A p-n junction NiO-CdS nanoparticles with enhanced photocatalytic activity: A response surface methodology study

Effects of coupling, calcination temperature and mole ratio of NiO and CdS nanoparticles (NPs) were studied on the enhancement of their photocatalytic activities towards methylene blue (MB). The coupled NiO-CdS NPs system had better activity than the monocomponent semiconductors. Among the NiO NPs calcined at different temperatures, the best activity was obtained for the prepared one at 200 °C. In changing the mole ratio of the semiconductors in the coupled systems, the best activity was obtained for mole ratio of 1:4 NiO/CdS. Electrochemical studies (electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV)) of the semiconductors-modified carbon paste electrodes (CPE) confirmed the best charge transfer for the NiO-CdS-CPE system with respect to the CPE containing the single semiconductors. The anodic charge transfer (αa) values of 0.65 and 0.52 for the NiO-CPE and NiO-CdS-CPE, confirmed that the rate constant of the anodic process for NiO-CdS-CPE is 164 times of the NiO-CPE. This was resulted to the better photocatalytic activity of the coupled system in MB photodegradation. Individual and interaction effects of the experimental factors in MB photodegradation were studied by designing the experiments by response surface methodology (RSM). The quadratic model was well fitted to the experimental data (with values of 0.9920 and 0.9833 for R2 and R2adj). The best degradation removal of MB was obtained at pH 3.5, catalyst dosage of 0.9 g L−1, 3.2 mg L−1 of MB at irradiation time of 83 min.