Eu2+/Eu3+ doped Al2O3 coatings formed on tungsten by plasma electrolytic oxidation: photoluminescent properties and application in luminescence thermometry

Plasma electrolytic oxidation (PEO) was employed to synthesize Eu2+/Eu3+ doped Al2O3 coatings on tungsten substrates using an alkaline phosphate electrolyte containing Eu2O3 particles at ambient conditions, eliminating the need for high-temperature or reducing atmospheres. The structural, morphological, and photoluminescence (PL) properties of the coatings were investigated. X-ray diffraction (XRD) confirmed the formation of alpha Al2O3 as the dominant phase. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis demonstrated uniform coatings with increasing europium content at higher Eu2O3 concentrations. The PL spectra exhibited characteristic emissions from both Eu2+ (broadband 4f65d1 → 4f7) and Eu3+ (sharp 5D0 → 7FJ) transitions, confirming the coexistence of both valence states. Eu3+ excitation spectra feature a charge transfer band and 4f-4f transitions. There is a negligible energy transfer between Eu2+ and Eu3+ ions. The local coordination of Eu3+ and Eu2+ ions in Al2O3 coatings is analyzed using the PL spectra, showing that both ions are in low-symmetry sites. The luminescence intensity ratio (LIR) of Eu2+/Eu3+ emissions was utilized for optical thermometry from 300 K to 650 K, showing a quasi-Boltzmann temperature dependence. The temperature-dependent PL studies revealed high absolute sensitivity (0.0008 K−1 @ 575 K), demonstrating the potential of Eu2+/Eu3+ doped Al2O3 coatings for contactless luminescence thermometry applications. © 2025 Elsevier B.V.