Author's personal copy Preparation and characterization of increased-efficiency photocatalytic TiO 2–2x N x thin films D. Luca a, ⁎ , C.-M. Teodorescu b , R. Apetrei a , D. Macovei b , D. Mardare a a Faculty of Physics, Alexandru Ioan Cuza University, 11 Carol I 700506, Iasi, Romania b National Institute of Materials Physics, Măgurele, P.O. Box MG 07, 077125, Romania Available online 1 April 2007 Abstract We report here on the characteristics of RF-sputtered 300 nm thick films of TiO 2–2x N x prepared on glass substrates at 350 °C, by adjusting the N 2 :Ar partial pressure ratio in the deposition chamber between 0.00 and 0.33. XRD, XPS, AFM and contact angle data were used to derive film structure, elemental composition and oxidation state of Ti, surface morphology and hydrophilicity, respectively. The band gap was derived from spectral data in the 350–450 nm range. Film structure and composition were changed by adjusting the partial pressure of the reactive gases during sputtering and by post-deposition annealing at 400 °C in air, for 90 min. The values of the contact angle of films' surface with de-ionized water and of surface free energy per unit area show that films are super-hydrophilic for high-nitrogen content. Correlations are made between film structure, elemental composition, electronic and wettability properties. © 2007 Elsevier B.V. All rights reserved. Keywords: TiO 2 ; Nitrogen doping; Anatase; Suboxides; Contact angle; Free surface energy 1. Introduction It is commonly recognized that anatase TiO 2 features both higher photocatalytic efficiency (PE) and super-hydrophilicity (SH), due to its highest band gap (3.2 eV) and suitable mechanisms of adsorption of OH - radicals at surface [1]. The improvement of PE and SH characteristics can be achieved via: (a) enhancing the anatase ordering, and (b) extension of the photo-sensitization wavelength range (PSWR) of the material towards 500 nm, for lower-intensity indoor illumination to become effective. To extend the PSWR, various materials have been used as dopants, including rare-earth elements [2] and transition metals [3]. Promising results have been reported with rare-earth dopants, while contradictory results have been reported with regard to transition metals [3,4]. One of the main drawbacks of the mentioned dopants is the thermal and conductivity instability, associated with the effect of the development of defect centers in the surface responsible for carrier recombination [4]. Reports on the beneficial effect of N doping of TiO 2 have been published recently by Asahi et al. [5]. The cited authors found that the optical absorption edge of the nitrogen doped TiO 2–2x N x films was shifted in the proximity of 500 nm for x =0.0075. To widen the information of a wider composition range we performed a more systematic study of the effect of N dopant. We report here on the characteristics of RF- sputtered 300 nm thick films of TiO 2–2x N x prepared by RF magnetron sputtering. This was acccomplished by adjusting the N 2 :Ar partial pressure ratios in the discharge between 0.00 and 0.33. Additionally, free surface energy measurements were performed to quantify film surface activity. 2. Experimental details The films were prepared by RF magnetron sputtering (Hüttinger PFG 300 RF, 13.56 MHz) in a stainless steel process chamber, with a base pressure of 1 × 10 - 5 mbar. A sintered TiO 2 disk (55 mm diameter, 3 mm thick, 99.99% purity, Furuuchi Chemicals) was used as sputtering target. The substrate holder was placed at 6 cm in front of the cathode. Substrate temperature was maintained (250 ± 1) °C. Six film samples were prepared on glass slides (Corning 7740 Pyrex® borosilicate). Before deposition, the substrates were successively sonicated in acetone and ethyl alcohol, then rinsed and soaked in dry air. The total pressure was adjusted using mass flow controllers (Tylan FC- Thin Solid Films 515 (2007) 8605 – 8610 www.elsevier.com/locate/tsf ⁎ Corresponding author. Tel.: +40 232 20 1179; fax: +40 232 20 1150. E-mail address: dumitru.luca@uaic.ro (D. Luca). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.115