Photocatalytic behavior of suspended and supported semiconductor particles in aqueous media: Fundamental aspects using catechol as model molecule

The interaction and photoreactivity of catechol with both anatase and WO3 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.