3. CNT: Role of one-dimensional catalyst support
3.1. Experiment
3.1.1. TEM grid selection
There were several trials and errors in finding a suitable TEM grid for the experiment. In particular, the purpose of this experiment was to place catalyst particles on CNTs and observe CNTs newly synthesized by their interactions, so there should be no interaction between catalyst particles and substrates. To solve this problem, a grid with a window among TEM grids was used. The TEM grid used at the beginning of the experiment was a TEM grid with a square window (200 um × 200 um).
CNT is a very strong material, so it is expected that it will withstand well without breaking even if the middle is suspended over the window. However, this was a completely wrong expectation, when the catalyst solution was dropped on the synthesized CNT, most of them were cut off. In addition, it was found that simple atmospheric flow could generate very severe shaking and breaking CNTs. Therefore, the next TEM grid candidate, the slit window TEM grid, was used. This is a TEM grid in which two 50 um × 1500 um windows are arranged side by side. It was expected that the stability would be high due to its relatively small width. The results were certainly able to synthesize more CNTs when observed with SEM. However, this TEM grids were not completely stable in the next step of raising the catalyst.
3.1.2. Catalyst deposition method
The method of placing a new catalyst on top of an already synthesized CNT is the most difficult part
140 160 180 200 220 240 260 280
0.8 1.0 1.2 1.4 1.6 1.8
Semiconducting CNT Metallic CNT
diameter (nm)
RBM (cm-1)
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of this experiment. The synthesized CNTs were very vulnerable to tension because only both sides of the TEM grid were fixed to the window frame and hung in the air. At first, the entire TEM grid in which CNT exists was immersed in a catalyst solution to deposit a catalyst. However, the loss of CNTs was too large to proceed with additional experiments. So, I decided to use the drop-casting technique. Drop- casting had relatively little loss of CNTs. Additionally, in order to confirm the synthesis of new CNTs from the catalyst above CNTs, the catalyst was placed only at half of the window. There have been various attempts here, too. Since CNT is simply placed on top of the TEM grid, masking techniques using photolithography and PR are not available. Therefore, a method of injecting droplets suitable for the volume of the window and a method of using a micropipette were attempted, but there were also many CNTs losses. The last method I tried was to drop only one droplet into the TEM grid window using a glass capillary and syringe and dry it. However, there was another problem. It was the wetting phenomenon caused by the high volatility of the catalyst solution. Once a droplet was located, it quickly spread throughout the TEM grid. In order to solve this problem, it was evaluated that the use of physical barriers was inevitable. Therefore, the absorbent paper was cut to install a physical barrier among the TEM grids. This worked very effectively. The SEM image also clearly showed that only half of the window area was contacted.
Figure 25 Preparation for drop CoNP solution
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Figure 26 SEM image of half-cat. treated TEM grid
In the case of iron catalysts, they were deposited to a thickness of 5 Å using thermal evaporation.
However, annealing at 550°C in the air did not proceed. This is because, due to the nature of CNTs, it is oxidized and lost when annealing is performed in the air.
3.1.3. Synthesis of carbon nanotube via CVD method
The CNT synthesis of the previously announced T-junction morphology is regarded as one CNT with branches connected to the main body. However, since the synthesis of T-junction CNTs presented in this paper consists of two CNTs, the synthesis process must also be performed twice. The first synthesis is not much different from the existing method. However, the direction of CNT growth across the window. The synthesis of the second CNT was carried out by placing a catalyst on the first CNT and then turning it 90°. If the synthesis of both steps is successful, the desired result, 'T-Junction branched CNT', can be obtained. All carbon nanotubes synthesized through CVD used the same synthesis conditions. However, only the TEM grid was put in the insert glass cylinder and tried to synthesize, but it did not grow properly. This was evaluated to be a problem caused by the change in substrate among the synthesized variables, and the synthesis method was changed. Same as a general method of synthesizing horizontal-aligned carbon nanotubes, the TEM grid was placed on a SiO2 substrate and the catalyst was placed in only one corner of SiO2. Then, as the desired result, the growth of CNTs starting from the catalyst passed over the TEM grid and proceeded in the opposite direction. The CNT synthesized in this way was first confirmed through SEM. Then, as a first step, the catalyst was placed on the CNT by the drop-casting method mentioned above, and the CNT on which the CoNP catalyst was raised was confirmed by SEM. Second, to perform CVD synthesis, CNTs were synthesized in the same method by turning 90° in the direction in which the synthesis was initially carried out, to perform CVD synthesis. The expected result at this stage is to observe that a new CNT grows in the direction of 90° in the CoNP particles on the existing CNT. There was another obstacle to synthesis. It is that CNTs
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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.