A method for effectively regulating the green emissions of ZnO through NiS@NiO/rGO

Highlights: • ZnO/NiS@NiO/rGO composite was successfully prepared by facial hydrothermal method. • The influence mechanism of NiS@NiO and rGO on the green emissions of ZnO was revealed. • The green emissions sources of ZnO/NiS@NiO/rGO were clarified. In this study, a ZnO/NiS@NiO/rGO (ZNNG) composite was characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy (SEM). Compared with simple ZnO, ZNNG exhibited stronger green emissions, but no ultraviolet (UV) emission peak was observed. Through a comprehensive analysis of ultraviolet–visible (UV–vis) absorption spectra, Mott Schottky (MS) plots, electron paramagnetic resonance (EPR) spectra, and X-ray photoelectron spectroscopy (XPS) results, it was proved that the stable green emissions of ZnO and its composites originated from the quenching of electrons between Zn interstitial (Zn_i) and surface oxygen defects. The introduction of NiS@NiO caused the conduction band (CB) and valence band (VB) of ZnO to change to lower energy states, effectively increasing the absorption intensities of ZnO in the visible light, while introducing numerous oxygen-related defects on the surface of ZnO. Further addition of rGO effectively promoted the migration of electrons from the CB to the Fermi energy (E_f). These effects effectively increased the green emission of ZnO and reduced the UV emission. Finally, a proper mechanism for the near-zero band gap NiS@NiO/rGO that can significantly enhance the green emission of ZnO was proposed.