On the hydrophilicity of nitrogen-doped TiO2 thin films

Nitrogen doping was proposed recently as a low-cost means to shift the absorption edge of titania towards 450-500nm. Results are reported here on the preparation of nitrogen-doped 250nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N2 partial pressure in the discharge from 0.00 to 0.27Pa. An additional amount of oxygen was added in the deposition chamber (pO2=0.11Pa) to compensate for oxygen depletion in the films. Surface composition and structure were derived from XPS and XRD data, respectively. XRD data revealed a mixture of amorphous phase and rutile polymorph in the as-deposited films, while an anatase-rich phase occurs in the samples deposited under N2 pressure of 0.05 and 0.16Pa. Above 0.16Pa, a rutile-rich phase is present. Surface wettability was investigated from contact angle data of de-ionized water with solid surface. The films deposited under mid-range N2 pressure in the discharge are super-hydrophilic. The photo-induced super-hydrophilicity decays after 36h. Film wettability is discussed in terms of the synergic effects of surface roughness (measured by AFM) and rutile-anatase TiO2 phase transition, induced by doping. The TiO2 surface super-hydrophilicity (and catalytic efficiency) are related to beneficial properties of anatase phase (Eg=3.2eV), associated with a low recombination rate of charge carriers in the surface.