Structural and photoluminescent behavior of Rare Earths doped Gd2O3 nanoparticles embedded into SiO2 amorphous matrix obtained by sol-gel method for energy conversion

Abstract

This work presents the synthesis, structural and spectroscopic characterization, and luminescence (upconversion and downshifting) properties of SiO₂-Gd₂O₃ materials doped with different proportions of Er³⁺, Yb³⁺, and Eu³⁺. The materials were synthesized via the sol-gel method and heat-treated for 8 h at 900 °C. XRD showed intense peaks corresponding to crystalline cubic Gd₂O₃ (space group Ia₃), confirmed by FTIR and Raman results, with no additional peaks, indicating successful doping. Rietveld refinement revealed a microstrain of 0.221–0.524 %, and crystallite sizes between 55.38 and 163.37 nm, confirmed by TEM images. TEM revealed crystalline Gd₂O₃ nanoparticles (NPs) dispersed in amorphous silica. UV–Vis spectra showed, absorption bands for both the matrix and dopants, including the O²⁻(2p) → Eu³⁺(4f⁶) charge transfer band, suggesting energy transfer. Using the Kubelka-Munk equation, the band gap was calculated to be between 4.98 and 5.18 eV. The emission spectra show the intraconfigurational 4f-4f bands of the Eu³⁺ ion between 469 and 710 nm arising from the ⁵D₂ → ⁷F₀, ⁵D₀ → ⁷F_0-5 transitions. Under 266 nm excitation, the Eu^{3+ 5}D₀ ^{→ 7}F₀ transition split, indicating two symmetry sites for Eu³⁺ ions (C₂ and S₆). Besides, the emission bands related to the Er³⁺ ion are also observed at 502, 512, 537, and 653 nm, associated with the transitions ⁴F_7/2 → ⁴I_15/2, ⁴H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, e ⁴F_9/2 → ⁴I_15/2, respectively. Under 800 nm excitation, the materials presented the upconversion phenomena, with Yb³⁺ ions acting as sensitizers. Strong green emission from Er³⁺ (525–575 nm) was observed. These results suggest potential applications energy conversion as multicolor emitters in the photonic and biophotonic fields for instance for singlete oxygen generation for PDT.