Investigation of photoinduced electron transfer and time-resolved fluorescence spectroscopy of ZnS@CdS inverted core/shell quantum dots

Core/shell colloidal quantum dots (QDs) are promising candidates for applications in photocatalysis because of their intense and tuneable absorption in the visible range, low photobleaching and flexible functionalization chemistry [1]. These structures are typically classified as type-I (the shell has both a higher conduction band (CB) and a deeper valence band (VB)), inverse type-I (a wide-gap semiconductor core is overcoated with a shell of a narrower gap) and type-II (one semiconductor has a higher CB, and the other has a deeper VB). Due to the energy level difference existing in the core-shell nanostructures, the photogenerated charge carriers are confined in the core (type-I), in the shell (inverse type-I) or spatially separated on the core and shell, respectively (type-II). The type-I core-shell structure does not have strong photocatalytic properties. This configuration is the most common in bioanalytical applications since it offers the best confinement of the exciton and the highest rates of radiative recombination (i.e., brighter photoluminescence). The type-II core/shell structure has been widely used to improve the activities of photocatalysts, but only limited effort has focused on the inverse type-I core-shell structure photocatalysts.From the synthesis point of view, the wet chemistry method for the fabrication of core/shell structures usually involves complex routes, where the core and shell are formed in at least two steps. Precise control of synthesis parameters in each step is crucial in order to obtain core/shell particles with high quality, which largely restricts the scale-up synthesis of core/shell photocatalysts. In this work, the ZnS@CdS core/shell QDs, which is conventionally considered as inverse type-I structure, were prepared through a facile sequential one-pot method. Then the prepared samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), UV-Vis, photoluminescence and Fourier Transform Infrared (FT-IR) spectroscopy. We thereafter investigated the interfacial photoinduced electron transfer and related secondary photochemical behavior in the system via time-resolved fluorescence spectroscopy. The increased lifetime of ZnS@CdS nanohybrid (0.45±0.04ns) compared with single ZnS QD (0.41±0.03ns) suggested the charge transfer process between the ZnS core and CdS nanoshell. Finally, the hybrid photocatalytic performance was successfully demonstrated towards photodecomposition of methylene blue under sunlight irradiation.