Core–shell structured Fe3O4@SiO2@CdS nanoparticles with enhanced visible-light photocatalytic activities

Abstract

A chelating-assistant growth route of CdS on the surface of Fe₃O₄@SiO₂ nanoparticles (NPs) was used to form a magnetically recoverable photocatalyst. Characterization by transmission electron microscopy, X-ray powder diffraction, Raman spectroscopy and a vibrating sample magnetometer reveals monodispersed superparamagnetic Fe₃O₄@SiO₂@CdS NPs (ca. 250 nm) have been formed with a uniform CdS shell thickness of ca. 20 nm, Brunauer–Emmett–Teller (BET) surface area of ca. 25.1 m² g⁻¹ and saturation magnetization of 22.02 emu g⁻¹. This composite shows excellent photocatalytic activity towards the degradation of methylene blue (MB) under visible-light irradiation with a reaction constant of 1.95 × 10⁻² min⁻¹ in spite of the low weight percentage of CdS (9.15%) as determined by the energy-dispersive X-ray spectroscopy, which is higher than those observed on Fe₃O₄@CdS (53.30%, 1.22 × 10⁻² min⁻¹) and CdS NPs (3.33 × 10⁻³ min⁻¹). Furthermore, Fe₃O₄@SiO₂@CdS can be quickly magnetically recovered within 30 s by applying an external magnetic field near the solution after the photocatalytic process, which still preserves the excellent particle monodispersity with the slightly reduced CdS thickness (ca. 15 nm), while Fe₃O₄@CdS and CdS NPs are severely photo-corroded and aggregated. The maximized specific surface area from uniform coating and the efficient generation of activated oxygen species from CdS shells might be responsible for the enhanced photoactivity.