Magnetically separable ZnFe2O4, Fe2O3/ZnFe2O4 and ZnO/ZnFe2O4 hollow nanospheres with enhanced visible photocatalytic properties

Magnetically separable ZnFe2O4-based composite hollow nanospheres with dimensions of 230 nm were successfully synthesized via an impregnating-calcination process using phenolic formaldehyde nanospheres (PFS) as a template. ZnFe2O4, Fe2O3/ZnFe2O4 and ZnO/ZnFe2O4 hollow nanospheres were synthesized by tuning the concentration of zinc salts. The samples were characterized by X-ray powder diffraction, energy dispersion spectroscopy, scanning electronic microscopy, transmission electron microscopy, nitrogen sorption measurements, UV-vis diffuse reflectance spectra and photoluminescence emission spectra. The results of photodegradation under visible light irradiation exhibited the order: ZnO/ZnFe2O4 > Fe2O3/ZnFe2O4 > ZnFe2O4 > bulk ZnFe2O4, and close investigation revealed that the high surface area, thin shell thicknesses, and matching heterostructures of the as-prepared ZnO/ZnFe2O4 and Fe2O3/ZnFe2O4 composites could dramatically improve the photocatalytic activity, which facilitates the efficient separation of photoinduced electron-hole pairs. Furthermore, the ZnFe2O4-based hollow nanospheres could be conveniently separated using an external magnetic field, and were chemically and optically stable after several repetitive tests. The study also provides a general and effective method towards the synthesis of composition-tunable hollow nanomaterials with sound heterojunctions that show a variety of potential applications.