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

Abstract

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 → ⁷F_J) 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.