Applied Catalysis B: Environmental 126 (2012) 334–341 Contents lists available at SciVerse ScienceDirect Applied Catalysis B: Environmental jo u r n al hom ep age: www.elsevier.com/locate/apcatb Photocatalytic properties of TiO 2 /WO 3 coatings formed by plasma electrolytic oxidation of titanium in 12-tungstosilicic acid S. Stojadinovi ´ c a,∗ , N. Radi ´ c b , R. Vasili ´ c c , M. Petkovi ´ c a , P. Stefanov d , Lj. Zekovi ´ c a , B. Grbi ´ c b a University of Belgrade, Faculty of Physics, Studentski trg 12-16, 11000 Belgrade, Serbia b University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, Njegoˇ seva 12, 11000 Belgrade, Serbia c Faculty of Environmental Governance and Corporate Responsibility, Educons University, Vojvode Putnika 87, Sremska Kamenica, Serbia d Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria a r t i c l e i n f o Article history: Received 16 June 2012 Received in revised form 21 July 2012 Accepted 31 July 2012 Available online xxx Keywords: Photocatalysis Plasma electrolytic oxidation TiO2/WO3 TiO2 a b s t r a c t In this paper, we have investigated photocatalytic properties of TiO 2 /WO 3 coatings formed by plasma electrolytic oxidation (PEO) of titanium in 12-tungstosilicic acid water solution and compared with pho- tocatalytic activity of pure TiO 2 coatings. The photocatalytic activity of the coatings is evaluated by measuring the degradation of methyl orange under simulated sunlight conditions. Photocatalytic activ- ity of TiO 2 /WO 3 coatings varied with duration of PEO process and higher photoactivity is obtained for a shorter process time. Photocatalytic activity of TiO 2 /WO 3 coatings is related to morphology and phase and elemental composition of coatings. The oxide coatings morphology is strongly dependent on PEO time. The elemental components of the coatings are Ti, W and O. The oxide coatings are partly crystal- lized and mainly composed of WO 3 and anatase. Decrease in the number of microdischarge channels and agglomeration of particles on the surface leads to a decrease in photocatalytic activity. The reduction of the photocatalytic activity with increased time of PEO process is accompanied with the increase of WO 3 concentration on the coatings’ surface. Diffuse reflectance spectroscopy has shown that coatings enriched with tungsten oxide exhibit significant red shift with respect to the pure TiO 2 coatings. TiO 2 /WO 3 coat- ings with improved catalytic activity, compared to pure TiO 2 coatings, are grouped around energy band gap of 2.6 eV. The results of PL measurements of TiO 2 /WO 3 coatings are in agreement with photocatalytic activities. The increase of PL intensity corresponds to decrease of photocatalytic activity of the coatings, indicating fast recombination of electron–hole pairs. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Plasma electrolytic oxidation (PEO), also called microarc oxi- dation (MAO) or anodic spark deposition (ASD), is high-voltage anodizing process which produces a stable oxide coating on the surfaces of number of metals, such as aluminum, mag- nesium, titanium, zirconium, tantalum, and their alloys [1–4]. Oxide coatings have controllable morphology and composition, excellent bonding strength to the metal, high microhardness, high- quality wear and corrosion resistance, etc. PEO process involves anodization of metals above the dielectric breakdown voltage where numerous transient, fine, short-lived discharges are gen- erated continuously over the coating surface, accompanied by gas evolution [5,6]. Plasma-chemical, thermal, and anodic oxida- tion processes are induced at the discharge sites due to increased local temperature and pressure modifying the structure, compo- sition, and morphology of such oxide coatings. The oxide coatings ∗ Corresponding author. Tel.: +381 11 7158161; fax: +381 11 3282619. E-mail address: sstevan@ff.bg.ac.rs (S. Stojadinovi ´ c). formed by PEO usually contain crystalline and amorphous phases with constituent species originating both from metal and elec- trolyte. Titanium dioxide (TiO 2 ) coatings obtained by PEO process of titanium have been widely investigated because of their potential applications, including biocompatible materials, structural ceram- ics, photocatalysis, sensors, optical coatings, etc. [7–11]. TiO 2 is an excellent photocatalyst, which causes the degradation of various organic pollutants under UV light irradiation due to its relatively wide band gap (∼3.2 eV) [12]. This wide band gap suggests that TiO 2 can only be excited by high energy UV irradiation with a wavelength below 387 nm. This practically rules out the use of sunlight as an energy source for the photoreaction. Also, a high rate of recombina- tion between excited electron/hole pairs limits the photocatalytic activity. Recently, a lot of research has been focused on improving the TiO 2 photocatalytic activity, increasing the absorption of TiO 2 under visible irradiation and decreasing electron/hole recombina- tion rate. It has been reported that the efficiency of a photocatalytic reaction can be improved by coupling TiO 2 with other semicon- ducting materials, such as CdSe [13], CdS [14], SnO 2 [15], WO 3 [16], ZrO 2 [17], and V 2 O 5 [18]. 0926-3373/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apcatb.2012.07.031