The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

The origins of the superior catalytic activity of poorly crystallized Ir-based oxide material for the OER in acid is still under debate. Here, authors synthesize porous IrMo oxides to deconvolute the effect of Ir oxidation state from short-range ordering and show the latter to be a key factor

Synthèse colloïdale et assemblages 2D/3D contrôlés de nanoparticules d'oxydes

Nanotechnology has become a key domain of technology in XXI century. The great development of the synthetic approaches toward nanoparticles (NPs) with desired composition, size and shape expose the potential of their use as building blocks for larger scale structures. It allows fabrication of functional materials and devices directly from colloids by bottom-up approach, thus involving possibility of material design over several length scales. The process is referred to NPs assembly or self-assembly and leads to materials with varying architectures as for instance 1D (rods), 2D (films) or 3D (superlattices or gels). However most of 3D assemblies are limited to the micrometric scale and are difficult to control. Practically the only route allowing preparation of macroscopic 3D structures from NPs is their gelation and preparation of aerogels. As an alternative, NPs can be embedded in some matrix creating bulk composite material, with homogenously distributed non-aggregated NPs.Therefore, this work is devoted to development of materials with different dimensionalities for various applications from metal oxides NPs (mainly Y3Al5O12:Ce and Li4Ti5O12). The first part describes the syntheses of YAG:Ce and LTO NPs by glycothermal approach. In the case of YAG:Ce, the reactions conditions were appropriately adjusted in order to obtain non-aggregated nanocrystals (NCs) of few nanometers. The colloidal solution containing such NCs with different concentration was used for fabrication of thin films with controllable thickness by spin-coating method. Contrary, the synthesis of LTO led to aggregated NPs with hierarchical structuration which was highly beneficial for Li-ion batteries. The large surface area and porosity ensured efficient exchange of Li ions between electrolyte and anode material. Furthermore, the YAG:Ce NCs were used for preparation of macroscopic monoliths with high porosity and transparency. For that reason, colloidal solution of NCs was gelled by the abrupt change of solvent dielectric constant. The gels were further supercritically dried yielding YAG:Ce NPs-based aerogels with high porosity and transparency. The same approach turned o be appropriate for other systems like GdF3 or hybrid aerogels of YAG:Ce and GdF3.Alternatively, YAG:Ce NPs were incorporated into silica aerogels forming robust macroscopic and highly transparent aerogels exhibiting properties of incorporated NPs. They served for novel type of sensors for low-energy ionizing radiation in liquids and gases. Their high porosity assured well-developed contact between radioactive emitter and the scintillator ensuring good harvesting of radioactive energy.La nanotechnologie est devenue un domaine clé de la technologie du XXIe siècle. L’important développement des approches pour la synthèse des nanoparticules (NPs) avec une composition, une taille et une forme désirées rend compte du potentiel de leur utilisation comme « blocs de construction » pour des structures de plus grande échelle. Cela permet d’envisager à la fois la fabrication de matériaux fonctionnels et de dispositifs directement à partir de colloïdes par approche ascendante et la conception de matériaux sur plusieurs échelles de grandeur. Le procédé utilise l'assemblage ou l'auto-assemblage de NPs et conduit à des matériaux avec des architectures différentes notamment 1D (bâtonnets), 2D (films) ou 3D (super-réseaux ou gels). Cependant, la plupart des assemblages 3D sont limités à l'échelle micrométrique et sont difficiles à contrôler. Pratiquement, la seule voie permettant la préparation de structures 3D macroscopiques à partir de NPs est la gélification et la préparation d'aérogels. Une voie alternative consiste à disperser les NPs dans une matrice, conduisant ainsi à un matériau composite massif, avec des NPs non agrégées distribuées de manière homogène.Le présent travail est consacré au développement de matériaux à partir de NPs d'oxydes métalliques (principalement Y3Al5O12: Ce et Li4Ti5O12) de différentes dimensions et pour diverses applications. La première partie de ce travail décrit la synthèse de NPs de YAG: Ce et de LTO par approche glycothermale. Dans le cas du YAG: Ce, les conditions de réaction ont été ajustées de façon appropriée pour obtenir des nanocristaux (NCs) non agrégés de quelques nanomètres. Des solutions colloïdales de différentes concentrations contenant de tels NCs ont été utilisées, pour la fabrication par la technique de « spin-coating », de films minces avec une épaisseur contrôlable. A l’inverse, la synthèse de LTO conduit à des NPs agrégées dans une structure hiérarchique très bénéfique pour les batteries au lithium. La grande surface spécifique et la porosité du matériau obtenu assurent en effet un échange efficace des ions lithium entre l'électrolyte et le matériau d'anode.Par ailleurs, les NCs de YAG: Ce ont été utilisés pour la préparation de matériaux monolithiques de grande taille avec une porosité et une transparence élevées. Pour cela, la solution colloïdale de NCs a été gélifiée par le changement brusque de la constante diélectrique du solvant de dispersion des NCs. Les gels ainsi obtenus ont été par la suite séchés de manière supercritique, donnant ainsi des aérogels à base de NPs de YAG:Ce, avec une porosité et une transparence élevées. La même approche s'est avérée appropriée pour d'autres systèmes à base de NPs de GdF3 ou de mélanges de NPS de YAG: Ce et de GdF3.Alternativement, les NPs de YAG: Ce ont été incorporées dans des aérogels de silice formant ainsi des aérogels macroscopiques robustes et hautement transparents présentant les propriétés des NPs incorporées. Ces aérogels composites ont été utilisés en tant que nouveaux types de capteurs pour les rayonnements ionisants de basse énergie dans les liquides ou les gaz. Leur porosité élevée permet un contact optimal entre l'émetteur radioactif et le scintillateur assurant ainsi une bonne récupération de l'énergie radioactive

