Photocatalytic methylene blue degradation on multilayer transparent TiO2 coatings

Abstract

This paper presents the influence of coating thickness on the photocatalytic activity of TiO2 coatings. Methylene blue decomposition under UV light was tested. Transparent anatase TiO2 thin films were prepared by the sol-gel method using alcoholic solutions. Thin films were deposited on a borosilicate glass substrate by the multiple dip-coating technique across a much wider range than is typically described in literature, specifically, from 1 to 12 dip-coating cycles. The degradation of MB dissolvent in water was measured by UV-Vis spectroscopy. The structure, morphology and optical properties of thin film coatings were determined by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and spectroscopic ellipsometry (SE). The crystalline structure of TiO2 films was identified as the anatase phase, consisting of uniform spherical particles from 25 to 32 nm depending on the number of dip-coatings cycles. Decreasing roughness and optical band gaps were also observed in the produced oxide films with increasing numbers of dip-coating cycles. The reaction rate of discoloration of the dye solution by photocatalytic degradation is strongly related to the thickness of the layer, the number of layers and its morphology. The thicker the oxide layer, the faster the reaction; however, this relationship is not linear. In addition, in this article, a methylene blue discoloration reaction model was proposed which takes into account stoichiometry, reagent electric charge balance and the REDOX type of reaction.