Displacement of $\mathrm{C}{\mathrm{O}}_2$ by Xe in single-walled carbon nanotube bundles

The displacement of $\mathrm{C}{\mathrm{O}}_2$ by Xe on single-walled nanotube bundles is investigated with Fourier transform infrared spectroscopy (FTIR) and grand canonical Monte Carlo (GCMC) simulations. The FTIR experiments show that $\mathrm{C}{\mathrm{O}}_2$ physisorption at $77\mathrm{K}$ produces an infrared peak at $2330{\mathrm{cm}}^{-1}$ for endohedral physisorption and at $2340{\mathrm{cm}}^{-1}$ for groove/external surface physisorption. Exposure to Xe causes a sequential displacement of $\mathrm{C}{\mathrm{O}}_2$ from these sites as shown by an intensity loss of the $2330{\mathrm{cm}}^{-1}$ peak, which precedes the loss at $2340{\mathrm{cm}}^{-1}$. The GCMC simulations on heterogeneous and homogenous bundles show that $\mathrm{C}{\mathrm{O}}_2$ in endohedral sites is initially displaced by Xe before that in groove/external surface sites. The $\mathrm{C}{\mathrm{O}}_2$ populations in each site of the bundle are taken from the GCMC simulations and used to model the variation of the FTIR intensities as a function of Xe pressure. The qualitative agreement between the simulated and experimental intensity changes is good, suggesting that the intensity changes seen in the experiments are related to $\mathrm{C}{\mathrm{O}}_2$ displacement from the sites indicated in the simulations.

Figure 1

Infrared spectra of $\mathrm{C}{\mathrm{O}}_2$ physisorbed on the SWNTs at $77\mathrm{K}$ after degassing at $373\mathrm{K}$ for $2\mathrm{h}$. A single peak is seen near $2340{\mathrm{cm}}^{-1}$ from the ${\nu }_3$ mode of $\mathrm{C}{\mathrm{O}}_2$, which gains intensity with pressure. The inset shows the integrated intensity of the IR spectra from the main figure. The solid line in the inset is only meant as a guide to the eye. The inset shows that the integrated intensity of the IR peak starts to saturate at around $4\times {10}^{-7}\text{Torr}$, indicating a filling of sites associated with the $2340{\mathrm{cm}}^{-1}$ peak.

Figure 2

Infrared spectra of $\mathrm{C}{\mathrm{O}}_2$ physisorbed on the SWNTs at $77\mathrm{K}$ after vacuum heating to $700\mathrm{K}$. Peaks at 2330 and $2340{\mathrm{cm}}^{-1}$ are seen before any gas phase $\mathrm{C}{\mathrm{O}}_2$ is introduced to the cell. These peaks are due to the physical entrapment of $\mathrm{C}{\mathrm{O}}_2$ in the bundle during the vacuum heating step. The inset shows the integrated intensity of $2330{\mathrm{cm}}^{-1}$ (solid circles), $2340{\mathrm{cm}}^{-1}$ (open squares), and their sum (solid triangles) as determined by deconvoluting the spectra from the main figure with a two Lorenztian fit.

Figure 3

GCMC simulation results for the coverage of $\mathrm{C}{\mathrm{O}}_2$ molecules in the endohedral (dotted lines and circles), interstitial (dashed lines and squares), and groove/external surface sites (solid lines and triangles) as a function of increasing Xe partial pressure. The $\mathrm{C}{\mathrm{O}}_2$ pressure is $7.0\times {10}^{-7}\text{Torr}$: (top) on heterogeneous bundle 1; (middle) on heterogeneous bundle 2; (bottom) on homogeneous bundle 3. The tubes making up these bundles are described in Table and their cross-sections are illustrated in the insets.

Figure 4

Representative IR experiments of Xe displacement at $77\mathrm{K}$ on a SWNT sample after vacuum heating to $700\mathrm{K}$. The sample cell is initially charged with $4.2\times {10}^{-7}\text{Torr}$ before introducing Xe. Xe exposure causes an initial loss of intensity at $2330{\mathrm{cm}}^{-1}$ with loss of intensity at $2340{\mathrm{cm}}^{-1}$ at higher pressures. For clarity only three pressures are shown from the experiment.

Figure 5

(a) Experimental difference IR spectra for the displacement of $\mathrm{C}{\mathrm{O}}_2$ by Xe from the SWNT sample with an initial $\mathrm{C}{\mathrm{O}}_2$ pressure of $7.0\times {10}^{-7}\text{Torr}$. The sample has been degassed at $373\mathrm{K}$ and has most of its endohedral and interstitial sites blocked. (b) Simulated IR difference spectra derived from the GCMC results on a fully open heterogeneous bundle (bundle 1 of Table ) by scaling the endohedral and interstitial populations by 0.05 to mimic a bundle with only a small fraction of these sites available for adsorption and displacement. The initial $\mathrm{C}{\mathrm{O}}_2$ pressure in the simulation is $7.0\times {10}^{-7}\text{Torr}$.

Figure 6

(a) Experimental difference IR spectra for the displacement of $\mathrm{C}{\mathrm{O}}_2$ by Xe after vacuum heating the sample to $700\mathrm{K}$. The experiment was run at $77\mathrm{K}$ with an initial $\mathrm{C}{\mathrm{O}}_2$ pressure of $4.0\times {10}^{-7}\text{Torr}$. (b) Simulated IR difference spectra from GCMC results on a fully open heterogenous bundle (bundle 1 of Table ), which was used to mimic the vacuum heated sample used in the experiments shown in (a). The starting $\mathrm{C}{\mathrm{O}}_2$ pressure in the simulation is $7.00\times {10}^{-7}\text{Torr}$.