Sur des oxydes de cérium contenant du fer nanostructurés et de morphologies contrôlées

Dans le cadre de ce travail, des composés de formulation Ce1 xFexO2 x2 ont été synthétisésà l aide de deux protocoles : co-précipitation et synthèse assistée par chauffage micro-ondes.L utilisation de cette dernière a ainsi conduit à l obtention de nanoparticules de morphologiescubiques ou bâtonnets et ceci pour des temps de synthèse relativement courts. L analysepar diffraction X a montré notamment que le paramètre de maille diminue en fonction de lateneur en fer, x. L environnement local et le degré d oxydation du fer ont été analysés parspectroscopies Mössbauer, RPE et XANES mettant ainsi en évidence la présence d ions Fe3+isolés au sein de sites octaédriques distordus et sous forme de clusters. Une comparaison entreles deux voies de synthèse a montré que des différences apparaissent à l échelle locale. Lessolutions solides obtenues ont ensuite été caractérisées au cours du traitement thermique etsous différentes atmosphères. Indépendamment de l atmosphère de recuit, une démixtion dela solution solide intervient pour des températures proches de 600C.This work deals with the synthesis and characterization of Ce1 xFexO2 x2 nanoparticles.Two different synthesis routes were used : the coprecipitation technique and the microwaveassisted synthesis route. This later allowed the obtention of controled morphologies such asnanocubic or nanorod particles, characterized by HRTEM. Compared to hydrothermal synthesisroute the time of reaction was limited to one hour. X-Ray Diffraction analysis showedthat the lattice parameter decreases versus the iron content. Local environment and oxidationstate of iron were analyzed by Mössbauer, EPR and XANES spectroscopies showingthe presence of two kinds of iron sites : isolated (distorted) octahedral sites and clusters. Acomparison between both synthesis routes shows clearly the differences at a local scale. Thenanoparticles were then further characterized during annealing and under different atmospheres.The solid solution undergoes a demixtion phenomenon around 600C which does notdepend on the atmosphere conditions.BORDEAUX1-Bib.electronique (335229901) / SudocSudocFranceF

Controlling the {111}/{110} Surface Ratio of Cuboidal Ceria Nanoparticles

The ability to control size and morphology is crucial in optimizing nanoceria catalytic activity as this is governed by the atomistic arrangement of species and structural features at the surfaces. Here, we show that cuboidal cerium oxide nanoparticles can be obtained via microwave-assisted hydrothermal synthesis in highly alkaline media. HRTEM revealed that the cube edges were truncated by CeO2{110} surfaces and the cube corners by CeO2{111} surfaces. When adjusting synthesis conditions by increasing NaOH concentration, the average particle size increased. Although this was accompanied by an increase of the cube faces, CeO2{100}, the cube edges, CeO2{110}, and cube corners, CeO2{111} remained of constant size. Molecular Dynamics (MD) was used to rationalise this behaviour and revealed that energetically, the corners and edges cannot be atomically sharp, rather they are truncated by {111} and {110} surfaces respectively to stabilise the nanocube; both experiment and simulation agreed a minimum size of ~1.6 nm associated with this truncation. Moreover, HRTEM and MD revealed {111}/{110} faceting of the {110} edges, which balances the surface energy associated with the exposed surfaces, which follows {111}>{110}>{100}, although only the {110} surface facets because of the ease of extracting oxygen from its surface, which follows {111}>{100}>{110}. Finally, MD revealed that the {100} surfaces are ‘liquid-like’ with a surface oxygen mobility 5 orders of magnitude higher than that on the {111} surfaces; this arises from the flexibility of the surface species network that can access many different surface arrangements due to very small energy differences. This finding has implications for understanding the surface chemistry of nanoceria and provides avenues to rationalize the design of catalytically active materials at the nanoscale