Optimizing the hydrothermal post-treatment process for a TiO₂/WO₃ hybrid coating to enhance the photocatalytic degradation of methylene blue under visible light

WO3 nanoparticles were grown hydrothermally on porous titania (TiO2) PEO-coated on Ti in an alkaline electrolyte using pulsed direct current mode. To achieve the best photodegradation of methylene blue (MB) mediated by the PEO/HT hybrid coatings under visible illumination, hydrothermal parameters including precursor concentration, temperature, and time were optimized. In this sense, increasing the Na2WO4 concentration resulted in a decrease in porosity percentage and an increase in average particle size. At an optimal precursor concentration of 0.005 M, the TiO2/WO3 hybrid coating exhibited 67% degradation compared to -47% degradation for a bare TiO2 coating. Based on the LaMer diagram, the growth mechanism of WO3 crystals under the hydrothermal process was described. The calculated band gap of the PEO/HT coatings reduced from 3.0 eV to 2.68 eV with an increase in W-based precursor content through the hydrothermal process. Consequently, the improvement in photocatalytic performance was attributed to the participation of more WO3 nanoparticles and the expansion of the light absorption range of TiO2 up to the visible light range. Increasing the hydrothermal temperature led to an increase in crystallinity and particle size due to growth, but a decrease in porosity. The optimal photocatalytic performance was achieved for the hybrid coatings at a concentration of 0.005 M, with a hydrothermal temperature of 120 degrees C and treatment time of 18 h, resulting in -88% degradation of methylene blue after 360 min of irradiation. The study of photoactivity on a pollutant model indicates that the resulting activity is primarily influenced by the morphology of hybrid coatings, followed by their specific surface area.