Water dynamics inside single-wall carbon nanotubes: NMR observations

The dynamics of water (${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$) molecules adsorbed inside single-wall carbon nanotubes (SWCNTs) of average diameter $13.5\mathrm{\AA }$ were investigated by means of ${}^2\mathrm{H}$- and ${}^1\mathrm{H}$-NMR between 100 and $300\mathrm{K}$. Above $220\mathrm{K}$, the NMR spectra were substantially narrowed, and indicated that the water is in a liquidlike state with translational and quasifree rotational motions. Below $220\mathrm{K}$, where water exhibits long range order inside SWCNTs, the large amplitude molecular motions start to freeze within a time scale of ${10}^{-6}\mathrm{s}$, while below around $120\mathrm{K}$, almost all of the protons become fixed around each atomic site. The results support the dynamic properties and ice-nanotube transition predicted by previous molecular dynamics calculations.

Figure 1

(Color online) Schematic illustration of the heptagonal ice-NTs with energetically equivalent proton arrangements. Larger spheres represent oxygen atoms and smaller spheres represent hydrogen atoms.

Figure 2

(Color online) $T$-evolution of ${}^2\mathrm{H}$ NMR spectrum for ${\mathrm{D}}_2\mathrm{O}$ confined inside SWCNTs. The dotted and dashed lines denote the single-peak and double-peaked components, respectively. The double-peaked components were obtained by subtracting the Lorentzian components from the observed ${}^2\mathrm{H}$ NMR sprctra. A calculated line shape with ${\overline{\nu }}_Q=6.8\mathrm{kHz}$ and $\eta =0$ is also displayed in the figure (bottom). The line shape is convoluted with a Lorentzian function with FWHM of $2.1\mathrm{kHz}$.

Figure 3

(a) $T$-dependence of the ${}^2\mathrm{H}$ NMR intensity and $T,I_S\times T$ (●) product. The $I_S\times T$ for the single Lorentzian shape (▿) and double-peaked structural (▵) components are also plotted in the figure. In the inset, the values of ${\overline{\nu }}_Q$ obtained from the double-peaked pattern are plotted. (b) $T$-dependence of the frozen water fraction estimated from ${}^1\mathrm{H}$ NMR (엯) and ${}^2\mathrm{H}$ NMR (●) intensities. (c) $T$-dependences of the XRD Bragg peak intensity arising from the ordered structures, heptagonal (▿) and octagonal (▵) ice-NTs. For comparison with the NMR results, the sum of the Bragg peak intensities (엯) is also plotted in the figure.

Figure 4

(Color online) $T$-evolution of ${}^1\mathrm{H}$ NMR spectrum for ${\mathrm{H}}_2\mathrm{O}$ confined inside SWCNTs. The upper scale indicates the shift $\left(\delta \right)$ measured relative to bulk water. The right-hand side is described as the downfield region. The dotted and dashed lines denote the Lorentzian and Gaussian components, respectively.