Formation of titanium oxide nanotubes using chemical treatments and their characteristic properties

Needle-shaped titanium oxide crystals with a diameter of 8 nm were obtained when titania nanopowders were treated chemically with NaOH aqueous solution and subsequently with HCl aqueous solution under various conditions (e.g., at 110 °C for 20 h). Transmission electron microscopy showed that the needle-shaped products have a tube structure with an inner diameter of approximately 5 nm and an outer diameter of approximately 8 nm. TiO2 nanotubes with a large specific surface area of ≈ 400 m2/g are expected to have great potentials for use as high-performance photocatalysts or adsorbents. The amount of residual Na+ ions in the nanotubes can be controlled by HCl treatment, resulting in the formation of Na–Ti–O titanate nanotubes. The titania and titanate nanotubes can also be modified during the treatment. When calcium acetate solution was used for the treatment, a new type of bioactive nanotube was prepared. An apatite layer was formed on a compact composed of the nanotubes within 1 day of soaking in simulated body fluid. An animal test using rats showed that new-bone-tissue formation around the nanotube compact started 3 days after implantation. When oxoacid solutions, such as perchloric acid, phosphoric acid or sulfuric acid, were used in the treatment, new types of nanotube showing proton conduction were prepared; one of the nanotube compacts showed a high electrical conductivity of 8 × 10− 2 S/cm at 150 °C. These nanotubes are expected to have applications in the fields of medicine and energy generation, as well as photocatalytic applications.