Papers by Author: C. Ángeles-Chávez

Abstract: This work is devoted to the structural and morphological study of co-doped Y2O3:Yb3+-Er3+ nanophosphors, with 1 mol% of Er2O3 and 2 mol% of Yb2O3, synthesized by hydrothermal precipitation method via cetyltrimethylammonium bromide (CTAB) surfactant complex for different ethanol/water solvent ratios ranging from 1:1 to 1:10. Structural characterization results showed the prevalence of cubic phase independent of the solvent ratio although particle size does depend on the solvent ratio. Concerning to morphology, microscopic characterization indicates that the content of water used in synthesis is determinant in the morphology of resulting samples: a high content of water favored the formation of nanorods/nanofibers while a low water content give rise to heterogeneous morphology with irregular particles. Also, solvent ratio determines the consistency of nanostructured materials; an intermediate solvent ratio leads to nanofibers while a high solvent ratio let to obtain nanorods with length/diameter ratio ~3-5. These results show that solvent concentration plays an important role in particle size and morphology of nanostructured materials while the annealing temperature determines the phase composition and crystallite size.

Abstract: Mechanical milled powders of boron nitride were obtained. The microstructural characterization of these milled powders was carried out using X-ray diffraction technique. Insights on the nature of the crystalline phases obtained in these milled powders were obtained between comparisons of theoretical and experimental X-ray diffraction patterns. Observations on the phase transformations have been carried out using calorimetry and thermogravity experiments. Morphological and microstructural characteristics of nanocrystals are obtained using SEM and HRTEM instruments.

Abstract: The concentration luminescence quenching of the NIR emission of Yb3+ in nanocrystalline ZrO2 is studied. It is found that the quenching is dominated by cooperative energy transfer processes from isolated Yb3+ ions to Yb-Yb pairs (Yb dimers). The Yb dimer concentration depends on the crystallite phase and size, which on time depends on Yb concentration. An extended energy transfer model was developed to predict the IR and cooperative visible fluorescence emissions by taking in to account the crystalline phase, the nanocrystals size, and the geometrical construction of Yb dimers. Our model succeeds to fit simultaneously both experimental VIS and NIR emissions, and the corresponding interaction parameters are reported.

Abstract: 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.