Chapter 3. Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
3.4 Carbon chain formation at growth front during CNT synthesis
3.4.3 Evolution of carbon chains at growth front with different carbon feeding rates
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chains are able to be converted into hexagons in the CNT wall because of the macro time scale of annealing.
3.4.3 Evolution of carbon chains at growth front with different carbon feeding
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Snapshots of the simulation (5th trace) shows it clearly that a lot of carbon chains formed as the MD simulation just began (Fig. 3.16c). These carbon chains soon formed a network with large hollows, grabbing the entire particle surface (Fig. 3.16d). Further carbon addition then filled the hollows, and there formed a defective encapsulation of the particle at the end of the simulation (Fig. 3.16f). Therefore, the CNT growth was quickly terminated as the fast addition of C sources immediately poisoned the catalyst particle.
Here the growth mode of CNT is quite abnormal. CNT growth is not by the propagation of the tube edge but by the entanglement of the carbon chains, which is similar to what we have observed for the CNT nucleation in Section 3.2. While, high concentration of C is required for nucleation but not for the elongation of CNT. It would even poison the elongation of CNT.
Figure 3.17 (a) Number of C atoms with 1 or 2 coordination during the 5 MD simulations with a carbon feeding rate of 1 C per 10 ps as a function of number of C added. As the initial structure was a (7,7) tube attached on a Ni72 particle, there were already 14 C atoms counted at the beginning. The simulations were performed at 1500 K with a time step of 0.5 fs, employing NVT ensembles. (b) Snapshots of the 1st MD simulation showing wetting of the metal particle with the addition of C atoms.
Decreasing the carbon feeding rate to 1 C/10 ps, we quickly observed a significant decrease in the number of unsaturated C atoms to averagely 31 at the end of the 5 MD simulations (Fig.
3.17a). It means the number of carbon chains decreased, which is also demonstrated by the snapshots of the MD simulations (Fig, 3.17c). There were no more encapsulations happened with this carbon feeding rate but one interesting thing occurred. We observed very obvious wetting of the catalyst particle into the tube wall just like shown in Fig. 3.17d. Carefully checking the tube wall, we found that it was because on the tube wall there were many defects, especially sp C atoms (Fig. 3.18a-b), which attracted the metal atoms.
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Figure 3.18 (a-b) sp carbon defects in the configurations during the MD simulations of CNT growth with a carbon feeding rate of 1 C/10 ps, where wetting of the metal particle into the tube happened. (c-f) and (g-j) Two series of snapshots showing dewetting of the metal particle happened after the healing of the sp carbon defects from the configurations in the MD simulations where wetting of the particle occurred.
We, therefore, removed all the sp C defects (defects like 5|7 were remained), and continued the MD simulation at the same temperature without the addition of C atoms for totally 900 ps. It was found that the metal particles gradually moved out of the tube (Fig 3.18c-g and g-j). So this means that wetting of the particle is a result of the high concentration of sp C defects in the tube wall.
The above simulation results show that with such a carbon feeding rate, there was no encapsulation any more, and the tubes could grow longer than those with higher feeding rates.
In addition, the formation of carbon chains at the CNT-catalyst interface was greatly suppressed as the number of unsaturated C atoms decreased. But the metal particles easily wetted into the CNT since the grown tube was highly defective under such a high carbon feeding rate.
Further decreasing the carbon feeding rate to 1 C/80 ps, average number of the unsaturated C atoms at the end of the simulations continued to decrease to only ~ 20, increasing totally 6 unsaturated C atoms compared with the initial 14 (Fig. 3.19a). This demonstrates the continuous suppression of the formation of carbon chains with the decreasing of the carbon feeding rate.
As shown in Fig. 3.19d, at the end of the simulations, the CNT-catalyst interface was very similar to pure zigzag edge but with very few short carbon chains. In addition, as the increasing of the tube wall quality, wetting of the particle into the tube was seldom observed again. To be noted, here high quality of the tube does not mean the disappearance of any defects. On the
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tube wall, defects like sp carbon were gone, while defects like 5|7 or other polygons other than hexagon still exist. But there were also something unexpected happening—the tube tended to grow narrower and close after a short growth time. Therefore, within the 5 cases, three of the simulations ended up with a closed fullerene-like structure. But after the closure, as the metal particle did not leave the tube structure, it restarted to grow the tube again, forming bamboo- like structure.
Figure 3.19 (a) Number of C atoms with 1 or 2 coordination during the 5 MD simulations with a carbon feeding rate of 1 C per 80 ps as a function of number of C added. As the initial structure was a (7,7) tube attached on a Ni72 particle, there were already 14 C atoms counted at the beginning. The simulations were performed at 1500 K with a time step of 0.5 fs, employing NVT ensembles. (b) Snapshots of the 1st MD simulation showing the growth of CNT on the metal particle with the addition of C atoms.
Figure 3.20 (a) Number of C atoms with 1 or 2 coordination during the 5 MD simulations with a carbon feeding rate of 1 C per 640 ps as a function of number of C added. As the initial structure was a (7,7) tube attached on a Ni72 particle, there were already 14 C atoms counted at the beginning. The simulations
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were performed at 1500 K with a time step of 0.5 fs, employing NVT ensembles. (b) Snapshots of the 3rd MD simulation showing the growth of the CNT on the metal particle with the addition of C atoms.
Decreasing the carbon feeding rate to 1 C/640 ps, it was more obvious that number of the unsaturated C was lowered to only 16, almost no increasing with the simulations (Fig. 3.20a).
At the interface, we observed only one C2 chain at one of the simulation result (Fig. 3.20d). But the narrowing of the tube was much more intense. Almost all of the simulations ended up with the closure of the tube. As we have discussed before, we believed that it was because the binding between Ni particle and the tube end was not strong enough to support the opening of the tube.
Perhaps the addition of H or O in the experimental conditions could solve this problem.
Figure 3.21 (a) Average number of C atoms with 1 or 2 coordination during each 5 MD simulations with different carbon feeding rates as a function of number of C added. As (b) the initial structure was a (7,7) tube attached on a Ni72 particle, there were already 14 C atoms counted at the beginning. The simulations were performed at 1500 K with a time step of 0.5 fs, employing NVT ensembles. (c-f) Final configurations of the MD simulations with carbon feeding rate of 1 C/ps, 1 C/10 ps, 1 C/80 ps, and 1 C/640 ps, respectively.
To summarize (Fig. 3.21), with the simulations of CNT growth under different carbon feeding rates, we demonstrated that it was because of the time-scale limitation of the MD simulation methods and the resulting high carbon feeding rate that the CNT-catalyst interface was always dirty with many carbon chains existing during the simulations. When decreasing the carbon feeding rate, the carbon chains would gradually disappear. It was believed that at experimental
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condition, with enough time for the annealing of the attached chains, the tube end would be regular edges, composing of only zigzag, armchair and kink site.