A lot of graphene layers have been found at CBs heated at 2300℃ (CB2300) from TEM images as shown in figure 3-8a. We confirmed that graphitization which is referred to converting amorphous carbon to graphite has been generated at extremely high temperature. It was found that the spherical- shape particles were crushed and shrunk, so that the size of CBs heated at 2300℃ was also reduced to 100nm compared with other CBs which are around 150nm of particle size as shown in figure 3-8b. That is because the layer planes and small crystallites were not only rearranged and displaced, but also internal pores were removed during heat-treatment at extremely high temperature.10 In addition, it seemed like that there was a phase separation between amorphous phase and graphene layers from TEM image at high magnification (right image in figure 3-8a).
The surface area of CBs heated at 2300℃ was dramatically reduced than CBs heated at 1000℃
as shown in figure 3-8c. The decrease of surface area can be elucidated with removal of the open, internal porosity during heat treatment while the increase of surface area is caused by removal of volatile species such as oxide of carbon and hydrocarbon, which generated narrow pores to expose the internal surface area. In other words, the reason why the surface area of the CBs heated at 2300℃ dramatically decreased is surface rearrangement and loss of porosity from heat-treatment at extremely high temperature. The significant reduce of surface area of CBs heated at 2300℃ would lead defects not to be exposed to oxygen and electrolyte in terms of ORR system whereas the CBs heated at 1000℃
having higher surface area would be well exposed to oxygen and electrolyte.
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Figure 3-8. a) TEM images of CBs heated at 2300℃. b) Size distribution of CBs heated at 2300℃. c) The surface area of CBs heated at 2300℃ by BET measurement and electrochemical analysis.
Since the graphene layers were found from CBs heated at 2300℃, X-ray diffraction (XRD) measurement was conducted to investigate the degree of graphitization of each CBs heated at various temperature as shown in figure 3-9.10 The XRD spectra of CBs heated at mild temperature showed broad (002) peak which were typically found from amorphous carbon materials whereas that of CBs heated at 2300 showed sharp peak intensity indicating (002) reflection and small peak indicating (100) reflection. This distinction of XRD peak indicated the degree of graphitization of CBs heated at 2300℃
remarkably increased due to the heat-treatment of extremely high temperature. In addition, the phase separation was also confirmed from (002) reflection of CBs heated at 2300℃ showing both broad peak indicating amorphous phase and sharp peak indicating crystalline phase.
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Crystallite parameters could be associated with the degree of defects in terms of the size of domain boundary.62 Typically, the material of smaller domain boundary has higher degree of defects.
The crystallite parameters of CBs with various heat-temperature were calculated by Sherrer’s equation.
𝐿𝑎= 1.84𝜆 𝐵 𝑐𝑜𝑠𝜃
𝐿𝑐 = 0.89𝜆 𝐵 𝑐𝑜𝑠𝜃
It has been found that the crystallite parameters associated size of domain boundary reduced by calculating La and Lc as annealing temperature of CBs increased indicating the defects density increased.
Figure 3-9. XRD spectra of CBs heated at 400℃, 600℃, 800℃, 1000℃, and 2300℃.
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To investigate degree of defects of CBs heated at 2300℃, Raman spectroscopy and XPS spectroscopy were absolutely conducted as like as the other CBs.57,63 Figure 3-10a showed increased peak sharpness of ID and IG. 2D peak located at 2680cm-1 which has not been showed up for other CBs was found indicating increased degree of graphitization. It is noticeable that ID/IG ratio of CBs heated at 2300℃ is much higher than the other CBs. That is because both existing defects which CBs originally possess and amorphous phase from phase separation were detected as ID peak. From the XPS data as shown in figure 3-10b, the FWHM of sp2 peak of CBs heated at 2300℃ got increased than that of CBs heated at 1000℃ indicating the both degree of defects and graphitization were increased.
Figure 3-10. a) Raman spectra of CBs heated at 2300℃ b) XPS-C1s spectra of each CBs heated at 2300℃.
Finally, electrochemical analysis have been conducted to compare ORR electroactivities of CBs heated at 2300℃ and the other CBs. The ORR activities of CBs increase as the heat treatment temperature increase in the range of 400℃ to 1000℃ except the ORR activities of CB2300 as shown in figure 3-11a. The ORR activity of CB2300 was reduced as almost same as that of CBs heated at 600℃
(CB600) even though the ID/IG ratio of CB2300 was much higher than that of CB600. The electron transfer number of CB2300 has a same tendency with the ORR activity of CB2300 showing reduced electron transfer number. The electron transfer number of CB2300 is around 3, which is much lower than that of CB800 and CB1000. That is because the shrunk and compacted structure of CB2300 would lead to make defects inaccessible which we called closed-defects. Since the structure of CB2300 was shrunk and compacted having high density of closed-defects, the ORR activity of CB2300 is lower than that of CB1000 which have more accessible defects. Hence, we can conclude that the amount of accessible defects considered as active sites is critical factor, not the total amount of defects.
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Figure 3-11. a) ORR polarization curves of each CBs heated at 400℃, 600℃, 800℃, 1000℃ and 2300℃. The amount of each CBs on rotating disk electrode was 0.48mg/cm2. b) Electron transfer number of each CBs heated at 400 ℃, 600℃, 800℃, 1000℃, and 2300℃.