Nb₂O₅ and AlNbO₄ coatings formed by plasma electrolytic oxidation of niobium: synthesis, characterization, and photocatalytic activity

This study describes the formation of efficient photocatalysts on a niobium substrate using plasma electrolytic oxidation (PEO) in an alkaline electrolyte both with and without the addition of NaAlO₂. The process of PEO was analyzed by using optical emission spectroscopy and real-time imaging. Scanning electron microscopy, energy-dispersive x-ray spectroscopy, x-ray diffraction, x-ray photoelectron spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy, and photoluminescence were used to characterize the formed photocatalysts. Photodegradation experiments in an aqueous solution of methyl orange were conducted to assess the photocatalytic activity (PA) of formed coatings. The PA of Nb₂O₅ coatings generated in pure alkaline electrolytes was found to be lower than that of AlNbO₄ coatings formed in alkaline electrolytes with added NaAlO₂. The significant PA of both Nb₂O₅ and AlNbO₄ coatings, due to their strong ultraviolet absorption, was associated with high concentrations of oxygen vacancy. After 8 h of irradiation, the PA for AlNbO₄ coating formed after 15 min in an alkaline electrolyte with 2 g/L NaAlO₂ reached around 81%. Furthermore, the study revealed that by adding ZnO and Eu₂O₃ particles into the alkaline electrolyte with + 2 g/L NaAlO_2, coatings such as AlNbO₄/ZnO and AlNbO₄:Eu³⁺/Eu2⁺ with improved PA comparing to AlNbO₄ alone could be produced.