Process modelling and analysis of plasma electrolytic oxidation of titanium for TiO2/WO3 thin film photocatalysts by response surface methodology

Plasma electrolytic oxidation of titanium in 12-tungstosilicic acid solution was investigated as a model reaction. The influence of the process parameters (concentration of 12-tungstosilicic acid solution, current density, and time) on the selected process response (efficiency of the methyl orange photodecolorization on the TiO2/WO3 coatings) was studied. A mathematical model was developed using a second-order response surface model with a central composite design incorporating the above-mentioned process parameters. The statistical analysis of experimental data indicates that a concentration of 12-tungstosilicic acid solution; process time, interaction between concentration and process time, and interaction between concentration and quadratic of process time have a significant effect on the methyl orange photodecolorization. Besides, the results show that obtained data were adequately fitted into the second-order polynomial model, since the calculated model F value (39.28) is higher than the critical F value. The values of coefficient of determination (R2 = 0.960) and adjusted coefficient of determination (RAdj.2 = 0.937) are close to 1, indicating a high correlation between the observed and the predicted values. This model can also be useful for setting an optimum value of the process parameters for achieving the maximum efficiency of the methyl orange photodecolorization. Under the optimum conditions established in the region of experimentation (C = 1.14 · 10− 3 M, j = 153 mA/cm2, and t = 58.9 s), a 38.6% (test duration of 8 h) efficiency of the methyl orange photodecolorization is obtained.