Photocatalytic Degradation of Methylorange Using TiO₂, WO₃ and Mixed Thin Films Under Controlled pH and H₂O₂

Wastewaters resulting from textile industry sector have a different chemistry compared with most of the other wastewaters. The different dyes in excess are usually very stable and even small quantities can have a major impact to the effluent. In order to treat these wastewaters, photodegradation is a largely investigated process that can be up-scaled. Photocatalysts based on wide band gap semiconductors can be used in heterogeneous photocatalysis and mostly reported are TiO₂ and WO₃. Under UV irradiation they form electron–hole pairs that produce active species that can oxidize the dye molecules. The electron–hole recombination represents the main cause for low efficiencies and is limited by the use of oxidant systems like H₂O₂. Doctor blade technique, a reproducible, up-scalable and low cost technique was used to obtain thin films. The reference dye, used in this experiment is methylorange in solution of 0.0125 mM, corresponding to average polluted water. In order to reduce the recombination in the catalysts, H₂O₂ is used. Another important aspect of the dye photocatalysis process, investigated in the paper is the adsorption of the dye molecule on the photocatalyst surface, strongly depending on pH which affects the dye's structure and the surface charge. Experiments are conducted at fixed pH values: 3, and respectively 7 covering values below and over the ZPC of the photocatalysts. The results show that TiO₂/WO₃ films have higher efficiency then the TiO₂ and WO₃ films, mainly due to the surface morphology of the films. By adding H₂O₂, higher efficiencies are obtained, confirming that the electron–hole pair recombination is reduced. From the point of view of pH, higher efficiencies are obtained in acidic solutions and the results are comparatively discussed considering the dye's ionic/neutral structure and the photocatalyst surface charge. The efficiency was calculated using UV-VIS spectrophotometer measurements of the solution and the thin films were characterized by AFM and XRD.