Thierry Le Mercier

Sustainable one-pot aqueous route to hierarchical carbon–MoO2 electrodes for Li-ion batteries

A route towards carbon–MoO2 core–shell spheres has been developed, through hydrothermal decomposition of ascorbic acid combined with precipitation of MoO2 nanoparticles. In this one-pot and green process, carbon spheres originating from ascorbic acid act as seeds for the in situ deposition of a corona made of 30 nm molybdenum dioxide particles. The as-obtained hierarchical nanostructured carbon–MoO2 core–shell spheres exhibit an ideal combination of electrical conductivity and lithium reactivity for Li-ion battery electrodes. This nanocomposite offers the opportunity to master the collector-active material and active material–electrolyte interfaces. Direct transfer “from the beaker to the battery” without any additives nor thermal treatment yields storage capacity values of ca. 600 mA h·g−1 at C/5 rate with excellent stability that challenges state-of-the-art molybdenum oxide-based batteries.

New luminescent rare earth activated oxynitridosilicates and oxynitridogermanates with the apatite structure

New luminescent oxynitrides based on the apatite structure doped with Eu2+ and Ce3+ have been prepared by ammonolysis of oxysilicate or oxygermanate precursors. The host compounds have the general formula La10−xSrx(Si/Ge)6O27−x/2−3y/2Ny. When doped with divalent europium or trivalent cerium they present blue, green, yellowish green or orange luminescence under UV/blue excitation. Remarkable long wavelength emissions centred beyond 500 nm are observed for the Ce3+ activated oxynitridosilicates and oxynitridogermanates, with compositions La8.9Ce0.1SrSi6N0.37O25.95, La8.9Ce0.1SrSi6N1.06O24.91 (λem = 545 nm) and La9.9Ce0.1Ge6N2.46O23.31 (λem = 590 nm). The treatment of precursor oxides with the required cationic ratios in NH3 gas was used as a convenient general method to prepare the oxynitride materials at moderate temperatures. This allowed decreasing the nitriding temperature from 1600–1400 °C to 1050–600 °C with respect to the method generally used for the synthesis of luminescent (oxy)nitride silicates, the solid state reaction between oxides, nitrides and carbonates under a N2 or N2–H2 flow.

Crystal structure of the end product of electrochemical lithium intercalation in V2(SO4)3

The crystal structure of Li2V2(SO4)3 obtained by electrochemical intercalation of lithium into V2(SO4)3 has been solved using high resolution synchrotron X-ray powder diffraction. Surprisingly, in light of its ‘soft chemistry’ synthesis, the structure of Li2V2(SO4)3 is derived from that of the starting material via rearrangement of strong V–O bonds, leading to a mixed corner sharing/edge sharing arrangement among the VO6 octahedra and SO4 tetrahedra. Results of in situ electrochemical Li intercalation/synchrotron powder diffraction studies, carried out in working batteries and with a time resolution of the order of minutes, indicate that the rearrangement is reversible, at least during the first cycle.

Phonon Scattering and Electron Doping by 2D Structural Defects in In/ZnO

In/ZnO bulk compounds have been synthesized using a simple solid-state process. In this study, both the structural features and thermoelectric properties of the Zn1-xInxO series with ultralow indium content (0 ≤ x ≤ 0.02) have been studied. High-angle annular dark-field scanning transmission electron microscopy analyses highlight that indium has the ability to create multiple basal plane and pyramidal defects that produce ZnO domains with inverted polarity starting from dopant concentrations as low as 0.25 atom %. Interestingly, the formation of parallel inversion boundaries consisting of InO6 octahedra in the ZnO4 tetrahedra matrix is responsible for phonon scattering while increasing electrical conductivity, thereby enhancing the thermoelectric properties. This effect of multiple extended two-dimensional defects on the thermoelectric properties of ZnO is reported for the first time with such low indium doping. On the chemistry side, the present results point toward a lack of In solubility in the ZnO structure. Moreover, this study is a step forward to the synthesis of other thermoelectric compounds where dopant-induced planar defects in bulk transition metal compounds have the potential to enhance both phonon scattering and electronic conductivity.

Highly Luminescent Composite Films from Core-Shell Oxide Nanocrystals

Luminescent nanocrystals can find interesting applications for the elaboration of light emitting transparent materials. The work described here is based on the use of lanthanide doped vanadate (YVO 4 :Eu) and phosphate (La, Ce, Tb)PO 4 -0, 7H 2 O nanoparticles grown through aqueous colloidal synthesis, with average sizes below 10 nm. The well-dispersed colloids are transparent and respectively exhibit red and green luminescence under U.V. excitation with high luminescence yields (20 – 50 %). Improvement of luminescence properties of the nanocrystals is achieved through the elaboration of core/shell nanostructures, obtained after the growth at the surface of an amorphous silica shell or a crystalline lanthanum phosphate shell. Surface derivatization is further achieved through the controlled growth of an organically modified silica coating using a functionalized silane precursor. Concentrated sols are obtained, which are highly luminescent and well-dispersed. They can be spin-coated on various substrates, leading to perfectly transparent and highly luminescent thin films. Multi-layers films and heating treatments are performed, leading to optimized materials.

Improving Blue Phosphor BAM:Eu for Fluorescent and LED Lighting

The effects of various fluoride fluxes on the crystallization of blue phosphor BaMgAl10O17:Eu2+ were characterized using a wide panel of experimental methods. LiF appeared as the most efficient, with a significant increase of the photoluminescence yield under excitation at 280 nm. Besides, the recrystallization resulted in a redistribution of the dopant among the three crystallographic sites, leading to a strong increase of the excitation in the near-UV domain. This allowed the emission to increase by 40 % under a 400 nm irradiation, typically of interest for possible LED-blue phosphor systems. At last, the shaping into hexagonal single crystals allowed both a reduction of the thermal degradation under air, and the discovery of a novel regeneration process at moderate temperature.