Optimization and modeling of efficient photocatalytic TiO₂-ZnO composite preparation parameters by response surface methodology

The major focus of this work is the design and optimization of the preparation conditions of binary titanium dioxide-zinc oxide (TiO₂-ZnO) composite using response surface methodology (RSM). Calcination temperature and ZnO weight percent (%) with respect to TiO₂ were chosen as the key parameters for optimizing the preparation conditions. The evaluation of photocatalytic nitrogen oxides (NO_x) removal efficiency was selected for modeling the RSM. Thirteen factorial design runs were first generated with one replicate and 5 center points in order to perform all the experiments. The experimental results on NO_x removal efficiency for the composites calcined at 747, 602, and 558 °C containing 5.48%, 3.48%, and 2.1% of ZnO with respect to the weight of TiO₂, designated as MaxT(Z%), MedT(Z%) and MinT(Z%) respectively, agree well with RSM predictions. The response coefficient difference between experimental and RSM results of MaxT(Z%), MedT(Z%), and MinT(Z%) were approximately 0.019, 0.044, and 0.11, respectively. The microstructural and optical properties as well the surface composition of MaxT(Z%), MedT(Z%), and MinT(Z%) were evaluated by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS), respectively. The results from the photodecomposition of NO_x gas under illuminated UV-light confirmed that the TiO₂-ZnO prepared with a 2.1 wt% of ZnO and calcined at 558 °C witnessed the best photocatalytic activity and the reasons for the excellent performance are corroborated with XRD, XPS, and DRS results. This study provides a simple and systematic approach for the optimization and modeling of the binary composite preparation for achieving enhanced photoactivity.