Higher Coverage Gas Adsorption on the Surface of Carbon Nanotubes: Evidence for a Possible New Phase in the Second Layer

We present adsorption isotherm results for Ne, $\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$, and Xe on bundles of close-ended single-wall carbon nanotubes, for coverages above the completion of the first layer. We find a small, sharp, substep present in the second-layer data for Ne and $\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$, and a weaker feature, that produces an isothermal compressibility peak, for Xe. The size and location of the feature allows its tentative identification as a new, second-layer, one-dimensional phase, in which the atoms sit atop high binding energy sites in the second layer.

Figure 1

Ne adsorption isotherms at 23.59, 23.59, 26.76, and 29.34 K. The coverage in $\mathrm{c}{\mathrm{m}}^{\mathrm{3}}\mathrm{T}\mathrm{o}\mathrm{r}\mathrm{r}$ ($\mathrm{1}\mathrm{c}{\mathrm{m}}^{\mathrm{3}}\mathrm{T}\mathrm{o}\mathrm{r}\mathrm{r}=3.54\times {10}^{16}$ molecules) is plotted as a function of the logarithm of the pressure (in Torr). Monolayer completion for the lower temperatures occurs near $\mathrm{14}\mathrm{500}\mathrm{c}{\mathrm{m}}^{\mathrm{3}}\mathrm{T}\mathrm{o}\mathrm{r}\mathrm{r}$; its location is marked by the thick horizontal arrow in the figure. The sharp substeps, indicated by the slanted arrows, correspond to the second-layer groove phase.

Figure 2

(a) $\mathrm{C}{\mathrm{H}}_{\mathrm{4}}$ adsorption isotherms for coverages beyond monolayer completion. Shown, from left to right, are data at 61.90, 65.29, 69.41, 72.78, and 76.58 K. A substep, which is most clearly visible in the 61.90 K isotherm, is present for the three lowest temperatures. The axes are the same as those for Fig. 1. (b) Logarithm of the derivative of the isotherm data shown in (a), plotted as a function of coverage. The data have been vertically displaced for the sake of clarity. The curves are arranged (top to bottom) in order of increasing temperatures. Arrows mark the location of an isothermal compressibility peak for all five temperatures.

Figure 3

(a) Xe isotherms for 112.07 and 118.06 K. Monolayer completion for Xe occurs at about $\mathrm{7}\mathrm{500}\mathrm{c}{\mathrm{m}}^{\mathrm{3}}$. The data start below monolayer completion and continue into the second layer. (b) Derivative of the isotherm data of (a). The broad compressibility peak at low coverages marks the completion of the first layer on the outer sites of the peripheral tubes. The smaller peak that occurs beyond monolayer completion (the horizontal bar indicates the width of the peak) corresponds to the formation of a second-layer groove phase.