While the behavior of water molecules confined in a nanospace is conjectured to have significant consequences in nanotechnology and nanobiotechnology, very little has been known about it.

Dr. Hiromichi Kataura, Senior Research Scientist of the Nanotechnology Research Institute (NRI), the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, and Dr. Yutaka Maniwa, Associate Professor, Graduate School of Science, Tokyo Metropolitan University (TMU), concurrently a guest researcher of the AIST and a scientist under CREST Program of the Japan Science and Technology Agency (JST), another independent administrative institution, with colleagues have identified the structure of water adsorbed in a single-walled carbon nanotube (SWCNT) through the X-ray diffraction experiment at the Photon Factory (PF) of the High Energy Accelerator Research Organization (KEK), an inter-university research institute corporation. It has been found that water within SWCNT forms tubular ice, named ice nanotube (Ice-NT) at lower temperatures, and its melting point varies depending upon the diameter of SWCNT, or Ice-NT. In particular, Ice-NT growing in SWCNT of diameter 1.17nm (1nm = 1/1,000,000,000m) has proved to be a tubular structure consisting of ring structures of 5 water molecules stacked together, with melting point 27°C. Up to now, ice was available only at room temperature under extremely high pressure, as high as 10,000 atm. However, the existence of ice at room temperature under subatmospheric pressure has been observed first in the world, providing new information on the behavior of water molecules confined in a nanospace. Moreover, in contrast to the existing empirical rule that the melting point of ice in fine glass tubing is lowered as the tube diameter is reduced, it has been found that the smaller the SWCNT diameter is, the higher the melting point of ice rises. While this effect is supposed to be closely related to the stability of ring cluster of water molecules, it may be expected that the behavior of water molecules will be fully understood through the further experiment in details.

When the temperature is raised to 45°C under subatmospheric pressure, water within SWCNT has been found to evaporate at once, to be emitted out of SWCNT. The nano-jet mechanism will find its application in the new generation ink jet and others.

The finding will be published in the journal, Chemical Physics Letters 401 (2005) pp. 533-537  under a title "Ordered water inside carbon nanotubes: Formation of pentagonal to octagonal ice-nanotubes"..

The research work is partly supported by a contract project "Development of Carbon Nanotube Materials for Non-Linear Optical Devices" under the FY03 Industrial Technology Research Grant Program of the New Energy and Industrial Technology Development Organization (NEDO), another independent administrative organization.

Figure  A model of room temperature Ice-NT grown in SWCNT, forming a tube by stacking 5-water molecule rings: Red, blue and gray spheres represent oxygen (O), hydrogen (H) and carbon (C) atoms, respectively.

The Earth is dubbed as "Water Planet", which is pervaded with water everywhere, and any object placed in the atmosphere is covered with numerous water molecules in adsorbed state. Even in a clean room designed dust-free contains a lot of water molecules, and the surface of a device fabricated in nanometer scale is covered with water molecules. Any reactions at molecular levels within a living cell also occur in an environment filled with water molecules. The behavior of water molecules confined in a nano-space is one of problems of great significance to be clarified immediately. However, as the means to study the behavior of water in extremely small space are limited, very little data have been available so far, and most of studies have based on the computer simulation. For instance, it has been known that melting point of ice within a fine tube of glass or other materials is lowered as the tube diameter is reduced, but no data are available for the case of nanometer scale. SWCNT holds a space of nanometer scale within which can accept certain molecules. Water molecules confined in this space are constrained in cluster formation. Under the aforesaid circumstances, the behavior study is of great consequence.

The formation of Ice-NT within a SWCNT under high pressure conditions was predicted in 2001 by Dr. Ken’ichiro Kouga, Associate Professor, Department of Education, Fukuoka University (currently, Associate Professor, Department of Chemistry, Science Faculty, Okayama University) and Dr. Hideki Tanaka, Professor, Department of Chemistry, Science Faculty, Okayama University through the computer simulation. The first experimental evidence for Ice-NT was obtained in 2002 by Dr. Yutaka Maniwa, Associate Professor of TMU-JST, and Dr. Hiromichi Kataura, Assistant Professor of TMU (the latter is currently a senior research scientist at NRI-AIST) through the precision X-ray structural analysis at temperatures below -38°C and subatmospheric pressure. Meanwhile, the NRI-AIST has been engaged in the development of technologies for precision diameter control and molecular inclusion with SWCNT to be used for non-linear optical devices under the support from the Industrial Technology Research Grant Program of the NEDO since fiscal 2003. On the basis of present results, 6 specimens of high purity SWCNT of different diameters have been prepared to examine in detail  how the creation and structure of Ice-NT are affected by diameter of SWCNT.

