Synthesis and Structural Transformation of Luminescent Nanostructured Gd2O3:Eu Produced by Solution Combustion Synthesis

ABSTRACTIn this work we explore the uniqueness of solution combustion synthesis (SCS) technique to produce luminescent nanostructured materials with metastable phases. We synthesized Gd2O3:Eu with the high-temperature phase and induced phase transformation toward the room temperature phase to investigate the effects of structural transformation on the luminescent properties. SCS is based on exothermic redox reactions that undergo self-sustaining combustion, yielding powders composed of agglomerates of nanocrystals with typical dimensions of tens of nanometers. Synthesis of materials through SCS occurs in conditions far from thermodynamic equilibrium and, due to the high temperatures achieved during combustion, metastable crystallographic phases can be formed. Eu-doped Gd2O3 was obtained with base-centered monoclinic structure and average nanocrystal size of 35 nm as determined by Debye-Scherre analysis. Phase transformation to the cubic structure was induced by isothermal annealing at 1000 oC for up to 152 hrs and followed by x-ray diffraction (XRD). Luminescence excitation and emission spectra were obtained as a function of annealing time. The transformation from monoclinic to cubic structure was followed by the behavior of the (111) monoclinic/(222) cubic intensity ratio. The ratio value for the as-prepared material is 6, decreasing fast to 3 after 5 hrs. annealing, and reaching a value of 0.1 after 152 hrs. Concomitant to the structural transformation, nanocrystal size was followed for both crystalline phases. The average nanocrystal size for the cubic phase increases from 27 to 47 nm from 1 to 152 hrs., respectively. On the other hand, nanocrystals with the monoclinic phase remained with a constant size around 38 nm. Overall, variation in size is small due to the low connectivity among nanocrystals resulting from the low isostatic pressure employed to prepare the pellets, together with the non-uniform shape of the agglomerates. Photoluminescence excitation spectra are dominated by a broad centered near 278 nm and assigned to the O2-Eu3+ charge transfer band. Photoluminescence emission results present the 5D0-7FJ (with J = 0-4) transitions of Eu3+ ions. The behavior of these bands was investigated as a function of annealing time and subsequently related to the structural changes.