Enhanced photocatalytic and photoelectrochemical activity in the ternary hybrid of CdS/TiO₂/WO₃ through the cascadal electron transfer

The composite of different semiconductor nanoparticles may facilitate the charge separation and transfer because the difference in the band edge positions creates the potential gradient at the composite interface. For this purpose, the CdS-TiO₂-WO₃ ternary hybrid was successfully synthesized and characterized for the structural, optical, and morphological properties by X-ray diffraction, diffuse reflectance UV/visible absorption spectroscopy, high-resolution transmission electron micrography, and energy-dispersive X-ray analysis. The photocatalytic activity was tested by monitoring the photoreduction of polyoxometalate (POM: PMo₁₂O ₄₀^3-) spectrophotometrically. The photoelectrochemical (PEC) property of the ternary hybrid electrode was also characterized by the linear sweep voltammetry, and the incident photon-to-current conversion efficiency was measured as a function of wavelength. The results of both the POM reduction and photocurrent tests indicated that the photocatalytic and PEC activities of the CdS-TiO₂-WO₃ ternary hybrid are much higher than those of bare CdS and any binary hybrids. The enhanced activity could be attributed to the cascadal electron transfer from CdS to TiO ₂ to WO₃ through the interfacial potential gradient in the ternary hybrid conduction bands. Such a cascadal electron transfer in the hybrid structure facilitated the charge separation and retarded the charge pair recombination. As a result, the CdS-TiO₂-WO₃ showed the maximum photocurrent density of 1.6 mA/cm² (at 0 V_Ag/AgCl) under visible light irradiation (λ > 495 nm), which is about 5 times larger than that of bare CdS and about 2-3 times larger than that of binary composites. The enhanced electron transfer within the CdS-TiO ₂-WO₃ composite was also confirmed by the electrochemical impedance spectroscopy.