Effect of Tb3+ doping on the photocatalytic activity of TiO2 coatings formed by plasma electrolytic oxidation of titanium

Tb doped TiO2 coatings are formed by plasma electrolytic oxidation (PEO) of titanium in water solution of 10 g/L Na3PO4.12H(2)O with addition of Tb4O7 powder. Surface morphology, chemical, and phase composition of obtained PEO coatings are investigated utilizing scanning electron microscopy equipped with energy dispersive x-ray spectroscopy and X-ray diffraction. Uniformly distributed Ti, O, P, and Tb are identified as the main constituents of the coatings. Diffraction patterns of obtained coatings show well pronounced reflections inherent to anatase phase of TiO2. Performed photoluminescence measurements indicate that incorporated Tb is in trivalent oxidation state. Photoluminescence emission spectra feature two overlapping regions: the first region is related to band of TiO2 host and the second region which features several emission bands is ascribed to f-f transitions of Tb3+ from excited level D-5(4) to lower levels F-7(J) (J = 3,4,5, and 6). The photocatalytic activity (PA) of Tb3+ doped TiO2 coatings is probed by measuring the photodegradation of methyl orange under simulated sunlight conditions, suggesting that the main factor affecting PA is concentration of Tb3+ ions incorporated into coatings. Tb3+ doped TiO2 coatings formed for short time have better PA than pure TiO2 coatings formed under the same conditions; the highest PA is observed for the coating formed after 1 min of PEO process. Presence of Tb3+ ions into TiO2 coatings in small concentration improves PA through effective suppression of electron/hole recombination