Plasma electrolytic synthesis of amorphous mono- and bimetallic (Zn, Co) tungstates on titanium with enhanced solar light-driven photocatalytic activity in the degradation of methyl orange dye

Highly efficient amorphous photocatalytic coatings on titanium with TiO 2 -WO 3 -(Zn,Co)WO 4 heterostructures were obtained by a single-stage plasma electrolytic oxidation method in electrolytes based on sodium tungstate with the addition of sodium phosphate and chelated zinc(II) and/or cobalt(II). X-ray diffraction analysis revealed the amorphous nature of the coatings, and EDX and XPS methods confirmed the presence of titanium and tungsten oxides, as well as metal tungstates, in the coatings. The optical band gap values were 3.1, 2.4 and 2.5 eV for coatings with ZnWO 4 , CoWO 4 and Zn 0.5 Co 0.5 WO 4 , respectively. In the Mott-Schottky diagrams, the samples with pristine ZnWO 4 show a positive slope, indicating n -type semiconductor behavior, while samples with pristine CoWO 4 and bimetallic Zn 0.5 Co 0.5 WO 4 , show a negative slope, indicating the behavior of a p -type semiconductor. In the presence of coatings containing ZnWO 4 , CoWO 4 , and Zn 0.5 Co 0.5 WO 4 , the degradation efficiency of MO in a neutral environment reached 94%, 96%, and 98% within 180 min of sunlight irradiation. GC-MS and IR spectroscopy studies of the MO solution after photocatalytic experiments confirmed the cleavage of bonds in MO and allowed the identification of intermediate MO decomposition products.