Photocatalytic behavior of suspended and supported semiconductor particles in aqueous media: Fundamental aspects using catechol as model molecule Teresa Lana-Villarreal * , Damia ´n Monllor-Satoca, Antonio Rodes, Roberto Go ´mez * Departament de Quı ´mica Fı ´sica i Institut Universitari d’Electroquı ´mica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain Available online 6 August 2007 Abstract The interaction and photoreactivity of catechol with both anatase and WO 3 nanoparticulate samples have been investigated by applying different experimental techniques. Infrared and Raman spectroscopies have been combined to study catechol adsorption from acidic aqueous solutions. Adsorbed (chelating) catecholate is detected on both oxides together with molecularly adsorbed catechol in the case of anatase. The adsorptive behavior in the dark is completed with a parallel study with a quartz crystal microbalance, showing that upon catechol adsorption the hydrophilic–hydrophobic properties of the oxide nanoporous film vary. Adsorbed catecholate behaves as a charge transfer complex, which can be excited with visible light, as evidenced by Raman Spectroscopy, making possible photocatalytic activity under visible illumination. The time evolution of the Raman spectra shows an increasing fluorescence indicating that, upon electron injection, polymerization of catechol takes place at the semiconductor surface. The polymerization rate is found to be orders of magnitude larger for sintered nanoparticulate thin films than for slurries. Laterally resolved photoelectrochemical experiments evidence that electrons photoinjected by the adsorbed catecholate are highly delocalized through the nanoporous matrix. The advantages of employing a multi-technique approach for investigating adsorptive and photocatalytic events from a physicochemical point of view are outlined. Particular attention is paid to both hydrophobic and electron delocalization effects as they contribute to confer to the nanoparticulate sintered films specific properties not found in suspensions to the same extent. # 2007 Elsevier B.V. All rights reserved. Keywords: Photocatalytic behavior; Semiconductor; Aqueous media; Catechol 1. Introduction Heterogeneous photocatalysis is one of the most promising oxidation technologies for wastewater treatment. An increasing attention has been devoted to this issue since the appearance of the first related publication in 1976 [1]. Nowadays several reviews are available regarding both fundamental and technological aspects [2–8]. Most of them focus on TiO 2 as photocatalyst, being used either suspended in solution or supported on a solid substrate. Often the immobilization of TiO 2 particles reduces their photocatalytic efficiency due to a diminution of active surface area [9] and to mass transport limitations [9,10]. However, this strategy shows several advantages from a practical viewpoint. First, photocatalysts can be used in continuous processes avoiding the filtration step for removal of the semiconductor particles after water purification. Secondly, it is feasible to work with configurations in which all the catalyst particles are illuminated [11]. Heterogeneous photocatalytic reactions are complex pro- cesses far from being fully understood. Not only oxidation and reduction reactions take place in the same particle surface (in the absence of an applied potential) but adsorption/desorption of reactants, intermediates and/or products may lead to different oxidation pathways. Therefore, it is advisable to independently study the different factors that determine the global process efficiency. Within this context, we believe that a multifaceted physicochemical investigation of model systems in heterogeneous photocatalysis is a valid approach that can contribute to improve both our understanding of the www.elsevier.com/locate/cattod Catalysis Today 129 (2007) 86–95 * Corresponding author. Fax: +34 965903537. E-mail addresses: Teresa.Lana@ua.es (T. Lana-Villarreal), Roberto.Gomez@ua.es (R. Go ´mez). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.06.050