Metal oxide, Mixed oxide, and hybrid metal@oxide nanocrystals : size-and shape-controlled synthesis and catalytic applications

Le contrôle de la taille et de la morphologie de nanocristaux d’oxydes métalliques simples, d’oxydes mixtes et d’oxydes métalliques hybrides est un sujet de grand intérêt. La dépendance de leur propriétés physio-chimiques avec leurs taille et morphologies, génèrent une variété de leur applications dans plusieurs domaines. Cependant, le dévellopement des nanocristaux en controllant la taille, la forme, l’assemblage et l’homogénéité de la composition chimique pour l’optimisation de propriété spécifiques demandent la combinaison de nombreux parametres de synthèse. Les trois différentes approches ont été développées dans le cadre de la thèse pour la synthèse d’une variété de nouveaux nanomatériaux d’oxydes simples, d’oxydes mixtes et d’oxydes métalliques hybrides dont la taille et la forme ont été bien controllées. Ces méthodes ont été nommées comme des méthodes solvo-hydrothermiques assistées par des molécules structurantes à l’état monophasique (eau ou eau/éthanol) et à l’état biphasique (eau-toluène). Nos approches de synthèse ont permi de préparer des nanocristaux des oxydes de métaux de transition (V, Cr, Mn, Co, Ni, In), et des terres rares (Sm, Ce, La, Gd, Er, Ti, Y, Zr), ainsi que des oxydes métalliques mixtes (tungstate, orthovanadate, molybdate). Ces nanomatériaux sont sous forme colloïdale mono-dispersée qui présente une cristallinité élevée. La taille et la forme de tels nanocristaux peuvent facilement être contrôlées par une simple variation des paramètres de synthèse telle que la concentration de précurseurs, la nature de la molécule structurante, la température et le temps de réaction. A large variété de techniques a été utilisée pour la caracterisation de ces nanomatériaux telles que TEM/HRTEM, SEM, SAED, EDS, XRD, XPS, FTIR, TGA-DTA, UV-vis, photoluminescence, BET. Les propriétés catalytiques de ces matériaux ont aussi été étudiées. Dans ce travail, le contrôle de la cinétique de croissance des nucléides ainsi que le mécanisme gouvernant la forme qui conduit à la taille et la morphologie finale du nanocrystal ont été proposé. L’effet de la taille et de la forme des nanoparticules d’oxyde métallique hybrides sur les propriétés catalytiques pour la réaction d’oxydation du CO et la photo-dégradation du bleue de méthylène a été aussi étudié. Car les catalyseurs existant actuellement à base de métaux nobles sont très couteux et en plus très sensibles à l’empoisonnement par le gas H2S ou les émissions polluantes de SOx. L’activité catalytique des nanocristaux d’oxydes métallique hybrides Cu@CeO2 de formes cubiques dans l’oxydation de CO et de Ag@TiO2 de formes de ceinture dans la photo dégradation du bleue de méthylène ont montré la dépendance de la taille et la forme des nanocristaux avec leur propriétés catalytiques.The ability to finely control the size and shape of metal oxide, mixed metal oxide, hybrid metal/oxide nanocrystals has become an area of great interest, as many of their physical and chemical properties are highly dependent on morphology, and the more technological applications will be possible for their use. Large-scale synthesis of such high-quality nanocrystals is the first and key step to this area of science. A tremendous effort has recently been spent in attempt to control these novel properties through manipulation of size, shape, structure, and composition. Flexibly nanocrystal size/shape control for both monodisperse single and multiple-oxide nanomaterial systems, however, remains largely empirical and still presents a great challenge. In this dissertation, new synthetic approaches have been developed and described for the synthetic design of a series of colloidal monodisperse metal oxide, mixed metal oxide, hybrid metal-oxide nanocrystals with controlled size and shape. These materials were generally characterized using TEM/HRTEM, SEM, SAED, EDS, XRD, XPS, FTIR, TGA-DTA, UV-vis, photoluminescence, BET techniques. Effect of the size and shape of these obtained hybrid metal-oxide nanocrystals on the catalytic properties is illustrated. We have developed three different new surfactant-assistant pathways for the large-scale synthesis of three types of nanomaterials including metal oxide, mixed metal oxide, hybrid noble-metal-oxide colloidal monodisperse nanocrystals. Namely, the solvo-hydrothermal surfactant-assisted methods in one-phase (water or water/ethanol) and two-phase (water-toluene) systems were used for the synthesis of metal oxide (transition metal-V, Cr, Mn, Co, Ni, In and rare earth-Sm, Ce, La, Gd, Er, Ti, Y, Zr) and mixed metal oxide (tungstate, orthovanadate, molybdate). The seed-media growth with the assistant of bifunctional surfactant was used for the synthesis of hybrid noble metal@oxide (Ag@TiO2, (Cu or Ag)@CeO2, Au/tungstate, Ag/molybdate, etc.) nanocrystals. A significant feature of our synthetic approaches was pointed out that most resulting nanocrystal products are monodisperse, high crystallinity, uniform shape, and narrow distribution. The size and shape of such nanocrystals can be controlled easily by simple tuning the reaction parameters such as the concentration of precursors and surfactants, the nature of surfactant, the temperature and time of synthetic reaction. The prepared nanocrystals with the functional surface were used as the building blocks for the self-assembly into hierarchical mesocrystal microspheres. The effective ways how to control the growth kinetics of the nuclei and the shape-guiding mechanisms leading to the manipulation of morphology of final products were proposed. Our current approaches have several conveniences including used nontoxic and inexpensive reagents (most using inorganic metal salts as starting precursors instead of expensive and toxic metallic alkoxides or organometallics), relatively mild conditions, high-yield, and large-scale production; in some causes, water or ethanol was used as environmentally benign reaction solvent. Catalytic activity and selectivity are governed by the nature of the catalyst surface, making shaped nanocrystals ideal substrates for understanding the influence of surface structure on heterogeneous catalysis at the nanoscale. Finally, this work was concentrated on demonstration of heterogeneous catalytic activity of hybrid metal-oxide nanomaterials (Cu@CeO2, Ag@TiO2) as a typical example. We synthesized the high-crystalline titanium oxide and cerium oxide nanocrystals with control over their shape and surface chemistry in high yield via the aqueous surfactant-assist method. The novel hybrid metal-oxide nanocrystals were produced by the depositing noble metal ion (Cu, Ag, Au) precursors on the pre-synthesized oxide seeds via seed-mediated growth. The catalytic activity of these metal-oxide nanohybrids of Cu@CeO2 nanocubes for CO oxidation conversion and Ag@TiO2 nanobelts for Methylene Blue photodegradation with size/shape-dependent properties were verified

