Nanotubes composed of various materials such as carbon, boron nitride, and oxides have been studied recently. In this report, the discovery of a new route for the synthesis of a nanotube made of titanium oxide is presented. Needle-shaped TiO2 crystals (anatase phase) with a diameter of ≈8 nm and a length of ≈100 nm were obtained when sol−gel-derived fine TiO2-based powders were treated chemically (e.g., for 20 h at 110 °C) with a 5−10 M NaOH aqueous solution. It was found by observation using a transmission electron microscope that the needle-shaped products have a tube structure. The TiO2 nanotubes have a large specific surface area of ≈400 m2·g-1. TiO2 nanotubes obtained in the present work are anticipated to have great potential for use in the preparation of catalysts, adsorbants, and deodorants with high activities, because their specific surface area is greatly increased. If metallic-, inorganic-, or organic-based materials can be inserted into the TiO2 nanotubes, novel characteristics such as electric, electromagnetic, or chemical properties may be induced in the TiO2 materials.

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TiO2 nanotubes (TNTs) with large aspect ratio and large specific surface area were prepared from P25 (Nippon Aerosil) and applied to dye-sensitized titanium dioxide solar cells (DSSCs). Optimization of fabrication conditions, i.e., pH of the starting paste, sintering temperature for the TiO2 electrodes, electrolyte compositions of DSSCs gave the high conversion efficiency with improved open circuit voltage (V(oc)) and fill factor (FF) when compared to DSSCs made of P25. The evaluation of dye adsorption and the photo-injected electron transport such as electron diffusion coefficient (D) and electron lifetime (tau) in TNTs electrodes revealed that the higher efficiency resulted from increase of electron density with keeping much longer tau in TNTs electrodes than in P25 electrodes.

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