Combining degradation and contact angle data in assessing the photocatalytic TiO2:N surface

Increasing the oxidative efficiency of titania by a better use of photoactivation energy from visible solar light has been the subject of major interest during the last decades. Doping titania materials with metals and nonmetals has been demonstrated as a means to enhancing the photocatalytic and hydrophilicity performances by shifting the optical absorption edge of the materials towards visible range. However, when using plasma vapor deposition techniques for film synthesis, incorporating higher amounts of certain dopants (such as nitrogen), in the films from the gas phase is less effective due to low affinity of Ti for them. Alternative means for trimming the N/O content ratio in the films remained an open issue till now. We propose here, a versatile new approach to synthesize TiO2:N thin films of controllable elemental composition by using a sintered TiN target as material source in a reactive RF discharge within the Ar-O-2 gas mixture. By adjusting the O-2 partial pressure between 4.2x10(-6) mbar and 1.3x10(-3) mbar, RF sputtered thin films have been prepared with N atomic concentration varying between 1.2-1.8 at.%. Film surface morphology, crystallinity, elemental composition and oxidation state data were inferred from AFM, XRD and XPS measurements. Large differences of atomic concentrations of the main elements in the surface have been found between the as-deposited and sputter-cleaned films, due to the effect of Ar+ ion bombardment. The rate constants of photoinduced hydrophilic conversion of the TiO2: N surface was inferred from water contact angle data taken during both surface activation and back-reaction time. The oxidative performance of the photocatalytic films was investigated using oleic acid (C18H34O2) as a test material. The results were discussed in terms of the effects of elemental composition, surface morphology and electronic structure of the films. Copyright (C) 2010 John Wiley & Sons, Ltd.