3. CNT: Role of one-dimensional catalyst support
3.2. Result
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are fractured or deviated due to high flow rates of gas. In particular, when using the CVD method, hydrogen is first injected at a high flow rate to remove oxygen present in the chamber. In addition, when annealing is performed in a nitrogen atmosphere in the last step, the nitrogen flow rate is not determined and is injected at a very high flow rate. In these two situations, CNTs were rearranged due to high flow rates. This means that CNTs can be broken making it impossible to observe. To improve this problem, the flow rate was adjusted. Therefore, in the second synthesis, the flow rate was adjusted to be similar to the hydrogen flow rate (4.2 sccm) in the initial and final stages.
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Most SEM images have been successful since the first synthesis. One problem is that the growth direction of CNTs grows randomly as if they were forming a network by several factors. In this case, it is difficult to check the directionality with the naked eye, so it is meaningless to rotate the sample at 90°
to proceed with the second synthesis. For this reason, when recycling the catalyst-stained TEM grid, some unstable synthesis should be expected unless the catalyst is completely dissolved and removed using a hydrofluoric acid solvent. CNTs that are intensively investigated in SEM images after the first synthesis are several strands located at both ends of the window. Even if the width of the window is 50 um, it is a very long length in terms of CNTs with an average diameter of around 2 nm. Therefore, it can be observed that most CNTs are broken when the catalyst solution is dropped. However, CNTs synthesized at both ends of the window are often formed at the corners, so it can be observed that they have a relatively short length. If branch CNTs are synthesized using the catalyst on the end located CNTs, more stable results can be obtained. In this process, a lot of effort was made to find the same CNT strands. It was difficult to engrave a marker on the TEM grid. Thus, a peripheral window usually changed in shape due to the influence of the catalyst was used.
Figure 29 SEM image of comparing between before and after dropping CoNP catalyst solution.
The round shape of the after image is the appearance of a catalyst deposited.
After the second synthesis, it was very difficult to capture the desired 'T-Junction' SEM image. In particular, tracking the same CNT has become the biggest obstacle in situations where most CNTs are fractured and rearranged. However, by chance, CNTs, which were not visible in SEM images after the first synthesis, were often seen as T-junction shapes. CNTs suspending in the air vary in distance depending on their position, so there are strands invisible from the SEM image. However, it is rearranged by factors such as catalyst, gas flow, and external impact, and is observed when the distance difference is corrected. As such, it was difficult to accurately confirm the presence or absence of
Before
After
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synthesis with SEM analysis, so TEM image photographing was performed.
Figure 30 Secondary CNTs synthesized from CoNP catalysts
Figure 31 SEM image; Synthesis of another CNT using previous CNT as a catalyst support.
CNT extending up and down in the image is strands that have existed before.
3.2.2. TEM image
Observing carbon nanotubes through TEM enables to find structural characteristics of individual nanotube with better magnification. Especially, metal catalyst nanoparticle which exist in the nanotubes could be identified through TEM. Impurities in the CNT walls including carbonaceous particles aroused by non-vaporized graphitic rod, amorphous carbon and opened tips could be also investigated (Goornavar, Jeffers, Biradar, & Ramesh, 2014). Some other structural parameters, such as the diameter and length of the tubes, number of walls, entanglement, and curvature, can also be observed using TEM (Chong et al., 2017). To decrease the number of impurities in walls of the carbon nanotube, Boncel et al. annealed MWCNTs at 2100 °C under argon atmosphere. The electron microscopy was used to characterize the MWCNT morphology. SEM could not present structural difference after the heat treatment. Meanwhile TEM showed eviction of the metal particles trapped in the carbon nanotube.
However, most of these studies are cases in which carbon nanotube synthesis was conducted on a
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substrate. The biggest difference in this experiment is the synthesis of new CNTs from CNTs suspended in the air, which became the biggest limitation when performing TEM imaging. In the case of TEM, like SEM, it is a device that shoots an electron beam into a sample and projects the current into an image.
Therefore, it is easy to observe only when the sample is fixed to some extent. However, in the case of CNTs hanging in the air, it is very shaky every time energy is applied, so it is difficult to obtain a clear image for TEMs that obtain high-resolution images. However, the difference from SEM imaging is that it can be visually confirmed that a cobalt catalyst exists at the junction of the newly synthesized CNT.
This is because the metal has a characteristic that is more clearly displayed when hit by an electron beam. However, as mentioned above, because it is very unstable, many CNTs were cut off even during TEM imaging. So, there's a problem with reproducibility.
Figure 32 TEM image after dropping CoNP catalyst solution on CNT. (a) The more amorphous carbon exists, the more the catalyst sticks. (b) The appearance of the catalyst clumping on CNTs with relatively little amorphous carbon.
(a)
(b)
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Figure 33 TEM image; Synthesis of another CNT using previous CNT as a catalyst support. The darker part of the yellow arrow means a metal catalyst.