Optical properties of rare-earth oxides and oxynitrides, .... towards "second generation" UV absorbers

International audience"Second-generation" UV-blocking materials are needed. Using the fluorite-type rare-earth tungstates R6WO12 as starting reference compounds, anionic or cationic substitutions were achieved in (R,R’)-(W,Mo)-(O,N) systems, thus delimiting large solid solution domains. The continuous shift of the absorption edge in diffuse reflectance spectra towards lower energy values makes it possible to tune it at the required value of 3.1 eV (400 nm), with in several cases a steep enough slope. Attempts were also carried out in perovskite (A,R)-Ti-(O,N) systems (A = alkaline-earth, R = rare-earth) to isolate suitable oxynitride compositions

Optical properties of oxynitride powders

International audience(Oxy)nitride materials have attractive properties directly related to the role played by nitrogen. A commonly used synthesis method consists of the thermal nitridation of an oxide precursor in flowing ammonia. As a consequence of the anionic N3-/O2- substitution results an increase in the covalent character, illustrated by a shift of the absorption edge towards higher wavelength values. Thus, oxynitrides offer potentialities as optical materials in the domain of colored pigments, UV absorbers and visible-light photocatalysts

Thermochemistry of a New Class of Materials Containing Dinitrogen Pairs in an Oxide Matrix

International audienceA series of N 2 -containing perovskite phases was prepared in the La-(Ba)-Ti-O system in order to study the dinitrogen retention phenomenon from a thermochemical viewpoint. High-temperature oxide melt solution calorimetry was undertaken to determine the energetics of the corresponding startingoxynitrides, intermediate phases, and oxides. Calorimetric results show that nitrogen is weakly bound within the oxide matrix and most of the enthalpy of oxidation of the intermediate phase is devoted to its structure change between the starting perovskite structure and the formation of a layered-perovskiteLa2Ti2O7 oxide

Synthesis of Ni-poor NiO nanoparticles for p-DSSC applications

International audienceTo improve the performances of p-Dye Sensitized Solar Cell (p-DSSC) for the future, the synthesis of modified p-type nickel oxide semiconductor, commonly used as photocathode in such devices, was initiated with Ni3O2(OH)4 as precursor. This specific nickel oxyhydroxide was first characterized by X-ray photo-electron spectroscopy and magnetic susceptibility measurements. Then its thermal decomposition was thoroughly studied in order to control the particles size of the as-prepared NiO nanopowders. Low temperature decomposition in air of this precursor allows the formation of Ni1-xO nanoparticles with a large amount of Ni vacancies and specific surface areas up to 250 m2.g-1. Its ammonolysis at 250°C leads to nanostructured N-doped NiO (NiO:N) materials

Synthesis of Ni-poor NiO nanoparticles for DSSC-p applications

International audienceOver the last decade, p-type semiconductors (SC) have known a renewed interest. Indeed these materials may have potentialapplications for light-emitting diodes, transistors, solar cells, etc. Since the achievement of the first Dye Sensitized Solar Cells (DSSC) by Grätzel in 1991 a new generation of solar cells has been developed where the n-type SC is replaced by a p-type one. This leads to the photo-injection of holes instead of electrons in the circuit. To date nickel oxide (NiO) is the reference p-type semiconductor.However yields are still far from those of n-DSSC and many studies aim to replace NiO by other systems such as CuAlO2 , CuGaO2,CuCrO2 or NiCo2O4 nanoparticles. Following our recent synthesis of N doped ZnO with stabilization of p-type charge carriers, wefocus now on the preparation of N doped NiO nanoparticles to improve the p-type conductivity of NiO. We study here the chemicalreactivity of a nickel oxyhydroxide precursor under air and ammonia that conducts to nanostructured Ni-poor NiO

