Applied Catalysis B: Environmental 132–133 (2013) 460–468 Contents lists available at SciVerse ScienceDirect Applied Catalysis B: Environmental jo u r n al hom ep age: www.elsevier.com/locate/apcatb Photocatalytic activity of N-doped and N–F co-doped TiO 2 and reduction of chromium(VI) in aqueous solution: An EPR study A.E. Giannakas, E. Seristatidou, Y. Deligiannakis, I. Konstantinou ∗ Department of Environmental and Natural Resources Management, University of Western Greece, G. Seferi 2, Agrinio 30100, Greece a r t i c l e i n f o Article history: Received 20 July 2012 Received in revised form 10 December 2012 Accepted 13 December 2012 Available online xxx Keywords: Photocatalysis N-doped TiO2 N–F co-doped TiO2 Cr(VI) reduction EPR N b • species Microwave saturation a b s t r a c t N-doped and N–F co-doped TiO 2 catalysts were prepared via a sol–gel method using NH 4 Cl and NH 4 F as N and N–F dopant precursors, respectively, having Ti:N and/or F molar ratios of 1:1, 1:2 and 1:3. The catalysts were tested for the photocatalytic reduction of Cr(VI) in the presence of oxalate ions. XRD analysis showed the formation of TiO 2 anatase phase in all cases. UV–vis DRS spectra showed that both N–F and N-doping resulted in a decrease in the band gap energy (E g ), at the values of 2.81 eV and 3.01 eV, respectively. Thus, N–F doped TiO 2 showed enhanced absorption at visible wavelengths. The structure and photodynamics of the TiO 2 catalysts was investigated in detail by electron paramagnetic resonance (EPR) spectroscopy. The EPR data showed that: [i] NO centers, N b • and O 2 -• radicals were formed. In addition, lattice Ti 3+ ions were detected in N–F co-doped solids; [ii] the N b • and Ti 3+ species were photoactive, while the NO species were non-photoactive. The photocatalytic efficiency for Cr(VI) reduction in the presence of oxalate ions, followed the trend TNF1 > TN1 > TNF2 > TN3 > TN2 > TNF3. Importantly, an apparent correlation between the catalytic effi- ciency and the concentration of N b • species was revealed by EPR. The location of N b • in the crystal lattice of TiO 2 has been assessed also by measuring their microwave saturation parameters P 1/2 . Electron capturing by O 2 and subsequent generation of O 2 -• was favored for N-doped catalysts. In contrast, in N–F co-doped catalysts, O 2 could not compete efficiently with Cr(VI) for the photogenerated electrons in energy states below the conduction band of TiO 2 , resulting in higher reduction efficiency for these catalysts. © 2012 Elsevier B.V. All rights reserved. 1. Introduction In recent years, catalytic techniques have been applied in vari- ous fields in order to solve the increasing environmental pollution [1]. Photocatalysis is a promising method that has a great potential for the conversion of solar photon energy into chemical energy and for the decomposition of pollutants in air and water [2]. Titanium dioxide (TiO 2 ) has been widely studied and used as a photocatalyst by virtue of its low cost, chemical stability, non-toxicity and favor- able optoelectronic properties. However, a serious disadvantage of TiO 2 as a wide band gap semiconductor (E g = 3.4 eV for anatase) is that absorbs only UV light ( ≤ 387 nm), which accounts for only a small fraction (3–5%) of solar energy. In order to improve the photocatalytic activity of TiO 2 under visible-light irradiation, many attempts have been made, including doping of TiO 2 with transition metals (Fe, Ce, La, etc.) [3–6]. How- ever, metal doping has several drawbacks i.e. the doped materials present low thermal stability, while metal leaching and possible ∗ Corresponding author. Tel.: +30 26410 74186; fax: +30 26410 74176. E-mail address: iokonst@cc.uoi.gr (I. Konstantinou). toxicity effects diminish the applicability potential for water- treatment applications. Furthermore, in many cases, the metal centers can act as deep electron-traps, reducing the photocatalytic efficiency [7]. Another approach to narrow the semiconductor’s band-gap is the replacement of lattice oxygen by anionic dopant species, such as N [7], C [8], S [9], I [10], F [11] or with co-doping such as N–I [12], N–S [13] and N–F [14]. In this way, the absorption edge of TiO 2 could be shifted to higher wavelengths, i.e. the doped TiO 2 can achieve significant photocatalytic activity under visible light irradiation [14–16]. Recent experimental studies have reported a remarkable pho- tocatalytic activity of nitrogen and fluorine co-doped TiO 2 under visible light [15,16]. N-doping resulted in improvement of visi- ble light absorption, as well as in the creation of photoinduced surface oxygen vacancies [17,18]. F-doping produced several ben- eficial effects such as the creation of surface oxygen vacancies, enhancement of surface acidity and formation of Ti 3+ cen- ters, i.e. conduction-band electrons e - CB [19]. Wang et al. [20], reported the unusual role of F in tailoring the band structure of the N-doped TiO 2 . By theoretical calculations and experi- ments, they demonstrated that incorporating F into N-doped TiO 2 enhances the oxidative power of the photoinduced holes in the 0926-3373/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2012.12.017