MgO/ZnO coatings formed on magnesium alloy AZ31 by plasma electrolytic oxidation: Structural, photoluminescence and photocatalytic investigation

Abstract

This paper presents the results of our recent investigation of MgO/ZnO coatings formed on AZ31 magnesium alloy by plasma electrolytic oxidation in phosphate-based alkaline electrolyte with varying concentration of ZnO particles. Surface morphology of obtained coatings is not significantly influenced by the addition of ZnO particles to the supporting electrolyte, while processing time has considerable influence on the morphology of formed coatings. Elemental mapping showed that elements are distributed rather uniformly across the obtained oxide coatings. The content of Zn increases with ZnO particle concentration in the supporting electrolyte as well as with PEO processing time. Incorporation of ZnO particles in obtained coatings was confirmed by X-ray diffraction and Raman spectroscopy. Photoluminescent emission spectra of MgO/ZnO coatings featured sharp band centered at about 380 nm and broad band centered at about 535 nm, with the leading contribution coming from ZnO deposited on the surface. Diffuse reflectance spectra revealed that absorption edge of MgO/ZnO coatings is positioned at about 385 nm. Photoactivity of obtained coatings increases with processing time and ZnO concentration up to 6 g/L.