Journal of Hazardous Materials 186 (2011) 2103–2109 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat Anodic titanium oxide as immobilized photocatalyst in UV or visible light devices M.V. Diamanti a,∗ , M. Ormellese a , E. Marin b , A. Lanzutti b , A. Mele a , M.P. Pedeferri a a Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Via Mancinelli 7, 20131 Milan, Italy b Università di Udine, Dipartimento di Scienze e Tecnologie Chimiche, Via del Cotonificio 108, 33100 Udine, Italy article info Article history: Received 15 November 2010 Accepted 27 December 2010 Available online 6 January 2011 Keywords: Anodizing Doping Organic dye Photocatalysis Titanium dioxide Visible light abstract Titanium anodizing can be a powerful technique to generate photoactive oxides, strongly adherent to the metallic substrate, and to modify their chemical composition by inducing doping effects. This work investigates the photocatalytic behavior of differently obtained anodic TiO 2 films under UV and visible light irradiation, so as to define the best treatment for wastewaters purifiers. Anodizing was performed in H 3 PO 4 and H 2 SO 4 mixtures or in fluoride containing electrolytes. Morphology, elemental composition and crystal structure of the anodic films were characterized by XDR, GDOES and SEM. When amorphous oxides were obtained, an annealing treatment was used to promote the formation of anatase crystals. Annealing was also performed in nitrogen atmosphere to induce nitrogen doping. The photocatalytic efficiency of anatase-enriched TiO 2 was investigated in rhodamine B photodegradation. Doping was induced not only by annealing but also directly by anodizing, and generated photoactivity in both the UV and Vis components of light. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Titanium dioxide is employed as photocatalytic substrate in het- erogeneous catalysis, specially in the development of Advanced Oxidation Processes (AOP) which exploit the synergetic use of UV irradiation and oxidizing compounds to increase the degradation efficiency of hazardous chemical compounds in wastewaters or in air [1–4]. Its main drawback is the band gap value of its crys- tal structures, equal to 3.02 eV for rutile and 3.20 eV for anatase, which correspond to the need for UV-A irradiation to activate its photocatalytic performances [5,6]. Nevertheless, the oxide structure can be modified to obtain a red-shift of light absorption. For example, a small fraction of non-stoichiometric nanocrystals in anatase or rutile matrix may enhance the photoactivity of the oxide, since these phases can alter the electronic structure of the resulting complex oxide by creat- ing accessible energy levels in the oxide band-gap. This principle is also exploited when TiO 2 is doped with interstitial or substitutional elements –e.g., nitrogen or carbon [7–11]. Most studies carried out in this field focus on the photocatalytic behavior of TiO 2 powders and the correlation between efficiency and physical properties of the TiO 2 particles – such as size, surface ∗ Corresponding author. Tel.: +39 0223993137; fax: +39 0223993180. E-mail address: mariavittoria.diamanti@polimi.it (M.V. Diamanti). area, crystal structure [12–14]. Yet, in wastewaters treatment the use of non-immobilized particles or poorly adherent films implies an onerous process of separation of catalyst from purified water. Creating TiO 2 films directly by oxidizing metallic titanium would provide a much stronger adhesion of the oxide to the substrate, and therefore improved stability to water flowing in photocatalytic devices and longer lifetime, while still being easy to control and implement even on an industrial scale [15,16]. Recently, new anodizing procedures have led to the obtain- ing of high surface area anodic films on titanium. Anodic Spark Deposition (ASD) processes in H 3 PO 4 and H 2 SO 4 mixtures cause the formation of partially crystalline oxides with enhanced sur- face area due to the formation of deep craters. Moreover, the oxide chemistry is partially modified because of the absorption of sulfur of phosphor ions from the electrolyte [17–19]. Deeper mor- phological modifications can be achieved in fluoride-containing electrolytes, where nanotubular amorphous oxides are obtained: these oxides can acquire photoactivity through annealing treat- ments, which modify the amorphous structure by inducing the formation of anatase crystals [20–25]. The exploitation of these surfaces in liquid phase heterogeneous catalysis could solve the immobilization issue while maintaining high photodegradation efficiency. This paper presents an investigation over the photocatalytic effi- ciency of anodic TiO 2 films in the degradation of organic pollutants in liquid phase: rhodamine B was chosen as model reactant [26–28]. 0304-3894/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.12.128