The formation of tungsten doped Al2O3/ZnO coatings on aluminum by plasma electrolytic oxidation and their application in photocatalysis

Abstract

Tungsten doped Al2O3 /ZnO coatings are formed by plasma electrolytic oxidation of aluminum substrate in supporting electrolyte (0.1 M boric acid + 0.05 M borax + 2 g/L ZnO) with addition of different concentrations of Na2WO4 center dot 2H(2)O. The morphology, crystal structure, chemical composition, and light absorption characteristics of formed surface coatings are investigated. The X-ray diffraction and X-ray photoelectron spectroscopy results indicate that formed surface coatings consist of alpha and gamma phase of A(2)O(3), ZnO, metallic tungsten and WO3. Obtained results showed that incorporated tungsten does not have any influence on the absorption spectra of Al2O3/ZnO coatings, which showed invariable band edge at about 385 nm. The photocatalytic activity of undoped and tungsten doped Al2O3 /ZnO coatings is estimated by the photodegradation of methyl orange. The photocatalytic activity of tungsten doped A(2)O(3)/ZnO coatings is higher thanof undoped Al2O3 /ZnO coatings; the best photocatalytic activity is ascribed to coatings formed in supporting electrolyte with addition of 0.3 g/L Na2WO4 center dot 2H(2)O. Tungsten in A(2)O(3)/ZnO coatings acts as a charge trap, thus reducing the recombination rate of photogenerated electron-hole pairs. The results of PL measurements are in agreement with photocatalytic activity. Declining PL intensity corresponds to increasing photocatalytic activity of the coatings, indicating slower recombination of electron-hole pairs.