm
−1 is symmetric and sharp; leading to
a high I 2D /I G value of 0.52, despite the fact that G band has also a high intensity in
this sample. It can be concluded from these results that the carbon product obtained
consisted of carbon crystallites of few-layers graphene [24, 53, 54].
The SEM micrographs of the nanostructured carbon extracted from the salt–
carbon mixture (Fig. 7.9, down panel) clearly show the presence of graphitic layers,
and demonstrate the occurrence and both graphitization and the surface exfoliation of
123
number of graphitized locations can be depicted on the smooth surface of the carbon
particles produced in molten NaCl, particularly, on the edge sites. For example, the
presence of graphite flakes on the edge sites of carbon particles can be clearly seen
from the SEM micrograph shown in Fig. 7.8b. Figure 7.8c shows a lower magnification SEM image demonstrating that these graphitized zones have also occasionally
developed from the edge sites toward the surface of irregular-shaped carbon particles.
In some areas, the carbon material is highly crystallized into graphene nanosheets,
such as shown in the SEM micrograph of Fig. 7.8d. These findings are interesting
since it is demonstrated that amorphous carbon materials can be graphitized under
the influence of molten NaCl at the relatively low temperature of 1100 °C.
In another experiment, the mixture of PET and NaCl was heated to a higher temperature of 1300 °C, by the same heating rate of 10 °C min
−1 , and the carbon material
obtained was characterized after the system was cooled down to the room temperature, and the salt content was washed away. Upper panel (left) in Fig. 7.9 shows the
alumina crucible and the mixture of carbon material dispersed in the solidified NaCl.
In order to evaluate the distribution of the carbon phase in the solidified NaCl, the
alumina crucible was broken, and the mixture of solidified salt and carbon was easily
retrieved from the crucible, as can be seen from Fig. 7.9, the upper panel (right). As
can be observed, the carbon material is entirely distributed into the solidified NaCl,
demonstrating the high dispersibility of the carbon product in molten NaCl, which
is remarkable. The high dispersibility observed greatly promotes the influence of
molten NaCl on the structural evolution of the PET-derived carbon materials. The
carbon–salt mixture was washed with distilled water, in order to dissolve the NaCl
content of the mixture. It was observed that the carbon material floats on the surface
of water due to its low density. After stirring for 20 min, the suspension was filtered,
and the carbon material obtained was dried overnight. Figure 7.7, upper panel, shows
the XRD analysis of the carbon material obtained, from which the presence of the
(002) reflection of hexagonal carbon at the 2θ value of 25.9° is evident. This value
represents an interlayer spacing of 3.44 Å. The broad peak with the maxima at the 2θ
value of 42.5° corresponds to overlapping (100) and (101) reflections. Moreover, the
diffraction peaks related to crystalline NaCl nearly vanished from the XRD pattern,
demonstrating an effective salt removal achieved during the washing process, due
to the exfoliated nature of the sample. Figure 7.7, down panel, exhibits the Raman
spectrum of the carbon material, providing interesting and rather important information about the quality of the carbon material produced. First of all, the relatively
small defect-induced D band observed at 1364 cm
−1 , together with the sharp and
distinguished graphite G band at 1590 cm
−1 could produce a low I D /I G value of 0.47,
revealing the presence of crystalline carbon domains with a low level of defects.
Moreover, the 2D band observed at 2723 cm
−1 is symmetric and sharp; leading to
a high I 2D /I G value of 0.52, despite the fact that G band has also a high intensity in
this sample. It can be concluded from these results that the carbon product obtained
consisted of carbon crystallites of few-layers graphene [24, 53, 54].
The SEM micrographs of the nanostructured carbon extracted from the salt–
carbon mixture (Fig. 7.9, down panel) clearly show the presence of graphitic layers,
and demonstrate the occurrence and both graphitization and the surface exfoliation of