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

Abstract

Nb2O5:Eu3+ and Nb2O5:Sm3+ coatings have been successfully deposited by plasma electrolytic oxidation of niobium in 0.1 M phosphoric acid containing either Eu2O3 or Sm2O3 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. Nb2O5:Eu3+ and Nb2O5:Sm3+ oxide coatings are found to be in low crystallinity pseudohexagonal Nb2O5 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 Eu3+ ions from excited level 5D0 to lower levels 7FJ (J = 0, 1, 2, 3, and 4) and Sm3+ ions from excited level 4G5/2 to the lower levels 6HJ (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 Eu3+ and Sm3+, i.e. time of PEO, but the peak positions remain virtually unchanged. The fact that in Nb2O5:Eu3+ and Nb2O5:Sm3+ coatings electrical dipole transition is much more intense than magnetic dipole transition indicates that Eu3+ and Sm3+ ions occupied non-inversion symmetry sites in Nb2O5:Eu3+ and Nb2O5:Sm3+ coatings, respectively.