Ab initio study of ${\mathrm{H}}_2\mathrm{O}$ and water-chain-induced properties of carbon nanotubes

We perform an ab initio study of the motion of the nano sized water dimer through a single-walled carbon nanotube (SWCNT), the stability of an encapsulated one-dimensional (1D) water chain inside SWCNT, and the ${\mathrm{H}}_2\mathrm{O}$-induced structural, energetic, electronic, and optical properties of the SWCNTs. The adsorption of the water molecules is caused by the dispersion forces, i.e., the van der Waals (vdW) interactions. Thus, the role of the vdW interactions in the estimation of the BE for the weakly bound adsorbates cannot be ignored as has been done in several earlier publications. We find that a single ${\mathrm{H}}_2\mathrm{O}$ molecule or single water dimer or a 1D chain of water dimers is trapped inside the medium-sized (6,6) carbon nanotube placed in vacuum. However, the ${\mathrm{H}}_2\mathrm{O}$ molecule or water dimer may be transmitted in case the tube is surrounded by water or water vapor at high vapor pressure at high temperatures. On the other hand, a chain of single ${\mathrm{H}}_2\mathrm{O}$ molecules or more number of the encapsulated ${\mathrm{H}}_2\mathrm{O}$ molecules is very weakly coupled to the wide (10,10) carbon nanotube and can, thus, easily transmit through the carbon nanotube in agreement with the recent experiments. Further, appreciable adsorption both inside and on the surface of the (10,10) carbon nanotube is predicted in concurrence with the experiments. The small (medium-sized) diameter tubes will adsorb strongly (accommodate) the water molecules outside (inside) the nanotubes. The ${\mathrm{H}}_2\mathrm{O}$ adsorption converts the conducting small-diameter zigzag (5,0) tube into a semiconductor. Further, the adsorption reduces the band gap of the semiconducting achiral zigzag (10,0) nanotube but increases the band gap of a chiral semiconducting (4,2) tube. The adsorbed ${\mathrm{H}}_2\mathrm{O}$ molecules increase the electrical conductivity in agreement with the experiment. The overall peak structure in the optical absorption for the pristine tube is not altered significantly by the adsorption except for small alterations in the energy locations and the relative intensities of the peaks in the achiral tubes.

Figure 1

(Color online) Actually calculated minimum-energy atomic configuration of a part of the optimized (6,6) carbon nanotube encapsulating water dimers, each dimer containing $2\text{−}{\mathrm{H}}_2\mathrm{O}$ molecules. The shaded circles denote the C atoms of the host (6, 6) carbon nanotube, the dark (red) circles represent the O atoms whereas the small open circles show the H atoms.

Figure 2

Variation of the total system energy with the location of the ${\mathrm{H}}_2\mathrm{O}$ molecule and the $2\text{−}{\mathrm{H}}_2\mathrm{O}$ water dimer lying on the axis of the (6,6) carbon nanotube of length of $6.2\mathrm{\AA }$ either outside or inside the tube.

Figure 3

Electronic structure and the density of electron states (DOS) in the vicinity of the Fermi level for the (5,0) nanotube. (a) Pristine and (b) Doped with $1\text{−}{\mathrm{H}}_2\mathrm{O}$ molecule on surface midhexagonal site. For the pristine tube, the top most filled and the lowest unfilled states have been marked as (38 to 40) and (41 to 45) near the center of the BZ. The numbering is different at various values of $k_z$. Some typical optical transitions have also been demonstrated.

Figure 4

Same as Fig. 3 but for the (10,10) nanotube. (a) Pristine tube. The doubly degenerate top most filled and the lowest unfilled states have been marked as (78, 79) and (82, 83) near the boundary of the BZ. Some typical optical transitions have also been drawn. (b) $1\text{−}{\mathrm{H}}_2\mathrm{O}$ molecule on surface midhexagonal site.

Figure 5

The excess electronic charge density of the $4\text{−}{\mathrm{H}}_2\mathrm{O}$ endohedrally doped (10,10) nanotube over that of the pristine tube in one $C$ plane. The charge contours are shown in the range of $0.0–0.75e∕{\left(\mathrm{a.u.}\right)}^3$ with an interval of $0.05e∕{\left(\mathrm{a.u.}\right)}^3$.

Figure 6

(Color online) Optical-absorption spectra for the pristine and the ${\mathrm{H}}_2\mathrm{O}$ molecule doped (a) (3,3), (b) (5,0), (c) (4,2), and (d) (6,6) carbon nanotubes in the energy range of $0–4.0\mathrm{eV}$, respectively.

Figure 7

Same as Fig. 6 but for the (a) pristine (10,0) nanotube and $1\text{−}{\mathrm{H}}_2\mathrm{O}$ molecule on surface site, (b) $1\text{−}{\mathrm{H}}_2\mathrm{O}$ molecule inside the (10, 0) tube, (c) pristine (10,10) nanotube and $1\text{−}{\mathrm{H}}_2\mathrm{O}$ molecule on surface site, and (d) $4\text{−}{\mathrm{H}}_2\mathrm{O}$ molecules inside the (10, 10) tube.