Effect of nano-confinement on the structure and properties of water clusters: An ab initio study

Abstract: An ab initio investigation on water clusters confined to armchair carbon nanotubes (CNT) with varying diameters has been performed using the density functional theory-based calculations. Different parameters have been investigated including structure, hydrogen bonding pattern and vibrational spectra of water-CNT complexes. Our results reveal that one-dimensional water chain parallel to CNT axis is formed in narrow nanotubes CNT(4,4) and CNT(5,5), whereas in CNT(6,6), zigzag structure is observed. An increase in the CNT diameter results in more symmetric structures similar to the gas phase. The vibrational analysis shows a redshift in stretching frequency of the hydrogen bond assisted O–H in CNT(6,6) due to the reduction in O—O separation whereas a significant blue shift in stretching frequency mode is observed in highly confined CNT(4,4) and CNT(5,5). It implies that the hydrogen bond strength between water molecules is strongest in CNT(6,6). It is also observed that water cluster tends to be near CNT wall due to H···π interaction between water molecule and the π-electron cloud of CNT. An inverse relation between the electronic charge transfer (from CNT to water) and the diameter is also established. This study demonstrates that the degree of confinement is extremely important in deciding the properties of confined water molecules. Graphic abstract: The structure and hydrogen bonding properties of the water clusters under nano-confined regime is investigated and the results reveal that the intramolecular charge separation for each water molecule increases under confinement and irrespective of the number of water molecules, the highest tube-water interaction energy is achieved in CNT(5,5) (diameter~7 Å). The important finding is that the degree of confinement (diameter of carbon nanotube) is extremely important in deciding the properties of confined water molecules.[Figure not available: see fulltext.]