Oriented single crystalline titanium dioxide nanowires.

We report the synthesis of oriented single crystalline titanium dioxide (TiO(2)) nanowire arrays on titanium foil. The synthesis method relies on the ability to grow single crystal sodium titanate (Na(2)Ti(2)O(5).H(2)O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth, the Na(2)Ti(2)O(5).H(2)O nanowires are converted to protonated bititanate (H(2)Ti(2)O(5).H(2)O) nanowires through an ion-exchange reaction without changing their morphology or crystal structure. Finally, the protonated bititanate nanowires are converted to single crystalline anatase TiO(2) nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpet of 2-50 microm long single crystalline nanowires oriented in the [100] direction and primarily normal to the titanium foil. Even longer nanowires can be grown. The single crystal TiO(2) nanowire arrays on flexible titanium substrate may be used in photocatalytic and photovoltaic devices such as dye-sensitized solar cells and may enhance their performance by providing fast electron transport. The nanowires can also be used as templates for producing hierarchical nanostructures such as nanowires decorated with nanoparticles on their periphery or nanotubes with walls made of nanoparticles.