7.3 Conversion of PET into Carbonaceous Nanomaterials
117
Among these techniques, Raman spectroscopy is widely recognized as a low cost
but powerful method for characterization of carbon nanostructures, since it provides
explicit insights into the layer structure, crystallinity and defects. In carbon materials, the possible presence of the main first-order Raman band formed by sp
2 -carbon
bonds, the G band; the disorder-induced Raman band, the D band, as well as the D
band overtone, the 2D band, is readily detectable by Raman spectroscopy. Generally
speaking, defects damage some attractive properties of carbon nanostructures, including their electrical and thermal conductivity. However, defects such as graphene edge
sites can be engineered to be favorable in specific applications, where the reactivity
of carbon nanostructures with their environment is important. An example of such
application was given in 6.5.
The intensity ratio of Raman bands I D /I G and I 2D /I G in graphite-based materials
corresponds to the density of defects and the quality of graphene flakes, respectively
[53, 54]. These parameters can also provide an estimation of the material’s conductivity [55, 56]. Generally, a lower I D /I G value is measured in graphitic materials with
higher values of electrical conductivity. Also, a higher I 2D /I G value together with a
symmetrical and sharp 2D band can be attributed to the presence of graphene with
fewer layers. For example, reduced graphene oxide (rGO) with an I D /I G value of 1.55,
1.19 and 1.02 exhibited Raman I 2D /I G values of 0.01, 0.07 and 0.14, and electrical
conductivity values of 69, 133 and 166 S m
−1 , respectively [57].
Based on the information mentioned above, the quality of PET-derived carbon
materials can be extracted from the literature [58–67]. The high-temperature pyrolysis of PET under N 2 and subsequent activation of the material obtained under steam,
CO 2 and/or KOH [58–62], and also the thermal treatment of PET under protective
atmospheres in various equipment such as arc discharge [63], chemical vapor deposition (CVD) [64, 65] and autoclave [66, 67] reactors can lead to the formation of
amorphous carbons, characterized by the presence of very weak Raman 2D bands
as well as I D /I G values of greater than unity.
For instance, the pyrolysis of PET under N 2 at 400 °C for 1 h, followed by a
heat treatment at 725 °C led to the formation of an amorphous carbonized char. This
material was then sieved to separate particles with the size of ≤0.15 mm, and the
sieved char particles were heated under N 2 to 925 °C. After 1 h, N 2 was replaced
by CO 2 and the heat treatment process was continued for another 2 h. The final
product was found to be an activated carbon material with a BET surface area of
around 660 m
2 g
−1 . The properties of the carbon product are summarized in Fig. 7.5.
The X-ray diffraction pattern of the activated carbon produced exhibits a broad
diffraction peak located at 2θ = 20°–30° which ascribed to reflections arisen from
hexagonal carbon (002) planes. Also, the peak observed at 2θ ~43° with low intensity
is attributed to the reflections of carbon (101) planes. This XRD pattern shows an
intermediate structure between crystalline graphitic and amorphous carbon [42].
Likewise, the Raman spectrum of the carbon product (Fig. 7.5b) shows the presence
of disordered graphitic crystallites. Particularly, the I D /I G has a high value of 1.04,
and there is no obvious sign of the Raman 2D band [42]. It should be mentioned that
such carbon materials with semi-crystalline structures inevitably suffer from lack of
reasonable electrical conductivity. At the present time, such a carbon material doesn’t
117
Among these techniques, Raman spectroscopy is widely recognized as a low cost
but powerful method for characterization of carbon nanostructures, since it provides
explicit insights into the layer structure, crystallinity and defects. In carbon materials, the possible presence of the main first-order Raman band formed by sp
2 -carbon
bonds, the G band; the disorder-induced Raman band, the D band, as well as the D
band overtone, the 2D band, is readily detectable by Raman spectroscopy. Generally
speaking, defects damage some attractive properties of carbon nanostructures, including their electrical and thermal conductivity. However, defects such as graphene edge
sites can be engineered to be favorable in specific applications, where the reactivity
of carbon nanostructures with their environment is important. An example of such
application was given in 6.5.
The intensity ratio of Raman bands I D /I G and I 2D /I G in graphite-based materials
corresponds to the density of defects and the quality of graphene flakes, respectively
[53, 54]. These parameters can also provide an estimation of the material’s conductivity [55, 56]. Generally, a lower I D /I G value is measured in graphitic materials with
higher values of electrical conductivity. Also, a higher I 2D /I G value together with a
symmetrical and sharp 2D band can be attributed to the presence of graphene with
fewer layers. For example, reduced graphene oxide (rGO) with an I D /I G value of 1.55,
1.19 and 1.02 exhibited Raman I 2D /I G values of 0.01, 0.07 and 0.14, and electrical
conductivity values of 69, 133 and 166 S m
−1 , respectively [57].
Based on the information mentioned above, the quality of PET-derived carbon
materials can be extracted from the literature [58–67]. The high-temperature pyrolysis of PET under N 2 and subsequent activation of the material obtained under steam,
CO 2 and/or KOH [58–62], and also the thermal treatment of PET under protective
atmospheres in various equipment such as arc discharge [63], chemical vapor deposition (CVD) [64, 65] and autoclave [66, 67] reactors can lead to the formation of
amorphous carbons, characterized by the presence of very weak Raman 2D bands
as well as I D /I G values of greater than unity.
For instance, the pyrolysis of PET under N 2 at 400 °C for 1 h, followed by a
heat treatment at 725 °C led to the formation of an amorphous carbonized char. This
material was then sieved to separate particles with the size of ≤0.15 mm, and the
sieved char particles were heated under N 2 to 925 °C. After 1 h, N 2 was replaced
by CO 2 and the heat treatment process was continued for another 2 h. The final
product was found to be an activated carbon material with a BET surface area of
around 660 m
2 g
−1 . The properties of the carbon product are summarized in Fig. 7.5.
The X-ray diffraction pattern of the activated carbon produced exhibits a broad
diffraction peak located at 2θ = 20°–30° which ascribed to reflections arisen from
hexagonal carbon (002) planes. Also, the peak observed at 2θ ~43° with low intensity
is attributed to the reflections of carbon (101) planes. This XRD pattern shows an
intermediate structure between crystalline graphitic and amorphous carbon [42].
Likewise, the Raman spectrum of the carbon product (Fig. 7.5b) shows the presence
of disordered graphitic crystallites. Particularly, the I D /I G has a high value of 1.04,
and there is no obvious sign of the Raman 2D band [42]. It should be mentioned that
such carbon materials with semi-crystalline structures inevitably suffer from lack of
reasonable electrical conductivity. At the present time, such a carbon material doesn’t