Synthesis and characterization of Al2O3/ZnO coatings formed by plasma electrolytic oxidation

• Al2O3/ZnO coatings are formed by plasma electrolytic oxidation (PEO).
• Amorphous alumina transforms to gamma and alpha alumina with increase of PEO time.
• Concentration of ZnO in oxide coatings increases with PEO time.
• Observed PL bands originate from oxygen defects in Al2O3 and ZnO.
• About 50% of methyl orange was photodecomposed after 12 h of irradiation.

Mixed Al2O3/ZnO coatings are prepared by conventional DC plasma electrolytic oxidation (PEO) of aluminum in boric acid and borax solution containing ZnO nanoparticles. Scanning electron microscopy equipped with energy dispersive x-ray spectroscopy, x-ray diffraction, and Raman spectroscopy were employed to monitor morphological, structural, and chemical changes of obtained oxide coatings. It was found that chemical and phase compositions strongly depend on PEO time. Photoluminescence and photodegradation of methyl orange served as a tool to estimate potential application of Al2O3/ZnO coatings. Photoluminescence measurements showed that clearly observable bands inherent to Al2O3 and ZnO are present in formed coatings, pointing at oxygen vacancies as the main source of photoluminescence. Longer PEO processing times proved beneficial for enhanced photocatalytic activity.