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.