94 Figure 3.13 Initial, transition state, and the final structures of the transformation of a (a) C3 or (c) C4 carbon chain to a hexagon at the kink site on Ni(111) surface. MEPs of the (b) first, (d) second, (f) third, and (h) fourth step of the CH4 decomposition reactions with (bottom with black arrowheads) and without oxygen (top with red arrowheads).
Background and Method
Prosperity of sp 2 carbon materials
After the discovery of fullerene – a spherical sp2-carbon hollow cage, a hollow cylinder also made of sp2-carbon (Fig. 1.3a, c-e) was later observed in 1991 under an electron microscope by S. In each of its unit cells, which are two characteristic atoms, A and B (blue and green dots respectively in Fig. 1.4c).
Trends and challenges in synthesis of nano-carbon materials
This limits the growth of extra layer of graphene after the formation of the first graphene layer. The twofold symmetry of the Ge(110) surface only allows the growth of graphene domains in one direction.
Current theories and simulations on the formation of nano-carbon materials
With higher density of bends at the CNT edge, the growth rate of the tube wall increases. The above prediction of CNT abundance depends on the maintenance of CNT chirality during growth.
Methods
- Density functional theory
- Density functional based tight binding
- Molecular dynamics simulation
- Nudged elastic band method
Thus, the accuracy of the MD simulation strongly depends on the way in which the potential energy is calculated. There are many different algorithms, of which Verlet is one of the most simple and effective. This new method, called the climbing picture NEB (CINEB), guarantees that the activation energy and the transition state configuration could be obtained after the convergence of the calculation (Fig. 1.30).
The CINEB used to find the MEP in the thesis is incorporated into the VASP code for DFT calculation with a spring constant of the forces between the configurations to be -5.0 eV/A2.
Arc discharge Fullerene Formation Mechanism
Introduction
An arc-jet model to simulate the carbon cluster evolution during fullerene formation
If helium is used as a buffer gas, the fullerene yield will first increase dramatically with increasing pressure and will reach its highest very soon. This different influence of the buffer gas is actually well explained by our collision model. As in equation (2.2), the rate of diffusion is inversely proportional to the pressure and the square root of the mass of the buffer gas.
As the pressure increases, the diffusion rate decreases and therefore the collision number decreases rationally, resulting in increased yield (Fig. 2.3a middle).
Free energy evolution from atomic carbon to buckyball fullerene during fullerene growth
- Collision between carbon clusters
- Transformation between non-IPR and IPR fullerenes
- Fullerene decomposition at high temperature
The addition of a single carbon atom to the non-IPR fullerene would greatly reduce the reaction energy in the first step (Fig. 2.15a). In the second step, the non-IPR fullerene was transformed into an IPR fullerene by the carbon chain (Fig. 2.16b). It was found that with the increase of the temperature, the transition barrier increases and.
In particular, the transition from non-IPR fullerene to IPR fullerene could be achieved with the help of the carbon chains/rings.
An advanced kinetic model to reveal the entire fullerene formation process
All reactions have a parameterized energy barrier 𝐸𝑏, as shown in Figure 2.3c, and a free energy of the reaction Δ𝐺 from the calculation in section 2.4. All responses were implanted into the new program and the flowchart of the program is shown in Figure. For example, it is shown that upon changing the initial carbon cluster rate (Figure 2.23c-e and f-h), the final distributions of the fullerenes are very different.
In addition, changing the initial carbon vapor concentration will also greatly alter the final batch distribution.
Conclusion
According to the free energy result, we found that at very high temperature, the stability of the carbon cluster decreases with increasing size. During the fullerene formation process, as the temperature decreases, the stability of the carbon chains/rings increases and they become the dominant species. As the temperature further decreases, the transition barrier decreases, so more and more carbon chains/rings can be converted into fullerenes.
More importantly, MD simulations indicated that the collisions between fullerenes and carbon chains/rings could aid the growth of the fullerene by transferring small carbon fragments (C1, C2, and C3) to the fullerene cages and aid in the growth and annealing of the non-IPR or odd-membered fullerenes to IPR fullerenes.
Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
Introduction
Nucleation of a SWCNT on iron catalyst
But we later realized that this could probably be the reason for stopping the formation of more polygons. But unfortunately, no breaking of the carbon chains was observed and the addition of H cannot promote the formation of polygons. We suspect that the addition of H at the beginning of the simulation is likely to prevent the formation of too many long carbon chains.
So the simulation result in this period proves our previous argument that the formation of long chains prevents the formation of the cap.
Stability of SWCNT on the catalyst during growth
- SWCNT growth on metal catalysts
- Effect of hydrogen on SWCNT growth
Strangely, the Fe particles seemed to carry these long carbon chains and still be able to aid in the rapid elongation of the tube. In the case of the Ni particle, its attachment to the tube is slightly smaller than that of Fe, which is why a small narrowing of the tube occurred. In the case of the Cu particle, its binding to the tube is much smaller and the narrowing can happen very quickly.
It is found that the addition of H was able to prevent the narrowing of the tube for those catalysts with weak binding to the tube as shown in Fig.
