Novel visible light active ternary TiO2-ZnO-WO3 composites: Facile preparation strategy and performance evaluation using the response surface design

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This work reports the design and optimization of the preparation conditions of high performance visible light active TiO2-ZnO-WO3 ternary 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 (NOx) 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-WO3 TC containing 3% of ZnO and 3% of WO3 and calcined at 600 °C (designated as MinT(Z3%,W3%)) is superior in photocatalytic NOx removal 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 Wx Ti1−x O2 via substitution of W ionic states in the crystal lattice of TiO2 crystal 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.