Designing materials for electrochemical carbon dioxide recycling

Electrochemical carbon dioxide recycling provides an attractive approach to synthesizing fuels and chemical feedstocks using renewable energy. On the path to deploying this technology, basic and applied scientific hurdles remain. Integrating catalytic design with mechanistic understanding yields scientific insights and progresses the technology towards industrial relevance. Catalysts must be able to generate valuable carbon-based products with better selectivity, lower overpotentials and improved current densities with extended operation. Here, we describe progress and identify mechanistic questions and performance metrics for catalysts that can enable carbon-neutral renewable energy storage and utilization

From binary to multinary copper based nitrides - unlocking the potential of new applications

This review summarizes the current knowledge on the chemistry of binary copper(I) nitride, Cu3N and its multinary derivatives containing either main group or transition metal elements. For many years, research in this area was focused on the development of copper nitride prepared in the form of thin films. Successful deposition of these materials has been achieved mainly by employing physical methods, which have provided materials suitable for potential application in optical data storage. However, for the last decade, attention has also been devoted to expanding the available options by which Cu3N can be synthesized and deposited. Consequently, the focus has switched to the development of chemical synthetic methods towards the fabrication of this semiconductor and to broadening the range of related compounds that might be discovered. Simultaneously, the formulation of novel techniques and the successful preparation of new nanostructured functional materials has resulted in the rapid evolution of new and relevant applications; e.g. catalytic and electrochemical. The overview presented here concentrates on the chemical methods that have been devised to synthesise both bulk samples and thin films of Cu3N. Our article also shows how these approaches have been developed to achieve significant progress in the creation of multinary copper based nitrides and in identifying their potential applications. It provides a concise history of previous copper nitride research and sets the context for the most current advances. These will no doubt provide the springboard for future research areas that will impact both transition metal nitride chemistry and materials science more generally