Visible-light-assisted photocatalytic removal of methylene blue over ternary ZnO/Zn2SnO4/rGO heterostructures: structural, optical, and charge-transport behavior

Abstract

Interfacial engineering of semiconductor heterostructures with conductive carbon materials offers a promising strategy for developing visible-light-assisted photocatalysts. In this study, ZnO/Zn2SnO4/rGO (ZSO/rGO) ternary composites with different rGO contents were synthesized via a hydrothermal method. XRD and Raman analyses confirmed the coexistence of hexagonal ZnO, cubic spinel Zn2SnO4, and rGO, while electron microscopy revealed oxide particles distributed in close contact with thin rGO sheets. XPS further indicated changes in the surface chemical and electronic environments after Zn2SnO4 and rGO incorporation. The optimized ZSO2.5 composite, containing 2.5 wt% rGO, exhibited the lowest photoluminescence intensity and a transient photocurrent response nearly twice that of binary ZnO/Zn2SnO4 (ZSO), supporting reduced radiative recombination and improved photoinduced carrier transport. ZSO2.5 achieved the highest methylene blue removal efficiency of 87.94% after 120 min of visible-light irradiation, with an apparent rate constant of 0.01764 min−1. Scavenger experiments indicated that ˙O2− made the most substantial contribution among the investigated reactive species, followed by ˙OH and h+. The enhanced performance is attributed to the combined effects of the ZnO/Zn2SnO4 interface, rGO-assisted light absorption and adsorption, and conductive pathways for carrier transport. However, the available results do not conclusively distinguish between Type-II and Z-scheme-like charge-transfer pathways. Because transformation products and mineralization were not directly analyzed, the observed process is described as methylene blue removal and decolorization rather than complete degradation. These findings demonstrate the potential of ZSO/rGO composites for visible-light-assisted water treatment.