Synthesis of the Nanoscale Phosphor (Y_0.95Tb_0.05)₂O₂CO₃ by Alkalide Reduction and Its Phase Transformation to (Y_0.95Tb_0.05)₂O₃

Abstract

Tb 0.05 ) 2 O 2 CO 3 nanoparticles have been synthesized by subambient homogeneous reduction using alkalide solutions and subsequent oxidation. As synthesized, the material consists of free flowing agglomerates of ill-defined, amorphous, or subnanocrystalline nanoparticles. Thermal analysis shows that 2.83 CO 2 molecules are adsorbed per Y 2 O 3 formula unit in the washed product. The samples annealed at 200 °C or greater are crystalline, consisting of agglomerated nanocrystals. The nanocrystals grow from an average of 15.3-17.6 nm, the agglomerates decrease in size, and the surface area decreases from 88 to 78 m 2 /g as the annealing temperature is raised from 400 to 700 °C. Annealing at 800 °C results in a phase change from the carbonate to (Y 0.95 Tb 0.05 ) 2 O 2 . The crystallite size of the oxide phase increases from 21.4 to 22.6 nm, and the surface area increases from 82 to 86 m 2 /g as the annealing temperature is increased from 800 to 1000 °C. Transmission electron microscopy observations show that the increase in the surface area, even as the average crystallite size increases, is consistent with the breaking up of agglomerates and the creation of a highly textured material, probably due to the release of CO 2 during the phase transition. The washed unannealed product displays luminescence typical of the Tb 3+ ion. The photoluminescence intensity of the green 5 D 4 -7 F 4 transition increases with increasing annealing temperature, from 5% for the unannealed product to 16% for the nanophosphor annealed at 1000 °C.

The observation of quantum confinement enhancement of the luminescence of nanocystalline (Y0.95Tb0.05)2O3 synthesized by alkalide reduction

“…11 The 248 nm peak in the excitation spectra of directly synthesized (Y 0.95 Tb 0.05 ) 2 O 3 annealed at 500 C is not present when the material is annealed at higher temperature but is similar to that observed at 243 nm in the excitation spectra of (Y 0.95 Tb 0.05 ) 2 O 3 made by the indirect method and that of (Y 0.95 Tb 0.05 ) 2 O 2 CO 3 at 247 nm. 13 A very similar feature was reported for nanocrystalline Y 2 O 3 :Tb made by an unspecified microemulsion technique and is absent in the spectra of bulk samples; it has been suggested that the feature arises from quantum confinement occurring in nanocrystals less than 5 nm in diameter. 10 It should be noted that the model of quantum confinement suggested is markedly different to that widely invoked to explain the behavior of semiconductor nanomaterials and the reader is cautioned to read the referenced literature 10 to fully understand the analysis in this report.…”

“…Note that the annealing of the previously unheated material evolves a significant quantity of CO 2 , which, as we previously reported, results in the synthesis of the carbonate (Y 0.95 Tb 0.05 ) 2 O 2 CO 3 if the headspace in the sealed ampoule is insufficient, causing a buildup of many atmospheres of pressure. 13 Avoiding the production of the carbonate and instead producing the oxide requires one to limit the pressure that is developed in the ampule as CO 2 is desorbed from the starting material during heating. By decreasing the sample mass to ampoule volume ratio from 0.033 g ml À1 in the previous study, to 0.0057 g ml À1 in the current study, and consequently the pressure from the evolution of CO 2 during heating from $4.2 to $0.72 atm at 400 C, the carbonate phase is avoided and the oxide results instead.…”

“…The unannealed material was found to be composed of ill-defined agglomerates in which it is difficult to distinguish individual nanoparticles as was seen in our previous studies of lanthanide doped yttria. 13,16 Annealing the material results in the break-up of these agglomerates to form more defined nanoparticles. Typical dimensions of the individual particles are $4-8 nm following annealing at 500 C (Fig.…”

“…Annealing of larger samples in the given volume resulted in the carbonate rather than the oxide. 13 Electron micrographs were obtained on a JEM-1200EX transmission electron microscope (TEM) operating at 80 keV. Samples for TEM were dispersed in MeOH by sonication and deposited on FormvarÔ holey film/carbon coated copper grids.…”

“…Earlier, we reported the phase transformation of nanocrystalline (Y 0.95 Tb 0.05 ) 2 O 2 CO 3 as an indirect route to nanocrystalline (Y 0.95 Tb 0.05 ) 2 O 3 . 13 Here we present a direct route to (Y 0.95 Tb 0.05 ) 2 O 3 by post-alkalide reduction oxidation of Y 0.95 Tb 0.05 alloy nanoparticles and a comparison of the luminescent properties of the material made by indirect and direct synthetic routes.…”