Programming crack patterns with light in colloidal plasmonic films

Abstract Crack formation observed across diverse fields like geology, nanotechnology, arts, structural engineering or surface science, is a chaotic and undesirable phenomenon, resulting in random patterns of cracks generally leading to material failure. Limiting the formation of cracks or “programming” the path of cracks is a great technological challenge since it holds promise to enhance material durability or even to develop low cost patterning methods. Drawing inspiration from negative phototropism in plants, we demonstrate the capability to organize, guide, replicate, or arrest crack propagation in colloidal films through remote light manipulation. The key consists in using plasmonic photothermal absorbers to generate “virtual” defects enabling controlled deviation of cracks. We engineer a dip-coating process coupled with selective light irradiation enabling simultaneous deposition and light-directed crack patterning. This approach represents a rare example of a robust self-assembly process with long-range order that can be programmed in both space and time

Chemical and structural characterization of amorphous and crystalline alumina obtained by alternative sol–gel preparation routes

Aluminum oxide is one of the most commonly used materials in the industry. It is used in the field of catalysis, refractories, and optics. Despite the fact that there are many techniques available, there is still a great challenge in obtaining a material with desired and designed properties. Nevertheless, there is a great flexibility in making customized alumina materials with desired physicochemical properties synthesized by sol–gel methods. This work consists in characterizing the physicochemical properties of sol–gel synthesized aluminum oxide using different sol–gel preparation routes. Three different sols were obtained by using organic precursors and underwent thermal treatment. The structure (Middle Infrared Spectroscopy, Diffused Reflectance Infrared Spectroscopy, X-ray Diffraction, Magic Angle Spinning Nuclear Magnetic Resonance) and microstructure (Scanning Electron Microscopy with Electron Dispersive Spectroscopy) tests of the materials were carried out. The specific surface area was determined by using the Brunauer–Emmett–Teller (BET) method. Thermal analysis was performed for all the powders, in order to analyze the specific temperature of materials transformation

From Nanoparticle Assembly to Monolithic Aerogels of YAG, Rare Earth Fluorides, and Composites

International audienc

Design and Application of High Optical Quality YAG:Ce Nanocrystal-Loaded Silica Aerogels

International audienceThe intrinsic properties of silica aerogels make them well suited for applications requiring high surface area. Therefore, the dispersion of functional nanoparticles (NPs) in these highly porous structures gives access to materials for wide range of applications such as catalysis, energy storage or sensing. The last one is particularly interesting if such composites possess good optical quality. Herein, the synthesis of monolithic and transparent silica aerogels highly loaded with Y3Al5O12:Ce nanocrystals (NCs) (up to 50 wt %) is reported. The developed composite aerogels can be impregnated with liquids, contrary to most of existing aerogels, which crack because of the strong capillary forces. Therefore, this system is designed as a novel concept of 3D porous scintillator, using the efficient photoluminescent and scintillating properties of Y3Al5O12:Ce. The investigated fluid containing low-energetic ionizing radiation emitters impregnates the material, which assures the efficient harvesting of radiation because of highly developed surface area. Such composites prove to be efficient new-type detectors of low-energy beta radiation both in liquids and gases