Carbon chain formation at growth front during CNT synthesis
- Stability of carbon chains at growth front
- Transformation between carbon chains and regular edges
- Evolution of carbon chains at growth front with different carbon feeding rates
By decreasing the carbon supply rate to 1 C/10 ps, we quickly observed a significant decrease in the number of unsaturated C atoms to an average of 31 by the end of the five MD simulations (Fig. 3.17a). It means that the number of carbon chains has decreased, which is also shown by the snapshots of the MD simulations (Fig. 3.17c). It was found that the metal particles gradually moved out of the tube (Fig. 3.18c-g and g-j).
This demonstrates the continuous suppression of the formation of carbon chains with decreasing carbon input rate.
Stability of various SWCNT-catalyst interfaces
- Global searching of interface configurations of (8,4) tube on flat solid surface
- Interface configurations of other tubes on solid surface
- Interface searching on liquid catalyst particles
It can be seen that the trend of the interface formation energy with different edge configurations is the same as the previous result with the short tube segment model, while the energy of the isolated A-Z edge (edge 1) rises relatively to be more higher than that of the conventional split edge A-Z (edge 8). Now the result of the interface formation energy between different edge configurations and the solid catalyst is more reasonable and convincing. Using exactly the same calculation method, we also checked the formation energy of the tube interface (12,6).
This clearly shows that the model with shorter tubes (grey dots) will overestimate the formation energy of the A-Z split edge interface due to slight distortion.
Role of alloy catalyst on SWCNT growth
- Phase separation of alloy catalyst on SWCNT
- Alloy catalyst without CNT
- Phase separation on large alloy catalyst
- CNT growth behavior with the phase separated alloy catalyst
And it is also clear from the figure that the added C atoms (indicated with red shading) mostly distributed on the Ni side of the particle, which was close to the tube edge. Thus, the ratio of the active metal in the alloy catalysts would be decisive for the CNT growth. In summary, we observed a CNT catalyst contact-induced phase separation of the alloy catalyst during CNT growth in our DFT-MD simulations.
The relatively active metal moves to the CNT-catalyst interface, and the less active metal to the other side, preventing deactivation of the catalyst from the alloy.
Chirality selectivity of SWCNT during nucleation
- Chirality assignment of SWCNT by the addition of the 6th pentagon into a graphitic
- Free energy during the nucleation of SWCNT
- Route towards selective growth of SWCNT
It is when the number of pentagons in the network reaches 6 (if no other polygons except pentagon and hexagon), the formation of the cap is completed and the elongation of the SWCNT starts. If the above discussion is correct, that the addition of the 6th pentagon is a kinetic process, we could imagine a picture like fig. At the onset of nucleation, with the addition of pentagons in the cap, the formation energy of the cap increases very rapidly.
We gradually added C onto the catalyst surface and calculated the interface formation energies for the corresponding caps.
Conclusion
Only when the excessively long carbon chains are removed can the formation of the nucleus be realized. Therefore, an etchant such as hydrogen must be added to control the concentration of the carbon chains. It was shown that the carbon chains at the CNT–catalyst interface observed in most MD simulations were due to the limited time scale of these simulation methods (<100 ns).
Through DFT-MD simulations, it was also found that the CNT–catalyst contact could induce the phase separation of the alloy catalyst during CNT growth.
New Mechanisms on Graphene Chemical Vapor Deposition Growth
Introduction
Elements modulation on graphene growth
- Oxygen assisted ultrafast growth of graphene
- Mechanism of fluorine modulated graphene growth
At the front of the Cu foil, there were only star-shaped graphene domains of ~20 μm size. First, we noticed that the domain shapes at the front and back of the Cu foil were the same as those in the graphene growth with oxygen. It can be seen that the reaction processes are very similar to those of the methane decomposition reactions with the help of O.
Later, we checked the influence of temperature on the barriers and reaction energies for the decomposition of the methane molecule (Figure 4.12).
Self-limited growth of graphene domains
Their data further showed that with increasing surface coverage, i.e. by reducing the exposed surface area of copper, the graphene growth rate decreased dramatically. Thus, they argued that the decrease in growth rate was due to the decrease in the exposed surface area of Cu, which is responsible for the catalyzed decomposition of methane. On the other hand, the growth rate is related to the total length of graphene domain edges.
The final state of our PFT simulation (Fig. 4.16f and p) also shows a similar behavior in that the widths of the narrow channels (labeled d) surrounding the graphene domains are almost the same.
Conclusion
Conclusions
E.; Liu, J., Synthesis of near-uniform single-walled carbon nanotubes using identical metal-containing molecular nanoclusters as catalysts. Shibuta, Y.; Maruyama, S., Molecular dynamics simulation of the formation process of single-walled carbon nanotubes by the CCVD method. Bolton, K., Importance of strong carbon-metal adhesion for catalytic nucleation of single-walled carbon nanotubes.
Zhao, Q.; Xu, Z.; Hu, Y.; Ding, F.; Zhang, J., Chemical vapor deposition synthesis of near-zigzag single-walled carbon nanotubes with stable tube-catalyst interface.
