Imaging the photoinduced charge injection in CdS/TiO₂ nanoparticles by the sequential fluorescence mapping method

The combination of a sensitizer and TiO₂ nanoparticles forming a photocatalytic material is a central issue in many fields of applied photochemistry. The charge injection of emissive sensitizers into the conduction band of the semiconductor TiO₂ may form a photoactive region that becomes dark, or it has a very low emission signal due to the generation of sensitizer radicals. However, by sequential coupling of a selected photoredox dye, such as resazurin, the dark region may become fluorescent at the interfaces where the charge injection has taken place due to the concomitant formation of fluorescent resorufin by cascade electron transfer. Using this strategy and a total internal reflection fluorescence microscopy (TIRFM) image, the charge injection in TiO₂/CdS and SiO₂/TiO₂/CdS nanoparticles is investigated The method allows the charge injection efficiency of the excited CdS into TiO₂ to be evaluated qualitatively, explaining the differences observed for these photocatalytic materials in H₂ generation.