Plasma electrolytic formation and characterization of MnWO4/WO3 film heterostructures

MnWO₄/WO₃ p-n heterojunction films were fabricated using a one-step method consisting of the plasma electrolytic oxidation (PEO) of titanium in homogeneous electrolytes containing paratungstate ions and stable water-soluble EDTA-chelated manganese. The influences of the formation current density and W:Mn molar ratio of the electrolyte, which was varied from 1:2 to 2:1, on the composition, morphology, and optical and photocatalytic properties of the resulting coatings were studied. X-ray diffraction analysis, scanning electron microscopy, energy dispersive X-ray analysis, Raman spectroscopy, and ultraviolet diffuse reflectance spectroscopy were used to characterize the formed composites. Regardless of the W:Mn ratio of the electrolyte, the coatings contained crystalline t-WO₃ and m-MnWO₄. Depending on the formation conditions, the optical band gap energies of the composites varied from 2.63 to 3.01 eV. The largest absorption red shift and lowest band gap energy were observed in the film composite formed in an electrolyte with W:Mn = 2:1, at a current density of 0.2 A cm⁻². Composites obtained in electrolytes with W:Mn ratios of 2:1 and 1:1 exhibited photocatalytic activity in the degradation of rhodamine C and methyl orange dyes in the presence of 10 mmol L^–1 H₂O₂ under ultraviolet and visible light irradiation. The role of hydrogen peroxide in this dye degradation on PEO-coated composites under light irradiation is discussed.