Chapter 3. Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
3.2 Nucleation of a SWCNT on iron catalyst
In this section, we, for the first time, report a complete cap nucleation process of CNT on an iron (Fe) particle using DFT-MD simulation, and reveal the importance of long carbon chains in CNT nucleation. Initially, the existence of long carbon chains facilitates CNT growth by triggering the nucleation of the first polygon in CNT cap. However, when there grows more chains, it in turn, interrupts the cap formation by preventing the increasing of polygons and poisoning the catalyst. We found that by breaking the long carbon chains into pieces, nucleation could be easily restarted, and subsequently, a complete cap formation was observed. Therefore, we propose that in experiments of growing CNT, appropriate partial pressure of gases (H2, and H2O etc.) other than the precursor is essential for cutting down the formed long carbon chains and facilitating the cap nucleation.
Figure 3.1 (a) Number of C added (left) and number of polygons formed (right) during 240 ps DFT-MD simulation. (b) Number of C-Fe bonds (red) and C inside Fe particle (black) in the simulation. Pink line is the original fluctuation of C-Fe bonds in the MD simulation and red line is the smoothed line of the pink line. (c) Snapshots with number of pentagon, hexagon, and heptagons denoted. Purple, grey, and red spheres represent Fe, C, and C in polygon.
The whole MD simulation was performed with an initial Fe30 catalyst particle for more than 240 ps long (time step = 1 fs) with 115 carbon atoms added in total. At the end of the simulation, 11 pentagons, 15 hexagons, and 5 heptagons were formed, consisting of a CNT cap with
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diameter of 0.9 – 1.0 nm. As shown in Fig 3.1c, firstly, C dissolved into the Fe particle, and gradually accumulated and formed long carbon chains at the particle surface. Later, with the increase of the chain density, long carbon chains entangled with each other and formed polygons (pentagons and hexagons). With the addition of more C, more polygons were introduced in and the cap lifted up. In the figure, we use (x,y,z) to denote the number of the pentagon, hexagon, and heptagon formed, respectively.
To illustrate the entire process clearly, we divided the whole process into four periods (see Fig.
3.1a, where 1-5 are labeled at the top of the figure): P1-P2 is the first period where the temperature remained 1800 K and during the 75 ps simulation totally 73 C were added; P2-P3 is the second period where the temperature was gradually annealed to 1300 K from 1800 K within 20 ps, and there was no further C addition; P3-P4 is the third period lasting for 28.1 ps with a steady temperature of 1300 K where no C added but the existing long carbon chains were broke into small carbon fragments such as C1 and C2; P4-P5 is the final period in which 42 more C were added at 1300 K and the cap formation was completed within 116.8 ps.
Fig. 3.1a shows the number of C added (black), and the number of pentagon (red), hexagon (yellow), and heptagon (blue). It is clear that the formation of pentagons is actually very steady while there is a dramatic increase in the number of hexagons at period P2-P4, which was because of the breaking of the long carbon chains as we will discuss later. The figure also confirms that the formation of pentagons is earlier than that of hexagons. In addition, the formation of heptagons is much slower than that of pentagons and hexagons. Fig. 3.1b shows the number of C-Fe bonds and C inside Fe particle. It can be seen that the number of C inside the catalyst particle is very steady while the number of C-Fe bonds fluctuates and would indicate us the lifting of the cap which we will talk about later.
Figure 3.2 (a) Snapshots during 75 ps DFT-MD simulation of Period 1 to 2 at 1800 K. Purple, grey, and red spheres represent Fe, C, and C in polygon. (b) Number of C added and polygons formed.
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During the first period P1-P2 (Fig. 3.2), totally 73 C were added gradually as shown in Fig.
3.2b (black line). From the snapshots of the MD simulation (Fig. 3.2a) we can see that it was after the saturation of the C in the Fe particle that carbon chains formed. In the second image of Fig. 3.2a, we observed more than two long carbon chains, each with more than 10 C. With the increase of these long carbon chains, they began to entangle with each other and this entanglement led to the formation of the first pentagon and the second pentagon. But the formation of the pentagons and hexagons at this stage was accompanied with the deformation, and therefore, at the end no more polygons appeared even when the particle was saturated with a lot of C already. To note, we performed the simulation at so high a temperature (1800 K) because high temperature could facilitate most of the process and we need to accelerate the nucleation due to the computational resource limitation. But we later realized that it could be probably the reason for the stop of the formation of more polygons.
