The formation of tungsten doped Al <inf>2</inf> O <inf>3</inf> /ZnO coatings on aluminum by plasma electrolytic oxidation and their application in photocatalysis

Abstract

Tungsten doped Al 2 O 3 /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 Na 2 WO 4 ·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 Al 2 O 3 , ZnO, metallic tungsten and WO 3 . Obtained results showed that incorporated tungsten does not have any influence on the absorption spectra of Al 2 O 3 /ZnO coatings, which showed invariable band edge at about 385 nm. The photocatalytic activity of undoped and tungsten doped Al 2 O 3 /ZnO coatings is estimated by the photodegradation of methyl orange. The photocatalytic activity of tungsten doped Al 2 O 3 /ZnO coatings is higher thanof undoped Al 2 O 3 /ZnO coatings; the best photocatalytic activity is ascribed to coatings formed in supporting electrolyte with addition of 0.3 g/L Na 2 WO 4 ·2H 2 O. Tungsten in Al 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.