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

Abstract

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.