Photocatalytic activity of ZnMn2O4/TiO2 heterostructure under solar light irradiation: Experimental and theoretical study

Abstract

This study focuses on the application of the hetero-junction ZnMn2O4/TiO2 as an efficient photocatalyst for the degradation of Rhodamine B (RhB), a cationic dye. ZnMn2O4 was synthesized by sol-gel auto-combustion, employing citric acid as fuel. The photocatalysts underwent characterization by X-ray diffraction, Fourier Transform infrared spectroscopy, Scanning Electronic Microscopy with integrated EDX and UV–Vis Diffuse Reflectance. Creating a p-n heterojunction could significantly boost photocatalytic efficiency. In this work, we use TiO2 as a n-type co-catalyst to enhance electron transfer efficiency. The ZnMn2O4 spinel exhibits direct optical transition at 1.97 eV, while TiO2 displays an indirectly allowed transition at 3.24 eV. Mott-Schottky plots revealed flat band potentials of -0.121 and +0.3 V/SCE for ZnMn2O4 and TiO2, respectively. Kinetic data of RhB adsorption were fitted using the pseudo-second order model. The heterojunction p-ZnMn2O4/n-TiO2 demonstrates a significant pollutant degradation rate, with 95% removal within 180 min under sunlight irradiation, surpassing the effectiveness of ZnMn2O4. To validate the experimental results, a theoretical investigation was conducted using density functional theory (DFT) and molecular dynamics (MD) simulations. Geometry optimization of the Rhodamine B molecule was studied in both gas and liquid phases, and various quantum chemical parameters were computed. Molecular dynamics simulations were additionally employed to examine the dynamic behavior of Rhodamine B adsorption on ZnMn2O4 (211) and TiO2 (101) surfaces in an aqueous environment.