Optically polarized ${}^{129}\text{X}\text{e}$ NMR investigation of carbon nanotubes

We demonstrate the utility of optically polarized ${}^{129}\text{X}\text{e}$ NMR in a convection cell for measuring the surface properties of materials. In particular, we show adsorption of xenon gas on oxidatively purified single- and multiwalled carbon nanotubes. The interaction between xenon and multiwalled nanotubes produced by chemical vapor deposition was stronger than that of single- or multiwalled nanotubes produced by carbon arc discharge. Xenon was observed in gas, liquid, and adsorbed phases. The large polarization and moderate pressures of xenon $\left(\sim 0.2\text{MPa}\right)$ allowed resolution of multiple lines in both the gas and condensed phases of xenon in contact with carbon nanotubes. Xe gas exchanges with physisorbed xenon in two different environments. Xe adsorbs preferentially on defects, but if the number of defects is not sufficient, it will also adsorb on surface and interstitial sites. Penetration of Xe in the tube interior was not observed.

Figure 1

Schematic drawing of the convection cell. The sample is located in the retort on the lower left, surrounded by the NMR coil. The cell is placed in a two-chambered housing, divided at the dotted line. The right half of the housing is maintained at 373 K; the left half of the housing is temperature controlled to the desired temperature between 163 and 273 K. The dashed lines on the gas retort show the alternate location for the NMR coil. Adapted with permission from Su et al. (Ref. 15)

Figure 2

TEM micrographs of the three samples at two magnifications. (a) MRSW: The dark particles are Ni- and Co-catalyst materials (left), and the tubes are bundled (right). (b) MRGC: At the lower magnification, the nanotubes are straight lines while the polyhedrons are nano-onions (left). Also notice the closed ends and bundling of the nanotubes (right). (c) MRMWC: Severe damage by the acid treatment opens the tubes (left) and introduces damage to the tube walls (right).

Figure 3

${}^{129}\text{X}\text{e}$ spectra of xenon in convection cells in close contact with carbon nanotubes at three temperatures: 273, 173, and 163 K. The bottom spectrum for each sample is the spectrum for the gas at 273 K recorded with the alternate NMR coil (see Fig. 1). Signal intensities have been scaled to account for the different number of scans used at each temperature, resulting in different noise levels. The number of scans for each spectrum is listed to the right. For ease of view, the gas signal has been reduced by the factor shown. The relaxation delay was based on the time for the convection cycle, 1 s. (a) MRSW, (b) MRGC, (c) MRMWC, and (d) MRGC adsorbed phase. Notice that the MRGC sample is the only one to show adsorbed peaks, $\sim 260\text{ppm}$, due to the low density of paramagnetic defect sites.

Figure 4

Ratio for xenon gas signal intensity in contact with CNTs over the signal intensity of the free gas for each of the samples at three different temperatures. MRSW is shown as circles, MRGC as squares, and MRMWC as triangles. The signal intensity ratio is a function of the strength of interaction between xenon and the nanotubes. MRMWC with the lowest signal intensity ratio has the strongest interaction at all temperatures.

Figure 5

${}^{129}\text{X}\text{e}$ NMR spectra of the adsorbed phase of MRGC at (a) 173 K and (b) 163 K. At 173 K the spectrum can be decomposed into 1 Lorentzian (1) and 2 Gaussian lines (2 and 3), while at 163 K, the spectrum is best fit with 2 Lorentzian (1 and 4) and 2 Gaussian lines (2 and 3). The experimental spectrum is shown at the top, the component lines on the bottom, and the sum of the components is the dashed line in the center.

Figure 6

${}^{129}\text{X}\text{e}$ NMR spectra of the gas phase for each of the samples at 273 K. Each spectrum can be decomposed into three lines: 1 Lorentzian (1) and 2 Gaussian (2 and 3). The experimental spectrum is shown at the top, the component lines on the bottom, and the sum of the components is the dashed line in the center. (a) MRSW. (b) MRGC. (c) MRMWC.

Figure 7

Schematic representation of the adsorbed phases and interparticle gas for (a) MRSW, (b) MRGC, and (c) MRMWC. The small filled circles are ${}^{129}\text{X}\text{e}$. Line 1 in the spectra is assigned to interparticle gas, line 2 to gas on the surface of the carbon nanotubes, and line 3 to the more confined interstitial sites, defect sites, or interiors of multiwalled nanotubes (c).

Figure 8

Plot of linewidth vs temperature for MRMWC (filled symbols) and MRGC (open symbols). Gas phase line 1, diamonds, Gaussian line 2, squares, and Gaussian line 3, circles. The width of the line of gas not in contact with carbon nanotubes is denoted by plusses for MRMWC and crosses for MRGC. Notice that lines 1 and 2 are constant in width within error. The linewidth of line 3 increases with decreasing temperature to the onset of adsorption (213 K for MRMWC, 173 K for MRGC) then begins to narrow.