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 450-500 run. Results are reported here on the preparation of nitrogen-doped 250 nm thick TiO2 films on glass substrates by RF magnetron sputtering. The nitrogen content in the films was modified by changing the N-2 partial pressure in the discharge from 0.00 to 0.27 Pa. An additional amount of oxygen was added in the deposition chamber (Po-2 = 0.11 Pa) 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 0.16 Pa. Above 0.16 Pa, 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 36 h. 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 (E-g = 3.2 eV), associated with a low recombination rate of charge carriers in the surface. (C) 2007 Elsevier B.V. All rights reserved.