Water-filled single-wall carbon nanotubes as molecular nanovalves

It is known that at low temperature, water inside single-wall carbon nanotubes (water–SWNTs) undergoes a structural transition to form tube-like solid structures. The resulting ice NTs are hollow cylinders with diameters comparable to those of typical gas molecules. Hence, the gas-adsorption properties of ice– and water–SWNTs are of interest. Here, we carry out the first systematic investigation into the stability of water–SWNTs in various gas atmospheres below 0.1 MPa by means of electrical resistance, X-ray diffraction, NMR measurements and molecular dynamics calculations. It is found that the resistivity of water–SWNTs exhibits a significant increase in gas atmospheres below a critical temperature Tc, at which a particular type of atmospheric gas molecule enters the SWNTs in an on–off fashion. On the basis of this phenomenon, it is proposed that water–SWNTs can be used to fabricate a new type of molecular nanovalve.