Utilization of Novel (KNbO3)1−x(Ba2FeNbO6)x (x = 0.1, 0.2, 0.3) Solid Solutions for Efficient Photo-Assisted Fenton Degradation of Methylene Blue Dye

Novel (KNbO₃)_1−x(Ba₂FeNbO₆)_x (x = 0.1, 0.2, 0.3) solid solutions corresponding to K_0.82Ba_0.18Fe_0.09Nb_0.91O₃, K_0.64Ba_0.36Fe_0.18Nb_0.82O₃, and K_0.46Ba_0.54Fe_0.27Nb_0.73O₃ compounds have been synthesized via molten salt method. X-ray diffraction confirms the formation of solid solutions, while transmission electron microscopy combined with energy-dispersive spectroscopy results demonstrates a homogeneous distribution of elements. The obtained solid solutions crystallized in a cubic crystal structure, whereas the parent KNbO₃ possesses an orthorhombic structure. The wide bandgap semiconductor KNbO₃ transformed into a visible-light-active material, with its bandgap energy reduced from 3.56 eV to ≈2.4 eV. The substitution of K in KNbO₃ with Ba is responsible for structural modification from orthorhombic to cubic symmetry, whereas both structural modification and the substitution of Nb with Fe correlated with optical properties. The photocatalytic activities of all obtained solid solutions are improved compared with the parent KNbO₃ and Ba₂FeNbO₆ compounds for photocatalytic degradation of methylene blue (MB) dye. Among the series of solid solutions, K_0.82Ba_0.18Fe_0.09Nb_0.91O₃ photocatalysts show the highest MB removal efficiency owing to its relatively higher surface area, suppressed charge carrier recombination, and more negative conduction band edge. Moreover, K_0.82Ba_0.18Fe_0.09Nb_0.91O₃ photocatalyst (0.1 g) combined with hydrogen peroxide (H₂O₂) to form a novel photo-Fenton system, achieving almost complete degradation of 100 mL of 10 mg L⁻¹ MB dye in 30 min.