Chapter 3. Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
3.6 Role of alloy catalyst on SWCNT growth
3.6.3 Phase separation on large alloy catalyst
Now let’s consider a model in larger system. As in real experiments the diameter of the CNT is several nm long, size of the catalyst particle would be hundreds or thousand times larger than those in the previous models. Such a huge particle is beyond the capacity of our simulation method.
In this case, we proposed a model that we focused on only a small section at the tube-catalyst interface as shown in Fig. 3.35a. If the particle is large enough, at the local interface, the curvature would be very small, and we could just mimic it with a perpendicular graphene sheet with its edge (armchair) attaching to a flat alloy surface (enlarged picture in Fig. 3.35a). With this model, after ~ 15 ps simulation (at 1800 K), the initially randomly distributed Ni atoms (Fig. 3.35b) accumulated around the graphene edge (Fig. 3.35e). The figure of the carbon-metal bond number demonstrates the phase separation process as the number of C-Ni increases and surpasses that of C-Au bonds (Fig. 3.35g). The energy evolution during the simulation also confirms this phase separation is energetically favorable.
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Figure 3.35 (a) Schematic diagram showing that in a large system, the interface between the catalyst particle and the CNT can be viewed as the interface between a metal flat surface and a perpendicularly attached graphene edge. (b-e) Snapshots of the DFT-MD simulation of a graphene armchair edge perpendicularly attached to the Au-Ni alloy surface at 1800 K at 0, 2, 6, and 14 ps, respectively. Blue and yellow spheres represent Ni and Au atoms. Grey and white sticks denote C and H atoms. (f) Energy evolution during the 14.978 ps MD simulation. Grey line shows the original energy fluctuation; black line is the smoothed line; red dashed line highlights the tendency of the energy evolution. (g) Number of C-Ni (blue) and C-Au (yellow) bonds during the MD simulation. Reproduced from Ref. 216. Copyright@2019, American Physical Society.
The above model is with an armchair edge of graphene attached on the Au-Ni alloy surface. We also performed simulations with zigzag edges attached on Au-Ni surface as well as on Cu-Ni surface (Fig. 3.36). Both simulations show the tendency that the active Ni atoms prefer to aggregate near the graphene edge. Firstly it shows that this oriented phase separation would happen with different edges, i.e. CNTs with different chiralities. Secondly, it tells that in a real system that is much larger than the previous models the active metal could still diffuse to the graphene-catalyst interface. Therefore, if as argued that only metals that have strong enough binding with CNT can sustain the CNT growth137, now taking into account our phase separation theory, we suspect that any metal can afford growing a tube as long as small amount of active
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metal is added into the catalyst particle. Because the active metal would accumulate and cover the whole tube end, making the binding between the tube and particle no difference from that between the tube and an entire active metal particle such as Fe, Co, and Ni. Considering the diameter of the normal SWCNT, ~ 200 active metal atoms are enough to terminate the entire tube rim.
Figure 3.36 (a) Energy evolution during the MD simulation of (c) a graphene zigzag edge perpendicularly attached to the Au-Ni alloy surface. (b) Number of C-Ni (blue) and C-Au (yellow) bonds during the simulation in (a). (d) Initial and (e) final configurations during the MD simulation in (a). Blue and yellow spheres represent Ni and Au atoms. Grey and white sticks denote C and H atoms. (f) Energy evolution during the MD simulation of (h) a graphene zigzag edge perpendicularly attached to the Cu- Ni alloy surface. (g) Number of C-Ni (blue) and C-Cu (red) bonds during the simulation in (f). (i) Initial and (j) final configurations during the MD simulation in (f). Blue and red spheres represent Ni and Cu atoms. Grey and white sticks denote C and H atoms. Reproduced from Ref. 216. Copyright@2019, American Physical Society.
The above simulations all show that the CNT-catalyst contact will induce a phase separation of the alloy catalyst during the CNT growth where the active metal atoms tend to accumulate in the vicinity of the CNT-catalyst interface. For a small catalyst (Fig. 3.37c top), the active metal aggregates to the bottom of the particle, interacting with the tube edge, and the less-active metal moves to the other side of the particle which is away from the tube; for a large catalyst (Fig.
3.37c bottom), the accumulation of the active metal happens at the CNT-catalyst interface, forming a circle at the tube end and matching with the tube rim.
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Figure 3.37 (a) Binding energies between Ni55, Cu55, and Au55 particles and a (9,1) tube. (b) Binding energies between C atom and Ni(111), Cu(111), and Au(111) surfaces. (c) Schematic diagram showing the phase separation in both small (diameter ~ 1 nm) and large (diameter > 2 nm) systems. Blue, red, and yellow spheres represent Ni, Cu, and Au atoms, respectively. Grey sticks denote C atoms. Reproduced from Ref. 216. Copyright@2019, American Physical Society.
It is actually not surprising to us for this phase separation in CNT growth. DFT calculations have shown that the binding between Ni55 and CNT is 28.99 eV, while it is 21.11 eV, and 19.02 eV for that between Cu55, Au55 and CNT, respectively (Fig. 3.37a). The strong interaction between Ni and CNT would drive the Ni atoms to the tube rim and therefore Au or Cu atoms are left on the other side of the particle. Considering the energy gain in the formation of strong C-Ni bond (Fig. 3.37b) compared to that in the formation of C-Au or C-Cu bond, the phase separated alloy catalyst in CNT growth is for sure more energetically favorable.
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