Phases of ethane adsorbed on purified HiPco single-walled carbon nanotubes

We have used adsorption isotherms to study ethane films on bundles of purified HiPco single-walled carbon nanotubes (SWNTs). We infer the existence of a possible phase transition in this system from a sudden change in the temperature dependence of the coverage interval corresponding to a feature present in the isotherms. The possible phase transition occurs near monolayer completion, at approximately $110\mathrm{K}$. We also measured isotherms at low fractional coverages and relatively high temperatures $(220–240\mathrm{K})$ in order to determine the binding energy of ethane on the SWNT bundles. We obtained a value of $308\mathrm{meV}$ for this quantity; this is 1.76 times larger than the binding energy of ethane on graphite. We used isotherm results obtained at all measured temperatures and coverages to determine the coverage dependence of the isosteric heat of adsorption for this system.

Figure 1

(Color online) Isotherms for ethane adsorption on single-walled nanotubes at 103, 105, 107, 110, 131, 150, 160, and $170\mathrm{K}$ (isotherm temperatures increase from left to right). The coverage, on the $Y$ axis in ${\mathrm{cm}}^3\text{Torr}$ ($1{\mathrm{cm}}^3\text{Torr}$ at $273\mathrm{K}$ is equal to $3.54\times {10}^{16}\text{molecules}$), is presented as a function of natural logarithm of pressure in Torr ($X$ axis).

Figure 2

(Color online) Full monolayer isotherm at $160\mathrm{K}$. The coverage in ${\mathrm{cm}}^3\text{Torr}$ ($Y$ axis) is presented as a function of the pressure in Torr ($X$ axis). After a low-pressure region of steep coverage increase, there is region where the coverage increases linearly with pressure; the lowest-pressure value where the isotherm deviates from linearity marks point B, as indicated by the segment parallel to the $X$ axis in the figure. This point is taken to coincide with monolayer completion.

Figure 3

(Color online) Isotherms for ethane adsorption on single-walled nanotubes at 103, 105, 107, and $110\mathrm{K}$. The coverage in ${\mathrm{cm}}^3\text{Torr}$ ($Y$ axis) is presented as a function of natural logarithm of pressure in Torr ($X$ axis). The height of the second substep at $110\mathrm{K}$ (${\mathrm{h}}_1$ in the figure) is compared with the height of the substep for 103, 105, and $107\mathrm{K}$ (${\mathrm{h}}_2$ in the figure).

Figure 4

(Color online) Low-coverage isotherms for ethane adsorption on single-walled nanotubes at 220, 230, and $240\mathrm{K}$. The coverage in ${\mathrm{cm}}^3\text{Torr}$ ($Y$ axis) is presented as a function of natural logarithm of pressure in Torr ($X$ axis).

Figure 5

(Color online) Coverage dependence of the isosteric heat of adsorption on monolayer fractional coverage. The isosteric heat in meV ($Y$ axis) is presented as a function of monolayer fractional coverage in layers ($X$ axis).