Order and disorder effects in nano-ZrO2 investigated by micro-Raman and spectrally and temporarily resolved photoluminescence

Abstract

Pure and europium (Eu³⁺) doped ZrO₂ synthesized by an oil-in-water microemulsion reaction method were investigated by in situ and ex situ X-ray diffraction (XRD), ex situ Raman spectroscopy, high-resolution transmission electron microscopy (HRTEM), steady state and time-resolved photoluminescence (PL) spectroscopies. Based on the Raman spectra excited at three different wavelengths i.e. 488, 514 and 633 nm and measured in the spectral range of 150–4000 cm⁻¹ the correlation between the phonon spectra of ZrO₂ and luminescence of europium is clearly evidenced. The PL investigations span a variety of steady-state and time resolved measurements recorded either after direct excitation of the Eu³⁺ f–f transitions or indirect excitation into UV charge-transfer bands. After annealing at 500 °C, the overall Eu³⁺ emission is dominated by Eu³⁺ located in tetragonal symmetry lattice sites with a crystal-field splitting of the ⁵D₀–⁷F₁ emission of 20 cm⁻¹. Annealing of ZrO₂ at 1000 °C leads to a superposition of Eu³⁺ emissions from tetragonal and monoclinic lattice sites with monoclinic crystal-field splitting of 200 cm⁻¹ for the ⁵D₀–⁷F₁ transition. At all temperatures, a non-negligible amorphous/disordered content is also measured and determined to be of monoclinic nature. It was found that the evolutions with calcination temperature of the average PL lifetimes corresponding to europium emission in the tetragonal and monoclinic sites and the monoclinic phase content of the Eu³⁺ doped ZrO₂ samples follow a similar trend. By use of specific excitation conditions, the distribution of europium on the amorphous/disordered surface or ordered/crystalline sites can be identified and related to the phase content of zirconia. The role of zirconia host as a sensitizer for the europium PL is also discussed in both tetragonal and monoclinic phases.