Fabrication of a ternary CdS/ZnIn₂S₄/TiO₂ heterojunction for enhancing photoelectrochemical performance: Effect of cascading electron-hole transfer

A novel, three-dimensional, ternary CdS/ZnIn₂S₄/TiO₂ heterojunction has been fabricated via a three-step facile hydrothermal method. In this structure, one-dimensional TiO₂ nanorods were directly grown on conductive fluorine-doped tin oxide (FTO) substrates. Next, to form a ternary heterojunction of CdS/ZnIn₂S₄/TiO₂, ZnIn₂S₄ nanosheets were designed on the TiO₂ nanorods and sensitized by CdS nanograins. A systematic photoelectrochemical study shows that the photocurrent density of the ternary heterojunction architecture is as high as 1.4 mA cm^-2 at a potential of 0.1 V versus Ag/AgCl (under optimized conditions). A more detailed study shows that the photocurrent density is more than two times higher than that of a single CdS/TiO₂ heterojunction (0.615 mA cm^-2) and three times higher than that of ZnIn₂S₄/TiO₂ (at 0.1 V vs. Ag/AgCl). This excellent photoelectrochemical performance is ascribed to the way that the band structure of TiO₂ nanorods synergistically cascades with ZnIn₂S₄ and CdS, which allows for the absorption of a wider portion of the solar spectrum and improves the effective separation of the generated electron-hole pairs. Electrochemical impedance spectroscopy (EIS) studies also reveal the significant changes in both the interface resistance and the charge transfer resistance of the CdS/ZnIn₂S₄/TiO₂ heterostructure. This can be attributed to the efficient hierarchical cascading that occurs during the electron-hole transfer from the excited CdS to TiO₂ through the enlarged interface of ZnIn₂S₄ upon visible light illumination.