Figure 3.3 (a) Snapshots (top and side views) during cooling process where the temperature decreases from 1800 K to 1300 K (P2-P3) and further to 800 K. (b) Number of C-Fe bonds (red) and C inside Fe (black). Pink line is the original fluctuation of C-Fe bonds in the MD simulation and red line is the smoothed line of the pink line. (c) Snapshots during 17.2 ps MD simulation when 20 H atoms were added to break the chains. Purple, grey, red, yellow and white spheres represent Fe, C, C in polygon, C in long chain, and H.
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Therefore, during the second period P2-P3 (Fig. 3.3), we stopped adding more C but decreased the temperature of the simulation gradually with a speed of 40 step per Kelvin decreasing. This process is called simulated annealing. We wanted to see the precipitation of C from the particle to form more polygons. But after forming three more polygons (2 hexagons and 1 pentagon) at 1300 K, further decreasing the temperature to 800 K could not lead to the formation of more polygons (Fig. 3.3a). Fig. 3.3b shows that there are more and more C-Fe bonds formed during the annealing but why it did not result in the formation of polygons? In the simulation, we observed that there were always several long carbon chains remaining there, very stable and stubborn. It was then argued that the formation of these long carbon chains took away the C and prevented the formation of polygons, which inhibited the cap formation or lifting.
After realizing the reason for the cease of polygon formation, we tried to break these long carbon chains by adding hydrogen (H) atoms. An additional simulation was done from P3 image. The simulation was performed for 17.23 ps with totally 20 H added (Fig. 3.3c). But unfortunately no breakage of the carbon chains was observed and the addition of H cannot promote the formation of polygons. The long carbon chains were not easily etched. We suspect that probably the addition of H at the beginning of the simulation could prevent the formation of too many long carbon chains. But once the long chains formed, it is hard to break them.
Figure 3.4 (a) Snapshots (top and side views) during MD simulation when the existing long chains were broken down and no more C added (P3-P4). Purple, grey, red, and yellow spheres represent Fe, C, C in polygon, and C in long chain. (b) Highlight part of Fig. 3.1a and b showing dramatic increase in hexagon number and decrease in surface C-Fe bonds number during simulation when the long chains were
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manually broken down. (c) Snapshots during 117 ps MD simulation of period 4 to 5. Purple, grey, and red spheres represent Fe, C, and C in polygon.
Because it costs too much to start a new simulation with H addition, to also verify that the formation of long carbon chains prevents the formation of polygons, we continued the simulation from P3 by artificially breaking the carbon chains and rearranging the C randomly on the particle surface. With twice of this breakage process and totally 28.1 ps simulation, the cap formation restarted and 7 more polygons formed including 2 pentagons, 4 hexagons and 1 heptagon (Fig. 3.4b), without adding more C (Fig. 3.4a). It can be seen that initially there were long carbon chains labeled in yellow in the first image in Fig. 3.4a. When they were broken, polygons formed very soon. And we could observe a small cap lifting already in the last image of Fig. 3.4a, which could be confirmed by the decreasing of the C-Fe bonds during the period in Fig. 3.4b (bottom). So the simulation result in this period demonstrates our previous argument that it is the formation of the long chains prevents the cap formation. The formation of long carbon chains is so essential that it leads to the formation of the first pentagon but it could also prevent the nucleation by poisoning the catalyst surface.
As the small cap had already lifted, we further added 42 more C gradually within a 116.8 ps subsequent simulation. The small cap was enlarged by the formation of 6 more pentagons, 9 more hexagons, and 4 more heptagons (Fig. 3.4c, second image). As a 5|7 pair could be regarded with no extra curvature created (equal to hexagons), totally 11 pentagons and 5 heptagons equals to simply 6 pentagons, which is just what is need for a cap. In addition, the steady increase of hexagons with the increase of C near the end of the simulation in Fig. 3.1a also confirms a typical elongation characteristic. Therefore, we believed that the cap formation was completed and there went the elongation of the CNT.
So in this work, we observed a complete cap formation process on Fe30 particle via DFT-MD simulation and found that: i) the entanglement of the long carbon chains formed at the beginning of the CNT synthesis initiates the formation of the first polygons; ii) the long carbon chains later poison the particle surface and prevent the cap enlargement; iii) artificially preventing chain formation could easily lead to the successful cap formation; iv) in real experiments, etching agent like hydrogen should be added to control the concentration of the carbon chains.
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