C. Angeles-Chavez

Role of Yb3+ and Er3+ concentration on the tunability of green-yellow-red upconversion emission of codoped ZrO2:Yb3+–Er3+ nanocrystals

Strong green and red visible emissions were obtained from ZrO2:Yb3+–Er3+ nanocrystals synthesized by sol-gel method and annealed at 1000 °C for 5 h. The average crystallite size was ∼70 nm with tetragonal phase for total concentration lower than 3 mol % and cubic phase for concentration higher than 5 mol %. The color coordinate of the upconverted signal was tailored by controlling the dopant composition that change the red/green ratio dominated by the cross relaxation and energy back transfer process as was demonstrated theoretically and confirmed experimentally. Both coefficients were calculated, C51∼1.02×10−16 and C5b∼6.04×10−17, from the theoretical model based on the rate equations. The highest energy transfer efficiency was η∼64% for 2 mol % of Yb and 2 mol % of Er3+. However, for the highest upconverted signal was only η∼29% obtained for 2 mol % Yb and 1 mol % Er with effective decay time τeff∼438 μs for red and τeff∼290 μs for green band.

Brilliant blue, green and orange–red emission band on Tm3+-, Tb3+- and Eu3+-doped ZrO2 nanocrystals

Tm3+-, Tb3+- and Eu3+-doped ZrO2 nanocrystals were prepared by a facile precipitation method with a hydrothermal process. Structural characterization showed a crystallite size ranging from 30 to 40 nm, and monoclinic and tetragonal zirconia phases were observed depending on the dopant concentration. The monoclinic phase was dominant for 0.5 mol% of Tb3+ and Eu3+, and the tetragonal phase was 100% stabilized for 2 mol% of Tm3+ and Tb3+. The structure of emission bands associated with Eu3+ confirms the substitution of Zr4+ located at C1 and D2h symmetry sites for the monoclinic and tetragonal phases. The emission of three primary colours, red, green and blue, was obtained from Eu3+, Tb3+ and Tm3+, respectively, which makes this nanophosphor an excellent candidate for use in photonics applications. The emitted signal was analysed as a function of ion concentration and the optimum concentration was determined.

Blue-green upconversion emission in ZrO2:Yb3+ nanocrystals

Strong blue-green cooperative upconversion emission was observed under infrared excitation in 70 nm average crystallite size ZrO2:Yb3+ nanocrystals prepared by sol-gel process. The structural characterization was performed by x-ray diffraction and high resolution transmission electron microscopy suggesting that crystalline phase of nanoparticles is controlled by active ion concentration. The cooperative absorption coefficient η∼2.5×10−22 is four orders of magnitude larger than the ones reported from bulk crystals. The highest emission intensity was obtained from the doped sample at 4 mol % and was the result of the simultaneous relaxation of two excited Yb ions. Decay time of the upconverted signal τCUC≈0.310 ms is half from the near infrared effective decay time confirming the cooperative process among Yb ions. Such strong cooperative effect is explained in terms of interaction enhancement due to the diminishing of Yb–Yb separation promoted partly by the surface recombination of nanocrystals.

Synthesis and photoluminescence of Y 2 O 3 : Yb 3+ -Er 3+ nanofibers

The influence of the solvent ratio in the hydrothermal synthesis of co-doped Y2O3:Yb3+-Er3+, via CTAB complex, is presented. The nanophosphors for two different ethanol/water solvent ratios, 1:1 and 1:3, shown the following features: the samples exhibit cubic phase, TEM image of the sample synthesized using a 1:3 solvent ratio shows a homogeneous nanofibers morphology in opposition with the sample obtained from a 1:1 solvent ratio which reveals different morphologies prevailing big irregular particles. In both cases the absorption spectra are similar as well as the upconversion emission. These results suggest that the solvent ratio plays an important role in the morphology and on the luminescence properties of the nanostructured materials.

Annealing effect on the luminescence properties of BaZrO3:Yb3+ microcrystals

Yb-doped barium zirconate (BaZrO3:Yb3+) was synthesized by a hydrothermal method at 100 °C. It presents a cubic perovskite crystalline phase, which is stable between 100 and 100 °C. Upon increasing the annealing temperature, two major changes are observed. (1) The near infrared (NIR) absorption band of Yb3+ decreases, and an enhancement in both the NIR emission of Yb3+ single ion and visible emission of Yb3+ pairs occurs, which is attributed to the reduction in impurities. (2) The annealing process induces new absorption bands in the ultraviolet-visible (UV-vis) region that increase as annealing temperature increases. These results suggest that reduction in Yb3+ to Yb2+ is taking place. The visible emission is considered to be a cooperative emission from Yb3+ pairs. Under UV and visible excitation, no emission of Yb2+ ions was observed. This suggests that the broadband absorption in the UV-vis region might be related to F color centers, which are responsible for the inhibition of the Yb2+ ion emission.

