Variation in defects and properties in composite of ZnO and α-Fe2O3 for sustainable wastewater treatment

Abstract

Defects in nanostructures play a pivotal role in determining their properties and performance in the desired applications. Herein, the defect states and structural properties of the bi-metal oxide composite of ZnO and α-Fe2O3 (ZF-W) are varied by annealing the composite at different temperatures. The changes in defects, structures, and phase are evaluated thoroughly using transmission electron microscopy, x-ray diffraction, photoluminescence, and Mössbauer spectroscopy techniques. The defect-rich ZF-W composite is found to be composed of defect-deficient ZnFe2O4 attaining the equilibrium state when as-synthesized ZF-W is annealed at 500 °C [ZF-W(500)]. Further annealing at 1000 °C, ZF-W(1000), a non-stoichiometric and highly defected ZnFe2O4 is evidenced in the composite. The changes in the composite with the annealing temperature are correlated with the cationic migration and evolution of defect states. Moreover, the transition associated with the vacancy defects, which trapped the excited electron and dispel the free electrons, thereby inhibiting fast electron–hole pair recombination, is corroborated from the photoluminescence spectra. When implemented for methyl blue adsorption/degradation without the assistance of any external sources, the degradation efficiency of ZF-W, ZF-W(300), ZF-W(500), and ZF-W(1000) is found to be 86%, 84%, 68%, and 82%, respectively. The prepared samples are highly stable and can be used repeatedly without losing effectiveness. The simultaneous evolution of defects and structural properties of the composite are attributed for the variation in methyl blue adsorption/degradation. The present study reveals the importance of defects present in the mixed metal oxide composite in obtaining high-performance dye degradation/adsorption properties for sustainable wastewater treatment.