Novel visible light active ternary TiO₂-ZnO-WO₃composites: Facile preparation strategy and performance evaluation using the response surface design

This work reports the design and optimization of the preparation conditions of high performance visible light active TiO₂-ZnO-WO₃ternary composite (TC) photocatalyst using minimum trial experiments based on response surface methodology (RSM). Calcination temperature, the weight percent (%) of ZnO and WO₃(with respect to TiO₂) and nitrogen oxides (NO_x) removal efficiency were chosen as the key variables toward optimizing the preparation conditions using RSM. Minitab Factorial design predicts trial experiments that are to be performed. The response coefficient difference between experimental and RSM and model verification approaches confirm that the TiO₂-ZnO-WO₃TC containing 3% of ZnO and 3% of WO₃and calcined at 600 °C (designated as MinT(Z3%,W3%)) is superior in photocatalytic NO_xremoval performances. The microstructural and optical properties as well the surface composition of the composites prepared at the Maximum, Medium and Minimum conditions as predicted by RSM were evaluated by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS) and the reasons for the excellent performance of MinT(Z3%,W3%) are corroborated with XRD, XPS, and DRS results. The generation of intermediate energy levels, formation of non-stochiometric solid solution like W_xTi_1-xO₂via substitution of W ionic states in the crystal lattice of TiO₂crystal and merging and reforming of particles contribute to visible light activation and high photocatalytic performance for MinT(Z3%,W3%). The possible photocatalytic mechanism informs that holes and bound hydroxyl radicals play predominant roles in the reaction pathways.