First reported by Fujishima and colleagues in the mid 1990s, upon exposure to UV light TiO2 changes into a ‘structurally metastable’ state with a remarkably different affinity for water. Hydrophilicity is commonly measured by the contact angle (CA) between the solid surface and liquid phase tangent line at the interface. Prior to UV light exposure, and depending largely on the surface roughness, TiO2 will typically have a contact CA of around 20-30°. Representative data show that over a period of 2 hour exposure to UV light, CA for TiO2 will decrease down to zero or very close to it. If stored in the dark, the TiO2 will not return to its original contact angle status for approximately 1500 hours, though this timing can be impacted by dark storage temperature. No other material is known to have such properties.
Initial studies used bulk TiO2, not the self-organized nanotube arrays. A relatively small number of reports in the literature have investigated TiO2 nanotube properties and how they impact the UV induced hydrophilicity phenomenon.
- For example, a comparison of TiO2 nanotubes prepared under different conditions (phosphate / HF electrolyte and ethylene glycol / HF electrolyte) at different anodization potentials were characterized. Interestingly, the TiO2 nanotube samples prepared in phosphate based electrolyte achieved a 0° contact angle after only 3-9 minutes of exposure to UV light, while the ethylene glycol prepared sample took 1.5 hours to achieve the same degree hydrophilicity. It should be noted that each TiO2 nanotube sample studied had different tube diameter, tube length, and surface roughness, so it was not clear which variable had the greatest impact impact.
- A recent study investigated annealing temperature and its impact on UV induced hydrophilicity.
Identical TiO2 nanotube arrays were prepared and annealed at varying temperatures (200, 400, 600, 800 °C). Samples un-annealed or annealed at 200°C remained amorphous. At 400°C anatase begins, and 600°C rutile phase appears, and at 800°C nanotubes are destroyed and a dense oxide rutile is seen. After 1 month of storage, all samples exhibited hydrophobic contact angle (generally > 130°), and after UV exposure they all show ~0° contact angle. The researchers note that anatase TiO2 nanotubes seem to show especially strong UV induced super hydrophilicity effect.
After the discovery of the ability to induce super hydrophilicity, some investigations have explored techniques to switch back and forth between being super hydrophilic and super hydrophobic (defined as having a contact angle > 150°). Recently, a strategy to modify TiO2 nanotube arrays with PTES was reported to achieve CA well above 150°, while nitrogen doping is another strategy reported to switch between super hydrophilic and super hydrophobic states in TiO2 nanotubes.