Super-hydrophilic and transparent thin films of TiO2 nanotube arrays by a hydrothermal reaction

Titanate nanotube arrays were grown on sapphire substrates by sputtering deposition of titanium films and a subsequent hydrothermal treatment in an aqueous NaOH solution. Post annealing in air transformed the crystals from a titanate structure to an anatase structure. Titanate and anatase nanotube arrays were optically transparent and adhered well to the substrates. Both titanate and anatase nanotube films exhibited high-hydrophilic states with water contact angles below 5 degrees even after dark storage for more than three months due to their surface nanostructures. UV illumination caused super-hydrophilic states with 0 degrees water contact angles, i.e. band-gap excitations decreased the initial water contact angles in dark conditions (similar to 3.5 degrees) to 0 degrees. The hydrophilicizing rate of the anatase nanotubes was faster than that of the titanate nanotubes because the anatase nanotubes had a higher crystallinity and more absorbed photons.