Synthesis of RE3+ (RE = Ho, Tb, Pr)-Doped Alumina Ceramic Coatings by Plasma Electrolytic Oxidation of Aluminum: Investigation of Photocatalytic Performance

Porous, crystalline gamma-Al2O3 coatings with a thickness of (6 ± 0.5) μm and a uniform distribution of rare earth (RE) dopants are synthesized by plasma electrolytic oxidation of aluminum at a current density of 150 mA/cm2 in a boric acid and borax (BB) solution containing added RE oxide particles (Ho2O3, Tb4O7, and Pr6O11) at concentrations of 1, 2, and 4 g/L. The concentration of RE oxide particles in the BB solution determines the amount of RE elements incorporated into the coatings but does not significantly affect their surface morphology, crystal structure, or light absorption properties. The coatings exhibit high absorption in the middle/near-ultraviolet region, characteristic of Al2O3. Typical 4f-4f transitions of Ho3+, Tb3+, and Pr3+ are observed in the photoluminescence spectra. Photocatalytic evaluations using methyl orange degradation under simulated solar irradiation show that RE doping significantly enhances photocatalytic efficiency. Peak degradation efficiencies are achieved at a concentration of 4 g/L for all RE oxides. After 8 h of irradiation, maximum degradation reaches 88%, 92%, and 85% with pseudo-first-order rate constants (kapp) of about 0.274 h−1, 0.339 h−1, and 0.232 h−1 for coatings synthesized in BB with 4 g/L Ho2O3, Tb4O7, or Pr6O11, respectively. In comparison, the pristine Al2O3 coating achieves only about 50% degradation (kapp ≈ 0.087 h−1). Photoluminescence indicates that RE3+ ions serve as effective charge-carrier traps, suppressing electron–hole pair recombination. RE-doped Al2O3 coatings demonstrate exceptional structural stability and reusability over six cycles, highlighting their potential for sustainable wastewater remediation.