Visible light activated photocatalytic behaviour of rare earth modified commercial TiO 2 D.M. Tobaldi a, *, R.C. Pullar a , A. Sever S ˇ kapin b , M.P. Seabra a , J.A. Labrincha a a Department of Materials and Ceramic Engineering/CICECO, University of Aveiro, Campus Universita ´rio de Santiago, 3810-193 Aveiro, Portugal b Slovenian National Building and Civil Engineering Institute, Dimicˇeva 12, SI-1000 Ljubljana, Slovenia 1. Introduction The contamination of natural water reserves, air (indoor and outdoor) and soil is currently one of the major global environmen- tal concerns. Therefore, to further the sustainable development of modern society, there is an urgent need for advances in green technologies to achieve environmental remediation. Environmental catalysis is one of the most studied processes in order, not only to reduce, but also to prevent the causes of pollution. Amongst these catalytic processes, photocatalysis is expected to be one playing an important role in the 21st century’s efforts in reducing environmental pollution [1]. Titanium dioxide (TiO 2 ) is the most investigated photocatalyst; the great interest in TiO 2 can be related to the work by Fujishima and Honda, published in 1972 [2]. This described the photo-assisted electrolysis of water upon irradiation of a single-crystal TiO 2 electrode (with a Pt counter-electrode), with photons of energy greater than the band gap of TiO 2 . However, this was not strictly catalysis, the reaction not being thermodynamically feasible without the photons – it was actually an energy storage reaction that can be termed photogalvanic [3]. The first reported works rigorously dealing with ‘‘photocatalysis’’ are those by Doerfler and Hauffe, published in 1964 [4,5]. In any case, we are still experiencing today an enormous boom in this field of research, with a great number of publications concerning photocatalysis appearing over the past 20 years. Photocatalysis can be described as that phenomenon in which a material (a semiconductor) modifies the rate of a reaction, via the action of light having a suitable wavelength [6,7]. When a semiconductor is irradiated with photons having energy higher than, or equal to, its energy band gap (E g ), an electron (e  ) is able to migrate from the valence band to the conduction band, leaving a hole (h + ) behind. Such a photo-generated couple (e  –h + ) is able to reduce and/or oxidise a pollutant adsorbed on the photocatalyst surface [8]. As a photocatalytic material, TiO 2 is chemically inert and non- toxic; the reactions take place at mild operating conditions (e.g. a low level of solar or artificial illumination, room temperature (RT) and atmospheric pressure); no chemical additive is necessary; Materials Research Bulletin 50 (2014) 183–190 A R T I C L E I N F O Article history: Received 16 April 2013 Received in revised form 2 October 2013 Accepted 17 October 2013 Available online 26 October 2013 Keywords: A. Oxides B. Phase transitions B. Optical properties C. Raman spectroscopy D. Catalytic properties A B S T R A C T A commercial TiO 2 nanopowder, Degussa P25, was modified with several rare earth (RE) elements in order to extend its photocatalytic activity into the visible range. The mixtures were prepared via solid- state reaction of the precursor oxides, and thermally treated at high temperature (900 and 1000 8C), with the aim of investigating the photocatalytic activity of the thermally treated samples. This thermal treatment was chosen for a prospective application as a surface layer in materials that need to be processed at high temperatures. The photocatalytic activity (PCA) of the samples was assessed in gas–solid phase – monitoring the degradation of isopropanol (IPA) – under visible-light irradiation. Results showed that the addition of the REs lanthanum, europium and yttrium to TiO 2 greatly improved its photocatalytic activity, despite the thermal treatment, because of the presence of more surface hydroxyl groups attached to the photocatalyst’s surface, together with a higher specific surface area (SSA) of the modified and thermally treated samples, with regard to the unmodified and thermally treated Degussa P25. The samples doped with La, Eu and Y all had excellent PCA under visible-light irradiation, even higher than the untreated Degussa P25 reference sample, despite their thermal treatment at 900 8C, with lanthanum producing the best results (i.e. the La-, Eu- and Y-TiO 2 samples, thermally treated at 900 8C, had, respectively, a PCA equal to 26, 27 and 18 ppm h 1 – in terms of acetone formation – versus 15 ppm h 1 for the 900 8C thermally treated Degussa P25). On the other hand, Ce–TiO 2 s had no significant photocatalytic activity. ß 2013 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +351 234 370 041. E-mail addresses: david.tobaldi@ua.pt, david@davidtobaldi.org (D.M. Tobaldi). Contents lists available at ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.10.033