Ultraviolet- and visible-light photocatalytic and sonophotocatalytic activities toward Congo red degradation using Ag/Mn₃O₄ nanocomposites

Herein, silver/manganese(II,III) oxide (Ag/Mn₃O₄) composites were used as photocatalysts to degrade the Congo red (CR) dye through photocatalysis (light) and sonophotocatalysis (light + ultrasound) under ultraviolet (UV)- and visible-light irradiation. Mn₃O₄ was prepared via a conventional sol–gel method, and the composites containing Ag were synthesized with various Ag-to-Mn₃O₄ molar ratios via a hydrothermal technique. The as-synthesized photocatalysts were characterized by X-ray diffraction, X-ray fluorescence, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, surface plasmonic resonance, Raman spectroscopy, nitrogen adsorption–desorption isotherm measurements, scanning electron microscopy, and transmission electron microscopy. The characterization results revealed that the Mn₃O₄ nanoparticles were successfully coupled with Ag nanoparticles and that the concentration of Ag nanoparticles influenced the composites’ physicochemical properties. The catalytic activity was observed under optimum pH conditions. A substantial enhancement of the removal of CR was observed when ultrasound was used during the photodegradation process. The samples exhibited excellent stability in four-cycle reusability tests. The addition of a scavenger was also studied to observe the contribution of reactive species.