UV–vis versus visible degradation of Acid Orange II in a coupled CdS/TiO2 semiconductors suspension

TiO2, CdS and coupled CdS/TiO2 powders with CdS percentage varied from 5 to 50% (w/w) have been prepared by a sol–gel method. These pure and composite semiconductors were tested for the photocatalytic degradation of an anionic azo-dye, i.e. Orange II in aerated aqueous suspension under UV–vis and visible (λ > 400 nm) illumination. The photocatalytic efficiency of the CdS/TiO2 couples is found to strongly depend on the proportion of CdS. Under UV–vis light, CdS/TiO2 couples containing a low proportion of CdS are found to be as efficient as pure TiO2 which shows the best photocatalytic activity. Under visible illumination, these formers exhibit faster degradation rate than both isolated components of the composite photocatalyst. The results obtained under both illumination conditions are compared and discussed in order to elucidate the respective role of Acid Orange II and CdS in the photosensitization mechanism of TiO2.