Photoluminescence properties of HfO2:RE3+ (RE = Eu, Sm, Tb, Dy) coatings formed by plasma electrolytic oxidation of hafnium in an electrolyte containing RE oxide particles

Abstract

This study investigates the morphology, composition, and photoluminescence (PL) properties of HfO2 coatings formed on hafnium by plasma electrolytic oxidation (PEO), with the addition of rare-earth (RE) oxide particles (Eu2O3, Sm2O3, Dy2O3, and Tb4O7) at various concentrations. The PEO coatings exhibited a characteristic microstructure with micropores and solidified molten oxide domains. EDS analysis confirmed the successful incorporation of RE elements from the electrolyte into the HfO2 layer, with the concentration of RE elements in the coating increasing as their concentration in the electrolyte increased. XRD showed that the pure HfO2 coating was exclusively monoclinic. Incorporation of RE3+ induced a phase transformation, stabilizing the tetragonal HfO2 at the expense of the monoclinic phase. The HfO2:RE3+ coatings displayed intense, characteristic PL emissions corresponding to the RE3+ intra-configurational 4f-4f transitions. Excitation occurs through both direct RE3+ 4f-4f transitions and a highly efficient charge transfer (CT) process from O2− to RE3+ (RE = Eu, Sm, Dy) and the 4f8 → 4f75d1 transition of Tb3+. Analysis of the asymmetric ratio (R) for Eu3+ and Sm3+, as well as the yellow/blue (Y/B) ratio for Dy3+ and green/blue (G/B) ratio for Tb3+, confirmed that the RE3+ ions occupy low-symmetry (non-centrosymmetric) sites in the monoclinic HfO2 phase, which is responsible for the highly efficient PL emission. The slight decrease in these R and Y/B values, as well as the increase in G/B values with increasing RE concentrations in HfO2 coatings, is related to the stabilization of the higher-symmetry tetragonal phase. This work demonstrates an effective PEO method to produce advanced RE-doped HfO2 phosphors on hafnium substrates with tunable crystalline phases and strong PL properties.