Study of the R-(Zr,W)-(O,N) (R = Y, Nd, Sm, Gd, Yb) oxynitride system

International audienceThe replacement of tantalum by the couple Zr/W within the RTa-O-N systems (R = Y, Nd, Sm, Gd, Yb), enables the preparation of novel oxide and oxynitride phases in the R-Zr-W-O-N system. R2Zr2xWxO7+x oxides exhibit the fluorite-type (x < 0.9) and scheelite (x 1) structures. Corresponding oxynitride compositions are of the fluorite-type and show different colors, for example in the case of ytterbium: pale yellow (x = 0.2 or 0.25), green (x = 0.5-0.8) and brown for the tungsten-rich samples (x = 0.9, 1). Photocatalytic activity measurements have been performed to investigate the overall water splitting behavior of these colored phases

Electronic Band Transitions in γ-Ge3N4

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. Support from Estonian Research Council grant PUT PRG 619 is gratefully acknowledged. The multi-anvil experiments at LMV were supported by the French Government Laboratory of Excellence initiative no ANR-10-LABX-0006, the Région Auvergne and the European Regional Development Fund (ClerVolc Contribution Number 478).Electronic band structure in germanium nitride having spinel structure, γ-Ge3N4, was examined using two spectroscopic techniques, cathodoluminescence and synchrotron-based photoluminescence. The sample purity was confirmed by x-ray diffraction and Raman analyses. The spectroscopic measurements provided first experimental evidence of a large free exciton binding energy De≈0.30 eV and direct interband transitions in this material. The band gap energy Eg = 3.65 ± 0.05 eV measured with a higher precision was in agreement with that previously obtained via XES/XANES method. The screened hybrid functional Heyd–Scuseria–Ernzerhof (HSE06) calculations of the electronic structure supported the experimental results. Based on the experimental data and theoretical calculations, the limiting efficiency of the excitation conversion to light was estimated and compared with that of w-GaN, which is the basic material of commercial light emitting diodes. The high conversion efficiency, very high hardness and rigidity combined with a thermal stability in air up to ~ 700 °C reveal the potential of γ-Ge3N4 for robust and efficient photonic emitters. © 2021, The Korean Institute of Metals and Materials. Published under the CC BY license.Euratom research and training programme 2014-2018 633053; Eesti Teadusagentuur ANR-10-LABX-0006, PUT PRG 619; ERDF; Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART2

Preparation of nitrogen doped zinc oxide nanoparticles and thin films by colloidal route and low temperature nitridation process

International audienceNitrogen doped zinc oxide (ZnO) nanoparticles have been synthesized using a colloidal route and low temperature nitridation process. Based on these results, 200 nm thick transparent ZnO thin films have been prepared by dip-coating on SiO2 substrate from a ZnO colloidal solution. Zinc peroxide (ZnO2) thin film was then obtained after the chemical conversion of a ZnO colloidal thin film by H2O2 solution. Finally, a nitrogen doped ZnO nanocrystalline thin film (ZnO:N) was obtained by ammonolysis at 250°C. All the films have been characterized by scanning electron microscopy, X-ray diffraction, X-Ray photoelectron spectroscopy and UV-Visible transmittance spectroscopy

Preparation of niobium based oxynitride nanosheets by exfoliation of Ruddlesden-Popper phase precursor.

International audienceA new oxynitride Ruddlesden-Popper phase K1.6Ca2Nb3O9.4N0.6 .1.1H2 O was synthesized by the topochemical ammonolysis reaction at 700 °C from the oxide Dion-Jacobson phase KCa2Nb3O10 in the presence of K2CO3. The oxynitride showed good stability with a little loss of nitrogen, even after a few months of exposure to air. Its crystal structure was solved by Rietveld refinement of X-ray powder diffraction data in space group P4/mmm and considering a two-phase mixture, due to the difference in the degree of hydration, with a = 3.894(2) Å and c = 17.90(8) Å for the most hydrated phase and a = 3.927(6) and c = 17.09(2) for the less one. Optical band gaps were measured by diffuse reflectance UV-Vis indicating a red shift of Eg to the visible region. The oxynitride layered perovskite was then protonated and exfoliated into nanosheets. TEM images and SAED patterns of the nanosheets proved that exfoliation was successful, showing lattice parameters quite compatible with the Rietveld refinement