Synergistic Role of NiO Loading and Oxygen Vacancies in p–n NiO/ZnO Heterojunctions for Visible-Light-Driven Photocatalytic Hydrogen Production

Hydrogen generation through photocatalytic water splitting is a promising approach, yet achieving high efficiency and long-term stability using cost-effective photocatalysts remains a significant challenge. In this study, a series of NiO/ZnO photocatalysts with varying NiO loadings (1 wt.%, 3 wt.%, and 5 wt.%) were synthesized via the impregnation method to construct a type-II heterojunction system. The physicochemical properties were characterized by X-ray Diffraction (XRD), UV–Vis Diffuse Reflectance Spectroscopy (DRS), X-ray Photoelectron Spectroscopy (XPS), Photoluminescence Spectroscopy (PL), and Field Emission Scanning Electron Microscopy–Energy Dispersive X-ray Spectroscopy (FESEM–EDX). The incorporation of NiO not only formed an efficient p–n heterojunction but also introduced abundant surface oxygen vacancies (Vo), which acted as active sites for charge trapping and water molecule activation, thereby suppressing electron–hole recombination. As a result, the optimized 1NiO/ZnO photocatalyst exhibited the highest hydrogen evolution rate of 170 μmol·h -1 ·g -1 under visible light irradiation (λ ≥ 400 nm) after 4 h of reaction, using methanol as a sacrificial agent. These findings highlight the synergistic role of p–n heterojunction formation and enhanced O v in promoting charge separation and surface reactivity, offering a promising strategy for designing efficient and stable photocatalysts for sustainable hydrogen production. • NiO/ZnO p–n heterojunction photocatalysts were fabricated via a simple impregnation route. • Optimal NiO loading induces abundant oxygen vacancies and efficient charge separation. • The p–n heterojunction significantly suppresses electron–hole recombination. • The optimized 1NiO/ZnO exhibits a hydrogen evolution rate of 170 μmol·h -1 ·g -1 . • Synergistic heterojunction and defect engineering enable sustainable photocatalytic H 2 production.