An in situ gelatin-assisted hydrothermal synthesis of ZnO–reduced graphene oxide composites with enhanced photocatalytic performance under ultraviolet and visible light

Abstract

A simple route for preparation of ZnO nanosphere–reduced graphene oxide (RGO) composites by an in situ gelatin-assisted hydrothermal synthesis was investigated. The chemical composition, morphology and structural features of the ZnO and ZnO–RGO samples were characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman microscopy, X-ray photoelectron spectroscopy (XPS), thermogravimatric analysis (TGA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption–desorption specific surface area measurements (BET) and diffuse reflectance absorption spectroscopy (DRS). The results showed that ZnO nanoparticles were well dispersed on the RGO nanosheets which serve as ZnO nanocrystal growing supports and the morphology controller. It was first found that through controlling the incorporated contents of RGO with the assistance of gelatin in hydrothermal synthesis, the morphology of ZnO, for example nanoflowers, nanorods and nanospheres, could be controlled. The ZnO–RGO composites were used as photocatalysts for degradation of methylene blue (MB) under ultraviolet (UV) and visible light irradiation. The results suggested that RGO improves the MB degradation performance under both UV and visible light irradiation, and the kinetic rate constants were both improved to 3 times higher by RGO incorporation. It was found that the mechanism of photocatalysis for ZnO–RGO composites under visible light irradiation is different from that under UV light. Under UV light, RGO serves as an electron reservoir to capture or shuttle photogenerated electrons from the ZnO and reduces electron–hole pair recombination; while under visible light, RGO behaves as a photosensitizing semiconductor and electron transformer to ZnO. Moreover, the large surface area and excellent electron transport of RGO also have some positive effects on the improvement of the photocatalytic performance of the composites.