Energy Barriers at the Ends of Carbon Nanotube Bundles: Effects on Interstitial Adsorption Kinetics

We investigate the presence of adsorption sites and energy barriers near the ends of carbon nanotube bundles to determine their consequences on gas adsorption in the interstitial channels between the tubes. In the case of ${\mathrm{H}}_2$ molecules adsorbing in bundles of closed (10,10) tubes, a high binding energy site followed by a potential barrier lie right before the entrance to the interstitial channels, producing a dramatic slowing down of interstitial adsorption kinetics.

Figure 1

Potential energy (in K) of a ${\mathrm{H}}_2$ molecule near the end of a bundle with three closed tubes. The origin of the $y$ axis lies at the center of the interstitial channel. The $z$ axis coincides with the axis of the channel and its origin indicates the entrance to the channel. Positive values of $z$ indicate positions outside the IC. See Fig. 2 for a more detailed picture of the region near the IC.

Figure 2

Ground state of a ${\mathrm{H}}_2$ molecule at the end of the bundle. Left panel: Potential energy (same geometry used in Fig. 1). Right panel: Probability density. Note that the probability maximum is shifted from the minimum of the potential energy to take advantage of the less confined environment.

Figure 3

Sensitivity of the presence of the outer adsorption site to the value of ${\sigma }_{\mathrm{gc}}$ and tube radius $R$. The region enclosed by the lines corresponds to systems that exhibit both IC and outer adsorption sites. For any given ${\sigma }_{\mathrm{gc}}$, if the radius is too large, the outer adsorption site disappears while no adsorption is possible inside the IC if the radius is too small. We indicate with star symbols the values of the parameters used in this work and the ones corresponding to Ref. 28 (WSW).

Figure 4

Mean coverage $n=⟨N⟩/N_s$ of ${\mathrm{H}}_2$ as a function of time. The dashed lines correspond to the homogeneous IC chain (no entry sites) and the full lines correspond to the IC with high binding energy sites at the ends. Top panel: $T=200\mathrm{K}$; the chemical potential is $\mu /(k_{B}T)=E_{\mathrm{IC}}/(k_{B}T)+0.5$ for the two upper curves ($n_{\mathrm{eq}}\approx 0.62$) and $\mu /(k_{B}T)=E_{\mathrm{IC}}/(k_{B}T)-0.5$ for the two lower curves ($n_{\mathrm{eq}}\approx 0.38$). Bottom panel: $T=100\mathrm{K}$ and $\mu =E_{\mathrm{IC}}$ ($n_{\mathrm{eq}}\approx 0.5$).