Visible photodecomposition of methylene blue over micro arc oxidized WO 3–loaded TiO 2 nano-porous layers

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WO 3-TiO 2 nano porous layers were synthesized by micro arc oxidation (MAO) process under different applied voltages in electrolytes containing sodium tungstate and phosphate salts with various concentrations. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive X-ray spectroscopy (EDS) techniques were employed to study phase structure and chemical composition of the layers. According to our data analysis, the nano porous layers consisted of anatase, rutile, and tungsten oxide phases with a varying fraction depending on the voltage and electrolyte concentration. Moreover, it was found that WO 3 not only dispersed in the TiO 2 matrix, but also doped into the TiO 2 lattice. Morphological and topographical investigations, carried out by scanning electron microscopy (SEM) and atomic force microscopy (AFM), revealed a porous structure with a rough surface. The pore size and the surface roughness of the layers increased with the applied voltage or the electrolyte concentration. The absorption edge of the layers, measured by a UV-Vis spectrophotometer, was observed to shift toward longer wavelengths when WO 3 was introduced to the TiO 2 layers. The band gap energy was determined as 3.21 eV and 2.88 eV for the MAO-grown TiO 2 and WO 3-TiO 2 systems, respectively. Furthermore, photocatalytic activity of the layers was also examined by measuring the decomposition rate of methylene blue under both ultraviolet and visible photo irradiations. It was observed that the composite layers were more photoactive as compared to MAO-synthesized pure TiO 2 layers. A possible growth mechanism was also proposed for the formation of WO 3-TiO 2 films via MAO process with emphasis on the electrochemical bases for the first time.