Structural, photoluminescent and photocatalytic properties of TiO <inf>2</inf> :Eu ³⁺ coatings formed by plasma electrolytic oxidation

Abstract

In this paper, we used plasma electrolytic oxidation (PEO) of titanium in water solution containing 10 g/L Na 3 PO 4 ·12H 2 O + 2 g/L Eu 2 O 3 powder for preparation of TiO 2 :Eu 3+ 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 TiO 2 phases, with anatase being the dominant one. X-ray photoelectron spectroscopy revealed that Ti 2p spin-orbit components of TiO 2 :Eu 3+ coatings are shifted towards higher binding energy, with respect to pure TiO 2 coatings, suggesting that Eu 3+ ions are incorporated into TiO 2 lattice. Diffuse reflectance spectroscopy showed that TiO 2 :Eu 3+ coatings exhibit evident red shift with respect to the pure TiO 2 coatings. Photoluminescence (PL) emission spectra of TiO 2 :Eu 3+ coatings are characterized by sharp emission bands in orange-red region ascribed to f-f transitions of Eu 3+ ions from excited level 5 D 0 to lower levels 7 F J (J = 0, 1, 2, 3, and 4). The excitation PL spectra of TiO 2 :Eu 3+ coatings can be divided into two regions: the broad band region from 250 nm to 350 nm associated with charge transfer state of Eu 3+ and the series of sharp peaks in the range from 350 nm to 550 nm corresponding to direct excitation of the Eu 3+ ions. It is observed that the intensity of peaks in excitation and emission PL spectra increases with the concentration of Eu 3+ , but the peak positions remain practically unchanged. The ratio of PL emission for electric and magnetic dipole transitions indicates highly asymmetric environment around Eu 3+ ions. The photocatalytic activity (PA) of TiO 2 :Eu 3+ 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 Eu 3+ incorporated into coatings is an important factor affecting PA; TiO 2 :Eu 3+ coating formed after 1 min of PEO time showed the highest PA.