Orange–red photoluminescence of Nb2O5:Eu3+, Sm3+ coatings formed by plasma electrolytic oxidation of niobium

Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings have been successfully deposited by plasma electrolytic oxidation of niobium in 0.1 M phosphoric acid containing either Eu 2 O 3 or Sm 2 O 3 powder. Surface morphology and chemical composition of obtained coatings are correlated to the mechanism of oxide growth during the PEO process. X-ray photoelectron spectroscopy revealed that Nb and P in the oxide coatings are in pentavalent, while Eu and Sm are in trivalent oxidation state. Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ oxide coatings are found to be in low crystallinity pseudohexagonal Nb 2 O 5 form. Photoluminescent emission spectra of either type of 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) and Sm 3+ ions from excited level 4 G 5/2 to the lower levels 6 H J (J = 5/2, 7/2, 9/2, and 11/2). It is observed that the intensity of peaks in excitation and emission photoluminescent spectra increases with the concentration of Eu 3+ and Sm 3+ , i.e. time of PEO, but the peak positions remain virtually unchanged. The fact that in Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings electrical dipole transition is much more intense than magnetic dipole transition indicates that Eu 3 + and Sm 3 + ions occupied non-inversion symmetry sites in Nb 2 O 5 :Eu 3+ and Nb 2 O 5 :Sm 3+ coatings, respectively.