Periodic nanoporous inorganic patterns directly made by self-ordering of cracks

International audienceSolution-processed inorganic nanoporous films are key components for the vast spectrum of applications ranging from dew harvesting to solar cells. Shaping them into complex architectures required for advanced functionality, often need time-consuming or expensive fabrication. In this work, we show how crack formation is harnessed to pattern porous inorganic films in a single step and without using lithography. We developed aqueous ink formulations that, in the presence of polymeric latexes, enable evaporation-induced, defect-free periodic arrays of cracks with tunable dimensions over several centimeters. The ink formulation strategy was generalized to more than ten inorganic materials including simple and binary porous oxide and metallic films covering a whole spectrum of properties including insulator, photocatalytic, electrocatalytic, conductive or electrochromic materials. Notably, this approach enables three-dimensional self-assembly of cracks by stacking several layers of different compositions, yielding periodic assemblies of polygonal shapes and Janus-type patterns. The crack patterned periodic arrays of nanoporous TiO2 diffract light, and are used as temperature-responsive diffraction grating sensors. More broadly, this method represents a unique example of self-assembly process leading to long-range order (over several cm) in a robust and controlled way

Hierarchically structured lithium titanate for ultrafast charging in long-life high capacity batteries

International audienceHigh-performance Li-ion batteries require materials with well-designed and controlled structures on nanometre and micrometre scales. Electrochemical properties can be enhanced by reducing crystallite size and by manipulating structure and morphology. Here we show a method for preparing hierarchically structured Li4Ti5O12 yielding nano- and microstructure well-suited for use in lithium-ion batteries. Scalable glycothermal synthesis yields well-crystallized primary 4–8 nm nanoparticles, assembled into porous secondary particles. X-ray photoelectron spectroscopy reveals presence of Ti+4 only; combined with chemical analysis showing lithium deficiency, this suggests oxygen non-stoichiometry. Electron microscopy confirms hierarchical morphology of the obtained material. Extended cycling tests in half cells demonstrates capacity of 170 mAh g−1 and no sign of capacity fading after 1,000 cycles at 50C rate (charging completed in 72 s). The particular combination of nanostructure, microstructure and non-stoichiometry for the prepared lithium titanate is believed to underlie the observed electrochemical performance of material

Electrochemical Active Surface Area Determination of Iridium‐Based Mixed Oxides by Mercury Underpotential Deposition

International audienceThe electrochemical surface area (ECSA) is a critical property to describe, analyze and compare electrocatalysts. The determination of the mass activity of a given catalyst is associated with this parameter which can thus lead to materials benchmarking. Reliable and robust methods to measure ECSA are needed, and those have to accommodate different structures, morphologies and compositions. In this study, we investigate mercury underpotential deposition (HgUPD) as a way to estimate ECSA for ultraporous electrocatalysts based on iridium and iridium-molybdenum electrocatalysts for the oxygen evolution reaction. Results reveal a clear agreement between physisorption measurements and HgUPD with excellent reproducibility. The method shows also that pre- and post-catalysis surface area measurements are not affected by the catalytic process on short timescale, opening the possibility of electrocalyst stability and degradation monitoring

Aerosol synthesis of thermally stable porous noble metals and alloys by using bi-functional templates

International audiencePorous noble metal nanostructures providing high surface areas and large pore volumes are attractive for numerous applications especially catalysis. However, the synthesis of such architectures with nanoscale features is challenging because of i.e high surface energy. Previous reports are based on multi-step synthetic approaches with low yield thus not compatible with industrial scale. Herein, we demonstrate that hierarchical nanostructured noble metals and alloys, with thermal stability up to 800 °C, can be fabricated by a high throughput, green and straightforward aerosol-assisted synthesis using bi-functional reducing templates. We unravel the reduction mechanism at the solid/solid interface at the single-particle level using state-of-the-art techniques such as in situ Transmission Electron Microscopy, in situ synchrotron-based X-ray absorption spectroscopy. We propose a mechanism based on a radical process thermally triggered by the depolymerization of organic components. From a fundamental point of view, this study sheds light on the reduction processes of noble metals in the presence of organic compounds with important fundamental implications for catalysis and electrocatalysis