Structural, photoluminescent and photocatalytic properties of TiO₂:Eu³⁺ coatings formed by plasma electrolytic oxidation

Graphical abstract: - Highlights: • TiO₂:Eu³⁺ coatings are formed by plasma electrolytic oxidation (PEO). • Photoluminescence is related to transitions of Eu³⁺ from level ⁵D₀ to levels ⁷F_J. • Eu³⁺ ions occupy non-inversion symmetry sites in the coatings. • PEO time is an important factor affecting photocatalytic activity. - Abstract: In this paper, we used plasma electrolytic oxidation (PEO) of titanium in water solution containing 10 g/L Na₃PO₄·12H₂O + 2 g/L Eu₂O₃ powder for preparation of TiO₂:Eu³⁺ coatings. The surfaces of obtained coatings exhibit a typical PEO porous structure. The energy dispersive X-ray spectroscopy analysis showed that the coatings are mainly composed of Ti, O, P, and Eu; it is observed that Eu content in the coatings increases with PEO time. The X-ray diffraction analysis indicated that the coatings are crystallized and composed of anatase and rutile TiO₂ phases, with anatase being the dominant one. X-ray photoelectron spectroscopy revealed that Ti 2p spin-orbit components of TiO₂:Eu³⁺ coatings are shifted towards higher binding energy, with respect to pure TiO₂ coatings, suggesting that Eu³⁺ ions are incorporated into TiO₂ lattice. Diffuse reflectance spectroscopy showed that TiO₂:Eu³⁺ coatings exhibit evident red shift with respect to the pure TiO₂ coatings. Photoluminescence (PL) emission spectra of TiO₂:Eu³⁺ coatings are characterized by sharp emission bands in orange–red region ascribed to f–f transitions of Eu³⁺ ions from excited level ⁵D₀ to lower levels ⁷F_J (J = 0, 1, 2, 3, and 4). The excitation PL spectra of TiO₂:Eu³⁺ coatings can be divided into two regions: the broad band region from 250 nm to 350 nm associated with charge transfer state of Eu³⁺ and the series of sharp peaks in the range from 350 nm to 550 nm corresponding to direct excitation of the Eu³⁺ ions. It is observed that the intensity of peaks in excitation and emission PL spectra increases with the concentration of Eu³⁺, but the peak positions remain practically unchanged. The ratio of PL emission for electric and magnetic dipole transitions indicates highly asymmetric environment around Eu³⁺ ions. The photocatalytic activity (PA) of TiO₂:Eu³⁺ coatings is evaluated by measuring the photodegradation of methyl orange under simulated sunlight conditions. It is shown that PEO time, i.e., the amount of Eu³⁺ incorporated into coatings is an important factor affecting PA; TiO₂:Eu³⁺ coating formed after 1 min of PEO time showed the highest PA.