ORIGINAL PAPER Photoelectrocatalytic activity of bi-layer TiO 2 /WO 3 coatings for the degradation of 4-chlorophenol: effect of morphology and catalyst loading J. Georgieva • S. Sotiropoulos • S. Armyanov • N. Philippidis • I. Poulios Received: 5 February 2010 / Accepted: 21 September 2010 / Published online: 9 October 2010 Ó Springer Science+Business Media B.V. 2010 Abstract WO 3 and bi-layer WO 3 /TiO 2 coatings of dif- ferent catalyst loadings were electrosynthesized on stain- less steel 304 (SS) substrates from acidic aqueous solutions by single-step and consecutive steps potentiostatic cathodic deposition. The resulting WO 3 /SS and TiO 2 /WO 3 /SS photoelectrodes were screened for their photoresponse under ultraviolet (UV) and visible light illumination by photovoltammetry and photoamperometry in sulphate solu- tions, in the absence and presence of 4-chlorophenol (4-CP). They were also evaluated for bulk photo-oxidation of 4-CP under constant potential, in the voltage range determined on the basis of the photovoltammetric tests. The optimal weight ratio between TiO 2 and WO 3 was also established, ensuring the best performance of these pho- toelectrodes for the photooxidation of 4-CP under UV and visible light irradiation. Keywords Tungsten trioxide coatings Á Titanium dioxide coatings Á Stainless steel Á 4-Chlorophenol Á Photodegradation 1 Introduction Heterogeneous photocatalysis has been suggested as an alternative method of hazardous waste treatment [1, 2]. One of the most popular photocatalysts is TiO 2 , a wide gap n-type semiconductor [3], because of its high photocata- lytic activity and chemical stability against photocorrosion. The use of TiO 2 slurries necessitates removal of the pho- tocatalyst by filtration. Replacing the slurries by photo- electrodes, wherein the semiconductor is immobilized on a conducting support, can avoid the filtration step of sus- pended particles but entails mass transfer and active area limitations. If the photoelectrode is subjected to an external positive bias (in an appropriate electrochemical cell), then the photogenerated electrons are drawn away from the catalyst surface through the external cell circuit, and photogenerated holes are transferred to the electrode sur- face. In this way, the rate of electron–hole recombination is decreased, and the rate of surface reactions increased. This process is known as electrically enhanced photocatalysis [4–9] or photoelectrocatalysis, which can overcome some of the above-mentioned disadvantages of supported phot- ocatalysts. However, the TiO 2 photocatalysts absorb only UV light, and thus it is now more desirable to develop photocatalysts which exhibit visible light response for practical applications [10, 11]. In that direction, minimiz- ing photogenerated electron–hole recombination rates and expanding semiconductor’s useful range of operation into visible light wavelengths are the main targets of research aiming at improving the efficiency of TiO 2 -based photo- catalysts. For example, doping with lanthanide ions [12–14] or coupling with WO 3 [15–25] is known to sup- press electron–hole recombination rates. On the other hand, introduction of a visible light active catalyst, such as WO 3 [15–18, 26–30], improves visible light utilization. Hence, coupling TiO 2 with WO 3 is a strategy for achieving both targets. Electrosynthesis of TiO 2 and electrodeposition of WO 3 have been proposed as an alternative route for the production of plain or mixed coatings on Pt, Au or opti- cally transparent electrodes [17–22, 24, 31, 32] and, more J. Georgieva (&) Á S. Armyanov Rostislaw Kaischew Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria e-mail: jenia@ipc.bas.bg S. Sotiropoulos Á N. Philippidis Á I. Poulios Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece 123 J Appl Electrochem (2011) 41:173–181 DOI 10.1007/s10800-010-0221-8