127 1. INTRODUCTION In the search for photocatalysts with extended visible light pho- tosensitivity, tungsten trioxide turns out to be an interesting candi- date. WO 3 is an n-type semiconductor with an indirect bandgap of 2.7 eV [1], and for polycrystalline material (thin films) a value of 2.6 eV has been measured at room temperature [2]. A conduction band position of around 0.5 V vs. NHE has been reported for pH 2 [3]. For polycrystalline materials, flatband potentials depend to a large extent on the way of preparation [4]. WO 3 is chemically stable in acid solution and cannot be oxidized [5]. Recently, the photodegradation of oxalic acid on WO 3 elec- trodes under electrical bias was reported [6]. Photocatalysis with transition metal oxides is a process based on their semiconducting nature [7] and junction behaviour can therefore be influenced by electrical bias if the material is immobilized in the form of a mac- roscopic electrode. Application of electrical bias in photocatalysis with immobilized semiconductors was shown to increase the effi- ciency of the process considerably [6], as charge carrier recombi- nation in the bulk of the semiconductor is suppressed. For n-type semiconductors, there are cases where oxidative photodegradation does not take place at all in the absence of bias, as for instance for WO 3 electrodes of a similar type as used in this work, and a test molecule, the azo-dye acid orange 7 (4-(2-hydroxy-1- naphthylazo)benzenesulfonic acid sodium salt, AO7) [8]. This was ascribed to the lack of a complementary reduction process (oxygen reduction) on the surface of the semiconductor, in contrast to what is observed with most TiO 2 catalysts. Several chemical methods for WO 3 thin film deposition have been used, viz. spray pyrolysis, electrodeposition [9], brush paint- ing, dip- and spin-coating. In spray pyrolysis, aqueous solvents tend to give strongly light scattering [10] and sometimes powdery deposits. If organic solvents are to be used, organometallic precur- sors have to be employed which are expensive and sensitive to humidity. In discontinuous (layer by layer) growth methods (brush painting [2], dip coating, and spin coating [2]), many applica- tion/annealing cycles are needed to build up a thick film and each of the annealing step needs slow heating-up / cooling-down in order to avoid breakage of large samples. Moreover, the total an- nealing time has an influence on the doping by oxygen vacancies. Especially in the case of WO 3 it has been shown that prolonged annealing at > 500°C leads to overdoping due to irreversible oxy- gen loss. Mesoporous WO 3 has been deposited from tungstic acid *To whom correspondence should be addressed: Email: mns@cnrs-bellevue.fr Photoelectrocatalysis with Drop-Cast Tungsten Trioxide Films M. Neumann-Spallart* and S. B. Sadale Groupe d’Étude de la Matière Condensée, C.N.R.S., 1, place Aristide Briand, 92195 Meudon CEDEX, France. Received: Nobember 23, 2009, Accepted: June 09, 2010 Abstract: Polycrystalline, monoclinic, uniform WO 3 films of up to 2.5 µm thickness were prepared by drop casting onto F:SnO 2 /glass using as precursor peroxo-tungstic acid and polyethylene glycol in water, and firing in air at 520°C. Under illumination of such n-type semiconducting electrodes in junctions with aqueous electrolytes, photocurrents were produced. Un- der depletion conditions, IPCEs (incident photon to current efficiencies) of up to 0.7 at 365 nm and 0.2 at 405 nm were obtained. The pho- tosensitivity extends into the visible with an onset at 470 nm. This opens the way for solar light powered electrochemical processes like pollutant degradation. The photocatalytic activity of such layers was examined using the azo-dye acid orange 7 (AO7) as a model pollutant. Degradation of 1 mM AO7 was carried out under backside UVA broadband illumination of electrodes of 62 cm 2 active surface area, using a thin film flow- through reactor equipped with a stainless steel counter electrode, rapid recirculation of the electrolyte, and electrical bias of 1.1 - 1.3 V. During illumination, the concentration of AO7 decreased with an apparent first order rate constant, k, of 1.44·10 -4 s -1 , corresponding to a specific decay constant of 2.3·10 -7 l·s -1 ·cm -2 for a light intensity of 4.7·10 -9 E·s -1 cm -2 . Keywords: photoelectrocatalysis, WO 3 , acid orange 7 Journal of New Materials for Electrochemical Systems 13, 127-131 (2010) © J. New Mat. Electrochem. Systems