Since a water molecule is composed of two light elements, hydrogen and oxygen, it is very difficult to directly observe water molecules in nanospace by the electron microscopy, and observation measures are limited. The experiment at the TMU-JST, however, revealed the possibility of analyzing configuration of water molecules within SWCNT through the high accuracy X-ray diffraction analysis with synchrotron orbital radiation (SOR), if a film specimen made of bundles or ropes of high purity SWCNTs with controlled diameter were used. The combination of SWCNT synthesis technology by the NRI-AIST with the precision structural analysis by the TMU-JST has achieved the success in systematic experimental study on the long-range ordering process of water within SWCNT.

The X-ray diffraction measurement was carried out at the experimental station BL1B in the Photon Factory (PF), Institute of Materials Structure Science at a temperature range from 90 K (-183°C) to 360 K (87°C). Six kinds of high purity SWCNT specimen with mean diameter from 1.17nm to 1.44nm were prepared, bored to make hollow tubes and enclosed in glass tubing with saturated water vapor for the X-ray diffraction experiment. When the specimen was heated from the room temperature to around 45°C, rapid vaporization of water enclosed in SWCNT was observed. The vaporizing temperature slightly varies depending upon the SWCNT diameter. While the overall vaporization of water enclosed in SWCNT may be assessed through the thermogravimetry, the present study is selectively targeted to water molecules enclosed in SWCNT.

Then, on lowering the temperature, new peaks appeared in the X-ray diffraction pattern to indicate regular alignment of water molecules (Fig. 1). The detailed analysis of X-ray diffraction pattern suggested that water within SWCNT was organized into Ice-NT consisting of ring-like structural units made of water molecules stacked together to form a tube. The structure of Ice-NT varies depending upon the SWCNT diameter: changing stepwise from 8-membered ring to 5-membered one as the diameter is reduced. At the same time, the solidifying temperature rose from 190K (-83°C) to 300K (27°C). See Figs. 1 and 2. As stated before, it is generally recognized that for water enclosed in fine glass tubing, the smaller the tube diameter is, the lower the melting point falls, and it is empirically understandable the melting point of 8-membered ring Ice-NT is -83°C, far lower than that of ordinary ice. What is problematic is an opposite trend of melting point rising as the diameter becomes thinner from 8-membered to 5-membered ring. Particularly, the melting point of 5-membered ring Ice-NT as high as 27°C under subatmospheric pressure, has not yet been observed, indicating a peculiar effect of water molecules confined in nanospace. The hitherto undescribed behavior of water molecules may be closely linked to the stability of ring-like water clusters, and the further detailed experiments is expected to lead to understanding of the mysterious behavior of water molecules.

Fig. 1. Temperature changes of X-ray diffraction peak intensity to represent regular arrangement of water molecules, suggesting the presence of four kinds of ice structures with different melting points (indicated by arrows). In case of SWCNT of 1.17nm diameter, the intensity rises at 300K to indicate that the melting point of Ice-NT in this SWCNT is 300K (27°C).

Fig. 2. Four models for ring-shaped Ice-NT from left to right: 5-membered ring (d = 0.476nm, MP = 27°C), 6-membered ring (d = 0.56nm, MP = 7°C), 7-membered ring (d = 0.645nm, MP = -53°C) and 8-membered ring (d = 0.732nm, MP = -83°C). [d: diameter, MP: melting point, red sphere: oxygen atom, blue sphere: hydrogen atom]

When SWCT is heated up, rapid vaporization occurred at around 45°C to emit a jet of water molecules, which may find its way of application as nano-jet. Since the heat capacity of SWCNY is so small that it can be heated up by illumination or other means. As SWCNTs of different structures absorb light of different wavelength, it is possible to heat up SWCNT of particular structure selectively by illuminating it with monochromatic laser light of specific wavelength. On the basis of this principle, it will be possible to make control in nanometer scale without focusing light to a single SWCNT. This mechanism may be applicable to nanometer ink jet or for driving a nanometer volume of matter.

Fig. 3. A schematic diagram of nano-jet created by illuminating a bundle of SWCNT with a monochromatic laser beam. Though the difference in wavelength of light absorbed depending upon the structural difference of SWCNT occurs in the region of invisible infrared radiation, laser beams are shown in different colors in this diagram. A jet of water molecules is emitted only from a SWCNT absorbing the illuminating light (purple in this illustration).