Visible light induced wetting of nanostructured N–F co-doped titania films

Nitrogen and fluorine co-doped TiO₂ films have been prepared by dip coating of a modified titania sol–gel based on a nitrogen precursor and a nonionic fluorosurfactant as pore template and fluorine source. The modified NF–TiO₂ films absorb in the visible spectral range, between 400–510 nm and undergo reversible hydrophilic conversion under visible light to a final contact angle of 8°, in contrast to the UV limited optical response of their undoped anatase TiO₂analogues. The phenomenon takes place at a rate slower than the corresponding one observed for the UV stimulated superhydrophilic effect. The wetting response of the N–F doped TiO₂ films correlates well with the variation of their optical properties and surface morphological characteristics and most importantly with their photocatalytic activity, rendering these materials very promising for self-cleaning applications under visible light.