Eu-Doped Cerium Oxide Nanoparticles Studied by Positron Annihilation

A.V. Thorata, b, T. Ghoshala, b, M.A. Morrisa, b and P.M.G. Nambissanc, aMaterials Research Group, Department of Chemistry and Tyndall National Institute, University College Cork, Cork, Ireland bCentre for Research on Adaptive Nanostructures and Nanodevices, Trinity College Dublin, Dublin, Ireland cApplied Nuclear Physics Division, Saha Institute of Nuclear Physics, Kolkata 700064, India

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The defect characteristics of cerium oxide (CeO2) nanoparticles prepared through a solvothermal process and doped by europium to different concentrations ([Eu] = 0, 0.1, 0.5, 1, ..., 50 wt%) were studied by positron lifetime and coincidence Doppler broadening measurements. The particle sizes estimated from X-ray diffraction showed a reducing trend with increasing doping concentration except an increase during [Eu] = 0.1-1 wt%. The latter effect is attributed to the reduction of Ce4+ ions to Ce3+ resulting into the release of vacancies and formation of Ce3+-vacancy associates. The lattice parameter increased with the decrease in particle size. Quantum confinement effects were observed in optical absorption studies as increase of band gap in particles of sizes below 7-8 nm. The vacancy-type defects were investigated by positrons. A lifetime of 176 ± 4 ps less than 187-189 ps reported for positrons in bulk CeO2 reveals trapping of positrons in vacancy-type defects within the nanocrystallites. The defect-specific positron lifetime is admixed with that at the crystallite surfaces and it increased due to vacancy agglomeration at higher doping concentrations. Coincidence Doppler broadening studies indicated positron annihilation in defects surrounded by oxygen ions and the S-W plot showed the effect of quantum confinement through a peak-like kink or shoulder in the plot. Optical absorption studies have supported this observation.

DOI: 10.12693/APhysPolA.125.756 PACS numbers: 61.46.Df, 78.20.Ci, 61.72.jd, 12.38.Aw, 78.70.Bj