Temperature dependence of water structure in carbon nanotubes

Water vapor adsorption on porous carbons has been actively studied because of its importance in various applications and basic science. However, the unique behavior and structure of water in carbon nanospaces have still been remained unclear. Carbon nanotubes inherently have rather restricted one-dimensional nanospaces and thus, force the adsorbed water to be aligned in them. The structure of water adsorbed in those nanospaces is ice-like even at ambient temperature. In this paper, we show the water structures in two-different single wall carbon nanotubes (CNTs) with average CNT diameters of 1.1 (narrow) and 2.5 (wide) nm at 260–300 K, which were evaluated by using in situ synchrotron X-ray diffraction (XRD). The water structures in the both CNTs were nano-ice-like forms at 300 K rather than a liquid-like. The ice-like structure in the wide CNTs was developed with decreasing temperature, although even at 260 K, sharp ice peaks were not observed in the XRD pattern. On the other hand, in the narrow CNTs, the water structure was gradually transformed from an ice-like form to a liquid-like form with decreasing temperature to 260 K. The structure transformation of solid-to-liquid with decreasing temperature is due to the fact that the highly restricted nanospaces obstruct the formation of ice-like clusters and/or hydrogen bonds. The anomalous structure transformation in both CNTs could be a clue to reveal one-dimensional water behavior in CNTs.