Ferromagnetic PEO coatings on titanium as photo-Fenton-like heterogeneous catalysts

Abstract

Ferromagnetic ‘Ti/Fe- and/or Co-containing TiO2-layer’ composites formed by plasma electrolytic oxidation (PEO) in electrolytes with dispersed particles of iron and/or cobalt hydroxides exhibited photocatalytic activity in methyl orange (MO) degradation under UV and visible light in the presence of hydrogen peroxide. The influence of Fe/Co ratio and additional annealing was studied on their chemical composition, optical, photocatalytic, and adhesion properties. With the proportion of cobalt in the coatings, their adhesion to titanium increased. After annealing, coatings with a higher proportion of iron had higher adhesion. The optical band gap energy Eg determined for direct allowed transitions was 1.37 eV and 1.61 eV for individual Fe-containing and Co-containing composites, respectively. In mixed Fe- and Co-containing PEO-composites, two direct allowed transitions were detected with Eg1 = 1.68 eV and Eg2 = 2.21–2.54 eV. Among initial PEO-coated samples, the most active and stable are Fe- and Co-containing PEO layers with a predominance of cobalt: MO degradation under 3-h irradiation with UV and visible light was 64 and 21 %, respectively. According to X-ray photoelectron spectroscopy, their surface layers contain a higher percentage of Fe2+ and Co2+ compared to Fe3+ and Co3+. Annealing the samples in air led to an increase in Eg values and a leveling off of photocatalytic activity as a result of the formation of crystalline NaTi2(PO4)3, FePO4, or CoPO4 phases and an increase in the proportion of oxidized iron. The main role in the degradation of MO was found to be played by hydroxyl radicals formed as a result of the interaction of hydrogen peroxide with photogenerated electrons.