Structural and photoluminescent properties of ZrO<inf>2</inf>:Tb³⁺ coatings formed by plasma electrolytic oxidation

Abstract

Tb3+ doped ZrO2 coatings were formed on zirconium substrate by plasma electrolytic oxidation (PEO) in electrolyte containing Tb4O7 powder. The evolution morphology, chemical composition, and crystalline structure of formed coatings as a function of PEO time are probed by scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The concentration of Tb incorporated into coatings increases with the time of PEO processing. Obtained coatings are crystallized and composed of monoclinic and tetragonal phases of ZrO2. The ratio of monoclinic and tetragonal phases, as well as crystallite size, is controlled by PEO time and concentration of Tb in coatings. Photoluminescence (PL) emission spectra of Tb doped ZrO2 coatings feature two distinct regions. The first region is related to ZrO2 PL band with a maximum positioned at about 490 nm, while the second region features several sharp emission bands which can be attributed to f-f transitions of Tb3+ from excited level 5D4 to lower levels 7FJ (J = 2,3,4,5, and 6). PL excitation spectra of Tb doped ZrO2coatings are characterized by broad band region from 250 nm to 350 nm with a maximum at around 280 nm originating from 4f8 → 4f75d1 transition of Tb3+ ions. The evolution of PL emission spectra shows that with increasing PEO time, i.e. increasing concentration of Tb in coatings, sharp emission bands of Tb3+ increase, while PL intensity of broad emission band of ZrO2 host decreases. These results indicate the existence of energy transfer from ZrO2 host to Tb3+ dopant.