Chapter 3. Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
3.3 Stability of SWCNT on the catalyst during growth
3.3.1 SWCNT growth on metal catalysts
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Snapshots of the simulations are shown in Fig. 3.5 from the very beginning (at 14.36 ps) to the end (174.475 ps). The time step of the simulation is 1 ps. The simulation began with a short (9,1) tube (terminated with H) attached to a Fe30 particle. Purple spheres represent the Fe atoms, grey and white sticks are the original C and H atoms, while red sticks denote the newly added C atoms. In the entire MD simulation, we gradually added 136 C, which is actually very fast feeding rate compared to that in real experiments. But due to the time scale limitation of this accurate MD simulation method, it is understandable.
It can be seen that after continuously adding ~ 45 C atoms (randomly added to the catalyst surface), the Fe catalyst particle was already saturated and the tube started to grow as some of the newly added C atoms already attached to the edge of the CNT (3rd configuration in Fig. 3.5).
Within this period, on the catalyst surface, there were mostly carbon dimers or very short carbon chains. Carbon atoms had been very efficiently delivered to the tube edge and incorporated into the CNT tube wall, which shows the high catalysis of Fe particle. But with further continuous adding of C atoms, the carbon chains on the catalyst surface grew longer and longer. For example, in the 7th configuration of Fig. 3.8 there were already more than one carbon chains with more than 5 C atoms. The carbon feeding rate was actually decreased compared to the initial adding rate while it was still too high that the carbon chains became longer. At the end, there were at least one carbon chain with more than 10 C atoms. It is strange that Fe particle seemed able to tolerate these long carbon chains and could still help the fast elongation of the tube. Within the 174.475 ps simulation, the tube elongated for a quite long segment as shown in the last configuration in Fig. 3.8 where the red part (the elongated part) of the tube is already longer than the grey part (the original part). The grown part of the tube, though with a large quantity of defects like pentagons and larger polygons, maintained a regular tube structure.
More importantly, the tube diameter remained almost the same as that of the initial.
Later, we did another CNT growth simulation using Ni30 particle as the catalyst (Fig. 3.6). Ni has also been widely used as catalyst to grow CNT though not as catalytic efficient as Fe. The MD simulation was also performed using DFT-LDA with a time step of 1 fs at 1800 K. Totally 99 C atoms were added during the 133.813 ps simulation time. Comparing the two simulations using Fe and Ni particle, there are very interesting differences. Firstly, although the two simulations are with similar carbon feeding rate (0.7 C per ps, after the initial feeding), it seems that there are more surface carbon on the Fe particle, like long carbon chains. More importantly, using Ni catalyst, the tube elongated more but the diameter of the tube decreased, while using Fe catalyst, the tube grew with the diameter maintained. Actually the newly grown parts of the two tubes are both defective with polygons other than hexagons. But the different behaviors in diameter control for the two catalysts raised our interests.
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Figure 3.6 Snapshots of a DFT-MD simulation of the SWCNT growth on a Ni30 catalyst particle by gradually adding 99 C at 1500 K. The total simulation time is 133.813 ps with a time step of 1 fs. Initially a small piece of a (9,1) tube (grey) is attached to the catalyst particle, which is terminated with H atoms (white). The newly added C are all labeled in red. Blue spheres represent the Ni atoms.
Figure 3.7 (a) Initial and final configurations of a DFT-MD simulation of the SWCNT growth on Fe30
particle at 1500 K. (b) Initial and final configurations of a DFT-MD simulation of the SWCNT growth on Ni30 particle at 1500 K. (c) Initial and final configurations of a DFT-MD simulation of the SWCNT growth on Cu30 particle at 1500 K. Initially a small piece of a (9,1) tube (grey) is attached to the catalyst particle, which is terminated with H atoms (white). The newly added C are all labeled in red. Purple, blue, and orange spheres represent the Fe, Ni, and Cu atoms, respectively.
Another MD simulation was then performed to further analyze the diameter control using different metal catalyst. In this simulation, we set the parameters exactly the same as the previous two simulations but only changed the catalyst to an even less-active Cu30 particle. It could be seen from the comparison of the results of the three simulations (Fig. 3.7) that from
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using Fe, Ni, to Cu, the grown tube is with smaller tube diameter. CNT growth on Fe could maintain the tube diameter; with Ni, the tube is gradually narrowing; using Cu, there is more intense narrowing in the tube growth. It has been argued that the binding between the tube and the catalyst would determine if the tube could stay open or if the catalyst could efficiently support the CNT growth137. Further DFT calculations show that the binding between the catalyst particle (containing 55 atoms) and the tube (9,1) drops from 32 eV, 29 eV, to 21 eV for Fe, Ni, and Cu, respectively. So, for Fe particle, its binding to the tube edge could sustain the tube growth and therefore the tube diameter did not change. For Ni particle, its binding to the tube is a little bit smaller than that for Fe, and therefore, a little narrowing of the tube happened.
For Cu particle, its binding to the tube is much lower, and the narrowing could happen very fast.
But when the tube narrows to a very small diameter, the high curvature of the small tube will prevent its further narrowing.
Here comes another problem: why in experiments, synthesis of CNT using Cu, Au and many other weak-binding catalysts is still possible if weak binding means the narrowing of the tube which leads to the termination of tube growth? In the real CNT growth, there are always some other elements in the gas phase that for now we have not added into the simulation systems.
Reports have shown that the addition of the other elements like O in CO2, and H in CH4, would greatly enhance the growth efficiency or change the chirality distribution81, 205. Therefore, addition of these elements in CNT growth simulation was considered later.