Wide-bandgap semiconductor nanotubes have potential as components in electronic and optical devices because of their transparency and high energies of excited charge-carriers. [1][2][3] Among the various semiconductor nanotubes with wide bandgaps, titanate-based nanotubes, which have been synthesized and characterized over the last few years, [4][5][6][7][8][9] have received a lot of attention because of their cheap fabrication and unique one-dimensional nanostructure. Recently, we reported the synthesis of a novel thin film of titanate nanotubes by an alternate adsorption method and its photochemical properties, in particular its photocatalytic oxidation activity and photoinduced hydrophilicity. [10] Thin-film fabrication of titanate nanotubes enables us to develop a new, functional, transparent device.
Herein, we focus on the electrochromic properties of titanate nanotubes. Electrochromism depends on the crystal structure, and cathodic coloration occurs in transition metal oxide semiconductors such as TiO 2 , WO 3 , MoO 3 , MnO 2 , and Nb 2 O 5 that are constructed from MO 6 octahedra. [11] Recently, several research groups have reported the electrochemical properties of layered materials made from these oxides; [12,13] however, the electrochromic properties of transparent nanotube thin films have yet to be disclosed. A titanate nanotube is composed of a one-dimensional nanostructure generated from nanosheets of TiO 6 octahedra, and therefore unique electrochromic properties are expected. In this study, transparent thin films of titanate nanotubes were deposited on electroconductive substrates by an alternate-layer deposition and their electrochromism was evaluated. The titanate nanotubes were found to exhibit significant electrochromism