Metal oxide, Mixed oxide, and hybrid metal@oxide nanocrystals : size-and shape-controlled synthesis and catalytic applications
|Abstract:||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.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||18 April 2018|
|Collection:||Thèses et mémoires|
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