Chapter 3. Growth of Single-Walled Carbon Nanotube on Metal Catalyst Particles
3.6 Role of alloy catalyst on SWCNT growth
3.6.1 Phase separation of alloy catalyst on SWCNT
In all the models discussed in this section, the catalysts, no matter monometallic or alloy ones, were consisting of 30 metal atoms, If with CNT, the catalysts were attached to a short (9,1) tube with totally 60 C. The C at the other side of the tube were saturated by H atoms. Considering the vibrations of the C and metal atoms, the time step for the simulation was set as 1 fs while the H atoms were fixed. All the simulations were performed at 1800 K.
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Figure 3.30 (a-c) Snapshots of the DFT-MD simulation of a (9,1) tube on Au15Ni15 alloy particle at 1800 K at 0.1, 6, and 17 ps, respectively. Blue and yellow spheres represent Ni and Au atoms. Grey and white sticks denote C and H atoms. (d) Initial and (e) final metal atom distribution along z direction. Blue and yellow represent Ni and Au, respectively. (f) Energy evolution during the 18.387 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 first model we presented was with a Au-Ni alloy consisting of 15 gold and 15 nickel atoms attaching on a (9,1) CNT. Initially the two kinds of metal were randomly distributed in the catalyst particle as shown in Fig. 3.30a. Around the tube end, there were both Au and Ni atoms forming bonds with C atoms. As the simulation goes, to our surprise, the change happened quite fast. Within 20 ps simulation, the relatively active Ni atoms quickly moved down towards the CNT and gradually accumulated around the rim of the tube, forming C-Ni bonds, as shown in Fig. 3.30b (snapshot of the MD simulation). At the end of this simulation (Fig. 3.30c), we can find a very clear phase separation of the alloy catalyst that at the bottom part of the particle is the active Ni atoms in blue while at the top part is the pushed up Au atoms in yellow.
Statistical analysis further confirms this separation of the two kinds of metal. Fig. 3.30d is the metal distribution along z direction at the beginning of the simulation (sum of the first 100 steps), while Fig. 3.30e is that of the end. Initial the Ni (blue) and Au (yellow) atoms were equally distributed at all heights. After ~ 18 ps, Au atoms positioned at the upper half of the particle and Ni the lower half. There is a clear boundary between the two colors in the catalyst.
To be mentioned, during the whole simulation process, the energy of the system first went down, and then vibrated and reached a dynamics equilibrium (Fig. 3.30f), demonstrating, at least, the final phase separated system is energetically preferred. In Fig. 3.30g, we also present the number of carbon-metal bonds during the simulation, showing that the initial competitive numbers of C-Ni and C-Au bonds changed significantly. The number of C-Ni bonds surpassed that of C-Au bonds and reached to about 10 when the number of C-Au was only around 1 or 2.
This means, at the end of the simulation, most of the C at the tube end were terminated by the active Ni atoms.
So with the first MD simulation, we observed a special side-by-side phase separation that in an alloy catalyst the more active metal (Ni) moved to the open end of the CNT while the less active metal (Au) was pushed up to the top of the catalyst. We chose the alloy catalyst consisting of Au and Ni because we knew that there is a so big activity difference between the two metals that the simulation result could be more obvious. The following two simulations show that this phase separation is not unique for only Au-Ni alloy but also alloy catalysts that have been
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widely used and reported for the CNT growth. Fig. 3.31a-c shows the snapshots during the MD simulation of a Cu15Ni15 alloy catalyst on the (9,1) tube. Blue and red spheres represent Ni and Cu atoms, respectively. The initially randomly distributed metal atoms soon separated, where Ni atoms mainly stayed at the bottom of the particle, attached to the tube end, and Cu atoms mostly at the top of the particle. There were also several Cu atoms trapped at the bottom but not attached to the tube end. The final metal distribution along z direction also indicates this phase separation as there are mostly red (Cu) on the top and blue (Ni) at the bottom (Fig. 3.31e).
Figure 3.31 (a-c) Snapshots of the DFT-MD simulation of a (9,1) tube on Cu15Ni15 alloy particle at 1800 K. Blue and red spheres represent Ni and Cu atoms. Grey and white sticks denote C and H atoms. (d) Initial and (e) final metal atom distribution along z direction. Blue and red represent Ni and Cu, respectively. (f) Initial and final configurations of the 20 ps DFT-MD simulation of a (9,1) tube on Fe15Ni15 alloy particle at 1800 K. Blue and purple spheres represent Ni and Fe atoms, respectively. Grey and white sticks denote C and H atoms. (g) Number of C-Ni (blue) and C-Fe (purple) bonds during the MD simulation. Reproduced from Ref. 216. Copyright@2019, American Physical Society.
Fe-Ni alloy catalyst is also commonly seen as Cu-Ni alloy while the activity difference of the two components is not very large. The snapshots of the MD simulation of Fe15Ni15 alloy on (9,1) tube show not very obvious phase separation between Fe and Ni atoms. But the number changes of C-Ni and C-Fe bonds during the simulation still provide us the evidence that there is still a slight phase separation happening within very short simulation time. So phase separation for
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CNT growth on alloy catalysts with relatively similar activities still exists but might take longer time which we cannot afford.
Figure 3.32 (a) Snapshots of the DFT-MD simulation of a (9,1) tube on Au10Ni20 alloy particle at 1800 K at 1, and 20.8 ps, respectively. Blue and yellow spheres represent Ni and Au atoms. Grey and white sticks denote C and H atoms. (b) Initial and (c) final metal atom distribution along z direction. Blue and yellow represent Ni and Au, respectively. (d) Energy evolution during the 20.799 ps MD simulation in (a). Grey line shows the original energy fluctuation; black line is the smoothed line; red dashed line highlights the tendency of the energy evolution. (e) Number of C-Ni (blue) and C-Au (yellow) bonds during the MD simulation in (a). (f) Energy evolution during the 13.556 ps MD simulation of a (9,1) tube on Cu20Ni10 alloy particle at 1800 K with the configurations at 0, 5, and 13 ps inserted. 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-Cu (red) bonds during the MD simulation. Reproduced from Ref. 216. Copyright@2019, American Physical Society.
Except for different types of alloy catalysts, we also performed MD simulations of CNTs on alloy catalysts of different composition ratios. Fig. 3.32a-e presents the simulation result of a
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(9,1) tube on Au10Ni20 alloy catalyst, which is with higher active component concentration.
After 20.8 ps simulation, most active metal (Ni) atoms accumulated to the part of the catalyst that connected with the tube (Fig. 3.32a and c). Number of C-Au bonds immediately dropped to 0 while number of C-Ni bonds reached around 12 (Fig. 3.32e).
For alloy catalysts with higher less-active component concentration, it is still the same. In the MD simulation of a (9,1) tube on Cu20Ni10 alloy catalyst, though only 10 Ni atoms could not cover the whole tube end up, most of the active Ni atoms moved to the tube edge and bonded with the dangling C at the rim (Fig. 3.32f). Therefore, even when there were less Ni atoms, there were more C-Ni bonds than C-Cu bonds (Fig. 3.32g).