MgAl and MgAl/ZnO coatings were prepared by plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy in aluminate electrolyte (5 g/L NaAlO₂) without and with addition of ZnO particles in various concentrations. The MgAl coating was partially crystallized and contained MgO and MgAl₂O₄ phases. The addition of ZnO particles to aluminate electrolyte had no significant effect on the surface morphology of formed coatings, while the Zn content increased with ZnO particle concentrations. X-ray diffraction confirmed the incorporation of ZnO particles in the coatings. The photodegradation of methyl orange (10 cm³ of 8 mg/L) was used to measure the photocatalytic activity (PA) of MgAl and MgAl/ZnO coatings. The PA of MgAl coating after 8 h of irradiation was around 58%, while the PA of MgAl/ZnO coatings formed in aluminate electrolyte with the addition of ZnO particles in concentrations of 4 g/L, 8 g/L, and 12 g/L were around 69%, 86%, and 97%, respectively.

The synthesis of Eu³⁺-doped MgAl oxide coatings containing MgO and MgAl₂O₄ was accomplished through plasma electrolytic oxidation of AZ31 magnesium alloy in aluminate electrolyte with the addition of Eu₂O₃ particles in various concentrations. Their morphological, structural, and above all, photoluminescent (PL) and photocatalytic activity (PA) were thoroughly investigated. PL emission spectra of MgAl oxide coatings feature characteristic emission peaks, which are ascribed to f–f transitions of Eu³⁺ ions from excited level ⁵D₀ to lower levels ⁷F_J. The charge transfer state of Eu³⁺ or direct excitation of the Eu³⁺ ground state ⁷F₀ into higher levels of the 4f-manifold are both attributed to the PL peaks that appear in the excitation PL spectra of the obtained coatings. The fact that the transition ⁵D₀ → ⁷F₂ (electrical dipole transition) in Eu³⁺-doped MgAl oxide coatings is considerably stronger than the transition ⁵D₀ → ⁷F₁ (magnetic dipole transition) indicates that Eu³⁺ ions occupied sites with non-inversion symmetry. Because of oxygen vacancy formation, the Eu³⁺-doped MgAl oxide coatings had a higher PA in the degradation of methyl orange than the pure MgAl oxide coating. The highest PA was found in Eu³⁺-doped MgAl oxide coating formed in an aluminate electrolyte with 4 g/L of Eu₂O₃ particles. The PA, morphology, and phase of Eu³⁺-doped MgAl oxide coatings did not change after several consecutive runs, indicating outstanding stability of these photocatalysts.

Ta₂O₅ coatings were created using micro-arc discharges (MDs) during anodization on a tantalum substrate in a sodium phosphate electrolyte (10 g/L Na₃PO₄·10H₂O). During the process, the size of MDs increases while the number of MDs decreases. The elements and their ionization states present in MDs were identified using optical emission spectroscopy. The hydrogen Balmer line H_β shape analysis revealed the presence of two types of MDs, with estimated electron number densities of around 1.1 × 10²¹ m⁻³ and 7.3 × 10²¹ m⁻³. The effect of MDs duration on surface morphology, phase and chemical composition, optical absorption, and photoluminescent, properties of Ta₂O₅ coatings, as well as their applications in photocatalytic degradation of methyl orange, were investigated. The created coatings were crystalline and were primarily composed of Ta₂O₅ orthorhombic phase. Since Ta₂O₅ coatings feature strong absorption in the ultraviolet light region below 320 nm, their photocatalytic activity is very high and increases with the time of the MDs process. This was associated with an increase of oxygen vacancy defects in coatings formed during the MDs, which was confirmed by photoluminescent measurements. The photocatalytic activity after 8 h of irradiation was around 69%, 74%, 80%, and 88% for Ta₂O₅ coatings created after 3 min, 5 min, 10 min, and 15 min, respectively.

MgAl oxide coatings composed of MgO and MgAl₂O₄ phases were doped with CeO₂ particles via plasma electrolytic oxidation (PEO) of AZ31 magnesium alloy in a 5 g/L NaAlO₂ water solution. Subsequently, particles of CeO₂ up to 8 g/L were added. Extensive investigations were conducted to examine the morphology, the chemical and phase compositions, and, most importantly, the photoluminescent (PL) properties and photocatalytic activity (PA) during the photodegradation of methyl orange. The number of CeO₂ particles incorporated into MgAl oxide coatings depends on the concentration of CeO₂ particles in the aluminate electrolyte. However, the CeO₂ particles do not significantly affect the thickness, phase structure, or surface morphology of the coatings. The PL emission spectrum of MgAl oxide coatings is divided into two bands: one in the 350–600 nm range related to structural defects in MgO, and another much more intense band in the 600–775 nm range attributed to the F⁺ centres in MgAl₂O₄. The incorporated CeO₂ particles do not have a significant effect on the PL intensity of the band in the red spectral region, but the PL intensity of the first band increases with the concentration of CeO₂ particles. The PA of MgAl/CeO₂ oxide coatings is higher than that of pure MgAl oxide coatings. The MgAl/CeO₂ oxide coating developed in aluminate electrolyte with a concentration of 2 g/L CeO₂ particles exhibited the highest PA. The MgAl/CeO₂ oxide coatings remained chemically and physically stable across multiple cycles, indicating their potential for applications.

ZnO/WO₃ coatings were synthesized by the plasma electrolytic oxidation of zinc in an alkaline phosphate electrolyte (supporting electrolyte, SE) with the addition of WO₃ particles or tungstosilicic acid (WSiA, H₄SiW₁₂O₄₀) at concentrations of up to 1.0 g/L. These coatings were intended for the decomposition of methyl orange (MO) through photocatalysis. The morphology, chemical composition, crystal structure and absorption properties of the coatings were investigated using scanning electron microscopy, energy dispersive X-ray spectroscopy, wavelength-dispersive X-ray spectroscopy, X-ray diffraction, photoelectron spectroscopy and diffuse reflectance spectroscopy. Under artificial sunlight, the PA of the coatings was investigated using MO decomposition. The photocatalytic activity (PA) of the ZnO/WO₃ coatings was higher than that of the ZnO obtained in SE. The decrease in the recombination rate of photo-generated electron/hole pairs due to the coupling of ZnO and WO₃ is related to the increased PA. The PA for ZnO and the most photocatalytically active ZnO/WO₃ was around 72% and 96%, respectively, after 8 h of irradiation. A mechanism for MO photo-degradation by the ZnO/WO₃ photocatalyst was also proposed.