Shell-thickness dependent electron transfer and relaxation in type-II core–shell CdS/TiO2 structures with optimized photoelectrochemical performance

Abstract

Shell-thickness dependent charge carrier dynamics and enhanced photoelectrochemical performance were studied in uniform core–shell CdS/TiO ₂ composites.

Abstract

Core–shell CdS/TiO ₂ structures are promising for solar-to-fuel conversion applications because their ideal type-II band alignment helps effective charge transfer to form the CdS ⁺/TiO ₂ ⁻ system. A better understanding of the charge carrier dynamics is critical to provide guiding principles for designing photoelectrochemical (PEC) devices. Hence, TiO ₂ shell-thickness dependent charge carrier dynamics and competition between electron relaxation in CdS ( e.g. recombination and trapping) and electron transfer from CdS to TiO ₂ were investigated using ultrafast transient absorption (TA) spectroscopy. The results indicate that the CdS/TiO ₂ nanocomposite with a molar ratio of 2 : 1 exhibits the highest electron transfer rate constant of k̄ _ET = 2.71 × 10 ¹⁰ s ⁻¹, along with an electron relaxation rate of k̄ _CdS/TiO2 = 3.43 × 10 ¹⁰ s ⁻¹, resulting in an electron transfer quantum efficiency of Q _ET = 79%, which also corresponds to the best PEC hydrogen generation in the CdS/TiO ₂ core–shell composites. However, the electron transfer rate decreases with increasing thickness of the TiO ₂ shell consisting of aggregated nanoparticles. One possible explanation is that the CdS and TiO ₂ form relatively larger, separate particles, or less conforming small particles, with poor interfaces with increasing TiO ₂, thereby reducing electron transfer from CdS to TiO ₂, which is supported by SEM, and TEM data and consistent with PEC results. The thickness and morphology dependence of electron transfer and relaxation provides new insight into the charge carrier dynamics in such composite structures, which is important for optimizing the efficiency of PEC for solar fuel generation applications.