Applied Catalysis B: Environmental 103 (2011) 232–239 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb The influence of the local structure of Fe(III) on the photocatalytic activity of doped TiO 2 photocatalysts—An EXAFS, XPS and Mössbauer spectroscopic study Éva G. Bajnóczi a , Nándor Balázs a , Károly Mogyorósi a , Dávid F. Srankó a , Zsolt Pap a , Zoltán Ambrus a , Sophie E. Canton b , Katarina Norén b , Ern ˝ o Kuzmann c,d , Attila Vértes c,d , Zoltán Homonnay d , Albert Oszkó e , István Pálinkó f , Pál Sipos a,∗ a Department of Inorganic and Analytical Chemistry, University of Szeged, Szeged, Hungary b Chemical Physics Department, Chemical Centre, Lund University, Lund, Sweden c Chemical Research Center, HAS, Laboratory of Chemistry, Budapest, Hungary d Institute of Chemistry, Lóránd Eötvös University, Budapest, Hungary e Department of Physical Chemistry and Materials Science, University of Szeged, Szeged, Hungary f Department of Organic Chemistry, University of Szeged, Szeged, Hungary article info Article history: Received 16 November 2010 Received in revised form 13 January 2011 Accepted 21 January 2011 Available online 27 January 2011 Keywords: TiO2 Fe(III) doping EXAFS XPS Mössbauer Local structure Photocatalysis abstract Fe(III)-doped TiO 2 based heterogeneous photocatalysts were prepared by the sol–gel technique (S sam- ples) or flame hydrolysis (F samples). In photocatalytic phenol decomposition, the undoped F-sample performed much better, than the undoped S one. However, for the S samples, photocatalytic activity first increased with the increasing Fe(III) concentration, and then passed through a maximum, while Fe(III)- doping in F samples significantly decreased it, even at the smallest dopant level. Since the same dopant caused opposite photocatalytic effects in the two series, their structure was systematically compared to identify the underlying chemical and/or physical reasons. The photocatalysts were first characterized by AAS, DRS, XRD and TEM methods and it has been shown that the differences in the photocatalytic activity cannot be explained by the minor variations in the bulk structural properties of TiO 2 . Mössbauer and XP spectroscopic measurements performed on representative samples qualitatively proved that the local structure of Fe(III) is different in the two series. To quantify these effects, Fe-K edge X-ray absorption measurements were performed. From the pre-edge and XANES region it was learnt that Fe(III) was present in a distorted octahedral environment in both series, however, the extent of distortion is much more significant within the S than within the F one. Information obtained from the EXAFS region indicated that the structure of Fe 2 O 3 was much more ordered in the F-series then in the S one and vacancies were more abundant in the S than in the F series. Moreover, the geometry around Fe(III) systematically varied within the S-series, which could explain, why photocatalytic activity passed through a maximum with the increasing Fe(III) concentration in these samples. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Titania (TiO 2 ) based semiconductor photocatalysis is one of the most active areas of materials science, both on fundamental and applied levels [1–4]. A common way of photosensitizing TiO 2 pho- tocatalyst is doping with metal ions (e.g., transition metal ions like Fe(III), V(V), Cr(III), Cu(II), Mn(II), Co(II), etc.) or nonmetal ions (e.g., I in various oxidation states, P(V), N(III), S(VI), etc.). Doping with metal ions may have both positive and negative effects on the pho- tocatalytic activity, because metal ions may decrease the band gap ∗ Corresponding author. Tel.: +36 62544338; fax: +36 62420505. E-mail address: sipos@chem.u-szeged.hu (P. Sipos). energy on one end, but may also act as recombination centers for the electrons and holes on the other. From theoretical calculations, Karvinen et al. predicted [5] that several metal ions (e.g., Ti 3+ ,V 3+ , Cr 3+ , Mn 3+ and Fe 3+ ) decreased the band gap energy in anatase but had no effect for rutile. Hoffmann and co-workers have experimen- tally studied the photocatalytic activity of TiO 2 doped with various metal ions [6,7] (Fe 3+ , Mo 5+ , Ru 3+ , Os 3+ , Re 5+ ,V 4+ and Rh 3+ ) and found both beneficial and non-beneficial effects. They hypothe- sized that the photocatalytic activity depended on several factors, such as the concentration and distribution of the dopant, its energy state in the lattice, the configuration of the d-electron, etc. Photo- catalytic activity of transition metal doped TiO 2 catalysts usually passes through a maximum with increasing dopant concentration, but excessive amount of dopant causes an adverse effect. In other 0926-3373/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2011.01.033