Honda Motor Co., Ltd. (Tokyo, JP) and Ushio (Ushiodenki Kabushiki Kaisha) (Tokyo, JP) discovered a method of manufacturing hydrophilic carbon nanotubes, which is capable of imparting hydrophilicity without damaging the surface of the carbon nanotubes. By irradiating carbon nanotubes with ultraviolet rays, hydrophilic functional group(s) are introduced into the surface of carbon nanotubes Hydrophilicity is imparted to the carbon nanotubes to such an extent that a contact angle of water is in the range of less than 130.degree. Ultraviolet ray is a far ultraviolet ray having a wavelength ranging from 1 to 190 nm. The irradiation with ultraviolet ray is conducted in the presence of oxygen and hydrogen or in the presence of ozone and hydrogen. The end part(s) of carbon nanotubes is opened by the irradiation with ultraviolet ray, according to inventors Toshiyuki Ohashi, Hiroshi Sugahara, and Mitsuaki Mitama in U.S. Patent 7,553,471.
The ultraviolet ray is a vacuum ultraviolet ray obtained by irradiation using a xenon gas-included dielectric barrier discharge excimer lamp. The hydrophilic functional group(s) produced are one or more groups including a group consisting of a hydroxyl group (--OH), a carbonyl group (--CO--), an aldehyde group (--CHO), a carboxyl group (--COOH), a nitro group (--NO2), and an amino group (--NH2). The carbon nanotubes are expected to have application as one-dimensional wires, catalysts, cold-cathode elements, and hydrogen storage substances owing to mechanical and structural properties derived from its specific structure.