Optimization of sol-gel synthesis parameters in the preparation of N-doped TiO ₂ using surface response methodology

Response surface methodology, Box–Behnken experimental design, was applied to investigate and find optimum synthesis parameters for preparing visible-light active nitrogen-doped titanium dioxide by sol–gel method. Nitrogen to titanium molar ratios, calcination temperature, and calcination time have been selected as the study parameters. X-ray diffraction crystal phase compositions, Brunauer–Emmett–Teller-specific surface area, and visible-light decolorization of methylene blue have been examined as experimental responses. A total of 15 tests were conducted, and all the samples have demonstrated different photoactivity under visible light. Furthermore, the important synthesis parameters that affect the three selected responses were investigated using the analysis of variance. Calcination temperature was found to be the most significant parameter that has direct influence on the crystal phase compositions, the specific surface area, and photoactivity of the synthesized catalysts. The model adequacy test and regression analysis have shown that the results were well fitted with quadratic model equations. Model predictions were in good agreement with experimental data with 96.68, 96.88, and 96.96% variability. N/Ti molar ratio of 6, calcination temperature of 400 °C, and calcination time of 3 h was found to be the optimum condition. Sample prepared at the optimum condition was characterized and compared with the undoped sample and results show the successful preparation of the nitrogen-doped titanium dioxide.