Effect of Visible and UV Illumination on the Water Contact Angle of TiO 2 Thin Films with Incorporated Nitrogen A. Borra ´ s, † C. Lo ´ pez, † V. Rico, † F. Gracia, † A. R. Gonza ´ lez-Elipe,* ,† E. Richter, ‡ G. Battiston, § R. Gerbasi, § N. McSporran, | G. Sauthier, | E. Gyo 1 rgy, | and A. Figueras | Instituto de Ciencia de Materiales de SeVilla (CSIC-UniV. SeVilla), AVda, Ame ´ rico Vespucio 49, 41092 SeVilla, Spain, Forschungszentrum Rossendorf, Institut fu ¨r Ionenstrahlphysik und Materialforschung, P.O. Box 510119, 01314 Dresden, Germany, ICTIMA-CNR, Corso Stati Uniti 4, 35127 Padua, Italy, and ICMAB-CSIC, Campus de la UAB, 08193 Bellaterra, Spain ReceiVed: August 21, 2006; In Final Form: NoVember 13, 2006 Doping TiO 2 with nitrogen is recognized as a procedure to get sensitization of this material with visible light. In the present work, incorporation of nitrogen within the structure of TiO 2 thin films has been accomplished by N 2 + ion implantation in TiO 2 anatase thin films (50 keV ion energy for doses of 3 × 10 16 ,6 × 10 16 , and 1.2 × 10 17 ions cm -2 ) and during preparation by metalorganic chemical vapor deposition (MOCVD) using nitrogen as carrier gas. The analysis of the samples by X-ray photoemission spectroscopy (XPS) and for the MOCVD samples also by secondary ion mass spectroscopy (SIMS) has shown that nitrogen, in the form of nitride-like species, (N/Ti ratios of 0.03 and 0.12 for the MOCVD and the implanted samples, respectively) has become effectively incorporated within the structure of TiO 2 . The water contact angle on the implanted thin films varied from about 80° to around 30° when illuminated with visible light, depending on the ion dose. Similarly, the MOCVD samples showed a sharp decrease in wetting contact angle under visible light from about 80° to 55°. In the two cases, the thin films reach total hydrophilicity by posterior UV irradiation. To account for these results, the possible existence of specific excitation mechanisms for visible or UV photons, the former involving the incorporated nitrogen atoms, is discussed. Introduction The activation of titanium oxide with light is a very interesting process with a large variety of applications, including the degradation of environmental pollutants in air and water, the recovery of metal cations from water, or the self-cleaning of glasses because of the modification of the wetting properties of its surface. 1 The wettability of materials is an important property in nature and technology. 2 Some years ago, Wang et al. 3 found that the surface of titanium dioxide thin films changed from hydrophobic to hydrophilic when subjected to UV irradia- tion. They also found that the normal contact angle of TiO 2 thin films (usually between 60° and 100°, depending on surface topography) was recovered when the films were illuminated with visible light or were stored in the dark, thus making reversible the change in wetting contact angle. Since this pioneering work, much effort has been dedicated to describe these UV-induced changes in hydrophilicity and to find the reasons for them. 4-8 At present, testing the wetting behavior of TiO 2 upon light irradiation is a common tool to check the photoactivity of this material. The most widely accepted model of the wetting behavior of illuminated TiO 2 suggests that the photogenerated electron- hole pairs migrate to its surface where they induce a series of reactions leading to the hydroxylation of the surface, a feature that promotes its hydrophilic character. However, this view is controversial and recently, Zubkov et al., 8 after a careful experiment carried out in an ultrahigh vacuum equipment where the cleaning conditions can be effectively controlled, have attributed the changes in water contact angle to the photo- oxidative removal of the carbon contamination layer always present on the air-exposed surfaces of these oxides. Whatever the type of surface processes involved, it seems that the only effect of visible light on undoped TiO 2 is the acceleration in the recovery of the original hydrophobic character because of the thermal effects resulting from irradiation. 7 The sensitization of TiO 2 with visible photons is a topic of the utmost interest 9 that has been induced by doping with transition-metal cations 10-11 or other atoms acting as anions. 12 In particular, Asahi et al. 12 have reported an improvement in the photocatalytic activity of TiO 2 doped with nitrogen, an effect confirmed by several other researchers. 13-18 It has been sug- gested that through both nitrogen doping and the presence of oxygen vacancies in its crystal structure, the activity of the TiO 2 photocatalyst in the visible region can be further enhanced. 19-21 Unfortunately, doping can also increase the number of electron- hole recombination centers and can reduce the thermal and mechanical stability of the material. 22 Addressing the wetting properties of TiO 2 , Premkumar 16 has reported that illumination with visible light of nitrogen-doped TiO 2 thin films prepared by magnetron sputtering results in the character of the material changing from hydrophobic to hydrophilic. This demonstrates that control of the wetting contact angle is possible without the assistance of UV photons. Some other recent investigations on TiO 2 thin films with incorporated nitrogen in their structure have also reported the possibility of inducing this change in water contact angle by illuminating with visible photons. 23,24 * Author to whom correspondence should be addressed. E-mail: arge@icmse.csic.es. † Instituto de Ciencia de Materiales de Sevilla. ‡ Institut fu ¨r Ionenstrahlphysik und Materialforschung. § ICTIMA-CNR. | ICMAB-CSIC. 1801 J. Phys. Chem. C 2007, 111, 1801-1808 10.1021/jp065392w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007