Influence of Mo on the Fe:Mo:C nanocatalyst thermodynamics for single-walled carbon nanotube growth

We explore the role of Mo in Fe:Mo nanocatalyst thermodynamics for low-temperature chemical-vapor deposition growth of single-walled carbon nanotubes (SWCNTs). By using the size-pressure approximation and ab initio modeling, we prove that for both Fe-rich ($\sim 80\%$ Fe or more) and Mo-rich ($\sim 50\%$ Mo or more) Fe:Mo clusters, the presence of carbon in the cluster causes nucleation of ${\text{Mo}}_2\text{C}$. This enhances the activity of the particle since it releases Fe, which is initially bound in a stable Fe:Mo phase, so that it can catalyze SWCNT growth. Furthermore, the presence of small concentrations of Mo reduces the lower size limit of low-temperature steady-state growth from $\sim 0.58\text{nm}$ for pure Fe particles to $\sim 0.52\text{nm}$. Our ab initio-thermodynamic modeling explains experimental results and establishes a direction to search for better catalysts.

Figure 1

(Color online) Ternary phase diagram for Fe-Mo-C nanoparticles of $R\sim \infty ,1.23,0.62,0.41\text{nm}$. Notations are explained in the text.

Figure 2

(Color online) Panel (a) Fraction of species for catalyst composition ${\left[{\text{Fe}}_{4/5}{\text{Mo}}_{1/5}\right]}_{1-x}{\text{-C}}_x$ and nanocatalyst of $R\gtrsim 0.62\text{nm}$. The dashed vertical line, labeled as “1,” represents ${\left[{\text{Fe}}_{4/5}{\text{Mo}}_{1/5}\right]}_{1-x}{\text{-C}}_x$ crossing the boundary phase $\text{Fe}\leftrightarrow {\text{Mo}}_2\text{C}$, as shown in Fig. 1c. Panel (b) Fraction of species for catalyst composition ${\left[{\text{Fe}}_{4/5}{\text{Mo}}_{1/5}\right]}_{1-x}{\text{-C}}_x$ and nanocatalyst of $R\lesssim 0.41\text{nm}$. The dashed vertical lines, labeled as “4” and “5,” represent ${\left[{\text{Fe}}_{4/5}{\text{Mo}}_{1/5}\right]}_{1-x}{\text{-C}}_x$ crossing the boundary phases $\text{Fe}\leftrightarrow {\text{Mo}}_2\text{C}$ and ${\tau }_3\leftrightarrow {\text{Mo}}_2\text{C}$, as shown in Fig. 1d.

Figure 3

(Color online) Panel (a) Fraction of species for catalyst composition ${\left[{\text{Fe}}_{1/2}{\text{Mo}}_{1/2}\right]}_{1-x}{\text{-C}}_x$ and nanocatalyst of $R\gtrsim 0.62\text{nm}$. The dashed vertical lines, labeled as “2” and “3,” represent ${\left[{\text{Fe}}_{1/2}{\text{Mo}}_{1/2}\right]}_{1-x}{\text{-C}}_x$ crossing the boundary phases ${\text{Fe}}_2\text{Mo}\leftrightarrow {\text{Mo}}_2\text{C}$ and $\text{Fe}\leftrightarrow {\text{Mo}}_2\text{C}$, as shown in Fig. 1c. Panel (b) Fraction of species for catalyst composition ${\left[{\text{Fe}}_{1/2}{\text{Mo}}_{1/2}\right]}_{1-x}{\text{-C}}_x$ and nanocatalyst of $R\lesssim 0.41\text{nm}$. The dashed vertical lines, labeled as “6,” “7” and “8,” represent ${\left[{\text{Fe}}_{1/2}{\text{Mo}}_{1/2}\right]}_{1-x}{\text{-C}}_x$ crossing the boundary phases ${\text{Fe}}_2\text{Mo}\leftrightarrow {\text{Mo}}_2\text{C}$, $\text{Fe}\leftrightarrow {\text{Mo}}_2\text{C}$, and ${\tau }_3\leftrightarrow {\text{Mo}}_2\text{C}$, as shown in Fig. 1d.