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

Abstract

Core–shell CdS/TiO₂ structures are promising for solar-to-fuel conversion applications because their ideal type-II band alignment helps effective charge transfer to form the CdS⁺/TiO₂⁻ system. A better understanding of the charge carrier dynamics is critical to provide guiding principles for designing photoelectrochemical (PEC) devices. Hence, TiO₂ shell-thickness dependent charge carrier dynamics and competition between electron relaxation in CdS (e.g. recombination and trapping) and electron transfer from CdS to TiO₂ were investigated using ultrafast transient absorption (TA) spectroscopy. The results indicate that the CdS/TiO₂ nanocomposite with a molar ratio of 2 : 1 exhibits the highest electron transfer rate constant of _ET = 2.71 × 10¹⁰ s⁻¹, along with an electron relaxation rate of _CdS/TiO2 = 3.43 × 10¹⁰ 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/TiO₂ core–shell composites. However, the electron transfer rate decreases with increasing thickness of the TiO₂ shell consisting of aggregated nanoparticles. One possible explanation is that the CdS and TiO₂ form relatively larger, separate particles, or less conforming small particles, with poor interfaces with increasing TiO₂, thereby reducing electron transfer from CdS to TiO₂, 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.