Gold Particle Size Determination on Au/TiO2-CeO2 Catalysts by Means of Carbon Monoxide, Hydrogen Chemisorption and Transmission Electron Microscopy

Au/TiO2 and Au/TiO2-CeO2 catalysts were prepared by the sol-gel method and carbon monoxide, hydrogen chemisorption and TEM spectroscopy have been exploited to determine the size of gold particles. The gold nanoparticles (8.1 to 2.1 nm) were deposited by using the deposition-precipitation method. The XRD characterization shows the presence of anatase as the TiO2 crystalline phase; while by XPS spectroscopy, the presence of Au°, Au2O3, Ce3+ and Ce4+ species co-existing in the Au/TiO2-CeO2 catalysts is shown. The characterizations by TPD-CO as well as by TPD-H2 (temperature programmed desorption) showed that on catalysts containing cerium, the gold particle size can be determined with great accuracy by using these chemisorption methods. The gold particle size calculated from either the CO or H2 thermodesorption values is in good agreement with that obtained by High Resolution Transmission Electron Microscopy (HRTEM) and Scanning Transmission Electron Microscopy (STEM) analyses. It was proposed that the TPD-CO and/or TPD-H2 techniques could be helpful for the characterization of the gold particles by TEM; especially when the high contrast in the pictures of the supports containing CeO2 prevents the particle size from being determined.

Synthesis of Hydroxylapatite from Sand Dollar and β-Tricalcium Phosphate by Solid-State Method

Abstract Synthesis of porous hydroxylapatite from the sand dollar (Mellita eduardobarrosoi sp. nov.) and β-tricalcium phosphate (β-TCP) by solid-state reaction was performed. Reaction conditions were explored at different molar ratios of the precursors and at different temperatures. X-ray diffraction was used to determine the crystalline structure of the sample at molar ratio of 1.25 TCP : 1 sand dollar at 1473 K. The morphology of the product obtained under these conditions is a porous structure, which is a requirement for better clinical application of the hydroxylapatite as a biomaterial. Chemical analysis carried out by energy-dispersive spectroscopy on the product presented a molar ratio of Ca/P closer to the stoichiometric molar ratio of the pure hydroxylapatite. This crystalline structure was analyzed by transmission electron microscopy, and large single hydroxylapatite crystals were observed. Electron diffraction patterns in different directions are discussed.

Effect of solvent on the up- and downconversion emissions of Y 2 O 3 :Yb 3+ −Er 3+ nanofibers synthesized by a hydrothermal method

We report on the structural, morphological, and luminescent properties of Y2O3:Yb3+ (2%)–Er3+ (1%) nanofibers synthesized by a hydrothermal method as a function of the solvent composition ethanol/water. The average length and diameter of the nanofibers ranges from 1.1 to 2.3 μm, and from 50 to 110 nm, respectively. A cubic crystalline structure was obtained, and no effect of the solvent was observed. However, the increment of OHs, because of the increment of water, modifies the quality of the nanofibers. Such impurities improve the emission bands under 940 and 490 nm excitation, especially the red band, via multiphonon relaxation. Relaxation dynamics is explained, based on direct and back energy transfer, multiphonon relaxation, and cross-relaxation. The direct energy transfer coefficients (Cb2, Cb4, and Cb5) calculated by the proposed theoretical model point out the fact that upconverted emissions are notably favored by the increment of OHs. The energy backtransfer (C5b) and cross-relaxation (C51) coefficients depend only on the ion concentration and not on the OH content.

Green upconverted emission enhancement of ZrO2 : Yb3+–Ho3+ nanocrystals

ZrO2 : Yb3+–Ho3+ nanocrystals with different concentrations of codopants were synthesized with a 0.0082 molar ratio of surfactant Pluronic F-127 using a sol–gel micelle method, and annealed at 1000 °C for 5 h. The dominant crystallite phase was tetragonal, with an average crystallite size of 20 nm. The upconverted signal produced by the codoped nanocrystals is an almost pure green emission centred at 540 nm caused by the radiation emitted by the 5F4 + 5S2 → 5I8 relaxation of Ho3+. The highest efficiency for the Ho3+ green emission, with a decay constant (τeff) of 360 µs, was obtained with a 2/0.001 mol% Yb/Ho composition. This yields a colour coordinate of (0.289, 0.699) with a maximum luminance of 8.7 × 10−3 lm after excitation with a 970 nm continuous wave laser diode. These results can be explained in terms of the energy transfer (ET) from the donor (Yb3+) to the acceptor (Ho3+). The ET efficiency (η) for this sample was only 5%, although an increase to 42% is possible for larger concentrations of Ho3+. Highly concentrated samples have shortened decay times, which cause highly saturated samples to exhibit larger ET efficiency values.

Structural and chemical characterization of Yb2O3-ZrO2 system by HAADF-STEM and HRTEM.

ZrO2:Yb3+ nanocrystalline phosphors with high concentrations of ytterbium ions were prepared using the sol-gel method. X-ray diffraction, high-angle annular-dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy (HRTEM) were used to characterize the nanocrystalline phosphors annealed at 1000 degrees C. Unit-cell distortion and changes in the crystalline structure of the monoclinic zirconia to tetragonal zirconia, and subsequently cubic zirconia, were observed with increased Yb concentration. Yb ions were randomly distributed into the lattice of the crystalline structure. No segregation of Yb2O3 phase was observed. The substitution of Zr atoms by Yb atoms on different crystalline phases was confirmed by the experimental results and theoretical simulations of HRTEM and HAADF-STEM.