Plasma Electrolytic Synthesis and Study of TiO2–WO3–ZnWO4 Film Heterostructures

Zn- and W-containing oxide heterostructures have been formed by plasma electrolytic oxidation of titanium in aqueous solutions of 0.1 mol/L Na2WO4 and 0.05 mol/L Zn(CH3COO)2 with addition of organic (0.05 mol/L H2C2O4 or 0.1 mol/L CH3COOH) and inorganic (0.05 mol/L H2SO4) acids. PEO coatings have been studied by X-ray diffraction, energy dispersive analysis, and scanning electron microscopy. Depending on the acid used, the coatings contain WO3 and/or NaxWO3 and Zn0.3WO3. Annealing in air at 500–700°С leads to crystallization of WO3 and ZnWO4 in their composition. WO3 polyhedra are formed on the surface of the PEO coatings obtained in electrolytes with oxalic and sulfuric acids. Elongated ZnWO4 crystals cover the surface of coatings formed in electrolytes with acetic acid. All resulting coatings exhibit photoactivity in the degradation of indigo carmine (10 mg/L, pH 5.9) under UV irradiation. The degree of degradation is 10, 38, and 35% for PEO coatings formed in electrolytes with the addition of CH3COOH, H2C2O4, and H2SO4, respectively. The photocatalytic activity of the samples can be regulated by controlling the shape, size, and composition of microcrystals growing on the surface of PEO coatings during annealing.